Mechanical Response of Fine-Grained Soil-Solid Waste Composites from Field Scale Direct Shear Tests

Mechanical Response of Fine-Grained Soil-Solid Waste Composites from Field Scale Direct Shear Tests | 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 Mechanical Response of Fine-Grained Soil-Solid Waste Composites from Field Scale Direct Shear Tests Mohsen Ajdari, Mohammadreza Arvin, Sina Zahmatkesh Astaneh, Kianoosh Hatami This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9013637/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The paper reports a study on the reinforcing effect of non-degradable municipal solid waste (MSW) in fine-grained soils with possible applications in reinforced soil construction. A low-plasticity silty soil was randomly mixed with five-year-old, non-degradable MSW at 0.0, 0.5, 1.0, and 2.0% concentrations by dry weight (wt%), and compacted in a large-scale split shear box of approximately one meter cubed volume. Specimens were consolidated under uniform normal stresses across their entire plan area for twenty-four hours and then sheared under drained, displacement-controlled conditions. Strength comparisons were made consistently at a large strain level to capture any reinforcing benefits of the soil-MSW mixtures under field conditions. The consolidation response of the composites exhibited a distinct threshold. Mixtures up to 1.0 wt% MSW showed consolidation behavior similar to that of a comparable unreinforced soil, whereas a 2.0 wt % concentration caused a slower consolidation rate and substantially larger settlements. The concurrent reduction in the virgin compression slope and increase in the recompression slope with reinforcement concentration indicates strain-dependent interactions, where interlocking between the soil and waste material mobilizes the tensile resistance of waste fragments anchored within the soil matrix. Results also show a significant (i.e. by as much as 50%) increase in the drained shear strength of the mixtures at large deformations relative to comparable raw samples. Variations in the shear strength properties of mixtures indicated two distinct trends in that, for up to 1.0% waste concentration, the increase in shear strength was essentially due to an increase in the apparent cohesion with minimal change in the friction angle, whereas at 2.0% concentration, the friction angle increased significantly while cohesion decreased, reflecting a transition to an interlocking-dominated reinforcement mechanism. Municipal Solid Waste Reinforced Soil Fine-grained Soil Large Direct Shear Device Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Urbanization and the resulting rise in the use of domestic and industrial materials and commercial products continuously lead to increased municipal solid waste (MSW) production worldwide [1]. A major portion of MSW is non-biodegradable plastics and polymeric products that can persist in landfills for decades, consuming land capacity and creating long-term environmental hazards [2]. Meanwhile, many infrastructure systems are built on weak and compressible fine-grained soils, which can lead to excessive settlements [3], cracking, and even instability in embankments, pavement subgrades, and retaining structures [4]. Even though conventional ground-improvement approaches, such as replacement with granular material, deep foundations, or chemical stabilization with cement or lime, are generally effective, they can be cost-prohibitive and could result in environmental concerns [5]. The above challenges necessitate sustainable ground-improvement strategies that can improve weak soils with the environmental benefits of reusing non-degradable solid waste materials. There have been several studies on the potential use of MSW as soil reinforcement in the past. However, a review of literature reveals the following key gaps: First, most of the studies to date have used plastic wastes that were processed into desired shapes and sizes, e.g. lab-prepared strips, fibers, or mats [6, 7] and then mixed uniformly or placed in layers in the specimens [8, 9]. This approach is not conducive to a direct and economical use of landfill-derived waste, which is typically irregular, multi-material, and partially degraded. Therefore, the mechanical response of fine-grained soils mixed with raw (i.e. unprocessed) landfill waste is still not well understood. Second, many studies are based on conventional small-scale laboratory tests (e.g., Proctor compaction, unconfined compression, small direct shear, or CBR), where specimen sizes are too small to be able to capture the interactions between large, stiff waste pieces and the surrounding soil matrix [10, 11]. Scale effects associated with waste particle size and shear-zone thickness can significantly influence measured strength and deformations. Third, the studies have largely been focused on sands, lateritic soils, or coarse subgrade materials, while much fewer studies have addressed cohesive or fine-grained soils reinforced with waste plastics [12, 13]. A review by Mishaal and Aldaood (2023) highlights the need for more work on cohesive soils, large-scale testing to clarify scale effects, and better understanding of soil-waste mixtures long-term performance in field conditions [13]. Finally, relatively few studies have examined older waste materials that have been buried in landfill conditions for several years, where aging, leaching, and partial degradation may modify their stiffness, surface characteristics, and soil-waste interaction mechanisms. The present study addresses the above gaps through an experimental investigation of a fine-grained soil reinforced with non-degradable MSW that collected from a landfill in the Province of Fars, Iran. Five-year-old undecomposed waste shreds were randomly mixed with a local fine-grained soil and compacted in a one meter-deep shear box with rigid walls, which allowed the behavior of the soil-waste composite to be examined at a scale comparable to field conditions. Large-scale direct shear tests were then performed on specimens with different MSW weight percentages under different normal stress levels to quantify how waste concentration could influence the shear strength and deformation characteristics of the composites. The specific objectives of the study were: (i) quantify changes in cohesion and internal friction angle of the fine-grained soil reinforced with aged, undecomposed landfill waste fibers in large-scale direct shear tests; (ii) assess the influence of waste fiber concentration on volume change during shearing; and (iii) identify a range of MSW concentrations that would result in increased shear strength within acceptable settlements. Background Over the past decades, the concept of fiber-reinforced soil has emerged as a practical and economical technique to improve the mechanical behavior of geomaterials [ 14 , 15 , 16 , 17 ]. Randomly distributed fibers, natural or synthetic, are mixed with soil to create a composite that exhibits enhanced strength and post-peak performance relative to the unreinforced matrix. A review of related studies indicates that the inclusion of synthetic fibers and plastic waste materials can significantly improve shear strength, stiffness, and bearing capacity and reduce brittleness and the likelihood of tensile cracks in soils. Mishaal and Aldaood (2023) summarized several studies and concluded that even waste fibers can provide substantial gains in strength and ductility, especially when they are well dispersed within the soil mass [ 13 ]. Accordingly, the use of plastic waste as soil reinforcement has become a desirable solution for soil improvement with a simultaneous benefit of waste reduction [ 18 , 19 ]. Iravanian and Haider (2020) reviewed the use of fibers produced from discarded plastic bottles and highlighted both the environmental urgency of plastic waste disposal and the engineering benefits of incorporating such materials into weak subgrade soils [ 20 ]. Their synthesis showed that adding up to 5% by weight of plastic bottle fibers can increase the shear strength and bearing capacity of the subgrade and potentially reduce pavement thickness in highway applications. Similar trends have been reported for plastic bag strips, carpets, tire shreds, and other polymeric inclusions, where improvements in strength parameters and deformation behavior are typically observed for an optimal range of fiber content and aspect ratio [ 21 ]. Recent experimental studies further demonstrate the versatility of plastic waste as reinforcement for different soil types and loading conditions. Marçal et al. (2020) evaluated the use of polypropylene waste strips from recycled packaging as reinforcement for compacted lateritic soils and observed increased unconfined compressive strength, higher apparent cohesion and friction angle in sandy clay, and up to 70% improvement in California bearing ratio for reinforced specimens [ 11 ]. Khokar et al. (2025) investigated the use of high-density polyethylene plastic bags in mat form as reinforcement for a fine soil from Oman, reporting more than threefold increases in compressive strength at an optimal number of reinforcement mats, along with changes in compaction characteristics that can be advantageous for field construction [ 7 ]. Other studies have focused on the behavior of fiber-reinforced, fine-grained soils under desiccation: Hernández et al. (2024) showed that recycled polyethylene terephthalate and polypropylene fibers can effectively control shrinkage and desiccation cracking in low-plasticity clays [ 21 ]. They also used image analysis and suction measurements to identify the fiber content that minimized crack intensity without significantly altering the soil-water retention curve. Large-Scale Direct Shear Apparatus The size of the test box was selected such that it would minimize scale effects for MSW pieces of up to 150 mm, and allow for the development of a representative shear zone during the tests. The test apparatus consists of a split steel shear box comprising a fixed lower half and a horizontally translating upper half. The box is 1 m × 1 m in plan area with a total height of nearly 1.1 m, all fabricated using 6 mm-thick steel plates (Fig. 1 ). The 0.7 m-tall upper part of box was stiffened against bulging under high normal stresses using welded gusset plates and steel braces to maintain consistent boundary conditions throughout the loading and shearing phases of the tests. Additional local stiffeners were welded to the box against the horizontal actuator to help distribute the applied load uniformly on the box and prevent local yielding or distortion in the box during the tests. The 0.4 m-tall lower part of the box was rigidly welded to the main chassis, with a 10 mm- steel base to help maintain a consistent specimen geometry. The box is equipped with four 10 mm-diameter drainage lines at its base plate. The drain lines can be used to control the moisture content and degree of saturation of the tested soil, and to carry out the tests in consolidated drained conditions and minimize any likelihood of excess pore-water pressure during the shearing phase of the tests. The box is protected using anti-corrosion coating to improve its durability and the repeatability of experiments across multiple tests. The shear box assembly is supported by a rigid load frame fabricated from welded channel sections. The external chassis plan dimensions are 2.50 m × 1.27 m, and base plates are installed beneath columns to distribute test reactions to the laboratory floor. Horizontal translation of the upper half is controlled by a guidance system integrated into the chassis. Steel rods and mating shafts engage with guide rails to constrain motion to a single horizontal degree of freedom, reduce parasitic friction, and maintain alignment under combined normal and shear loading. A nominal 10-mm gap was selected to ensure the upper and lower boxes remain mechanically decoupled during translation, thereby permitting relative displacement and the formation of a well-defined target shear plane (Fig. 1 -d). The gap size can influence measured response because an undersized gap may promote unintended contact and parasitic friction/alignment constraints, whereas an oversized gap can increase particle extrusion and alter the effective shear area and stress distribution (and thus the magnitude of any required area correction), potentially biasing interpreted shear strength and volumetric trends. To prevent specimen loss along the split interface during large displacements, a removable steel sheet is installed along the rear edge of the interface which remains in place during loading and shearing and can be removed after testing to facilitate specimen recovery and cleaning. Normal stress is applied through a vertical manual hydraulic jack positioned above the specimen. The total stroke of the 100 kN jack is 270 mm (including screw-driven adjustment and hydraulic