Monotonic and cyclic behaviour of sand under torsional loading

Comprehensive tests were performed to obtain the stress–strain and stiffness characteristics of a fine uniform sand in a hollow cylinder apparatus under monotonic and cyclic torsional loading. Unstable response with a sudden reduction in shear strength after peak was observed under undrained monotonic loading. After a quasi-steady state the specimens showed a phase transformation and a tendency to dilation. Drained torsional hollow cylinder tests showed an equivalent characteristic state marking the transition between contractant and dilatant behaviour and the same line at phase transformation as their undrained counterparts. The instability line, defined under monotonic loading, appears to form a boundary to the behaviour of sand under cyclic loading. Unstable behaviour is introduced when the effective stress path followed by the specimen during cyclic loading reaches the instability line at a shear strain of approximately 0·5%. The first loading cycle is at variance with the following cycles in terms of the stress–dilatancy relationship in drained or the excess pore water pressure generation in undrained torsional hollow cylinder tests. The liquefaction, stiffness and damping characteristics of the sand have been defined.