Dynamic Response and Pore Evolution Mechanism of Composite Improved Loess Using an Eco-Friendly Curing Agent and Cement: A Macroscopic and Microscopic Experimental Study

Dynamic Response and Pore Evolution Mechanism of Composite Improved Loess Using an Eco-Friendly Curing Agent and Cement: A Macroscopic and Microscopic Experimental Study | 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 Article Dynamic Response and Pore Evolution Mechanism of Composite Improved Loess Using an Eco-Friendly Curing Agent and Cement: A Macroscopic and Microscopic Experimental Study Xiangming Lv, Chongliang Luo, Fei Ma, Baocheng Wang, Xin Wang, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8980134/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 12 You are reading this latest preprint version Abstract To address the loose structure and insufficient dynamic stability of collapsible loess subgrades in Gangu, Gansu Province, as well as the engineering and environmental concerns associated with conventional stabilizers (high energy consumption and carbon emissions), this study proposes a composite stabilization strategy using an eco-friendly curing agent (EFCA) and P·O 42.5 Portland cement. Unconfined compressive strength (UCS) tests and dynamic triaxial tests were performed, and scanning electron microscopy (SEM) image processing together with fractal theory was employed to systematically elucidate the macroscopic dynamic response and the microscopic pore-reconstruction and evolution mechanisms of the composite-improved loess. The results indicate that the optimal mix proportion determined by an orthogonal design is “6% cement + 0.02% curing agent”, yielding a 28-day UCS of 2.51 MPa, which is 483% higher than that of the untreated loess. The dynamic resilient modulus ( E d ) increases markedly and reaches 826.49 MPa under a confining pressure of 60 kPa and a dynamic stress of 30 kPa (an 8.07-fold increase). Nonlinear regression analysis confirms that the Ni model, by jointly accounting for the coupled effects of mean and deviatoric stresses, provides exceptionally high predictive accuracy for E d of the composite-improved loess, with an average relative error of only 0.026. Quantitative microstructural analysis reveals that the synergistic effects of chemical cementation and hydration products promote the transformation of loess particles into dense aggregates, resulting in a decrease in the pore fractal dimension ( D ) from 1.323 to 1.249. This topological reconstruction from connected macropores to discrete micropores fundamentally reduces the structural complexity of the soil. The study clarifies a cross-scale physical-mechanical mechanism whereby “microstructural pore-fractal dimensionality reduction” drives a “macroscopic surge in dynamic stiffness”, providing theoretical and data support for green, low-carbon subgrade construction and long-term dynamic stability evaluation in loess regions. Physical sciences/Engineering Physical sciences/Materials science Collapsible loess Eco-friendly curing agent Dynamic resilient modulus Dynamic constitutive prediction model Microstructural evolution Fractal dimension Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 1 Introduction Loess in China covers an area of 640,000 km² and is widely distributed in the northwestern region [ 1 ] . In particular, loess in the Tianshui area of Gansu Province, characterized by typical macropores, pronounced collapsibility, and susceptibility to disintegration, poses major challenges to transportation projects associated with the Belt and Road Initiative and often triggers hazards such as road collapses and slope slides [ 2 – 6 ] . To address the shortage of high-quality subgrade fill materials in loess regions, improving in situ undisturbed loess to enhance its engineering performance is a key approach to overcoming regional bottlenecks in transportation construction [ 7 – 9 ] . Moreover, subgrades are subjected to complex cyclic traffic-induced dynamic loads during long-term service; therefore, an in-depth investigation of the dynamic characteristics and dynamic constitutive response of improved loess is of decisive importance for ensuring the long-term safety and stability of subgrade engineering. At present, loess improvement has become a research hotspot in the academic community. Conventional chemical stabilizers (e.g., cement, lime, and fly ash) can substantially increase soil strength, yet they commonly face technical bottlenecks such as sensitivity to shrinkage cracking and slow early-age strength development [ 10 – 13 ] . Meanwhile, the production of traditional silicate-based cementitious materials involves very high energy consumption and carbon emissions, which runs counter to the current engineering trend toward “green and low-carbon construction”. With advances in polymer materials science and the concept of sustainable construction, novel curing agents have emerged in geotechnical engineering [ 14 – 17 ] . Improving loess fill using an EFCA not only enables the use of locally available materials to reduce engineering costs but also reduces the consumption of traditional binders such as lime and cement, which is of great significance for achieving carbon-emission reduction. Ma et al. [ 18 ] showed that a polymer-based curing agent (PBCA) can effectively improve the water sensitivity and densification of loess through ion exchange and by altering the electrical double layer (EDL) structure. However, a single novel curing agent often suffers from limitations such as a restricted degree of improvement or a lack of rigid skeletal support [ 14 ] . Therefore, exploring the composite improvement mechanism of a novel EFCA combined with conventional cement-so as to account for both rigid skeletal strength and flexible crack resistance while achieving a balance between economy and environmental protection-has become a new trend in the development of subgrade improvement materials. In geotechnical dynamic evaluation, the dynamic resilient modulus ( E d ) is a key parameter used to characterize the elastic recovery capacity of subgrade soils under cyclic loading. In recent years, extensive studies have investigated E d for various soil types. Based on dynamic triaxial test (DTT), Li et al. [ 19 ] examined the effects of compaction degree, water content, and stress state on the E d of Malan loess. Liu Weizheng et al. [ 20 ] introduced matric suction into a prediction model to analyze the evolution of E d in compacted red clay. Gao et al. [ 21 ] developed an E d prediction model for subgrade clay by considering loading frequency. Qian et al. [ 22 ] conducted DTT on soil-rock mixture materials, analyzed the influences of mean stress, octahedral shear stress (OSS), and rock content on E d , and further established relationships between the dynamic resilient modulus and factors such as rock content and stress state using a new prediction model. Dong Cheng et al. [ 23 ] developed a three-parameter composite dynamic constitutive model for cement-improved high liquid limit clay. Although substantial progress has been made in macro-scale prediction using dynamic constitutive models (e.g., the NCHRP 1-28A model and the N i model that couple mean stress and deviatoric stress), research on the dynamic resilient response and accurate prediction of composite-improved collapsible loess under complex stress states remains relatively limited. More importantly, the leap in the macroscopic dynamic performance of improved soils is essentially the macroscopic manifestation of microstructural pore reconstruction and the evolution of inter-particle contact relationships. In recent years, multi-scale microstructural testing techniques have been progressively introduced to investigate the mechanisms underlying the engineering behavior of loess. For example, Gao et al. [ 24 ] revealed the distinctive loose particle skeleton and macropore distribution of natural Malan loess and identified these features as fundamental causes of collapsibility and mechanical instability. Using SEM, Ni et al. [ 25 ] quantitatively analyzed the microstructural evolution of particle rearrangement and pore reduction in Malan loess under dynamic compaction. Regarding the mechanism of improved loess, Ma et al. [ 18 , 26 ] combined SEM, nuclear magnetic resonance (NMR), and fractal dimension (FD) analyses to quantitatively evaluate the transformation pathway from connected macropores to isolated micropores in polymer-cement composite-improved loess, confirming that composite improvement can significantly reduce pore complexity. However, existing studies on microstructural geometric fractal characteristics have mostly focused on interpreting static indices such as UCS, and systematic reports on cross-scale macro-micro coupling mechanisms that integrate the quantitative evolution of pore fractal characteristics with the macroscopic E d and dynamic constitutive models remain scarce. How, precisely, do the coupled hydration/polymerization reactions between an eco-friendly curing agent and cement reconstruct the fragile connected pore network within loess? How does the evolution of microstructural fractal geometry, in turn, drive the soil to exhibit higher deformation-resistant stiffness under cyclic dynamic loading? These issues remain theoretical blind spots that constrain the wider application of eco-friendly composite stabilization technologies. Accordingly, this study targets collapsible loess from a distressed road section in Gangu County, Gansu Province, and proposes composite stabilization using an EFCA and P·O 42.5 Portland cement. UCS tests and DTT were conducted to determine the optimal composite mix proportion, to investigate the evolution of E d under different confining pressures and dynamic stress levels, and to comparatively evaluate the predictive accuracy of the NCHRP 1-28A and N i models. On this basis, SEM observations, ImageJ-based image processing, and microstructural pore fractal theory were integrated to quantitatively analyze the evolution of the internal pore structure of loess before and after composite stabilization. This study aims to elucidate the intrinsic mechanism underlying the enhanced dynamic performance of composite-improved loess through a full-chain framework of “macroscopic mechanical prediction-microscopic quantitative characterization-physical cementation mechanism”, thereby providing solid theoretical support and scientific evidence for durability-oriented design and low-carbon construction of subgrade engineering in loess regions. 2 Experimental program 2.1 Testing of physical and mechanical properties The soil samples were collected from the subgrade and slope of a severely distressed section of the X503 highway reconstruction project in Gangu County, Gansu Province, at a sampling depth of 0.5-3 m. The specimens were yellowish-brown and exhibited typical structural looseness, with well-developed pores and pronounced vertical joints. Laboratory tests were conducted in accordance with the Test Methods of Soils for Highway Engineering (JTG 3430 − 2020) [ 26 ] , and the main physical and mechanical parameters are listed in Table 1 . The test results show that the collapsibility coefficient ( δ s ) is greater than 0.015; according to the specification [ 27 ] , the soil is identified as a low-plasticity silt and collapsible loess, classified as poorly graded ( C u = 8.24, C c = 0.64), with a dominant sand-size fraction (0.075-2 mm) accounting for more than 50%. Therefore, this undisturbed loess requires stabilization when used as subgrade fill. Table 1 Physical and mechanical properties of the soil samples ρ max (g/cm 3 ) ω opt (%) w L (%) w p (%) δ s C u C c 1.84 13.8 23.83 10.41 0.04 8.24 0.64 2.2 Experimental scheme The test plan includes the UCS test plan and the E d test plan. The specific introduction is as follows: 2.2.1 UCS test scheme In this study, an EFCA (KJD-II ionic liquid soil curing agent; pH = 11.26; density = 1.393 g·cm⁻³; solid content = 47.43%) and P·O 42.5 silicate Portland cement (PC) were used to composite-stabilize collapsible loess. To determine the optimal composite proportion, an orthogonal experimental design (OED) was adopted, with EFCA dosages of 0%, 0.01%, 0.015%, 0.02%, and 0.025%, cement dosages of 4%, 6%, and 8%, and curing ages of 7 and 28 days. According to the Technical Specification for Expressway Subgrade Construction in Loess Areas [ 28 ] and considering the actual engineering conditions, the specimen preparation parameters were controlled at a compaction degree of 94% and a water content of 13.82% (optimum water content). The dosages of each component in the loess-EFCA-cement-water system were determined by mass-ratio calculations; after thorough mixing, standard specimens with a diameter of Φ 150 mm were prepared using the static compaction method. After curing at 20 ± 2°C, UCS tests were performed using an electronic universal testing machine (Fig. 1 ). 2.2.2 E d test scheme After the optimal composite stabilization scheme was selected based on UCS results, E d was measured for specimens with the optimal mix proportion using a British GDS system. The system integrates a dynamic stress loading module (maximum load: 20 kN; frequency: 0–10 Hz), a confining-pressure control unit, and a data acquisition device (Fig. 2 ). Considering the stress state and typical stress levels of highway subgrades in China, the lateral pressure acting on subgrade soils is relatively small (generally 0–60 kPa), and the dynamic stress experienced by most subgrade soils does not exceed 110 kPa [ 29 ] . Accordingly, in conjunction with the relevant provisions of JTG 3430 − 2020 [ 26 ] , the loading sequence was set as listed in Table 2 , comprising a preloading stage and an E d -testing stage, and an HWF was applied to approximate traffic loading at 10 Hz with a loading duration of 0.1 s and a rest duration of 0.9 s. In the preloading stage, a pre-confining pressure of 30 kPa was applied for 1000 load cycles, whereas in the E d -testing stage, each loading condition was applied for 100 cycles. Table 2 Dynamic triaxial test loading sequence Stress loading path 𝜎 3 (kPa) 𝜎 𝑐 (kPa) 𝜎 d (kPa) θ (kPa) τ oct (kPa) N 0-Preloading stage 30 6 55 145 25.93 1000 1 60 12 30 210 14.14 100 2 45 9 30 165 14.14 100 3 30 6 30 120 14.14 100 4 15 3 30 75 14.14 100 5 60 12 55 235 25.93 100 6 45 9 55 190 25.93 100 7 30 6 55 145 25.93 100 8 15 3 55 100 25.93 100 9 60 12 75 255 35.36 100 10 45 9 75 210 35.36 100 11 30 6 75 165 35.36 100 12 15 3 75 120 35.36 100 13 60 12 105 285 49.50 100 14 45 9 105 240 49.50 100 15 30 6 105 195 49.50 100 16 15 3 105 150 49.50 100 注: 𝜎 3 denotes the confining pressure; 𝜎 𝑐 denotes the contact stress; 𝜎 d denotes the dynamic stress; θ denotes the mean stress; τ oct denotes the octahedral shear stress. 