Search Results (189)

Seasonal permafrost areas undergo long-term freeze–thaw cycles, severely compromising the strength of foundation soils. Consequently, deformation and settlement under long-term cyclic traffic loads are greater than in normal temperature areas, leading to potential safety hazards. This study focuses on soft clay soils in seasonal permafrost areas. Remoulded soft clay is subjected to freeze–thaw cycles, followed by a series of long-term cyclic traffic load tests using the GDS dynamic triaxial testing system and pore size analyses using the nuclear magnetic resonance (NMR) technology. The study aims to investigate the effects of varying freeze–thaw cycles, compaction coefficients, and types of curing agents on the cumulative plastic strain of soft clay. The findings indicate that under identical freeze–thaw conditions, both the presence of curing agents and the increase of the soil’s compaction coefficient significantly restrain the deformation of freeze–thawed soils. In the micro perspective, freeze–thaw cycles cause irreversible fracturing of the soil’s internal framework, while the addition of curing agents effectively mitigates the pore enlargement effect. The resulting pore size distribution differs by about 4% from the original distribution, which is consistent with the patterns observed in dynamic triaxial tests. Full article

Dynamic characterization of calcareous (carbonate) sands is essential for performance-based design of offshore foundations, coastal reclamation, and marine infrastructure in tropical and subtropical regions. In contrast to silica sands, carbonate sediments are biogenic and typically comprise angular, irregular grains with intra-particle voids and fragile skeletal microstructure. These traits promote grain crushing and fabric evolution at relatively low-to-moderate confinement, leading to pronounced stress dependency, strong nonlinearity with strain amplitude, and substantial scatter in laboratory stiffness and damping measurements. Consequently, empirical correlations calibrated primarily on quartz sands may yield biased estimates when transferred to carbonate environments. This study presents an ML-driven, leakage-aware benchmarking framework for predicting two key dynamic parameters of biogenic calcareous sands, damping ratio $D$ and shear modulus $G$, using standard tabular descriptors commonly available in geotechnical practice. Two consolidated experimental databases were curated from resonant column and cyclic triaxial measurements ($D$: $n=890$; $G$: $n=966$), spanning mean effective confining stress $25 \le {\sigma }_m^{\prime }\le 1600 \mathrm{k}$Pa and a wide range of density and gradation conditions. To emphasize transferability, explicit deposit/site labels were excluded, and missingness arising from heterogeneous reporting was handled through a consistent preprocessing pipeline (training-only imputation, categorical encoding, and scaling). Eleven regression algorithms were evaluated, covering linear baselines, regularized regression, neighborhood learning, single trees, bagging and boosting ensembles, kernel regression, and a feedforward neural network. Performance was assessed using $R^2$, RMSE, and MAE on training/validation/test splits, and engineering credibility was supported through explainability-based diagnostics to verify mechanically plausible sensitivities. Results show that ensemble-tree models (Extra Trees and Random Forest) provide the most reliable accuracy–robustness balance across both targets, consistently outperforming linear models and the tested SVR configuration and exhibiting stable validation-to-test behavior. The explainability audit confirms physically meaningful separation of governing controls: stiffness is primarily stress-controlled (${\sigma }_m^{\prime }$ dominant for $G$), whereas damping is primarily strain-controlled ($\gamma$ dominant for $D$). The proposed framework supports practical deployment as a fast surrogate for generating $G\hspace{0.17em}-\hspace{0.17em}\gamma$ and $D\hspace{0.17em}-\hspace{0.17em}\gamma$ curves within the training domain and for guiding targeted laboratory test planning in carbonate settings. Full article

During reservoir stimulation and long-term operation of Enhanced Geothermal Systems (EGSs), repeated injection of cold fluids induces cyclic thermal shock in the surrounding rock mass, leading to progressive modification of mechanical properties and fracture behavior. However, the combined effects of cyclic thermal shock and true-triaxial stress conditions on granite strength and failure characteristics remain inadequately quantified. In this study, a series of true-triaxial compression tests were conducted on granite specimens subjected to cyclic thermal shock at 400 °C. Thermal shock cycles of 0, 1, 5, 10, and 15 were considered in conjunction with intermediate principal stress levels of 5, 20, 30, and 50 MPa to systematically evaluate their coupled influence on characteristic stresses and macroscopic failure behavior. The results show that the peak intensity increases with the rise of the intermediate principal stress, but with the increase in the number of thermal shocks, it first increases and then decreases. Macroscopic failure is dominated by asymmetric V-shaped fracture surfaces, roughly oriented along the ${\sigma }_2$ direction. As the intermediate principal stress increases, the failure mode transitions from tensile–shear mixed failure to shear-dominated failure, whereas thermal cycling promotes the persistence of tensile–shear cracking even under relatively high ${\sigma }_2$ conditions. Based on these observations, a modified Mogi–Coulomb strength criterion that accounts for thermal shock-induced damage is proposed to describe granite strength under true-triaxial stress conditions. The research results can provide a theoretical basis for optimizing the design of hydraulic fracturing in hot dry rock and evaluating reservoir stability. Full article

