ABSTRACT This study investigates the dynamic stability of frozen soil slopes subjected to strong disturbances such as blasting. Dynamic compression tests were conducted on saturated frozen clay specimens using a split Hopkinson pressure bar system. The effects of strain rate (100–700 s−1), temperature (−15°C, −23°C, and −30°C), and pore ratio (0.30, 0.24, and 0.18) on the mechanical behavior were systematically examined. The propagation of stress waves and the dynamic stress–strain responses were analyzed. Results show that as the strain rate increases, the arrival times of the incident, reflected, and transmitted wave peaks advance, and the time to reach peak energy, stress, and strain decreases. This trend is consistent across all tested temperatures and pore ratios. Both lower temperatures and higher pore ratios lead to increased specimen strength and a marked shortening of the plastic plateau stage in the stress–strain curves. Based on the experimental results and the effective stress principle for saturated soils, a damage-enhanced constitutive model was developed within the Zhu-Wang-Tang constitutive framework by incorporating a wave-impedance term. This term links microstructural changes (ice content and cementation) to macroscopic strength, effectively characterizing the coupled effects of strain rate, temperature, and pore ratio. The model predictions show good agreement with the experimental data, providing a theoretical basis for the dynamic analysis of frozen soil engineering.

This study investigates geogrid reinforcement performance using a discrete element and finite difference coupled method to simulate the interactions between a calibrated precise-shape geogrid and ballast. A comparative analysis of experimental and numerical simulation results for single- and double-layer geogrid pullout behaviors was conducted, evaluating the pullout resistance-displacement evolution, internal contact force distribution, and particle displacement patterns. The influence of different geogrid layouts on the mechanical properties of the geogrid-ballast interface was also investigated. The results show that the single-layer geogrid at 300 mm generated the highest pullout resistance, while the double-layer geogrids significantly improved the reinforcement efficiency compared to the single-layer geogrid, consistent with experimental results. Furthermore, the coefficient of double-layer reinforcement effect was proposed to assess the capacity of double-layer reinforcement. It reached a maximum of 1.52 when the double-layer geogrid was laid at 200-300 mm. The interlayer zone exhibits combined interlocking effects, where overlapping contact forces stabilize particles and reduce ballast movement. This interaction strengthens trackbed integrity by minimizing particle rearrangement under load. Findings underscore the superiority of double-layer layouts in optimizing geogrid-ballast interfacial mechanics through controlled force interference and enhanced interlocking.

This study examines the effect of ballast gradation on the repose angle of ballast piles through large-scale hopper flow tests conducted across 158 gradation group within and exceed the Chinese Railway Ballast Specifications. The results indicate that within the specified limits, ballast piles exhibit higher repose angles. When d50 is fixed, the repose angle first increases and then decreases with increasing uniformity coefficient (C ), peaking at d50 = 40.0 mm and C =1.46, suggesting an optimal gradation for well-graded ballast. Generally, increases in d100 and C reduce the repose angle, with the minimum values occurring when both parameters are large. The upper and lower span of the gradation exhibit a nonlinear effect on the repose angle, indicating a synergistic influence of the particle size spans on ballast pile stability. A predictive equation for the repose angle, incorporating the combined effects of key gradation parameters (d30, d50, d80, d100, and C ), is proposed. For practical application in railway, the ballast gradation range specified in current standards has been further refined to identify an optimized gradation region for superior ballast quality. The findings provide a rapid evaluation method for ballast degradation and ballast gradation optimization in railway engineering.

Longitudinal resistance is critical for ballasted track stability. An innovative X-shaped sleeper was developed to enhance this performance. A coupled DEM-MBD model of a full-scale ballast-sleeper-subgrade system was established to investigate the effect of sleeper geometry on longitudinal resistance and mechanisms. Results show a single X-shaped sleeper increases longitudinal resistance by approximately 27.2% over double I-shaped sleepers. Both types exhibit comparable end and bottom resistance, but the X-shaped sleeper's V-shaped arms significantly improve ballast interlocking, nearly doubling side frictional resistance. Microscopic analysis indicates the X-shaped sleeper mobilizes a broader range of particles, engaging more ballast in resisting movement. Stress analysis reveals stress under I-shaped sleepers concentrates beneath the crib, whereas the X-shaped sleeper transfers stress into shoulder ballast and deeper subgrade, enabling subballast and subgrade layers to enhance trackbed stability through compaction and disturbance. These findings support the application and optimization of X-shaped sleepers in ballasted tracks.

