Waves can cause significant accumulation of pore water pressure and liquefaction in seabed soils, leading to instability of foundations of marine hydrokinetic devices (MHKs). Geostatic shear stresses (existing around foundations, within slopes, etc.) can substantially alter the rate of pore pressure buildup, further complicating the liquefaction susceptibility assessments. In this study, the development of wave-induced residual pore water pressure and liquefaction within sandy seabed slopes supporting MHK structures is evaluated. Unlike most earlier studies that excluded the impact of shear stress ratios (SSR) on the residual pore pressure response of sloping seabeds, asymmetrical cyclic loadings are considered herein for a range of SSRs. To obtain wave-induced loading in the seabed (and cyclic shear stress ratios, CSRs), the poroelasticity equations governing the seabed response, coupled with those for fluid and structure domains, are solved simultaneously. Utilizing an experimental model based on anisotropic cyclic triaxial test data that includes CSR and SSR impacts, an equation for the rate of pore pressure buildup is developed and added as a source term to the 2D consolidation equation. Numerical investigations were performed by developing finite element models in time domain. The models were calibrated using particle swarm optimization method and validated against wave flume experimental data. The results indicate that the consideration of static shear stresses has led to sudden rise in residual pore pressures followed by fast dissipations at early and late time steps, respectively, beneath the structure. The exclusion of SSR is shown to cause significant overestimation of pore pressure accumulations at late cycles, potentially causing significant overdesign of MHK foundations. The impact of proximity to the free drainage boundary, CSR amplitude, and loading frequency on the accumulation of residual pore pressure is illustrated. The residual liquefaction susceptibility of the seabed is shown to decline by increase of the seabed slope angle.

In the dynamic response analysis of slopes, the displacement of sliding surfaces is an important indicator for assessing stability. However, due to the uniform dynamic parameters of the Newmark slide block method, it is difficult to accurately analyze the displacements of large-scale slopes. To address this issue, the spatial distribution characteristics of dynamic parameters need to be considered to accurately analyze the stability of slopes. Under the combined action of rainfall and reservoir water level change, the Shiliushubao old landslide in the Three Gorges Reservoir area remains stable. To investigate the seismic stability of slopes, simulated seismic waves were employed. Firstly, the dynamic triaxial test, designed with cyclic loading, was employed to investigate the variation rules of the dynamic parameters of slope soil, and to establish a functional relationship. Then, the stochastic seismic motion model was used to generate artificially seismic waves in the Three Gorges Reservoir Area. Finally, to assess the stability of the old landslide, finite element software, GeoStudio 2018 was used to obtain the spatial distribution characteristics of the dynamic parameters and to calculate the permanent displacements of the reservoir bank slope by inputting random ground motion loads and dynamic characteristic functions. It is demonstrated that under the most unfavorable working conditions of heavy rainfall and the earthquake in the specific region, the permanent displacement of the Shiliushubao old landslide will be less than the critical permanent displacement, the old landslide is not to experience instability again.

Based on the discrete element particle flow program PFC3D, an undrained cyclic triaxial numerical model is established to investigate the large strain dynamic characteristics and liquefaction behaviors of the loose sand under stress amplitude-controlled and strain amplitude-controlled tests. The results demonstrate that the value of micro parameters at the initial liquefaction moment are the same under the two control modes. The whole cyclic loading process of both loading control methods can be divided into different zones based on the evolution of the micro parameters. In studying the movement state of soil particles after initial liquefaction, the strain amplitude-controlled test is more comprehensive to observe the development process of microstructure. The peak value of the damping ratio calculated by the typical symmetrical hysteresis loop method is around 0.5% of the deviatoric strain, while is around 1.0% of the deviatoric strain when considering the asymmetry of the stress-strain hysteresis loop. In stress amplitude-controlled tests, the phase transition and large flow-slip behavior of the loose sand will result in an unclear peak of the damping ratio. In this context, strain amplitude-controlled tests can be advantageous for the study of loose sand.