To investigate the effects of repeated drying and wetting on the mechanical properties and meso-fabric of metal tailings, lead-zinc tailings from Hunan Province were studied. A self-developed apparatus was used to simulate the cyclic drying-wetting processes. Combined with triaxial shear tests and stereomicroscopic image analysis, the changes in macroscopic mechanical properties and meso-fabric, as well as their correlation mechanisms, were investigated. The results indicate that the wet-dry cycles did not alter the strain-softening characteristics of the tailings’ stress-strain curves; however, they significantly intensified the degree of softening during the later stages of cycling (4–6 cycles). The static strength exhibited a trend characterized by “initial gradual degradation → temporary recovery → further deterioration” with an increasing number of cycles. After six cycles, the strength was significantly reduced compared to the initial state. The effective cohesion (c′) fluctuated markedly, with an amplitude of 31.1%, while the variation in the effective internal friction angle (φ′) was only 6.02%, indicating that dry-wet cycles have a more pronounced effect on the cohesion of tailings. At the microscopic level, the dry-wet cycling process promoted the upward migration of fine particles ranging from 0 to 60 µm, resulting in a decrease in the proportion of smaller particles in the lower layer. The porosity increased overall, with the lower layer rising from 44.06% to 54.26%. Pore evolution was dominated by the enlargement of pores larger than 150 µm. The macro-meso correlation analysis revealed that “fine particle migration → expansion of pores → loss of cementitious material” was the core driving factor for the deterioration of macroscopic mechanics, and the macroscopic mechanical response was the external manifestation of the adjustment of the microscopic structure. This research can provide certain theoretical support for the long-term safe operation and stability improvement of tailings dams. Full article

In the process of continuous mining and continuous backfilling (CMCB) in close-distance coal seams, the supporting unit (CMCBSU), composed of coal pillar and filling body, is affected by mining-induced disturbances from adjacent coal seams. This study establishes a mechanical model for the CMCBSU, revealing that the coordination of the CMCBSU depends on the similarity degree of elastic modulus of the components. Subsequently, numerical simulations were conducted to analyze the stress conditions. The results showed that the ${\sigma }_1$ and ${\sigma }_3$ exhibited cyclic loading and unloading characteristics. Based on the stress paths, conventional triaxial compression tests were performed on coal (CTC-coal), filling body, and the CMCBSU, as well as triaxial cyclic loading and unloading tests on coal (TCLU-coal). The results indicated that coal exhibited significant brittleness, the filling body demonstrated strain-softening characteristics, and the CMCBSU showed strain-softening behavior. Hysteresis loops were observed in the elastic region of the TCLU-coal. The failure characteristics of the specimens indicated that the shear stress was the primary cause of specimen failure. After testing, the filling body exhibited radial fish-scale-like wrinkles on the specimen surface, the coal and the CMCBSU showed primary shear cracks. In the CMCBSU, the primary shear crack generated on the filling body side relates to that on the coal side. In contrast, secondary cracks on the filling body side rarely penetrate the coal side. Excluding the influence of internal weak planes on specimen failure, cyclic loading and unloading within the elastic region of the coal reduced its internal friction angle. Mechanical parameters indicate that the weaker load-bearing medium determined the load-bearing capacity of the CMCBSU, the medium with a higher elastic modulus primarily determined the CMCBSU’s resistance to elastic deformation, and the cyclic loading and unloading caused by CMCBSU in close-distance coal seams had minimal impact on the coal’s resistance to elastic deformation. Full article

