Effect of Initial Shear Stress on liquefaction failure and shear strain development by Hollow Cylindrical Torsional Shear Tests

Soil liquefaction under initial shear stress induced by nearby slopes, embankments and superstructures causes large shear failure in the ground, leading to sliding, tilting and uneven settlement of the structures. In this study, a hollow torsional shear apparatus is used to investigate the effect of initial shear stress acting on horizontal plane on liquefaction behavior of medium to loose sand of relative density D r ≒ 30, 50% and fines content F c =0 ～ 30%. A series of test results have revealed that the liquefaction failure is categorized into five types; cyclic failure (CF) for α =0, cyclic biased failure (CBF) for small α , biased gradual failure (BGF) and biased sudden limited failure (BSLF) and biased sudden failure (BSF) for larger α . Among the 5 failure types, it is important to pay special attention to BSF type, which may develop abrupt strain increase due to initial shear stress by cyclic loading. In comparing with undrained monotonic loading tests, the mechanism of the abrupt BSF has been clarified. The angle of a yielding surface φ y ´ corresponding to the BSF has been studied under the effect of initial stresses for test specimens with different contractive behavior caused by parametrically changing fines content and relative density. It has been demonstrated that the BSF type failure is controlled by the angle φ y ´ reflecting contractive behavior of sand, irrespective of the magnitude of initial stress.