In practical engineering, earthquake-induced liquefaction can occur more than once in sandy soils. The existence of low-permeable soil layers, such as clay and silty layers in situ, may hinder the dissipation of excess pore pressure within sand (or reconsolidation) after the occurrence of liquefaction due to the mainshock and therefore weaken the reliquefaction resistance of sand under an aftershock. To gain more mesomechanical insights into the reduced reliquefaction resistance of the reconsolidated sand under aftershock, a series of discrete element simulations of undrained cyclic simple shear tests were carried out on granular specimens with different degrees of reconsolidation. During both the first (mainshock) and second (aftershock) cyclic shearing processes, the evolution of the load-bearing structure of the granular specimens was quantified through a contact-normal-based fabric tensor. The interplay between mesoscopic structure evolutions and external loadings can well explain the decrease in reliquefaction resistance during an aftershock.

Laboratory experiments have shown that the proportional shearing of granular materials along arbitrary strain path directions will lead to stress states that converge asymptotically to proportional stress paths with constant stress ratios. The macro- and microscopic characteristics of this asymptotic behaviour, as well as the existence of asymptotic states exhibiting a constant stress ratio and a steady strain-rate direction, have been studied using the discrete element method (DEM). Proportional shearing along a wide range of strain-rate directions and from various initial stress/density states has been conducted. The simulation results suggest that general contractive asymptotic states (except for isotropic states) do exist but may be practically unattainable. Dilative strain path simulations, on the other hand, result in continuously changing stress ratios until static liquefaction occurs, indicating the absence of dilative asymptotic states. Despite this difference, a unique relationship between the stress increments and the current stress ratio gradually emerges from all strain path simulations, regardless of strain path direction and initial stress/density conditions. At the particle scale, the granular assembly sheared along proportional strain paths exhibits a constant partition ratio between strong and weak contacts. Although general proportional strain paths are associated with changing geometric and mechanical anisotropies, the rates of change in these anisotropies for contractive strain paths are synchronised to maintain a constant ratio of their contributions to the mobilised shear strength of the material, with a higher proportion being contributed by geometric anisotropy for more dilative strain paths.

Earthquake-induced soil liquefaction causes ground and foundation failures, and it challenges the scientific community to explore the liquefaction problem in deep deposit under strong shaking. Due to the capacity limitation of physical modelling facility, it is difficult to reproduce soil liquefaction response of deep sand ground by centrifuge shaking table test. To address this problem, a suite of centrifuge model tests were conducted with the aid of Iai's Type III generalized scaling law (i.e., GSL) to observe the liquefaction response of deep sand ground, where Models 1 and 2 were used to validate the GSL and Model 3 with the validated GSL stands for the deep sand ground with prototype depth of 80 m. The test results of Models 1 and 2 indicate that GSL generally performs well for small-strain shear modulus, nonlinear dynamic response of acceleration and the generation of excess pore water pressure, but leaves considerable errors for post-shaking dissipation process and ground settlement with large plastic strain. The test results of Model 3 indicate that liquefaction is also possible in depth of 30-40 m under shaking event of PBA = 0.4 g and Mw = 7.5. For deeper depth without triggering of liquefaction, a depthdependent power function relationship between the peak excess pore water pressure and Arias intensity has been established. The test results also revealed that consolidation and earthquake shaking history contribute to the development of soil anisotropy in a deep ground, leading to a continuous increase of anisotropy degree, which could be evaluated using the small-strain shear moduli in different stress planes under orthogonal stress conditions.

Gap-graded soil, characterized by the absence of certain particle sizes, is commonly used in infrastructure projects such as dams and roadbeds. A comprehensive understanding of both the macro- and micro-mechanical behaviors of discontinuously graded soils is essential for their effective use in engineering applications. In this study, drainage triaxial compression tests were conducted on four gap-graded soil samples with different fine-grain contents mainly using the DEM method, whereas the flexible boundary part was performed using the FDM-DEM method. The contacts were classified based on the magnitude of contact forces between coarse and fine particles, considering the coordination number of the particles involved and the normal angular distribution of these contacts. This classification enabled a detailed analysis of how fine particles contribute to stress transmission and structural evolution during shearing. The fabric tensor for these contact types provided further insights into the anisotropy of samples during shearing. On the microscopic scale, the evolution of contact numbers was found to closely align with the observed stress-strain behaviors. Increasing fine particle content significantly altered the role of fine particles in the stress transmission process. With low content of finer particles, initially, fine particles were situated within the voids formed by coarse particles, and the fine particles are gradually embedded into the coarse particles during the loading process. With the increase of fine particle content, fine particles constantly aggregate to block coarse particles and become the main medium of stress transmission.

Soil elements in situ are subjected to multidirectional shearing during earthquakes. Ignoring the effect of two horizontal shear components generally results in an underestimation of the liquefaction resistance of soils during earthquakes. The actual earthquake sequence generally consists of a mainshock and subsequent aftershocks. Soils may experience liquefaction during the mainshock and then reliquefy again during the subsequent aftershocks. Previous studies on multidirectional loading paths have mainly focused on single liquefaction events. This study employs 3D discrete element modeling to simulate reliquefaction behavior of sands with various multidirectional cyclic simple shear loading histories. The specimens are initially subjected to various strain histories under multidirectional loading paths and then reconsolidated to initial stress states. Subsequently, each soil specimen is subjected to unidirectional cyclic loading in two different directions in the reliquefaction tests. The influence of multidirectional cyclic loading histories on the post-liquefaction drainage compression and reliquefaction resistance are analyzed. Moreover, the evolution of soil fabrics and interaction between fabric orientation and loading direction in the reliquefaction test are investigated. The results highlight that reliquefaction behavior of soils depends on both the fabric and the interaction between the fabric orientation and the loading direction. This study aims to provide micromechanical insight for understanding the effects of multidirectional shearing histories on reliquefaction resistance of sands.