In cold regions, flexible pavements are vulnerable to frost-induced damage, necessitating effective insulation strategies. Foam glass aggregate (FGA) insulation layers, made from recycled glass, offer promising thermal insulation properties but are mechanically fragile and susceptible to permanent deformation under repeated loading. Manufacturers provide technical recommendations, particularly regarding load limits for installation and the dimensions of the thermal protection layer. These are considered insufficient to assist pavement designers in their work. The definition of critical criteria for permissible loads was deemed necessary to design mechanically durable structures using this alternative technology. This study investigates the critical stress conditions that FGA layers can tolerate within flexible pavement systems to ensure long-term structural integrity. Laboratory cyclic triaxial tests and full-scale accelerated pavement testing using a heavy vehicle simulator were conducted to evaluate the resilient modulus and permanent deformation behavior of FGA. The results show that FGA exhibits stress-dependent elastoplastic behavior, with resilient modulus values ranging from 70 to 200 MPa. Most samples exhibited plastic creep or incremental collapse behavior, underscoring the importance of careful stress management. A strain-hardening model was calibrated using both laboratory and full-scale data, incorporating a reliability level of 95%. This study identifies critical deviatoric stress thresholds (15–25 kPa) to maintain stable deformation behavior (Range A) under realistic confining pressures. FGA performs well as a lightweight, insulating, and draining layer, but design criteria remain to be defined for the design of multi-layer road structures adapted to local materials and traffic conditions. Establishing allowable critical stress levels would help designers mechanically validate the geometry, particularly the adequacy of the overlying layers. These findings support the development of mechanistic design criteria for FGA insulation layers, ensuring their durability and optimal performance in cold climate pavements. Full article

Offshore wind turbines are subjected to environmental loads such as wind and ocean waves throughout their entire service lives. Saturated sandy soils experience liquefaction under cyclic shear stresses induced by earthquakes or strong wave actions, which can result in the tilting, settlement, or even overturning of structures. This study investigates the effect of fine content (FC) on the liquefaction resistance (CRR) of saturated sandy soils with different density states. Sandy soils with varying FC values are examined under three scenarios: (1) constant relative density; (2) constant void ratio; and (3) constant skeleton void ratio. A series of undrained cyclic triaxial tests are conducted on sandy soils with different FC and density states (D_r, e, and e_sk). The results indicate that an increase in FC leads to a decrease in CRR at constant D_r or e, whereas CRR at constant e_sk increases with increasing FC. No clear correlation is observed between D_r, e, or e_sk and CRR for saturated sandy soils with varying FC. Since e_sk does not account for the effect of fine particles on the contact state of skeleton particles, the equivalent skeleton void ratio (e_sk^*) is introduced to describe the particle contact state of sandy soils with different fine contents (FCs), considering the degree of fine particle participation. In addition, the test data reveal that the CRR of sandy soils with different FC and density states decreases with increasing e_sk^*, and a power relationship between the reduction in CRR and the increase in e_sk^* is established. This finding indicates that e_sk^*, which considers the proportion of fines contributing to the load-sustaining framework, serves as a reliable index for evaluating the CRR of various sandy soils. We find that grain shape plays a significant role in influencing CRR, and the overall CRR of sandy soils increases as the grain shape changes from spherical to angular, compared to the published test results for other sandy soils. Full article

Three-dimensionally printed (3DP) samples with quartz sand effectively avoid the heterogeneity of reservoir rocks in underground gas storage (UGS), providing reliable supports for rock mechanics research under cyclic injection–production pressures. A study on the mechanical properties of 3DP rock samples was conducted by coupling triaxial tests with discrete element method (DEM) simulation. Key results are as follows: (1) The graded particle model (GPM) based on actual particle size distribution (PSD) closely matched experimental data, with an average peak strength error of 1.13%. (2) Cyclic saturation post-processing with silica sol significantly enhanced mechanical properties, increasing peak strength from 5.70 to 52.84 MPa and inducing a plastic-to-brittle failure transition. A power-law relationship was identified between saturation cycles and macroscopic strength. (3) DEM simulations revealed that bond effective modulus linearly controls Young’s modulus. The influence of cohesion on peak strength is greater than that of the friction angle, and the bond stiffness ratio regulates shear failure threshold. The cohesion force is 50 MPa, and the peak strength has been increased to 107.89 MPa. (4) Enhancing particle cohesive strength was key to improving the mechanical properties of 3DP rock samples. This study provides a reliable framework for customized 3DP rock preparation and UGS-related mechanical simulations. Full article

