[Objective] Existing grouting theories and engineering experience are mostly based on hydrostatic or weakly flowing water conditions, making it difficult to accurately describe the diffusion and evolution characteristics of grout under dynamic water-flow environments. It is necessary to systematically reveal the diffusion mechanisms of grout in water-flowing fractures so as to provide theoretical support for grouting design under complex water inrush conditions in water-sealed caverns. In response to engineering conditions involving stable flowing water in a single fracture, this study aims to: (1) reveal the influence mechanisms of fracture geometric characteristics and construction parameters on grout diffusion behavior; (2) quantitatively analyze the controlling effects of key factors on grout diffusion distance, diffusion time, and sealing efficiency; and (3) clarify the relative importance of different influencing factors in the grouting of water-flowing fractures, thereby providing a basis for optimization of grouting parameters and construction decision-making for water-sealed caverns. [Methods] Based on the grout-water two-phase flow theory, a numerical model of grouting in a single fracture with flowing water was established using the finite element method. Variations in water flow velocity within the fracture and the driving effect of grouting pressure were comprehensively considered, and the diffusion, advection, and deposition processes of grout within the fracture were simulated. Through parametric comparative analysis, the effects of fracture aperture, fracture inclination, grouting pressure, flowing water velocity, and fracture boundary extent on the evolution of grout diffusion were systematically investigated. On this basis, a sealing efficiency index was introduced to comprehensively evaluate the grouting performance under different working conditions. [Results] Under flowing water conditions, the grout diffusion pattern, stabilization time, and final sealing performance within fractures were jointly controlled by multiple coupled factors. (1) Fracture inclination had a significant inhibiting effect on grout diffusion. As the fracture inclination increased, the coupling between the gravitational component and the flowing water direction was enhanced, causing the grout to more easily deviate along the down-dip direction. As a result, the ability of grout to migrate against the water flow was weakened, and the diffusion range was markedly restricted. (2) The time required for grout diffusion to reach a stable state increased significantly with an increase in the extent of the fracture domain, because a larger boundary extent provided a greater seepage space for grout diffusion. In contrast, increasing grouting pressure effectively accelerated the advance of the grout diffusion front and shortened the stabilization time, exhibiting a pronounced accelerating effect. (3) In terms of diffusion distance, the effective diffusion distance of grout was inversely proportional to fracture boundary extent and flowing water velocity. Higher flowing water velocity resulted in stronger scouring and transport effects on the grout, thereby reducing its retention capacity within the fracture. Conversely, increases in fracture aperture and grouting pressure facilitated the grout in overcoming water flow resistance, enabling longer diffusion distances and more sufficient fracture filling. (4) Comparative analysis of the influence degrees of various factors indicated that fracture aperture had the most significant effect on grout sealing efficiency, followed in descending order by flowing water velocity, grouting pressure, and fracture boundary extent. This demonstrated that fracture geometric characteristics and hydrodynamic conditions were the key factors controlling the success or failure of grouting under flowing water conditions. [Conclusion] Overall, in strong flowing water environments, relying solely on increasing grouting pressure does not significantly improve grouting performance, and comprehensive design must be carried out by jointly considering fracture aperture characteristics and groundwater hydrodynamic conditions. For areas with larger fracture apertures and higher flowing water velocities, measures such as staged grouting or advance water reduction should be preferentially adopted to enhance grout retention and sealing capacity within fractures.

[Objective] This study aims to investigate the influence of super-large-diameter double-circular pipe jacking construction on the settlement deformation of underground pipelines. Taking the water quality assurance project of Tiegang-Shiyan Reservoir in Shenzhen as the background, and based on the modified Peck formula, this study uses a combination of numerical simulation and field monitoring to systematically analyze the effects of multiple factors such as pipeline burial depth, material, pipe diameter, pipe jacking spacing, and spatial position. [Methods] The variable normalization method was used to analyze the influence degree of each factor on the pipeline, and the safety performance of the pipeline was evaluated. [Results] When the jacking pipes vertically crossed under the pipeline, the induced settlement range was the smallest, indicating a relatively reasonable construction method. When the pipeline burial depth, material, or pipe diameter was changed, the stratum displacement field ultimately showed a “V”-shaped distribution. However, when the spacing between the two jacking pipes increased to 1.5 times the jacking pipe diameter (i.e., 6 m), the displacement field shape transformed into a “W”-shaped pattern, and the influence range of pipeline deformation significantly expanded. Sensitivity analysis showed that the spacing between the two jacking pipes was the most significant factor affecting pipeline settlement (sensitivity=0.54), while pipeline diameter had the least influence (sensitivity=0.06), and pipeline burial depth had a moderate influence (sensitivity=0.40). Furthermore, the safety state of the sewage pipeline was evaluated using the allowable joint rotation angle. The calculated joint rotation angle under field monitoring conditions was 0.54°, which was lower than the standard control value of 1.15°, indicating that the pipeline joints remained in a safe state during construction and did not suffer damage due to uneven settlement. [Conclusion] Currently, there is considerable research on settlement deformation of underground pipelines caused by single-line pipe jacking construction, but research on the influence of super-large-diameter double-line pipe jacking with shallow burial depth is limited. This study clarifies the influencing mechanisms of key construction parameters, providing theoretical basis and data support for engineering practices involving large-diameter pipe jacking undercrossing existing pipelines.