A series of 1-g shaking table model tests were carried out on two rectangular subway station structures embedded in kaolin clay bed, and the test results suggested that the two ends of the middle column and the lower corner between roof and sidewall experienced evidently much larger bending moments and tensile strains, indicating that these locations are most likely to be damaged during seismic shakings. Furthermore, using a validated numerical simulation program, a suite of three-dimensional (3D) finite element (FE) analyses were performed to systematically study the factors of seismic intensity, flexural rigidity of middle column and sidewall thickness on the seismic responses of two-storey two-span subway station structures installed in clayey ground. It was found that the inter-storey drift ratio (IDR), damage extent and middle column’s bending moment were comparatively larger for the lower storey, which was largely due to the inertial effect arising from the upper storey and its surrounding clays. In addition, the indicators of both IDR and damage factor were employed to quantitatively evaluate the seismic damage extents of the two-storey two-span subway station structures installed in soft clayey strata. Some viable suggestions were raised for systematically assessing and reducing the seismic damages of subway station structures in clayey ground.

Natural granite residual soil (GRS) usually exhibits a metastable structure characterized by an inherited relict structure from the parent rock, cemented by free iron oxides and clay, which confers a complex mechanical behavior. However, existing research has rarely established constitutive models that account for the unique structured characteristics of GRS. Accordingly, this study introduces a constitutive model for GRS, referred to as the Modified Cam Clay (MCC) model, to simulate the mechanical behavior considering the degradation of inherited structure and cementation. In this model, a non-linear failure envelope is formulated to merge with the Critical State Line of the remolded soil at high stress levels in order to capture the degradation of inherited structure. And a damage function is introduced to capture the cementation degradation during shearing. Finally, four structured GRS and three tuff residual soils are employed to validate the model’s accuracy through triaxial experiments and true triaxial experiments. Simulation results show the effect of stress level, and the deformation change from dilation to compression can be successfully reproduced. The introduced model can accurately capture the mechanical behavior of not only structured GRS but also other structured residual soils, proving its extended suitability, which could provide more guidance for engineering design and construction in weathered strata.

To investigate the dynamic response and load transfer in ballasted tracks under impact loading, this study developed full-scale three-dimensional discrete element-multi-body dynamics (DEM-MBD) coupled models for I-shaped and X-shaped sleeper systems. Macroscopic responses, including wheel-rail impact forces, sleeper and subgrade surface accelerations, and subgrade stress distribution, were evaluated, while mesoscopic behavior focused on sleeper-ballast contact distribution, ballast force chains, and particle motion. Results indicate that increasing the wheelset drop height from 10 mm to 30 mm nonlinearly amplified key responses: peak sleeper loads increased from 110.6 kN to 188.8 kN, mean ballast contact forces rose from 38.37 N to 72.16 N, and subgrade surface accelerations escalated from 33.4 m/s² to 78.0 m/s². Compared with conventional I-shaped sleepers, the X-shaped design reduced peak sleeper acceleration by 23% and average ballast contact forces by 40%. Mesoscopic analysis revealed that impact loads were transmitted through concentrated force chains beneath the sleepers, forming a trapezoidal stress diffusion zone. The X-shaped sleeper fundamentally optimizes load transfer by transforming sparse, high-intensity force chains (I-shaped) into a dense network of finer force chains. This enhances particle interlocking, promotes multi-directional stress diffusion, and reduces localized stress concentrations in both the ballast and subgrade, leading to superior impact energy dissipation and overall bearing stability. These findings demonstrate that the X-shaped sleeper improves load transfer efficiency, impact resistance, and vibration mitigation in ballasted track systems.

