THE ORIENTATION OF SHEAR BANDS IN BIAXIAL TESTS

Shear band formation in planar deformation of soil samples is studied theoretically to explain the experimental finding that orientation angles depend on particle sizes. A review of such data shows that steep Coulomb-type shear bands occur in fine sands, whereas less inclined Roscoe-type shear bands are observed for coarse material. A bifurcation analysis for the onset of shear-banding appears to be non-decisive, as all orientations within the Roscoe-Coulomb range are found to be admissible. The new element of this theoretical analysis is that the material outside the shear band is allowed to unload elastically. Hereafter the analysis is extended to the post-peak regime. Post-peak, the Coulomb-type shear band appears to be the weakest failure mode with the lowest residual strength. This explains the frequent occurrence of such shear bands. However, Coulomb-type shear bands are shown to involve stress discontinuities which can hardly occur in samples of coarse sand. This explains the occurrence of Roscoe-type shear bands as the latter do not involve stress discontinuities. -Author