Effect of vertical effective and initial static shear stresses on the liquefaction resistance of sands in cyclic direct simple shear tests

To investigate the effect of the vertical effective and initial static shear stresses on cyclic resistance under cyclic loading with principal stress rotation, a series of cyclic direct simple shear tests were conducted on loose and dense Nakdong River sands. The results revealed that the increasing vertical effective stress (σ′_v0) often decreased the cyclic resistance, and the decrease was more evident for a high initial static shear ratio (α = 0.2). The degree of reduction of the correction factor K_σ predicted by recommended methods in practice mostly showed more conservative results than the measured K_σ values in this study. Increasing the initial static shear stress decreased the cyclic resistance of the loose sand. For the dense sand, the cyclic resistance either increased or decreased, depending on the combined effect of the σ′_v0 and α levels. The correction factor K_α decreased minimally to 0.689 for the loose sand and rose maximally to 1.175 for the dense sand. When the relative density (D_r) increased from 40% to 80%, the increase in the cyclic resistance for a high α (=0.2) was considerably greater than that for α = 0. The cyclic response of the loose sand was due to sudden flow deformation without any apparent shear strain accumulation, regardless of the α value; while the cyclic response of the dense sand indicated plastic strain accumulation. Thus, the cyclic resistance of sand is influenced by the combined effect of D_r, σ′_v0, and α, with the potential influence of principal stress rotation.