Unloading Elastic Behavior of Sand in Cyclic Triaxial Tests

In this paper, digital image technology was used in the surface-deformation measurement of soil, making it possible to monitor the axial, radial, and volumetric deformations and the deformation distribution over the entire surface of specimens in triaxial tests. A series of cyclic triaxial tests on silica powder and Fujian sand were conducted to investigate the stress-strain behavior of sand in pre-failure deformation, particularly in the unloading elastic behavior. The sand gradually tended toward the stable state of deformation after it was subjected to a certain number of cyclic loadings. The sand appeared to be entirely nonlinear elastic, with unchanged and closed hysteresis loops of the stress-strain curve. In addition, the elastic parameters—Young's modulus and Poisson's ratio—of the specimens during the unloading process of the stable state of deformation were carefully studied. The test results suggested that the unloading tangent Young's modulus of sand was determined by the stress state and void ratio but was not affected by the stress amplitude of cyclic loading. However, Young's modulus depended more definitely on the axial stress than on the lateral stress when the deviatoric stress increment was applied in the triaxial stress condition. Moreover, the unloading average Poisson's ratio was dependent on the stress state and void ratio.