Undrained behaviour of granular material and the role of fabric in isotropic and K-0 consolidations: DEM approach

摘要

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 K-0 consolidated prior to undrained shearing up to 30% axial strain to reach steady-state conditions. Aunique 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, F-vM, also reached steady-state values. The normalised anisotropic fabric variable, A, and the trace of the joint stress-fabric tensor, K-F, evolved during undrained simulation and reached the steady-state value at the end of shearing. This forms a unique relationship in K-F-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, psi, and stress ratio at instability, eta(IS)=(q/p') IS, were dependent on consolidation path, and the difference was not related to coordination number, but to F-vM. However, a new phenomenon was observed that stress ratio, eta, at the end of K-0 consolidation and eta(IS) 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.