UNDRAINED MULTIDIRECTIONAL DIRECT SIMPLE SHEAR BEHAVIOR OF COHESIVE SOIL

The foundation soil of an offshore Arctic gravity structure is subject ed to complex loading conditions. In addition to radial shear stresses induced by the weight of a structure, large horizontal shear forces f rom ice loads can produce large rotations and even reversal in the dir ection of horizontal shear stresses acting on soil elements within the foundation soil. The Multidirectional Direct Simple Shear (MDSS) appa ratus was developed to simulate, at the element level, these states of stress. The MDSS first consolidates a circular specimen under both a vertical stress (sigma(vc))' and a horizontal shear stress (tau(hc) = tau(1); setdown conditions). The specimen is then sheared undrained by applying a second independent horizontal shear stress (tau(2); ice lo ading) at an angle theta relative to the horizontal shear stress tau(1 ). The experimental program of MDSS and Geonor Direct Simple Shear tes ts on normally consolidated Boston Blue Clay used tau(hc) = tau(1) = 0 .2 sigma(vc)' and varied theta in 30 degrees increments from zero (she ar in the same direction) to 180 degrees (shear in the opposite direct ion). The peak undrained resistance decreased almost twofold from thet a = 0 degrees to theta = 120 degrees, and the deformation behavior var ied from very brittle at low theta angles to becoming ductile at highe r angles. These dramatic changes in foundation response are an importa nt design issue.