Based on the micro-mechanical model recently developed by Nemat-Nasser S. (J. Mech. Phys. Solids 48 (2000) 1541), a three-dimensional continuum mechanics model is presented for the deformation of granular materials which carry the applied load through frictional contacts. The model incorporates the anisotropy (or fabric) which develops as a frictional granular mass is deformed in shear, and includes the coupling between shearing and volumetric straining (or dilatancy). The model parameters are estimated, based on the results of a series of cyclic shearing experiments on large hollow cylindrical samples of silica sand. Then, the model is used to predict other experimental results, arriving at good correlation. #