Stress-dilation of undisturbed sand samples in drained and undrained triaxial shear

Stress-dilation behavior of undisturbed sand samples tested in both drained and undrained triaxial shear has been studied. The stress-dilation relation is recognized as being a basic component of the stress-strain behavior of granular materials. The dilation angle, psi commonly used to represent the dilation characteristics of the soil, is defined clearly for plane strain conditions. However, the paper discusses confusion regarding its definition under triaxial loading conditions, and adopts the definition sin psi = - d epsilon(p)(v)/d gamma(p)(max). Drained triaxial tests performed on specimens obtained from undisturbed block samples of sand indicated that the undisturbed material exhibits a well defined stress-dilation relation. By referring to plastic (irrecoverable) components of strain, it was found that this relation was also compatible to results of tests in undrained triaxial shear. Demonstration of this compatibility required that the small membrane penetration effects in the undrained triaxial shear tests, resulting from changing effective confining stress, be taken into account. From the results of the present investigation, and of other studies reported in the literature, it was found that the relation between friction angle, phi' and dilation angle, psi, under axi-symmetric conditions, as defined above, can be reasonably expressed by the empirical expression: phi' approximate to 0.4 psi + phi'(cv).