Undrained behaviour of granular material and the role of fabric in isotropic and K0 consolidations: DEM approach

Abstract

The behaviour of granular material was investigated by simulating an undrained triaxial compression test for three-dimensional assembly of ellipsoid particles over a wide range of void ratio, e, and mean confining stress, p′. The assembly was either isotropically consolidated or K0 consolidated prior to undrained shearing up to 30% axial strain to reach steady-state conditions. A unique steady-state line was obtained, irrespective of the consolidation path. The micro-mechanical quantities, such as coordination number and von Mises fabric in terms of second invariant of deviatoric fabric, FvM, also reached steady-state values. The normalised anisotropic fabric variable, A, and the trace of the joint stress–fabric tensor, KF, evolved during undrained simulation and reached the steady-state value at the end of shearing. This forms a unique relationship in KF–p′–e space and the projection of this relationship in e–log(p′) space is the classical steady-state line. This underpins the concept of fabric evolution and steady-state fabric in anisotropic critical state theory. The relationships of state parameter, ψ, and stress ratio at instability, ηIS = (q/p′)IS, were dependent on consolidation path, and the difference was not related to coordination number, but to FvM. However, a new phenomenon was observed that stress ratio, η, at the end of K0 consolidation and ηIS may reach beyond the steady-state stress ratio, M. Experimental data are needed to verify this; until then such behaviour should be regarded as unproven.