Existing literature on true triaxial and torsional shear tests indicate that the mechanical response of a granular assembly is significantly influenced by the magnitude of the intermediate principal stress ratio. The present study aims to explore the mechanism behind such effects in reference to the particle-level interaction using 3D DEM simulations. In this regard, true triaxial numerical simulations have been carried out with constant minor principal stress and varying b\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$b$$\end{document} values employing rolling resistance-type contact model to mimic particle shape. The numerical simulations have been validated against the true triaxial experiments reported in the literature for dense Santa Monica beach sand. The macro-level shearing response of the granular assembly has been examined in terms of the evolution of stress ratio and volumetric strain for different rolling resistance coefficients. Further, such macro-level response has been assessed in reference to the micro-scale attributes, e.g. average contact force, number of interparticle contacts, mechanical coordination number, contact normal orientation, and fabric tensor as well as meso-scale attribute like strong contact force network. Lade's failure surface has been adopted to represent the stress and fabric at peak state in the octahedral plane, and mathematical expressions have been proposed relating the failure surface parameters to the rolling resistance coefficient.

This study investigates, for the first time ever, the suffusion on gap-graded granular soils under torsional shear conditions from a microscopic perspective. A numerical model of the hollow cylinder torsional shear test (HCTST) using the discrete element method (DEM) is first developed, where an algorithm for simulating the real inner and outer rubber membranes of the hollow cylinder apparatus (HCA) is introduced. After the validation, the computational fluid dynamics (CFD) approach is introduced for the coupling between the particle and fluid phases. Then, a series of the coupled CFD-DEM suffusion simulations considering the rotation of the major principal stress axis (alpha) and intermediate principal stress ratio (b) are conducted. It is found that more fine particles are eroded in cases having smaller alpha and b, and the clogging phenomenon in the middle zones becomes more significant as both alpha and b increase. From the microscopic perspective, the specimens whose contact anisotropy principal direction is close to the fluid direction will lose more fines, and the anisotropy magnitude also plays an important role. In addition, the differences in structure and vertical connectivity of the pores in HCTST samples under various complex loading conditions cause fine particles to have different migration paths, further resulting in different fines mass loss.

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.