3 Results and discussion 3.1 UCS test results 3.1.1 Failure patterns of specimens The failure patterns of specimens in the UCS tests are shown in Fig. 3 , and the typical feature is a shear failure plane at approximately 45°. The tests show that, at the initial loading stage (Fig. 3 a), the specimen remains in the elastic deformation regime under a low stress level, the stress–strain curve increases linearly, and no surface damage is observed. As the axial stress increases to the peak value (Fig. 3 b), cracks initiate at the specimen ends, indicating a transition from elastic to elasto-plastic behavior. With continued loading (Fig. 3 c), the cracks propagate toward the core and are accompanied by local spalling, and the stress exhibits a nonlinear decay as damage accumulates. At the final loading stage (Fig. 3 d), microcracks rapidly coalesce to form a macroscopic slip surface, and the specimen exhibits a typical hourglass-shaped shear failure pattern (shear angle ≈ 45°), at which point the stress enters the residual stage and drops sharply. This evolution process clearly reflects the progressive mechanical response of composite-improved loess, from linear elastic deformation to the accumulation of plastic damage and ultimately to macroscopic instability and failure. 3.1.2 UCS analysis of composite-improved loess Based on the UCS results, the UCS of untreated loess at 7 and 28 days is 0.41 MPa and 0.43 MPa, respectively, which is substantially lower than the minimum strength threshold of 0.5 MPa specified for subgrade fill in the Design Specification for Expressway Subgrades in Loess Areas of Gansu Province (JTG/T D31-05-2017) [ 28 ] , confirming that the undisturbed loess must be modified before it can be used in subgrade engineering. The tests show that, after composite improvement using EFCA (dosage: 0-0.025%) and P·O 42.5 cement (dosage: 4–8%), all specimens achieve 7-day strengths of 0.96–1.99 MPa and 28-day strengths of 1.34–2.85 MPa, representing increases of 134–385% and 212–563% relative to untreated loess, respectively, fully meeting the requirements of the specification [ 28 ] . The analysis indicates that the highly alkaline environment provided by the KJD-II curing agent promotes ion exchange and cementation of clay minerals in loess, while its high solid content supplies additional cementitious substances, thereby enhancing inter-particle bonding. In addition, the hydration of P·O 42.5 silicate cement produces hydration products such as calcium silicate hydrate (C-S-H) gel and ettringite (AFt), which fill soil pores and form a rigid skeletal structure [ 30 ] . Moreover, the combined action of the curing agent and cement further optimizes the soil microstructure and, through a dual cementation effect, significantly improves soil compactness and integrity, thereby effectively suppressing collapsibility and enhancing its mechanical performance. As shown in Figs. 6 and 7 , when the P·O 42.5 cement dosage increased from 4% to 6%, the 7-day and 28-day UCS increased by 119.51% and 62.79%, respectively, and the corresponding marginal benefit rate (MBR) was 59.76%/% and 31.40%/% (MBR = Δ strength growth rate/Δ dosage). When the dosage increased further to 8%, the increases decreased to 78.05% and 16.28%, and the MBR decreased to 39.02%/% and 8.14%/%, showing a pronounced diminishing-return trend with attenuation rates of 34.7% and 74.1%, respectively. Specimen observations indicate that when the cement dosage exceeds 6%, the density of drying-shrinkage cracks on the specimen surface increases, and the engineering cost rises markedly. In the 6% cement system, at 7 days, as the EFCA dosage increased from 0 to 0.01%, 0.015%, 0.02%, and 0.025%, the strength growth rates were 17.0%, 17.1%, 21.95%, and 9.76%, respectively, and the corresponding MBR values were 7.0, 14.0, 18.0, and 8.0 MPa/% (MBR = Δ strength/Δ dosage), showing a clear increase-then-decrease pattern with the peak MBR at an EFCA dosage of 0.02%. The 28-day data also verify this pattern. This dosage minimizes the increase in material cost, while the 28-day UCS reaches 2.51 MPa, which is 5.02 times the specified value (0.5 MPa), and it also keeps the material cost controllable, consistent with the principle of balancing specification-based strength requirements and economy. Therefore, a P·O 42.5 cement dosage of 6% and an EFCA dosage of 0.02% constitute the optimal composite stabilization scheme. 3.2 DTT results E d is a key index for characterizing the dynamic response of geomaterials, and it is physically defined as the ratio of axial dynamic stress to resilient strain, which directly reflects the elastic recovery capacity of the material under cyclic loading. A larger E d indicates a stronger elastic load-bearing capacity of the soil. Based on the DTT results, the data from the last five cycles under each loading sequence were extracted, E d was calculated according to Eq. ( 1 ), and the average over the five cycles was taken as the E d value of the subgrade fill [ 23 ] . $${E_{\text{d}}}=\frac{{{\sigma _{\text{d}}}}}{{{\varepsilon _{\text{r}}}}}$$ 1 where E d is the dynamic resilient modulus of the soil (kPa), σ d is the cyclic stress (kPa), and ε r is the axial strain (%). 3.2.1 Effect of stress state on E d After loess was composite-stabilized with 6% P·O 42.5 silicate cement and 0.02% EFCA, E d increased markedly, and E d was strongly affected by σ 3 and σ d . For untreated loess (7-day curing) at σ d = 30 kPa, when σ₃ increased from 15 to 60 kPa, E d increased from 85.36 to 102.36 MPa (19.9% increase); however, the stage-wise marginal gain rate (Δ E d /Δ σ 3 ) first increased and then decreased with σ 3 (15→30→45→60 kPa) (0.24→0.59→0.31 MPa/kPa), indicating that the lateral confinement effect was most pronounced at a relatively low confining pressure of 30 kPa. When σ 3 = 30 kPa, increasing σ d from 30 to 105 kPa reduced E d from 88.91 to 63.87 MPa (28.2% decrease), with an attenuation gradient of 0.33 MPa/kPa. In contrast, the composite-improved loess exhibited a pronounced enhancement effect; at σ₃ = 60 kPa and σ d = 30 kPa, E d reached 826.49 MPa, which is 8.07 times that of untreated loess. Dynamic-stress sensitivity analysis shows that for the composite-improved loess at σ 3 = 30 kPa, increasing σ d from 30 to 105 kPa led to a 20.9% decrease in E d (723.31→572.36 MPa), with an attenuation gradient of 2.01 MPa/kPa. The analysis indicates that as σ₃ increases, the lateral confinement on the specimen is enhanced, its ability and stiffness to resist resilient deformation increase, the resilient strain decreases, and E d increases. In contrast, increasing σ d weakens the soil’s resistance to resilient deformation, leading to an increase in resilient strain and a decrease in E d , which is consistent with the findings reported in Refs. [ 20 , 29 ] . The responses of E d to mean stress θ differ markedly between untreated loess and composite-improved loess (6% P·O 42.5 cement + 0.02% EFCA). For untreated loess at σ ₃ = 30 kPa, when θ increased from 75 to 150 kPa, E d decreased from 85.36 to 56.14 MPa (a 34.2% reduction), with an attenuation gradient of 0.39 MPa/kPa (attenuation gradient = Δ E d /Δ θ ), and the marginal gain rate increased with increasing σ₃ (from 4.2% to 4.8% as σ₃ increased from 15 to 60 kPa). In contrast, the composite-improved loess exhibits superior performance: at a high confining pressure ( σ ₃ = 60 kPa), E d reaches 826.49 MPa, which is 8.07 times that of untreated loess, and the confining-pressure gain rate remains stable (average gain rate = Δ E d /Δ σ ₃ ≈ 5.1 MPa/kPa). This phenomenon is attributed to the synergistic action of cement hydration products (C-S-H gel) and the silica-alumina gel from the curing agent, which forms an interwoven rigid-flexible structure that effectively suppresses stiffness degradation induced by increasing θ , and the cemented structure effectively mitigates the decline in the confining-pressure gain rate. 4 Prediction of Ed models In geotechnical engineering, prediction models for the E d of subgrade soils are commonly classified into three categories according to the stress variables considered: models considering only shear effects, models considering only confinement effects, and composite models considering both shear and confinement effects. Given that Ed increases with increasing confining pressure and decreases with increasing shear stress, it indicates that E d is jointly influenced by mean stress and shear stress or deviatoric stress. Therefore, incorporating the effects of mean stress and shear stress into an Ed model can more realistically and comprehensively reflect the mechanical behavior of subgrade soils. For calculating E d of subgrade soils, prediction models that simultaneously consider the effects of shear stress and confining stress are particularly favored. Such models can not only capture the variation of subgrade soils under multiple levels of confining pressure but also highlight the role of shear stress. Under the loading conditions and typical stress levels of highway subgrades in China, the Highway Subgrade Design Specification: JTG 3430 − 2020 recommends using the NCHRP 1-28A model for prediction. This model comprehensively considers the effects of octahedral shear stress and the lateral confinement associated with mean stress on Ed, and it can realistically reflect the influence of complex stress states on E d in subgrades [ 31 ] . In addition, the N i model establishes relationships between confining pressure and deviatoric stress and subgrade soils, providing a more direct and comprehensive approach to describing the Ed characteristics of subgrade soils [ 32 ] . The application of the above models is of great significance for accurately predicting Ed of subgrade soils. NCHRP 1-28A model: $${E_{\text{d}}}={k_1}{{\text{p}}_{\text{a}}}{\left( {\frac{\theta }{{{{\text{p}}_{\text{a}}}}}} \right)^{{k_2}}}{\left( {\frac{{{\tau _{{\text{oct}}}}}}{{{{\text{p}}_{\text{a}}}}}+1} \right)^{{k_3}}}$$ 2 $$\theta ={\sigma _1}+{\sigma _2}+{\sigma _3}$$ 3 $${\tau _{{\text{oct}}}}=\frac{{\sqrt {{{\left( {{\sigma _1} - {\sigma _2}} \right)}^2}+{{\left( {{\sigma _1} - {\sigma _3}} \right)}^2}+{{\left( {{\sigma _2} - {\sigma _3}} \right)}^2}} }}{3}$$ 4 N i model: $${E_{\text{d}}}={k_1}{{\text{p}}_{\text{a}}}{\left( {\frac{{{\sigma _3}}}{{{{\text{p}}_{\text{a}}}}}+1} \right)^{{k_2}}}{\left( {\frac{{{\sigma _{\text{d}}}}}{{{{\text{p}}_{\text{a}}}}}+1} \right)^{{k_3}}}$$ 5 where k 1 , k 2 , and k 3 are model fitting parameters, the values of which are related to the physical and mechanical state of the subgrade soil; P a is the standard atmospheric pressure with a value of 101.325 kPa; θ is the mean stress (kPa); τ oct is the octahedral shear stress (kPa); and σ 2 is the intermediate principal stress of the soil (kPa). Based on the Ed test data of untreated loess and composite-improved loess (6% P·O 42.5 cement + 0.02% EFCA) from Gangu, Gansu Province, this study performed parameter fitting for the NCHRP 1-28A and N i models using nonlinear regression analysis in SPSS. The fitted model parameters are listed in Tables 3 and 4 , and the coefficients of determination ( R ²) for the fitted Ed are shown in Figs. 12 and 13 , respectively. The results show that the R ² values of the NCHRP 1-28A model for fitting E d of untreated loess and composite-improved loess are 0.960 and 0.950, respectively. Correspondingly, the R ² values of the N i model are 0.955 and 0.969. Given that the R² values of both models exceed 0.95, both the NCHRP 1-28A and N i models can effectively simulate the variation trend of E d for untreated loess and composite-improved loess. This finding provides an important theoretical basis for the selection and design of subgrade fill materials in loess regions. Table 3 Fitted parameters and fitting results of the NCHRP 1-28A model for E d Fill material type Curing age k 1 k 2 k 3 Untreated loess 7d 1.108 0.201 -1.713 Composite-improved loess 7d 7.756 0.382 -1.44 Table 4 Fitted parameters and fitting results of the N i model for E d Fill material type Curing age k 1 k 2 k 3 Untreated loess 7d 0.982 0.534 -0.803 Composite-improved loess 7d 5.997 0.997 -0.481 To accurately evaluate the prediction accuracy of the NCHRP 1-28A and N i models for E d of untreated loess and composite-improved loess, this study adopted the relative error formula as the evaluation metric. Relative error quantifies the percentage of relative deviation between the model-predicted values and the measured observations, providing an effective measure of model prediction accuracy [ 33 ] . The relative error is defined as: $${S_{{\text{err}}}}=\sqrt {\frac{{\mathop \sum \limits_{{i=1}}^{n} {{({N_{p,i}} - {N_{m,i}})}^2}}}{{\mathop \sum \limits_{{i=1}}^{n} N_{{m,i}}^{2}}}}$$ 6 where S err is the relative error; N p,i is the i- th predicted value (kPa); and N m,i is the i- th measured value (kPa). Table 5 presents a detailed comparison of the relative errors of the NCHRP 1-28A and N i models in predicting E d for untreated loess and composite-improved loess. Analysis of the data in Table 5 shows that, for both loess types, the mean relative error of the N i model is slightly lower than that of the NCHRP 1-28A model. The comparison further indicates that the mean relative error of the N i model is markedly lower than that of the NCHRP 1-28A model. This is mainly attributed to the formation of a rigid-flexible cemented network within the composite-improved loess, which makes it more sensitive to variations in deviatoric stress. By more directly incorporating the coupling term between confining pressure and deviatoric stress, the N i model accurately captures the stiffness-degradation characteristics of this cemented structure under cyclic shearing, thereby exhibiting better applicability for evaluating the dynamic mechanical behavior of improved loess. Therefore, the N i model shows higher applicability in describing the dynamic mechanical behavior of subgrades composed of untreated loess and composite-improved loess. This finding provides valuable reference for guiding loess subgrade engineering design in the Tianshui region and helps improve the scientific rigor and accuracy of engineering design. Table 5 Prediction accuracy evaluation of the NCHRP 1-28A and N i models Fill material type Curing age NCHRP1-28A models N i models Untreated loess 7d 0.031 0.032 Composite-improved loess 7d 0.033 0.026 Mean