Effects of confining stress and roughness on mechanical behavior of sand-steel structure interface

Effects of confining stress and roughness on mechanical behavior of sand-steel structure interface | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effects of confining stress and roughness on mechanical behavior of sand-steel structure interface Zilong Zhou, Yiqun Su, Shaofeng Wang, Jinbiao Wu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7800502/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 01 Apr, 2026 Read the published version in Granular Matter → Version 1 posted You are reading this latest preprint version Abstract This study presents an experimental investigation into the macro- and micro-scale shear mechanical behavior of sand-structure interfaces. Objectives include characterizing stress-displacement and volumetric responses, identifying optimal interface roughness, and understanding strain localization and kinematics failure mechanisms within shear and dilation zones. Direct shear tests were performed on sand interfaced with steel plates exhibiting varying, well-defined trapezoidal sawtooth roughness profiles ( R n ranging from 0 to 21.6) under normal stresses from 50 to 350 kPa. A modified direct shear apparatus integrated with PIV technology enabled real-time, non-contact monitoring and quantitative analysis of the sand deformation field, correlating macroscopic mechanical responses with microscale observations. Results showed that interface peak shear strength decreased in stress ratio ( τ η /σ η ) with increasing normal stress, with R n = 1.35 yielding the highest strength. Volumetric behavior transitioned from dilative to contractive-dilative modes as normal stress increased, with peak contraction near peak strength. Interface shear strength efficiency ( α ) generally decreased with increasing normal stress, indicating a transition from internal shearing within adjacent sand to predominantly interfacial sliding failure mode. PIV analysis provided direct visualization and quantification of shear band and dilation zone formation and evolution. The thickness and morphology of these zones were affected by both normal stress and interface roughness; higher normal stress generally suppressed dilatancy, while specific roughness profiles modulated strain localization. Microscale kinematics observations confirmed non-uniform deformation patterns, highlighting the critical role of particle overriding and rearrangement. The findings underscore the importance of integrating macro- and meso-scale to achieve a comprehensive understanding of sand-structure interface behavior. sand-structure interface interface roughness shear zone mechanical behavior Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 1. Introduction Soil-structure interfaces (SSIs) are integral to numerous geotechnical engineering structures, including pile foundations, retaining walls, reinforced soil structures, underground pipelines, and tunnels, where they play the critical role of transferring structural and environmental loads to the surrounding soil. For piles, for example, a substantial portion of capacity is mobilized through via skin friction at the pile-soil interface, as schematically illustrated in Fig. 1 . The performance of these structures largely depends on effective stress transfer between the soil and the structural elements, which is governed by the mechanical behavior of SSI (Chen et al., 2020 ; Liu et al., 2025 ; Wang et al., 2019 ; Xu et al., 2024 ). Accordingly, reliable prediction of interface shear behavior at SSIs, particularly for sandy soils against typical structural surfaces, is essential for stability and long-term performance of these geotechnical systems (Paikowsky et al., 1995 ; Zhang et al., 2006 ; Zhang and Zhang, 2009 ). Geometric characteristics of the structural surface directly govern the particle interlocking patterns, contact mechanics, and frictional mobilization, thereby shaping interface strength and dilatancy (Hryciw and Irsyam, 1993 ; Martinez and Frost, 2017 ). Shear deformation at SSI is inherently non-uniform, with strains concentrating in a narrow zone adjacent to the interface, forming distinct shear bands (Iwashita and Oda, 2000 ; Wang et al., 2007 ). The formation and evolution of these shear bands reflect complex internal deformation mechanisms and are key to interpreting the macroscopic mechanical behavior and ultimate failure modes (Alshibli and Sture, 2000 ). Therefore, investigating how structure surface geometry influences shear bands development and their associated mechanical behavior is essential for understanding the overall interfacial shear behavior and its fundamental mechanisms (Hu et al., 2004; Wang and Jiang, 2011 ; Zhang et al., 2006 ). In geotechnical engineering practice, structural surfaces range from the relatively smooth to rough surface with distinct macroscopic geometric configurations (Su et al., 2025 ; Zhou et al., 2019 ). Examples of the latter include the helical grooves on pile foundations, the profiled surfaces of pipe jacking segments, and, relevant to this study, trapezoidal sawtooth steel structural surfaces. These geometric configurations and their associated roughness significantly affect soil–structure interaction by altering particle contact behavior and interlocking mechanisms (Frost et al., 2002 ; Su et al., 2018 ; Uesugi and Kishida, 1986 ). Extensive studies have investigated the influence of interfacial geometric characteristics on the macroscopic mechanical response of SSIs using direct shear tests and found that rougher interfaces (inclusion of ribs, grooves, or specific textures) typically mobilize higher shear resistance and more pronounced dilatancy phenomena (Hu Liming and Pu Jialiu, 2004). Researchers have further introduced a critical or optimal roughness concept for sand–structure interfaces, beyond which increasing in interface roughness yield limited gains in friction angle, and the dominant failure mode may shift from interfacial sliding to shearing within the soil (Porcino et al., 2003 ). These findings highlight that geometry affects not only interface shear strength but also failure mechanisms. Shear zone formation is a prominent features of SSI failure and have been extensively verified through experimental observations and numerical simulations (Chen et al., 2023 ; Zhou et al., 2019 ). The formation and evolution of the shear bands largely govern the non-linear mechanical response of the interface. Numerous studies have aimed to estimate shear bands thickness, revealing that it is closely linked to the mean particle diameter and further influenced by factors such as soil density, confining pressure, and particle shape (Alshibli and Sture, 2000 ; Sadrekarimi and Olson, 2010 ; Zhou et al., 2019 ). Shear band development is typically accompanied by volumetric changes, expressed as either dilations (volume expansion) or contractions (volume reduction). For dense sandy soils, or under certain stress path conditions, interfacial shearing often induces dilative behavior, with localized volumetric expansion forming what is known as a dilation zone near the interface(Rowe, 1962 ; Oda and Kazama, 1998 ). Although conventional direct shear tests can provide the overall volumetric changes from vertical displacement measurement, they are unable to capture the internal shearing structure, the dynamic shearing zone formation, or the spatial extent of the dilation zone. Microscale studies indicate that dilatancy results from particle overriding, rearrangement, and rotation, which increase local porosity. These mechanisms are tightly coupled with the evolution of shear bands and are key to linking microstructural behavior with macroscopic interface response (DeJong and Westgate, 2009 ; Gu et al., 2014 ; Su et al., 2018 ). Although the presence of shear and dilation zones at soil–structure interface (SSIs) is well established, their underlying mechanisms remain challenging to fully understand. Current research largely relies on macroscopic experiments and numerical simulations. Conventional direct shear tests provide valuable data on interface shear strength and volumetric response but cannot reveal internal microscale deformation or failure processes due to their "black-box" limitations. Numerical approaches, particularly the Discrete Element Method (DEM), offer insights into particle-scale behaviors, such as force chains evolution and particle movements. However, most DEM studies employed idealized particle shapes (e.g., circular or spherical particles), which may not fully capture the complex interlocking effects and rotational constraints of real sand grains (Chen et al., 2023 ; Su et al., 2025 ). Thus, results are also sensitive to modelling assumptions, including particle size, contact models, and boundary conditions, underscoring the need for experimental validation. For specific regular shape of sand-structure interfaces involving with varying degrees of roughness, detailed experimental data on evolution of shear and dilation zones from microscale initiation to macroscale stabilization, remain scarce. Key gaps include accurate measurements of shear zone thicknesses and high-resolution internal deformation fields. Moreover, the physical mechanisms by which interface roughness and morphology govern the shear zone development, internal particulate kinematics and volumetric behavior are not yet fully elucidated. Establishing direct and quantitative links between these microscale evolutionary characteristics of shear and dilation zones and the macroscopic mechanical response of the sand-structure interface remains a significant challenge in the field. To address the aforementioned research gaps, this study investigates the macro- and micro-scale shear behavior of sand-steel structure interfaces by conducting shearing tests with a modified direct shear apparatus integrated with Particle Image Velocimetry (PIV) technique. Tests were conducted in dense sand against trapezoidal sawtooth steel surfaces across a range of normal stresses and relative roughness. This approach allows to directly capture microscopic sand-steel interface shearing kinematics, quantify shear zone thickness with objective criteria, and relate these kinematic metrics to macroscopic mechanical response under changing surface geometry and confinement stress. Thus, this study first introduces the physical properties of sand materials, the profiled steel geometries, and the modified PIV-integrated shear apparatus. Then macroscopic results, including stress-displacement behavior, volumetric response, and peak shear strength are analyzed. Furthermore, microscale analysis focuses on displacement and shear strain fields, and the quantitative evaluation of shear and dilation zone thicknesses. Finally, by integrating macroscopic mechanical responses and microscale observations, this study aims to clarify the combined influence of normal stress and interface roughness on the interfacial shear behavior, and to reveal the intrinsic physical mechanisms that govern these phenomena. 2. Methods and Materials 2.1 Test apparatus To facilitate the observation of sand deformation, a conventional direct shear test apparatus was modified, as depicted in Fig. 2 . The original equipment comprised a commercial strain-controlled Direct Shear Test (DST) system (Model: ShearTrac II, Manufacturer: Geocomp), which featured a rectangular shear box with dimensions of 100 mm × 100 mm × 42 mm. These shear box dimensions are in compliance with the requirements stipulated by ASTM (2004) D3080-04, wherein the dimensions are specified to be between six and ten times the maximum particle diameter. A transparent observation window made of high strength annealed glass was integrated into the apparatus to support Particle Image Velocimetry (PIV) measurements, thereby enabling researchers to visually and quantitatively assess the complex shear behavior at the sand-steel structure interface. Critically, annealed glass possesses low coefficient of friction, which can effectively minimize the frictional interference between the window and the enclosed sand mass, consequently reducing end-boundary effects. By minimizing such boundary influences, the experimental conditions more closely approximate an idealized plane strain state, thereby enhancing the accuracy and reliability of deformation field observations and measurements. 2.2 materials properties The granular material used in this study was Stockton Beach sand, hereinafter referred to as STK sand for brevity. This material is a washed, uniform quartz silica sand, characterized by a quartz content exceeding 98%. The particle size distribution of the sand is presented in Fig. 3 , which can be classified as fine sand, with a median particle size (D 50 ) of 0.37 mm and a maximum particle size (D max ) of 0.62 mm. In accordance with the Unified Soil Classification System (USCS; ASTM 2011), the sand is classified as poorly graded sand (SP); under Australian Standard AS-1289.3.6.1 (Standards Australia 2009), it is classified as medium sand. The minimum and maximum dry unit weights, determined as per Australian Standard AS-1289.5.5.1 (Standards Australia 1998), are 14.5 kN/m³ and 17.2 kN/m³, respectively. The specific gravity of the soil solids (Gs), measured using a gas pycnometer, was determined to be 2.65. Other gradation characteristics of the STK sand are summarized in Table 1 . Table 1 Grading properties of STK sand D 10, mm D 30, mm D 60, mm C u C c e min γ max, kN/m 3 e max γ min, kN/m 3 0.10 0.37 0.41 4.10 2.34 0.51 17.2 0.79 14.5 Note: D 10 , D 30 , D 60 , soil particle diameter at which 10%, 30%, and 60% of the mass of a soil specimen is finer, respectively; C u , uniformity coefficient; C c , coefficient of curvature; e min , e max , minimum and maximum void ratios associated with the maximum, γ max , and minimum, γ min , unit weights, respectively. The sand-steel structure interface model was constituted by STK sand in contact with a steel surface, the three-dimensional schematic representation of which, along with the defined coordinate axes, is depicted in Fig. 2 . This structured surface features an isosceles trapezoidal profile with base angles of 45°, satisfying the geometric condition S 1 = S 2 = peak-to-valley height (h). This configuration creates a typical periodic, non-clogging surface, where the volume of the interfacial depressions is invariably equal to the volume of the protrusions. Consequently, surface roughness can be systematically varied by adjusting by only the height parameter h . In this study, six rough surfaces were prepared with h values of 0.25, 0.5, 1, 2, 4, and 8 mm. An additional surface with h value of 0 mm, representing a nominally smooth interface, was included to provide a baseline for comparative analysis. Following the definition by Uesugi and Kishida ( 1986 ), the relative roughness ( R n ) of surface is expressed as R n = R max / D 50 , where R max represents the maximum vertical distance between the highest and lowest points of the interface surface profile, and D 50 is the mean particle diameter. In this case, R max is equivalent to h . The calculated relative roughness R n values for all tested surfaces are listed in Table 2 . Table 2 Roughness of sand-steel structure interface tested h , mm 0 0.25 0.5 1 2 4 8 R max , mm 0 0.25 0.5 1 2 4 8 R n 0 0.68 1.35 2.7 5.4 10.8 21.6 2.3 experimental procedures In this study, both Direct Shear Tests (DSTs) on sand and Sand-steel structure Interface (SSI) tests were conducted. Dense sand specimens with a relative density D r = 92% were prepared using the sand pluviation method. The core principle of this technique involves controlling the height of fall of the sand, allowing it to naturally deposit and achieve the target density within the shear box. Preliminary calibration tests were performed to determine the optimal pluviation height required to obtain the desired relative density. During specimen preparation, sand was uniformly and continuously pluviated from the calibrated height into the shear box. Upon completion of sand filling, the specimen surface was carefully leveled by removing any excess sand. For SSI tests, steel interface plates with varying surface roughness were placed in the lower half of the shear box, and dry sand was subsequently pluviated into the upper half. For the pure sand DSTs, the sand was pluviated directly into the assembled shear box without any interface plate. All tests were performed under vertical normal stresses σ n of 50, 150, 250, and 350 kPa. The parameters for the experimental conditions are detailed in Table 3 . The shear rate of 0.5 mm/min was applied in the shear test until a horizontal shear displacement ( u ) of 10 mm was reached. To capture the deformation near the interface, a high-resolution camera was used to record images of the sand particles in the vicinity of the interface at 15-second intervals. These images were subsequently analyzed using PIVlab software to compute the horizontal displacement field ( u ), vertical displacement field ( v ), and shear strain rate field of the sand particles, thereby quantifying the particulate kinematics at the interface. Table 3 Experimental test conditions Test No. R n σ n (kPa) D r (%) 1 0 50 91.68 2 150 90.44 3 250 91.25 4 350 91.94 5 0.68 50 91.24 6 150 90.54 7 250 91.46 8 350 89.50 9 1.35 50 90.58 10 150 90.71 11 250 91.69 12 350 91.05 13 2.7 50 89.68 14 150 90.60 15 250 92.46 16 350 91.64 17 5.4 50 91.52 18 150 90.19 19 250 91.14 20 350 91.12 21 10.8 50 90.18 22 150 90.89 23 250 90.38 24 350 89.59 25 21.6 50 92.44 26 150 90.96 27 250 91.01 28 350 91.06 3. Results and analysis 3.1 Macroscopic Shear Behavior and Interface Strength The relationships between the stress ratio ( τ n / σ n ) and shear displacement ( u ), as well as the vertical displacement ( δ v ) and shear displacement ( u ) obtained from the direct shear tests on sand are presented in Fig. 4 . The stress ratio τ n / σ n reflects the intrinsic macroscopic shear characteristics of the sand material itself, embodying its capacity to resist internal shear slip along a predefined shear plane under the applied normal stress. This ratio effectively reflects the operative internal friction coefficient of the sand. The dilatancy behavior of sand is primarily influenced by the applied normal stress. As the normal stress increases from 50 kPa to 350 kPa, the associated dilatant volume change monotonically decreases, indicating a progressive suppression of dilatancy. Notably, the dilatancy curves corresponding to normal stresses of 250 kPa and 350 kPa are closely aligned, suggesting that beyond a certain threshold, additional increases in normal stress yield only marginal reductions in dilative behavior. The peak shear strength of the sand comes from two key components: the resistance required to overcome inter-particle friction and interlocking, and the energy expanded against dilatancy. When the normal stress increases from 50 kPa to 150 kPa, enhanced compaction of the particles occurs, leading to improved inter-particle contact and interlocking effect. Although dilatancy begins to be inhibited at this stage, its influence remains relatively minor. As a result, the peak stress ratios at 50 kPa and 150 kPa are comparable. However, as the normal stress is further increase to 250 kPa and 350 kPa, the dilatancy suppression effect becomes significantly more pronounced. The increased confinement significantly restricts particle movement and volumetric expansion, leading to a noticeable decline in the contribution of dilatancy to peak strength and, consequently, a reduction in the peak stress ratio. The residual shear strength characterizes the sand’s fundamental frictional behavior after it has undergone structural degradation due to shearing. At this stage, the inter-particle interlocking mechanisms are largely destroyed and dilatancy has essentially ceased. The magnitude of the residual shear strength is primarily governed by the sliding friction between particles and their final arrangement after shearing stabilization. Under a normal stress of 50 kPa, the inter-particle contact forces are relatively low, resulting in a relatively loose structure after rearrangement. This limited normal confinement stress is insufficient to fully mobilize the potential frictional resistance, leading to lower residual shear strength. As the normal stress increases to 150 kPa, the residual shear strength improves. At this stage, sand experiences significant dilatancy and particle rearrangement, and forms a denser structure that is more favorable to shear resistance. However, at even higher normal stresses, increased particle alignment in the shear direction and greater plastic deformation may slightly reduce the residual strength, despite higher contact stresses. Fig. 5 presents the shear stress ratio versus shear displacement (τn/σn-δh) and vertical displacement versus shear displacement (δv-δh) curves for the sand-steel structure interface with