Apparent kinetic viscosity of saturated Nanjing sand due to liquefaction-induced large deformation in torsional shear tests

The post-liquefaction dynamic flow deformation of saturated sand may cause severe damages on the ground and underground structures. To investigate the dynamic flow characteristics of the post-liquefied saturated sand, a series of undrained cyclic torsional shear tests on saturated Nanjing sand are conducted using the hollow column torsional shear apparatus by applying and measuring double amplitude shear strain up to about 100%. Based on the test results, how the effective confining pressure and the initial shear stress affect its dynamic flow characteristics in the liquefaction state is analyzed. The results show that the sample is actually in the dilation state at the end of "zero effective stress" determined by the excess pore pressure ratio. In other word, compared with the post-liquefaction "zero effective stress" state, the response of the excess pore pressure is hysteretic. The initial static shear stress has great effect on the apparent viscosity-strain rate relationship curves of saturated Nanjing sand in the "zero effective stress" state and the dilation state, while it has little effect on the apparent viscosity-strain rate relationship curves of saturated Nanjing sand in the reversal unloading state. Meanwhile, the easy flow property of saturated Nanjing sand under cyclic loading is obvious stronger than that under monotonic loading.