Evaluating impact of various direct simple shear machine compliances on constant volume cyclic strength of Nevada Sand
The results from cyclic direct simple shear tests are presented in this thesis. The tests were performed under undrained constant volume conditions using Nevada Sand to evaluate the effectiveness of the recent modifications on reducing the impact of machine compliance (particularly, in terms of vertical deformations during shearing) on the results. The specimens were reconstituted with the water sedimentation technique at two relative densities: loose and dense. Furthermore, the cyclic resistance responses for dense and loose specimens are presented, including a detailed analysis of the various measured axial deformations to determine their impact on the results. Measuring axial strains during shearing is critical for tests sheared under constant volume conditions. For such tests to be considered undrained, the volumetric change and, therefore the axial strains, should not exceed 0.05% as stated by ASTM D6528-17. The results from a range of tests showed that some axial strains measures, like the average of the peak-to-peak axial strains and the difference and the average of the axial strain slopes, reflected more accurately what occurred while shearing the specimens. The average peak-to-peak axial strain captured the rocking or tilting behavior while the difference in axial strain slopes showed that at higher CSRs, higher bending is observed in the equipment, and therefore higher distortions in the specimens resulting in a lower number of cycles to liquefaction. The improvements to the equipment showed a major decrease in rocking, particularly at the level of the bottom plate of the 4-piston assembly. This resulted in a 10-times decrease in the rocking at the platen level. However, there is still some more room for improvement to further lower the rocking through a more rigid connection between the top platen and the load cell. The screw-nut jacks worked well in locking the top platen in place and minimizing vertical displacements during shearing. The larger jacks and higher torques results in lower vertical strains, up to 20 times lower than the current recommendations of 0.05%. Lastly, the modifications resulted in a much stiffer system for resisting the lateral forces with minimal horizontal displacement in the top platen, and thus eliminating the major drawback of the old system.