Experimental Study of 3D Failure Surface for Cross-Anisotropic Sand Deposits During Stress Rotation
A torsion shear apparatus developed at The Catholic University of America, Washington, DC was used to conduct experiments on Fine Nevada Sand in order to study the effects of cross-anisotropy, shear banding and stress rotation under three independent principal stresses on stress-strain behavior. Drained torsion shear tests were performed with constant b-values, major principal stress directions and mean confining stress. A series of 18 drained tests using a true triaxial apparatus were conducted for comparison with certain torsion shear results. Drained triaxial tests with varying mean confining stress were also performed. Additional drained conventional triaxial tests were also performed to determine parameters needed for modeling. The 3D failure surface of Fine Nevada sand is presented. Shear banding patterns and analysis is presented for the true triaxial and torsion shear tests. The failure conditions from the torsion shear results are compared to a newly developed failure criterion. The effects of the intermediate principal stress on the failure surface are analyzed. Results clearly show the effects of cross-anisotropy in Fine Nevada Sand. This is seen in variation of friction angle with differing conditions and from strain analysis in different stress paths. Non-associated flow is observed and shear band inclinations in torsion shear specimens, although scattered, follow the Coulomb equation prediction. Data from these tests provide a solid foundation for future developments of cross-anisotropic constitutive models for frictional materials.
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