Shear strength and small-strain stiffness of a natural clay under general stress conditions

Abstract

In this paper the results of an experimental investigation into the mechanical behaviour of a natural stiff clay are presented. A set of drained true triaxial tests was performed, following rectilinear stress paths with constant mean effective stress, having different directions in the deviatoric stress plane. In addition, the small-strain shear stiffness of the soil in the vertical and horizontal planes was measured in the triaxial apparatus, during isotropic and anisotropic compression. To this purpose, three pairs of differently oriented bender elements were used. True triaxial failure data made it possible to plot the failure envelope in the deviatoric plane, and showed that the clay is slightly anisotropic with respect to strength. The analysis of the stress–strain response showed that the soil stiffness depends on the stress-path direction. For shear strains smaller than about 0·01% it was observed that, for monotonic loading conditions, the stress–strain relationships can be described with reasonable accuracy using a non-linear cross-anisotropic elastic model. Bender element test results confirmed that the clay is anisotropic with respect to stiffness, and that the small-strain shear modulus in the horizontal plane is larger, by about 22%, than that measured in the vertical plane. Such results also demonstrated that the small-strain stiffness increases with increasing values of the effective stress ratio experienced by the samples during constant-η compression paths, though this effect was not large because of the overconsolidated state of the soil. A reasonable agreement was finally observed between the shear stiffness of the clay obtained, at small strains, from true triaxial and bender element tests.