This paper presents the results of 3D discrete element modeling of monotonic constant volume simple shear test on Pea gravel. 3D DEM simulations were validated using results from large-scale stacked-ring simple shear laboratory tests on real soils, where each particle was accounted for and was characterized by size and shape using the translucent segregation table (TST) test. To acknowledge and incorporate both the irregularity and non-uniformity of particle shapes in real soil specimen and providing a realistic representation of soil assembly in the numerical simulations, a non-uniform distribution of rolling resistance (obtained from the particle shape characterization using TST) was assigned to the spherical particles in the simulated specimens. Different aspects of soil behavior at micro- and mesoscale such as non-coaxiality, stress-induced fabric anisotropy and validity of boundary measurements in evaluating the soil response were investigated. It is shown that boundary measurements (as generally done in laboratory) lead to a conservative estimate of the soil strength and generated pore pressure inside the specimen.

This paper aims to investigate the role of bi-directional shear in the mechanical behaviour of granular materials and macro-micro relations by conducting experiments and discrete element method (DEM) modelling. The bi-directional shear consists of a static shear consolidation and subsequent shear under constant vertical stress and constant volume conditions. A side wall node loading method is used to exert bi-directional shear of various angles. The results show that bi-directional shear can significantly influence the mechanical behaviour of granular materials. However, the relationship between bidirectional shear and mechanical responses relies on loading conditions, i.e. constant vertical stress or constant volume conditions. The stress states induced by static shear consolidation are affected by loading angles, which are enlarged by subsequent shear, consistent with the relationship between bidirectional shear and principal stresses. It provides evidence for the dissipation of stresses accompanying static liquefaction of granular materials. The presence of bi-directional principal stress rotation (PSR) is demonstrated, which evidences why the bi-directional shear of loading angles with components in two directions results in faster dissipations of stresses with static liquefaction. Contant volume shearing leads to cross-anisotropic stress and fabric at micro-contacts, but constant vertical stress shearing leads to complete anisotropic stress and fabric at micro-contacts. It explains the differentiating relationship between stress-strain responses and fabric anisotropy under these two conditions. Micromechanical signatures such as the slip state of micro-contacts and coordination number are also examined, providing further insights into understanding granular behaviour under bi-directional shear. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting 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/).

The swelling soils, also known as expansive soils, increase in volume due to an increase in moisture content. The settlement of expansive soils could be the main reason for considerable damage to roads, highways, structures, irrigation channel covers, and the protective shell of tunnels that use bentonite for wall stability. Therefore, it is important to determine the amount of swelling pressure in expansive soils. This research uses two laboratory swelling test methods with constant volume (CVS) and ASTM-4546-96 standard, the swelling pressure of lime-stabilized bentonite soil has been estimated. Based on the key findings of this study, the swelling pressure values of pure bentonite samples tested using the ASTM-4546-96 method, compared to the constant volume swelling test, show an approximately 170% increase.

Conventional CFD (Computational Fluid Dynamics)-DEM (Discrete Element Method) coupling methods encounter apparent difficulties in addressing the large deformation exhibited by soils with arbitrarily shaped fluid domains for undrained triaxial shear tests with flexible membranes. Herein, a novel CFD-DEM coupling method is proposed to address the main challenges of dynamically reproducing complex external boundaries and mapping for fluid fields. The workflow of surface mesh construction, mesh coarsening, and internal volume division is proposed to generate required meshes. The mapping of fluid information between updated and original meshes is implemented by a distance-weighted interpolation strategy. The coupling method is subsequently applied to investigate the effect of flexible membranes with and without clamped ends on undrained triaxial shear characteristics of soils after its comparison to the constant volume method for validation. The flexible membranes without clamped ends are proven to delay the shear dilation and weaken the inter-particle contact force. Moreover, they enable the free development of the shear band and induce significant octahedral shear strain at both ends of the band. The fluid pressure distributions of both boundary types are uniform and a vortex-shaped velocity field for the fluid is obtained due to the effect of the particle-fluid interaction.