Undrained Multidirectional Direct Simple Shear Behavior of Cohesive Soil

Abstract

The foundation soil of an offshore Arctic gravity structure is subjected to complex loading conditions. In addition to radial shear stresses induced by the weight of a structure, large horizontal shear forces from ice loads can produce large rotations and even reversal in the direction of horizontal shear stresses acting on soil elements within the foundation soil. The Multidirectional Direct Simple Shear (MDSS) apparatus was developed to simulate, at the element level, these states of stress. The MDSS first consolidates a circular specimen under both a vertical stress (σ′\Dvc\N) and a horizontal shear stress (τ\Dhc\N = τ\D1\N; setdown conditions). The specimen is then sheared undrained by applying a second independent horizontal shear stress (τ\D2\N; ice loading) at an angle θ relative to the horizontal shear stress τ\D1\N. The experimental program of MDSS and Geonor Direct Simple Shear tests on normally consolidated Boston Blue Clay used τ\Dhc\N = τ\D1\N = 0.2σ′\Dvc\N and varied θ in 30° increments from zero (shear in the same direction) to 180° (shear in the opposite direction). The peak undrained resistance decreased almost twofold from θ = 0° to θ = 120°, and the deformation behavior varied from very brittle at low θ angles to becoming ductile at higher angles. These dramatic changes in foundation response are an important design issue.