RELATIVE DENSITY EFFECTS ON DRAINED SAND BEHAVIOR AT HIGH PRESSURES

Abstract

A high-pressure triaxial compression and extension apparatus was employed to investigate the effects of initial relative density on the drained behavior of sand. Experiments were performed on cylindrical specimens of Cambria sand with initial relative densities of 30% and 60%, and compared with previously published results for 90%. Confining pressures in the range of 0.25 to 60 MPa were employed to obtain the characteristic behavior of the sand over a large range of pressures. As the confining pressure increased particle crushing increasingly influenced the stress-strain and volume change relations, producing increasing strains to failure and increasingly contractive volume changes. Beyond certain high values of mean normal stress at failure, increasing with initial relative density, the volumetric contraction and strains to failure began to decrease. The Mohr-Coulomb failure envelopes show the greatest curvature for the highest density, but the failure envelopes merge at high pressures where, due to particle crushing, the relations between void ratio and isotropic pressure are described by a single curve. The experiments show clearly that the rates of dilation at failure and the friction angles are directly related to each other, as has been proposed by many investigators. The friction angles are higher in extension than in compression for lower confining pressures, but a crossover occurs and the friction angles are higher in compression than in extension for higher confining pressures.