[Objective] In recent years, the increasing number of buildings constructed on soft ground has made the treatment of soft soil foundations particularly important. Investigating the strength variation characteristics of silt foundations under different types of stabilizing agents and curing ages,as well as exploring a preliminary method for identifying the strength level of non-standard soil samples obtained in the field,is of great practical significance for engineering applications. [Methods] Representative silty soil layers from the Zhongshan area were selected. Cement-only mixing tests were first conducted to optimize cement content (ratio of cement mass to the mass of treated wet soil). Subsequently,silt was stabilized using lignosulfonate acid (LA),triethanolamine (TEA),and alkali-activated sodium silicate (AS) as stabilizing agents,respectively. Scanning electron microscopy (SEM) tests were then carried out to analyze the microstructures and stabilization mechanisms of untreated and stabilized soils. Finally,ArcGIS was used to construct independent elevation models from the SEM images of untreated and stabilized soils and to process them into three-dimensional images. Scatter plots were plotted in double-logarithmic coordinates,and the soil porosity and particle fractal dimension were further calculated and analyzed. [Results] 1) Using the unconfined compressive strength of specimens as the evaluation index,under the same curing age, the cement content was positively correlated with the unconfined compressive strength. Before a curing age of 7 days, the unconfined compressive strength increased rapidly; during 7-14 days, the growth rate slowed down; and from 14 to 28 days, the unconfined compressive strength continued to increase. Considering economic cost and code requirements, the optimal cement content was 18% of the wet soil mass. 2) At certain mixing ratios, single incorporation of LA, TEA, and AS all enhanced the strength of cement-stabilized soil and could be used as stabilizing agents. Based on the optimal contents of the three stabilizers, ternary mixing stabilization tests were conducted. The results showed that single incorporation of AS exhibited better stabilization performance than the other single-additive groups and the ternary mixing group. When the cement content was 18% and the AS content was 0.9%, the unconfined compressive strength of the stabilized soil reached a maximum value of 2.39 MPa. 3) SEM tests indicated that the specimens of the 18S blank group failed to generate sufficient gel-like hydration products (C-S-H) and needle-like Aft crystals. As a result, limited cementitious material existed between soil particles, and numerous pores were observed. After stabilization with 18S-0.9AS, a large amount of gel-like C-S-H hydration products and needle-like Aft crystals were rapidly generated, which interwove to form a large-area spatial network structure and initially formed a skeleton. This process led to particle bonding and aggregation and filled the interparticle pores. Overall, the 18S-0.9AS group exhibited the best stabilization effect. 4) Three-dimensional SEM images of untreated and stabilized soils were constructed and processed using ArcGIS. Data calculation and analysis showed that the untreated soil had 97 663 226.34 pore pixels, accounting for 53.22% of the total image pixels (porosity), with a fractal dimension of 1.399 8. The 18S-0.9AS stabilized soil had 50 153 642.75 pore pixels, accounting for 27.27% of the total image pixels (porosity), decreased by 25.95 percentage points compared with the untreated soil. The fractal dimension of this group was 1.853 5, and the unconfined compressive strength reached the maximum value of 2.39 MPa. [Conclusion] 1) The larger the particle fractal dimension of a specimen, the more complex the particle structure, the higher the surface roughness, the lower the porosity, and the higher the compressive strength. 2) The porosity of specimens and the compressive strength exhibit a nonlinear decreasing relationship, whereas the particle fractal dimension and the compressive strength exhibit a nonlinear increasing relationship. 3) When standard specimens cannot be obtained at construction sites, the compressive strength of specimens can be preliminarily inferred by using the porosity and particle fractal dimension of non-standard specimens, based on the physical significance and correlations of fractal dimensions among different specimens.

[Objective] Microwave-assisted rock breaking technology shows promising application potential in hard rock tunneling. The effectiveness of microwave irradiation on rocks is significantly influenced by their microstructure, particularly the mineral particle size distribution. Existing studies mostly focus on single mineral components or simple binary combinations, whereas systematic investigations into how the heterogeneity of complex mineral particle size distributions in natural rocks affects the microwave rock breaking remain limited. This study aims to quantify the heterogeneity of mineral particle size distribution through experiments and numerical simulations, and to reveal its influence mechanisms on the thermal-physical response, damage evolution, and mechanical property degradation of rocks. [Methods] Seven groups of standard granite specimens (Φ50 mm × 100 mm) with different mineral particle size heterogeneity coefficients were selected and subjected to microwave irradiation tests. Surface temperature variations of the specimens during irradiation were monitored, longitudinal wave velocities before and after irradiation were measured, and the peak strengths were obtained through uniaxial compression tests. Image processing techniques were used to extract the surface mineral distributions of the specimens, and quantitative indicators characterizing the heterogeneity of particle size distributions were defined and calculated. Using COMSOL software, a multi-physics numerical model coupling electromagnetic fields, heat conduction, and solid mechanics was established. The model precisely reconstructed the real mineral distributions with different heterogeneity coefficients, and simulated and analyzed the dynamic evolution process of the temperature fields, stress fields, and plastic damage zones of the specimens under microwave irradiation. [Results] (1) Thermal response: under identical irradiation conditions, the heterogeneity of mineral particle size distribution significantly affected the thermal response of the specimens. With the increase of heterogeneity, the temperature rise of the rock became more pronounced. The specimen with the highest heterogeneity (H=0.78) exhibited a final temperature approximately 44 ℃ higher than that of the most homogeneous specimen (H=0.34).(2) Damage and weakening: with increasing heterogeneity coefficient, the number of microwave-induced surface microcracks increased significantly. The reduction in longitudinal wave velocity intensified, with a maximum difference reaching 30%. The uniaxial compressive strength loss rate increased from 11.2% to 29.6%, with a maximum difference of 18.4%. These results indicated that the more heterogeneous the mineral distribution was, the more severe the internal damage induced by microwaves and the more significant the weakening effect on macroscopic mechanical performance became.(3) Mechanism: stronger heterogeneity led to more intense temperature gradients and thermal stress concentrations at the interface between strong microwave-absorbing minerals (such as potassium feldspar) and weak microwave-absorbing minerals (such as quartz). This was because specimens with higher heterogeneity contained larger potassium feldspar particles, which had stronger microwave absorption capacity, resulting in a rapid local temperature rise. Both the area proportion of the critical tensile stress zones (>15 MPa) and the area proportion of the plastic zones increased monotonically with the increase of the heterogeneity coefficient. The plastic zones first appeared at the contact interface between potassium feldspar and quartz and expanded over time. [Conclusion] The heterogeneity of mineral particle size distribution is a key microstructural factor controlling the effectiveness of microwave-assisted rock breaking. The defined quantitative heterogeneity coefficient can effectively predict the outcomes of microwave irradiation: rocks with higher heterogeneity are more likely to experience uneven heat accumulation and large interfacial thermal stresses under microwave irradiation, thereby leading to more extensive microcrack initiation, more significant wave velocity reduction, and more significant strength loss. This study identifies the potassium feldspar-quartz interface as the preferential site for damage initiation.

