Deformation of water-saturated sand .1. Uniform undrained deformation and shear banding

Experimental results from undrained plane-strain (biaxial) compression tests on water-saturated St Peter sandstone sand are analysed within the framework of continuum mixtures theory for elastoplastic materials with internal friction and dilatancy, The simple theoretical frame of non-associative plasticity is shown to be adequate for reproducing the main features of the stress-strain curves and the effective stress paths of uniformly deformed undrained specimens under axial displacement control and constant confining pressure. The outstanding stress states, such as the states of stationary deviators, the states of zero plastic volumetric strain rates and the states of zero pore-water pressure production, are defined precisely and characterized, In displacement-controlled tests and for significant portions of the effective stress path the second order plastic work is negative without visible deviations from homogeneous isochoric deformation, It is also shown that in ordinary soil mechanics testing situations no pore-water pressure shocks can develop across shear-band boundaries, and thus locally undrained shear banding at the state of the maximum deviator is impossible.