extension), which is sufficient for specimen seating, consolidation compression, and subsequent shear-induced volumetric change. Normal load is transferred through a stiff steel loading platen that covers nearly the entire inside plan area of the test box (approximately 0.98 m × 0.98 m). The platen is reinforced with steel ribs to minimize bending and promote uniform stress distribution over the specimen surface. Because the platen and box components are heavy at this scale, a pulley and cable assembly is used to lift, position, and remove the platen in a controlled manner, reducing eccentricities during its placement during the tests. During shearing, volumetric contraction may occur and can reduce the applied normal force if the loading system is not adjusted. In this configuration, the vertical jack can be advanced during the test to compensate for settlement and to maintain a practically constant normal stress. The same vertical loading configuration can also be used to conduct large-scale one-dimensional consolidation within the box prior to shearing, enabling testing of specimens with a defined stress history and density state. Shear displacement is imposed on the upper box by a horizontal hydraulic jack connected to the chassis. The actuator provides a shear force capacity of approximately 70 kN and a maximum horizontal travel of about 300 mm, allowing observation of peak strength, post-peak softening, and residual behavior at large displacements. The jack is supplied by a hydraulic power unit equipped with a 30 L oil reservoir and a 1 hp electric motor. Shear direction, forward and backward, is controlled through a hydraulic valve system. An inverter is used to regulate motor speed and, consequently, the hydraulic flow rate, enabling stable strain-controlled loading and prescribed constant displacement rates. The system allows a broad range of shear rates, including lower rates on the order of 7 mm/min to ensure drained response based on the material properties of the tested specimen. Normal and shear forces are measured using two S-shape, 100 kN load cells in both the vertical and horizontal load paths. The sensors are connected to digital readout units that provide real-time measurements and enable calibration based on sensor sensitivity. Mechanical connections between the jacks, load cells, and the shear box assembly are made using threaded connectors and bolts to maintain an axial load path and minimize eccentricity. Horizontal and vertical displacements are monitored directly on the apparatus using mounted LVDTs referenced to the fixed chassis and the moving upper box. The horizontal displacement reading provides the imposed shear displacement, while the vertical reading captures consolidation compression and shear-induced contraction/dilation of the specimen Materials Fine-grained soil The base geomaterial in this study was a fine-grained soil that had been collected from the sediments accumulated on a plain located in south of Iran and was transported to the laboratory in the form of a disturbed bulk sample. The soil was thoroughly mixed with MSW and compacted in the laboratory to an optimum unit weight from the Standard Proctor test in the test box. The large size of the specimen was beneficial in accommodating the inherent heterogeneity and anisotropy of the soil-MSW mixture in the experiments. Undecomposed buried MSW The reinforcing phase of the mixtures was produced from the MSW that was recovered from a landfill in the province of Fars south of Iran. Five year old non-biodegradable fraction of buried waste layers were sampled as representatives of the material that had remained after the decomposition of readily degradable components (Fig. 2 ). Visual inspection during sampling and subsequent handling indicated that the recovered MSW was essentially free of organic matter. Therefore, the reinforcement was treated as an aged MSW residue dominated by persistent constituents (e.g., plastics and other durable fragments, Fig. 3 ). This is important from a materials standpoint because the mechanical contribution of MSW to a soil composite highly depends on its degradation state. Eventually, when organics are absent and the remaining fraction is primarily non-degradable, the reinforcement behavior is more stable over the duration of laboratory testing and less influenced by time-dependent biodegradation or gas generation. Table 1 Selected properties of the tested soil Property/Characteristic Test Value Fines Content (%) Sieve Analysis 78 Sand Content (%) 20 Specific Gravity Pycnometer 2.7 Optimum Water Content (%) Standard Proctor 15.2 Maximum Dry Unit Weight (kN/m 3 ) 17.3 Liquid Limit (%) Atterberg Limits 32 Plasticity Index 6 Unified Soil Classification System (USCS) ML Liquidity Index -1 Sieve analysis results indicated that the soil was a low-plasticity silt with sand (ML) according to the USCS with selected properties as listed in Table 1 . The sampling procedure targeted distinct waste layers to obtain a separable MSW fraction, which was physically isolated from adjacent soil layers. Although the waste was separated as carefully as possible, MSW is inherently heterogeneous and anisotropic at multiple scales. The relative proportions of plastics, textiles, paper-like residues, thin metallic pieces and glass fragments can vary spatially and directionally within the same landfill cell. As a result, the recovered MSW cannot be characterized by a distinct composition that is uniform throughout the cell. Instead, it should be considered as a representative batch of aged waste fragments with a degree of variability typical of landfill-derived materials. This intrinsic variability was addressed in the experimental design by using relatively small reinforcement concentrations and by adopting controlled preparation procedures to maximize repeatability. Mix Design, Specimen Conditioning and Test Procedure Because the landfill is constructed with alternating layers of soil and waste, the retrieved MSW was separated from surrounding soil, cleaned of adhering fines by manual sorting, and prepared for mixing. Prior to blending, chunks of fine-grained soil were pneumatically crushed as necessary and passed through sieves to remove stones and cohesive clods and obtain repeatable mixing and reduce local heterogeneity caused by oversized fragments. Reinforced mixtures were produced by randomly distributing the prepared MSW within the soil at four different waste concentrations by mass of 0% (unreinforced control), 0.5%, 1.0%, and 2.0% MSW relative to the total mass of the mixture. These lower concentration levels were selected so that the presence of MSW in the mixtures would be in the form of discrete inclusions that modify the composite shear response through frictional interlocking, tensile bridging, and constrained deformations while avoiding an inadvertent transformation of the material into a waste-dominant matrix. For large-scale direct shear specimens, the conditioning objective was to create a uniform, saturated soil-MSW composite consistent with drained shearing while minimizing specimen-to-specimen variability introduced by moisture gradients. The MSW inclusions were introduced such that they remained randomly oriented and dispersed within each lift rather than being segregated within discrete layers. This approach was intended to emulate field mixing scenarios in which reinforcement elements are distributed throughout the soil mass and to avoid the formation of preferential planes. During the specimen setup, care was taken to maintain uniformity along the plan area of the box by leveling each lift prior to compaction and by avoiding localized clumping of MSW inclusions. Water was added incrementally to the prepared soil (and to the soil-MSW blend after inclusion) while mixing thoroughly to achieve the target state that was visually uniform and void of any dry clumps. After thorough mixing, the material for each specimen (i.e. soil, or soil-MSW mix) was staged next to the test box and covered with plastic sheets for a minimum of 16 hours for moisture equilibration before it was placed in the test box in lifts to prepare the specimens for load testing. The test material was compacted in 250 mm-lifts statically using the vertical loading system of the apparatus until the entire 1-m-high specimen was properly placed in the test box. After compacting each lift, the soil was sampled for its void ratio and moisture content to ensure that the mixture remained consistent in its target saturated condition. If the calculated moisture content did not meet the target degree of saturation, the material was removed and replaced with added moisture as necessary and compacted before placing the next lift. This procedure helped to minimize unintended variations in the degree of saturation and variability in the stiffness and strength of the specimens. Immediately after completing the final lift, the specimen surface was prepared for seating of the loading plate, and the assembly was set for consolidation under the target normal stress. The normal load, corresponding to target interim vertical stresses (e.g., 1, 2, 4, 8, 16, 20, 30, 40 kPa) was applied gradually using a manual hydraulic jack while continuously monitoring the load cell readout. Each applied normal stress was maintained constant for a 24-hour consolidation period. During this phase, the jack was adjusted as needed to compensate for settlement so that the measured normal load remained at the target value. Consolidation drainage was provided through the drainage outlets at the base of the lower box, and vertical displacement during consolidation was recorded to quantify settlement and stabilization. The required drainage time during the subsequent shear stage was estimated from the time to 50% consolidation (t₅₀) following the procedure described in ASTM D3080, supporting selection of a displacement rate consistent with drained response. After specimens with different MSW inclusions were consolidated under vertical pressures of 20, 30, and 40 kPa, they were sheared at a constant displacement rate of 7 mm/min. Shear stress was computed from the measured horizontal force divided by the corrected plan area of the shear plane, and stress-displacement curves were used to compare the performances of different mixtures. In total, 12 large-scale shear tests were carried out involving three (3) different normal stresses and four (4) MSW concentrations (including unreinforced, or 0% MSW). Results One-dimensional Consolidation Response Figure 4 shows consolidation responses of soil-MSW mixtures at different MSW concentrations. Results show a distinct threshold response with MSW content (by dry weight, wt%) in that the consolidation curves for mixtures up to 1% are bundled in the proximity of the curve for the unreinforced specimen, indicating that low concentrations do not influence the compressibility of the soil skeleton significantly. This observation indicates that at low concentrations below a threshold value, MSW fragments are too sparse to disrupt the continuity of the soil fabric and introduce a connected network of deformable inclusions that would influence the consolidation response of the soil-MSW mixtures significantly. Results also show that the consolidation lines for higher MSW concentrations shift downward, indicating that for a given effective vertical stress, specimens with higher MSW content consistently exhibit lower void ratios. Table 2 shows the oedometric consolidation parameters for the large-scale specimens examined in this study. Results show that the coefficient of consolidation (C v ) is a. In contrast, C v decreases consistently with MSW concentration. This reduction indicates that higher MSW concentrations lead to slower dissipation of excess pore water pressure and hence, a slower rate of consolidation in the corresponding specimens. From a mechanistic standpoint, this trend is consistent with a composite system where deformable, irregular inclusions increase the overall compressibility and potentially create more tortuous drainage paths, both of which reduce the characteristic rate of consolidation under the same drainage boundary conditions. Meanwhile, the preconsolidation stress ( \({\sigma}_{c}{\prime})\) calculated using the Casagrande method in Table 3 increases slightly but consistently with MSW concentration. However, C r increases with MSW concentration while C c decreases, both of which show more significant changes for the 2% concentration specimen, consistent with the results shown in Fig. 4 . The increase in C r indicates a softer elastic response during reloading (or reduced stiffness in the pre-yield regime), which can be attributed to the presence of inclusions that accommodate small strains through local deformation and rearrangement. The reduction in C c at the 2 wt% MSW concentration suggests that the normally consolidated response is modified from pure soil skeleton compression toward