Cyclic behaviour of loose anisotropically consolidated Ottawa sand under undrained torsional loading

Abstract

The behaviour of loose, anisotropically consolidated Ottawa sand was examined under undrained cyclic loading in the hollow cylinder apparatus. The stress level, the cyclic stress level and the consolidation stress ratio were varied. The instability line of anisotropically consolidated loose Ottawa sand, defined under monotonic loading, is shown to form a boundary in the vicinity of which a sudden increase in the rate of excess pore water pressure and strain accumulation is observed, and the sand strain-softens. However, this unstable response is arrested at phase transformation, and the sand specimens show a stable or temporarily stable effective stress cycle at low and high consolidation stress ratios respectively. Similar behaviour is observed for isotropically consolidated sand, which exhibits initial liquefaction after strain-softening. The terminal excess pore water pressure associated with this final stage is described as a function of consolidation stress ratio, while the development of excess pore water pressure during cyclic loading is uniquely defined for all tests as a function of normalised shear work. The cyclic resistance of the sand decreases with decreasing consolidation stress ratio. Anisotropic consolidation significantly affects the stiffness and damping ratio values of loose Ottawa sand.