[Objective] As a new type of support anchor, the Glass Fiber-Reinforced Plastic (GFRP) screw anchor is increasingly used in engineering practice due to its simple construction, light weight, high strength, corrosion resistance, and cost-effectiveness. Previous calculation formulas for uplift bearing capacity of screw anchors mostly based on artificially defined boundaries between shallow and deep embedment, resulting in large differences in critical embedment depth ratio H/D (where H is the embedment depth and D is the anchor plate diameter) given by different scholars. This study aims to propose a calculation formula for uplift bearing capacity based on the generalized unified failure surface morphology, providing theoretical support for the design of GFRP screw anchors in engineering practice. [Methods] Using finite element numerical simulation software ABAQUS, numerical simulations of the uplift performance of GFRP screw anchors under different embedment ratios were conducted. The maximum uplift bearing capacity of vertical GFRP screw anchors in soil and the evolution of the failure surface morphology of anchored soil were investigated. [Results and Conclusion] (1) The load-displacement curves of GFRP screw anchor during uplift in anchored soil could be divided into three stages: the elastic stage, the local plastic stage, and the penetration failure stage. In the elastic stage, the load and displacement showed linear relationship, with the slope of the curve representing the equivalent stiffness of the anchor plate-soil system. In the local plastic stage, plastic deformation occurred in the local soil zone, deformation of the anchored soil gradually increased, system stiffness decreased, load-displacement relationship became non-linear (indicated by a gradually decreasing curve slope), and the interaction between the GFRP screw anchor and the anchored soil began to exhibit non-linear characteristics. In the penetration failure stage, the resistance of the anchored soil reached its ultimate value, cracks in the anchored soil became interconnected, the soil failure surface formed, and the curve entered a stable phase where the load no longer changed with displacement increase. The uplift bearing capacity factor N_γ showed a trend of initial increase followed by gradual stabilization with embedment ratio, which matched well with experimental results from other scholars, thereby verifying the simulation’s validity. The turning point occurred near H/D=9, suggesting that an embedment ratio H/D=9 could be considered as the critical embedment ratio distinguishing shallow from deep embedment. (2) Numerical simulation showed that during uplift, the anchored soil could be divided into three zones based on stress state: conical active zone formed by compaction above the GFRP screw anchor, passive zone extending outward along screw anchor edge influenced by extrusion from soil within failure surface, and transition zone between active and passive zones in plastic state. The soil within the failure surface consistently extruded the soil outside failure surface. Except for the outermost region, which might be in a state of at-rest earth pressure, the soil on the failure surface was generally in a state of passive earth pressure. With increasing embedment ratio, the failure surface morphology of the anchored soil underwent a dynamic evolution from “trumpet” shape➝“goblet” shape➝“light bulb” shape. No distinct “critical embedment ratio” between shallow and deep embedment for GFRP screw anchors existed. (3) By integrating the maximum uplift bearing capacities obtained from numerical simulations and the evolution patterns of the failure morphology in the anchored soil, a generalized unified failure surface model that did not require distinguishing between shallow and deep embedment for the soil anchored by GFRP screw anchors was proposed. This model was derived by introducing a cubic term based on the “trumpet” shape (with inclined linear boundaries) failure surface. On this basis, a calculation formula for the uplift bearing capacity under different embedment depths of GFRP screw anchors was derived, and the formula was compared and validated with numerous experimental results from domestic and international studies. The results demonstrated that the proposed calculation formula could effectively predict the maximum uplift bearing capacity under different embedment depth ratios.

[Objective] To explore the impact of freeze-thaw environments on the strength of guar gum-modified loess, direct shear test, unconfined compression strength test, Brazilian splitting test, and bender element small-strain test are conducted to examine the strength degradation and deterioration characteristics of guar gum-modified loess before and after freeze-thaw cycles. [Methods] In this experiment, cationic guar gum was selected for specimen preparation. The specimens were subjected to freeze-thaw cycles in a sealed environment. In the bender element tests, a single-pulse sine wave was used as the input signal at a frequency of 1 kHz. The initial arrival method was adopted to determine the travel time, from which the wave velocity was calculated. [Results] After freeze-thaw cycles, the tensile strength, uniaxial compressive strength, and shear strength of guar gum-modified loess decreased, but all remained greater than that of untreated loess subjected to freeze-thaw cycles. The strength degradation of guar gum-modified loess after freeze-thaw cycles was influenced by the coupling effects of water content and guar gum content, with water content having a more pronounced effect than guar gum content. Microscopic analysis revealed that guar gum formed filamentous cement by bonding with water molecules, which linked soil particles and restrained their movement. Consequently, the strength of the modified loess was notably enhanced. However, during the freeze-thaw cycles, the simultaneous damage of the above-mentioned cement and loess particles was the main reason for the more pronounced strength degradation of guar gum-modified loess than that of untreated loess. The shear wave velocity of loess modified by guar gum before and after freeze-thaw cycles was measured using bender element tests. The functional relationships of shear wave velocity with compressive strength and tensile strength of loess were established, revealing a good correlation with uniaxial compressive strength. [Conclusion] The research findings systematically reveal the influence of freeze-thaw action on guar gum soil stabilization technology, and a novel method is proposed to evaluate the strength degradation of modified loess with wave velocity. This provides environmentally friendly materials and new insights for soil stabilization in the loess regions of northwestern China, while offering a theoretical basis for the further application of guar gum in geotechnical engineering.

[Objective] This study investigates how soluble salts in clay redistribute under an applied direct-current electric field during electro-osmotic drainage and how the redistribution affects dewatering and consolidation efficiency. The study quantifies the spatiotemporal evolution of salt content through bulk electrical conductivity, distinguishes the individual effects of salinity and water content on conductivity, and infers ion-migration trends and their implications for the combined dewatering and desalination performance of saline clays. [Methods] Laboratory conductivity calibrations were conducted on remolded clay across practical ranges of water content and soluble-salt concentration. Based on these data, an empirical relationship was established that linked soil bulk conductivity to pore-fluid salinity while explicitly incorporating water content, enabling the conversion of measured conductivities into estimates of salt content. Subsequently, a one-dimensional electro-osmotic consolidation test was conducted. Segmented voltage, local conductivity, cumulative drainage, and current were monitored and recorded. Using this calibration, time-lapse conductivity profiles were processed to reconstruct salt-content distributions and their evolution. This method could provide a framework to monitor and interpret coupled ionic transport and water removal during electro-osmosis. [Results] Calibration showed that conductivity increased with both salinity and water content. However, when water content was considered, salinity accounted for a larger share of the variance in bulk conductivity. Accordingly, conductivity served as a reliable in-situ indicator of salt content during electro-osmosis. The electro-osmotic test revealed a distinct zonation of salt content consistent with electromigration toward the cathode. At the anode, salt content declined rapidly during the first 2 hours and then stabilized at approximately 2.0 g/L until the end of the test. In the mid-section, salt content also decreased over the first 2 hours, showing the smallest reduction among the three regions, followed by an increase and subsequent decline. By 6 hours, it temporarily exceeded the initial salinity. This peak reflected the convergence in the middle zone of cation fluxes migrating from anode to cathode and anion fluxes moving in the opposite direction. After 6 hours, the mid-section salinity decreased progressively and, at the end of the test, fell below that of the anode region. The cathode experienced the most pronounced change, showing a continuous decline throughout energization. By 8 hours, the cathodic zone had nearly approached a salt-free state. During electro-osmosis, the soil potential field was strongly modulated by both water content and salinity, producing spatially differentiated potential distributions that evolved over time. Water content and drainage rate exhibited non-uniform dynamics among regions and ultimately formed a moisture gradient of Anode < Middle < Cathode. Salinity exerted a pronounced control on potential pathways and transmission efficiency. Therefore, its evolution should be incorporated explicitly in design to optimize treatment outcomes. The combined evidence demonstrated that electro-osmotic drainage in saline clay could achieve two outcomes simultaneously: accelerated consolidation and effective removal of soluble salts. The latter mitigated adverse effects of high salinity on subsequent construction, including corrosion risk and strength variability, thereby improving the suitability of the treated ground. [Conclusion] This study delineates the migration and distribution patterns of soluble salts in high-salinity clays under electro-osmotic drainage, offering a new perspective for treatment and practical guidance for engineering application. Operationally, a critical point is reached when salinity in the cathodic zone drops to a very low level. Continuing energization beyond this point leads to sharply diminished drainage efficiency and disproportionately increased energy consumption. At the design stage, measuring soil electrical conductivity and conducting pre-tests to characterize the salinity-moisture relationship are recommended, thereby informing the required energization time. In practice, continuous conductivity monitoring provides a comprehensive indicator of overall dewatering progress. Wider adoption of these insights is expected to facilitate broader and more effective application of electro-osmosis in geotechnical engineering.