a mixed mechanism in which inclusion rearrangement and local void redistribution increasingly contribute to the measured volumetric strain. From these two trends (i.e. the reduction in C c and the increase in C r with MSW concentration), it can be concluded that the reinforcement effect of MSW becomes more apparent after sufficient deformation develops and MSW fragments begin to interlock with the soil matrix. This interlocking provides an anchorage framework that serves as a mechanical hinge, enabling the MSW pieces to mobilize tensile resistance. The resulting tensile contribution increases the overall stiffness of the composite and its resistance to additional compressive deformations. Table 2 Oedometric consolidation parameters for large-scale specimens of the tested materials Specimen \({\sigma}_{c}{\prime}\) (kPa) C r C c C v (cm 2 /h) Plain 25 0.005 0.06 49 0.5% MSW 26 0.006 0.06 44 1% MSW 27 0.006 0.06 41 2% MSW 29 0.01 0.03 37 The practical effect of these changes is captured by consolidation settlement estimates under the tested vertical stresses. Settlement increases markedly with MSW content across the stress range. The unreinforced soil shows the smallest settlements, while the 2% MSW mixture exhibits the largest settlements, on the order of roughly two to three times those of the base soil under comparable stress. This confirms that higher MSW contents can significantly compromise serviceability by increasing long-term compression, particularly in applications where deformation control is critical. Overall, the consolidation results indicate that MSW inclusion above approximately 1% substantially increases compressibility and settlement, while lower contents produce minimal change in the one-dimensional volumetric response. Therefore, applications involving substantial MSW contents should be designed to include a pre-service waiting preloading period, allowing the majority of consolidation settlement to occur before the MSW-reinforced earthwork is placed into service. Drained Shear Behavior and Mohr-Coulomb Strength Parameters Figure 5 shows shear-displacement responses of plain and MSW-reinforced specimens at different net normal stresses. Results for all cases presented show that undecomposed MSW inclusions improve the shear resistance of the soil tested and this improvement is consistently greater at higher concentrations and larger deformations. Results also show that higher-concentration mixtures exhibited greater shear resistance at the start of the experiments, which was also greater at lower normal stresses. A closer inspection of the results in Fig. 5 indicates that the shear response of all reinforced specimens can be considered as essentially bilinear, consisting of a short initial interval of high stiffness followed by strain hardening up to about 20% relative horizontal displacement. This behavior is attributed to the initial predominantly elastic response of the soil skeleton, after which the MSW inclusions progressively contribute to shear resistance. With increasing displacement, tensile forces in the MSW inclusions are gradually mobilized as they interlock with the surrounding soil fabric and begin to bridge the shear zone, leading to sustained hardening over a wide range of strains. Results in Fig. 5 also show that the shear stress-displacement response of the plain soil starts to plateau at approximately 25% relative displacement. In contrast, reinforced specimens do not reach a true critical-state condition within the displacement range investigated in the experiments. Instead, they continue to exhibit a strain hardening response up to a relative displacement of 27%, which is attributed to the progressive mobilization of tensile resistance in MSW inclusions that become interlocked with the soil skeleton and bridge across the developing shear planes. With increasing shear displacement, these inclusions elongate and carry load in tension, thereby sustaining additional shear resistance beyond the amount expected from soil friction alone. The elongation is accompanied by irreversible deformations of the MSW pieces, consistent with post-test observations of visibly stretched and deformed inclusions recovered from the shear plane. Figure 6 shows the Mohr-Coulomb failure envelopes for the tested specimens based on the data from Fig. 5 at 20% ‘shear’ strain. The shear strain in the present analysis is defined as the shear (i.e. relative) displacement of the upper box divided by the dimension of the specimen along the shearing force (i.e. 1.00 m). The 20% strain was used based on the data shown in Fig. 5 as a reasonable choice to represent the largest strain attained in all test cases for failure envelope calculations and consistent comparisons. Results in Fig. 6 provide a clear picture that mixing undecomoposed MSW with soil can indeed lead to significant increases in the shear strength of the mix. The increase in shear strength is consistently greater at higher concentrations and larger deformations. Results in Fig. 6 also show that MSW increases the shear strength of the mixtures in two different ways at different concentrations. At low MSW concentration of less than 1%, the internal friction angle of the mixture remains practically constant within 16°-17°, while its cohesion intercept increases with MSW concentration from 26 kPa to 34 kPa. This indicates that MSW reinforcement at lower concentrations contributes primarily to the “apparent cohesion” of the mixture. In this case, isolated MSW fragments intersect the developing shear band and mobilize tensile resistance and interface bonding that increases the cohesion intercept of the strength envelope without substantially changing the frictional slope. Strength gains occur with limited disruption to the soil skeleton, consistent with the earlier observation on the consolidation behavior of the specimens that low concentrations of the MSW do not significantly influence volumetric behavior. At 2% MSW, the shear response of mixture shifts to a friction-dominated regime. Cohesion slightly decreases compared with that of the 1% mixture, but the friction angle increases significantly (i.e. by ~ 70% from 17º to 29º). This transition suggests that, at higher MSW concentrations, the shear mechanism changes from primarily matrix-controlled shearing with added bridging to a composite interlocking mechanism where the MSW fragments contribute strongly to dilation and frictional resistance. At the same time, the higher MSW concentration tends to separate soil aggregates and loosen the soil matrix, which explains the observed increase in compressibility and settlement. Therefore, the 2% mixture exhibits a coupled response: reduced stiffness and increased volumetric deformation under one-dimensional loading, but increased shear resistance under drained shearing due to enhanced interlocking and reinforcement effects. Nevertheless, when the Mohr-Coulomb parameters are used to determine the shear strength of an MSW mixture at normal stresses representative of field applications, it is observed in Fig. 6 that the overall shear resistance increases with MSW concentration despite a slight reduction in cohesion at 2% MSW concentration. The significant increase in friction angle at 2% outweighs the reduced cohesion, producing the greatest magnitudes of shear strength among the mixtures investigated. This confirms that the principal contribution in shear strength at higher inclusion concentrations arises from frictional and geometric interlocking mechanisms rather than cohesion-type bonding. Strength-Settlement Tradeoff and Performance Window From a mechanistic perspective, aged MSW inclusions contribute to composite behavior through (i) mobilization of tensile resistance in elongated fragments (ii) enhanced interlocking and frictional interaction between waste fragments and the surrounding soil matrix and (iii) alteration of deformation localization during shearing. Compared with conventional polymer fibers, landfill-derived MSW may exhibit wider variability in aspect ratio, surface roughness, thickness, and stiffness. These differences can influence the efficiency of stress transfer at the soil-MSW interface and the degree to which inclusions bridge developing shear bands. For low reinforcement contents, the dominant effect is typically the creation of a distributed network of discrete inclusions that restrains relative displacement, delays strain softening, and can increase peak shear resistance under drained conditions, depending on normal stress and the mixture density. The contrasting influences of MSW concentration on the consolidation and shear responses of soil-MSW mixtures as observed in Figs. 4 through 6 suggests the following strength-settlement tradeoff in the practical use of undecomposed MSW in soil improvement applications: Low MSW concentrations (≤ 1%) provide measurable shear strength gains primarily through increased apparent cohesion while maintaining consolidation behavior that is comparable to that of the unreinforced soil. This range of concentration is therefore suited for applications where both the strength and serviceability of the reinforced mass are of concern, such as in shallow foundations, pavement subgrades, and engineered fills where deformation control is important. Higher MSW concentration (≥ 1%) yield greater improvements in shear strength, primarily due to greater interlocking interactions leading to a significant rise in friction angle of the mixture. However, they also lead to substantially higher consolidation settlements and a slower consolidation rate. This combination can be advantageous in deformation-tolerant systems where stability is the dominant design requirement and larger settlements can be accommodated or mitigated in slope stabilization, temporary embankments and retaining structures in construction, rapid deployment of troops or first responders in disaster-stricken areas, rural areas, or in applications where post-construction deformations of reinforced embankments can take place for a period of time before the project is put in service. Conclusions This study examined the mechanical performance of a fine-grained soil reinforced with low concentrations of aged, non-degradable MSW fragments (i.e. 0–2% by dry mass), using consolidation and large-scale drained shear testing under low normal stresses (≤ 40 kPa) representative of shallow earthwork structures. The results show a distinct threshold behavior of the mixtures as a function of their MSW concentration, and a clear strength-settlement tradeoff that can determine their practical applications. Results showed that the one-dimensional consolidation response of soil-MSW mixtures was not significantly different from that of the unreinforced specimen at low MSW concentrations of up to 1 wt%. However, consolidation responses of the mixtures still showed a slight but consistent trend of reduced void ratio at higher MSW concentrations even at those low values. This observation indicated that sparse MSW inclusions were not able to alter the soil skeleton’s volumetric compression mechanism significantly. At 2% MSW, the consolidation rate decreased while settlements increased significantly, demonstrating that higher concentrations of MSW inclusions make the composite more compressible and more time dependent. The concurrent reduction in the compression index and an increase in the recompression index with MSW concentration pointed to a strain-dependent reinforcement mechanism: MSW fragments contribute meaningfully only after sufficient magnitudes of deformation take place in the mixtures, when interlocking with the soil matrix provides anchorage and enables the mobilization of the inclusions tensile resistance. Drained direct shear results showed that MSW inclusions increase mobilized shear resistance at large deformations. The nature of increase in shear strength was found to depend on MSW concentration in that, at low concentrations of up to 1%, the friction angle of the mixtures remained nearly constant while cohesion increased, indicating an apparent cohesion/bridging contribution. At 2% MSW, the increase in shear strength was found to be friction controlled, involving a significant increase in the friction angle accompanied by a slight reduction in the cohesion intercept, indicative of stronger interlocking and frictional engagement between the MSW inclusions and the soil matrix. From a practical standpoint, MSW concentrations ≤ 1% seem to offer the most balanced improvement, providing strength gains with negligible adverse effects on soil settlements. Higher MSW concentrations can improve shear strength further but could only be beneficial if increased amounts of settlements could be accommodated in the project requirements. If the project timeline permits, a significant portion of the anticipated consolidation settlements may be allowed to take place first before earthwork construction is completed and the supported