The mechanical behavior of sand is characterized by significant nonlinearity, anisotropy, and state-dependence, which poses considerable challenges to traditional constitutive models under complex loading conditions. To address the limitations of existing models in terms of parameter complexity and numerical efficiency, this paper introduces a simple sand model grounded in a modified hyperbolic equation and the Anisotropic Critical State Theory (ACST). The model first unifies the description of strain hardening and softening by incorporating a virtual peak deviatoric stress into the hyperbolic equation, avoiding an explicit yield surface. Subsequently, leveraging the ACST, it captures the effects of void ratio, mean effective stress, and fabric evolution through a dilatancy state parameter defined as a function of the fabric tensor. Furthermore, based on the elastoplastic theory, the model is extended from principal stress space to a general stress space. The proposed model is implemented in ABAQUS via a user-defined material subroutine (UMAT) using a modified Euler stress integration algorithm. Its performance is rigorously validated against extensive experimental data, including triaxial, hollow cylinder torsional shear, plane strain, and strip footing bearing capacity tests. The results highlight the critical influence of anisotropic fabric on sand behavior. With only 13 parameters, the model demonstrates a significantly enhanced capability to predict anisotropic mechanical responses under complex stress paths, offering a reliable computational tool for the numerical analysis of granular materials under monotonic loading.

Particle breakage governs the degradation of railway ballast under repeated loading; however, the influence of fracture configuration in the Bonded Particle Model (BPM) on macro-micro responses remain insufficiently understood. This study compares three BPM configurations-Uniform Bond Breakage Model (UBBM), Corner Bond Breakage Model (CBBM), and Diverse Bond Breakage Model (DBBM)-using coupled Finite Difference Method (FDM)- Discrete Element Method (DEM) triaxial simulations with a flexible membrane boundary. The results show that all breakable models exhibit similar responses at the lowest confining pressure of 60 kPa, whereas clear divergence emerges at higher confinement (90-240 kPa). Compared with the rigid model, the breakable specimen under a confining pressure of 60 kPa exhibits an approximately 50% reduction in peak deviatoric stress. Particle breakage also suppresses dilatancy, and this suppression becomes more pronounced under higher confining pressures. At the microscale, breakage promotes fragment redistribution and drives the transition of force chain structures from anisotropic to more isotropic patterns. Gradation analysis indicates that fragmentation is localized within the shear zone and intensifies with increasing confining pressure and axial strain. Overall, the results demonstrate that bond strength heterogeneity and fracture topology jointly govern the mechanical response and structural evolution of ballast. Among the examined configurations, the DBBM provides more consistent agreement with experimental observations, highlighting the importance of incorporating non-uniform bond structures in DEM simulations.

Coral reef limestone (CRL) exhibits mechanical anisotropy due to its porous structure and oriented growth lines, yet replicating such heterogeneous pore systems in numerical simulation remains challenging: 3D micro-CT imaging is costly and provides only structural geometry, while discontinuous pores hinder conventional discrete element models from building realistic 3D representations. To address this issue, a refined discrete element modeling approach is proposed to reproduce the anisotropic pore structure of CRL, characterized by honeycomb-like and oriented tubular features. A coupled discrete element method (DEM)-finite difference method (FDM) approach is further employed to simulate Split Hopkinson Pressure Bar (SHPB) tests, with model parameters calibrated against experimental results to ensure quantitative accuracy. The effects of growth line inclination on dynamic micro-mechanical behavior are systematically investigated. Results show that while dynamic strength increases with strain rate, its rate sensitivity diminishes at higher loading rates. As the inclination increases from 0 degrees to 90 degrees , the dynamic peak stress and elastic modulus decrease significantly by up to 62.0 % and 58.5 %, respectively. Micromechanical analysis reveals that this degradation is governed by a transition in load transfer mechanisms from axial compression to oblique shear along weaker structural interfaces. These findings provide new insights into the micro-mechanical behavior of porous CRL.

While screw-shaft piles are widely used in fine-grained soils, their application in coarse-grained soils remains limited.Coarse particles make the pile-soil interaction more complex and obscure the load transfer mechanism, leading to considerable errors in existing calculation methods. To address this gap, based on a practical project, this study takes gravelly soil as a typical coarse-grained soil and systematically investigates the vertical bearing performance of a single screw-shaft pile. Drawing on a real-world engineering project, a numerical model is established and subsequently validated by laboratory model tests. Numerical simulations are conducted to analyze the load transfer mechanism, soil displacement distribution, and the effects of pile length, pile diameter, thread segment ratio, thread width, and thread pitch on bearing characteristics. The results demonstrate that under ultimate conditions, the influence radius of soil displacement is about 1D–2D at the pile side and 1D at the tip (D = pile diameter). As pile length increases from 800 mm to 1400 mm, the ultimate bearing capacity increases by 20.2

