Strain rate-dependent undrained response of a coral sand

The strain rate-dependent response of sand is influenced by particle crushing. However, such a conclusion is drawn in the previous studies, primarily based on drained triaxial or 1D compression tests on crushable sand. This study explores the interplay of strain rate and crushing behaviour of sand under undrained triaxial (CU) conditions. We perform CU tests on coral sand under different confining pressures and relative densities. Two different axial strain rates in the quasi-static range are used to explore the rate-sensitive response of coral sand. The undrained experiments reveal that a slow strain rate and high confining pressure lead to higher crushing, as commonly observed in drained triaxial experiments since dilatancy diminishes. However, CU tests on dense samples also demonstrate higher crushing than medium dense samples, which is a counterintuitive observation. Later, it is analysed that crushing evolution before and after phase transformation and corresponding pore pressure generation plays a pivotal role in the undrained response. In addition, strain rate also governs the particle crushing and peak strength of the sand samples. This study proposes a generalized rate-dependent breakage index that satisfactorily predicts the correlation between relative breakage and plastic work under different rates, relative densities, and confining pressures.