structures are put in service. In summary, the study results indicate that undecomposed MSW has the potential for use as an effective soil reinforcement agent, but its beneficial contributions in shear strength must be balanced against its potential adverse effects on compressibility. The data presented herein could be used as preliminary guidance for this purpose, while more accurate and reliable design recommendations await further studies on a broader range of MSW material types and concentrations, time scales and confining stresses, among other factors. Declarations Author Contribution Mohsen Ajdari conceived the central research idea and defined the study objectives; conceptualized the experimental program; designed and developed the large-scale direct shear device and the overall test methodology; supervised the technical direction of the work; interpreted the experimental trends and discussed the geotechnical implications; and drafted the original version of the manuscript. Mohammadreza Arvin curated the dataset; processed, reduced, and organized the raw measurements; performed the primary quantitative analyses and prepared analysis-ready tables/plots; contributed to interpretation of results; and led substantive manuscript revision and editing. Sina Zahmatkesh Astaneh performed the laboratory testing program, including specimen preparation and execution of the direct shear tests; ensured proper documentation and quality control during testing; and compiled and delivered the raw data and test records. Kianoosh Hatami contributed to data interpretation and verification, including review of analysis assumptions and consistency checks; strengthened the discussion of findings and their relevance to practice; and provided critical revisions to improve the manuscript’s technical clarity, structure, and presentation. References Dehdari, V., Ajdari, M. and Rostami, A. (2021) ‘Experimental study on shear strength parameters of a municipal solid waste employing a large direct shear apparatus’, Geomechanics and Geoengineering , Published online 27 May. Kaboudani, A., Ajdari, M., Maleki, S., Esfandiari, Z. and Shafiee, A. (2023) ‘Roles of ageing on the physical, chemical and mechanical parameters of a prototype municipal solid waste’, Geomechanics and Geoengineering , pp. 1–17. Akbari, I., Ajdari, M. and Shafiee, A. (2022) ‘Mechanical properties of landfill components under low to medium stress levels’, Bulletin of Engineering Geology and the Environment , 81(9), pp. 1–17. Esfandiari, Z., Ajdari, M. and Vahedifard, F. (2021) ‘Time-dependent deformation characteristics of an unsaturated sand–bentonite mixture under drying–wetting cycles’, Journal of Geotechnical and Geoenvironmental Engineering (ASCE) , 147(3), 04020172. Ajdari, M. and Bahmyari, H. (2015) ‘Oedometric response of an artificially prepared sand–bentonite mixture improved by potassium silicate’, Scientia Iranica , 22(2), pp. 367–372. Dhatrak, A. and Konmare, S.D. (2015) ‘Performance of randomly oriented plastic waste in flexible pavement’, International JPRET , 3(9), pp. 193–202. Khokar, I.A., Al-Saidi, K.S., Ali, S.A. and Hayder, G. (2025) ‘Utilizing recycled vegetable plastic bags as an innovative and sustainable material for soil reinforcement applications’, International Journal of Environmental Impacts , 8(5), pp. 1094–1101. Ghiassian, H., Poorebrahim, G. and Gray, D.H. (2004) ‘Soil reinforcement with recycled carpet wastes’, Waste Management & Research , 22(2), pp. 108–114. Ghazavi, M. and Sakhi, M.A. (2005) ‘Influence of optimized tire shreds on shear strength parameters of sand’, International Journal of Geomechanics , 5(1), pp. 58–65. Choudhary, A.K., Jha, J.N. and Gill, K.S. (2010) ‘A study on CBR behavior of waste plastic strip reinforced soil’, Emirates Journal for Engineering Research , 15(1), pp. 51–57. Marçal, R., Lodi, P.C., de Souza Correia, N., Giacheti, H.L., Rodrigues, R.A. and McCartney, J.S. (2020) ‘Reinforcing effect of polypropylene waste strips on compacted lateritic soils’, Sustainability , 12. Benson, C.H. and Khire, M.V. (1994) ‘Reinforced sand with strips of reclaimed high-density polyethylene’, Journal of Geotechnical Engineering (ASCE) , 120(5), pp. 838–855. Mishaal, F.Z. and Aldaood, A.H. (2023) ‘Soil reinforcement with synthetic fibers and plastic waste materials: a review’, Al-Rafidain Engineering Journal , 28(2), pp. 33–47. Nataraj, M.S. and McManis, K.L. (1997) ‘Strength and deformation properties of soil reinforced with fibrillated fibers’, Geosynthetics International , 4(1), pp. 65–79. Gregory, G.H. (2006) Shear strength, creep and stability of fiber-reinforced soil slopes . PhD thesis. Oklahoma State University, Stillwater, OK. Li, J., Tang, C., Wang, D., Pei, X. and Shi, B. (2014) ‘Effect of discrete fibre reinforcement on soil tensile strength’, Journal of Rock Mechanics and Geotechnical Engineering , 6, pp. 133–137. Hatami, K., Gregory, G. H., and Garland Jr, G. S. (2019). Guidelines for the Use of Fiber Reinforced Soil (FRS) in Highway Construction. Foose, G.J., Benson, C.H. and Bosscher, P.J. (1996) ‘Sand reinforced with shredded waste tires’, Journal of Geotechnical Engineering , 122(9), pp. 760–767. Sivakumar Babu, G.L., Chouksey, S., Anoosha, G. and Geetha Manjari, K. (2010) ‘Strength and compressibility response of plastic waste mixed soil’, in Proceedings of the Indian Geotechnical Conference 2010 (GEOtrendz) , 16–18 December, Mumbai, India. Mumbai: IGS Mumbai Chapter and IIT Bombay. Iravanian, A. and Haider, A.B. (2020) ‘Soil stabilization using waste plastic bottles fibers: a review paper’, IOP Conference Series: Earth and Environmental Science , 614, 012082. Consoli, N.C., Vendruscolo, M.A. and Prietto, P.D.M. (2010) ‘Behavior of plate load tests on soil layers improved with cement and fiber’, Journal of Geotechnical and Geoenvironmental Engineering (ASCE) , 129(1), pp. 96–101. Hernández, C., Beltrán, G. and Botero, E. (2024) ‘Use of recycled plastic fibers to control shrinkage and desiccation cracking in clayey soils’, Sustainability , 16, 3853. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted 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. 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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-9013637","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":612783849,"identity":"b7865088-9261-481c-8159-f8d67a58cf81","order_by":0,"name":"Mohsen Ajdari","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABDUlEQVRIiWNgGAWjYFACHgjF2JDAwJBQwcDAxg6TOUCUljNALczEagGqB+prA9KEtOi29x7+8HEHgzxze/KzDw/nbZPnY2Zgk+ZtY5Dju5GAVYvZmXNpkjPPMBg29jwznpG47bZhG1SLsSQuLTdyzJiBChgbZyQYMwC1MIK03JzZxpC4AbcW489/2xjsG2ekf2ZInHPbHqalHo8WA2mgrxMbZ+QAbWm4nQjScuNjG0OCAU6/nDGT7G2TSG7seVPMkHDsdnIbM2P7jw/nJAxnnnmAXcvxHuMPP9tsbDe2p29m/FFz23Z+e/Nhg4QyG3m+49htgQIJBsMGOIexASxCEMgTVjIKRsEoGAUjFQAAnBdhPLKKWBwAAAAASUVORK5CYII=","orcid":"","institution":"Awest Virginia University Institute of Technology","correspondingAuthor":true,"prefix":"","firstName":"Mohsen","middleName":"","lastName":"Ajdari","suffix":""},{"id":612783850,"identity":"107d3357-96b9-4f37-9480-838023275677","order_by":1,"name":"Mohammadreza Arvin","email":"","orcid":"","institution":"Fasa University","correspondingAuthor":false,"prefix":"","firstName":"Mohammadreza","middleName":"","lastName":"Arvin","suffix":""},{"id":612783851,"identity":"87c0ed05-626c-496e-b821-70ec994bd26f","order_by":2,"name":"Sina Zahmatkesh Astaneh","email":"","orcid":"","institution":"Fasa University","correspondingAuthor":false,"prefix":"","firstName":"Sina","middleName":"Zahmatkesh","lastName":"Astaneh","suffix":""},{"id":612783852,"identity":"16254e1e-be17-400c-923d-69feedc1e716","order_by":3,"name":"Kianoosh Hatami","email":"","orcid":"","institution":"University of Oklahoma","correspondingAuthor":false,"prefix":"","firstName":"Kianoosh","middleName":"","lastName":"Hatami","suffix":""}],"badges":[],"createdAt":"2026-03-02 20:39:19","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9013637/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9013637/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":105557605,"identity":"df6e7574-2c92-4a1b-b130-129332ea9255","added_by":"auto","created_at":"2026-03-27 11:21:43","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1131551,"visible":true,"origin":"","legend":"\u003cp\u003eLarge-scale direct shear apparatus used in the study: (a) overall view (b) vertical loading system (c) horizontal loading system; and (d) guide rail and slider assembly that separates the upper and lower shear boxes and facilitates relative motion\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9013637/v1/5b46e5c36d2b5f84ed627d13.png"},{"id":105557606,"identity":"797deebd-a013-433d-b102-3d63d22df26e","added_by":"auto","created_at":"2026-03-27 11:21:43","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":431680,"visible":true,"origin":"","legend":"\u003cp\u003eMSW specimen used as reinforcement material for the studied soil after five years of burial\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9013637/v1/7172ee900027da1940b4e692.png"},{"id":105567419,"identity":"5b9360e8-c575-40a6-9281-580228877419","added_by":"auto","created_at":"2026-03-27 12:59:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":50403,"visible":true,"origin":"","legend":"\u003cp\u003eMass composition of the studied undecomposed buried MSW (percent)\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9013637/v1/8b1424ec00f3b3aecb8fa98a.png"},{"id":105557607,"identity":"4668442b-f215-4173-bd59-71969c085fbd","added_by":"auto","created_at":"2026-03-27 11:21:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":42355,"visible":true,"origin":"","legend":"\u003cp\u003eConsolidation responses of plain and MSW-reinforced soils at different MSW contents\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9013637/v1/1fad3ee48c1c34f197137fec.png"},{"id":105557602,"identity":"1a927a06-15c3-4e9a-9d20-a25e7eb48c56","added_by":"auto","created_at":"2026-03-27 11:21:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":131752,"visible":true,"origin":"","legend":"\u003cp\u003eShear response of plain and MSW-reinforced large-scale specimens at different MSW contents, σ_(v,net)= (a) 20 kPa (b) 30 kPa (c) 40 kPa\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9013637/v1/bbe416df35afb21267e1d65c.png"},{"id":105566632,"identity":"8877eca3-925e-4578-a2ce-768d241e9848","added_by":"auto","created_at":"2026-03-27 12:56:51","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":22839,"visible":true,"origin":"","legend":"\u003cp\u003eMohr-Coulomb failure envelopes for large-scale plain and MSW-reinforced specimens at different MSW concentrations\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-9013637/v1/6dff88fc13858099433f5376.png"},{"id":105570359,"identity":"9cb58144-092c-4ce1-b765-ca3a2f558a47","added_by":"auto","created_at":"2026-03-27 13:16:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2759092,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9013637/v1/0228f58e-947a-41af-8fa1-30afe57419cd.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Mechanical Response of Fine-Grained Soil-Solid Waste Composites from Field Scale Direct Shear Tests","fulltext":[{"header":"Introduction","content":"\u003cp\u003eUrbanization and the resulting rise in the use of domestic and industrial materials and commercial products continuously lead to increased municipal solid waste (MSW) production worldwide [1]. A major portion of MSW is non-biodegradable plastics and polymeric products that can persist in landfills for decades, consuming land capacity and creating long-term environmental hazards [2]. Meanwhile, many infrastructure systems are built on weak and compressible fine-grained soils, which can lead to excessive settlements [3], cracking, and even instability in embankments, pavement subgrades, and retaining structures [4]. Even though conventional ground-improvement approaches, such as replacement with granular material, deep foundations, or chemical stabilization with cement or lime, are generally effective, they can be cost-prohibitive and could result in environmental concerns [5]. The above challenges necessitate sustainable ground-improvement strategies that can improve weak soils with the environmental benefits of reusing non-degradable solid waste materials.