Triaxial tests on an unbonded locked sand

This paper describes the results of a series of triaxial tests carried out to investigate the influence of fabric structure on the yield and failure behaviour of intact Reigate silver sand—a locked sand characterised by minimal cement content, large grain contacts, and a relative density index of 136%. Three distinct limiting stress envelopes are identified, based on (a) the start of dilation, (b) the maximum stress ratio during dilation, and (c) possible critical states. At effective cell pressures of 100 kPa and below, the stress ratio at the start of dilation is close to the maximum during subsequent dilation and much greater than at the critical state. At higher cell pressures the material behaves more conventionally, in that maximum stress ratios are more clearly linked to maximum rates of dilation. The role and significance of dilation in a locked sand are discussed with reference to the development of strain localisations into shear planes, thin sections from samples subjected to differing degrees of shear, and tests on unstructured sands.