a relatively roughness Rn=0. Due to the extremely smooth nature of the structure surface at this condition, the interface exhibits minimal shear strength, and shear failure readily occurs within a thin layer of sand immediately adjacent to the interface. This localized zone of sand quickly reaches a residual state under shearing. During this internal shearing process, only minor volumetric changes and characteristic of the sand inherent shearing behavior occurs together with dilatancy developing slowly and gradually stabilizing. As shear displacement increases, boundary constraint effects near the ends of the shear box become more pronounced. The soil near the leading edge of the shear box experiences progressive compression, while the soil at the trailing edge undergoes internal readjustment due to the forward movement of the sand mass. This results in non-uniform deformation across the specimen, which is reflected in the vertical displacement measurements. In later stages of loading, these boundary-induced distortions lead to fluctuations and an apparent uplift in the δv-δh curve. Figure 6 presents the relationships between the stress ratio ( τ n / σ n ) and shear displacement ( δ h ), as well as the vertical displacement ( δ v ) and shear displacement ( δ h ), for sand-interface interface tests conducted at various relative interface roughness values ( R n = 0.68, 1.35, 2.7, 5.4, 10.8, and 21.6) under four normal stress levels ( σ n = 50, 150, 250, and 350 kPa). The corresponding peak and residual friction angles derived from these tests are also shown in Fig. 7 . The peak shear stress to normal stress ratio ( τ n /σ n ) of the interface exhibits a clear stress dependency on both normal stress and interface roughness. In general, τ n / σ n decreases with increasing normal stress, indicating that shear strength becomes progressively less sensitive to interface characteristics under higher confinement. At a low normal stress of 50 kPa, all roughness interface profiles mobilize substantial shear resistance. In particular, under the condition of R n = 1.35, the interface mobilizes the highest peak strength, with a τ n / σ n value exceeding 1.0. However, as the normal stress increases, peak shear strength diminishes for all roughness levels. The peak shear strength also displays a non-monotonic relationship with the relative interface roughness, revealing an optimal relative roughness of. Across all interfaces, this specific interface configuration ( R n = 1.35) consistently yields the highest peak strength for all tested normal stress levels. Deviations from this optimal roughness value, whether toward smoother or rougher surfaces, lead to reduced shear resistance. Additionally, the effect of roughness becomes less pronounced at higher normal stresses, suggesting that the superiority of the optimal roughness ( R n = 1.35) is attenuated as confinement stress increases. A distinct strain-softening phenomenon, characterized by peak strengths significantly exceeding residual strengths, is observed for all experimental conditions. This softening most prominent under low normal stress and at the optimal roughness ( R n = 1.35). With continued shearing beyond the peak shear strength, the shear stress ratio τ n /σ n converges towards a relatively stable residual value, typically in the range of 0.6 to 0.8. These residual values reflects that the interfacial behavior ultimately tends to be governed by the critical state characteristics of the sand itself. Nevertheless, the residual friction angle still exhibits a slight stress dependency. Furthermore, the volumetric deformation mode of the interface is also dependent on normal stress and interface roughness. At normal stress of 50 kPa, all interfaces, irrespective of their roughness, exhibit continuous dilatant behavior throughout the entire shearing process. Similar to peak strength, the magnitude of dilatancy also shows a non-monotonic relationship on roughness levels, with the maximum dilatancy occurring at the optimal roughness of R n = 1.35. At higher normal stresses, particularly for rougher interfaces, the behavior transitions to a more complex contractive–dilative mode. In this case, initial volumetric contraction is followed by dilation, with the point of maximum contraction occurring near the peak shear strength. This pattern indicates an intrinsic link between shear strength mobilization and the transition in volumetric change mechanisms. To further evaluate the shear capacity of the sand-steel structure interface and to identify the associated failure modes under different interface roughness conditions, the interface shear strength efficiency ( α ) is introduced. This parameter is defined as the ratio of the peak shear strength of the soil-structure interface ( τ s−g ) to the peak shear strength of the sand itself ( τ s ) under the same normal pressure: \(\:\alpha\:=\frac{{\tau\:}_{s-g}}{{\tau\:}_{s}}\) . Figure 8 presents the variation of α across different interface roughness values and normal stresses levels. This efficiency coefficient ( α ) directly reflects how effectively the interface, transmits shear stress compared to the surrounding soil mass itself. It also serves as an indicator for inferring the potential failure mode. Specifically, α > 1 indicates high interfacial shear efficiency, where the interface mobilizes greater shear strength than the soil itself, suggesting that failure likely occurs within the sand mass adjacent to the interface; α ≈ 1 implies comparable shear strength between the interface and the sand, making the failure mode uncertain – it may occur at the interface, within the soil, or through a combination mechanism; α < 1 suggests relatively low interfacial efficiency, implying that the interface itself acts as a weak plane where failure is prone to occur along the contact surface or within a very thin adjacent sand layer. For all tested relative roughness levels ( R n ), α generally decreases with increasing normal stress. For instance, in the case of R n = 1.35, α decreases from 1.31 at 50 kPa to 1.07 at 350 kPa. This trend indicates that even at high stress levels, this particular interface retains sufficient interlocking and dilatancy effects to cause the failure path to deviate slightly from the interface into the adjacent soil, albeit this tendency is considerably less pronounced than at lower stress levels. Among all roughness conditions, R n = 1.35 generally exhibits the highest α values all normal stress levels. Under low normal stress conditions, α exceeds 1.0 for all roughness levels, suggesting that the potential failure mode is more likely to involve shearing within the sand adjacent to the interface and develop a relatively thicker shear band. As normal stress increases, α values for some roughness conditions may decrease to below 1.0. For interfaces where α approaches or falls below 1.0, the failure mode is more likely to transit toward interface-dominated sliding failure or localized shearing within a very thin layer adjacent to the contact surface. 3.2 Mesoscopic Visualization Analysis of Interfacial Shear Deformation The macroscopic direct shear test results have comprehensively characterized the mechanical response of the sand-steel structure interface, demonstrating clear dependencies of peak strength and volumetric deformation behavior on both normal stress and relative interface roughness. Several key phenomena observed in these tests warrant deeper investigation. These include: (1) the existence of an optimal roughness that maximizes interfacial performance; (2) the transition in volumetric deformation behavior with increasing stress levels, specifically the complex contractive-dilative mode observed under high stress, which is closely associated with peak strength mobilization; (3) the relationship between the interface strength efficiency and the failure mode evolution. While these macroscopic findings delineate the overall performance characteristics of the interface, they cannot directly reveal the underlying physical mechanisms driving these complex behaviors. To overcome the inherent black-box limitations of macroscopic testing and to gain insight into the localized deformation and failure processes, this study employs Particle Image Velocimetry (PIV) for microscale analysis (Stanier et al., 2016 ). PIV is a two-dimensional digital image processing technique that enables non-intrusive measurement of particulate motion within the observation plane. Subsequent processing of the observational data using PIVlab software focuses on analyzing the critical mechanical mechanism transitions and complex interactions manifested in the aforementioned macroscopic phenomena. Therefore, typical experimental conditions representative of these core phenomena were selected for detailed investigation. Specifically, deformation fields of the optimal roughness interface ( R n = 1.35) under different normal stresses (150 kPa and 250 kPa) were compared to directly observe how increasing confining stress inhibits dilatancy and triggers a transition toward the contractive-dilative behavior. Furthermore, tests at representative high normal stress (250 kPa) were selected to compare the interfaces behavior with different roughness ( R n = 1.35, 2.7, and 10.8). This comparison aimed to clarify how geometric features influence strain localization, particulate kinematics, and potential failure modes development, thus offering new insights into the origins of the non-monotonic roughness-shear strength relationship. Figure 9 presents the evolution of particle displacement fields ( u , v ) and shear strain rate fields, as captured through PIV analysis under various normal stress and interface roughness conditions. In direct shear tests on sand-steel structure interfaces, shear deformations is primarily localized within a narrow interaction zone adjacent to the structural surface, referred to as the shear band. This localized zone represents the primary area of strain concentration and deformation. For the optimal roughness interface ( R n = 1.35), the horizontal displacement field ( u ) reveals a shear band characterized by well-defined morphology and smooth, coherent boundaries under different normal stresses. Initiating from the horizontal edges of the specimen, the shear band exhibits a distinct upward trajectory, deviating from the interface profile and propagating into the overlying soil mass. Ultimately, a uniformly developed localized shear zone is formed at a certain distance above from the physical interface. In contrast, for interface with R n = 2.7 and R n = 10.8, the shear band initial development closely aligns with the geometric profile of the rough physical interface, particularly near the specimen's horizontal boundaries. Only in the central portion of the specimen does the shear band observes a limited upward extension. As the roughness increases from R n = 2.7 to R n = 10.8, the upward propagation appears more constrained, and the main body of the shear band tends to follow the interface more closely. Methodologies based on particle displacement gradients for the quantification of localized shear bands have been widely employed in the study of soil-structure interaction problems. Analysis of the vertical displacement field ( v ) indicates the influence of stress and deformation behavior. Under higher normal stress, significant particle rearrangement and densification occur even in the initial stages of shearing, leading to an overall contractive response in the sand specimen. Nonetheless, under all tested conditions, pronounced dilatancy is observed immediately adjacent to the interface roughness elements. The shape of reflects the underlying configuration of the interface, often displaying an undulatory or sawtooth morphology. A comparison between results at 150 kPa and 250 kPa reveals that although particle displacement gradients or relative movements within the dilation zone may appear more intense at 250 kPa, the overall vertical extent of dilatant zone (i.e., the thickness of the dilation zone) is slightly less than that observed at 150 kPa. This observation provides direct visual confirmation of the dilatancy-suppressing effect of increased normal stress. Notably, the interface with R n = 2.7 exhibits a comparatively larger dilation zone height at 250 kPa, which contrasts with cumulative dilatancy trends observed at the macroscopic scale. For the R n = 10.8 interface, dilatancy phenomenon is predominantly concentrated within troughs between the roughness elements, resulting in a limited upward spread and a comparatively thinner overall dilation zone. The shear strain rate field clearly depict the formation path and temporal evolution of the shear band. Under the optimal roughness condition, the high shear rate zone demonstrates a clear tendency to propagate into the soil mass from the initial stages of shearing, with the core of the ultimately formed shear band being relatively distant from the physical interface. Conversely, for interfaces with roughness values of R n = 2.7 and R n = 10.8, the initial shear band nucleate at the horizontal boundaries of the specimen and develop along the rough surface toward the center. In these higher roughness interfaces, shear bands is tend form at smaller shear displacements and remain more confined to the vicinity of the interface. The aforementioned visual analysis of the PIV-derived displacement fields ( u , v ) and shear rate fields provides intuitive and valuable insights into the location, shape of the shear band, and the distribution characteristics of the dilation zone under various experimental conditions. However, qualitative visual observation alone is insufficient for the accurate and objective quantification of the thicknesses of these localized deformation zones, nor does it enable rigorous comparisons across different interface configurations and confinement stress levels. To overcome these limitations and enable a more precise characterization of shear and volumetric deformation, this study introduces a quantitative approach based on averaged displacement component profiles as key indicators. Specifically, the spatial distributions of normalized displacement is analyzed as functions of distance from the sand-steel structure interface, and regions exhibiting significant changes in displacement gradients can be identified. Smooth curves, f(d) , are obtained through quadratic spline interpolation. The first derivative, f'(d) , and the second derivative, f''(d) , can then be computed using finite difference methods. Finally, the curvature, κ , can be calculated according to the formula: \(\kappa =\frac{{|f^{\prime\prime}(d)|}}{{{{\left( {1+{{[f^{\prime}(d)]}^2}} \right)}^{3/2}}}}\) . Conventionally, the boundaries of the shear band and dilation zone can be defined as the locations where the normalized horizontal or vertical displacement gradient reaches its maximum value. Based on this procedure, two quantitative parameters are obtained: the shear band thickness ( δ s ) and dilation zone thickness ( δ d ). These parameters enable consistent, objective comparison across experimental conditions and allow for robust evaluation of how interface roughness and normal stress influence the spatial extent of localized deformation. Figure 10 illustrates the progressive determination of the soil-structure interface shear zone at various shear displacement stages under a normal stress = 150 kPa and interface relative roughness R n = 1.35. The identification process incorporates particle movement tracking, average displacement profiles, and the curvature analysis to define the shear band boundaries at each stage of deformation. Subsequently, Fig. 11 presents the evolution of the normalized shear band thickness as a function of shear displacement for this same condition. In this analysis, a Gaussian kernel density estimation method was employed to compute the two-dimensional spatial density values of particles undergoing shear displacement at various locations. The color intensity of the scattered points represents the data points concentration in that region, with a density value approaching 1 indicating a high data points density. Notably, the regions of high data point density strongly coincide with the distribution curve of the normalized displacement component versus distance from the contact surface. This agreement indirectly validates the effectiveness of using averaged displacement distribution curve method to identify regions characterized by significant changes in displacement gradients. Based on the observed shear strain rate field, the formation of a fully developed shear band is considered to occurred when a continuous high shear rate zone extends across the specimen. In all tests, this continuous shear band was observed to be fully developed when the shear displacement was between 20% and 40% of the total applied displacement. Accordingly, the quantitative analysis of shear band thickness evolution commences from a shear displacement corresponding to 40% of the total shear displacement. The evolution of shear band thickness displays a non-linear increase trend with shear displacement. However, the rate of this increase gradually diminishes and eventually stabilizes, indicating convergence towards a consistent shear band thickness. This behavior suggests that the particle rearrangement process ultimately culminates in the formation of a complete and stable shear band. Figure 12 illustrates the evolution of normalized shear band thickness with shear displacement across a range of normal stress levels and surface roughness conditions. Following this, the final value of normalized shear band thickness attained under each condition is summarized in Fig. 13 . A systematic trend is observed where the normalized shear band thickness decreases with an increase in the applied normal stress. For instance, under the optimal roughness condition of R n = 1.35, the shear band thickness reduces from approximately 7.19 D 50 at 50 kPa to approximately 6.28 D 50 at 350 kPa. Similarly, for the highest roughness condition of R n = 21.6, the shear band thickness also decreases from approximately 4.60 D 50 at 50 kPa to approximately 3.64 D 50 at 350 kPa. In addition to the stress-dependent behavior, a significant non-monotonic relationship is observed between normalized shear band thickness and the relative interface roughness. At each normal stress level, the thickest shear band consistently occur at the optimal relative roughness of R n = 1.35, indicating an optimal roughness condition that mobilizes more distributed strain localization. For example, under a normal stress of 250 kPa, the shear band thickness corresponding to R n = 1.35 is 6.68 D 50 , whereas for R n = 0.68 and R n = 21.6, the thicknesses decrease to 5.16 D 50 and 4.12 D 50 , respectively. Figure 14 presents the evolution of dilation zone thickness with shear displacement under a specific condition ( σ n = 150 kPa, R n = 1.35). Based on the understanding of dilation zone development, Fig. 15 extends this comparison to summarize the final normalized dilation zone thickness across a varying range of normal stress levels and surface roughness conditions. The dilation zone is generally considered to be spatially coincident with or encompassed within the shear band. Experimental observations confirm that the dilatancy phenomenon predominantly occurs within the localized shear band region. For all tested relative roughness values, the normalized dilation zone thickness exhibits a pronounced decreasing trend with increasing normal stress. For example, in the case of R n = 2.7, the dilation zone thickness decreases from approximately 5.96 D 50 at 50 kPa to approximately 4.11 D 50 at 350 kPa. Similarly, for R n = 1.35, the thickness significantly reduces from approximately 6.45 D 50 at 50 kPa to approximately 3.07 D 50 at the same stress level. Under low normal stress conditions (e.g., 50 kPa), the dilation zone thickness increases with R n , reach a peak value (approximately 6.45 D 50 ) at R n = 1.35, and then decreases with further increases in R n . This pattern aligns with the observed macroscopic dilatancy and peak strength behavior under the same stress condition. However, at higher normal stress levels (150–350 kPa), a shift in the optimal roughness corresponding to the maximum dilation zone thickness is observed. Specifically, under relatively higher stress conditions, R n = 2.7 exhibits the maximum dilation zone thickness, while previous optimal R n = 1.35 yields comparatively lower values, even falling below the value observed for R n = 0.68 at 350 kPa. For structure surface with significantly higher roughness (e.g., R n = 10.8, 21.6), the dilation zone thickness remains generally low across all normal stress level. These results suggest that excessive roughness may suppress dilative deformation by confining particle movement near the interface. Volumetric expansion during shearing is an inherent aspect of the overall shear deformation mechanism. Consequently, the region experiencing significant dilatancy is, in spatial terms, expected to closely overlap with the zone of intense shear deformation. Accordingly, the dilation zone is generally considered to be contained within the broader shear band. A comparison between the normalized shear band thickness and normalized dilation zone thickness across all tested combinations of relative interface roughness ( R n ) and normal stress ( σ n ) confirms this spatial relationship. In every case, the calculated shear band thickness values consistently exceed the corresponding dilation zone thickness. For instance, under the optimal roughness condition of R n = 1.35 and a normal stress of 250 kPa, the shear band thickness is approximately 6.68 D 50 , whereas the dilation zone thickness is approximately 4.47 D 50 . This quantitative disparity directly supports the conclusion the volumetric expansion is indeed spatially contained within the broader zone of shear band. Visual analysis of the shear strain rate field and the vertical displacement field ( v ) under identical experimental conditions further support this interpretation. A strong spatial correlation is observed between the regions of concentrated shear strain and regions where significant dilatancy occurs. The sawtooth or undulatory distribution characteristic of the dilatancy is invariably observed within the zone influenced by high shear rates and closely follows the geometric profile of the interface. These observations confirm that shear-induced dilatancy does not occur in isolation. Rather, it emerges as a secondary effect within regions undergoing substantial shear deformation. In essence, dilatancy is both initiated and constrained by the extent of strain localization imposed by the interface roughness and applied stress conditions. 