This study systematically investigates the evolution of mechanical damage and the permeability response of tight sandstone under triaxial compression and alternating load conditions, with a focus on the safety and stability of deep underground tight sandstone gas storage reservoirs in China subjected to complex geological environments and alternating stress conditions. By integrating conventional triaxial testing, cyclic loading experiments, CT scanning, and fractal dimension analysis, this study elucidates the enhancement effects and transformation mechanisms of confining pressure on the strength behavior and failure patterns of sandstone. It identifies the influence mechanisms of fault roughness on permeability and its convergence behavior under high-stress conditions and comprehensively characterizes the three-stage evolution of sandstone damage at the microscale under cyclic loading. Experimental results showed that with increasing confining pressure, both the peak strength and elastic modulus of sandstone displayed an increasing trend. With confining pressure increasing from 10 MPa to 40 MPa, the peak deviatoric stress increased from 98.42 MPa to 171.00 MPa and the elastic modulus rose from 8.70 GPa to 12.65 GPa. The failure mode transitioned from brittle shear failure under low confining pressure to a ductile-plastic failure pattern under high confining pressure. Alternating loading resulted in a 17.23% reduction in sandstone strength (from 98.42 MPa to 81.46 MPa at 10 MPa confining pressure). At confining pressures > 25 MPa, the permeability differences among faults with different roughness converged to within 10%. These research findings offer a robust experimental foundation and theoretical framework for evaluating the long-term stability and predicting the sealing performance of deep underground gas storage reservoirs. Full article

This study examines the seepage and damage behavior of coal under cyclic loading and unloading, typical in multi-layer coal seam mining. Four stress paths were designed: isobaric, stepwise, incrementally increasing, and cross-cyclic, based on real-time stress monitoring in protected coal strata. Seepage tests on gas-bearing coal were conducted using a fluid–solid coupled triaxial apparatus. The results show that axial compression most significantly affects axial strain, followed by volumetric strain, with minimal impact on radial strain. Permeability variation closely follows the stress–strain curve. Under isobaric cyclic loading (below specimen failure strength), specimens with higher initial damage (0.6) exhibit a sharp permeability decrease (75.47%) after the first cycle, with gradual recovery in subsequent cycles. In contrast, samples with lower initial damage (0.05) show higher permeability during loading, which eventually reverses, with unloading permeability surpassing loading permeability. Across all paths, a significant increase in residual deformation and permeability recovery exceeding 100% indicate the onset of instability. Continued cyclic loading increases damage accumulation, with different evolution patterns based on initial damage levels. These findings provide valuable insights into the pressure-relief permeability enhancement mechanism in coal seam mining and inform optimal gas drainage borehole design. Full article

Fatigue damage is critical for 0Cr17Ni4Cu4Nb stainless-steel components that may operate near yield under stress-controlled cycles and occasional peak holds. This work investigates the cyclic response of 0Cr17Ni4Cu4Nb stainless-steel under near-yield-stress-controlled (NYSC) loading and proposes a unified damage framework that bridges monotonic ductile fracture, near-yield stress-controlled fatigue. Building on the Enhanced Lou-Yoon model, an elastic-damage term is introduced and embedded within a continuum damage mechanics framework, allowing elastic (sub-yield) and plastic (post-yield, Ultra-Low-Cycle-Fatigue/Low-Cycle-Fatigue (ULCF/LCF)) damage to be treated in a unified, path-averaged stress-state description defined by stress triaxiality and the Lode parameter. Five stress-controlled test groups are examined, with applied load amplitudes from 20.6 to 25.1 kN (equivalent stress amplitudes 858~1044 MPa) yielding fatigue lives ranging from 32 to 13,570 cycles. The extended model captures the evolution of damage origin mechanisms from elasticity-dominated to plasticity-dominated as loading severity increases, demonstrating a unified elastic-plastic damage modeling approach. As a result, it accurately predicts fatigue lives spanning two orders of magnitude with an average absolute percentage error of approximately 14.5% across all conditions. Full article

To investigate the seepage and deformation failure characteristics of deep unloaded rock mass under cyclic loading and unloading disturbance, a series of triaxial cyclic loading and unloading tests were conducted on granite. These tests were performed under varying seepage pressures and unloading conditions to analyze the mechanical properties, seepage behavior, and fracture failure characteristics of the material. The findings indicate the following: (1) An increase in seepage pressure and unloading magnitude results in pronounced radial expansion characteristics in the rock specimens following cyclic loading and unloading. Additionally, the axial, radial, and volumetric residual strains exhibit a nonlinear acceleration in growth as the number of cyclic loading and unloading applications increases. (2) The elastic modulus of rocks exhibits two distinct phases: an initial rapid decline followed by a steady-state decrease. Concurrently, Poisson’s ratio demonstrates an initial decrease, which is subsequently followed by a consistent increase. Furthermore, when considering the effects of unloading, the inflection point of the Poisson’s ratio curve will occur earlier. (3) The interplay between seepage pressure and unloading conditions markedly exacerbates the damage and degradation of the rock. Specifically, under conditions of 70% unloading and a seepage pressure of 4 MPa, the peak stress of the rock specimen is reduced by 21.90%, and the peak intensity permeability increases by 446.70%. (4) Under conditions of high confining pressure and elevated seepage pressure, V-shaped conjugate shear fracture surfaces are likely to develop during the cyclic loading failure of granite, accompanied by a limited number of secondary shear cracks. Concurrently, tensile failure surfaces that are parallel to the maximum principal stress are also observed under the influence of unloading. Full article

