The foundation soil below the structure usually bears the combined action of initial static and cyclic shear loading. This experimental investigation focused on the cyclic properties of saturated soft clay in the initial static shear stress state. A range of constant volume cyclic simple shear tests were performed on Shanghai soft clay at different initial static shear stress ratios (SSR) and cyclic shear stress ratios (CSR). The cyclic behavior of soft clay with SSR was compared with that without SSR. An empirical model for predicting cyclic strength of soft clay under various SSR and CSR combinations was proposed and validated. Research results indicated that an increase of shear loading level, including SSR and CSR, results in a larger magnitude of shear strain. The response of pore water pressure is simultaneously dominated by the amplitude and the duration of shear loading. The maximum pore water pressure induced by smaller loading over a long duration may be greater than that under larger loading over a short duration. The initial static shear stress does not necessarily have a negative impact on cyclic strength. At least, compared to cases without SSR, the low-level SSR can improve the deformation resistance of soft clay under the cyclic loading. For the higher SSR level, the cyclic strength decreases with the increase of SSR.

There are currently two main criteria to identify the triggering time of soil liquefaction, namely when the excess pore water pressure reaches vertical effective overburden stress or the double-amplitude axial strain reaches 5 %. However, several researchers have pointed out that the excess pore water pressure may not reach confining pressure at some certain conditions, and the cycle numbers reaching liquefaction obtained by adopting two criteria for calcareous sand specimens are inconsistent, which may lead to overestimation or underestimation of the liquefaction resistance of calcareous sand. Therefore, this study introduces a parameter with physical meaning, secant shear modulus to evaluate the liquefaction potential of soil. To do that, a series of undrained shear tests were conducted on three types of sand. Firstly, the experimental results demonstrated that the difference in cycle numbers to liquefaction obtained by the two criteria increases with the increase of relative density. In addition, the study found that the degradation law of secant shear modulus with the number of cycles is not affected by loading conditions, initial state of soil, and soil type. On this basis, based on the relationship between secant shear modulus gradient and pore pressure ratio, it is highlighted that the liquefaction process can be quantitatively divided into three stages and the moment of liquefaction triggering can be correctly identified. Finally, the proposed liquefaction criterion is compared with widely used traditional criteria and latest apparent viscosity-based criterion, and the results showed that the liquefaction resistance obtained by the proposed criterion was more conservative, which benefits for reducing the occurrence of large strain development.

The delayed breakage of particles significantly affects the long-term mechanical properties of rockfill materials. This study examines the effects of particle strength dispersion on the distribution of time-dependent strength using fracture mechanics and probabilistic methods. Subsequently, the distribution of normalized maximum contact force (NMCF), defined as the ratio of the maximum contact force to instantaneous strength, for specimens with uniform particle size is derived using extreme value theory and Discrete Element Method (DEM). Based on this analysis, the probabilities of delayed breakage in rockfill specimens over various time intervals are calculated using a joint probability delayed breakage criterion. The feasibility of the proposed method is validated by comparing theoretical calculation with DEM triaxial creep simulation results that accounted for particle breakage. The findings offer innovative tools and theoretical insights for understanding and predicting the particle delayed breakage behavior of rockfill materials and for developing macro-micro creep crushing constitutive models.

True triaxial tests were conducted on artificially frozen sand. The effects of the intermediate principal stress coefficient, temperature and confining pressure on the strength of frozen sand were studied. The stress-strain curves under different initial conditions indicated a strain hardening. In response to increases of either the intermediate principal stress coefficient or the confining pressure or to a decrease of temperature, the strength typically increased. Furthermore, a new strength criterion was proposed to describe the strength of artificially frozen sand under a constant b-value stress path, combining the strength function in the p-q and pi planes. Considering the low confining pressure, the strength criterion in the p-q plane fitted the linear relationship in the parabolic strength criterion well. The strength criterion in the pi plane was combined with stress invariants, and a new strength criterion was established. This criterion considers unequal tension and compression strength, and integrates temperature. Test results indicated its validity. All parameters of the strength criterion could be easily determined from the triaxial compression and triaxial tensile tests.

