Effects of Principal Stress Directions on 3D Failure Conditions in Cross-Anisotropic Sand: Fid-Bau Portal

Effects of Principal Stress Directions on 3D Failure Conditions in Cross-Anisotropic Sand

Lade, Poul V. / Rodriguez, Nina M. / Van Dyck, Eugene J.

Abstract

An experimental program was carried out to study the variation of shear strength of cross-anisotropic deposits of fine Nevada sand under three-dimensional (3D) conditions using a recently developed torsion shear apparatus. A total of 44 drained torsion shear tests were performed at a constant mean confining stress, $σ_{m}$ , constant intermediate principal stress ratios, as indicated by $b = (σ_{2} - σ_{3}) / (σ_{1} - σ_{3})$ , and principal stress directions, $α$ . 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 fine Nevada sand is presented with discrete combinations of $α$ and intermediate principal stress. The effects of these two variables on the shape of the failure surface are presented. The friction angles are highest for $α = 0 °$ for all $b$ -values and show a dip in strength with the lowest values near $α = 67.5 °$ , at which the major principal stress is inclined such that the potential shear band directions coincide with the bedding planes, which exhibit the lowest strengths. Overall, the friction angle varies from 57 to 32°, while the conventional friction angle from a triaxial compression test on a vertical specimen produced 41°, which is often employed in a given geotechnical engineering project.