relative error 0.032 0.029 5 Microstructural analysis of improved loess Improvements in macroscopic mechanical performance are essentially the outcome of the coupled evolution of inter-particle contact modes, cementation continuity, and the geometric characteristics of the pore network within the soil [ 18 , 24 , 34 ] . To elucidate the micro-mechanisms underlying the significant increase in Ed of loess composite-improved with an eco-friendly curing agent (KJD-II) and cement, SEM observations were conducted, and ImageJ-based image processing together with microstructural pore fractal theory was employed to qualitatively and quantitatively analyze the inter-particle contact characteristics, pore morphology, and pore-structure complexity of untreated loess and composite-improved loess at different curing ages. 5.1 Qualitative evolution of loess microstructure before and after composite stabilization Figure 14 presents the SEM images of untreated loess and the corresponding ImageJ processing results (original image → pore identification → binarization); overall, the untreated loess exhibits a typical granular skeletal structure, with relatively clean surfaces of silt and clay particles, predominantly point–point or point–surface contacts between particles, and a lack of effective cementing substances to form a continuous skeletal framework (Fig. 14 a). Meanwhile, its pore distribution is markedly heterogeneous, featuring a wide range of pore sizes, irregular boundaries, and numerous connected channels, showing a structural characteristic of a “loose skeleton with coexisting macropores and overhead pores” (Fig. 14 c). Such a fragile skeletal structure is highly prone to stress concentration, particle sliding, and structural yielding under external dynamic stress, which is the fundamental reason for its low E d . Figure 15 shows the SEM morphologies of composite-improved loess at different curing ages. Overall, with increasing curing age, both the number of pores and the pore-size scale continuously decrease, and soil particles gradually transform from a loose packing state to a cemented dense structure, exhibiting a typical evolution pathway of “early formation-mid-term filling-late densification”. ①1 day: Compared with untreated loess, initial cementation and filling effects appear, some larger connected pores are segmented, and the pore distribution tends to become more uniform (Fig. 15 a). This stage corresponds to the initial period of cement hydration and curing-agent participation, during which reaction products preferentially form bridging and coating at particle contact points, thereby beginning to alter the original skeleton connectivity dominated by point contacts. ②.7 days: Cementation and aggregation become more evident, inter-particle connections tend to be continuous, and pore connectivity further decreases (Fig. 15 b). As hydration/reaction proceeds, the filling of skeletal pores and the coating of particles by cementing products are strengthened, promoting a transition of the structure from “point contact/point–surface contact” to “surface–surface contact”. ③.14 days: Pores are further refined, the proportion of small pores increases, and the overall structure becomes denser (Fig. 15 c). This stage typically corresponds to the continued deposition of reaction products and effective filling of skeletal pores, during which the pore network transforms from “connected” to “discrete”. ④.28 days: Macropores largely disappear, pore boundaries become relatively smooth, pore size and connectivity further decrease, and the structure tends toward a stable dense state (Fig. 15 d). The reaction products of the curing agent and cement effectively fill inter-particle pores and coat particles, promoting the formation of aggregates and leading to tighter particle packing, thereby increasing structural compactness and providing microstructural support for the improvement of macroscopic strength and dynamic performance. 5.2 Quantitative analysis of microstructural pore fractal characteristics of composite-improved loess Although qualitative SEM observations can visually reflect the evolution of soil structure, they are insufficient for precisely describing the complexity of the spatial pore distribution and the densification process. Therefore, the fractal dimension D was introduced to quantitatively evaluate microstructural pore morphology: ImageJ was used to identify the pore equivalent perimeter P and equivalent area A , a double-logarithmic plot of log P versus log A was constructed, and D was obtained by fitting to calculate the pore fractal dimension of the soil before and after composite stabilization. The magnitude of D directly reflects the irregularity of pore boundaries and the complexity of the structure; D typically ranges from 1 to 2, and a smaller D indicates fewer macropores within the soil, simpler and more regular pore morphology, and a denser overall soil skeleton [ 26 , 35 ] . \({\text{lg }}P=\frac{D}{2}\lg A+C\) (7) where P is the equivalent pore perimeter (µm), A is the equivalent pore area (µm²), C is a constant, and D is the fractal dimension. Figure 16 shows that the microstructural pore fractal dimension of untreated loess is D = 1.323, quantitatively confirming the presence of a large number of morphologically complex and highly connected pore networks. Figure 17 further reveals that the pore fractal dimension of composite-improved loess decreases in a strictly monotonic manner with curing age. In the early stage of improvement (curing age: 1 day), physical filling and initial hydration preliminarily segment macropores. From 7 to 14 days, D decreases from 1.279 to 1.257, indicating that as the hydration/curing reactions accelerate, continuously generated cementitious products occupy a large portion of the pore space, thereby reducing the irregularity of pore boundaries. By 28 days, D reaches a minimum value of 1.249, representing a decrease of approximately 5.6% relative to untreated loess (1.323). This continuous decreasing trend of “1.323 → 1.249” provides strong quantitative evidence for the dual effects of “pore filling” and “cementation/solidification” in the composite stabilization system. Gel-like products continuously fill the primary pore network, progressively segmenting and refining the originally complex, irregular, large-scale backbone pores and ultimately transforming them into tiny and relatively isolated closed micropores. SEM observations show that with increasing curing age, pore number and connectivity decrease, inter-particle connections tend to become continuous, and the structure transforms toward a cemented dense state (Fig. 15 ). Consistently, the fractal dimension from the microstructural analysis before and after composite stabilization decreases significantly (Figs. 16 and 17 ), indicating reduced complexity of pore boundaries and the pore network. Together, these results mutually corroborate that composite stabilization weakens structural defects and enhances skeletal integrity and deformation-resistant stiffness through a “cementation-filling-densification” pathway, thereby providing a structural explanation for the improvement in dynamic performance. Consistent with this, the macroscopic tests in this study show that Ed of composite-improved loess can reach 826.49 MPa under high confining pressure and low dynamic stress, which is 8.07 times that of untreated loess, demonstrating a pronounced enhancement in dynamic stiffness. 5.3 Micro-mechanism discussion: structural contribution of the composite cemented structure to dynamic resilient behavior The enhancement in dynamic resilient performance of composite-improved loess ultimately arises from a systematic reconstruction of the internal structure from “loose particles-connected pores” to “cemented skeleton–discrete micropores”. Based on the SEM microstructural observations and pore-fractal results (Figs. 15 – 17 ), the contribution of the composite cemented structure to dynamic resilient behavior can be summarized into three coupled micro-mechanisms, as illustrated in Fig. 18 . (1) Construction of a cemented network: C-S-H gel and AFt generated by cement hydration deposit on particle surfaces and grow into pore spaces, exerting “coating-bridging-void-filling” effects that gradually transform weak particle contacts into a continuous structure bonded by cementitious products, thereby forming a rigid skeleton capable of transmitting and dispersing dynamic stress. Meanwhile, the highly alkaline environment provided by the EFCA promotes ion exchange of clay minerals and strengthens cementation, and its high solid content supplies additional cementitious substances to the system, further enhancing inter-particle bonding and interfacial resistance to sliding. (2) Reshaping of the pore network: In the early stage of composite stabilization, reaction products preferentially form at particle contacts and develop bridging and coating, causing the original connected pores to be segmented into multiple pore units of smaller scales. In the middle-to-late stage, cementitious products continue to deposit and fill skeletal pores, further reducing pore connectivity and gradually converting the pore network from connected to discrete, which macroscopically manifests as a pronounced reduction of macropores and overall densification (Fig. 15 ). This process reduces the freedom of particle rearrangement and the space available for pore compression, thereby weakening the accumulation of irrecoverable deformation under cyclic loading. (3) Reduction of structural complexity: Quantitative fractal-dimension results show that after composite stabilization, the pore fractal dimension decreases with curing age from 1.323 for untreated loess to 1.249 (Figs. 16 and 17 ), indicating that the irregularity of pore boundaries and the geometric complexity of the pore network are significantly reduced, pore morphology tends to become more regular, and the structure shifts from a heterogeneous, disturbance-prone state toward a more homogeneous and more integral dense skeleton. A reduction in structural complexity implies fewer potential stress-concentration points and “weak links”, which facilitates more uniform diffusion and transmission of dynamic stress within the skeleton. In summary, the construction of a cemented network, reshaping of the pore network, and reduction of structural complexity jointly promote the formation of a more stable “cemented-skeletal” system in composite-improved loess. Under cyclic loading, the continuous cemented network can effectively restrain particle sliding and local structural rearrangement, reduce pore compression and strain accumulation, and limit resilient deformation, thereby macroscopically manifesting as a significant increase in Ed accompanied by a tendency toward stabilization of the confining-pressure gain rate. 6 Conclusions This study investigated collapsible loess from Gangu, Gansu Province, and systematically examined the evolution of static/dynamic performance and the microstructural strengthening mechanisms of loess composite-improved with EFCA and P·O 42.5 cement through UCS tests, E d tests, and quantitative microstructural analyses. The main conclusions are as follows: (1) Orthogonal test results indicate that the optimal mix proportion is 6% P·O 42.5 cement and 0.02% EFCA, for which the 28-day UCS reaches 2.51 MPa, representing a 483% increase relative to untreated loess and meeting the specification requirements for subgrade fill. The failure process of the improved soil underwent four stages “elastic deformation-crack initiation-plastic yielding-residual strength” and ultimately exhibited a typical hourglass-shaped shear failure (shear angle ≈ 45°), indicating that composite stabilization effectively mitigated the brittle disintegration characteristics of untreated loess and endowed the soil with stronger deformation resistance. (2) Under the optimal mix proportion, the composite-improved loess exhibits excellent dynamic stability. At σ ₃ = 60 kPa and σ d = 30 kPa, Ed reaches 826.49 MPa, which is 8.07 times that of untreated loess (91.12 MPa). E d increases markedly with increasing σ ₃ (confinement-strengthening effect) and exhibits a nonlinear decrease with increasing σ d (stiffness-softening effect). The interwoven “rigid-flexible” structure formed in the composite stabilization system effectively suppresses stiffness degradation. (3) A comparative analysis of the NCHRP 1-28A and Ni models shows that both can well capture the nonlinear dynamic response characteristics of subgrade soils ( R ² > 0.95). However, the Ni model not only considers the coupling effect between mean stress and deviatoric stress but also exhibits higher prediction accuracy in describing the hardening behavior of composite-improved loess, making it more suitable for dynamic settlement calculation and design prediction for composite-improved loess subgrades in this region. (4) SEM observations and quantitative fractal-dimension analysis indicate that composite stabilization fundamentally reconstructs the micro-topological structure of loess. With increasing curing age, cement hydration products (C-S-H gel) and curing-agent polymerization products synergistically fill the primary macropores, driving particle contacts to shift from “point contact” to tight “surface contact” and aggregate cementation. The microstructural pore fractal dimension decreases significantly from 1.323 for untreated loess to 1.249 after stabilization (28 days), confirming a densification transition of the pore network from “connected and complex” to “discrete and regular”. This increased microstructural homogeneity and reduced defects effectively interrupt the internal damage path of dynamic stress within the soil, constituting the fundamental physical mechanism for the substantial increase in macroscopic E d . (5) Introducing EFCA effectively reduces reliance on traditional cement materials and, while ensuring subgrade dynamic stability, not only lowers engineering costs but also aligns with the low-carbon and emission-reduction trend in transportation infrastructure development, indicating strong potential for broader application. Declarations Funding This research was funded by the following projects: the National Natural Science Foundation of China (Project No. 52068063); the Scientific Research Project of "Young Doctor Fund" of Gansu Provincial Universities (Project No. 2026QB-082); the Innovation Fund Project of Gansu Provincial Universities (2022A-107, 2025A-150); the Special Project for the Construction of Scientific Research and Innovation Platforms of Tianshui Normal University in 2023 (PTJ2023-07); and the School - level Project of Lanzhou Bowen University of Science and Technology (2025BWKY029). Data availability The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request. Author contributions Xiangming Lv, Chongliang Luo, Fei Ma, Baocheng Wang and Xin Wang wrote the main manuscript text. All authors reviewed the manuscript. Competing interests The authors declare no competing interests. References Liu, Y., Xu, Y. & Liu, Y. Population growth and spatiotemporal differentiation in the Loess Plateau region since 2000. Prog Geogr. 