Investigating the Combined Effects of Inherent and Stress-Induced Anisotropy on the Mechanical Behavior of Granular Materials Using Three-Dimensional Discrete Element Method

Xinran Chen,Lei Zhang,Jianming Ling,Jinsong Qian

doi:10.1155/2020/7841824

Abstract

The three-dimensional discrete element method (DEM) was employed to investigate the combined effects of inherent and stress-induced anisotropy of granular materials. The particles were modeled following real particle shapes. Both isotropic and inherently anisotropic specimens were prepared, and then true triaxial numerical tests were conducted using different intermediate principle stress ratios (b). The results indicate that the oriented particles in the anisotropic specimens form strong contacts in their long axis direction in the early stages of shearing, which restrains the contraction of the specimens. As the strain increases, the oriented particles start to rotate and slide, which results in shorter contraction stages and fewer number of interparticle contacts with peak values compared to the isotropic specimens. In addition, the increase inbvalues aggravates the rotating and sliding of particles in the inherently anisotropic specimens and restrains the contraction of the granular and the increase of contact forces. As a result, the inherent anisotropy reduces the effects of stress-induced anisotropy on the mechanical behavior of granular materials.

Highlights

Granular materials are commonly used in the base layers of typical pavement structures. e directional distribution of particles in granular materials significantly affects their mechanical properties [1], which leads to an anisotropic phenomenon. e anisotropy of granular material can be divided into inherent anisotropy and stress-induced anisotropy [2]
Stress-induced anisotropy is caused by the nonuniform distribution of external loading, which leads to the rearrangement of the particles [3]. To investigate these two kinds of anisotropy, the long axis orientation of granular particles is widely used for inherent anisotropy analysis, and the intermediate principle stress ratio, denoted by b, which represents the relative magnitude of the intermediate principle stress, is used for stress-induced anisotropy analysis [4, 5]
For selected small b values, the long axis of the particles in the anisotropic specimens are initially perpendicular to the σ1 direction to carry most of the external load [21] and resist the sliding tendency in σ1 direction

Summary

Introduction

Granular materials are commonly used in the base layers of typical pavement structures. e directional distribution of particles in granular materials significantly affects their mechanical properties [1], which leads to an anisotropic phenomenon. e anisotropy of granular material can be divided into inherent anisotropy and stress-induced anisotropy [2]. Some experimental studies have shown uncertain relationships among the angle of shear resistance, b values, and other controversial behavior [8, 9], as laboratory test results can be affected by the initial fabric difference [10, 11], different sample preparation methods [12, 13], different boundary conditions, etc. Hosseininia [21] studied the inherent anisotropy of granular soils using a two-dimensional DEM and found that the initial distribution of elongated particles and associated voids vary during shear deformation. Erefore, the combined effects of the two types of anisotropy may affect the mechanical behavior of granular materials simultaneously and needs further investigations For this purpose, 3D DEM simulations were conducted for this study.

Numerical Model

Macroscopic Analysis

Microscopic Analysis