[Objective] This study systematically investigates the influence of particle shape on the mechanical properties of soil-rock mixtures, with a particular focus on strength and deformation characteristics. It aims to address the current knowledge gap regarding the specific mechanisms through which particle shape affects the mechanical behavior of such materials. By establishing quantitative relationships between shape parameters and mechanical response, the study aims to provide a scientific basis for predicting and controlling the settlement of soil-rock mixture subgrades in engineering practice. [Methods] High-strength α-hemihydrate gypsum powder was utilized to fabricate rock-like particles with controlled shapes. The Brazilian shape parameter (Y) was employed to quantitatively characterize particle morphology. A series of consolidated-drained triaxial tests were conducted using a medium-pressure triaxial apparatus to systematically evaluate the mechanical properties of soil-rock mixtures containing particles with different shape coefficients. The testing program included comprehensive measurements of peak deviatoric stress, internal friction angle, cohesion, and other shear strength parameters under different confining pressures. Microstructural analysis was performed to observe particle breakage patterns and stress transmission mechanisms. [Results] The experimental results revealed significant shape-dependent mechanical behavior. As the particle shape coefficient Y increased, the peak deviatoric stress of the specimens initially increased and then tended to stabilize. With increasing Y, the internal friction angle and initial shear angle φ₀ gradually decreased, while the cohesion exhibited a corresponding increase. The increment of shear angle Δφ showed a non-monotonic trend, first decreasing and then increasing. The shape coefficient was found to alter the stress transmission path within the particle skeleton, leading to preferential breakage of large-sized particles. When Y increased from 0.63 to 0.73, the failure mode transitioned from edge damage to localized rupture, and ultimately to complete fragmentation. Ellipsoidal particles (Y≥0.69) exhibited stress concentration at the long-axis ends, resulting in localized fracture concentrated in the shear band region. In contrast, near-spherical particles demonstrated uniform stress distribution and exhibited surface spalling. [Conclusion] This study successfully establishes the quantitative relationship between particle shape and mechanical properties of soil-rock mixtures, revealing the underlying mechanisms of shape effects. The findings demonstrate that particle shape significantly influences the strength, deformation, and failure characteristics through its control on stress transmission and particle breakage patterns. The study provides a scientific basis for the prediction and design of soil-rock mixture subgrade settlement, offering practical guidance for engineering applications. Future research should focus on extending these findings to field-scale conditions and developing predictive models that incorporate shape effects for improved design accuracy.

[Objective] During the parameter inversion process of groundwater models, frequent calls to the forward model result in excessive computational demand and prolonged processing time, which severely limits their practical applicability. To address the high time consumption in groundwater model inversion, this study proposes a coupled inversion algorithm capable of rapidly identifying unsteady permeability coefficients. [Methods] The proposed algorithm coupled the parameter inversion process with reduced-order model training, where the reduced-order model was employed instead of the original model for parameter inversion calculation, thereby reducing total inversion time. During the iteration process, the sum of squared errors between the reduced-order model calculation values and observations was used as the objective function, and an improved differential evolution algorithm with strong global search capability was employed as the optimization method for parameter inversion. In each iteration, the parameters that best matched the observations were identified as optimal, and snapshots of these optimal parameters were calculated to train the reduced-order model, thereby enhancing the inversion accuracy in the next generation. To enable the reduced-order model to accurately capture the time-domain response characteristics of the original model, a uniform snapshot strategy was employed to collect time-step snapshots. Based on the characteristics of the coupled inversion algorithm, the relative error of the reduced-order solution corresponding to the optimal parameters was calculated at all nodes within the time domain, and iteration was terminated when the maximum error fell below a preset threshold. [Results] Taking a two-dimensional pumping well model as an example, the proposed method was compared with an inversion approach based on the original model. The results indicated that: (1) For unsteady seepage parameter inversion, compared with the optimal time snapshot strategy, using a uniform snapshot collection strategy to construct the reduced-order model could achieve higher computational accuracy, while the reduced-order model had a lower average order. (2) While maintaining inversion accuracy comparable to that of the original model, the proposed algorithm could reduce computational time by approximately 95.37%. (3) Near the optimal parameters, the reduced-order model obtained by the proposed method showed almost identical responses to the original model. However, the error increased significantly when moving away from the optimal solution. (4) The effects of observation error, mesh density, and inversion dimensionality on the inversion accuracy of the proposed algorithm were consistent with those of the original model, but the computational time of the proposed algorithm was less than 5% of that of the original model. (5) The proposed algorithm was less affected by the order of the original model, and the increase in the computational time was proportionally smaller than the increase in model order, indicating higher computational efficiency for high-order models than for low-order ones. (6) Compared with low-dimensional inversion problems, the proposed algorithm exhibited greater time-saving efficiency in handling high-dimensional cases, suggesting stronger robustness against the curse of dimensionality. (7) Under different convergence accuracies, the proposed algorithm could consistently reproduce the results of the original model without a significant increase in computational time even as accuracy improved. [Conclusion] The proposed coupled inversion algorithm in this study, as a deterministic finite element-based inversion framework, innovatively couples the training process of the reduced-order model with the parameter inversion process and significantly improves the computational efficiency of parameter inversion.Characterized by a simple structure,ease of implementation, and no need for posterior error calculation,the algorithm has significant engineering application value and promising prospects for broad application.

