Effects of Stress Rotation and Changes of B-Values on Cross-Anisotropic Behavior of Natural, K0-Consolidated Soft Clay

Abstract

ABSTRACT A series of consolidated-undrained torsion shear tests was performed on large, hollow cylindrical specimens of undisturbed San Francisco Bay Mud to investigate the influence of stress rotation and changes in b-values on the stress-strain, pore pressure, and strength characteristics of a cross-anisotropic, natural clay. The specimens were first K0-consolidated and then sheared along various undrained stress paths to study the clay behavior in the full range of stress rotation from 0° to 90° relative to vertical. For this cross-anisotropic clay, the test results indicate that the major principal strain increment directions in physical space do not coincide with the directions of major principal stress at failure. The deviation between these two directions varied continuously and the maximum deviation was measured at approximately 20°. The normalized, undrained shear strength decreases systematically with inclination of the major principal stress at failure, ψ, from triaxial compression with ψ = 0° and b = 0 to triaxial extension with ψ = 90° and b = l. The effective friction angle remains constant at approx. 35° in the range of ψ from 0° to 30° after which increases gradually to almost 50° at ψ = 90°. These results are unlike those obtained for a laboratory prepared deposit of kaolin, which tended to behave more like an isotropic material despite the initial A0-consolidation. Thus, effects of aging are very important in the behavior of clays, and it may not be possible to simulate such effects with sufficient accuracy and reliability with laboratory prepared clay deposits.