Rockfill, a coarse granular material commonly used in dam construction, exhibits complex mechanical behavior under generalized stress conditions. This paper investigates the mechanical properties of rockfill through a series of stress-path tests conducted on a self-developed, large-scale true triaxial apparatus with cubical specimens of 60 x 30 x 30cm. Three test series are carried out by varying the mean effective stress, the deviator stress and the Lode's angle, respectively. An elastoplastic constitutive model is presented to describe the behavior of rockfill. An improved dilatancy equation is introduced by considering the phase transformation stress ratio instead of the critical stress ratio.

Frozen mixed soils are widely distributed in the strata and slopes of permafrost regions. This paper aims to study the strength criterion and elastoplastic constitutive model for frozen mixed soils from micro to macro scales. Based on the knowledge of mathematical set theory and limit analysis theory, the support function of frozen soils matrix is derived. The concept of local equivalent strain is proposed to solve the problem of nonuniform deformation caused by rigid inclusions in frozen mixed soils. According to the nonlinear homogenization theory and the Mori-Tanaka method in micromechanics, the strength criterion of frozen mixed soils is established, which can consider coarse particle contents. By introducing the concepts of equivalent yield stress and equivalent plastic deformation, the elastoplastic constitutive model is proposed by the associated flow rule, which can also consider the influence of coarse particle contents. Finally, using the data in the literature, the proposed strength criterion and elastoplastic constitutive model for frozen mixed soil are verified, respectively. The effects of coarse particle contents on the mechanical properties of frozen mixed soils are discussed.

Despite over six decades of field and laboratory investigations, theoretical studies, and advances in constitutive modeling, questions remain on the fundamental issues concerning liquefaction mechanisms, the collective influence of multiple factors on excess pore water pressure (EPWP) generation, and liquefaction triggering criteria. This paper presents the general apparent viscosity-and average flow coefficient-based methodology for quantifying the solid-liquid phase-change process of liquefiable soil under undrained cyclic loading. The analysis reveals that the evolution of the soil particle-fabric system is the fundamental physico-mechanical mechanism behind EPWP generation in a liquefiable soil, with the accompanying change in soil physical state serving as the intrinsic mechanism driving EPWP generation. The study further identifies the physico-mechanical foundations of EPWP generation, as well as the inherent causes and a unified quantitative characterization of the coupled influences of multiple factors on EPWP generation. This work presents the novel observation that the marginal peak excess pore pressure ratio (ru,pm) between the solid-liquid mixed phase and the liquid phase of liquefiable soil can be identified accurately and that ru,pm is characterized by its inherent robustness. A ru,pm value of 0.90 can be used as a liquefaction triggering criterion for soils both in laboratory element tests and in the field. Another original finding is that the liquefaction triggering resistance curve is the threshold state curve between solid-liquid mixed phase and transiently liquid phase of a liquefiable soil and is unique for a specific initial physical state. The definitions of liquefaction triggering and corresponding liquefaction triggering resistance are clear and unambiguous and have the same physico-mechanical basis. The insights obtained in this paper will potentially enable the scientific and engineering communities to reinterpret the liquefaction mechanism, its evaluation, and liquefaction mitigation strategies.

