Strain Localization and Undrained Steady State of Sand

Experimental results are presented that characterize the behavior of a loose, fine-grained, water-saturated sand tested under globally undrained conditions is a plane strain apparatus. Together with local measurements of boundary forces and deformations, stereophotogrammetry is used to track the progressive localization of strain. The constitutive behaviors in the tests prior to localization can be characterized as either undrained local softening or undrained load softening and subsequent rehardening. In both cases, a consistent pattern of onset of the formation of the persistent shear band, mobilization of the maximum effective friction, and the complete formation of the band was observed. Conditions approximating steady state only were observed in the shear bands. The effect of strain localization on the conventionally derived steady state line was found to be minimal when undrained load softening behavior was observed, but significantly different steady-state lines were found from results of plane strain and axisymmetric tests. The differences between tests conducted under these two stress conditions assessed to evaluate the practical implications of the observed differences.