A laboratory study of slip surface formation in an intact natural stiff clay

The paper presents the results of drained constant-p′ tests on natural undisturbed specimens of Corinth Marl. The specimens were instrumented with local axial strain gauges and pore pressure probes. The relationship between deviator stress q and overall axial strain is characterised by a flat peak followed by a small reduction in strength. The specimen dilates strongly throughout this phase of the test. A steep drop in strength then occurs, resulting in the formation of a thin slip surface and the cessation of dilation. Careful analysis of the results from the local strain gauges indicates that strain localisation initiates after peak strength but before the commencement of the steep drop in q. Most of the reduction in shear strength takes place for relative displacements of less than 0·5 mm, after which stable post-rupture strength is reached. The pore pressure probes show that, during the initial stages of drained shearing, a small reduction in local pore pressure occurs in the middle of the specimen relative to the ends, which is consistent with dilation. This pore pressure difference reduces rapidly during the formation of the slip surface, reflecting the cessation of dilation of the material on either side of the slip surface.