Dynamic triaxial tests were conducted to clarify the dynamic deformation characteristics of silty clay in flood irrigation areas under cyclic loading, using single-sample stepwise and multiple samples of constant amplitude. The effects of confining pressure, bias consolidation ratio, drainage conditions, dynamic load frequency, and cyclic stress ratio on the development law of cumulative plastic strain and residual dynamic pore pressure, the evolution characteristics of the hysteresis curve, and the change law of softening index of silty clay were studied. The results show that the development of cumulative plastic strain and residual dynamic pore pressure of soil under dynamic load is consistent. According to the stability theory, the dynamic behavior of samples under different test conditions can be divided into three typical cases: plastic stability, plastic creep and incremental failure. Under the basic conditions of this test, the boundary cyclic stress ratios of the three dynamic states of plastic stability, plastic creep, and incremental failure are around 0.30 and 0.40, respectively. The hysteresis characteristics of undrained specimens in the plastic stable state are obvious, and the hysteresis curve shows an S shape. With the progression of loading, soil experiences stiffness degradation. The cumulative plastic deformation of undrained specimens is smaller than that of drained specimens, and the softening index of soil under drained and undrained conditions remains stable at around 1.15 and 0.91, respectively, under lower cyclic stress ratios. Through grey relational analysis, it is found that the cyclic stress ratio has the greatest influence on the cumulative plastic strain and pore pressure ratio. The confining pressure exerts the greatest influence on the softening index. The parameters of the cumulative plastic strain model suitable for silty clay in flood irrigation areas have been determined, and the prediction effect is good.

Soil instability and potential failure under principal stress rotation require greater attention than ever before due to increased operation of heavier and longer high-speed trains. This study focuses on the interplay between cyclic vertical stress and torsional shear stress on the failure condition of a low-plasticity subgrade soil, facilitated by a hollow cylinder apparatus. Combined vertical and torsional loading significantly influences strain response, with increasing torsional stress leading to higher strain accumulation. Moreover, the data indicate that an increase in torsional shear stress is generally accompanied by a swift rise in the EPWP and a corresponding decrease in the soil stiffness. In view of this, a novel parameter, the overall stiffness degradation index (delta o) that simultaneously captures both the vertical and torsional shear effects under principal stress rotation is proposed as an early indicator of instability. In addition, a normalised torsional stress ratio (NTSR), which is the ratio of the amplitude of torsional shear stress to the confining pressure, is introduced to assess the impact of torsional shear stress. Whereby, higher NTSR values correlate with premature inception of failure. These experimental results provide new insights for a better understanding of soil instability under simulated railway loading.

This article presents a series of cyclic triaxial tests to investigate the particle breakage characteristics of coarse-grained filler under heavy-haul train load. The results show that the main patterns of particle breakage for large-sized particles (the particle size between 22.4 and 31.5 mm) are fracture and abrasion, and the particle breakage makes the outer contour of the particle closer to the sphere. The particle breakage is found in the process of vibratory compaction of specimens, and the particle breakage caused by isotropic consolidation under low confining pressure (no more than 300 kPa) can be ignored. It is also found that significant particle breakage occurs during cyclic loading, characterized by the reduction of the large-sized particles (the particle size between 16 and 31.5 mm) and the increase of fines content. In addition, further particle breakage is caused by the increase in the cyclic stress ratio. Based on the test results, a power function equation of Marsal's breakage factors and cyclic stress ratio is proposed.