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThere have been several studies on the potential use of MSW as soil reinforcement in the past. However, a review of literature reveals the following key gaps: First, most of the studies to date have used plastic wastes that were processed into desired shapes and sizes, e.g. lab-prepared strips, fibers, or mats [6, 7] and then mixed uniformly or placed in layers in the specimens [8, 9]. This approach is not conducive to a direct and economical use of landfill-derived waste, which is typically irregular, multi-material, and partially degraded. Therefore, the mechanical response of fine-grained soils mixed with raw (i.e. unprocessed) landfill waste is still not well understood. Second, many studies are based on conventional small-scale laboratory tests (e.g., Proctor compaction, unconfined compression, small direct shear, or CBR), where specimen sizes are too small to be able to capture the interactions between large, stiff waste pieces and the surrounding soil matrix [10, 11]. Scale effects associated with waste particle size and shear-zone thickness can significantly influence measured strength and deformations. Third, the studies have largely been focused on sands, lateritic soils, or coarse subgrade materials, while much fewer studies have addressed cohesive or fine-grained soils reinforced with waste plastics [12, 13]. A review by Mishaal and Aldaood (2023) highlights the need for more work on cohesive soils, large-scale testing to clarify scale effects, and better understanding of soil-waste mixtures long-term performance in field conditions [13]. Finally, relatively few studies have examined older waste materials that have been buried in landfill conditions for several years, where aging, leaching, and partial degradation may modify their stiffness, surface characteristics, and soil-waste interaction mechanisms.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe present study addresses the above gaps through an experimental investigation of a fine-grained soil reinforced with non-degradable MSW that collected from a landfill in the Province of Fars, Iran. Five-year-old undecomposed waste shreds were randomly mixed with a local fine-grained soil and compacted in a one meter-deep shear box with rigid walls, which allowed the behavior of the soil-waste composite to be examined at a scale comparable to field conditions. Large-scale direct shear tests were then performed on specimens with different MSW weight percentages under different normal stress levels to quantify how waste concentration could influence the shear strength and deformation characteristics of the composites. The specific objectives of the study were: (i) quantify changes in cohesion and internal friction angle of the fine-grained soil reinforced with aged, undecomposed landfill waste fibers in large-scale direct shear tests; (ii) assess the influence of waste fiber concentration on volume change during shearing; and (iii) identify a range of MSW concentrations that would result in increased shear strength within acceptable settlements.\u003c/p\u003e"},{"header":"Background","content":"\u003cp\u003eOver the past decades, the concept of fiber-reinforced soil has emerged as a practical and economical technique to improve the mechanical behavior of geomaterials [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Randomly distributed fibers, natural or synthetic, are mixed with soil to create a composite that exhibits enhanced strength and post-peak performance relative to the unreinforced matrix. A review of related studies indicates that the inclusion of synthetic fibers and plastic waste materials can significantly improve shear strength, stiffness, and bearing capacity and reduce brittleness and the likelihood of tensile cracks in soils. Mishaal and Aldaood (2023) summarized several studies and concluded that even waste fibers can provide substantial gains in strength and ductility, especially when they are well dispersed within the soil mass [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAccordingly, the use of plastic waste as soil reinforcement has become a desirable solution for soil improvement with a simultaneous benefit of waste reduction [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. Iravanian and Haider (2020) reviewed the use of fibers produced from discarded plastic bottles and highlighted both the environmental urgency of plastic waste disposal and the engineering benefits of incorporating such materials into weak subgrade soils [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Their synthesis showed that adding up to 5% by weight of plastic bottle fibers can increase the shear strength and bearing capacity of the subgrade and potentially reduce pavement thickness in highway applications. Similar trends have been reported for plastic bag strips, carpets, tire shreds, and other polymeric inclusions, where improvements in strength parameters and deformation behavior are typically observed for an optimal range of fiber content and aspect ratio [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eRecent experimental studies further demonstrate the versatility of plastic waste as reinforcement for different soil types and loading conditions. Mar\u0026ccedil;al et al. (2020) evaluated the use of polypropylene waste strips from recycled packaging as reinforcement for compacted lateritic soils and observed increased unconfined compressive strength, higher apparent cohesion and friction angle in sandy clay, and up to 70% improvement in California bearing ratio for reinforced specimens [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Khokar et al. (2025) investigated the use of high-density polyethylene plastic bags in mat form as reinforcement for a fine soil from Oman, reporting more than threefold increases in compressive strength at an optimal number of reinforcement mats, along with changes in compaction characteristics that can be advantageous for field construction [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Other studies have focused on the behavior of fiber-reinforced, fine-grained soils under desiccation: Hern\u0026aacute;ndez et al. (2024) showed that recycled polyethylene terephthalate and polypropylene fibers can effectively control shrinkage and desiccation cracking in low-plasticity clays [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. They also used image analysis and suction measurements to identify the fiber content that minimized crack intensity without significantly altering the soil-water retention curve.\u003c/p\u003e\n\u003ch3\u003eLarge-Scale Direct Shear Apparatus\u003c/h3\u003e\n\u003cp\u003eThe size of the test box was selected such that it would minimize scale effects for MSW pieces of up to 150 mm, and allow for the development of a representative shear zone during the tests. The test apparatus consists of a split steel shear box comprising a fixed lower half and a horizontally translating upper half. The box is 1 m \u0026times; 1 m in plan area with a total height of nearly 1.1 m, all fabricated using 6 mm-thick steel plates (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe 0.7 m-tall upper part of box was stiffened against bulging under high normal stresses using welded gusset plates and steel braces to maintain consistent boundary conditions throughout the loading and shearing phases of the tests. Additional local stiffeners were welded to the box against the horizontal actuator to help distribute the applied load uniformly on the box and prevent local yielding or distortion in the box during the tests.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe 0.4 m-tall lower part of the box was rigidly welded to the main chassis, with a 10 mm- steel base to help maintain a consistent specimen geometry. The box is equipped with four 10 mm-diameter drainage lines at its base plate. The drain lines can be used to control the moisture content and degree of saturation of the tested soil, and to carry out the tests in consolidated drained conditions and minimize any likelihood of excess pore-water pressure during the shearing phase of the tests. The box is protected using anti-corrosion coating to improve its durability and the repeatability of experiments across multiple tests.\u003c/p\u003e \u003cp\u003eThe shear box assembly is supported by a rigid load frame fabricated from welded channel sections. The external chassis plan dimensions are 2.50 m \u0026times; 1.27 m, and base plates are installed beneath columns to distribute test reactions to the laboratory floor. Horizontal translation of the upper half is controlled by a guidance system integrated into the chassis. Steel rods and mating shafts engage with guide rails to constrain motion to a single horizontal degree of freedom, reduce parasitic friction, and maintain alignment under combined normal and shear loading. A nominal 10-mm gap was selected to ensure the upper and lower boxes remain mechanically decoupled during translation, thereby permitting relative displacement and the formation of a well-defined target shear plane (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e-d). The \u003cb\u003egap size\u003c/b\u003e can influence measured response because an undersized gap may promote unintended contact and parasitic friction/alignment constraints, whereas an oversized gap can increase particle extrusion and alter the effective shear area and stress distribution (and thus the magnitude of any required area correction), potentially biasing interpreted shear strength and volumetric trends. To prevent specimen loss along the split interface during large displacements, a removable steel sheet is installed along the rear edge of the interface which remains in place during loading and shearing and can be removed after testing to facilitate specimen recovery and cleaning.\u003c/p\u003e \u003cp\u003eNormal stress is applied through a vertical manual hydraulic jack positioned above the specimen. The total stroke of the 100 kN jack is 270 mm (including screw-driven adjustment and hydraulic extension), which is sufficient for specimen seating, consolidation compression, and subsequent shear-induced volumetric change. Normal load is transferred through a stiff steel loading platen that covers nearly the entire inside plan area of the test box (approximately 0.98 m \u0026times; 0.98 m). The platen is reinforced with steel ribs to minimize bending and promote uniform stress distribution over the specimen surface. Because the platen and box components are heavy at this scale, a pulley and cable assembly is used to lift, position, and remove the platen in a controlled manner, reducing eccentricities during its placement during the tests.\u003c/p\u003e \u003cp\u003eDuring shearing, volumetric contraction may occur and can reduce the applied normal force if the loading system is not adjusted. In this configuration, the vertical jack can be advanced during the test to compensate for settlement and to maintain a practically constant normal stress. The same vertical loading configuration can also be used to conduct large-scale one-dimensional consolidation within the box prior to shearing, enabling testing of specimens with a defined stress history and density state.\u003c/p\u003e \u003cp\u003eShear displacement is imposed on the upper box by a horizontal hydraulic jack connected to the chassis. The actuator provides a shear force capacity of approximately 70 kN and a maximum horizontal travel of about 300 mm, allowing observation of peak strength, post-peak softening, and residual behavior at large displacements. The jack is supplied by a hydraulic power unit equipped with a 30 L oil reservoir and a 1 hp electric motor. Shear direction, forward and backward, is controlled through a hydraulic valve system. An inverter is used to regulate motor speed and, consequently, the hydraulic flow rate, enabling stable strain-controlled loading and prescribed constant displacement rates. The system allows a broad range of shear rates, including lower rates on the order of 7 mm/min to ensure drained response based on the material properties of the tested specimen.\u003c/p\u003e \u003cp\u003eNormal and shear forces are measured using two S-shape, 100 kN load cells in both the vertical and horizontal load paths. The sensors are connected to digital readout units that provide real-time measurements and enable calibration based on sensor sensitivity. Mechanical connections between the jacks, load cells, and the shear box assembly are made using threaded connectors and bolts to maintain an axial load path and minimize eccentricity. Horizontal and vertical displacements are monitored directly on the apparatus using mounted LVDTs referenced to the fixed chassis and the moving upper box. The horizontal displacement reading provides the imposed shear displacement, while the vertical reading captures consolidation compression and shear-induced contraction/dilation of the specimen\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterials\u003c/h2\u003e \u003cdiv id=\"Sec4\" class=\"Section3\"\u003e \u003ch2\u003eFine-grained soil\u003c/h2\u003e \u003cp\u003eThe base geomaterial in this study was a fine-grained soil that had been collected from the sediments accumulated on a plain located in south of Iran and was transported to the laboratory in the form of a disturbed bulk sample. The soil was thoroughly mixed with MSW and compacted in the laboratory to an optimum unit weight from the Standard Proctor test in the test box. The large size of the specimen was beneficial in accommodating the inherent heterogeneity and anisotropy of the soil-MSW mixture in the experiments.