Pore Pressures and Strains After Repeated Loading of Saturated Clay

Abstract

Repeated and sustained loading tests on undrained samples of normally consolidated lacustrine clay are used to prove the existence of a relationship between pore water pressures and axial strains.The behavior of the clay is studied by investigating the pore pressure and strain responses under repeated loading, and comparing them with the responses from the more usual sustained loading. Samples under repeated loading fail at a stress below the compressive strength of the material as obtained from a standard strength test.It has been shown that, for a repeated stress level below a critical value a nonfailure equilibrium state is reached, closed stress–strain hysteresis loops occur and the soil behavior is essentially elastic. For a repeated stress above the critical value, the effective stress failure envelope is reached and each loading cycle leads to further nonrecoverable deformations and ultimate failure.The pore pressures and axial strains produced by loading may be divided into recoverable and nonrecoverable components; both components of pore water pressure are linearly related to the axial strain provided the sample is not close to failure and there is a gradual increase in axial strains and pore pressures with time. This relationship is explained by a simple mechanistic picture.The pore pressures due to the application of a shear stress may be considered as a combination of an elastic recoverable component, due to the elastic response of the soil grain structure, and a plastic nonrecoverable component due to a partial collapse of the grain structure with subsequent transfer of stresses from the failed grain contacts to the pore water. There are coresponding axial strains occurring due to the elastic response of the soil grain structure and the partial collapse of the structure.Experimental proof of the pore pressure versus strain relationship is presented by means of a series of loading tests on normally consolidated lacustrine silty clay samples tested under undrained triaxial conditions.