The soft clay under the road ground will suffer cyclic torsional shear stress encountered traffic load in addition to axial stress, which will cause further deformation of clay. To investigate the effect of torsional shear stress on the cumulative axial strain of clay, a series of undrained tests under cardioid stress path were performed on K0 consolidated undisturbed samples by using a hollow cylinder apparatus (HCA). The effect of vertical cyclic stress ratio (VCSR) and shear stress ratio (eta) on the deformation and degradation characteristics of clay was investigated. The results indicate that there is inconsistency between the strain path and stress path. The increase in eta further accelerates the accumulation of axial strain, which resulted from the degradation of clay induced by torsional shear stress. Considering the VCSR and eta, a calculation method of degradation index was developed. Furthermore, a cumulative axial strain prediction model of clay under the cardioid stress path was established considering the degradation. This model addresses the limitation of traditional prediction models by considering the impact of torsional shear stress on cumulative axial strain.

Red mudstone is a problematic soil that is easily subjected to weathering, disintegrating, and swelling. In this study, a series of large-scale cyclic triaxial tests were performed to investigate the cumulative deformation behavior of red mudstone clay mixed with weathered red mudstone gravel as an improved coarse-grained red mudstone soil (IRMS). The influences of compaction moisture content and confining pressure were investigated. The cyclic loading was applied from 25 to 225 kPa with an increment of 25 kPa and 1,000 or 2,000 cycles for each stage at a frequency of 2 Hz. The experimental results indicate that the strains at the onset of failure are approximately 1% for the optimal moisture content (OMC) with the number of cycles N = 14,000-16,000, and the strains are approximately 1% for the moisture content 2% dry of OMC with N = 12,000-14,000, while the strains exceed 10% for the moisture content 2% wet of OMC with N = 3,000-4,000. The cumulative strain decreases with increasing confining pressure from 20 to 50 kPa, but the influence becomes more significant under higher dynamic stress. A prediction model is proposed for the evolution of cumulative strain under cyclic loading. The IRMS could be used as a construction material for railway subgrade with proper control of field compaction moisture content.

The traffic loading is a typical cyclic loading with variable confining pressure and always lasts long, and is believed to have a significant effect on the subgrade soil, especially for the subgrade filled with soft clay. However, the mechanics have yet to be fully understood. Given that the duration of traffic loading lasts long enough, the partially drained conditions should be considered for the soft clay under the long-term cyclic loading, rather than the undrained conditions adopted commonly by most previous researches. In this study, 28 cyclic tests were conducted on the remolded saturated soft clay, utilizing both constant confining pressure and variable confining pressure under partially drained and undrained conditions. The effect of cyclic confining pressure and different drainage conditions is analyzed in relation to the evolution of pore pressure and deformation behaviors. Incorporating both the cyclic confining pressure and cyclic stress ratio, a concise pre-diction model of permanent strain is proposed and validated by the experimental results.

Saturated sand foundations are susceptible to liquefaction under dynamic loads. This can result in roadbed subsidence, flotation of underground structures, and other engineering failures. Compared with the traditional foundation reinforcement technology, enzyme-induced calcium carbonate precipitation technology (EICP) is a green environmental protection reinforcement technology. The EICP technology can use enzymes to induce calcium carbonate to cement soil particles and fill soil pores, thus effectively improving soil strength and inhibiting sand liquefaction damage. The study takes EICP-solidified standard sand as the research object and, through the dynamic triaxial test, analyzes the influence of different confining pressure (sigma 3) cementation times (CT), cyclic stress ratio (CSR), dry density (rho d), and vibration frequency (f) on dynamic strength characteristics. Then, a modified dynamic strength model of EICP-solidified standard sand was established. The results show that, under the same confining pressure, the required vibration number for failure decreases with the increase in dynamic strength, and the dynamic strength increases with the rise in dry density. At the same number of cyclic vibrations, the greater the confining pressure and cementation times, the greater the dynamic strength. When the cementation times are constant, the dynamic strength of EICP-solidified sand decreases with the increase in the vibration number. When cementation times are 6, the dynamic strength of the specimens with CSR of 0.35 is 25.9% and 32.4% higher than those with CSR of 0.25 and 0.30, respectively. The predicted results show that the model can predict the measured values well, which fully verifies the applicability of the model. The research results can provide a reference for liquefaction prevention in sand foundations.