The application of reinforcing geogrid is a simple, cost-effective method for reducing permanent deformation in the ballast layer. Understanding the behavior of the ballast/geogrid system can lead to improved railway design and lower maintenance. A composite element test (CET), under simplified full-scale field conditions, was simulated by coupling discrete element method and the finite difference method. This study investigated the dynamic response and deformation behavior of geogrid-reinforced ballast under cyclic loading, focusing on variations in subgrade stiffness, geogrid location, and boundary conditions within the CET. Results indicate that greater subgrade stiffness increases the compressive force borne by the subgrade. Conversely, as subgrade stiffness decreases, the upper load is more evenly distributed to the bottom. The deployment of geogrids effectively constrains ballast particles, disperses upper loads, and reduces contact force at the model base, thereby minimizing sleeper settlement. Moreover, geogrid reinforcement is more significant for soft subgrade than for stiff subgrade. Simultaneously, the sleeper settlement under confined conditions is significantly smaller than that under unconfined condition. These results contributed to a comprehensive analysis of the mechanical properties of ballasted bed under dynamic loads, offering insights from both micro and macroperspectives. Additionally, the study clarifies the mechanism of geogrid-reinforced ballast, offering valuable insights for practical geogrid applications.

To address the long-term settlement of embankments over structured soft soil during the in-service stage, artificial structured soils with different interparticle bonding strengths and initial void ratios were prepared, and repeated triaxial loading tests were conducted to investigate the effects of bonding strength, initial void ratio, stress amplitude and cycle number on the accumulative deformation characteristics. The results show that the relationship between the accumulative plastic strain and cycle number can be classified into stable, critical and destructive types, and an empirical relationship between the stress sensitivity and dynamic stress ratio is established. Furthermore, two different empirical models for accumulative plastic strain are presented that incorporate soil structure. Reasonable agreement between the model predictions and the experimental results for different natural soft soils demonstrate that the proposed models can accurately capture the accumulative deformation behaviour of structured soils. In addition, considering the accumulated plastic deformation of soil subjected to cyclic loading as static creep, a simplified method for calculating three-dimensional cyclic accumulative deformation is proposed by implementing the proposed model in a finite-element simulation utilizing an implicit stress integration algorithm. Finally, the effects of the dynamic stress level and structural strength on the accumulative deformation are analyzed. This has important implications in controlling the long-term settlement of embankment in soft soil area.

In marine infrastructures, pile foundations are widely employed as the supporting systems, which often face threatening arising from geological hazards such as earthquakes and submarine landslides. Using coupled smoothed particle hydrodynamics (SPH) and finite element method (FEM), a series of numerical simulations were performed to examine the performances of both regular monopile and hybrid monopile (with friction wheel) in soft marine clay subjected to submarine debris flows. Some important factors of hybrid monopile including radius, buried depth and thickness of the friction wheel, as well as the effects of initial velocity and thickness of debris flow, were taken into account. The investigation results suggested that the hybrid monopile had evidently more promising performance against debris flows than the regular monopile, which could be generally improved by increasing the friction wheel's radius or thickness. However, increasing the buried depth of friction wheel did not necessarily yield an improved performance. Besides, the improving effects of friction wheel were found to be dependent on the initial velocity and thickness of debris flow to varying extents. This study provides a useful reference for the design of pile foundation against debris flow risks in marine engineering.

Variable section pile foundation has been increasingly employed in engineering practice due to their ability of high vertical bearing capacity and strong bending resistance, while its seismic behavior has yet to be well investigated. In this study, a series of 1-g shaking table model tests were carried out on variable section pile-cap systems installed in sandy bed. The test results indicated that the using of variable piles could effectively reduce the acceleration response at pile cap and pile curvature, although the pile bending moment became larger. A validated three-dimensional (3D) finite element (FE) modelling procedure was employed for performing a suite of numerical parametric analyses, accounting the effects of variable diameter ratio, variable thickness ratio and pile flexural rigidity. Parametric numerical analyses suggested that there were optimal variable diameter ratio and variable thickness ratio of about 1.2 and 3, respectively, at which both the maximum pile curvature and curvature at pile top were significantly smaller than that of equal section pile. Besides, the improving effect of using variable section pile against seismic shakings was found to be dependent on the pile flexural rigidity, which was more significant for relatively flexible piles. The results obtained from this study can provide useful reference for the using of variable section piles against seismic risks.