[Objective] This study aims to establish a field-based method that uses heavy dynamic cone penetration test (DCPT) energy index P_index to quantify relative density (D_r) of sandy-cobble soil and to link D_r to deformation moduli E₅₀ and E_ur, thereby overcoming the long-standing difficulties of retrieving undisturbed samples and calibrating parameters for this material. [Methods] (1) A Φ600 mm × 600 mm calibration chamber was fabricated to enable precise reconstitution of specimens at D_r=0.40, 0.55, 0.70, and 0.85. (2) A series of 63.5 kg DCPTs were performed, and penetration resistance-depth curves were analyzed to extract P_index. (3) Triaxial consolidated-drained and unloading-reloading tests were conducted on the same D_r specimens to obtain E₅₀ and E_ur. (4) A three-tier Bayesian-Bootstrap model “P_index-D_r-modulus” was developed and coded into PLAXIS-Hardening-Soil model. (5) The method was validated using field monitoring data of an adjacent excavation. [Results] (1) P_index decreased with D_r following a power law (R²≥0.93). When D_r increased from 0.40 to 0.85, P_index dropped by 62%, outliers decreased by 47%, and repeatability error remained <3%. (2) E₅₀=112.4 D_r^1.87 MPa and E_ur=318.6 D_r ^1.64 MPa. E_ur/E₅₀ decreased exponentially from 2.8 to 2.1. (3) Ten-fold cross-validation yielded a mean absolute error of D_r =0.028. The relative errors of the predicted E₅₀ and E_ur were <8% and <7%, respectively. (4) FE simulations using the predicted moduli yielded an average relative displacement error of 6.1% compared to 18.4% (Mohr-Coulomb) and 12.7% (Modified Mohr-Coulomb), and the maximum vertical displacement deviation of the station reduced from 5.2 mm to 1.7 mm. (5) The proposed method was applicable to sandy-cobble layers in the upper and middle reaches of the Yongding River alluvial fan, western Beijing (D_r=0.35-0.90, cobble content ≤70%). [Conclusion] The study presents the first continuous field method linking DCPT impact energy, relative density, and deformation moduli for sandy-cobble soil without undisturbed sampling. The compact power-exponential model can be directly implemented in commercial software, providing in-situ parameters for deformation analysis of excavations and tunnels in such formations. The significant improvement in deformation prediction accuracy provides immediate advantages for risk control and support optimization during tunnelling or excavation near existing metro structures.

[Objective] Reverse-graded deposits are prone to geological disasters under rainfall conditions. To comprehensively understand their seepage characteristics and disaster-causing mechanism, the indoor seepage test is conducted using typical deposits as the research object. The variation patterns of permeability across different deposit layers and the migration characteristics of fine particles were studied under different fine particle contents, dry densities, and infiltration heads. [Methods] Based on the Taheman deposit landslide, constant-head seepage experiments were performed using a seepage erosion device under different fine particle contents, dry densities, and infiltration heads. The influences of external hydraulic conditions and reverse-graded deposit characteristics on the permeability of each deposit layer were analyzed, and the migration patterns of fine particles within each layer were revealed. [Results] Fine particle content, dry density, and infiltration head exerted the greatest influence on the permeability variations in the upper rock-soil layers, followed by the middle rock-soil layers, with the least influence on the lower rock-soil layers. Higher fine particle content facilitated the accumulation and deposition of fine particles in the upper and middle rock-soil layers, resulting in a decrease in the soil permeability. Under varying infiltration heads, the permeability of the upper layers exhibited either an increase or decrease, while the middle and lower layers consistently showed permeability decline. Regardless of influencing factors, the permeability of the lower layers uniformly decreased. Fine particles(<0.075 mm) had obvious migration, loss, and deposition processes in the middle and upper rock-soil layers. The lower the soil layer was, the smaller the particle size of the fine particles it could retain. [Conclusion] The middle and upper parts of reverse-graded deposits exhibit weaker erosion resistance, where fine particles are prone to migration and loss, leading to reduced permeability. The erosion resistance of the middle section is stronger than that of the upper part, while the lower section demonstrates the highest erosion resistance. Fine particles are easily subject to deposition and accumulation, which reduces permeability. Factors such as fine particle content, dry density, and infiltration head have the greatest impact on permeability variations in the upper section, followed by the middle section, with the least effect on the lower section. When the seepage direction is top-down, significant particle migration occurs between the upper and middle layers, but fewer fine particles migrate and deposit into the lower layers. The ability to retain fine particles decreases with proximity to the lower part of the deposit. Due to the coarse-upper and fine-lower structure of reverse-graded deposits, particle migration and loss characteristics vary with different seepage directions, warranting further investigation in subsequent experiments.

[Objective] Rock material typically exhibits nonlinear strength characteristics under complex loading conditions, and stress-drop can be observed during shear failure while retaining part of the residual strength. To investigate the mechanical properties of carbonaceous shale, conventional triaxial compression tests were conducted, and a new rock damage model was established based on continuum damage mechanics to describe the stress-strain curves. [Methods] The proposed model first utilized a nonlinear exponential strength criterion to describe the micro-elements of rocks, considering the material heterogeneity, and assuming the micro-element strength followed a Weibull distribution. The damage variable was derived from the accumulated failure proportion of micro-elements. Subsequently, the model employed a modified Lemaitre equivalent strain assumption to capture the stress-drop effect and residual strength, allowing for the entire stress-strain curve to be represented. Model parameters were determined using the extremum method. Finally, the model’s predictions were compared with conventional triaxial compression test results from different rock types to verify its validity. [Results] Results showed that the established rock damage model accurately described the entire stress-strain relationship of rock samples under various confining pressures. During the post-peak deformation stage, the shear strength of the rock samples dropped rapidly and eventually approached the residual strength due to the stress-drop effect, and the rock samples became fully damaged. The comparisons also suggested that the exponential strength criterion was generally suitable for various rocks; however, both the axial strain corresponding to peak strength and the residual strength varied approximately linearly with confining pressure. [Conclusion] The established exponential damage model of rock has good prospects for theoretical application.