31 , 156–166. https://doi.org/10.11820/dlkxjz.2012.02.004 (2012). (in Chinese). Tian, Y. et al. Deformation mechanism and evolutionary process of the Tianshui forging machine plant landslide in Gansu. J. Geomech. 21 , 298–308 (2015). (in Chinese). Zhao, C. et al. Analysis of microtremor characteristics of slopes in loess areas of Tianshui City. J. Eng. Geol. 24 (S1), 478–488. https://doi.org/10.13544/j.cnki.jeg.2016.s1.069 (2016). (in Chinese). Lei, T., Wu, Z. & Chen, T. 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Mechanism of composite improvement of loess based on quantitative analysis of microstructure and mechanical strength. Constr. Build. Mater. 379 , 131215. https://doi.org/10.1016/j.conbuildmat.2023.131215 (2023). Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 06 Apr, 2026 Reviews received at journal 02 Apr, 2026 Reviews received at journal 27 Mar, 2026 Reviewers agreed at journal 26 Mar, 2026 Reviewers agreed at journal 25 Mar, 2026 Reviewers agreed at journal 24 Mar, 2026 Reviewers agreed at journal 20 Mar, 2026 Reviewers invited by journal 20 Mar, 2026 Editor assigned by journal 20 Mar, 2026 Editor invited by journal 19 Mar, 2026 Submission checks completed at journal 12 Mar, 2026 First submitted to journal 11 Mar, 2026 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-8980134","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":610279829,"identity":"9aa6299b-0faf-4832-be0a-d048b6687dc4","order_by":0,"name":"Xiangming Lv","email":"","orcid":"","institution":"Tianshui Normal University","correspondingAuthor":false,"prefix":"","firstName":"Xiangming","middleName":"","lastName":"Lv","suffix":""},{"id":610279831,"identity":"56b4be91-c60e-4c01-955a-5f424b86f89c","order_by":1,"name":"Chongliang 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\u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eE\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and stress state for composite-improved loess (curing age: 7 days)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/810ad4cae5b65eb848c9a6d7.jpg"},{"id":105410106,"identity":"cc320848-4d0f-44c9-8434-5b6898eeeb7b","added_by":"auto","created_at":"2026-03-25 17:15:13","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":57007,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRelationship between \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eE\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eθ\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e for untreated loess (curing age: 7 days)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/93f8acdd5d74eea921bed990.jpg"},{"id":105565956,"identity":"b6bc490d-03f1-45e6-b4c7-8b2c09ac5f3e","added_by":"auto","created_at":"2026-03-27 12:54:53","extension":"jpg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":55818,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eRelationship between \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eE\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e and \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eθ\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e for composite-improved loess (curing age: 7 days)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/921579029efb72fabd6a59ad.jpg"},{"id":105410098,"identity":"279372a3-6ecb-41af-8e07-546d31498b3c","added_by":"auto","created_at":"2026-03-25 17:15:12","extension":"jpg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":57260,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePredicted and measured \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eE\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e for untreated loess\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"12.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/d7a1de58e08c7c24473cce68.jpg"},{"id":105565744,"identity":"e48dd3c7-c3c9-4858-a522-3d2d23754385","added_by":"auto","created_at":"2026-03-27 12:54:16","extension":"jpg","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":66672,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePredicted and measured \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eE\u003c/strong\u003e\u003c/em\u003e\u003csub\u003e\u003cstrong\u003ed\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e for composite-improved loess\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"13.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/b50be180cea9de433b9c5f0d.jpg"},{"id":105410100,"identity":"3f007b34-667a-41d2-ad61-38467a037eb5","added_by":"auto","created_at":"2026-03-25 17:15:12","extension":"jpg","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":257558,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eProcessing results of loess SEM microstructural images.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"14.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/a86d4f485cf316a6287ad9fc.jpg"},{"id":105410102,"identity":"64d92df9-2220-4983-876b-c2e5f2a5637f","added_by":"auto","created_at":"2026-03-25 17:15:12","extension":"jpg","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":472492,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSEM microstructures of loess composite-improved with EFCA and cement at different curing ages.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"15.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/65191bceb4880bdfd1643c69.jpg"},{"id":105410103,"identity":"26bc72c0-831f-4ad9-bc70-8b9ea983c6e1","added_by":"auto","created_at":"2026-03-25 17:15:12","extension":"jpg","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":98831,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eD\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e of microstructural pores in loess\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"16.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/3a4731a48ebee621268fb4eb.jpg"},{"id":105410101,"identity":"75f93fa1-1f2e-4c35-8207-3554dcf2d509","added_by":"auto","created_at":"2026-03-25 17:15:12","extension":"jpg","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":236964,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003eD\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e of microstructural pores in loess composite-improved with EFCA and cement at different curing ages.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"17.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/623c581ec4a268619ea0ce80.jpg"},{"id":105410104,"identity":"d4ae7397-e576-4826-8cc8-d4a96f50f310","added_by":"auto","created_at":"2026-03-25 17:15:12","extension":"jpg","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":271644,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic illustration of the cross-scale physical-mechanical mechanism for the enhanced dynamic resilient performance of composite-improved loess.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"18.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/c4603c26e1298c054f93375d.jpg"},{"id":105570062,"identity":"f85d1564-6f55-4d63-846a-0f025d168b02","added_by":"auto","created_at":"2026-03-27 13:14:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4196471,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8980134/v1/a235fcdc-1394-4533-8dc3-0059b5aec233.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Dynamic Response and Pore Evolution Mechanism of Composite Improved Loess Using an Eco-Friendly Curing Agent and Cement: A Macroscopic and Microscopic Experimental Study","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eLoess in China covers an area of 640,000 km\u0026sup2; and is widely distributed in the northwestern region \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. In particular, loess in the Tianshui area of Gansu Province, characterized by typical macropores, pronounced collapsibility, and susceptibility to disintegration, poses major challenges to transportation projects associated with the Belt and Road Initiative and often triggers hazards such as road collapses and slope slides \u003csup\u003e[\u003cspan additionalcitationids=\"CR3 CR4 CR5\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. To address the shortage of high-quality subgrade fill materials in loess regions, improving in situ undisturbed loess to enhance its engineering performance is a key approach to overcoming regional bottlenecks in transportation construction \u003csup\u003e[\u003cspan additionalcitationids=\"CR8\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. Moreover, subgrades are subjected to complex cyclic traffic-induced dynamic loads during long-term service; therefore, an in-depth investigation of the dynamic characteristics and dynamic constitutive response of improved loess is of decisive importance for ensuring the long-term safety and stability of subgrade engineering.\u003c/p\u003e \u003cp\u003eAt present, loess improvement has become a research hotspot in the academic community. Conventional chemical stabilizers (e.g., cement, lime, and fly ash) can substantially increase soil strength, yet they commonly face technical bottlenecks such as sensitivity to shrinkage cracking and slow early-age strength development \u003csup\u003e[\u003cspan additionalcitationids=\"CR11 CR12\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Meanwhile, the production of traditional silicate-based cementitious materials involves very high energy consumption and carbon emissions, which runs counter to the current engineering trend toward \u0026ldquo;green and low-carbon construction\u0026rdquo;. With advances in polymer materials science and the concept of sustainable construction, novel curing agents have emerged in geotechnical engineering \u003csup\u003e[\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. Improving loess fill using an EFCA not only enables the use of locally available materials to reduce engineering costs but also reduces the consumption of traditional binders such as lime and cement, which is of great significance for achieving carbon-emission reduction. Ma et al. \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e showed that a polymer-based curing agent (PBCA) can effectively improve the water sensitivity and densification of loess through ion exchange and by altering the electrical double layer (EDL) structure. However, a single novel curing agent often suffers from limitations such as a restricted degree of improvement or a lack of rigid skeletal support \u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. Therefore, exploring the composite improvement mechanism of a novel EFCA combined with conventional cement-so as to account for both rigid skeletal strength and flexible crack resistance while achieving a balance between economy and environmental protection-has become a new trend in the development of subgrade improvement materials.\u003c/p\u003e \u003cp\u003eIn geotechnical dynamic evaluation, the dynamic resilient modulus (\u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e) is a key parameter used to characterize the elastic recovery capacity of subgrade soils under cyclic loading. In recent years, extensive studies have investigated \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e for various soil types. Based on dynamic triaxial test (DTT), Li et al. \u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e examined the effects of compaction degree, water content, and stress state on the \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of Malan loess. Liu Weizheng et al. \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e introduced matric suction into a prediction model to analyze the evolution of \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e in compacted red clay. Gao et al. \u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e developed an \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e prediction model for subgrade clay by considering loading frequency. Qian et al. \u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e conducted DTT on soil-rock mixture materials, analyzed the influences of mean stress, octahedral shear stress (OSS), and rock content on \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e, and further established relationships between the dynamic resilient modulus and factors such as rock content and stress state using a new prediction model. Dong Cheng et al. \u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e developed a three-parameter composite dynamic constitutive model for cement-improved high liquid limit clay. Although substantial progress has been made in macro-scale prediction using dynamic constitutive models (e.g., the NCHRP 1-28A model and the N\u003csub\u003ei\u003c/sub\u003e model that couple mean stress and deviatoric stress), research on the dynamic resilient response and accurate prediction of composite-improved collapsible loess under complex stress states remains relatively limited.\u003c/p\u003e \u003cp\u003eMore importantly, the leap in the macroscopic dynamic performance of improved soils is essentially the macroscopic manifestation of microstructural pore reconstruction and the evolution of inter-particle contact relationships. In recent years, multi-scale microstructural testing techniques have been progressively introduced to investigate the mechanisms underlying the engineering behavior of loess. For example, Gao et al. \u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e revealed the distinctive loose particle skeleton and macropore distribution of natural Malan loess and identified these features as fundamental causes of collapsibility and mechanical instability. Using SEM, Ni et al. \u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e quantitatively analyzed the microstructural evolution of particle rearrangement and pore reduction in Malan loess under dynamic compaction. Regarding the mechanism of improved loess, Ma et al. \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e combined SEM, nuclear magnetic resonance (NMR), and fractal dimension (FD) analyses to quantitatively evaluate the transformation pathway from connected macropores to isolated micropores in polymer-cement composite-improved loess, confirming that composite improvement can significantly reduce pore complexity. However, existing studies on microstructural geometric fractal characteristics have mostly focused on interpreting static indices such as UCS, and systematic reports on cross-scale macro-micro coupling mechanisms that integrate the quantitative evolution of pore fractal characteristics with the macroscopic \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e and dynamic constitutive models remain scarce. How, precisely, do the coupled hydration/polymerization reactions between an eco-friendly curing agent and cement reconstruct the fragile connected pore network within loess? How does the evolution of microstructural fractal geometry, in turn, drive the soil to exhibit higher deformation-resistant stiffness under cyclic dynamic loading? These issues remain theoretical blind spots that constrain the wider application of eco-friendly composite stabilization technologies.