Anisotropic Coulomb failure criterion: From DEM to experiments

摘要

National Technical University of Athens, Greece Sand deposited under gravity possesses significant inherent fabric anisotropy and thereby direction-dependent strength. Sand's shear strength anisotropy has been traditionally characterized by anisotropic Mohr-Coulomb criterion with friction angle depending on the angle between the bedding plane and principal stress direction. We argue, instead, that the generalization of an isotropic strength criterion to an anisotropic form should be based on the original Coulomb criterion, and the geometrical descriptor should be the angle Ψ_b, between the bedding plane and the failure plane. We employ a two-pillar approach to validate this argument. First, we use microstructure-conscious Discrete Element Method (DEM) simulation to create a master specimen of elongated virtual particles with natural fabric as a result of simulated gravity deposition. Direct shear and biaxial compression tests are simulated on virtual specimens "cut" out at various angles from the master specimen. Shear strength as a function of Ψ_b over its full range between 0° and 180° was revealed, for the first time, to be not symmetrical in regards to Ψ_b = 90, and the new relationship successfully predicts the emerging of of two types of failure planes that had been observed in biaxial compression tests but could not be explained by the anisotropic Mohr-Coulomb criterion. Second, we subsequently performed full-blown direct shear laboratory tests on three materials with distinct particle characteristics. Apart from uncovering a rich set of material behaviors related to strength anisotropy, in particular for the untested range of Ψ_b from 90° to 180°, the laboratory test results' high resemblance to DEM simulation results demonstrates the great power of micro-scale simulation in the study of complex and unknown material responses. This demonstration is especially intriguing because here the DEM prediction of the Ψ_b-strength curve shape was made and published before any laboratory results on real materials were available.