[Objective] Due to the excellent load-bearing performance, geosynthetic reinforced soil (GRS) composites have been widely adopted in the construction of load-bearing GRS bridge abutments. Unlike conventional gravity or cantilever retaining walls, GRS abutments are required to bear significantly higher vertical loads transferred from the superstructure. Therefore, it is essential to investigate the ultimate bearing behavior of GRS composites to ensure the safety and reliability of the structures. [Methods] In this study, a series of plane strain model tests were conducted to evaluate the ultimate bearing capacity of GRS composites. Nine groups of tests were designed and conducted using geotextile as the reinforcement material, incorporating four types of backfill material gradations and three reinforcement spacings. The gradation of the backfill materials primarily varied in particle size distribution within the range of 1-8 mm, while the reinforcement spacing was set at 20 cm, 25 cm, and 33.3 cm. The test results were compared with those of unreinforced soil and analytical predictions based on the Federal Highway Administration (FHWA) design guidelines. [Results] The experimental results demonstrated that reinforcement significantly enhanced the ultimate bearing capacity of GRS composites. Under the same backfill material condition, the incorporation of reinforcement led to significant increases in ultimate bearing capacity compared with the unreinforced test. Specifically, with reinforcement spacings of 20 cm and 25 cm, the ultimate bearing capacity increased by 87.5% and 62.5%, respectively. These results clearly indicated that the reinforcement spacing played a critical role in the bearing performance of GRS composites. In addition, smaller spacings resulted in greater overall stiffness of the composite system. When the reinforcement spacing was constant and the backfill particle size ranged between 1 mm and 8 mm, the effect of gradation on the ultimate bearing capacity was relatively minor. However, differences in backfill material gradation led to noticeable variations in the overall stiffness of GRS composites. When the experimental results were compared with predictions obtained from the FHWA-recommended method for GRS composite bearing capacity, a significant discrepancy was observed. The FHWA method considerably underestimated the ultimate bearing capacity in all test cases. Therefore, it was not recommended to calculate the ultimate bearing capacity of GRS composites with finer graded backfill materials by directly applying the FHWA method. During post-test inspection, the locations of geosynthetic rupture were identified and analyzed. The observed failure surfaces within the reinforced soil mass approximately corresponded to a Rankine failure plane. The results indicated that the obvious composite behaviors were demonstrated in the GRS composites. [Conclusion] This experimental study provides a systematic analysis of the ultimate bearing capacity of GRS composites under plane strain conditions, emphasizing the roles of reinforcement spacing and backfill material gradation. The findings confirm that geosynthetic reinforcement can significantly enhance both the strength and stiffness of the soil composites, with closer reinforcement spacing resulting in better performance. The study reveals that the current design guidelines recommended by FHWA significantly underestimate the actual ultimate bearing capacity, particularly when the backfill material gradation differs from the recommended values. These findings offer valuable reference for future engineering design and construction, promoting more efficient and reliable use of fine-grained or narrowly graded soil in reinforced soil structures.

[Objective] Current research on the damping ratio evolution of silty fine sand under varying saturation conditions remains relatively limited, and in particular, systematic investigations revealing the mechanisms by which saturation variations influence damping ratios within the framework of unsaturated soil mechanics are still insufficient. To address this gap, typical loose silty fine sand from the Hangzhou Bay area is selected as the research subject. Utilizing a self-improved unsaturated soil resonant column testing system, the damping ratio evolution characteristics and underlying mechanisms along the desaturation path are thoroughly investigated. The findings aim to provide a scientific basis for analyzing dynamic properties and engineering applications of unsaturated soils. [Methods] An improved unsaturated soil resonant column apparatus was employed to conduct small-strain dynamic property tests under controlled drainage paths and saturation conditions. Test specimens were initially prepared with varying saturation levels and stress states. A stepwise desaturation process under constant net stress conditions was then applied to establish a series of samples with different saturation states. Subsequently, shear modulus and damping ratio measurements were performed across varying saturation levels and net stress conditions via resonant column loading. By integrating theoretical analyses of soil-water characteristic curves (SWCC) and pore water distribution states, distinct stages of damping ratio evolution were identified, and damping mechanisms induced by saturation variations were elucidated. [Results] The experimental results indicated that: (1) under identical saturation conditions, the damping ratio of silty fine sand gradually decreased with increasing net stress. (2) During saturation evolution, the damping ratio exhibited distinct stage-wise variations: when the soil approached full saturation, the damping ratio remained relatively stable. As saturation declined, the damping ratio rapidly attenuated and reached a minimum near the optimal saturation Sr_opt. Further saturation reduction led to a slight rebound in the damping ratio. (3) Based on the SWCC and pore water morphology evolution, the saturation-dependent damping ratio variations were categorized into three stages: boundary effect stage, transition stage, and residual saturation stage. [Conclusion] This study elucidates the damping ratio evolution mechanism of loose silty fine sand along desaturation paths. It demonstrates that the damping ratio is influenced not only by net stress but also closely associated with pore water distribution states, and that its minimum damping ratio occurs at the optimal saturation Sr_opt. The results reveal that saturation-dependent damping ratio changes can be categorized into three stages, each corresponding to distinct pore water morphology evolution processes. By employing the improved unsaturated soil resonant column, the complete damping ratio evolution process with saturation variation is observed. The principle of minimum damping ratio under “optimal saturation” control is proposed, and the three-stage evolution mechanism is revealed through SWCC analysis. These findings provide a foundation for refining unsaturated soil dynamic models and enhancing the scientific rigor of related engineering designs.

[Objective] Large-scale excavation unloading above an existing subway tunnel, resulting from activities such as river dredging and widening or foundation pit excavation, induces unloading deformation of the soil mass, which threatens the stability of the tunnel structure. Therefore, measures are required to limit the soil deformation. Given the limited research on the impact of river dredging and excavation on existing tunnels, this paper, taking the Nanjing Jiuxiang River Widening Project as a case study, investigates the impact of the reinforcement project’s foundation pit excavation on the shield tunnel under different working conditions. It also predicts the effectiveness of the reinforcement structure in suppressing tunnel uplift, thereby providing a reference for subsequent construction. [Methods] A pressure slab structure, consisting of a slab, bored piles, and a diaphragm wall, was proposed to protect the tunnel, and the preliminary design parameters and construction plan for these components were determined. Based on this, the Finite Element Method (FEM) was employed to predict the impact of the reinforcement measures on the existing tunnel section. First, a three-dimensional finite element analysis model was established based on the fundamental assumption of compatibility between tunnel displacement and soil deformation. The parameters for the reinforcement zone and the structure were determined based on engineering experience and field-measured data. Then, a finite element analysis of the vertical displacement of the tunnel segments was conducted under various working conditions, and the contour maps of segment vertical displacement were obtained. Subsequently, the longitudinal deformation of the shield tunnel was analyzed using the calculation results from the most unfavorable working condition. A curve fitting regression analysis was performed using the Gaussian function on the tunnel segment deformation to establish its longitudinal distribution pattern. Finally, a verification calculation of the internal forces within the tunnel structure was performed. [Results] (1) Working Condition 5 was identified as the most unfavorable case, with a maximum uplift deformation of the tunnel segments of 17.8 mm. (2) The tunnel’s vertical displacement peaked near the intersection of the river and tunnel centerlines. This displacement decreased rapidly as the distance from the intersection increased, diminishing to nearly zero at a distance of approximately 90 m. (3) For Working Condition 5, the maximum negative bending moment per linear meter of the tunnel segment was 179.2 kN·m with a corresponding axial force of 724 kN, and the maximum positive bending moment was 161.3 kN·m with a corresponding axial force of 556 kN. The calculated crack widths were 0.170 mm and 0.187 mm, respectively. [Conclusions] (1) The pile-wall-slab structure is proven to be effective in restraining the tunnel’s uplift deformation. (2) The performance of the pile-wall-slab structure is closely related to the excavation method of the foundation pit. Adopting a strip excavation by sections can effectively control ground disturbance and, in turn, tunnel deformation. (3) Overall ground reinforcement is significantly effective in suppressing tunnel uplift deformation, whereas the effect of using counterweights inside the tunnel is relatively limited.

