Shear characteristics and stress-dilation relation of medium sand under normal to high pressures

The triaxial shear tests on the sands with various relative densities under normal to high pressures are carried out to investigate the effects of density and confining pressure on sand strength and deformation characteristics. Their mechanical properties are analyzed. Among the pressures ranging from normal to medium (0.8 MPa≤${\sigma }_3$≤2 MPa), the strain-stress curves show strain softening to different extents, and the shear dilatancy increases with the increase of relative density and the decrease of confining pressure. When entering high confining pressure range (${\sigma }_3$> 2 MPa), the strain-stress curves gradually turn into strain hardening type and the sample volume gradually turns into shear contraction. Prominent particle breakages will occur in the later shearing period of medium and dense sands at high confining pressure, which may lead to the secondary phase transition in the shearing process. The failure friction angles decrease with the logarithmic confining pressure linearly, and the attenuation rates of failure friction angles for different density sands are determined by linear fitting. Based on the Bolton’s stress-dilation relation, the critical state friction angle of the sand is determined, and the relationship among dilatancy index, initial relative density and average effective stress is established to provide strength parameters for stability analysis of sandy soil foundation under normal to high pressures.