\u003c/p\u003e \u003cp\u003eAccordingly, this study targets collapsible loess from a distressed road section in Gangu County, Gansu Province, and proposes composite stabilization using an EFCA and P\u0026middot;O 42.5 Portland cement. UCS tests and DTT were conducted to determine the optimal composite mix proportion, to investigate the evolution of \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e under different confining pressures and dynamic stress levels, and to comparatively evaluate the predictive accuracy of the NCHRP 1-28A and N\u003csub\u003ei\u003c/sub\u003e models. On this basis, SEM observations, ImageJ-based image processing, and microstructural pore fractal theory were integrated to quantitatively analyze the evolution of the internal pore structure of loess before and after composite stabilization. This study aims to elucidate the intrinsic mechanism underlying the enhanced dynamic performance of composite-improved loess through a full-chain framework of \u0026ldquo;macroscopic mechanical prediction-microscopic quantitative characterization-physical cementation mechanism\u0026rdquo;, thereby providing solid theoretical support and scientific evidence for durability-oriented design and low-carbon construction of subgrade engineering in loess regions.\u003c/p\u003e"},{"header":"2 Experimental program","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Testing of physical and mechanical properties\u003c/h2\u003e \u003cp\u003eThe soil samples were collected from the subgrade and slope of a severely distressed section of the X503 highway reconstruction project in Gangu County, Gansu Province, at a sampling depth of 0.5-3 m. The specimens were yellowish-brown and exhibited typical structural looseness, with well-developed pores and pronounced vertical joints. Laboratory tests were conducted in accordance with the Test Methods of Soils for Highway Engineering (JTG 3430\u0026thinsp;\u0026minus;\u0026thinsp;2020) \u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e, and the main physical and mechanical parameters are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The test results show that the collapsibility coefficient (\u003cem\u003eδ\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e) is greater than 0.015; according to the specification \u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e, the soil is identified as a low-plasticity silt and collapsible loess, classified as poorly graded (\u003cem\u003eC\u003c/em\u003e\u003csub\u003eu\u003c/sub\u003e = 8.24, \u003cem\u003eC\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e = 0.64), with a dominant sand-size fraction (0.075-2 mm) accounting for more than 50%. Therefore, this undisturbed loess requires stabilization when used as subgrade fill.\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\u003ePhysical and mechanical properties of the soil samples\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eρ\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e(g/cm\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eω\u003c/em\u003e\u003csub\u003eopt\u003c/sub\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003ew\u003c/em\u003e\u003csub\u003eL\u003c/sub\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003ew\u003c/em\u003e\u003csub\u003ep\u003c/sub\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eδ\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eC\u003c/em\u003e\u003csub\u003eu\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003eC\u003c/em\u003e\u003csub\u003ec\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e13.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e23.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10.41\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e8.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Experimental scheme\u003c/h2\u003e \u003cp\u003eThe test plan includes the UCS test plan and the \u003cb\u003eE\u003c/b\u003e\u003csub\u003e\u003cb\u003ed\u003c/b\u003e\u003c/sub\u003e test plan. The specific introduction is as follows:\u003c/p\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 UCS test scheme\u003c/h2\u003e \u003cp\u003eIn this study, an EFCA (KJD-II ionic liquid soil curing agent; pH\u0026thinsp;=\u0026thinsp;11.26; density\u0026thinsp;=\u0026thinsp;1.393 g\u0026middot;cm⁻\u0026sup3;; solid content\u0026thinsp;=\u0026thinsp;47.43%) and P\u0026middot;O 42.5 silicate Portland cement (PC) were used to composite-stabilize collapsible loess. To determine the optimal composite proportion, an orthogonal experimental design (OED) was adopted, with EFCA dosages of 0%, 0.01%, 0.015%, 0.02%, and 0.025%, cement dosages of 4%, 6%, and 8%, and curing ages of 7 and 28 days. According to the Technical Specification for Expressway Subgrade Construction in Loess Areas \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e and considering the actual engineering conditions, the specimen preparation parameters were controlled at a compaction degree of 94% and a water content of 13.82% (optimum water content). The dosages of each component in the loess-EFCA-cement-water system were determined by mass-ratio calculations; after thorough mixing, standard specimens with a diameter of \u003cem\u003eΦ\u003c/em\u003e150 mm were prepared using the static compaction method. After curing at 20\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u0026deg;C, UCS tests were performed using an electronic universal testing machine (Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2 \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e test scheme\u003c/h2\u003e \u003cp\u003eAfter the optimal composite stabilization scheme was selected based on UCS results, \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e was measured for specimens with the optimal mix proportion using a British GDS system. The system integrates a dynamic stress loading module (maximum load: 20 kN; frequency: 0\u0026ndash;10 Hz), a confining-pressure control unit, and a data acquisition device (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Considering the stress state and typical stress levels of highway subgrades in China, the lateral pressure acting on subgrade soils is relatively small (generally 0\u0026ndash;60 kPa), and the dynamic stress experienced by most subgrade soils does not exceed 110 kPa \u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. Accordingly, in conjunction with the relevant provisions of JTG 3430\u0026thinsp;\u0026minus;\u0026thinsp;2020 \u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e, the loading sequence was set as listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, comprising a preloading stage and an \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e-testing stage, and an HWF was applied to approximate traffic loading at 10 Hz with a loading duration of 0.1 s and a rest duration of 0.9 s. In the preloading stage, a pre-confining pressure of 30 kPa was applied for 1000 load cycles, whereas in the \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e-testing stage, each loading condition was applied for 100 cycles.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDynamic triaxial test loading sequence\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStress loading path\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003e\u0026#120590;\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e(kPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003e\u0026#120590;\u003c/em\u003e\u003csub\u003e\u0026#119888;\u003c/sub\u003e(kPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003e\u0026#120590;\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e(kPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eθ\u003c/em\u003e(kPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eτ\u003c/em\u003e\u003csub\u003eoct\u003c/sub\u003e(kPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003eN\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0-Preloading stage\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e145\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e25.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e1000\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e210\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e14.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e165\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e14.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e14.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e14.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e235\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e25.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e190\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e25.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e145\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e25.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e25.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e255\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e35.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e210\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e35.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e165\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e35.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e35.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e105\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e285\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e49.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e105\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e240\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e49.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e105\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e195\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e49.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e105\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e150\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e49.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e100\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\u003e注: \u0026#120590;\u003csub\u003e3\u003c/sub\u003e denotes the confining pressure; \u0026#120590;\u003csub\u003e\u0026#119888;\u003c/sub\u003e denotes the contact stress; \u003cem\u003e\u0026#120590;\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e denotes the dynamic stress; \u003cem\u003eθ\u003c/em\u003e denotes the mean stress; \u003cem\u003eτ\u003c/em\u003e\u003csub\u003eoct\u003c/sub\u003e denotes the octahedral shear stress.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 UCS test results\u003c/h2\u003e \u003cdiv id=\"Sec9\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1 Failure patterns of specimens\u003c/h2\u003e \u003cp\u003eThe failure patterns of specimens in the UCS tests are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e3\u003c/span\u003e, and the typical feature is a shear failure plane at approximately 45\u0026deg;. The tests show that, at the initial loading stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), the specimen remains in the elastic deformation regime under a low stress level, the stress\u0026ndash;strain curve increases linearly, and no surface damage is observed. As the axial stress increases to the peak value (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), cracks initiate at the specimen ends, indicating a transition from elastic to elasto-plastic behavior. With continued loading (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), the cracks propagate toward the core and are accompanied by local spalling, and the stress exhibits a nonlinear decay as damage accumulates. At the final loading stage (Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), microcracks rapidly coalesce to form a macroscopic slip surface, and the specimen exhibits a typical hourglass-shaped shear failure pattern (shear angle\u0026thinsp;\u0026asymp;\u0026thinsp;45\u0026deg;), at which point the stress enters the residual stage and drops sharply. This evolution process clearly reflects the progressive mechanical response of composite-improved loess, from linear elastic deformation to the accumulation of plastic damage and ultimately to macroscopic instability and failure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2 UCS analysis of composite-improved loess\u003c/h2\u003e \u003cp\u003eBased on the UCS results, the UCS of untreated loess at 7 and 28 days is 0.41 MPa and 0.43 MPa, respectively, which is substantially lower than the minimum strength threshold of 0.5 MPa specified for subgrade fill in the Design Specification for Expressway Subgrades in Loess Areas of Gansu Province (JTG/T D31-05-2017) \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e, confirming that the undisturbed loess must be modified before it can be used in subgrade engineering. The tests show that, after composite improvement using EFCA (dosage: 0-0.025%) and P\u0026middot;O 42.5 cement (dosage: 4\u0026ndash;8%), all specimens achieve 7-day strengths of 0.96\u0026ndash;1.99 MPa and 28-day strengths of 1.34\u0026ndash;2.85 MPa, representing increases of 134\u0026ndash;385% and 212\u0026ndash;563% relative to untreated loess, respectively, fully meeting the requirements of the specification \u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. The analysis indicates that the highly alkaline environment provided by the KJD-II curing agent promotes ion exchange and cementation of clay minerals in loess, while its high solid content supplies additional cementitious substances, thereby enhancing