Orientation of locally drained shear bands in contractant clays

Abstract

A slip surface or shear band orientation of 45° from the minor principal stress is obtained when contractant clays are numerically analyzed while assuming undrained conditions. This orientation occurs because the shear band is modeled as undrained and with no volume change. However, thin shear bands induce the dissipation of pore water to the outside body, and depending on the degree of local drainage, the incipient shear bands are then no longer at 45°. In drained conditions, the incipient shear band inclination to the minor principal stress is controlled by the friction angle, the dilatancy angle or a combination of both. This paper examines the orientation of a developing shear band, accounting for the effects of local pore water pressure flow away from the shear band to investigate whether the orientation of the band is controlled by the degree of drainage (i.e. volume change) or the mechanisms of shear band orientation under fully drained conditions. The numerical results show that the band’s inclination can be >45° for globally undrained contractant clays sheared at rates of ≤10%/h, rates that are normal in laboratory testing.