Z. X. Yang

Critical State for Anisotropic Granular Materials: A Discrete Element Perspective

AbstractCritical-state failure is a salient feature for particular materials, such as granular media and soils, whereas the critical-state theory (CST) is widely accepted for understanding and interpreting the behavior of a range of materials under complex loading conditions. In elastoplastic soil modeling, the critical state is also a favorable reference state for capturing and simulating the soils’ responses, and thus many constitutive models have been developed within the CST framework. Notwithstanding this, the uniqueness of the critical state has evoked increasing debate for many years and remains an unresolved issue. The central difficulty lies in how to integrate fabric anisotropy into the delineation of the critical-state failure. Motivated by the recent development of the anisotropic critical-state theory, this paper presents a discrete element investigation of the critical-state behavior of anisotropic granular materials. Samples with different degrees of initial fabric anisotropy are sheared to...

Drained Instability in Loose Granular Material

AbstractThe constant shear drained (CSD) test is considered an appropriate test to simulate the instability behavior of soils in slopes under water infiltration conditions or lateral stress relief. However, contradiction of the drained instability in loose granular material exists in the reported test results, which is considered because of the condition of constant deviatoric stress (q) not strictly satisfied in the literature. Because the variation of q during the CSD test has significant effects on the test results, more data from tests under a true constant q condition are needed to clarify this subject. In this paper, CSD and conventional undrained triaxial tests in both compressive and extensive loading were performed to investigate the instability behavior of loose sand. All the CSD tests were conducted by advanced variable confining pressure triaxial apparatus, in which a true constant q was maintained during the test; thus, more reliable results were obtained. It is observed experimentally that w...

The Influence of Particles’ Aspect Ratio on the Shear Behaviour of Granular Materials

The mechanical response of granular material depends both on the material properties (e.g. stiffness, anisotropy, permeability, etc.) and particle geometry (shape, roughness, etc.). This paper presents a study of the effect of particle shape (i.e. aspect ratio) on the mechanical behaviour of a granular assembly using DEM (discrete element method). In this study, the numerical simulations employ samples with different particle aspect ratios but identical particle size distribution (PSD). Conventional triaxial compression DEM simulations are carried out under both ‘drained’ and ‘undrained’ (constant volume) conditions. The shear behaviour of the assemblies and the evolution of their microstructure under shearing are examined in detail. At the macroscopic level, the test results show that the particle aspect ratio has a significant effect on the stress-strain curve, peak strength, dilatancy characteristics and critical state behaviour. In particular, the samples with lower aspect ratios can lead to higher peak/residual shear strengths, and higher angles of shearing resistance. The critical state occurs at a higher position in a void ratio versus mean normal stress plot for lowest particle aspect ratio. The results from numerical analysis compare reasonably well with the available experimental data. At the microscopic level, the fabric evolution is greatly affected by the aspect ratio, and the critical state fabric is also examined.