Impact of height control on constant volume simple shear testing on sand
Liquefaction is a failure behavior of saturated sand under undrained condition experiencing transient, usually seismic, loading. Current laboratory procedures for evaluating liquefaction behaviors and potentials include using simple shear tests to simulate a vertical propagating seismic wave or a monotonic load. To better simulate field conditions, the sand specimens are tested at saturation under undrained conditions. However, undrained condition can be simulated by keeping the volume of the specimen constant during the test. When performing simple shear tests under constant volume condition, ASTM Standard D6528 allows up to 0.05% volume change of the specimen to be considered a valid equivalent undrained test. Volumetric change of the specimen during constant volume shear impacts the changes in the normal stresses caused by the contractive or dilative behaviors of sand under constant volume condition. This impact on the change in the normal stress, even while the vertical deformations are within the allowed range, can be significant and is a function of the specimen properties. For this research, simple shear tests were run on different sand specimens using the modified UT Cyclic Simple Shear device. The previous version of the device tested saturated sand specimens under truly undrained conditions, while the modifications allowed dry sand specimens to be tested under constant volume conditions to simulate undrained condition. The modifications to the device were to increase the rigidity of the system and overall quality of the results. Monotonic and cyclic loading test results were obtained from the modified testing device. Analyzing the monotonic test results found that even small height changes to the specimen will affect the contractive and dilative behaviors of the specimen. Contractive height change of loose specimens decreased the amount of generated positive change in the axial stress during contraction, while the dilative height change to the dense specimens increased the dilative stress changes at high shear strains. Cyclic test results were compared to the results from corresponding tests obtained from the previous version of the testing device to verify the effect of the modification, as well as comparing the results of constant volume and undrained conditions.The results show that specimens tested under constant volume condition displayed lower strength than the specimen under undrained condition. The dense specimens showed a more significant reduced strength with the difference increasing with the amplitude of vertical movement of the top platen during cyclic loading. Finally, methods of remediating the stress change were proposed. These methods, while experimental, could be modified and used to improve the quality of the simple shear test results under constant volume condition.