3.3 Influence of Normal Stress and Contact Surface Roughness A comprehensive set of direct shear tests was conducted on sand-steel structure interfaces under varying normal stresses (50–350 kPa) and relative interface roughness values ( R n = 0.68–21.6). The results, derived from both macroscopic mechanical responses (e.g. stress ratio – shear displacement and vertical displacement-shear displacement curves) and microscale deformation fields (via PIV-observed u , v , and shear strain rate), along with quantitative metrics (normalized shear band and dilation zone thickness), reveals a complex and coupled influence of normal stress and interface roughness on interfacial shear behavior. These findings are summarized as follows. 3.3.1 Influence on Peak and Residual Shear Strength Normal stress and relative interface roughness jointly and significantly influence both peak and residual shear strengths of the sand-steel structure interface. However, the governing mechanisms and sensitivity of these two strength parameters to normal stress and roughness are different. These differences lie in the microscale interaction mechanisms between particles and the rough surface in the vicinity of the interface, primarily involving particle overriding, interlocking, and embedment effects. The peak shear strength of the interface demonstrates a high sensitivity to both normal stress ( σ n ) and relative roughness ( R n ). It generally decreases with increasing σ n , a trend primarily attributed to the suppression effect of confinement stress on interfacial dilatancy. As depicted in Fig. 16 (c), high normal stress impedes particle overriding by suppressing upward particle movement, thereby directly reducing the dilatancy-driven component of shear resistance that is more easily mobilized under low normal stress conditions, as shown in Fig. 16 (b)). In addition, the peak shear strength exhibits a clear non-monotonic relationship with the relative interface roughness ( R n ). For all tested normal stress levels, an optimal relative roughness ( R n = 1.35) is identified, at which the interface is capable of mobilizing the highest peak strength. This suggests that R n = 1.35 provides a geometric scale that most effectively match the particulate characteristics of the employed STK sand, enabling optimal interlocking. As depicted in Fig. 16 (b), this condition facilitates a tight interlocking arrangement between sand particles and structure interface, thereby most effectively activating the particle overriding mechanism and the internal shear resistance near the interface. When the interface roughness deviates from this optimal value, either becoming too smooth (leading to insufficient interlocking and increased interfacial sliding) or too rough (causing particles partially to embed deeper troughs of the roughness elements and the shear band to elevate above the interlocking zone), the interlocking efficiency decreases, consequently resulting in reduced peak shear strength. It is noteworthy that the influences of normal stress and roughness are coupled. At high normal stress conditions, the universal and pronounced suppression of dilatancy (compare to the relatively unconstrained overriding with the inhibited overriding in Fig. 16 (c)), the differences in peak strength among interfaces with varying roughness are relatively diminished. Although higher normal stress can enhance interlocking between particles (see Fig. 16 (c)), sand particle overriding motion required to overcome this strong interlocking becomes more difficult, thereby dampening the advantage of the optimal roughness at high stress levels. In contrast, the residual shear strength, which emerges after large shear displacements, tends to stabilize and shows weaker sensitivity to both interface roughness and normal stress compared to the peak shear strength. This characteristic strongly indicates that the interfacial behavior in the residual stage is predominantly governed by the critical state properties of the sand itself. At this state, the residual strength primarily reflects the fundamental frictional properties of the sand under steady-state shearing conditions. Although the residual strength still displays a slight tendency to decrease with increasing normal stress and exhibits a rather complex dependency on interface roughness. This may be attributed to final the stable particle fabric formed after large shearing, residual interfacial geometric effects, or subtle ongoing particle rearrangements within the shear band. 3.3.2 Influence on Shear Band Characteristics The normalized shear band thickness consistently decreases with increasing normal stress, a trend directly linked to the suppression of dilatancy under high confining stress. As illustrated Fig. 16 (c), high normal confining stress impedes the upwards overriding of sand particles, thereby forcing them to rearrange and slide within a more constrained space. This results in a more localized concentration of shear deformation and a correspondingly thinner shear band. With respect to interface roughness, the shear band thickness exhibits a non-monotonic relationship, reaching a maximum at the optimal roughness of R n = 1.35. PIV observations clearly demonstrate that at this optimal roughness, sand particles form an efficient interlocking state with the interface roughness elements (as depicted in Fig. 11 ). This enhanced interlocking necessitates that the shear failure path develops via a combination of particle overriding (as shown in the effective overriding mode in Fig. 16 (b)) and internal shearing, thereby extending the shear band upward into the soil mass above the interface. This resulting deformation process mobilizes a larger volume of sand particles through complex rearrangement, rolling, and sliding, thus forming the thickest shear band. At this stage, the interface shear strength efficiency α exceeds 1.0, indicating that the interface shear resistance, primarily induced by interlocking and dilatancy, is significantly greater than that of the sand. Thus, the shearing failure tends to occur within the soil adjacent to the interface rather than along the contact surface. However, as the normal stress increases or the roughness deviates from the optimal value, the interface shear strength efficiency α decreases. When α approaches or falls below 1.0 (as shown by PIV visualizations in Fig. 9 ), the failure mode is more likely to occur along the interface or in a thin zone immediately adjacent to it. For example, at higher roughness levels ( R n >1.35), particles may partially embed into the deeper troughs of the rough surface. In such scenarios, the shear band tends to form above the rough surface, bypassing the primary interlocking zone and thus reducing interface interlocking efficiency. Meanwhile, the initial development of the shear band at the specimen ends is more closely aligned with the interface and involves a greater degree of interfacial sliding rather than complex and extensive particle shearing within the soil mass. Mechanistically, the shear band thickness is governed by two primary factors, which are the intensity of particle interlocking and the magnitude of particle overriding. The former determines how many particles must be mobilized to overcome geometric constraints and initiate shear deformation. Stronger interlocking requires more particles involved in the rearrangement process, thus contributes to form a thicker shear band. The latter dictates the vertical deformation space required to accommodate sand particles movement within the shear band during shearing. Increasing normal stress suppresses sand particles overriding (see in Fig. 16 (c)), which reduces the space available for deformation and narrows the shear band. Overall, relative interface roughness R n governs shear band thickness in a non-monotonic fashion, as modulates both interlocking efficiency and overriding patterns. The present study identifies an optimal R n = 1.35, at which the balance between particle interlocking and overriding yields maximum mobilized volume of soil in shear deformation and consequently resulting in the thickestshear band. 3.3.3 Influence on Volumetric Strain Behavior The normal stress level is a critical determinant of the volumetric deformation mode. Under low stress (e.g. 50 kPa), the interface exhibits purely dilative behavior, as relatively low confining stress allows sand particles to readily rearrange and expand volumetrically. In contrast, at relatively higher stress levels (≥ 150 kPa), the volumetric response transits to an initial contractancy followed by subsequent dilatancy. This shift reflects the initial densification and rearrangement of particles driven by under higher confining stress, with sand specimen dilatancy manifesting only after sufficient shear displacement to overcome this resistance. Interface roughness further activate dilative behavior by introducing geometric obstructions that promote particle interlocking and overriding. Under low normal stresses, the interface with roughness R n = 1.35 yields the maximum macroscopic dilatancy and the thickest dilation zone, consistent with its corresponding peak shear strength. However, at higher normal stresses, although overall macroscopic dilatancy tendencies are suppressed, PIV-based observations and measurements of normalized dilation zone thickness indicate that R n = 2.7 develops the thickest apparent dilation zone. This finding suggests that under high confining stress, the spatial extent of dilatancy may shift away from roughness values ( R n = 1.35) that corresponded to optimal peak strength. For interface with extremely high roughness values (e.g. R n ≥ 10.8), dilatancy is significantly constrained. In these cases, volumetric expansion is primarily confined to narrow troughs of structure roughness elements, resulting in the smallest dilation zone thickness. This indicates that excessive roughness may hinder effective particle mobility and restrict volumetric deformation, rather than enhancing it. Also, a close coupling is observed between the mobilization of peak shear strength and development of dilatancy behavior. At higher normal stress levels, the occurrence of maximum contractancy (i.e. minimum volumetric strain) often coincides with the mobilization of peak shear strength. This correspondence marks a transition from a densification-dominated behavior to dilatancy-driven behavior, where particles begin to override geometric constraints despite the confining stress. Throughout all conditions, the dilation zone primarily occurs within the shear band, with both exhibiting a high degree of spatial overlap, reflecting the fact that dilatancy is an intrinsic volumetric response to shear deformation. The volumetric strain behavior observed in this study is the result of a dynamic competition and transition between two governing mechanisms: the normal stress-induced tendency for densification and embedment, and the interface roughness-induced tendency for particle overriding. At low normal stresses, sand particles overriding is readily occurs, leading to predominant dilatative behavior. At high normal stresses, the initial shear phase is dominated by particle densification, leading to contraction. In this case, sand particle overriding is suppressed and delayed until sufficient shear energy has accumulated to overcome the confining stress. Relative roughness Rn governs this interplay by modulating both the mechanical threshold for overriding and the spatial potential for particle rearrangement and embedment. 4. Conclusions This study investigates the coupled effects of normal stress (σn) and relative interface roughness ( R n ) on the shear behavior of sand-steel structure interfaces through a comprehensive experimental framework, including macroscopic direct shear tests, interface shear strength efficiency analysis, PIV-based microscale observation, and quantitative characterization of shear/dilation zone development. The findings highlight how stress conditions and surface geometry jointly influence interfacial strength, deformation patterns, and failure mechanisms. Three micromechanical processes, particle overriding, interlocking, and embedment emerge as the fundamental drivers of these behaviors. (1) Peak shear strength of sand-steel structure interface decreases with increasing σn, and shows a non-monotonic relationship with R n , peaking at R n = 1.35 attaining its maximum at the optimal roughness of R n = 1.35. This is due to the combined effect of dilatancy suppression under high stress and geometric interlocking. Residual shear strength, in contrast, is less affected by σn and Rn, reflecting the sand’s critical state behavior. Practical evaluation should therefore distinguish between peak and residual strength when assessing bearing capacity and stability. (2) The interface shear strength efficiency ( α ) reflects failure mode tendencies. α > 1 implies failure is more likely to occur within the sand mass adjacent to the interface, while α < 1 indicates interface itself constitutes a plane of weakness, and failure tends to occur along the interface. Its variation with σ n and R n aligns well with PIV-observed shear band shifts, making α a valuable design indicator for interface optimization. (3) Both normalized shear band thickness ( δ s ) and dilation zone thickness ( δ d ) decrease with increasing normal stress and vary non-monotonically with roughness R n . The maximum δ s occurs at R n = 1.35, while the optimal R n for δ d may shift under high normal stress. Importantly, δ d remains within δ s , confirming that dilatancy is a localized volumetric response within the shear band. (4) Interfacial shear behavior is predominantly governed by three core mechanisms: particle overriding, interlocking, and embedment effects. High normal confining stress inhibits overriding and promotes densification; structure interface with varying Rn shapes interlocking and deformation patterns. These findings underscore the need to consider both roughness and stress conditions in design, and support the development of stress- and geometry-aware interface models for soil-structure interaction. Declarations CRediT authorship contribution statement Zilong Zhou: Supervision, Conceptualization. Yiqun Su: Investigation, Writing –original draft. Shaofeng Wang: Review & Editing, Funding acquisition. Jinbiao Wu: Review & Editing, Funding acquisition, Methodology. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability The data t used in the current study are available from the corresponding author upon reasonable request. Acknowledgements The work described in this paper was supported by the National Natural Science Foundation of China and the Central South University. References Alshibli, K.A., Sture, S., 2000. Shear Band Formation in Plane Strain Experiments of Sand. J. Geotech. Geoenvironmental Eng. 126, 495–503. https://doi.org/10.1061/(ASCE)1090-0241(2000)126:6(495) Chen, W.-B., Zhou, W.-H., Dos Santos, J.A., 2020. Analysis of consistent soil–structure interface response in multi–directional shear tests by discrete element modeling. Transp. Geotech. 24, 100379. https://doi.org/10.1016/j.trgeo.2020.100379 Chen, W.-B., Zhou, W.-H., Yin, Z.-Y., 2023. Recent Development on Macro–Micro Mechanism of Soil-Structure Interface Shearing Through DEM. Arch. Comput. Methods Eng. 30, 1843–1862. https://doi.org/10.1007/s11831-022-09854-0 DeJong, J.T., Westgate, Z.J., 2009. Role of Initial State, Material Properties, and Confinement Condition on Local and Global Soil-Structure Interface Behavior. J. Geotech. Geoenvironmental Eng. 135, 1646–1660. https://doi.org/10.1061/(ASCE)1090-0241(2009)135:11(1646) Frost, J.D., DeJong, J.T., Recalde, M., 2002. Shear failure behavior of granular–continuum interfaces. Eng. Fract. Mech. 69, 2029–2048. https://doi.org/10.1016/S0013-7944(02)00075-9 Gu, X., Huang, M., Qian, J., 2014. DEM investigation on the evolution of microstructure in granular soils under shearing. Granul. Matter 16, 91–106. https://doi.org/10.1007/s10035-013-0467-z Hryciw, R.D., Irsyam, M., 1993. Behavior of Sand Particles Around Rigid Ribbed Inclusions During Shear. Soils Found. 33, 1–13. https://doi.org/10.3208/sandf1972.33.3_1 Hu Liming, Pu Jialiu, 2004. Testing and Modeling of Soil-Structure Interface. J. Geotech. Geoenvironmental Eng. 130, 851–860. https://doi.org/10.1061/(ASCE)1090-0241(2004)130:8(851) Iwashita, K., Oda, M., 2000. Micro-deformation mechanism of shear banding process based on modified distinct element method. Powder Technol. 109, 192–205. https://doi.org/10.1016/S0032-5910(99)00236-3 Liu, K., Wu, R., Ma, B., Zhang, P., Zeng, C., 2025. Macro/meso-evolution of shear behavior of sand-steel interface in pipe jacking via digital image correlation (DIC) technology. Tunn. Undergr. Space Technol. 155, 106197. https://doi.org/10.1016/j.tust.2024.106197 Martinez, A., Frost, J.D., 2017. The influence of surface roughness form on the strength of sand–structure interfaces. Géotechnique Lett. 7, 104–111. https://doi.org/10.1680/jgele.16.00169 Oda, M., Kazama, H., 1998. Microstructure of shear bands and its relation to the mechanisms of dilatancy and failure of dense granular soils. Géotechnique 48, 465–481. https://doi.org/10.1680/geot.1998.48.4.465 Paikowsky, S., Player, C., Connors, P., 1995. A Dual Interface Apparatus for Testing Unrestricted Friction of Soil Along Solid Surfaces. Geotech. Test. J. 18, 168–193. https://doi.org/10.1520/GTJ10320J Porcino, D., Fioravante, V., Ghionna, V., Pedroni, S., 2003. Interface Behavior of Sands from Constant Normal Stiffness Direct Shear Tests. Geotech. Test. J. 26, 289–301. https://doi.org/10.1520/GTJ11308J Rowe, P.W., 1962. The stress-dilatancy relation for static equilibrium of an assembly of particles in contact. Proc R Soc Lond 269, 500–527. https://doi.org/10.1098/rspa.1962.0193 Sadrekarimi, A., Olson, S.M., 2010. Shear Band Formation Observed in Ring Shear Tests on Sandy Soils. J. Geotech. Geoenvironmental Eng. 136, 366–375. https://doi.org/10.1061/(ASCE)GT.1943-5606.0000220 Stanier, S.A., Blaber, J., Take, W.A., White, D.J., 2016. Improved image-based deformation measurement for geotechnical applications. Can. Geotech. J. 53, 727–739. https://doi.org/10.1139/cgj-2015-0253 Su, D., Wu, D., Fan, M., Zhang, R., Chen, J., Xiong, H., Chen, X., 2025. DEM investigation into the coupling effects of particle asphericity and interface roughness on shear behaviour of soil-structure interface. Particuology 97, 39–57. https://doi.org/10.1016/j.partic.2024.11.019 Su, L.-J., Zhou, W.-H., Chen, W.-B., Jie, X., 2018. Effects of relative roughness and mean particle size on the shear strength of sand-steel interface. Measurement 122, 339–346. https://doi.org/10.1016/j.measurement.2018.03.003 Uesugi, M., Kishida, H., 1986. Frictional Resistance at Yield between Dry Sand and Mild Steel. Soils Found. 26, 139–149. https://doi.org/10.3208/sandf1972.26.4_139 Wang, H.-L., Zhou, W.-H., Yin, Z.-Y., Jie, X.-X., 2019. Effect of Grain Size Distribution of Sandy Soil on Shearing Behaviors at Soil–Structure Interface. J. Mater. Civ. Eng. 31, 04019238. https://doi.org/10.1061/(ASCE)MT.1943-5533.0002880 Wang, J., Gutierrez, M.S., Dove, J.E., 2007. Numerical studies of shear banding in interface shear tests using a new strain calculation method. Int. J. Numer. Anal. Methods Geomech. 