Flow Deformation of Sands Subjected to Principal Stress Rotation

Abstract

In order to investigate the undrained deformation behaviour of sand subjected to principal stress rotation, two series of tests were performed using a torsional cylinder shear apparatus. The following conclusions were obtained from the tests. Pore pressures and strains in excess of 5% were accumulated under the cyclic rotation of principal stresses despite the deviatoric stress remaining constant. The form of deformation could be classified into non-flow, limited flow or full flow deformation. Flow deformation occurred when the effective stress state of samples reached a critical stress ratio. If the effective stress reached the steady state line after the critical stress ratio was reached, samples demonstrated full flow deformation. If on the other hand, the effective stress reached the phase transformation line after the critical stress ratio was attained, samples demonstrated limited flow deformation. The critical stress ratio was defined as the stress ratio at which strain softening commenced, and was obtained from undrained shear tests at various relative densities and principal stress directions. It was observed that the existence of a critical stress ratio was a function of both relative density and principal stress direction. The test results in this paper demonstrate the importance of investigating the effects not only of the density and confining pressure, but also of the principal stress direction and its rotation on the flow deformation of sand.