inter-particle bonding. In addition, the hydration of P\u0026middot;O 42.5 silicate cement produces hydration products such as calcium silicate hydrate (C-S-H) gel and ettringite (AFt), which fill soil pores and form a rigid skeletal structure \u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. Moreover, the combined action of the curing agent and cement further optimizes the soil microstructure and, through a dual cementation effect, significantly improves soil compactness and integrity, thereby effectively suppressing collapsibility and enhancing its mechanical performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and \u003cspan refid=\"Fig22\" class=\"InternalRef\"\u003e7\u003c/span\u003e, when the P\u0026middot;O 42.5 cement dosage increased from 4% to 6%, the 7-day and 28-day UCS increased by 119.51% and 62.79%, respectively, and the corresponding marginal benefit rate (MBR) was 59.76%/% and 31.40%/% (MBR\u0026thinsp;=\u0026thinsp;Δ strength growth rate/Δ dosage). When the dosage increased further to 8%, the increases decreased to 78.05% and 16.28%, and the MBR decreased to 39.02%/% and 8.14%/%, showing a pronounced diminishing-return trend with attenuation rates of 34.7% and 74.1%, respectively. Specimen observations indicate that when the cement dosage exceeds 6%, the density of drying-shrinkage cracks on the specimen surface increases, and the engineering cost rises markedly. In the 6% cement system, at 7 days, as the EFCA dosage increased from 0 to 0.01%, 0.015%, 0.02%, and 0.025%, the strength growth rates were 17.0%, 17.1%, 21.95%, and 9.76%, respectively, and the corresponding MBR values were 7.0, 14.0, 18.0, and 8.0 MPa/% (MBR\u0026thinsp;=\u0026thinsp;Δ strength/Δ dosage), showing a clear increase-then-decrease pattern with the peak MBR at an EFCA dosage of 0.02%. The 28-day data also verify this pattern. This dosage minimizes the increase in material cost, while the 28-day UCS reaches 2.51 MPa, which is 5.02 times the specified value (0.5 MPa), and it also keeps the material cost controllable, consistent with the principle of balancing specification-based strength requirements and economy. Therefore, a P\u0026middot;O 42.5 cement dosage of 6% and an EFCA dosage of 0.02% constitute the optimal composite stabilization scheme.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.2 DTT results\u003c/h2\u003e \u003cp\u003e \u003cem\u003eE\u003c/em\u003e \u003csub\u003ed\u003c/sub\u003e is a key index for characterizing the dynamic response of geomaterials, and it is physically defined as the ratio of axial dynamic stress to resilient strain, which directly reflects the elastic recovery capacity of the material under cyclic loading. A larger \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e indicates a stronger elastic load-bearing capacity of the soil. Based on the DTT results, the data from the last five cycles under each loading sequence were extracted, \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e was calculated according to Eq.\u0026nbsp;(\u003cspan refid=\"Equ7\" class=\"InternalRef\"\u003e1\u003c/span\u003e), and the average over the five cycles was taken as the \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e value of the subgrade fill \u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${E_{\\text{d}}}=\\frac{{{\\sigma _{\\text{d}}}}}{{{\\varepsilon _{\\text{r}}}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e is the dynamic resilient modulus of the soil (kPa), \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e is the cyclic stress (kPa), and \u003cem\u003eε\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e is the axial strain (%).\u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Effect of stress state on \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e\u003c/h2\u003e \u003cp\u003eAfter loess was composite-stabilized with 6% P\u0026middot;O 42.5 silicate cement and 0.02% EFCA, \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e increased markedly, and \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e was strongly affected by \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e and \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e. For untreated loess (7-day curing) at \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 kPa, when σ₃ increased from 15 to 60 kPa, \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e increased from 85.36 to 102.36 MPa (19.9% increase); however, the stage-wise marginal gain rate (Δ \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e/Δ\u003cem\u003eσ\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e) first increased and then decreased with \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e (15\u0026rarr;30\u0026rarr;45\u0026rarr;60 kPa) (0.24\u0026rarr;0.59\u0026rarr;0.31 MPa/kPa), indicating that the lateral confinement effect was most pronounced at a relatively low confining pressure of 30 kPa. When \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 kPa, increasing \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e from 30 to 105 kPa reduced \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e from 88.91 to 63.87 MPa (28.2% decrease), with an attenuation gradient of 0.33 MPa/kPa. In contrast, the composite-improved loess exhibited a pronounced enhancement effect; at σ₃ = 60 kPa and \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 kPa, \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e reached 826.49 MPa, which is 8.07 times that of untreated loess. Dynamic-stress sensitivity analysis shows that for the composite-improved loess at \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 kPa, increasing \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e from 30 to 105 kPa led to a 20.9% decrease in \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e (723.31\u0026rarr;572.36 MPa), with an attenuation gradient of 2.01 MPa/kPa. The analysis indicates that as σ₃ increases, the lateral confinement on the specimen is enhanced, its ability and stiffness to resist resilient deformation increase, the resilient strain decreases, and \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e increases. In contrast, increasing \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e weakens the soil\u0026rsquo;s resistance to resilient deformation, leading to an increase in resilient strain and a decrease in \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e, which is consistent with the findings reported in Refs. \u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe responses of \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e to mean stress \u003cem\u003eθ\u003c/em\u003e differ markedly between untreated loess and composite-improved loess (6% P\u0026middot;O 42.5 cement\u0026thinsp;+\u0026thinsp;0.02% EFCA). For untreated loess at \u003cem\u003eσ\u003c/em\u003e₃ = 30 kPa, when \u003cem\u003eθ\u003c/em\u003e increased from 75 to 150 kPa, \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e decreased from 85.36 to 56.14 MPa (a 34.2% reduction), with an attenuation gradient of 0.39 MPa/kPa (attenuation gradient\u0026thinsp;=\u0026thinsp;Δ \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e /Δ\u003cem\u003eθ\u003c/em\u003e), and the marginal gain rate increased with increasing σ₃ (from 4.2% to 4.8% as σ₃ increased from 15 to 60 kPa). In contrast, the composite-improved loess exhibits superior performance: at a high confining pressure (\u003cem\u003eσ\u003c/em\u003e₃ = 60 kPa), \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e reaches 826.49 MPa, which is 8.07 times that of untreated loess, and the confining-pressure gain rate remains stable (average gain rate\u0026thinsp;=\u0026thinsp;Δ \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e /Δ\u003cem\u003eσ\u003c/em\u003e₃ \u0026asymp; 5.1 MPa/kPa). This phenomenon is attributed to the synergistic action of cement hydration products (C-S-H gel) and the silica-alumina gel from the curing agent, which forms an interwoven rigid-flexible structure that effectively suppresses stiffness degradation induced by increasing \u003cem\u003eθ\u003c/em\u003e, and the cemented structure effectively mitigates the decline in the confining-pressure gain rate.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4 Prediction of Ed models","content":"\u003cp\u003eIn geotechnical engineering, prediction models for the \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of subgrade soils are commonly classified into three categories according to the stress variables considered: models considering only shear effects, models considering only confinement effects, and composite models considering both shear and confinement effects. Given that Ed increases with increasing confining pressure and decreases with increasing shear stress, it indicates that \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e is jointly influenced by mean stress and shear stress or deviatoric stress. Therefore, incorporating the effects of mean stress and shear stress into an Ed model can more realistically and comprehensively reflect the mechanical behavior of subgrade soils.\u003c/p\u003e \u003cp\u003eFor calculating \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of subgrade soils, prediction models that simultaneously consider the effects of shear stress and confining stress are particularly favored. Such models can not only capture the variation of subgrade soils under multiple levels of confining pressure but also highlight the role of shear stress. Under the loading conditions and typical stress levels of highway subgrades in China, the Highway Subgrade Design Specification: JTG 3430\u0026thinsp;\u0026minus;\u0026thinsp;2020 recommends using the NCHRP 1-28A model for prediction. This model comprehensively considers the effects of octahedral shear stress and the lateral confinement associated with mean stress on Ed, and it can realistically reflect the influence of complex stress states on \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e in subgrades \u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. In addition, the N\u003csub\u003ei\u003c/sub\u003e model establishes relationships between confining pressure and deviatoric stress and subgrade soils, providing a more direct and comprehensive approach to describing the Ed characteristics of subgrade soils \u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. The application of the above models is of great significance for accurately predicting Ed of subgrade soils.\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eNCHRP 1-28A model:\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$${E_{\\text{d}}}={k_1}{{\\text{p}}_{\\text{a}}}{\\left( {\\frac{\\theta }{{{{\\text{p}}_{\\text{a}}}}}} \\right)^{{k_2}}}{\\left( {\\frac{{{\\tau _{{\\text{oct}}}}}}{{{{\\text{p}}_{\\text{a}}}}}+1} \\right)^{{k_3}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\theta ={\\sigma _1}+{\\sigma _2}+{\\sigma _3}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$${\\tau _{{\\text{oct}}}}=\\frac{{\\sqrt {{{\\left( {{\\sigma _1} - {\\sigma _2}} \\right)}^2}+{{\\left( {{\\sigma _1} - {\\sigma _3}} \\right)}^2}+{{\\left( {{\\sigma _2} - {\\sigma _3}} \\right)}^2}} }}{3}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eN\u003csub\u003ei\u003c/sub\u003e model:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$${E_{\\text{d}}}={k_1}{{\\text{p}}_{\\text{a}}}{\\left( {\\frac{{{\\sigma _3}}}{{{{\\text{p}}_{\\text{a}}}}}+1} \\right)^{{k_2}}}{\\left( {\\frac{{{\\sigma _{\\text{d}}}}}{{{{\\text{p}}_{\\text{a}}}}}+1} \\right)^{{k_3}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere k\u003csub\u003e1\u003c/sub\u003e, k\u003csub\u003e2\u003c/sub\u003e, and k\u003csub\u003e3\u003c/sub\u003e are model fitting parameters, the values of which are related to the physical and mechanical state of the subgrade soil; P\u003csub\u003ea\u003c/sub\u003e is the standard atmospheric pressure with a value of 101.325 kPa; \u003cem\u003eθ\u003c/em\u003e is the mean stress (kPa); τ\u003csub\u003eoct\u003c/sub\u003e is the octahedral shear stress (kPa); and \u003cem\u003eσ\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e is the intermediate principal stress of the soil (kPa).\u003c/p\u003e \u003cp\u003eBased on the Ed test data of untreated loess and composite-improved loess (6% P\u0026middot;O 42.5 cement\u0026thinsp;+\u0026thinsp;0.02% EFCA) from Gangu, Gansu Province, this study performed parameter fitting for the NCHRP 1-28A and N\u003csub\u003ei\u003c/sub\u003e models using nonlinear regression analysis in SPSS. The fitted model parameters are listed in Tables\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, and the coefficients of determination (\u003cem\u003eR\u003c/em\u003e\u0026sup2;) for the fitted Ed are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e and \u003cspan refid=\"Fig25\" class=\"InternalRef\"\u003e13\u003c/span\u003e, respectively. The results show that the \u003cem\u003eR\u003c/em\u003e\u0026sup2; values of the NCHRP 1-28A model for fitting \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of untreated loess and composite-improved loess are 0.960 and 0.950, respectively. Correspondingly, the \u003cem\u003eR\u003c/em\u003e\u0026sup2; values of the N\u003csub\u003ei\u003c/sub\u003e model are 0.955 and 0.969. Given that the R\u0026sup2; values of both models exceed 0.95, both the NCHRP 1-28A and N\u003csub\u003ei\u003c/sub\u003e models can effectively simulate the variation trend of \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e for untreated loess and composite-improved loess. This finding provides an important theoretical basis for the selection and design of subgrade fill materials in loess regions.