In this study, a series of undrained multidirectional cyclic simple shear tests were conducted using the discrete element method. Various stress paths, including figure-8, circular, teardrop, and straight-line shapes, were considered. Realistic and irregular particles were generated by integrating the theory of random fields for spherical topology with the Fourier-shape-based method. The influence of particle shape irregularity was assessed using a synthetic parameter derived from four common descriptors: aspect ratio, roundness, convexity, and sphericity. The study revealed that the liquefaction resistance of samples subjected to a constant cyclic shear stress ratio predominantly depended on the stress trajectory and particle shape. Numerical results demonstrated that the sample undergoing the unidirectional simple shear exhibited the highest liquefaction resistance, whereas the figure-8 shape exhibited the lowest. Furthermore, greater irregularity in particle shape corresponded to increased resistance to failure. Additionally, microstructural evolutions of granular samples were quantified throughout the simulation using the contact-normal-based fabric tensor. This allowed for a comprehensive exploration of the interplay between internal structure and external loading, leading to a more comprehensive understanding of the macroscopic observations discussed above.

The mechanical behavior of the sand is affected by anisotropy. This paper presents a novel constitutive model for anisotropic sand that accounts for fabric evolution. In this proposed model, a novel hardening parameter and a new state variable are introduced to capture the effects of the evolving anisotropic fabric. A universal fabric tensor evolution law, independent of specific fabric tensors, is proposed based on the characteristics of the unified hardening model and the findings from discrete element simulations. Additionally, a dilatancy anisotropy compression line (DACL) is defined to compute the state variable, ensuring the uniqueness of the critical state line (CSL). The proposed model has been validated through a large number of monotonic shear datasets obtained from experiments and DEM simulations, while parameters in this proposed model are physically meaningful and easy to be determined. Analysis of fabric evolution under different loading paths indicates that the undrained triaxial compression test is the most effective for reaching the critical state, providing a useful reference for the critical state soil mechanics.

Considering fabric evolution effects is crucial for accurately describing the macroscopic mechanical behavior of cohesionless soil under cyclic loading. Building upon the nonlinear dilatancy equation established for sand-gravel composites under monotonic loading, a fabric-dilatancy internal variable, which accounts for fabric evolution during the dilatancy stage under cyclic loading, is introduced. An elastoplastic constitutive model based on the generalized plasticity framework is proposed to capture the full range of mechanical behaviors of sand-gravel composites under both static and liquefaction conditions. By comparing the liquefaction deformation, stress paths, and excess pore water pressure development of sand-gravel composites before and after considering fabric evolution effects, the significance of fabric evolution effects in simulating the liquefaction response of sand-gravel composites is demonstrated. The model's performance is validated through a series of large-scale triaxial tests on sand-gravel composites under both static and dynamic loading conditions, as well as by comparing with test results from relevant literature. The results show that the model generally provides a reasonable representation of the stress-strain-volume behavior of sand-gravel composites under static drained conditions, as well as the accumulation and dissipation of excess pore water pressure, stress path evolution, and liquefaction deformation during liquefaction. This model can serve as a powerful tool for numerical simulation in sand-gravel composites engineering.

The main objective of this study was to investigate the response of uniform sand under constant volume (i.e., undrained) conditions and how it is influenced by the initial anisotropy induced in the soil fabric due to preshearing stress history. The experimental program explored a range of parameters, including stress-strain response, tendency to volume change, phase transformation, flow instability, noncoaxiality between stress and strain rate, and the critical state line. To induce initial anisotropy, samples were presheared along different directions and subsequently tested using an Swedish Geotechnical Institute (SGI)-type bidirectional direct simple shear apparatus. The testing program focused on the effects of initial anisotropy that were induced by preshearing, resulting from the application of initial shear stress in various directions relative to the subsequent shearing direction. To interpret the variations of stresses within the samples, Budhu's approach for stress state determination in simple shear specimens was adopted. The results demonstrate that the stress-strain behavior and global volume change tendency of the soil are heavily influenced by the magnitude and direction of the preshearing stress history. Furthermore, the study reveals that the effects of stress history significantly diminish at large shear strains as the samples approach the critical state.

In this paper, a series of true triaxial tests with different intermediate principal stress ratios are conducted on both the lunar soil simulant and the sandy soils on earth using the discrete element method. An advanced discrete element servomechanism based on polyhedral specimen configuration is implemented such that true triaxial loading paths can be implemented under low confining pressure without introducing severe stress concentration. The high frictional angle and apparent cohesion of the lunar simulant are captured by employing a highly efficient contact model that fuses rolling resistance and van der Waals forces. The employed micro-scale parameters are calibrated based on the triaxial test results of the CSU-LRS-1 lunar soil simulant. The simulation results show that the lunar soil simulant exhibits lower shear strength with an increasing intermediate principal stress ratio. Generally, although the lunar soil simulant has a greater void ratio than that of sandy soils, the former exhibits significantly stronger shear-induced dilatancy and higher shear strength. The evolution of the load-bearing structure is quantified through a contact-normal-based fabric tensor. The interplay between internal structure evolution and external loadings can well explain the difference in mechanical behavior between lunar soil simulant and sandy soils on earth.