Undrained anisotropy and rotational shear in granular soil

The impact of fabric anisotropy on the behaviour of granular soil remains a subject of great interest. In particular, the effects of principal stress rotation on the undrained response of saturated sand are not fully understood. This paper describes an experimental investigation conducted in an automated hollow cylinder apparatus into the undrained anisotropic behaviour of saturated sand in rotational shear, which is defined as a class of non-proportional loading with a continuous rotation of the principal stress directions but a constant deviatoric stress. Special attention in this investigation was placed on the influence of the relative magnitude of the intermediate principal stress, characterised by the parameter b = (σ₂ − σ₃)/(σ₁ − σ₃), on the pore pressure response and deformation characteristics. The experimental observations indicate that soil specimens, even in very dense state, were weakened by the build-up of pore water pressure in rotational shear. The intermediate principal stress parameter b was not a negligible factor for the behaviour of saturated sand in the rotation shear, but had significant impact. Under otherwise identical conditions, the soil specimen rotationally sheared under the condition b = 0 exhibited a much stronger resistance to pore pressure build-up and a less dramatic stiffness degradation than that sheared under the condition b = 1·0. This observation is considered to be directly associated with the soil fabric anisotropy.