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFitted parameters and fitting results of the NCHRP 1-28A model for \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFill material type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCuring age\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ek\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ek\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ek\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUntreated loess\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.108\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.201\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-1.713\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComposite-improved loess\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e7.756\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.382\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-1.44\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\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFitted parameters and fitting results of the N\u003csub\u003ei\u003c/sub\u003e model for \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFill material type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCuring age\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ek\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ek\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ek\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUntreated loess\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.982\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.534\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-0.803\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComposite-improved loess\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e5.997\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.997\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e-0.481\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\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo accurately evaluate the prediction accuracy of the NCHRP 1-28A and N\u003csub\u003ei\u003c/sub\u003e models for \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of untreated loess and composite-improved loess, this study adopted the relative error formula as the evaluation metric. Relative error quantifies the percentage of relative deviation between the model-predicted values and the measured observations, providing an effective measure of model prediction accuracy \u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e. The relative error is defined as:\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$${S_{{\\text{err}}}}=\\sqrt {\\frac{{\\mathop \\sum \\limits_{{i=1}}^{n} {{({N_{p,i}} - {N_{m,i}})}^2}}}{{\\mathop \\sum \\limits_{{i=1}}^{n} N_{{m,i}}^{2}}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eS\u003c/em\u003e\u003csub\u003eerr\u003c/sub\u003e is the relative error; \u003cem\u003eN\u003c/em\u003e\u003csub\u003ep,i\u003c/sub\u003e is the \u003cem\u003ei-\u003c/em\u003eth predicted value (kPa); and \u003cem\u003eN\u003c/em\u003e\u003csub\u003em,i\u003c/sub\u003e is the \u003cem\u003ei-\u003c/em\u003eth measured value (kPa).\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e presents a detailed comparison of the relative errors of the NCHRP 1-28A and N\u003csub\u003ei\u003c/sub\u003e models in predicting \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e for untreated loess and composite-improved loess. Analysis of the data in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows that, for both loess types, the mean relative error of the N\u003csub\u003ei\u003c/sub\u003e model is slightly lower than that of the NCHRP 1-28A model. The comparison further indicates that the mean relative error of the N\u003csub\u003ei\u003c/sub\u003e model is markedly lower than that of the NCHRP 1-28A model. This is mainly attributed to the formation of a rigid-flexible cemented network within the composite-improved loess, which makes it more sensitive to variations in deviatoric stress. By more directly incorporating the coupling term between confining pressure and deviatoric stress, the N\u003csub\u003ei\u003c/sub\u003e model accurately captures the stiffness-degradation characteristics of this cemented structure under cyclic shearing, thereby exhibiting better applicability for evaluating the dynamic mechanical behavior of improved loess. Therefore, the N\u003csub\u003ei\u003c/sub\u003e model shows higher applicability in describing the dynamic mechanical behavior of subgrades composed of untreated loess and composite-improved loess. This finding provides valuable reference for guiding loess subgrade engineering design in the Tianshui region and helps improve the scientific rigor and accuracy of engineering design.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003ePrediction accuracy evaluation of the NCHRP 1-28A and N\u003csub\u003ei\u003c/sub\u003e models\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFill material type\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCuring age\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNCHRP1-28A models\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN\u003csub\u003ei\u003c/sub\u003e models\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eUntreated loess\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.031\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.032\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eComposite-improved loess\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e7d\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.033\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.026\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e \u003cp\u003eMean relative error\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.032\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.029\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"5 Microstructural analysis of improved loess","content":"\u003cp\u003eImprovements in macroscopic mechanical performance are essentially the outcome of the coupled evolution of inter-particle contact modes, cementation continuity, and the geometric characteristics of the pore network within the soil \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e. To elucidate the micro-mechanisms underlying the significant increase in Ed of loess composite-improved with an eco-friendly curing agent (KJD-II) and cement, SEM observations were conducted, and ImageJ-based image processing together with microstructural pore fractal theory was employed to qualitatively and quantitatively analyze the inter-particle contact characteristics, pore morphology, and pore-structure complexity of untreated loess and composite-improved loess at different curing ages.\u003c/p\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e5.1 Qualitative evolution of loess microstructure before and after composite stabilization\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig26\" class=\"InternalRef\"\u003e14\u003c/span\u003e presents the SEM images of untreated loess and the corresponding ImageJ processing results (original image \u0026rarr; pore identification \u0026rarr; binarization); overall, the untreated loess exhibits a typical granular skeletal structure, with relatively clean surfaces of silt and clay particles, predominantly point\u0026ndash;point or point\u0026ndash;surface contacts between particles, and a lack of effective cementing substances to form a continuous skeletal framework (Fig.\u0026nbsp;\u003cspan refid=\"Fig26\" class=\"InternalRef\"\u003e14\u003c/span\u003ea). Meanwhile, its pore distribution is markedly heterogeneous, featuring a wide range of pore sizes, irregular boundaries, and numerous connected channels, showing a structural characteristic of a \u0026ldquo;loose skeleton with coexisting macropores and overhead pores\u0026rdquo; (Fig.\u0026nbsp;\u003cspan refid=\"Fig26\" class=\"InternalRef\"\u003e14\u003c/span\u003ec). Such a fragile skeletal structure is highly prone to stress concentration, particle sliding, and structural yielding under external dynamic stress, which is the fundamental reason for its low \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003e shows the SEM morphologies of composite-improved loess at different curing ages. Overall, with increasing curing age, both the number of pores and the pore-size scale continuously decrease, and soil particles gradually transform from a loose packing state to a cemented dense structure, exhibiting a typical evolution pathway of \u0026ldquo;early formation-mid-term filling-late densification\u0026rdquo;. ①1 day: Compared with untreated loess, initial cementation and filling effects appear, some larger connected pores are segmented, and the pore distribution tends to become more uniform (Fig.\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003ea). This stage corresponds to the initial period of cement hydration and curing-agent participation, during which reaction products preferentially form bridging and coating at particle contact points, thereby beginning to alter the original skeleton connectivity dominated by point contacts. ②.7 days: Cementation and aggregation become more evident, inter-particle connections tend to be continuous, and pore connectivity further decreases (Fig.\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003eb). As hydration/reaction proceeds, the filling of skeletal pores and the coating of particles by cementing products are strengthened, promoting a transition of the structure from \u0026ldquo;point contact/point\u0026ndash;surface contact\u0026rdquo; to \u0026ldquo;surface\u0026ndash;surface contact\u0026rdquo;. ③.14 days: Pores are further refined, the proportion of small pores increases, and the overall structure becomes denser (Fig.\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003ec). This stage typically corresponds to the continued deposition of reaction products and effective filling of skeletal pores, during which the pore network transforms from \u0026ldquo;connected\u0026rdquo; to \u0026ldquo;discrete\u0026rdquo;. ④.28 days: Macropores largely disappear, pore boundaries become relatively smooth, pore size and connectivity further decrease, and the structure tends toward a stable dense state (Fig.\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003ed). The reaction products of the curing agent and cement effectively fill inter-particle pores and coat particles, promoting the formation of aggregates and leading to tighter particle packing, thereby increasing structural compactness and providing microstructural support for the improvement of macroscopic strength and dynamic performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e5.2 Quantitative analysis of microstructural pore fractal characteristics of composite-improved loess\u003c/h2\u003e \u003cp\u003eAlthough qualitative SEM observations can visually reflect the evolution of soil structure, they are insufficient for precisely describing the complexity of the spatial pore distribution and the densification process. Therefore, the fractal dimension \u003cem\u003eD\u003c/em\u003e was introduced to quantitatively evaluate microstructural pore morphology: ImageJ was used to identify the pore equivalent perimeter P and equivalent area \u003cem\u003eA\u003c/em\u003e, a double-logarithmic plot of log \u003cem\u003eP\u003c/em\u003e versus log \u003cem\u003eA\u003c/em\u003e was constructed, and \u003cem\u003eD\u003c/em\u003e was obtained by fitting to calculate the pore fractal dimension of the soil before and after composite stabilization. The magnitude of \u003cem\u003eD\u003c/em\u003e directly reflects the irregularity of pore boundaries and the complexity of the structure; \u003cem\u003eD\u003c/em\u003e typically ranges from 1 to 2, and a smaller \u003cem\u003eD\u003c/em\u003e indicates fewer macropores within the soil, simpler and more regular pore morphology, and a denser overall soil skeleton \u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003c/span\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\({\\text{lg }}P=\\frac{D}{2}\\lg A+C\\)\u003c/span\u003e \u003c/span\u003e (7)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eP\u003c/em\u003e is the equivalent pore perimeter (\u0026micro;m), \u003cem\u003eA\u003c/em\u003e is the equivalent pore area (\u0026micro;m\u0026sup2;), \u003cem\u003eC\u003c/em\u003e is a constant, and \u003cem\u003eD\u003c/em\u003e is the fractal dimension.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig29\" class=\"InternalRef\"\u003e16\u003c/span\u003e shows that the microstructural pore fractal dimension of untreated loess is \u003cem\u003eD\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1.323, quantitatively confirming the presence of a large number of morphologically complex and highly connected pore networks. Figure\u0026nbsp;\u003cspan refid=\"Fig30\" class=\"InternalRef\"\u003e17\u003c/span\u003e further reveals that the pore fractal dimension of composite-improved loess decreases in a strictly monotonic manner with curing age. In the early stage of improvement (curing age: 1 day), physical filling and initial hydration preliminarily segment macropores. From 7 to 14 days, D decreases from 1.279 to 1.257, indicating that as the hydration/curing reactions accelerate, continuously generated cementitious products occupy a large portion of the pore space, thereby reducing the irregularity of pore boundaries. By 28 days, \u003cem\u003eD\u003c/em\u003e reaches a minimum value of 1.249, representing a decrease of approximately 5.6% relative to untreated loess (1.323). This continuous decreasing trend of \u0026ldquo;1.323 \u0026rarr; 1.249\u0026rdquo; provides strong quantitative evidence for the dual effects of \u0026ldquo;pore filling\u0026rdquo; and \u0026ldquo;cementation/solidification\u0026rdquo; in the composite stabilization system. Gel-like products continuously fill the primary pore network, progressively segmenting and refining the originally complex, irregular, large-scale backbone pores and ultimately transforming them into tiny and relatively isolated closed micropores. SEM observations show that with increasing curing age, pore number and connectivity decrease, inter-particle connections tend to become continuous, and the structure transforms toward a cemented dense state (Fig.\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003e). Consistently, the fractal dimension from the microstructural analysis before and after composite stabilization decreases significantly (Figs.\u0026nbsp;\u003cspan refid=\"Fig29\" class=\"InternalRef\"\u003e16\u003c/span\u003e and \u003cspan refid=\"Fig30\" class=\"InternalRef\"\u003e17\u003c/span\u003e), indicating reduced complexity of pore boundaries and the pore network. Together, these results mutually corroborate that composite stabilization weakens structural defects and enhances skeletal integrity and deformation-resistant stiffness through a \u0026ldquo;cementation-filling-densification\u0026rdquo; pathway, thereby providing a structural explanation for the improvement in dynamic performance. Consistent with this, the macroscopic tests in this study show that Ed of composite-improved loess can reach 826.49 MPa under high confining pressure and low dynamic stress, which is 8.07 times that of untreated loess, demonstrating a pronounced enhancement in dynamic stiffness.