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e\n\u003ch3\u003eUndecomposed buried MSW\u003c/h3\u003e\n\u003cp\u003eThe reinforcing phase of the mixtures was produced from the MSW that was recovered from a landfill in the province of Fars south of Iran. Five year old non-biodegradable fraction of buried waste layers were sampled as representatives of the material that had remained after the decomposition of readily degradable components (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Visual inspection during sampling and subsequent handling indicated that the recovered MSW was essentially free of organic matter. Therefore, the reinforcement was treated as an aged MSW residue dominated by persistent constituents (e.g., plastics and other durable fragments, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This is important from a materials standpoint because the mechanical contribution of MSW to a soil composite highly depends on its degradation state. Eventually, when organics are absent and the remaining fraction is primarily non-degradable, the reinforcement behavior is more stable over the duration of laboratory testing and less influenced by time-dependent biodegradation or gas generation.\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\u003eSelected properties of the tested soil\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eProperty/Characteristic\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTest\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eValue\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFines Content (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSieve Analysis\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e78\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSand Content (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSpecific Gravity\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePycnometer\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOptimum Water Content (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eStandard Proctor\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e15.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMaximum Dry Unit Weight (kN/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e17.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLiquid Limit (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAtterberg Limits\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlasticity Index\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUnified Soil Classification System (USCS)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eML\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLiquidity Index\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eSieve analysis results indicated that the soil was a low-plasticity silt with sand (ML) according to the USCS with selected properties as listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe sampling procedure targeted distinct waste layers to obtain a separable MSW fraction, which was physically isolated from adjacent soil layers. Although the waste was separated as carefully as possible, MSW is inherently heterogeneous and anisotropic at multiple scales. The relative proportions of plastics, textiles, paper-like residues, thin metallic pieces and glass fragments can vary spatially and directionally within the same landfill cell. As a result, the recovered MSW cannot be characterized by a distinct composition that is uniform throughout the cell. Instead, it should be considered as a representative batch of aged waste fragments with a degree of variability typical of landfill-derived materials. This intrinsic variability was addressed in the experimental design by using relatively small reinforcement concentrations and by adopting controlled preparation procedures to maximize repeatability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eMix Design, Specimen Conditioning and Test Procedure\u003c/h3\u003e\n\u003cp\u003eBecause the landfill is constructed with alternating layers of soil and waste, the retrieved MSW was separated from surrounding soil, cleaned of adhering fines by manual sorting, and prepared for mixing. Prior to blending, chunks of fine-grained soil were pneumatically crushed as necessary and passed through sieves to remove stones and cohesive clods and obtain repeatable mixing and reduce local heterogeneity caused by oversized fragments. Reinforced mixtures were produced by randomly distributing the prepared MSW within the soil at four different waste concentrations by mass of 0% (unreinforced control), 0.5%, 1.0%, and 2.0% MSW relative to the total mass of the mixture. These lower concentration levels were selected so that the presence of MSW in the mixtures would be in the form of discrete inclusions that modify the composite shear response through frictional interlocking, tensile bridging, and constrained deformations while avoiding an inadvertent transformation of the material into a waste-dominant matrix.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor large-scale direct shear specimens, the conditioning objective was to create a uniform, saturated soil-MSW composite consistent with drained shearing while minimizing specimen-to-specimen variability introduced by moisture gradients. The MSW inclusions were introduced such that they remained randomly oriented and dispersed within each lift rather than being segregated within discrete layers. This approach was intended to emulate field mixing scenarios in which reinforcement elements are distributed throughout the soil mass and to avoid the formation of preferential planes. During the specimen setup, care was taken to maintain uniformity along the plan area of the box by leveling each lift prior to compaction and by avoiding localized clumping of MSW inclusions. Water was added incrementally to the prepared soil (and to the soil-MSW blend after inclusion) while mixing thoroughly to achieve the target state that was visually uniform and void of any dry clumps. After thorough mixing, the material for each specimen (i.e. soil, or soil-MSW mix) was staged next to the test box and covered with plastic sheets for a minimum of 16 hours for moisture equilibration before it was placed in the test box in lifts to prepare the specimens for load testing.\u003c/p\u003e \u003cp\u003eThe test material was compacted in 250 mm-lifts statically using the vertical loading system of the apparatus until the entire 1-m-high specimen was properly placed in the test box. After compacting each lift, the soil was sampled for its void ratio and moisture content to ensure that the mixture remained consistent in its target saturated condition. If the calculated moisture content did not meet the target degree of saturation, the material was removed and replaced with added moisture as necessary and compacted before placing the next lift. This procedure helped to minimize unintended variations in the degree of saturation and variability in the stiffness and strength of the specimens. Immediately after completing the final lift, the specimen surface was prepared for seating of the loading plate, and the assembly was set for consolidation under the target normal stress.\u003c/p\u003e \u003cp\u003eThe normal load, corresponding to target interim vertical stresses (e.g., 1, 2, 4, 8, 16, 20, 30, 40 kPa) was applied gradually using a manual hydraulic jack while continuously monitoring the load cell readout. Each applied normal stress was maintained constant for a 24-hour consolidation period. During this phase, the jack was adjusted as needed to compensate for settlement so that the measured normal load remained at the target value. Consolidation drainage was provided through the drainage outlets at the base of the lower box, and vertical displacement during consolidation was recorded to quantify settlement and stabilization. The required drainage time during the subsequent shear stage was estimated from the time to 50% consolidation (t₅₀) following the procedure described in ASTM D3080, supporting selection of a displacement rate consistent with drained response.\u003c/p\u003e \u003cp\u003eAfter specimens with different MSW inclusions were consolidated under vertical pressures of 20, 30, and 40 kPa, they were sheared at a constant displacement rate of 7 mm/min. Shear stress was computed from the measured horizontal force divided by the corrected plan area of the shear plane, and stress-displacement curves were used to compare the performances of different mixtures. In total, 12 large-scale shear tests were carried out involving three (3) different normal stresses and four (4) MSW concentrations (including unreinforced, or 0% MSW).\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003eOne-dimensional Consolidation Response\u003c/h2\u003e\n \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows consolidation responses of soil-MSW mixtures at different MSW concentrations. Results show a distinct threshold response with MSW content (by dry weight, wt%) in that the consolidation curves for mixtures up to 1% are bundled in the proximity of the curve for the unreinforced specimen, indicating that low concentrations do not influence the compressibility of the soil skeleton significantly. This observation indicates that at low concentrations below a threshold value, MSW fragments are too sparse to disrupt the continuity of the soil fabric and introduce a connected network of deformable inclusions that would influence the consolidation response of the soil-MSW mixtures significantly.\u003c/p\u003e\n \u003cp\u003eResults also show that the consolidation lines for higher MSW concentrations shift downward, indicating that for a given effective vertical stress, specimens with higher MSW content consistently exhibit lower void ratios.\u003c/p\u003e\n \u003cp\u003eTable \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the oedometric consolidation parameters for the large-scale specimens examined in this study. Results show that the coefficient of consolidation (C\u003csub\u003ev\u003c/sub\u003e) is a. In contrast, C\u003csub\u003ev\u003c/sub\u003e decreases consistently with MSW concentration. This reduction indicates that higher MSW concentrations lead to slower dissipation of excess pore water pressure and hence, a slower rate of consolidation in the corresponding specimens. From a mechanistic standpoint, this trend is consistent with a composite system where deformable, irregular inclusions increase the overall compressibility and potentially create more tortuous drainage paths, both of which reduce the characteristic rate of consolidation under the same drainage boundary conditions.\u003c/p\u003e\n \u003cp\u003eMeanwhile, the preconsolidation stress (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma}_{c}{\\prime})\\)\u003c/span\u003e\u003c/span\u003e calculated using the Casagrande method in Table 3 increases slightly but consistently with MSW concentration. However, C\u003csub\u003er\u003c/sub\u003e increases with MSW concentration while C\u003csub\u003ec\u003c/sub\u003e decreases, both of which show more significant changes for the 2% concentration specimen, consistent with the results shown in Fig. \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The increase in C\u003csub\u003er\u003c/sub\u003e indicates a softer elastic response during reloading (or reduced stiffness in the pre-yield regime), which can be attributed to the presence of inclusions that accommodate small strains through local deformation and rearrangement. The reduction in C\u003csub\u003ec\u003c/sub\u003e at the 2 wt% MSW concentration suggests that the normally consolidated response is modified from pure soil skeleton compression toward a mixed mechanism in which inclusion rearrangement and local void redistribution increasingly contribute to the measured volumetric strain. From these two trends (i.e. the reduction in C\u003csub\u003ec\u003c/sub\u003e and the increase in C\u003csub\u003er\u003c/sub\u003e with MSW concentration), it can be concluded that the reinforcement effect of MSW becomes more apparent after sufficient deformation develops and MSW fragments begin to interlock with the soil matrix. This interlocking provides an anchorage framework that serves as a mechanical hinge, enabling the MSW pieces to mobilize tensile resistance. The resulting tensile contribution increases the overall stiffness of the composite and its resistance to additional compressive deformations.