While methods like cyclic triaxial testing and p-y model updating theory exist in geotechnical and offshore wind engineering, they have not been systematically applied to solve the specific deformation problems of offshore PV piles. This study investigates a specific offshore photovoltaic (PV) project in Qinhuangdao City, Hebei Province. Initially, field tests of horizontal static load on steel pipe pile foundations were conducted. A finite element model (FEM) of single piles was subsequently developed and validated. Further analysis examined the failure modes, initial stiffness, and ultimate resistance of offshore PV single piles in sandy soil foundations under varying pile diameters and embedment depths. The hyperbolic p-y curve model was modified by incorporating pile diameter size effects and embedment depth considerations. Key findings reveal the following: (1) The predominant failure mechanism of fixed offshore PV monopiles manifests as wedge-shaped failure in shallow soil layers. (2) Conventional API specifications and standard hyperbolic models demonstrate significant deviations in predicting p-y (horizontal soil resistance-pile displacement) curves, whereas the modified hyperbolic model shows good agreement with field measurements and numerical simulations. This research provides critical data support and methodological references for calculating the horizontal bearing capacity of offshore PV steel pipe pile foundations. Full article

The growing urgency for transportation network development in seasonally frozen regions brings attention to two critical symmetrical factors: cyclic loading and freeze–thaw cycles. In saline soil areas, these symmetrical mechanical and environmental processes, along with varying salt content, significantly affect soil mechanical properties, posing considerable challenges for engineering design. In this study, the dynamic triaxial tests were conducted on a type of carbonate saline soil considering four factors, including moisture content, salt content, freeze–thaw cycle and confining pressure, and the variations in dynamic parameters, including dynamic strength and dynamic elastic modulus, with the above four factors were studied, and the influential mechanisms of four factors were fully discussed. The results demonstrated that the variations in dynamic strength (τ_d) versus vibration cycles (N_F) were better fitted by logarithmic functions than by a linear one. An increase in moisture content, salt content, and freeze–thaw cycle all reduced the τ_d and dynamic elastic modulus (E_d); in addition, the E_d decreased significantly when the dynamic axial strain was less than 0.2%, and then stabilized with further increases in dynamic axial strain. The dynamic parameters of saline soil became nearly constant after undergoing five freeze–thaw cycles, and increased significantly with increasing confining pressure. Moreover, the relationship between the maximum dynamic elastic modulus (E_dmax) and the four factors could be described by power functions. These findings could provide certain references for addressing the combined effects of symmetrical cyclic loading and freeze–thaw cycles in subgrade design for saline soil regions. Full article

To address the challenge of long-term stiffness retention of subgrades in humid–hot climates, this study evaluates expansive soil stabilized with construction and demolition waste (CDW), focusing on the resilient modulus (M_r) under coupled stress states and wetting–drying histories. Basic physical and swelling tests identified an optimal CDW incorporation of about 40%, which was then used to prepare specimens subjected to controlled. Wetting–drying cycles (0, 1, 3, 6, 10) and multistage cyclic triaxial loading across confining and deviatoric stress combinations. M_r increased monotonically with both stresses, with stronger confinement hardening at higher deviatoric levels; with cycling, M_r exhibited a rapid then gradual degradation, and for most stress combinations, the ten-cycle loss was 20%–30%, slightly mitigated by higher confinement. Grey relational analysis ranked influence as follows: the number of wetting–drying cycles > deviatoric stress > confining pressure. A Lytton model, based on a modified prediction method, accurately predicted M_r across conditions (R² ≈ 0.95–0.98). These results integrate stress dependence with environmental degradation, offering guidance on material selection (approximately 40% incorporation), construction (adequate compaction), and maintenance (priority control of early moisture fluctuations), and provide theoretical support for durable expansive soil subgrades in humid–hot regions. Full article