31, 1349–1366. https://doi.org/10.1002/nag.589 Wang, J., Jiang, M., 2011. Unified soil behavior of interface shear test and direct shear test under the influence of lower moving boundaries. Granul. Matter 13, 631–641. https://doi.org/10.1007/s10035-011-0275-2 Xu, M., Wang, Lizhong, Wang, Lilin, Guo, Z., Zhou, W., 2024. Influence of bounding surface plasticity-based soil-structure interaction model on integrated dynamic behaviour of jacket offshore wind turbines. Ocean Eng. 298, 117204. https://doi.org/10.1016/j.oceaneng.2024.117204 Zhang, G., Liang, D., Zhang, J.-M., 2006. Image analysis measurement of soil particle movement during a soil–structure interface test. Comput. Geotech. 33, 248–259. https://doi.org/10.1016/j.compgeo.2006.05.003 Zhang, G., Zhang, J., 2009. State of the art: Mechanical behavior of soil–structure interface. Prog. Nat. Sci. 19, 1187–1196. https://doi.org/10.1016/j.pnsc.2008.09.012 Zhou, W.-H., Jing, X.-Y., Yin, Z.-Y., Geng, X., 2019. Effects of particle sphericity and initial fabric on the shearing behavior of soil–rough structural interface. Acta Geotech. 14, 1699–1716. https://doi.org/10.1007/s11440-019-00781-2 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 01 Apr, 2026 Read the published version in Granular Matter → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7800502","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":534850161,"identity":"b22a66b3-9ac5-401b-838d-d1d0a1a1adc0","order_by":0,"name":"Zilong Zhou","email":"","orcid":"","institution":"Central South University","correspondingAuthor":false,"prefix":"","firstName":"Zilong","middleName":"","lastName":"Zhou","suffix":""},{"id":534850162,"identity":"c42cef77-ac78-4ee6-8d4e-5c51ade27774","order_by":1,"name":"Yiqun Su","email":"","orcid":"","institution":"Central South University","correspondingAuthor":false,"prefix":"","firstName":"Yiqun","middleName":"","lastName":"Su","suffix":""},{"id":534850163,"identity":"de45f908-fe26-4504-aabf-8adac345c43e","order_by":2,"name":"Shaofeng Wang","email":"","orcid":"","institution":"Central South University","correspondingAuthor":false,"prefix":"","firstName":"Shaofeng","middleName":"","lastName":"Wang","suffix":""},{"id":534850164,"identity":"52ab8a99-1775-4efd-acd0-b0fb02030cea","order_by":3,"name":"Jinbiao Wu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA20lEQVRIiWNgGAWjYBACxmYGNoYEMJP5AEToAPFa2BKI0wJSCqV5DIjTwtzOe+zBgxobBoMbOR8//mxjkOO7kcD4uQCvw/jSDRKOpQG15G6W5m1jMJa8kcAsPQOvFh4ziQS2wwxmN3K3MTO2MSRuuJHAxsxDUMs/kJacZ4xAh9UTpyWxDayFjQHosAQDIrSYGyT2pfHYn3lmLM1zTsJw5pmHzdL4tBj2nzF7+OObjZxke/LDjz/KbOT5jicf/IxXSwOEhqmRANncgEcDA4M8XtlRMApGwSgYBSAAADD1RkqYTDOtAAAAAElFTkSuQmCC","orcid":"","institution":"Central South University","correspondingAuthor":true,"prefix":"","firstName":"Jinbiao","middleName":"","lastName":"Wu","suffix":""}],"badges":[],"createdAt":"2025-10-07 14:23:32","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7800502/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7800502/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10035-026-01636-w","type":"published","date":"2026-04-01T15:59:28+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":94856575,"identity":"2add0f88-91c0-456a-8e60-cd454aed0866","added_by":"auto","created_at":"2025-10-31 12:18:45","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":4176077,"visible":true,"origin":"","legend":"","description":"","filename":"manuscriptGM.docx","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/a95ed0f1bcbc23e8c8a91eb8.docx"},{"id":94985596,"identity":"1343a1a3-9a25-4b88-a0f1-cf1a8aefa0ba","added_by":"auto","created_at":"2025-11-03 06:58:28","extension":"json","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":6235,"visible":true,"origin":"","legend":"","description":"","filename":"030019cb2c5c498ab2795acfa117a373.json","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/b3bb95ea66968884ba8ccf5c.json"},{"id":94856582,"identity":"301ed5cb-eaa0-495e-8a74-a5af5efc9ba9","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"xml","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":144925,"visible":true,"origin":"","legend":"","description":"","filename":"030019cb2c5c498ab2795acfa117a3731enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/683c5f1fcbf56a7b2b7d641b.xml"},{"id":94985528,"identity":"e50d9903-8455-4202-a6f4-b7ea7dbbbc62","added_by":"auto","created_at":"2025-11-03 06:58:21","extension":"png","order_by":3,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":59515,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/1c00f7c4e37c14b800154d4e.png"},{"id":94986552,"identity":"784d00bf-4a33-461a-a0c6-d220f6f6cbf1","added_by":"auto","created_at":"2025-11-03 07:00:25","extension":"png","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":447266,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/ab898c687efe56d88dad8c00.png"},{"id":94856584,"identity":"95db284d-17c8-4aca-9173-772c804dd3c8","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"emf","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":153180,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage11.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/94789d5097a90c6b2dbd8755.emf"},{"id":94856591,"identity":"8e47f66e-4750-409f-9b9f-981883c66771","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"emf","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":161952,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage12.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/a169cb17ccf2ed3a29acb769.emf"},{"id":94985642,"identity":"38d40794-a645-44e0-924d-9e2f8ece97d6","added_by":"auto","created_at":"2025-11-03 06:58:32","extension":"emf","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2758580,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage13.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/e0cbb0ce9157e720449749b8.emf"},{"id":94856592,"identity":"651c7a21-0417-4af3-9687-3374c3d66539","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"emf","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":143224,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage14.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/b81efa69d590fce698f6ef52.emf"},{"id":94856594,"identity":"bdd5f6d3-6512-4469-b905-432731d91e17","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"emf","order_by":9,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2751408,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage15.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/5941d5a5f206f55e6e5534a8.emf"},{"id":94856587,"identity":"e8344ea4-7204-412a-aa03-08485738499d","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":62218,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage16.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/dc6c5fb7de420527f74b2835.png"},{"id":94985825,"identity":"6efe820a-f520-4eea-8703-3c5f1a660dde","added_by":"auto","created_at":"2025-11-03 06:59:03","extension":"png","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":812324,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/50511041417a67813b62503e.png"},{"id":94856595,"identity":"d4e0d537-8127-4846-9ab8-c5111b962649","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"emf","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":137168,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage3.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/9313de0a8aa0d530597bd50c.emf"},{"id":94985860,"identity":"6762c39c-a233-4e6a-b9fa-61c701ee9a3e","added_by":"auto","created_at":"2025-11-03 06:59:08","extension":"emf","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":195252,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage4.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/74dcb723049757bed7c57da0.emf"},{"id":94856613,"identity":"af5e1580-da0a-4b27-9fe3-5b4a48dd2d90","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"emf","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":197392,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage5.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/47062d0b756eb761f05e630e.emf"},{"id":94856618,"identity":"2073edbb-9f4e-41b8-8a91-df20a937e7df","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"emf","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1139484,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage6.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/559a233eada9e94edc36e528.emf"},{"id":94856602,"identity":"9a20e9ba-d9db-430f-99b3-b6b7f06d08bd","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":255496,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/789c76a19db1b8e7a0814fd1.png"},{"id":94856608,"identity":"b4fe30c1-1934-4216-82c8-d934758e3e37","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"emf","order_by":17,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":5060420,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage8.emf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/c40f47df2ee1c092b1a1bd15.emf"},{"id":94856625,"identity":"e50ee7b1-9fa4-46ff-8e67-4815756013c0","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"jpeg","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1332732,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/08735e092a2adb3d83836bf2.jpeg"},{"id":94985093,"identity":"8a5e38c5-3e3c-4ecd-82eb-a14187c389a2","added_by":"auto","created_at":"2025-11-03 06:57:25","extension":"png","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":15385,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/eec4fc06bd831e4587ae7e77.png"},{"id":94856603,"identity":"945e61bb-e765-4033-b80a-a03e55301d1e","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":20,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":85325,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/cfd3dffedee7758573d43377.png"},{"id":94985717,"identity":"30618794-0395-47f7-8867-c7f314661cec","added_by":"auto","created_at":"2025-11-03 06:58:45","extension":"png","order_by":21,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":61482,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/61fa35984b77af14a09f4afc.png"},{"id":94985798,"identity":"563cc6e9-a706-45ff-a169-007511bfa718","added_by":"auto","created_at":"2025-11-03 06:59:02","extension":"png","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":135812,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/3712042eb924032535838ac6.png"},{"id":94856611,"identity":"f582c44d-8951-4680-a7d3-b71314d39546","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":358143,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage13.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/31d0c6b3df56226ee3546c45.png"},{"id":94856609,"identity":"a5e697ce-d6a4-41a3-b8af-92188ff67571","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":67565,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage14.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/310a02fc4f32049a7210024c.png"},{"id":94856599,"identity":"689ee682-0e88-4d9d-8d4e-70525cdb1e30","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":336467,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage15.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/7208411de2ff9a4b387b99a1.png"},{"id":94856616,"identity":"ffc86428-1676-4abe-ac37-b2404ef7beab","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"png","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":18316,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage16.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/e3c82782c53107f2757c69e8.png"},{"id":94856623,"identity":"f5ab1d2a-c71a-4a82-bc2d-8d7d3f7e8998","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"png","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":99828,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/661439675651197790517f07.png"},{"id":94856605,"identity":"b35b5140-946a-4ae3-b599-da819bb044c6","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":28,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":39838,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/96c1b2f2557454242173bbbc.png"},{"id":94986518,"identity":"7445a0c1-9522-42e1-9263-8763159fb6d4","added_by":"auto","created_at":"2025-11-03 07:00:23","extension":"png","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":153081,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/9956d503af465f0c72509c6d.png"},{"id":94856621,"identity":"0cae6ad4-977a-439f-ba0b-39183a9dfe69","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"png","order_by":30,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":154628,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/ef78bb21b6c9cf1f452a518a.png"},{"id":94856617,"identity":"1e02ccd1-e246-4223-899c-97b78da5b013","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"png","order_by":31,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":595030,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/7cd335525b655bcca7f6cff2.png"},{"id":94856612,"identity":"ecd7b098-efb0-4aa4-b091-b9ea0cd1b74e","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"png","order_by":32,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":63151,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/2edd41e354af52e1f8d28478.png"},{"id":94985894,"identity":"ab0f6d5f-38ae-43c6-a08d-34b5948786c7","added_by":"auto","created_at":"2025-11-03 06:59:14","extension":"png","order_by":33,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":425118,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/4e320420e81a0cae9770c481.png"},{"id":94856615,"identity":"6dabe833-de96-4706-8755-42d970eac5dd","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"png","order_by":34,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":188010,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/7d10b6a413da72ad2c7487ba.png"},{"id":94856624,"identity":"1c731d7c-e00e-4d23-8a34-a2e235abfc53","added_by":"auto","created_at":"2025-10-31 12:18:47","extension":"xml","order_by":35,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":143968,"visible":true,"origin":"","legend":"","description":"","filename":"030019cb2c5c498ab2795acfa117a3731structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/80616cbc1c34732fe1d226d6.xml"},{"id":94985883,"identity":"48570d04-9e41-4a3a-a927-c53acc8156bc","added_by":"auto","created_at":"2025-11-03 06:59:11","extension":"html","order_by":36,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":156379,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/72bae32e0b94adb307efbef9.html"},{"id":94856577,"identity":"d61e7cce-daf9-414d-b38c-813744c3868c","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":119024,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of soil-structure interface\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/a42369b059a864c911b58fd8.png"},{"id":94985869,"identity":"123a481c-9811-4d7b-82fe-f35bf057b1fd","added_by":"auto","created_at":"2025-11-03 06:59:09","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":739693,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of direct shear test: (a) Schematic of the Modified ShearTrac-II Consolidated Shear Device (b) Three-dimensional sketch of the structure surface geometry (c)Photograph of the Experimental Setup (d) Photographs of Soil-Interface Samples in Shear Box\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/58d96c10ebe774c76b11a59a.png"},{"id":94856572,"identity":"3f456ae8-d624-4356-bb90-4b2856c0384d","added_by":"auto","created_at":"2025-10-31 12:18:45","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":53301,"visible":true,"origin":"","legend":"\u003cp\u003eParticle size distribution of STK sand.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/d958672391982a62b755b718.png"},{"id":94856573,"identity":"0b85f278-60d8-4136-a1da-bc98dfab8c84","added_by":"auto","created_at":"2025-10-31 12:18:45","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":75762,"visible":true,"origin":"","legend":"\u003cp\u003eResults of direct shear tests on sand under different normal stress\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/5d20ef1cd40bcb649b897338.png"},{"id":94856578,"identity":"6b8f09c3-1fda-43c5-b114-82ceca753d74","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":69117,"visible":true,"origin":"","legend":"\u003cp\u003eResults of direct shear tests on Sand-steel structure Interface under different normal stress with \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e =0\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/32bc5ec4aee796a20039bc92.png"},{"id":94984876,"identity":"c0ae49ba-e96f-452d-8761-7a9dd210bad0","added_by":"auto","created_at":"2025-11-03 06:56:44","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":185780,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of shear behavior for different \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e values at the sand-steel structure interface under various normal stresses (a) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003csub\u003e \u003c/sub\u003e= 50 kPa; (b) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003csub\u003e \u003c/sub\u003e= 150 kPa; (c) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003csub\u003e \u003c/sub\u003e= 250 kPa; (d) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003csub\u003e \u003c/sub\u003e= 350 kPa\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/6b33756ee6c28c0ec826d234.png"},{"id":94856581,"identity":"439d34ee-35c4-4223-92d2-c12f58ea9058","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":418286,"visible":true,"origin":"","legend":"\u003cp\u003eFriction angle obtained in the DST at different \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e \u003c/em\u003evalues (a) peak friction angle; (b) residual friction angle\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/1fd86312f606c5eac7ae096e.png"},{"id":94985991,"identity":"e3d20203-b5e1-4d9f-b4c7-1c3b12f725fc","added_by":"auto","created_at":"2025-11-03 06:59:30","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":282026,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of interface shear strength coefficient from DST\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/816dc2244b05332d0a29c766.png"},{"id":94856597,"identity":"24537872-e1ec-46cb-8838-4402d54c5ad8","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":401883,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of PIV-Analyzed Kinematic Fields: (a) Displacement component \u003cem\u003eu\u003c/em\u003e (b) Displacement component \u003cem\u003ev\u003c/em\u003e (c) Shear strain rate\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/e4ae6590dd89b85d0cc98402.png"},{"id":94856586,"identity":"5cd35608-3141-4604-98b9-b87464558bfe","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":581951,"visible":true,"origin":"","legend":"\u003cp\u003eDetermination of soil-structure interface shear zone under a normal stress of 150 kPa and relative roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35: (a)\u003cem\u003e δ = \u003c/em\u003e4%W (b) \u003cem\u003eδ\u003c/em\u003e = 6%W (c) \u003cem\u003eδ\u003c/em\u003e = 8%W (d) \u003cem\u003eδ\u003c/em\u003e = 10%W\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/0575dea173baa321878b482d.png"},{"id":94986487,"identity":"0f3982cc-c950-4995-8237-eb068b1005ce","added_by":"auto","created_at":"2025-11-03 07:00:21","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":63142,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of normalized shear band thickness with shear displacement under a normal stress of 150 kPa and relative roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/742cf071dc8cc22b8d4a044f.png"},{"id":94856588,"identity":"06a68c23-3c6a-4d44-a281-7ff522eec501","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":106588,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of normalized shear zone thickness with shear displacement under different normal stress levels and surface roughness conditions: (a) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003csub\u003e \u003c/sub\u003e= 50kPa (b) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 150kPa (c) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 250kPa (d) \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 350kPa\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/29824f4f174a1bffb89408e6.png"},{"id":94856598,"identity":"fb5b976a-a79b-4696-bccb-5babedf24411","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":143197,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of final normalized shear zone thickness under varying normal stress levels and surface roughness conditions\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/32ffc00de8b3756f2b3dd6ae.png"},{"id":94985576,"identity":"7affbdeb-92c8-4501-8e55-43006a30f367","added_by":"auto","created_at":"2025-11-03 06:58:26","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":82983,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of dilation zone thickness with shear displacement under a normal stress of 150 kPa and relative roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/4d92770ee61ef1e696188b19.png"},{"id":94856606,"identity":"b3ca9992-a126-4e52-9ea5-0081323d29a2","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":198030,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of final normalized dilation zone thickness under varying normal stress levels and surface roughness conditions\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/8385b680243d59484daa9517.png"},{"id":94856593,"identity":"2b1713c3-1b4a-41b0-8350-1d24dfb9257a","added_by":"auto","created_at":"2025-10-31 12:18:46","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":179630,"visible":true,"origin":"","legend":"\u003cp\u003eShear mechanisms at particle-structured rough interfaces: (a) Load-transfer mechanisms (b) Particle-surface interaction under different relative roughness (c) Particle-surface interaction under different normal stress.