Deformation and cyclic resistance of sand in large-strain undrained torsional shear tests with initial static shear stress

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This paper presents the findings from an experimental study focusing on the undrained cyclic behavior of sand in the presence of initial static shear stress. A series of undrained cyclic torsional shear tests was performed on saturated air-pluviated Toyoura sand specimens up to single amplitude shear strain (γSA) exceeding 50%. Two types of cyclic loading conditions, namely, stress reversal (SR) and stress non-reversal (SNR), were employed by changing the amplitude of the combined initial static shear and cyclic shear stresses. The tests covered a broad range of initial states in terms of relative density (Dr = 20–74%) and the initial static shear stress ratio (α = 0–0.30). The following five distinct modes of deformation were identified from the tests based on the density state, the transient undrained peak shear stress, and the combined cyclic and static shear stresses: 1) static liquefaction, 2) cyclic liquefaction, 3) cyclic mobility, 4) shear deformation failure, and 5) limited deformation. Of these, cyclic liquefaction and static liquefaction are the most critical. They occur in very loose sand (Dr ≤ 24%) under SR and SNR, respectively, and are characterized by abrupt flow-type shear deformation. Cyclic mobility occurs under SR in loose to dense sand with Dr ≥ 24%. Contrarily, shear deformation failure typically occurs under SNR in sand with 24 < Dr < 65%, and limited deformation may take place in dense sand with Dr ≥ 65%. In this paper, a stress-void ratio-based predictive method is proposed to identify the likely mode of deformation/failure in sand under undrained shear loading with static shear. Furthermore, the cyclic resistance is evaluated at three different levels of γSA (i.e., small, γSA = 3%; moderate, γSA = 7.5%; and large, γSA = 20%). The results show that, independent of the density state, the cyclic resistance continuously decreases with an increase in α at the small γSA level, while it first decreases and then increases for both loose and dense sand at the moderate and large γSA levels.