Effects of Principal Stress Directions and Mean Normal Stress on Failure Criterion for Cross-Anisotropic Sand

An experimental program consisting of triaxial compression tests and torsion shear tests was designed and carried out to study the variation of shear strength in cross-anisotropic deposits of fine Nevada sand under two- and three-dimensional (3D) conditions using triaxial compression and a recently developed torsion shear apparatus. Triaxial compression tests were performed on vertical and horizontal specimens with cross-anisotropic fabric to obtain the shear strength and its variation with mean normal stress and with testing direction. The results formed the basis for characterizing the variation of curvature in meridian planes with mean normal stress. Furthermore, 44 drained torsion shear tests were performed at constant mean confining stress sigma m; constant intermediate principal stress ratios, as indicated by b=(sigma 2-sigma 3)/(sigma 1-sigma 3); and principal stress directions . The experiments were performed on large hollow cylinder specimens deposited by dry pluviation and tested in an automated torsion shear apparatus. The specimens had a height of 40 cm, an average diameter of 20 cm, and a wall thickness of 2 cm. The 3D failure surface of the fine Nevada sand is presented with discrete combinations of principal stress direction and the intermediate principal stress, and the effects of these two variables on the shape of the failure surface are presented.