\u003c/p\u003e","description":"","filename":"16.png","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/ad9da3c441898a959c7761ee.png"},{"id":106344981,"identity":"5124e335-2979-446e-aa9d-967b892fd87e","added_by":"auto","created_at":"2026-04-07 16:17:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4243505,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7800502/v1/aa2b3440-bd49-42c7-9857-c6883a38f7ad.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effects of confining stress and roughness on mechanical behavior of sand-steel structure interface","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSoil-structure interfaces (SSIs) are integral to numerous geotechnical engineering structures, including pile foundations, retaining walls, reinforced soil structures, underground pipelines, and tunnels, where they play the critical role of transferring structural and environmental loads to the surrounding soil. For piles, for example, a substantial portion of capacity is mobilized through via skin friction at the pile-soil interface, as schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The performance of these structures largely depends on effective stress transfer between the soil and the structural elements, which is governed by the mechanical behavior of SSI (Chen et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Xu et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Accordingly, reliable prediction of interface shear behavior at SSIs, particularly for sandy soils against typical structural surfaces, is essential for stability and long-term performance of these geotechnical systems (Paikowsky et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1995\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Zhang and Zhang, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Geometric characteristics of the structural surface directly govern the particle interlocking patterns, contact mechanics, and frictional mobilization, thereby shaping interface strength and dilatancy (Hryciw and Irsyam, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Martinez and Frost, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Shear deformation at SSI is inherently non-uniform, with strains concentrating in a narrow zone adjacent to the interface, forming distinct shear bands (Iwashita and Oda, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). The formation and evolution of these shear bands reflect complex internal deformation mechanisms and are key to interpreting the macroscopic mechanical behavior and ultimate failure modes (Alshibli and Sture, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Therefore, investigating how structure surface geometry influences shear bands development and their associated mechanical behavior is essential for understanding the overall interfacial shear behavior and its fundamental mechanisms (Hu et al., 2004; Wang and Jiang, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Zhang et al., \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn geotechnical engineering practice, structural surfaces range from the relatively smooth to rough surface with distinct macroscopic geometric configurations (Su et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Examples of the latter include the helical grooves on pile foundations, the profiled surfaces of pipe jacking segments, and, relevant to this study, trapezoidal sawtooth steel structural surfaces. These geometric configurations and their associated roughness significantly affect soil\u0026ndash;structure interaction by altering particle contact behavior and interlocking mechanisms (Frost et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Su et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Uesugi and Kishida, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1986\u003c/span\u003e). Extensive studies have investigated the influence of interfacial geometric characteristics on the macroscopic mechanical response of SSIs using direct shear tests and found that rougher interfaces (inclusion of ribs, grooves, or specific textures) typically mobilize higher shear resistance and more pronounced dilatancy phenomena (Hu Liming and Pu Jialiu, 2004). Researchers have further introduced a critical or optimal roughness concept for sand\u0026ndash;structure interfaces, beyond which increasing in interface roughness yield limited gains in friction angle, and the dominant failure mode may shift from interfacial sliding to shearing within the soil (Porcino et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). These findings highlight that geometry affects not only interface shear strength but also failure mechanisms.\u003c/p\u003e\u003cp\u003eShear zone formation is a prominent features of SSI failure and have been extensively verified through experimental observations and numerical simulations (Chen et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). The formation and evolution of the shear bands largely govern the non-linear mechanical response of the interface. Numerous studies have aimed to estimate shear bands thickness, revealing that it is closely linked to the mean particle diameter and further influenced by factors such as soil density, confining pressure, and particle shape (Alshibli and Sture, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Sadrekarimi and Olson, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Zhou et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Shear band development is typically accompanied by volumetric changes, expressed as either dilations (volume expansion) or contractions (volume reduction). For dense sandy soils, or under certain stress path conditions, interfacial shearing often induces dilative behavior, with localized volumetric expansion forming what is known as a dilation zone near the interface(Rowe, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e1962\u003c/span\u003e; Oda and Kazama, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). Although conventional direct shear tests can provide the overall volumetric changes from vertical displacement measurement, they are unable to capture the internal shearing structure, the dynamic shearing zone formation, or the spatial extent of the dilation zone. Microscale studies indicate that dilatancy results from particle overriding, rearrangement, and rotation, which increase local porosity. These mechanisms are tightly coupled with the evolution of shear bands and are key to linking microstructural behavior with macroscopic interface response (DeJong and Westgate, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Gu et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Su et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAlthough the presence of shear and dilation zones at soil\u0026ndash;structure interface (SSIs) is well established, their underlying mechanisms remain challenging to fully understand. Current research largely relies on macroscopic experiments and numerical simulations. Conventional direct shear tests provide valuable data on interface shear strength and volumetric response but cannot reveal internal microscale deformation or failure processes due to their \"black-box\" limitations. Numerical approaches, particularly the Discrete Element Method (DEM), offer insights into particle-scale behaviors, such as force chains evolution and particle movements. However, most DEM studies employed idealized particle shapes (e.g., circular or spherical particles), which may not fully capture the complex interlocking effects and rotational constraints of real sand grains (Chen et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Su et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Thus, results are also sensitive to modelling assumptions, including particle size, contact models, and boundary conditions, underscoring the need for experimental validation. For specific regular shape of sand-structure interfaces involving with varying degrees of roughness, detailed experimental data on evolution of shear and dilation zones from microscale initiation to macroscale stabilization, remain scarce. Key gaps include accurate measurements of shear zone thicknesses and high-resolution internal deformation fields. Moreover, the physical mechanisms by which interface roughness and morphology govern the shear zone development, internal particulate kinematics and volumetric behavior are not yet fully elucidated. Establishing direct and quantitative links between these microscale evolutionary characteristics of shear and dilation zones and the macroscopic mechanical response of the sand-structure interface remains a significant challenge in the field.\u003c/p\u003e\u003cp\u003eTo address the aforementioned research gaps, this study investigates the macro- and micro-scale shear behavior of sand-steel structure interfaces by conducting shearing tests with a modified direct shear apparatus integrated with Particle Image Velocimetry (PIV) technique. Tests were conducted in dense sand against trapezoidal sawtooth steel surfaces across a range of normal stresses and relative roughness. This approach allows to directly capture microscopic sand-steel interface shearing kinematics, quantify shear zone thickness with objective criteria, and relate these kinematic metrics to macroscopic mechanical response under changing surface geometry and confinement stress. Thus, this study first introduces the physical properties of sand materials, the profiled steel geometries, and the modified PIV-integrated shear apparatus. Then macroscopic results, including stress-displacement behavior, volumetric response, and peak shear strength are analyzed. Furthermore, microscale analysis focuses on displacement and shear strain fields, and the quantitative evaluation of shear and dilation zone thicknesses. Finally, by integrating macroscopic mechanical responses and microscale observations, this study aims to clarify the combined influence of normal stress and interface roughness on the interfacial shear behavior, and to reveal the intrinsic physical mechanisms that govern these phenomena.\u003c/p\u003e"},{"header":"2. Methods and Materials","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Test apparatus\u003c/h2\u003e\u003cp\u003eTo facilitate the observation of sand deformation, a conventional direct shear test apparatus was modified, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The original equipment comprised a commercial strain-controlled Direct Shear Test (DST) system (Model: ShearTrac II, Manufacturer: Geocomp), which featured a rectangular shear box with dimensions of 100 mm \u0026times; 100 mm \u0026times; 42 mm. These shear box dimensions are in compliance with the requirements stipulated by ASTM (2004) D3080-04, wherein the dimensions are specified to be between six and ten times the maximum particle diameter. A transparent observation window made of high strength annealed glass was integrated into the apparatus to support Particle Image Velocimetry (PIV) measurements, thereby enabling researchers to visually and quantitatively assess the complex shear behavior at the sand-steel structure interface. Critically, annealed glass possesses low coefficient of friction, which can effectively minimize the frictional interference between the window and the enclosed sand mass, consequently reducing end-boundary effects. By minimizing such boundary influences, the experimental conditions more closely approximate an idealized plane strain state, thereby enhancing the accuracy and reliability of deformation field observations and measurements.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 materials properties\u003c/h2\u003e\u003cp\u003eThe granular material used in this study was Stockton Beach sand, hereinafter referred to as STK sand for brevity. This material is a washed, uniform quartz silica sand, characterized by a quartz content exceeding 98%. The particle size distribution of the sand is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, which can be classified as fine sand, with a median particle size (D\u003csub\u003e50\u003c/sub\u003e) of 0.37 mm and a maximum particle size (D\u003csub\u003emax\u003c/sub\u003e) of 0.62 mm. In accordance with the Unified Soil Classification System (USCS; ASTM 2011), the sand is classified as poorly graded sand (SP); under Australian Standard AS-1289.3.6.1 (Standards Australia 2009), it is classified as medium sand. The minimum and maximum dry unit weights, determined as per Australian Standard AS-1289.5.5.1 (Standards Australia 1998), are 14.5 kN/m\u0026sup3; and 17.2 kN/m\u0026sup3;, respectively. The specific gravity of the soil solids (Gs), measured using a gas pycnometer, was determined to be 2.65. Other gradation characteristics of the STK sand are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eGrading properties of STK sand\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"9\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eD\u003csub\u003e10,\u003c/sub\u003e mm\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eD\u003csub\u003e30,\u003c/sub\u003e mm\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eD\u003csub\u003e60,\u003c/sub\u003e mm\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eC\u003csub\u003eu\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eC\u003csub\u003ec\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003ee\u003csub\u003emin\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003eγ\u003csub\u003emax,\u003c/sub\u003e kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003ee\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003eγ\u003csub\u003emin,\u003c/sub\u003e kN/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.37\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.41\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e4.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e0.51\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e17.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.79\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e14.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"9\"\u003eNote: D\u003csub\u003e10\u003c/sub\u003e, D\u003csub\u003e30\u003c/sub\u003e, D\u003csub\u003e60\u003c/sub\u003e, soil particle diameter at which 10%, 30%, and 60% of the mass of a soil specimen is finer, respectively; C\u003csub\u003eu\u003c/sub\u003e, uniformity coefficient; C\u003csub\u003ec\u003c/sub\u003e, coefficient of curvature; e\u003csub\u003emin\u003c/sub\u003e, e\u003csub\u003emax\u003c/sub\u003e, minimum and maximum void ratios associated with the maximum, γ\u003csub\u003emax\u003c/sub\u003e, and minimum, γ\u003csub\u003emin\u003c/sub\u003e, unit weights, respectively.\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe sand-steel structure interface model was constituted by STK sand in contact with a steel surface, the three-dimensional schematic representation of which, along with the defined coordinate axes, is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. This structured surface features an isosceles trapezoidal profile with base angles of 45\u0026deg;, satisfying the geometric condition S\u003csub\u003e1\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;S\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;peak-to-valley height (h). This configuration creates a typical periodic, non-clogging surface, where the volume of the interfacial depressions is invariably equal to the volume of the protrusions. Consequently, surface roughness can be systematically varied by adjusting by only the height parameter \u003cem\u003eh\u003c/em\u003e. In this study, six rough surfaces were prepared with \u003cem\u003eh\u003c/em\u003e values of 0.25, 0.5, 1, 2, 4, and 8 mm. An additional surface with \u003cem\u003eh\u003c/em\u003e value of 0 mm, representing a nominally smooth interface, was included to provide a baseline for comparative analysis. Following the definition by Uesugi and Kishida (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1986\u003c/span\u003e), the relative roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) of surface is expressed as \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003emax\u003c/em\u003e\u003c/sub\u003e / \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e, where \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003emax\u003c/em\u003e\u003c/sub\u003e represents the maximum vertical distance between the highest and lowest points of the interface surface profile, and \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e is the mean particle diameter. In this case, \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003emax\u003c/em\u003e\u003c/sub\u003e is equivalent to \u003cem\u003eh\u003c/em\u003e. The calculated relative roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e values for all tested surfaces are listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eRoughness of sand-steel structure interface tested\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"8\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eh\u003c/em\u003e, mm\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.5\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003emax\u003c/em\u003e\u003c/sub\u003e, mm\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.68\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1.35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e2.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e5.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e10.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e21.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 experimental procedures\u003c/h2\u003e\u003cp\u003eIn this study, both Direct Shear Tests (DSTs) on sand and Sand-steel structure Interface (SSI) tests were conducted. Dense sand specimens with a relative density \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e = 92% were prepared using the sand pluviation method. The core principle of this technique involves controlling the height of fall of the sand, allowing it to naturally deposit and achieve the target density within the shear box. Preliminary calibration tests were performed to determine the optimal pluviation height required to obtain the desired relative density. During specimen preparation, sand was uniformly and continuously pluviated from the calibrated height into the shear box. Upon completion of sand filling, the specimen surface was carefully leveled by removing any excess sand. For SSI tests, steel interface plates with varying surface roughness were placed in the lower half of the shear box, and dry sand was subsequently pluviated into the upper half. For the pure sand DSTs, the sand was pluviated directly into the assembled shear box without any interface plate. All tests were performed under vertical normal stresses \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e of 50, 150, 250, and 350 kPa. The parameters for the experimental conditions are detailed in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The shear rate of 0.5 mm/min was applied in the shear test until a horizontal shear displacement (\u003cem\u003eu\u003c/em\u003e) of 10 mm was reached. To capture the deformation near the interface, a high-resolution camera was used to record images of the sand particles in the vicinity of the interface at 15-second intervals. These images were subsequently analyzed using PIVlab software to compute the horizontal displacement field (\u003cem\u003eu\u003c/em\u003e), vertical displacement field (\u003cem\u003ev\u003c/em\u003e), and shear strain rate field of the sand particles, thereby quantifying the particulate kinematics at the interface.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eExperimental test conditions\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTest No.