[Objective] This paper aims to investigate the mechanical properties and damage evolution law of lime-solidified saline soil under freeze-thaw cycles, with a focus on analyzing the effects of lime content, curing age, and the number of freeze-thaw cycles on its unconfined compressive strength (UCS). In addition, a damage constitutive model based on statistical distribution is established to predict the stress-strain response and performance degradation of solidified saline soil under freeze-thaw conditions. [Methods] Saline soil from Zhenlai County in western Jilin Province was selected as the research object. Lime was added at proportions of 3%, 6%, and 9% as the curing agent, and specimens were prepared under the optimum moisture content (20%) and a degree of compaction of 90%. The specimens were cured for 7 days and 28 days and subjected to 0-60 freeze-thaw cycles. Unconfined compressive strength (UCS) tests and scanning electron microscopy (SEM) analyses were conducted to examine the macroscopic mechanical properties and microstructural changes. Based on the Weibull distribution function, a damage evolution model was established using the experimental data. [Results] The optimum lime content was 6%, and the unconfined compressive strength reached 835.01 kPa after 28 days of curing, which was more than four times that of the untreated soil. Both untreated and solidified saline soils exhibited strain-softening behavior and brittle failure after freeze-thaw cycles. With the increase in freeze-thaw cycles, the strength of the solidified soil gradually decreased but still remained significantly higher than that of the untreated soil. SEM images showed that lime treatment effectively reduced crack development and improved the integrity of the microstructure. The established Weibull damage model accurately simulated the entire stress-strain process under different numbers of freeze-thaw cycles, and the fitting accuracy improved with an increasing number of cycles. [Conclusion] Lime solidification significantly enhances the strength and freeze-thaw resistance of saline soil, with the optimum effect achieved at a 6% lime content and 28 days of curing. The damage model based on the Weibull distribution can effectively characterize the mechanical behavior and damage evolution of solidified saline soil during freeze-thaw processes. The research findings provide theoretical and technical support for the solidification treatment of saline soil in cold regions. The innovation lies in correlating microstructural changes with macroscopic mechanical responses and establishing a statistical damage prediction model suitable for freeze-thaw conditions.

[Objective] The identification and grouping of rock mass structural planes are essential prerequisites for conducting stability analysis. Existing clustering methods are highly sensitive to initial cluster centers, resulting in suboptimal grouping results and relatively low efficiency. To overcome these limitations, this study proposes a rock mass structural plane clustering method based on an improved sparrow search algorithm (KS-SSA). [Methods] First, SPM chaotic mapping was used to initialize the sparrow population, and then the step factor of the SSA was modified to improve the optimization speed. The improved SSA algorithm was used to optimize the initial cluster centers, achieving a global optimal solution. Subsequently, the K-medoids algorithm was applied for the final grouping of structural plane orientation data. Silhouette coefficient (SC) was used to evaluate the clustering performance. Four sets of artificial structural planes were generated using Monte Carlo simulation to validate the proposed algorithm, and comparative analyses were conducted with classical KPSO, FCM, spectral clustering, and traditional sparrow algorithms. [Results] The differences among different algorithms were primarily concentrated at the boundary points. The proposed algorithm could accurately identify all boundary data points, demonstrating good clustering performance with higher recognition accuracy and better clustering results. Using the silhouette coefficient, the structural planes could be accurately divided into four groups, which was consistent with the actual conditions. Furthermore, the sparrow population size and the number of iterations were identified as two key parameters for clustering analysis. Therefore, based on the artificial data, the optimal population size was determined to be 50 by analyzing the variation of fitness curves. Setting the number of iterations to 50 was appropriate while ensuring computational efficiency. The application of the proposed method to the publicly available Shanley dataset further validated its effectiveness. The new algorithm was applied to 201 structural plane data from a rock slope in the Nujiang River. By calculating the silhouette coefficients, three dominant structural plane groups were identified, which were generally consistent with the field investigation results. In addition, the KPSO algorithm was also employed in the field structural plane data analysis, and computational efficiency of the new method was discussed. The KS-SSA algorithm achieved stable clustering results within just 50 iterations, whereas the KPSO algorithm required 110 iterations for convergence. The runtime of the KS-SSA and KPSO algorithms was 45.97 s and 69.95 s, respectively, indicating that the new method had significantly higher computational efficiency. [Conclusion] The KS-SSA algorithm initializes the sparrow population based on SPM chaotic mapping, effectively preventing the clustering results from falling into local optima. Meanwhile, the step factor of the sparrow algorithm is modified, enabling the KS-SSA algorithm to dynamically adjust the step size and improve the optimization speed. Comparative analysis of three case studies demonstrates that compared with traditional algorithms, the proposed method exhibits superior performance in boundary data identification and improved robustness. This study discusses and clarifies the parameter selection and computational efficiency of the new method. The new method demonstrates promising application prospects and can be applied to large-scale data processing and analysis, providing a theoretical basis for the numerical simulation of three-dimensional networks of rock mass structural planes and rock mass stability analysis.

[Objective] During tunnel excavation, mining roadway development, or other underground construction processes, the rock mass of underground engineering is typically subjected to multi-stage permanent dynamic and repetitive stress disturbances. Moreover, cyclic stress amplitude is recognized as one of the primary factors influencing the occurrence of dynamic disasters in underground engineering. This study aims to investigate the influence of stress amplitude on the mechanical behavior and fracture modes of red sandstone under multi-stage constant-amplitude cyclic loading. [Methods] Utilizing an MTS-816 rock mechanics servo-testing system and real-time acoustic emission (AE) monitoring technology, the strength and deformation behaviors of red sandstone specimens during multi-stage cyclic loading were analyzed in detail. The evolution patterns of AE parameters under varying cyclic stress amplitudes and the final failure modes of the red sandstone specimens were elucidated. [Results] Lower cyclic stress amplitudes were found to impart a certain strengthening effect on red sandstone specimens, whereas higher stress amplitudes induced more severe deterioration. The increase in cyclic stress amplitude exhibited a hardening effect on the sandstone. Under multi-stage constant-amplitude cyclic loading, the elastic modulus and elastic strain energy of the red sandstone specimens were generally enhanced. Within a single fatigue loading stage, the AE signals gradually transitioned from active to stable with the increase in loading cycles. Under lower stress amplitude conditions, the red sandstone specimens exhibited a higher number of AE ring-count events during the loading process, and the cumulative AE curve showed only a sudden increase at the moment of failure. In contrast, for specimens under higher stress amplitudes, the AE activity remained relatively stable in the early loading stage but became more intense at the time of failure. An increase in stress amplitude led to the formation of larger axial primary cracks at specimen failure, and the secondary cracks and local spalling phenomena caused by crack coalescence became more severe. Specimens under high stress amplitudes were more prone to splitting failure dominated by overall tensile cracks. [Conclusion] The cyclic stress amplitude has a significant impact on the safety and stability of underground engineering. Therefore, the dynamic cyclic stress amplitude should be emphasized as a key factor during the operation and maintenance of underground engineering.