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e5.3 Micro-mechanism discussion: structural contribution of the composite cemented structure to dynamic resilient behavior\u003c/h2\u003e \u003cp\u003eThe enhancement in dynamic resilient performance of composite-improved loess ultimately arises from a systematic reconstruction of the internal structure from \u0026ldquo;loose particles-connected pores\u0026rdquo; to \u0026ldquo;cemented skeleton\u0026ndash;discrete micropores\u0026rdquo;. Based on the SEM microstructural observations and pore-fractal results (Figs.\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig30\" class=\"InternalRef\"\u003e17\u003c/span\u003e), the contribution of the composite cemented structure to dynamic resilient behavior can be summarized into three coupled micro-mechanisms, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig31\" class=\"InternalRef\"\u003e18\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e(1) Construction of a cemented network: C-S-H gel and AFt generated by cement hydration deposit on particle surfaces and grow into pore spaces, exerting \u0026ldquo;coating-bridging-void-filling\u0026rdquo; effects that gradually transform weak particle contacts into a continuous structure bonded by cementitious products, thereby forming a rigid skeleton capable of transmitting and dispersing dynamic stress. Meanwhile, the highly alkaline environment provided by the EFCA promotes ion exchange of clay minerals and strengthens cementation, and its high solid content supplies additional cementitious substances to the system, further enhancing inter-particle bonding and interfacial resistance to sliding.\u003c/p\u003e \u003cp\u003e(2) Reshaping of the pore network: In the early stage of composite stabilization, reaction products preferentially form at particle contacts and develop bridging and coating, causing the original connected pores to be segmented into multiple pore units of smaller scales. In the middle-to-late stage, cementitious products continue to deposit and fill skeletal pores, further reducing pore connectivity and gradually converting the pore network from connected to discrete, which macroscopically manifests as a pronounced reduction of macropores and overall densification (Fig.\u0026nbsp;\u003cspan refid=\"Fig28\" class=\"InternalRef\"\u003e15\u003c/span\u003e). This process reduces the freedom of particle rearrangement and the space available for pore compression, thereby weakening the accumulation of irrecoverable deformation under cyclic loading.\u003c/p\u003e \u003cp\u003e(3) Reduction of structural complexity: Quantitative fractal-dimension results show that after composite stabilization, the pore fractal dimension decreases with curing age from 1.323 for untreated loess to 1.249 (Figs.\u0026nbsp;\u003cspan refid=\"Fig29\" class=\"InternalRef\"\u003e16\u003c/span\u003e and \u003cspan refid=\"Fig30\" class=\"InternalRef\"\u003e17\u003c/span\u003e), indicating that the irregularity of pore boundaries and the geometric complexity of the pore network are significantly reduced, pore morphology tends to become more regular, and the structure shifts from a heterogeneous, disturbance-prone state toward a more homogeneous and more integral dense skeleton. A reduction in structural complexity implies fewer potential stress-concentration points and \u0026ldquo;weak links\u0026rdquo;, which facilitates more uniform diffusion and transmission of dynamic stress within the skeleton. In summary, the construction of a cemented network, reshaping of the pore network, and reduction of structural complexity jointly promote the formation of a more stable \u0026ldquo;cemented-skeletal\u0026rdquo; system in composite-improved loess. Under cyclic loading, the continuous cemented network can effectively restrain particle sliding and local structural rearrangement, reduce pore compression and strain accumulation, and limit resilient deformation, thereby macroscopically manifesting as a significant increase in Ed accompanied by a tendency toward stabilization of the confining-pressure gain rate.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"6 Conclusions","content":"\u003cp\u003eThis study investigated collapsible loess from Gangu, Gansu Province, and systematically examined the evolution of static/dynamic performance and the microstructural strengthening mechanisms of loess composite-improved with EFCA and P\u0026middot;O 42.5 cement through UCS tests, \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e tests, and quantitative microstructural analyses. The main conclusions are as follows:\u003c/p\u003e \u003cp\u003e(1) Orthogonal test results indicate that the optimal mix proportion is 6% P\u0026middot;O 42.5 cement and 0.02% EFCA, for which the 28-day UCS reaches 2.51 MPa, representing a 483% increase relative to untreated loess and meeting the specification requirements for subgrade fill. The failure process of the improved soil underwent four stages \u0026ldquo;elastic deformation-crack initiation-plastic yielding-residual strength\u0026rdquo; and ultimately exhibited a typical hourglass-shaped shear failure (shear angle\u0026thinsp;\u0026asymp;\u0026thinsp;45\u0026deg;), indicating that composite stabilization effectively mitigated the brittle disintegration characteristics of untreated loess and endowed the soil with stronger deformation resistance.\u003c/p\u003e \u003cp\u003e(2) Under the optimal mix proportion, the composite-improved loess exhibits excellent dynamic stability. At \u003cem\u003eσ\u003c/em\u003e₃ = 60 kPa and \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;30 kPa, Ed reaches 826.49 MPa, which is 8.07 times that of untreated loess (91.12 MPa). \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e increases markedly with increasing \u003cem\u003eσ\u003c/em\u003e₃ (confinement-strengthening effect) and exhibits a nonlinear decrease with increasing \u003cem\u003eσ\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e (stiffness-softening effect). The interwoven \u0026ldquo;rigid-flexible\u0026rdquo; structure formed in the composite stabilization system effectively suppresses stiffness degradation.\u003c/p\u003e \u003cp\u003e(3) A comparative analysis of the NCHRP 1-28A and Ni models shows that both can well capture the nonlinear dynamic response characteristics of subgrade soils (\u003cem\u003eR\u003c/em\u003e\u0026sup2; \u0026gt; 0.95). However, the Ni model not only considers the coupling effect between mean stress and deviatoric stress but also exhibits higher prediction accuracy in describing the hardening behavior of composite-improved loess, making it more suitable for dynamic settlement calculation and design prediction for composite-improved loess subgrades in this region.\u003c/p\u003e \u003cp\u003e(4) SEM observations and quantitative fractal-dimension analysis indicate that composite stabilization fundamentally reconstructs the micro-topological structure of loess. With increasing curing age, cement hydration products (C-S-H gel) and curing-agent polymerization products synergistically fill the primary macropores, driving particle contacts to shift from \u0026ldquo;point contact\u0026rdquo; to tight \u0026ldquo;surface contact\u0026rdquo; and aggregate cementation. The microstructural pore fractal dimension decreases significantly from 1.323 for untreated loess to 1.249 after stabilization (28 days), confirming a densification transition of the pore network from \u0026ldquo;connected and complex\u0026rdquo; to \u0026ldquo;discrete and regular\u0026rdquo;. This increased microstructural homogeneity and reduced defects effectively interrupt the internal damage path of dynamic stress within the soil, constituting the fundamental physical mechanism for the substantial increase in macroscopic \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e(5) Introducing EFCA effectively reduces reliance on traditional cement materials and, while ensuring subgrade dynamic stability, not only lowers engineering costs but also aligns with the low-carbon and emission-reduction trend in transportation infrastructure development, indicating strong potential for broader application.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was funded by the following projects: the National Natural Science Foundation of China (Project No. 52068063); the Scientific Research Project of \u0026quot;Young Doctor Fund\u0026quot; of Gansu Provincial Universities (Project No. 2026QB-082); the Innovation Fund Project of Gansu Provincial Universities (2022A-107, 2025A-150); the Special Project for the Construction of Scientific Research and Innovation Platforms of Tianshui Normal University in 2023 (PTJ2023-07); and the School - level Project of Lanzhou Bowen University of Science and Technology (2025BWKY029).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eXiangming Lv, Chongliang Luo, Fei Ma, Baocheng Wang and Xin Wang wrote the main manuscript text. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLiu, Y., Xu, Y. \u0026amp; Liu, Y. Population growth and spatiotemporal differentiation in the Loess Plateau region since 2000. \u003cem\u003eProg Geogr.\u003c/em\u003e \u003cb\u003e31\u003c/b\u003e, 156\u0026ndash;166. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.11820/dlkxjz.2012.02.004\u003c/span\u003e\u003cspan address=\"10.11820/dlkxjz.2012.02.004\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2012). (in Chinese).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTian, Y. et al. Deformation mechanism and evolutionary process of the Tianshui forging machine plant landslide in Gansu. \u003cem\u003eJ. 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Mechanism of composite improvement of loess based on quantitative analysis of microstructure and mechanical strength. \u003cem\u003eConstr. Build. Mater.\u003c/em\u003e \u003cb\u003e379\u003c/b\u003e, 131215. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.conbuildmat.2023.131215\u003c/span\u003e\u003cspan address=\"10.1016/j.conbuildmat.2023.131215\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (2023).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Collapsible loess, Eco-friendly curing agent, Dynamic resilient modulus, Dynamic constitutive prediction model, Microstructural evolution, Fractal dimension","lastPublishedDoi":"10.21203/rs.3.rs-8980134/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8980134/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo address the loose structure and insufficient dynamic stability of collapsible loess subgrades in Gangu, Gansu Province, as well as the engineering and environmental concerns associated with conventional stabilizers (high energy consumption and carbon emissions), this study proposes a composite stabilization strategy using an eco-friendly curing agent (EFCA) and P\u0026middot;O 42.5 Portland cement. Unconfined compressive strength (UCS) tests and dynamic triaxial tests were performed, and scanning electron microscopy (SEM) image processing together with fractal theory was employed to systematically elucidate the macroscopic dynamic response and the microscopic pore-reconstruction and evolution mechanisms of the composite-improved loess. The results indicate that the optimal mix proportion determined by an orthogonal design is \u0026ldquo;6% cement\u0026thinsp;+\u0026thinsp;0.02% curing agent\u0026rdquo;, yielding a 28-day UCS of 2.51 MPa, which is 483% higher than that of the untreated loess. The dynamic resilient modulus (\u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e) increases markedly and reaches 826.49 MPa under a confining pressure of 60 kPa and a dynamic stress of 30 kPa (an 8.07-fold increase). Nonlinear regression analysis confirms that the Ni model, by jointly accounting for the coupled effects of mean and deviatoric stresses, provides exceptionally high predictive accuracy for \u003cem\u003eE\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e of the composite-improved loess, with an average relative error of only 0.026. Quantitative microstructural analysis reveals that the synergistic effects of chemical cementation and hydration products promote the transformation of loess particles into dense aggregates, resulting in a decrease in the pore fractal dimension (\u003cem\u003eD\u003c/em\u003e) from 1.323 to 1.249. This topological reconstruction from connected macropores to discrete micropores fundamentally reduces the structural complexity of the soil. The study clarifies a cross-scale physical-mechanical mechanism whereby \u0026ldquo;microstructural pore-fractal dimensionality reduction\u0026rdquo; drives a \u0026ldquo;macroscopic surge in dynamic stiffness\u0026rdquo;, providing theoretical and data support for green, low-carbon subgrade construction and long-term dynamic stability evaluation in loess regions.\u003c/p\u003e","manuscriptTitle":"Dynamic Response and Pore Evolution Mechanism of Composite Improved Loess Using an Eco-Friendly Curing Agent and Cement: A Macroscopic and Microscopic Experimental Study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-25 17:15:07","doi":"10.21203/rs.3.rs-8980134/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-06T06:31:27+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-02T12:01:34+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-27T23:52:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"25866400485907528991791329899925120062","date":"2026-03-26T13:26:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"272835154163512525977643280750918994165","date":"2026-03-25T06:55:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"150310389011017458579649826461617028843","date":"2026-03-25T03:18:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"24411409340850127137042724464599405328","date":"2026-03-20T17:38:26+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-20T17:34:32+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-20T12:38:27+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-03-19T13:47:45+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-12T07:43:35+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2026-03-11T12:31:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"98d17ea2-3851-480e-b07f-0a40369a4565","owner":[],"postedDate":"March 25th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":64931143,"name":"Physical sciences/Engineering"},{"id":64931144,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-05-12T07:27:02+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-25 17:15:07","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8980134","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8980134","identity":"rs-8980134","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}