\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e (kPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003er\u003c/em\u003e\u003c/sub\u003e (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.68\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e250\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.25\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.94\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.68\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.24\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.54\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e250\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.46\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e89.50\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.35\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.58\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.71\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e250\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.69\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.05\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e13\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e2.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e89.68\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.60\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e250\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e92.46\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.64\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e17\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e5.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.52\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e18\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.19\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e250\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.14\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.12\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e10.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.18\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e22\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.89\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e250\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.38\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e89.59\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e25\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e21.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e92.44\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e26\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e150\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e90.96\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e250\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.01\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e28\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e350\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e91.06\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and analysis","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Macroscopic Shear Behavior and Interface Strength\u003c/h2\u003e\u003cp\u003eThe relationships between the stress ratio (\u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) and shear displacement (\u003cem\u003eu\u003c/em\u003e), as well as the vertical displacement (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e) and shear displacement (\u003cem\u003eu\u003c/em\u003e) obtained from the direct shear tests on sand are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The stress ratio \u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e reflects the intrinsic macroscopic shear characteristics of the sand material itself, embodying its capacity to resist internal shear slip along a predefined shear plane under the applied normal stress. This ratio effectively reflects the operative internal friction coefficient of the sand. The dilatancy behavior of sand is primarily influenced by the applied normal stress. As the normal stress increases from 50 kPa to 350 kPa, the associated dilatant volume change monotonically decreases, indicating a progressive suppression of dilatancy. Notably, the dilatancy curves corresponding to normal stresses of 250 kPa and 350 kPa are closely aligned, suggesting that beyond a certain threshold, additional increases in normal stress yield only marginal reductions in dilative behavior. The peak shear strength of the sand comes from two key components: the resistance required to overcome inter-particle friction and interlocking, and the energy expanded against dilatancy. When the normal stress increases from 50 kPa to 150 kPa, enhanced compaction of the particles occurs, leading to improved inter-particle contact and interlocking effect. Although dilatancy begins to be inhibited at this stage, its influence remains relatively minor. As a result, the peak stress ratios at 50 kPa and 150 kPa are comparable. However, as the normal stress is further increase to 250 kPa and 350 kPa, the dilatancy suppression effect becomes significantly more pronounced. The increased confinement significantly restricts particle movement and volumetric expansion, leading to a noticeable decline in the contribution of dilatancy to peak strength and, consequently, a reduction in the peak stress ratio.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe residual shear strength characterizes the sand\u0026rsquo;s fundamental frictional behavior after it has undergone structural degradation due to shearing. At this stage, the inter-particle interlocking mechanisms are largely destroyed and dilatancy has essentially ceased. The magnitude of the residual shear strength is primarily governed by the sliding friction between particles and their final arrangement after shearing stabilization. Under a normal stress of 50 kPa, the inter-particle contact forces are relatively low, resulting in a relatively loose structure after rearrangement. This limited normal confinement stress is insufficient to fully mobilize the potential frictional resistance, leading to lower residual shear strength. As the normal stress increases to 150 kPa, the residual shear strength improves. At this stage, sand experiences significant dilatancy and particle rearrangement, and forms a denser structure that is more favorable to shear resistance. However, at even higher normal stresses, increased particle alignment in the shear direction and greater plastic deformation may slightly reduce the residual strength, despite higher contact stresses.\u003c/p\u003e\u003cp\u003eFig. 5 presents the shear stress ratio versus shear displacement (τn/σn-δh) and vertical displacement versus shear displacement (δv-δh) curves for the sand-steel structure interface with a relatively roughness Rn=0. Due to the extremely smooth nature of the structure surface at this condition, the interface exhibits minimal shear strength, and shear failure readily occurs within a thin layer of sand immediately adjacent to the interface. This localized zone of sand quickly reaches a residual state under shearing. During this internal shearing process, only minor volumetric changes and characteristic of the sand inherent shearing behavior occurs together with dilatancy developing slowly and gradually stabilizing. As shear displacement increases, boundary constraint effects near the ends of the shear box become more pronounced. The soil near the leading edge of the shear box experiences progressive compression, while the soil at the trailing edge undergoes internal readjustment due to the forward movement of the sand mass. This results in non-uniform deformation across the specimen, which is reflected in the vertical displacement measurements. In later stages of loading, these boundary-induced distortions lead to fluctuations and an apparent uplift in the δv-δh curve.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;6 presents the relationships between the stress ratio (\u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) and shear displacement (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e), as well as the vertical displacement (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003ev\u003c/em\u003e\u003c/sub\u003e) and shear displacement (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003eh\u003c/em\u003e\u003c/sub\u003e), for sand-interface interface tests conducted at various relative interface roughness values (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 0.68, 1.35, 2.7, 5.4, 10.8, and 21.6) under four normal stress levels (\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;50, 150, 250, and 350 kPa). The corresponding peak and residual friction angles derived from these tests are also shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The peak shear stress to normal stress ratio (\u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/σ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) of the interface exhibits a clear stress dependency on both normal stress and interface roughness. In general, \u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e decreases with increasing normal stress, indicating that shear strength becomes progressively less sensitive to interface characteristics under higher confinement. At a low normal stress of 50 kPa, all roughness interface profiles mobilize substantial shear resistance. In particular, under the condition of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, the interface mobilizes the highest peak strength, with a \u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e value exceeding 1.0. However, as the normal stress increases, peak shear strength diminishes for all roughness levels.\u003c/p\u003e\u003cp\u003eThe peak shear strength also displays a non-monotonic relationship with the relative interface roughness, revealing an optimal relative roughness of. Across all interfaces, this specific interface configuration (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35) consistently yields the highest peak strength for all tested normal stress levels. Deviations from this optimal roughness value, whether toward smoother or rougher surfaces, lead to reduced shear resistance. Additionally, the effect of roughness becomes less pronounced at higher normal stresses, suggesting that the superiority of the optimal roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35) is attenuated as confinement stress increases.\u003c/p\u003e\u003cp\u003eA distinct strain-softening phenomenon, characterized by peak strengths significantly exceeding residual strengths, is observed for all experimental conditions. This softening most prominent under low normal stress and at the optimal roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35). With continued shearing beyond the peak shear strength, the shear stress ratio \u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/σ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e converges towards a relatively stable residual value, typically in the range of 0.6 to 0.8. These residual values reflects that the interfacial behavior ultimately tends to be governed by the critical state characteristics of the sand itself. Nevertheless, the residual friction angle still exhibits a slight stress dependency. Furthermore, the volumetric deformation mode of the interface is also dependent on normal stress and interface roughness. At normal stress of 50 kPa, all interfaces, irrespective of their roughness, exhibit continuous dilatant behavior throughout the entire shearing process. Similar to peak strength, the magnitude of dilatancy also shows a non-monotonic relationship on roughness levels, with the maximum dilatancy occurring at the optimal roughness of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35. At higher normal stresses, particularly for rougher interfaces, the behavior transitions to a more complex contractive\u0026ndash;dilative mode. In this case, initial volumetric contraction is followed by dilation, with the point of maximum contraction occurring near the peak shear strength. This pattern indicates an intrinsic link between shear strength mobilization and the transition in volumetric change mechanisms.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo further evaluate the shear capacity of the sand-steel structure interface and to identify the associated failure modes under different interface roughness conditions, the interface shear strength efficiency (\u003cem\u003eα\u003c/em\u003e) is introduced. This parameter is defined as the ratio of the peak shear strength of the soil-structure interface (\u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u0026minus;g\u003c/em\u003e\u003c/sub\u003e) to the peak shear strength of the sand itself (\u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e) under the same normal pressure:\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\alpha\\:=\\frac{{\\tau\\:}_{s-g}}{{\\tau\\:}_{s}}\\)\u003c/span\u003e\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents the variation of \u003cem\u003eα\u003c/em\u003e across different interface roughness values and normal stresses levels. This efficiency coefficient (\u003cem\u003eα\u003c/em\u003e) directly reflects how effectively the interface, transmits shear stress compared to the surrounding soil mass itself. It also serves as an indicator for inferring the potential failure mode. Specifically, \u003cem\u003eα\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;1 indicates high interfacial shear efficiency, where the interface mobilizes greater shear strength than the soil itself, suggesting that failure likely occurs within the sand mass adjacent to the interface; \u003cem\u003eα\u003c/em\u003e\u0026thinsp;\u0026asymp;\u0026thinsp;1 implies comparable shear strength between the interface and the sand, making the failure mode uncertain \u0026ndash; it may occur at the interface, within the soil, or through a combination mechanism; \u003cem\u003eα\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1 suggests relatively low interfacial efficiency, implying that the interface itself acts as a weak plane where failure is prone to occur along the contact surface or within a very thin adjacent sand layer.\u003c/p\u003e\u003cp\u003eFor all tested relative roughness levels (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e), α generally decreases with increasing normal stress. For instance, in the case of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, \u003cem\u003eα\u003c/em\u003e decreases from 1.31 at 50 kPa to 1.07 at 350 kPa. This trend indicates that even at high stress levels, this particular interface retains sufficient interlocking and dilatancy effects to cause the failure path to deviate slightly from the interface into the adjacent soil, albeit this tendency is considerably less pronounced than at lower stress levels. Among all roughness conditions, \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 generally exhibits the highest α values all normal stress levels. Under low normal stress conditions, \u003cem\u003eα\u003c/em\u003e exceeds 1.0 for all roughness levels, suggesting that the potential failure mode is more likely to involve shearing within the sand adjacent to the interface and develop a relatively thicker shear band. As normal stress increases, \u003cem\u003eα\u003c/em\u003e values for some roughness conditions may decrease to below 1.0. For interfaces where α approaches or falls below 1.0, the failure mode is more likely to transit toward interface-dominated sliding failure or localized shearing within a very thin layer adjacent to the contact surface.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Mesoscopic Visualization Analysis of Interfacial Shear Deformation\u003c/h2\u003e\u003cp\u003eThe macroscopic direct shear test results have comprehensively characterized the mechanical response of the sand-steel structure interface, demonstrating clear dependencies of peak strength and volumetric deformation behavior on both normal stress and relative interface roughness. Several key phenomena observed in these tests warrant deeper investigation. These include: (1) the existence of an optimal roughness that maximizes interfacial performance; (2) the transition in volumetric deformation behavior with increasing stress levels, specifically the complex contractive-dilative mode observed under high stress, which is closely associated with peak strength mobilization; (3) the relationship between the interface strength efficiency and the failure mode evolution.\u003c/p\u003e\u003cp\u003eWhile these macroscopic findings delineate the overall performance characteristics of the interface, they cannot directly reveal the underlying physical mechanisms driving these complex behaviors. To overcome the inherent black-box limitations of macroscopic testing and to gain insight into the localized deformation and failure processes, this study employs Particle Image Velocimetry (PIV) for microscale analysis (Stanier et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). PIV is a two-dimensional digital image processing technique that enables non-intrusive measurement of particulate motion within the observation plane. Subsequent processing of the observational data using PIVlab software focuses on analyzing the critical mechanical mechanism transitions and complex interactions manifested in the aforementioned macroscopic phenomena. Therefore, typical experimental conditions representative of these core phenomena were selected for detailed investigation. Specifically, deformation fields of the optimal roughness interface (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35) under different normal stresses (150 kPa and 250 kPa) were compared to directly observe how increasing confining stress inhibits dilatancy and triggers a transition toward the contractive-dilative behavior. Furthermore, tests at representative high normal stress (250 kPa) were selected to compare the interfaces behavior with different roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, 2.7, and 10.8). This comparison aimed to clarify how geometric features influence strain localization, particulate kinematics, and potential failure modes development, thus offering new insights into the origins of the non-monotonic roughness-shear strength relationship.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e presents the evolution of particle displacement fields (\u003cem\u003eu\u003c/em\u003e, \u003cem\u003ev\u003c/em\u003e) and shear strain rate fields, as captured through PIV analysis under various normal stress and interface roughness conditions. In direct shear tests on sand-steel structure interfaces, shear deformations is primarily localized within a narrow interaction zone adjacent to the structural surface, referred to as the shear band. This localized zone represents the primary area of strain concentration and deformation. For the optimal roughness interface (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35), the horizontal displacement field (\u003cem\u003eu\u003c/em\u003e) reveals a shear band characterized by well-defined morphology and smooth, coherent boundaries under different normal stresses. Initiating from the horizontal edges of the specimen, the shear band exhibits a distinct upward trajectory, deviating from the interface profile and propagating into the overlying soil mass. Ultimately, a uniformly developed localized shear zone is formed at a certain distance above from the physical interface. In contrast, for interface with \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 2.7 and \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 10.8, the shear band initial development closely aligns with the geometric profile of the rough physical interface, particularly near the specimen's horizontal boundaries. Only in the central portion of the specimen does the shear band observes a limited upward extension. As the roughness increases from \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 2.7 to \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 10.8, the upward propagation appears more constrained, and the main body of the shear band tends to follow the interface more closely. Methodologies based on particle displacement gradients for the quantification of localized shear bands have been widely employed in the study of soil-structure interaction problems. Analysis of the vertical displacement field (\u003cem\u003ev\u003c/em\u003e) indicates the influence of stress and deformation behavior. Under higher normal stress, significant particle rearrangement and densification occur even in the initial stages of shearing, leading to an overall contractive response in the sand specimen. Nonetheless, under all tested conditions, pronounced dilatancy is observed immediately adjacent to the interface roughness elements. The shape of reflects the underlying configuration of the interface, often displaying an undulatory or sawtooth morphology.