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eOedometric consolidation parameters for large-scale specimens of the tested materials\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSpecimen\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\sigma}_{c}{\\prime}\\)\u003c/span\u003e\u003c/span\u003e (kPa)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003eC\u003csub\u003er\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003eC\u003csub\u003ec\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eC\u003csub\u003ev\u003c/sub\u003e (cm\u003csup\u003e2\u003c/sup\u003e/h)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003ePlain\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e0.005\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e49\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e0.5% MSW\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e26\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e0.006\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e44\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e1% MSW\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e0.006\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e0.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e41\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e2% MSW\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e0.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e0.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\n \u003cp\u003e37\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003eThe practical effect of these changes is captured by consolidation settlement estimates under the tested vertical stresses. Settlement increases markedly with MSW content across the stress range. The unreinforced soil shows the smallest settlements, while the 2% MSW mixture exhibits the largest settlements, on the order of roughly two to three times those of the base soil under comparable stress. This confirms that higher MSW contents can significantly compromise serviceability by increasing long-term compression, particularly in applications where deformation control is critical. Overall, the consolidation results indicate that MSW inclusion above approximately 1% substantially increases compressibility and settlement, while lower contents produce minimal change in the one-dimensional volumetric response. Therefore, applications involving substantial MSW contents should be designed to include a pre-service waiting preloading period, allowing the majority of consolidation settlement to occur before the MSW-reinforced earthwork is placed into service.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eDrained Shear Behavior and Mohr-Coulomb Strength Parameters\u003c/h3\u003e\n\u003cp\u003eFigure 5 shows shear-displacement responses of plain and MSW-reinforced specimens at different net normal stresses. Results for all cases presented show that undecomposed MSW inclusions improve the shear resistance of the soil tested and this improvement is consistently greater at higher concentrations and larger deformations. Results also show that higher-concentration mixtures exhibited greater shear resistance at the start of the experiments, which was also greater at lower normal stresses.\u003c/p\u003e\n\u003cp\u003eA closer inspection of the results in Fig.\u0026nbsp;5 indicates that the shear response of all reinforced specimens can be considered as essentially bilinear, consisting of a short initial interval of high stiffness followed by strain hardening up to about 20% relative horizontal displacement. This behavior is attributed to the initial predominantly elastic response of the soil skeleton, after which the MSW inclusions progressively contribute to shear resistance. With increasing displacement, tensile forces in the MSW inclusions are gradually mobilized as they interlock with the surrounding soil fabric and begin to bridge the shear zone, leading to sustained hardening over a wide range of strains.\u003c/p\u003e\n\u003cp\u003eResults in Fig.\u0026nbsp;5 also show that the shear stress-displacement response of the plain soil starts to plateau at approximately 25% relative displacement. In contrast, reinforced specimens do not reach a true critical-state condition within the displacement range investigated in the experiments. Instead, they continue to exhibit a strain hardening response up to a relative displacement of 27%, which is attributed to the progressive mobilization of tensile resistance in MSW inclusions that become interlocked with the soil skeleton and bridge across the developing shear planes. With increasing shear displacement, these inclusions elongate and carry load in tension, thereby sustaining additional shear resistance beyond the amount expected from soil friction alone. The elongation is accompanied by irreversible deformations of the MSW pieces, consistent with post-test observations of visibly stretched and deformed inclusions recovered from the shear plane.\u003c/p\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the Mohr-Coulomb failure envelopes for the tested specimens based on the data from Fig. 5 at 20% \u0026lsquo;shear\u0026rsquo; strain. The shear strain in the present analysis is defined as the shear (i.e. relative) displacement of the upper box divided by the dimension of the specimen along the shearing force (i.e. 1.00 m). The 20% strain was used based on the data shown in Fig. 5 as a reasonable choice to represent the largest strain attained in all test cases for failure envelope calculations and consistent comparisons. Results in Fig. \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e provide a clear picture that mixing undecomoposed MSW with soil can indeed lead to significant increases in the shear strength of the mix. The increase in shear strength is consistently greater at higher concentrations and larger deformations.\u003c/p\u003e\n\u003cp\u003eResults in Fig. \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e also show that MSW increases the shear strength of the mixtures in two different ways at different concentrations. At low MSW concentration of less than 1%, the internal friction angle of the mixture remains practically constant within 16\u0026deg;-17\u0026deg;, while its cohesion intercept increases with MSW concentration from 26 kPa to 34 kPa. This indicates that MSW reinforcement at lower concentrations contributes primarily to the \u0026ldquo;apparent cohesion\u0026rdquo; of the mixture. In this case, isolated MSW fragments intersect the developing shear band and mobilize tensile resistance and interface bonding that increases the cohesion intercept of the strength envelope without substantially changing the frictional slope. Strength gains occur with limited disruption to the soil skeleton, consistent with the earlier observation on the consolidation behavior of the specimens that low concentrations of the MSW do not significantly influence volumetric behavior.\u003c/p\u003e\n\u003cp\u003eAt 2% MSW, the shear response of mixture shifts to a friction-dominated regime. Cohesion slightly decreases compared with that of the 1% mixture, but the friction angle increases significantly (i.e. by ~\u0026thinsp;70% from 17\u0026ordm; to 29\u0026ordm;). This transition suggests that, at higher MSW concentrations, the shear mechanism changes from primarily matrix-controlled shearing with added bridging to a composite interlocking mechanism where the MSW fragments contribute strongly to dilation and frictional resistance. At the same time, the higher MSW concentration tends to separate soil aggregates and loosen the soil matrix, which explains the observed increase in compressibility and settlement. Therefore, the 2% mixture exhibits a coupled response: reduced stiffness and increased volumetric deformation under one-dimensional loading, but increased shear resistance under drained shearing due to enhanced interlocking and reinforcement effects.\u003c/p\u003e\n\u003cp\u003eNevertheless, when the Mohr-Coulomb parameters are used to determine the shear strength of an MSW mixture at normal stresses representative of field applications, it is observed in Fig. \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e that the overall shear resistance increases with MSW concentration despite a slight reduction in cohesion at 2% MSW concentration. The significant increase in friction angle at 2% outweighs the reduced cohesion, producing the greatest magnitudes of shear strength among the mixtures investigated. This confirms that the principal contribution in shear strength at higher inclusion concentrations arises from frictional and geometric interlocking mechanisms rather than cohesion-type bonding.\u003c/p\u003e\n\u003ch3\u003eStrength-Settlement Tradeoff and Performance Window\u003c/h3\u003e\n\u003cp\u003eFrom a mechanistic perspective, aged MSW inclusions contribute to composite behavior through (i) mobilization of tensile resistance in elongated fragments (ii) enhanced interlocking and frictional interaction between waste fragments and the surrounding soil matrix and (iii) alteration of deformation localization during shearing. Compared with conventional polymer fibers, landfill-derived MSW may exhibit wider variability in aspect ratio, surface roughness, thickness, and stiffness. These differences can influence the efficiency of stress transfer at the soil-MSW interface and the degree to which inclusions bridge developing shear bands. For low reinforcement contents, the dominant effect is typically the creation of a distributed network of discrete inclusions that restrains relative displacement, delays strain softening, and can increase peak shear resistance under drained conditions, depending on normal stress and the mixture density.\u003c/p\u003e\n\u003cp\u003eThe contrasting influences of MSW concentration on the consolidation and shear responses of soil-MSW mixtures as observed in Figs. \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e through \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e6\u003c/span\u003e suggests the following strength-settlement tradeoff in the practical use of undecomposed MSW in soil improvement applications:\u003c/p\u003e\n\u003cp\u003eLow MSW concentrations (\u0026le;\u0026thinsp;1%) provide measurable shear strength gains primarily through increased apparent cohesion while maintaining consolidation behavior that is comparable to that of the unreinforced soil. This range of concentration is therefore suited for applications where both the strength and serviceability of the reinforced mass are of concern, such as in shallow foundations, pavement subgrades, and engineered fills where deformation control is important.\u003c/p\u003e\n\u003cp\u003eHigher MSW concentration (\u0026ge;\u0026thinsp;1%) yield greater improvements in shear strength, primarily due to greater interlocking interactions leading to a significant rise in friction angle of the mixture. However, they also lead to substantially higher consolidation settlements and a slower consolidation rate. This combination can be advantageous in deformation-tolerant systems where stability is the dominant design requirement and larger settlements can be accommodated or mitigated in slope stabilization, temporary embankments and retaining structures in construction, rapid deployment of troops or first responders in disaster-stricken areas, rural areas, or in applications where post-construction deformations of reinforced embankments can take place for a period of time before the project is put in service.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThis study examined the mechanical performance of a fine-grained soil reinforced with low concentrations of aged, non-degradable MSW fragments (i.e. 0\u0026ndash;2% by dry mass), using consolidation and large-scale drained shear testing under low normal stresses (\u0026le;\u0026thinsp;40 kPa) representative of shallow earthwork structures. The results show a distinct threshold behavior of the mixtures as a function of their MSW concentration, and a clear strength-settlement tradeoff that can determine their practical applications.\u003c/p\u003e \u003cp\u003eResults showed that the one-dimensional consolidation response of soil-MSW mixtures was not significantly different from that of the unreinforced specimen at low MSW concentrations of up to 1 wt%. However, consolidation responses of the mixtures still showed a slight but consistent trend of reduced void ratio at higher MSW concentrations even at those low values. This observation indicated that sparse MSW inclusions were not able to alter the soil skeleton\u0026rsquo;s volumetric compression mechanism significantly.\u003c/p\u003e \u003cp\u003eAt 2% MSW, the consolidation rate decreased while settlements increased significantly, demonstrating that higher concentrations of MSW inclusions make the composite more compressible and more time dependent. The concurrent reduction in the compression index and an increase in the recompression index with MSW concentration pointed to a strain-dependent reinforcement mechanism: MSW fragments contribute meaningfully only after sufficient magnitudes of deformation take place in the mixtures, when interlocking with the soil matrix provides anchorage and enables the mobilization of the inclusions tensile resistance.