Calibration of an Anisotropic Sand Model

Dilatancy is an essential sand response. Experimental and microstructural investigations have shown that whether a sand is in a dilative or contractive state depends on its density, confining pressure, stress ratio, as well as the soil fabric. In the past decade, a number of researchers have tackled the dilatancy issue from the perspective of constitutive modeling. It has been shown by Manzari and Dafalias and Li and Dafalias that, in order to model sand behavior over a full range of density states, additional dependence of dilatancy on the internal state of the sand is needed, and the material state must be described in reference to the critical/steady state line in the e-p-η space. This concept is referred to as the state-dependent dilatancy. In addition, due mainly to the process of deposition under earth gravity, the behavior of sand is very likely to be inherently anisotropic, meaning the stress-strain-strength relations for the same sand may vary as the stress tensor rotates in reference to the orientation of soil fabric. Experimental investigations in the past 15 years on flow liquefaction of earth structures have revealed that the influence of inherent fabric anisotropy on the residual strength of a granular soil is so drastic that the inherent anisotropy can no longer be ignored. On the basis of these observations, Li & Dafalias proposed an anisotropic sand model aimed at describing these features. This paper briefly reviews the issue of sand dilatancy and the impact of fabric anisotropy, and the model developed by Li and Dafalias. The focus will be placed on the calibration and determination of several important material constants, including the critical state parameters and the fabric anisotropic index.