\u003c/p\u003e\u003cp\u003eA comparison between results at 150 kPa and 250 kPa reveals that although particle displacement gradients or relative movements within the dilation zone may appear more intense at 250 kPa, the overall vertical extent of dilatant zone (i.e., the thickness of the dilation zone) is slightly less than that observed at 150 kPa. This observation provides direct visual confirmation of the dilatancy-suppressing effect of increased normal stress. Notably, the interface with \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 2.7 exhibits a comparatively larger dilation zone height at 250 kPa, which contrasts with cumulative dilatancy trends observed at the macroscopic scale. For the \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 10.8 interface, dilatancy phenomenon is predominantly concentrated within troughs between the roughness elements, resulting in a limited upward spread and a comparatively thinner overall dilation zone. The shear strain rate field clearly depict the formation path and temporal evolution of the shear band. Under the optimal roughness condition, the high shear rate zone demonstrates a clear tendency to propagate into the soil mass from the initial stages of shearing, with the core of the ultimately formed shear band being relatively distant from the physical interface. Conversely, for interfaces with roughness values of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 2.7 and \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 10.8, the initial shear band nucleate at the horizontal boundaries of the specimen and develop along the rough surface toward the center. In these higher roughness interfaces, shear bands is tend form at smaller shear displacements and remain more confined to the vicinity of the interface.\u003c/p\u003e\u003cp\u003eThe aforementioned visual analysis of the PIV-derived displacement fields (\u003cem\u003eu\u003c/em\u003e, \u003cem\u003ev\u003c/em\u003e) and shear rate fields provides intuitive and valuable insights into the location, shape of the shear band, and the distribution characteristics of the dilation zone under various experimental conditions. However, qualitative visual observation alone is insufficient for the accurate and objective quantification of the thicknesses of these localized deformation zones, nor does it enable rigorous comparisons across different interface configurations and confinement stress levels. To overcome these limitations and enable a more precise characterization of shear and volumetric deformation, this study introduces a quantitative approach based on averaged displacement component profiles as key indicators. Specifically, the spatial distributions of normalized displacement is analyzed as functions of distance from the sand-steel structure interface, and regions exhibiting significant changes in displacement gradients can be identified. Smooth curves, \u003cem\u003ef(d)\u003c/em\u003e, are obtained through quadratic spline interpolation. The first derivative, \u003cem\u003ef'(d)\u003c/em\u003e, and the second derivative, \u003cem\u003ef''(d)\u003c/em\u003e, can then be computed using finite difference methods. Finally, the curvature, \u003cem\u003eκ\u003c/em\u003e, can be calculated according to the formula:\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\kappa =\\frac{{|f^{\\prime\\prime}(d)|}}{{{{\\left( {1+{{[f^{\\prime}(d)]}^2}} \\right)}^{3/2}}}}\\)\u003c/span\u003e\u003c/span\u003e. Conventionally, the boundaries of the shear band and dilation zone can be defined as the locations where the normalized horizontal or vertical displacement gradient reaches its maximum value. Based on this procedure, two quantitative parameters are obtained: the shear band thickness (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e) and dilation zone thickness (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003ed\u003c/em\u003e\u003c/sub\u003e). These parameters enable consistent, objective comparison across experimental conditions and allow for robust evaluation of how interface roughness and normal stress influence the spatial extent of localized deformation.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e illustrates the progressive determination of the soil-structure interface shear zone at various shear displacement stages under a normal stress\u0026thinsp;=\u0026thinsp;150 kPa and interface relative roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35. The identification process incorporates particle movement tracking, average displacement profiles, and the curvature analysis to define the shear band boundaries at each stage of deformation. Subsequently, Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e presents the evolution of the normalized shear band thickness as a function of shear displacement for this same condition. In this analysis, a Gaussian kernel density estimation method was employed to compute the two-dimensional spatial density values of particles undergoing shear displacement at various locations. The color intensity of the scattered points represents the data points concentration in that region, with a density value approaching 1 indicating a high data points density. Notably, the regions of high data point density strongly coincide with the distribution curve of the normalized displacement component versus distance from the contact surface. This agreement indirectly validates the effectiveness of using averaged displacement distribution curve method to identify regions characterized by significant changes in displacement gradients.\u003c/p\u003e\u003cp\u003eBased on the observed shear strain rate field, the formation of a fully developed shear band is considered to occurred when a continuous high shear rate zone extends across the specimen. In all tests, this continuous shear band was observed to be fully developed when the shear displacement was between 20% and 40% of the total applied displacement. Accordingly, the quantitative analysis of shear band thickness evolution commences from a shear displacement corresponding to 40% of the total shear displacement. The evolution of shear band thickness displays a non-linear increase trend with shear displacement. However, the rate of this increase gradually diminishes and eventually stabilizes, indicating convergence towards a consistent shear band thickness. This behavior suggests that the particle rearrangement process ultimately culminates in the formation of a complete and stable shear band.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e illustrates the evolution of normalized shear band thickness with shear displacement across a range of normal stress levels and surface roughness conditions. Following this, the final value of normalized shear band thickness attained under each condition is summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e. A systematic trend is observed where the normalized shear band thickness decreases with an increase in the applied normal stress. For instance, under the optimal roughness condition of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, the shear band thickness reduces from approximately 7.19 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at 50 kPa to approximately 6.28 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at 350 kPa. Similarly, for the highest roughness condition of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 21.6, the shear band thickness also decreases from approximately 4.60 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at 50 kPa to approximately 3.64 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at 350 kPa. In addition to the stress-dependent behavior, a significant non-monotonic relationship is observed between normalized shear band thickness and the relative interface roughness. At each normal stress level, the thickest shear band consistently occur at the optimal relative roughness of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, indicating an optimal roughness condition that mobilizes more distributed strain localization. For example, under a normal stress of 250 kPa, the shear band thickness corresponding to \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 is 6.68 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e, whereas for \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 0.68 and \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 21.6, the thicknesses decrease to 5.16 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e and 4.12 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e, respectively.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e14\u003c/span\u003e presents the evolution of dilation zone thickness with shear displacement under a specific condition (\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;150 kPa, \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35). Based on the understanding of dilation zone development, Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e15\u003c/span\u003e extends this comparison to summarize the final normalized dilation zone thickness across a varying range of normal stress levels and surface roughness conditions. The dilation zone is generally considered to be spatially coincident with or encompassed within the shear band. Experimental observations confirm that the dilatancy phenomenon predominantly occurs within the localized shear band region. For all tested relative roughness values, the normalized dilation zone thickness exhibits a pronounced decreasing trend with increasing normal stress. For example, in the case of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 2.7, the dilation zone thickness decreases from approximately 5.96 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at 50 kPa to approximately 4.11 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at 350 kPa. Similarly, for \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, the thickness significantly reduces from approximately 6.45 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at 50 kPa to approximately 3.07 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e at the same stress level. Under low normal stress conditions (e.g., 50 kPa), the dilation zone thickness increases with \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e, reach a peak value (approximately 6.45 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e) at \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, and then decreases with further increases in \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e. This pattern aligns with the observed macroscopic dilatancy and peak strength behavior under the same stress condition. However, at higher normal stress levels (150\u0026ndash;350 kPa), a shift in the optimal roughness corresponding to the maximum dilation zone thickness is observed. Specifically, under relatively higher stress conditions, \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 2.7 exhibits the maximum dilation zone thickness, while previous optimal \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 yields comparatively lower values, even falling below the value observed for \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 0.68 at 350 kPa. For structure surface with significantly higher roughness (e.g., \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 10.8, 21.6), the dilation zone thickness remains generally low across all normal stress level. These results suggest that excessive roughness may suppress dilative deformation by confining particle movement near the interface.\u003c/p\u003e\u003cp\u003eVolumetric expansion during shearing is an inherent aspect of the overall shear deformation mechanism. Consequently, the region experiencing significant dilatancy is, in spatial terms, expected to closely overlap with the zone of intense shear deformation. Accordingly, the dilation zone is generally considered to be contained within the broader shear band. A comparison between the normalized shear band thickness and normalized dilation zone thickness across all tested combinations of relative interface roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) and normal stress (\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) confirms this spatial relationship. In every case, the calculated shear band thickness values consistently exceed the corresponding dilation zone thickness. For instance, under the optimal roughness condition of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 and a normal stress of 250 kPa, the shear band thickness is approximately 6.68 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e, whereas the dilation zone thickness is approximately 4.47 \u003cem\u003eD\u003c/em\u003e\u003csub\u003e\u003cem\u003e50\u003c/em\u003e\u003c/sub\u003e. This quantitative disparity directly supports the conclusion the volumetric expansion is indeed spatially contained within the broader zone of shear band. Visual analysis of the shear strain rate field and the vertical displacement field (\u003cem\u003ev\u003c/em\u003e) under identical experimental conditions further support this interpretation. A strong spatial correlation is observed between the regions of concentrated shear strain and regions where significant dilatancy occurs. The sawtooth or undulatory distribution characteristic of the dilatancy is invariably observed within the zone influenced by high shear rates and closely follows the geometric profile of the interface. These observations confirm that shear-induced dilatancy does not occur in isolation. Rather, it emerges as a secondary effect within regions undergoing substantial shear deformation. In essence, dilatancy is both initiated and constrained by the extent of strain localization imposed by the interface roughness and applied stress conditions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Influence of Normal Stress and Contact Surface Roughness\u003c/h2\u003e\u003cp\u003eA comprehensive set of direct shear tests was conducted on sand-steel structure interfaces under varying normal stresses (50\u0026ndash;350 kPa) and relative interface roughness values (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 0.68\u0026ndash;21.6). The results, derived from both macroscopic mechanical responses (e.g. stress ratio \u0026ndash; shear displacement and vertical displacement-shear displacement curves) and microscale deformation fields (via PIV-observed \u003cem\u003eu\u003c/em\u003e, \u003cem\u003ev\u003c/em\u003e, and shear strain rate), along with quantitative metrics (normalized shear band and dilation zone thickness), reveals a complex and coupled influence of normal stress and interface roughness on interfacial shear behavior. These findings are summarized as follows.\u003c/p\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e3.3.1 Influence on Peak and Residual Shear Strength\u003c/h2\u003e\u003cp\u003eNormal stress and relative interface roughness jointly and significantly influence both peak and residual shear strengths of the sand-steel structure interface. However, the governing mechanisms and sensitivity of these two strength parameters to normal stress and roughness are different. These differences lie in the microscale interaction mechanisms between particles and the rough surface in the vicinity of the interface, primarily involving particle overriding, interlocking, and embedment effects. The peak shear strength of the interface demonstrates a high sensitivity to both normal stress (\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) and relative roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e). It generally decreases with increasing \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e, a trend primarily attributed to the suppression effect of confinement stress on interfacial dilatancy. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(c), high normal stress impedes particle overriding by suppressing upward particle movement, thereby directly reducing the dilatancy-driven component of shear resistance that is more easily mobilized under low normal stress conditions, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(b)). In addition, the peak shear strength exhibits a clear non-monotonic relationship with the relative interface roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e). For all tested normal stress levels, an optimal relative roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35) is identified, at which the interface is capable of mobilizing the highest peak strength. This suggests that \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 provides a geometric scale that most effectively match the particulate characteristics of the employed STK sand, enabling optimal interlocking. As depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(b), this condition facilitates a tight interlocking arrangement between sand particles and structure interface, thereby most effectively activating the particle overriding mechanism and the internal shear resistance near the interface. When the interface roughness deviates from this optimal value, either becoming too smooth (leading to insufficient interlocking and increased interfacial sliding) or too rough (causing particles partially to embed deeper troughs of the roughness elements and the shear band to elevate above the interlocking zone), the interlocking efficiency decreases, consequently resulting in reduced peak shear strength.\u003c/p\u003e\u003cp\u003eIt is noteworthy that the influences of normal stress and roughness are coupled. At high normal stress conditions, the universal and pronounced suppression of dilatancy (compare to the relatively unconstrained overriding with the inhibited overriding in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(c)), the differences in peak strength among interfaces with varying roughness are relatively diminished. Although higher normal stress can enhance interlocking between particles (see Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(c)), sand particle overriding motion required to overcome this strong interlocking becomes more difficult, thereby dampening the advantage of the optimal roughness at high stress levels. In contrast, the residual shear strength, which emerges after large shear displacements, tends to stabilize and shows weaker sensitivity to both interface roughness and normal stress compared to the peak shear strength. This characteristic strongly indicates that the interfacial behavior in the residual stage is predominantly governed by the critical state properties of the sand itself. At this state, the residual strength primarily reflects the fundamental frictional properties of the sand under steady-state shearing conditions. Although the residual strength still displays a slight tendency to decrease with increasing normal stress and exhibits a rather complex dependency on interface roughness. This may be attributed to final the stable particle fabric formed after large shearing, residual interfacial geometric effects, or subtle ongoing particle rearrangements within the shear band.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e3.3.2 Influence on Shear Band Characteristics\u003c/h2\u003e\u003cp\u003eThe normalized shear band thickness consistently decreases with increasing normal stress, a trend directly linked to the suppression of dilatancy under high confining stress. As illustrated Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(c), high normal confining stress impedes the upwards overriding of sand particles, thereby forcing them to rearrange and slide within a more constrained space. This results in a more localized concentration of shear deformation and a correspondingly thinner shear band.\u003c/p\u003e\u003cp\u003eWith respect to interface roughness, the shear band thickness exhibits a non-monotonic relationship, reaching a maximum at the optimal roughness of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35. PIV observations clearly demonstrate that at this optimal roughness, sand particles form an efficient interlocking state with the interface roughness elements (as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e). This enhanced interlocking necessitates that the shear failure path develops via a combination of particle overriding (as shown in the effective overriding mode in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(b)) and internal shearing, thereby extending the shear band upward into the soil mass above the interface. This resulting deformation process mobilizes a larger volume of sand particles through complex rearrangement, rolling, and sliding, thus forming the thickest shear band. At this stage, the interface shear strength efficiency \u003cem\u003eα\u003c/em\u003e exceeds 1.0, indicating that the interface shear resistance, primarily induced by interlocking and dilatancy, is significantly greater than that of the sand. Thus, the shearing failure tends to occur within the soil adjacent to the interface rather than along the contact surface. However, as the normal stress increases or the roughness deviates from the optimal value, the interface shear strength efficiency α decreases. When α approaches or falls below 1.0 (as shown by PIV visualizations in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e), the failure mode is more likely to occur along the interface or in a thin zone immediately adjacent to it. For example, at higher roughness levels (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e \u0026gt;1.35), particles may partially embed into the deeper troughs of the rough surface. In such scenarios, the shear band tends to form above the rough surface, bypassing the primary interlocking zone and thus reducing interface interlocking efficiency. Meanwhile, the initial development of the shear band at the specimen ends is more closely aligned with the interface and involves a greater degree of interfacial sliding rather than complex and extensive particle shearing within the soil mass.