[Objective] In recent years, with frequent extreme climate events, the northwestern region of China has experienced large diurnal temperature difference in summer, with daytime high temperatures continuously rising and persisting for extended periods. The resulting water vapor migration significantly affects the subgrade moisture content, thereby influencing the engineering performance of the subgrade. Investigating the water vapor migration patterns in loess subgrades under sealed pavement structures subjected to high temperatures holds substantial practical significance and application value for scientifically predicting the engineering performance of loess subgrades and ensuring their long-term stability. [Methods] A self-developed one-dimensional soil column test apparatus was employed, with compacted loess columns from the suburbs of Xi’an as the study subjects. The MTD15 temperature and moisture sensors were utilized to monitor the temperature and moisture variations within the loess columns under a boundary heating condition of 55 ℃. The water vapor migration patterns in one-dimensional sealed soil columns were analyzed under two heating modes: diurnal temperature cycling and continuous heating. Furthermore, the effects of different heating modes on the temperature and moisture distribution characteristics of the soil columns were explored. [Results] Boundary heating caused the temperature of the soil columns to rise, with a faster increase in the upper part and a slower increase at the bottom. During the heating phase, the temperature distribution along the height exhibited an approximately linear pattern. Under the cyclic heating of diurnal temperature difference, the internal temperature of the soil columns dropped rapidly after the heating was stopped. The cooling rate in the upper part was significantly higher than that in the middle and lower parts. By 08:00 the next day, the soil column temperature ranged between 22-25 ℃, with the middle part slightly warmer than the two ends. At 20:00 each day and 8:00 the following day, the temperature distribution along the depth of the soil columns remained basically the same. Under continuous heating, the soil column temperature reached dynamic equilibrium after 10 days of heating, exhibiting a two-segment, piecewise linear distribution. The variation trends in the moisture content distribution curves of the soil columns under the two heating methods were basically the same—namely, a decrease in moisture content in the upper part, an increase in the middle, and a decrease in the lower part, with these trends becoming more pronounced as the experiment progressed. However, within the upper 22.5 cm of the soil columns, the moisture content under the cyclic heating of diurnal temperature difference was higher than that under continuous high-temperature heating. [Conclusion] Under boundary heating conditions, the moisture of sealed soil columns is primarily governed by the combined effects of liquid water vaporization and moisture migration within the pores, water vapor migration driven by vapor concentration gradients, and condensation of vapor in the pores when the vapor pressure exceeds the saturated vapor pressure. These mechanisms collectively result in an inverse “S”-shaped moisture distribution. For subgrades with cover layers, 30 days of cyclic heating of diurnal temperature difference during summer induces moisture at 5 cm depth to fluctuate between -2% and +2% of the initial moisture content. This cyclic fluctuation of the upper subgrade moisture induced by diurnal temperature difference will significantly affect the mechanical properties of the subgrade. Under extreme climate conditions characterized by continuously rising heating temperatures and prolonged duration, the moisture content variations in the subgrade become more pronounced. These significant variations in moisture will lead to dehydration, shrinkage, and cracking in the upper subgrade layers, thereby compromising the service performance and lifespan of the subgrade and pavement structures.

[Objective] Extensive red sandstone spoil generated from highway tunnel projects in central-southern China poses significant disposal challenges due to its high water content, susceptibility to slaking and disintegration, low strength, and high compressibility—characteristics that fail to meet subgrade material standards. To realise the goal of “treating waste with waste and turning waste into treasure”, we use quicklime, calcined coal gangue powder, fly ash and cinder powder as multi-source solid-waste curing agents to explore the optimal ratio and mechanism for co-curing red sandstone spoil, providing a basis for resource utilisation. [Methods] Red sandstone spoil was served as the base material, quicklime, calcined coal gangue powder, grade-III fly ash and cinder powder were selected as curing agents, with their chemical compositions determined by X-ray fluorescence spectroscopy. Single-mix tests were conducted to investigate the effect of each curing agent alone on the strength of the red sandstone spoil and to determine appropriate dosage ranges. Then orthogonal experimental design was employed, using 7-day unconfined compressive strength (UCS) as the index to obtain the optimal ratio. Based on the optimal mix ratio, the red sandstone spoil was modified with curing agent dosages of 8%,10%,12%,and 14%,and cured for 3,7,14,and 28 days respectively. Unconfined compressive strength (UCS) was tested. Water stability coefficients were measured through water immersion tests. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were conducted to examine changes in microstructure and mineral phases before and after curing, revealing the stabilization mechanism. [Results] Optimal mix proportions and dosages:the optimal mass ratio was quicklime∶fly ash∶calcined coal gangue∶cinder powder=4∶8∶8∶7. The strength showed an initial increase followed by a decline with increasing total dosage, with 12% total dosage yielding the highest strength. Macro-scale performance improvement: at 12% dosage, 7-day UCS reached 1.659 MPa (16 times that of raw spoil) and 28-day UCS 2.255 MPa. Water stability was significantly improved, with the coefficient reaching 59.37% at 12% and 65.90% at 14%, with strength increasing over time, particularly rapidly between 3-14 days. Microscopic mechanism: XRD analysis showed that the contents of orthoclase and albite decreased in the improved red sandstone spoil, while the diffraction peaks of quartz and calcite were enhanced, indicating that ion exchange and pozzolanic reactions occurred, generating cementitious products such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H). SEM observation revealed that the soil particles were loose and porous before improvement, while after improvement, the cementitious products bonded the particles to form a dense skeletal structure, which became more complete with curing age. [Conclusions] The optimal co-curing ratio is quicklime∶fly ash∶calcined coal gangue∶cinder powder=4∶8∶8∶7 at 12% dosage, delivering the best mechanical properties and water stability. The mechanism is a lime-centred synergistic reaction: lime hydration provides an alkaline environment and Ca²⁺, triggering ion exchange (soil particles changing from potassium/sodium type to calcium type) and pozzolanic reactions (forming cementitious products), while solid waste particles fill pores to reinforce the soil skeleton. This offers an economical and eco-friendly route for red sandstone spoil utilisation, with both engineering and ecological value.