\u003c/p\u003e \u003cp\u003eDrained direct shear results showed that MSW inclusions increase mobilized shear resistance at large deformations. The nature of increase in shear strength was found to depend on MSW concentration in that, at low concentrations of up to 1%, the friction angle of the mixtures remained nearly constant while cohesion increased, indicating an apparent cohesion/bridging contribution. At 2% MSW, the increase in shear strength was found to be friction controlled, involving a significant increase in the friction angle accompanied by a slight reduction in the cohesion intercept, indicative of stronger interlocking and frictional engagement between the MSW inclusions and the soil matrix.\u003c/p\u003e \u003cp\u003eFrom a practical standpoint, MSW concentrations\u0026thinsp;\u0026le;\u0026thinsp;1% seem to offer the most balanced improvement, providing strength gains with negligible adverse effects on soil settlements. Higher MSW concentrations can improve shear strength further but could only be beneficial if increased amounts of settlements could be accommodated in the project requirements. If the project timeline permits, a significant portion of the anticipated consolidation settlements may be allowed to take place first before earthwork construction is completed and the supported structures are put in service.\u003c/p\u003e \u003cp\u003eIn summary, the study results indicate that undecomposed MSW has the potential for use as an effective soil reinforcement agent, but its beneficial contributions in shear strength must be balanced against its potential adverse effects on compressibility. The data presented herein could be used as preliminary guidance for this purpose, while more accurate and reliable design recommendations await further studies on a broader range of MSW material types and concentrations, time scales and confining stresses, among other factors.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eMohsen Ajdari conceived the central research idea and defined the study objectives; conceptualized the experimental program; designed and developed the large-scale direct shear device and the overall test methodology; supervised the technical direction of the work; interpreted the experimental trends and discussed the geotechnical implications; and drafted the original version of the manuscript. Mohammadreza Arvin curated the dataset; processed, reduced, and organized the raw measurements; performed the primary quantitative analyses and prepared analysis-ready tables/plots; contributed to interpretation of results; and led substantive manuscript revision and editing. Sina Zahmatkesh Astaneh performed the laboratory testing program, including specimen preparation and execution of the direct shear tests; ensured proper documentation and quality control during testing; and compiled and delivered the raw data and test records. Kianoosh Hatami contributed to data interpretation and verification, including review of analysis assumptions and consistency checks; strengthened the discussion of findings and their relevance to practice; and provided critical revisions to improve the manuscript\u0026rsquo;s technical clarity, structure, and presentation.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eDehdari, V., Ajdari, M. and Rostami, A. (2021) \u0026lsquo;Experimental study on shear strength parameters of a municipal solid waste employing a large direct shear apparatus\u0026rsquo;, \u003cem\u003eGeomechanics and Geoengineering\u003c/em\u003e, Published online 27 May.\u003c/li\u003e\n\u003cli\u003eKaboudani, A., Ajdari, M., Maleki, S., Esfandiari, Z. and Shafiee, A. (2023) \u0026lsquo;Roles of ageing on the physical, chemical and mechanical parameters of a prototype municipal solid waste\u0026rsquo;, \u003cem\u003eGeomechanics and Geoengineering\u003c/em\u003e, pp. 1\u0026ndash;17.\u003c/li\u003e\n\u003cli\u003eAkbari, I., Ajdari, M. and Shafiee, A. (2022) \u0026lsquo;Mechanical properties of landfill components under low to medium stress levels\u0026rsquo;, \u003cem\u003eBulletin of Engineering Geology and the Environment\u003c/em\u003e, 81(9), pp. 1\u0026ndash;17.\u003c/li\u003e\n\u003cli\u003eEsfandiari, Z., Ajdari, M. and Vahedifard, F. (2021) \u0026lsquo;Time-dependent deformation characteristics of an unsaturated sand\u0026ndash;bentonite mixture under drying\u0026ndash;wetting cycles\u0026rsquo;, \u003cem\u003eJournal of Geotechnical and Geoenvironmental Engineering (ASCE)\u003c/em\u003e, 147(3), 04020172.\u003c/li\u003e\n\u003cli\u003eAjdari, M. and Bahmyari, H. (2015) \u0026lsquo;Oedometric response of an artificially prepared sand\u0026ndash;bentonite mixture improved by potassium silicate\u0026rsquo;, \u003cem\u003eScientia Iranica\u003c/em\u003e, 22(2), pp. 367\u0026ndash;372.\u003c/li\u003e\n\u003cli\u003eDhatrak, A. and Konmare, S.D. (2015) \u0026lsquo;Performance of randomly oriented plastic waste in flexible pavement\u0026rsquo;, \u003cem\u003eInternational JPRET\u003c/em\u003e, 3(9), pp. 193\u0026ndash;202.\u003c/li\u003e\n\u003cli\u003eKhokar, I.A., Al-Saidi, K.S., Ali, S.A. and Hayder, G. (2025) \u0026lsquo;Utilizing recycled vegetable plastic bags as an innovative and sustainable material for soil reinforcement applications\u0026rsquo;, \u003cem\u003eInternational Journal of Environmental Impacts\u003c/em\u003e, 8(5), pp. 1094\u0026ndash;1101.\u003c/li\u003e\n\u003cli\u003eGhiassian, H., Poorebrahim, G. and Gray, D.H. (2004) \u0026lsquo;Soil reinforcement with recycled carpet wastes\u0026rsquo;, \u003cem\u003eWaste Management \u0026amp; Research\u003c/em\u003e, 22(2), pp. 108\u0026ndash;114.\u003c/li\u003e\n\u003cli\u003eGhazavi, M. and Sakhi, M.A. (2005) \u0026lsquo;Influence of optimized tire shreds on shear strength parameters of sand\u0026rsquo;, \u003cem\u003eInternational Journal of Geomechanics\u003c/em\u003e, 5(1), pp. 58\u0026ndash;65.\u003c/li\u003e\n\u003cli\u003eChoudhary, A.K., Jha, J.N. and Gill, K.S. (2010) \u0026lsquo;A study on CBR behavior of waste plastic strip reinforced soil\u0026rsquo;, \u003cem\u003eEmirates Journal for Engineering Research\u003c/em\u003e, 15(1), pp. 51\u0026ndash;57.\u003c/li\u003e\n\u003cli\u003eMar\u0026ccedil;al, R., Lodi, P.C., de Souza Correia, N., Giacheti, H.L., Rodrigues, R.A. and McCartney, J.S. (2020) \u0026lsquo;Reinforcing effect of polypropylene waste strips on compacted lateritic soils\u0026rsquo;, \u003cem\u003eSustainability\u003c/em\u003e, 12.\u003c/li\u003e\n\u003cli\u003eBenson, C.H. and Khire, M.V. (1994) \u0026lsquo;Reinforced sand with strips of reclaimed high-density polyethylene\u0026rsquo;, \u003cem\u003eJournal of Geotechnical Engineering (ASCE)\u003c/em\u003e, 120(5), pp. 838\u0026ndash;855.\u003c/li\u003e\n\u003cli\u003eMishaal, F.Z. and Aldaood, A.H. (2023) \u0026lsquo;Soil reinforcement with synthetic fibers and plastic waste materials: a review\u0026rsquo;, \u003cem\u003eAl-Rafidain Engineering Journal\u003c/em\u003e, 28(2), pp. 33\u0026ndash;47.\u003c/li\u003e\n\u003cli\u003eNataraj, M.S. and McManis, K.L. (1997) \u0026lsquo;Strength and deformation properties of soil reinforced with fibrillated fibers\u0026rsquo;, \u003cem\u003eGeosynthetics International\u003c/em\u003e, 4(1), pp. 65\u0026ndash;79.\u003c/li\u003e\n\u003cli\u003eGregory, G.H. (2006) \u003cem\u003eShear strength, creep and stability of fiber-reinforced soil slopes\u003c/em\u003e. PhD thesis. Oklahoma State University, Stillwater, OK.\u003c/li\u003e\n\u003cli\u003eLi, J., Tang, C., Wang, D., Pei, X. and Shi, B. (2014) \u0026lsquo;Effect of discrete fibre reinforcement on soil tensile strength\u0026rsquo;, \u003cem\u003eJournal of Rock Mechanics and Geotechnical Engineering\u003c/em\u003e, 6, pp. 133\u0026ndash;137.\u003c/li\u003e\n\u003cli\u003eHatami, K., Gregory, G. H., and Garland Jr, G. S. (2019). Guidelines for the Use of Fiber Reinforced Soil (FRS) in Highway Construction.\u003c/li\u003e\n\u003cli\u003eFoose, G.J., Benson, C.H. and Bosscher, P.J. (1996) \u0026lsquo;Sand reinforced with shredded waste tires\u0026rsquo;, \u003cem\u003eJournal of Geotechnical Engineering\u003c/em\u003e, 122(9), pp. 760\u0026ndash;767.\u003c/li\u003e\n\u003cli\u003eSivakumar Babu, G.L., Chouksey, S., Anoosha, G. and Geetha Manjari, K. (2010) \u0026lsquo;Strength and compressibility response of plastic waste mixed soil\u0026rsquo;, in \u003cem\u003eProceedings of the Indian Geotechnical Conference 2010 (GEOtrendz)\u003c/em\u003e, 16\u0026ndash;18 December, Mumbai, India. Mumbai: IGS Mumbai Chapter and IIT Bombay.\u003c/li\u003e\n\u003cli\u003eIravanian, A. and Haider, A.B. (2020) \u0026lsquo;Soil stabilization using waste plastic bottles fibers: a review paper\u0026rsquo;, \u003cem\u003eIOP Conference Series: Earth and Environmental Science\u003c/em\u003e, 614, 012082.\u003c/li\u003e\n\u003cli\u003eConsoli, N.C., Vendruscolo, M.A. and Prietto, P.D.M. (2010) \u0026lsquo;Behavior of plate load tests on soil layers improved with cement and fiber\u0026rsquo;, \u003cem\u003eJournal of Geotechnical and Geoenvironmental Engineering (ASCE)\u003c/em\u003e, 129(1), pp. 96\u0026ndash;101.\u003c/li\u003e\n\u003cli\u003eHern\u0026aacute;ndez, C., Beltr\u0026aacute;n, G. and Botero, E. (2024) \u0026lsquo;Use of recycled plastic fibers to control shrinkage and desiccation cracking in clayey soils\u0026rsquo;, \u003cem\u003eSustainability\u003c/em\u003e, 16, 3853.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"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":"Municipal Solid Waste, Reinforced Soil, Fine-grained Soil, Large Direct Shear Device","lastPublishedDoi":"10.21203/rs.3.rs-9013637/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9013637/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe paper reports a study on the reinforcing effect of non-degradable municipal solid waste (MSW) in fine-grained soils with possible applications in reinforced soil construction. A low-plasticity silty soil was randomly mixed with five-year-old, non-degradable MSW at 0.0, 0.5, 1.0, and 2.0% concentrations by dry weight (wt%), and compacted in a large-scale split shear box of approximately one meter cubed volume. Specimens were consolidated under uniform normal stresses across their entire plan area for twenty-four hours and then sheared under drained, displacement-controlled conditions. Strength comparisons were made consistently at a large strain level to capture any reinforcing benefits of the soil-MSW mixtures under field conditions.\u003c/p\u003e \u003cp\u003eThe consolidation response of the composites exhibited a distinct threshold. Mixtures up to 1.0 wt% MSW showed consolidation behavior similar to that of a comparable unreinforced soil, whereas a 2.0 wt % concentration caused a slower consolidation rate and substantially larger settlements. The concurrent reduction in the virgin compression slope and increase in the recompression slope with reinforcement concentration indicates strain-dependent interactions, where interlocking between the soil and waste material mobilizes the tensile resistance of waste fragments anchored within the soil matrix.\u003c/p\u003e \u003cp\u003eResults also show a significant (i.e. by as much as 50%) increase in the drained shear strength of the mixtures at large deformations relative to comparable raw samples. Variations in the shear strength properties of mixtures indicated two distinct trends in that, for up to 1.0% waste concentration, the increase in shear strength was essentially due to an increase in the apparent cohesion with minimal change in the friction angle, whereas at 2.0% concentration, the friction angle increased significantly while cohesion decreased, reflecting a transition to an interlocking-dominated reinforcement mechanism.\u003c/p\u003e","manuscriptTitle":"Mechanical Response of Fine-Grained Soil-Solid Waste Composites from Field Scale Direct Shear Tests","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-27 11:21:38","doi":"10.21203/rs.3.rs-9013637/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":"54b0ef10-903a-4d25-a92b-c984f9328927","owner":[],"postedDate":"March 27th, 2026","published":true,"recentEditorialEvents":[{"type":"editorInvitedReview","content":"","date":"2026-05-18T14:44:07+00:00","index":30,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-11T02:00:38+00:00","index":29,"fulltext":""},{"type":"reviewerAgreed","content":"167808483821459116690922798668320516740","date":"2026-05-07T00:50:35+00:00","index":28,"fulltext":""},{"type":"reviewerAgreed","content":"91403604227405905923723287074487654551","date":"2026-05-05T20:17:47+00:00","index":27,"fulltext":""},{"type":"reviewerAgreed","content":"23835802529128548787117913249071096590","date":"2026-05-05T15:42:57+00:00","index":26,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-03-27T11:21:38+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-27 11:21:38","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9013637","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9013637","identity":"rs-9013637","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}