\u003c/p\u003e\u003cp\u003eMechanistically, the shear band thickness is governed by two primary factors, which are the intensity of particle interlocking and the magnitude of particle overriding. The former determines how many particles must be mobilized to overcome geometric constraints and initiate shear deformation. Stronger interlocking requires more particles involved in the rearrangement process, thus contributes to form a thicker shear band. The latter dictates the vertical deformation space required to accommodate sand particles movement within the shear band during shearing. Increasing normal stress suppresses sand particles overriding (see in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e16\u003c/span\u003e(c)), which reduces the space available for deformation and narrows the shear band. Overall, relative interface roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e governs shear band thickness in a non-monotonic fashion, as modulates both interlocking efficiency and overriding patterns. The present study identifies an optimal \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, at which the balance between particle interlocking and overriding yields maximum mobilized volume of soil in shear deformation and consequently resulting in the thickestshear band.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e3.3.3 Influence on Volumetric Strain Behavior\u003c/h2\u003e\u003cp\u003eThe normal stress level is a critical determinant of the volumetric deformation mode. Under low stress (e.g. 50 kPa), the interface exhibits purely dilative behavior, as relatively low confining stress allows sand particles to readily rearrange and expand volumetrically. In contrast, at relatively higher stress levels (\u0026ge;\u0026thinsp;150 kPa), the volumetric response transits to an initial contractancy followed by subsequent dilatancy. This shift reflects the initial densification and rearrangement of particles driven by under higher confining stress, with sand specimen dilatancy manifesting only after sufficient shear displacement to overcome this resistance. Interface roughness further activate dilative behavior by introducing geometric obstructions that promote particle interlocking and overriding. Under low normal stresses, the interface with roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 yields the maximum macroscopic dilatancy and the thickest dilation zone, consistent with its corresponding peak shear strength. However, at higher normal stresses, although overall macroscopic dilatancy tendencies are suppressed, PIV-based observations and measurements of normalized dilation zone thickness indicate that \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 2.7 develops the thickest apparent dilation zone. This finding suggests that under high confining stress, the spatial extent of dilatancy may shift away from roughness values (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35) that corresponded to optimal peak strength.\u003c/p\u003e\u003cp\u003eFor interface with extremely high roughness values (e.g. \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e \u0026ge; 10.8), dilatancy is significantly constrained. In these cases, volumetric expansion is primarily confined to narrow troughs of structure roughness elements, resulting in the smallest dilation zone thickness. This indicates that excessive roughness may hinder effective particle mobility and restrict volumetric deformation, rather than enhancing it. Also, a close coupling is observed between the mobilization of peak shear strength and development of dilatancy behavior. At higher normal stress levels, the occurrence of maximum contractancy (i.e. minimum volumetric strain) often coincides with the mobilization of peak shear strength. This correspondence marks a transition from a densification-dominated behavior to dilatancy-driven behavior, where particles begin to override geometric constraints despite the confining stress.\u003c/p\u003e\u003cp\u003eThroughout all conditions, the dilation zone primarily occurs within the shear band, with both exhibiting a high degree of spatial overlap, reflecting the fact that dilatancy is an intrinsic volumetric response to shear deformation. The volumetric strain behavior observed in this study is the result of a dynamic competition and transition between two governing mechanisms: the normal stress-induced tendency for densification and embedment, and the interface roughness-induced tendency for particle overriding. At low normal stresses, sand particles overriding is readily occurs, leading to predominant dilatative behavior. At high normal stresses, the initial shear phase is dominated by particle densification, leading to contraction. In this case, sand particle overriding is suppressed and delayed until sufficient shear energy has accumulated to overcome the confining stress. Relative roughness Rn governs this interplay by modulating both the mechanical threshold for overriding and the spatial potential for particle rearrangement and embedment.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis study investigates the coupled effects of normal stress (σn) and relative interface roughness (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e) on the shear behavior of sand-steel structure interfaces through a comprehensive experimental framework, including macroscopic direct shear tests, interface shear strength efficiency analysis, PIV-based microscale observation, and quantitative characterization of shear/dilation zone development. The findings highlight how stress conditions and surface geometry jointly influence interfacial strength, deformation patterns, and failure mechanisms. Three micromechanical processes, particle overriding, interlocking, and embedment emerge as the fundamental drivers of these behaviors.\u003c/p\u003e\u003cp\u003e(1) Peak shear strength of sand-steel structure interface decreases with increasing σn, and shows a non-monotonic relationship with \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e, peaking at \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 attaining its maximum at the optimal roughness of \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35. This is due to the combined effect of dilatancy suppression under high stress and geometric interlocking. Residual shear strength, in contrast, is less affected by σn and Rn, reflecting the sand\u0026rsquo;s critical state behavior. Practical evaluation should therefore distinguish between peak and residual strength when assessing bearing capacity and stability.\u003c/p\u003e\u003cp\u003e(2) The interface shear strength efficiency (\u003cem\u003eα\u003c/em\u003e) reflects failure mode tendencies. \u003cem\u003eα\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;1 implies failure is more likely to occur within the sand mass adjacent to the interface, while \u003cem\u003eα\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1 indicates interface itself constitutes a plane of weakness, and failure tends to occur along the interface. Its variation with \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e aligns well with PIV-observed shear band shifts, making \u003cem\u003eα\u003c/em\u003e a valuable design indicator for interface optimization.\u003c/p\u003e\u003cp\u003e(3) Both normalized shear band thickness (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e) and dilation zone thickness (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003ed\u003c/em\u003e\u003c/sub\u003e) decrease with increasing normal stress and vary non-monotonically with roughness \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e. The maximum \u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e occurs at \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35, while the optimal \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e for \u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003ed\u003c/em\u003e\u003c/sub\u003e may shift under high normal stress. Importantly, \u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003ed\u003c/em\u003e\u003c/sub\u003e remains within \u003cem\u003eδ\u003c/em\u003e\u003csub\u003e\u003cem\u003es\u003c/em\u003e\u003c/sub\u003e, confirming that dilatancy is a localized volumetric response within the shear band.\u003c/p\u003e\u003cp\u003e(4) Interfacial shear behavior is predominantly governed by three core mechanisms: particle overriding, interlocking, and embedment effects. High normal confining stress inhibits overriding and promotes densification; structure interface with varying Rn shapes interlocking and deformation patterns. These findings underscore the need to consider both roughness and stress conditions in design, and support the development of stress- and geometry-aware interface models for soil-structure interaction.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCRediT authorship contribution statement\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eZilong Zhou:\u003c/strong\u003e Supervision, Conceptualization. \u003cstrong\u003eYiqun Su:\u0026nbsp;\u003c/strong\u003eInvestigation, Writing \u0026ndash;original draft. \u003cstrong\u003eShaofeng Wang:\u003c/strong\u003e Review \u0026amp; Editing, Funding acquisition. \u003cstrong\u003eJinbiao Wu:\u0026nbsp;\u003c/strong\u003eReview \u0026amp; Editing, Funding acquisition, Methodology.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data t\u0026nbsp;used in the current study are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe work described in this paper was supported by the National Natural Science Foundation of China and the Central South University.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAlshibli, K.A., Sture, S., 2000. Shear Band Formation in Plane Strain Experiments of Sand. J. Geotech. Geoenvironmental Eng. 126, 495\u0026ndash;503. https://doi.org/10.1061/(ASCE)1090-0241(2000)126:6(495)\u003c/li\u003e\n\u003cli\u003eChen, W.-B., Zhou, W.-H., Dos Santos, J.A., 2020. Analysis of consistent soil\u0026ndash;structure interface response in multi\u0026ndash;directional shear tests by discrete element modeling. Transp. Geotech. 24, 100379. https://doi.org/10.1016/j.trgeo.2020.100379\u003c/li\u003e\n\u003cli\u003eChen, W.-B., Zhou, W.-H., Yin, Z.-Y., 2023. Recent Development on Macro\u0026ndash;Micro Mechanism of Soil-Structure Interface Shearing Through DEM. Arch. Comput. Methods Eng. 30, 1843\u0026ndash;1862. https://doi.org/10.1007/s11831-022-09854-0\u003c/li\u003e\n\u003cli\u003eDeJong, J.T., Westgate, Z.J., 2009. Role of Initial State, Material Properties, and Confinement Condition on Local and Global Soil-Structure Interface Behavior. J. Geotech. Geoenvironmental Eng. 135, 1646\u0026ndash;1660. https://doi.org/10.1061/(ASCE)1090-0241(2009)135:11(1646)\u003c/li\u003e\n\u003cli\u003eFrost, J.D., DeJong, J.T., Recalde, M., 2002. Shear failure behavior of granular\u0026ndash;continuum interfaces. Eng. Fract. Mech. 69, 2029\u0026ndash;2048. https://doi.org/10.1016/S0013-7944(02)00075-9\u003c/li\u003e\n\u003cli\u003eGu, X., Huang, M., Qian, J., 2014. DEM investigation on the evolution of microstructure in granular soils under shearing. Granul. Matter 16, 91\u0026ndash;106. https://doi.org/10.1007/s10035-013-0467-z\u003c/li\u003e\n\u003cli\u003eHryciw, R.D., Irsyam, M., 1993. Behavior of Sand Particles Around Rigid Ribbed Inclusions During Shear. Soils Found. 33, 1\u0026ndash;13. https://doi.org/10.3208/sandf1972.33.3_1\u003c/li\u003e\n\u003cli\u003eHu Liming, Pu Jialiu, 2004. Testing and Modeling of Soil-Structure Interface. J. Geotech. Geoenvironmental Eng. 130, 851\u0026ndash;860. https://doi.org/10.1061/(ASCE)1090-0241(2004)130:8(851)\u003c/li\u003e\n\u003cli\u003eIwashita, K., Oda, M., 2000. Micro-deformation mechanism of shear banding process based on modified distinct element method. Powder Technol. 109, 192\u0026ndash;205. https://doi.org/10.1016/S0032-5910(99)00236-3\u003c/li\u003e\n\u003cli\u003eLiu, K., Wu, R., Ma, B., Zhang, P., Zeng, C., 2025. Macro/meso-evolution of shear behavior of sand-steel interface in pipe jacking via digital image correlation (DIC) technology. Tunn. Undergr. Space Technol. 155, 106197. https://doi.org/10.1016/j.tust.2024.106197\u003c/li\u003e\n\u003cli\u003eMartinez, A., Frost, J.D., 2017. The influence of surface roughness form on the strength of sand\u0026ndash;structure interfaces. G\u0026eacute;otechnique Lett. 7, 104\u0026ndash;111. https://doi.org/10.1680/jgele.16.00169\u003c/li\u003e\n\u003cli\u003eOda, M., Kazama, H., 1998. Microstructure of shear bands and its relation to the mechanisms of dilatancy and failure of dense granular soils. G\u0026eacute;otechnique 48, 465\u0026ndash;481. https://doi.org/10.1680/geot.1998.48.4.465\u003c/li\u003e\n\u003cli\u003ePaikowsky, S., Player, C., Connors, P., 1995. A Dual Interface Apparatus for Testing Unrestricted Friction of Soil Along Solid Surfaces. Geotech. Test. J. 18, 168\u0026ndash;193. https://doi.org/10.1520/GTJ10320J\u003c/li\u003e\n\u003cli\u003ePorcino, D., Fioravante, V., Ghionna, V., Pedroni, S., 2003. Interface Behavior of Sands from Constant Normal Stiffness Direct Shear Tests. Geotech. Test. J. 26, 289\u0026ndash;301. https://doi.org/10.1520/GTJ11308J\u003c/li\u003e\n\u003cli\u003eRowe, P.W., 1962. The stress-dilatancy relation for static equilibrium of an assembly of particles in contact. Proc R Soc Lond 269, 500\u0026ndash;527. https://doi.org/10.1098/rspa.1962.0193\u003c/li\u003e\n\u003cli\u003eSadrekarimi, A., Olson, S.M., 2010. Shear Band Formation Observed in Ring Shear Tests on Sandy Soils. J. Geotech. Geoenvironmental Eng. 136, 366\u0026ndash;375. https://doi.org/10.1061/(ASCE)GT.1943-5606.0000220\u003c/li\u003e\n\u003cli\u003eStanier, S.A., Blaber, J., Take, W.A., White, D.J., 2016. Improved image-based deformation measurement for geotechnical applications. Can. Geotech. J. 53, 727\u0026ndash;739. https://doi.org/10.1139/cgj-2015-0253\u003c/li\u003e\n\u003cli\u003eSu, D., Wu, D., Fan, M., Zhang, R., Chen, J., Xiong, H., Chen, X., 2025. DEM investigation into the coupling effects of particle asphericity and interface roughness on shear behaviour of soil-structure interface. Particuology 97, 39\u0026ndash;57. https://doi.org/10.1016/j.partic.2024.11.019\u003c/li\u003e\n\u003cli\u003eSu, L.-J., Zhou, W.-H., Chen, W.-B., Jie, X., 2018. Effects of relative roughness and mean particle size on the shear strength of sand-steel interface. Measurement 122, 339\u0026ndash;346. https://doi.org/10.1016/j.measurement.2018.03.003\u003c/li\u003e\n\u003cli\u003eUesugi, M., Kishida, H., 1986. Frictional Resistance at Yield between Dry Sand and Mild Steel. Soils Found. 26, 139\u0026ndash;149. https://doi.org/10.3208/sandf1972.26.4_139\u003c/li\u003e\n\u003cli\u003eWang, H.-L., Zhou, W.-H., Yin, Z.-Y., Jie, X.-X., 2019. Effect of Grain Size Distribution of Sandy Soil on Shearing Behaviors at Soil\u0026ndash;Structure Interface. J. Mater. Civ. Eng. 31, 04019238. https://doi.org/10.1061/(ASCE)MT.1943-5533.0002880\u003c/li\u003e\n\u003cli\u003eWang, J., Gutierrez, M.S., Dove, J.E., 2007. Numerical studies of shear banding in interface shear tests using a new strain calculation method. Int. J. Numer. Anal. Methods Geomech. 31, 1349\u0026ndash;1366. https://doi.org/10.1002/nag.589\u003c/li\u003e\n\u003cli\u003eWang, J., Jiang, M., 2011. Unified soil behavior of interface shear test and direct shear test under the influence of lower moving boundaries. Granul. Matter 13, 631\u0026ndash;641. https://doi.org/10.1007/s10035-011-0275-2\u003c/li\u003e\n\u003cli\u003eXu, M., Wang, Lizhong, Wang, Lilin, Guo, Z., Zhou, W., 2024. Influence of bounding surface plasticity-based soil-structure interaction model on integrated dynamic behaviour of jacket offshore wind turbines. Ocean Eng. 298, 117204. https://doi.org/10.1016/j.oceaneng.2024.117204\u003c/li\u003e\n\u003cli\u003eZhang, G., Liang, D., Zhang, J.-M., 2006. Image analysis measurement of soil particle movement during a soil\u0026ndash;structure interface test. Comput. Geotech. 33, 248\u0026ndash;259. https://doi.org/10.1016/j.compgeo.2006.05.003\u003c/li\u003e\n\u003cli\u003eZhang, G., Zhang, J., 2009. State of the art: Mechanical behavior of soil\u0026ndash;structure interface. Prog. Nat. Sci. 19, 1187\u0026ndash;1196. https://doi.org/10.1016/j.pnsc.2008.09.012\u003c/li\u003e\n\u003cli\u003eZhou, W.-H., Jing, X.-Y., Yin, Z.-Y., Geng, X., 2019. Effects of particle sphericity and initial fabric on the shearing behavior of soil\u0026ndash;rough structural interface. Acta Geotech. 14, 1699\u0026ndash;1716. https://doi.org/10.1007/s11440-019-00781-2\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"sand-structure interface, interface roughness, shear zone, mechanical behavior","lastPublishedDoi":"10.21203/rs.3.rs-7800502/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7800502/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study presents an experimental investigation into the macro- and micro-scale shear mechanical behavior of sand-structure interfaces. Objectives include characterizing stress-displacement and volumetric responses, identifying optimal interface roughness, and understanding strain localization and kinematics failure mechanisms within shear and dilation zones. Direct shear tests were performed on sand interfaced with steel plates exhibiting varying, well-defined trapezoidal sawtooth roughness profiles (\u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e ranging from 0 to 21.6) under normal stresses from 50 to 350 kPa. A modified direct shear apparatus integrated with PIV technology enabled real-time, non-contact monitoring and quantitative analysis of the sand deformation field, correlating macroscopic mechanical responses with microscale observations. Results showed that interface peak shear strength decreased in stress ratio (\u003cem\u003eτ\u003c/em\u003e\u003csub\u003e\u003cem\u003eη\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/σ\u003c/em\u003e\u003csub\u003e\u003cem\u003eη\u003c/em\u003e\u003c/sub\u003e) with increasing normal stress, with \u003cem\u003eR\u003c/em\u003e\u003csub\u003e\u003cem\u003en\u003c/em\u003e\u003c/sub\u003e = 1.35 yielding the highest strength. Volumetric behavior transitioned from dilative to contractive-dilative modes as normal stress increased, with peak contraction near peak strength. Interface shear strength efficiency (\u003cem\u003eα\u003c/em\u003e) generally decreased with increasing normal stress, indicating a transition from internal shearing within adjacent sand to predominantly interfacial sliding failure mode. PIV analysis provided direct visualization and quantification of shear band and dilation zone formation and evolution. The thickness and morphology of these zones were affected by both normal stress and interface roughness; higher normal stress generally suppressed dilatancy, while specific roughness profiles modulated strain localization. Microscale kinematics observations confirmed non-uniform deformation patterns, highlighting the critical role of particle overriding and rearrangement. The findings underscore the importance of integrating macro- and meso-scale to achieve a comprehensive understanding of sand-structure interface behavior.\u003c/p\u003e","manuscriptTitle":"Effects of confining stress and roughness on mechanical behavior of sand-steel structure interface","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-31 12:18:41","doi":"10.21203/rs.3.rs-7800502/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"3d04596a-6821-4641-9c57-fae1d2f15b5e","owner":[],"postedDate":"October 31st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-04-07T16:15:01+00:00","versionOfRecord":{"articleIdentity":"rs-7800502","link":"https://doi.org/10.1007/s10035-026-01636-w","journal":{"identity":"granular-matter","isVorOnly":false,"title":"Granular Matter"},"publishedOn":"2026-04-01 15:59:28","publishedOnDateReadable":"April 1st, 2026"},"versionCreatedAt":"2025-10-31 12:18:41","video":"","vorDoi":"10.1007/s10035-026-01636-w","vorDoiUrl":"https://doi.org/10.1007/s10035-026-01636-w","workflowStages":[]},"version":"v1","identity":"rs-7800502","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7800502","identity":"rs-7800502","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}