The structural characteristics of soil-rock mixture (SRM) slopes, including the content, shape, size, and spatial distribution of rock blocks, can significantly influence their failure mechanisms and factor of safety (FOS). Defining the structural characteristics of SRM slopes for stability analysis remains challenging. This study proposes a method for establishing random models and evaluating the statistical properties of the FOS values of SRM slopes. Accordingly, the SRM slope models were constructed by considering the random properties of the shape, size, and spatial distribution of rock blocks in the slope domain. A slope failure criterion based on energy changes and the combined subroutines of USDFLD and URDFIL was implemented in the ABAQUS finite element software to determine the FOS values of the SRM slopes. Monte Carlo simulations were performed to assess the statistical properties of the FOS for random SRM slopes varying rock block properties. The results indicated that when the rock block content was greater than 30%, the stability of SRM slopes considerably increased. For a rock block content of 40%, the effect of rock block size on the SRM slope stability followed two different trends: the mean FOS value tended to decline and subsequently increased as rock block size increased. However, this trend was not observed on SRM slopes with a 30% rock block content. Besides, the dispersion of the FOS values gradually increased with increasing rock content and rock block size. Furthermore, the soil-rock interface strength affected the stability and failure mechanism of SRM slopes. These findings enhance comprehension of the SRM slope stability assessment and demonstrate improved accuracy in predicting and mitigating damage.

A series of true triaxial unloading tests are conducted on sandstone specimens with a single structural plane to investigate their mechanical behaviors and failure characteristics under different in situ stress states. The experimental results indicate that the dip angle of structural plane (B) and the intermediate principal stress (o2) have an important influence on the peak strength, cracking mode, and rockburst severity. The peak strength exhibits a first increase and then decrease as a function of o2 for a constant B. However, when o2 is constant, the maximum peak strength is obtained at B of 90 degrees, and the minimum peak strength is obtained at B of 30 degrees or 45 degrees. For the case of an inclined structural plane, the crack type at the tips of structural plane transforms from a mix of wing and anti-wing cracks to wing cracks with an increase in o2, while the crack type around the tips of structural plane is always anti-wing cracks for the vertical structural plane, accompanied by a series of tensile cracks besides. The specimens with structural plane do not undergo slabbing failure regardless of B, and always exhibit composite tensile-shear failure whatever the o2 value is. With an increase in o2 and B, the intensity of the rockburst is consistent with the tendency of the peak strength. By analyzing the relationship between the cohesion (c), internal friction angle (4), and B in sandstone specimens, we incorporate B into the true triaxial unloading strength criterion, and propose a modified linear Mogi-Coulomb criterion. Moreover, the crack propagation mechanism at the tips of structural plane, and closure degree of the structural plane under true triaxial unloading conditions are also discussed and summarized. This study provides theoretical guidance for stability assessment of surrounding rocks containing geological structures in deep complex stress environments. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Liquefaction and shear sliding (i.e., slides) are common failure modes of cohesionless seafloor under ocean waves. However, existing research has rarely focused on shear-sliding failure, especially when considering wave- induced residual pore water pressure. Additionally, the relationship between shear-sliding failure and liquefaction is not well understood. In this study, a slice method is developed to assess the shear-sliding failure in cohesionless seafloor under non-linear waves, incorporating the effect of both oscillatory and residual seabed responses. The applicability of various liquefaction criteria is discussed, based on the interrelation between the shear-sliding and liquefaction zones. The results indicate that the seabed soil is more prone to shear-sliding failure than liquefaction under wave-induced pore water pressure. When only oscillatory pore water pressure is considered, the liquefaction criteria, assuming the initial vertical effective stress vanishes due to the excess pore water pressure, better identify the liquefaction zone, which is enveloped by and overlaps with the shear- sliding zone at a factor of safety of zero. In cases where both oscillatory and residual pore pressure coexist, the unified liquefaction criterion, which also assumes onset of liquefaction at zero vertical effective stress, provides more reliable predictions of the liquefaction zone. As residual pore pressure accumulates, the difference between shear sliding and liquefaction depths becomes more pronounced. A sensitivity analysis of shear-sliding depth with varying soil parameters indicates that relative density exerts the most significant influence, followed by the effective internal friction angle, while the shear modulus has the least effect. The effect of variations in soil parameters on shear-sliding depth diminishes to some extent with prolonged wave action.