Shallow gas can cause many disasters, and it is reported in many marine engineering constructions. For this, it is imperative to understand the impact of gas on the mechanical behaviors of soil. This study investigated the influence of undrained triaxial compression tests on dense gassy sand commonly encountered in coastal areas. Triaxial tests were performed on specimens with saturations of 100%, 99.8%, 95.9%, and 92.7% under confining pressures of 50 kPa and 200 kPa by a self-developed multi-purpose integrated triaxial apparatus (MITA) for gassy soil. The results are presented in terms of monotonic stress‒strain behavior, volumetric behavior, shear strength, and excess pore water pressure (EPWP). The occurrence of gas bubbles has different effects on loose and dense sands, augmenting the undrained shear strength of loose sand while concurrently diminishing that of dense sand. The deviatoric stress of dense sand increases during shear shrinkage, which is similar to the characteristics of loose sand under the influence of gas bubbles. However, following sand dilation, the effect of gas bubbles on deviatoric stress manifests in an antithetical manner. With elevated gas content, the shear strength of dense sand decreases, accompanied by a deceleration in the development of EPWP and a notable increase in volumetric changes. To this end, a microscopic explanation concerning the deformation and evolution of gas bubbles within sand during the shear process was presented to reveal the macroscopic laws governing the undrained shear attributes of dense gassy sand.

This study investigates the reinforcement mechanism of double-layer geogrid-reinforced ballast through a series of single-layer and double-layer pullout tests. The effects of geogrid layers, aperture size, and reinforcement depth were analyzed in detail. A coefficient of double-layer reinforcement effect (η) was introduced to evaluate the capacity of double-layer reinforcement and identify optimal laying depths. Results indicate that the peak pullout resistance at a depth of 300 mm is significantly higher than at shallower depths (100 mm and 200 mm). Additionally, the 65 mm aperture geogrid demonstrated a 33–44

This paper presents a rigorous, semi-analytical solution for the drained cylindrical cavity expansion in transversely isotropic sand. The constitutive model used for the sand is the SANISAND-F model, which is developed within the anisotropic critical state theory framework that can account for the essential fabric anisotropy of soils. By introducing an auxiliary variable, the governing equations of the cylindrical expansion problem are transformed into a system of ten first-order ordinary differential equations. Three of these correspond to the stress components, three are associated with the kinematic hardening tensor, three describe the fabric tensor, and the last one represents the specific volume. The solution is validated through comparison with finite element analysis, using Toyoura sand as the reference material. Parametric analyses and discussion on the impact of initial void ratio, initial mean stress level, at-rest earth pressure coefficient and initial fabric anisotropy intensity are presented. The results demonstrate that the fabric anisotropy of sand significantly influences the distribution of stress components and void ratio around the cavity. When fabric anisotropy is considered, the solution predicts lower values of radial, circumferential and vertical stresses near the cavity wall compared to those obtained without considering fabric anisotropy. The proposed solution is expected to enhance the accuracy of cavity expansion predictions in sand, which will have significant practical applications, including interpreting pressuremeter tests, predicting effects of driven pile installation, and improving the understanding of sand mechanics under complex loading scenarios.

The soil arching effect is one of the most important load transfer mechanisms for piled embankments. This study performed three-dimensional discrete element method-based trapdoor tests to investigate the evolution of soil arching in unreinforced piled embankments with different fill heights and pile coverage ratios. This study measured vertical and horizontal earth pressures at different locations and calculated the lateral earth pressure coefficients. Based on the variations of vertical pressures and lateral earth pressure coefficients, this study determined the soil arch heights and proposed an empirical formula to calculate soil arch heights. The distributions of contact force chains and principal stress vectors were evaluated to reveal the microscopic behavior of soil arching. Results showed that piled embankments with a higher relative fill height had a higher soil arching effect and higher resistance to the soil arching degradation, while piled embankments with a lower pile coverage ratio had lower resistance to the soil arching degradation. The soil arching model in DEM-simulated tests with high pile coverage ratios was composed of a spherical dome arch, four plane arches, and four pyramid rigid cores above the piles. The DEM-simulated tests with low pile coverage ratios had same soil arching model as those with high pile coverage ratios except that the dome arch and the plane arch have supports in the middle due to the reduced pile zone.