Influences of confining pressure and shear rate on stress characteristics of triaxial drainage tests on calcareous sand

The triaxial consolidation drainage tests with different shear rates are carried out to study the shear stress characteristics of the calcareous sand in Xisha Islands, Sansha City, Hainan Province of China. The test results show that the larger the confining pressure is, the later the peak value of the deviator stress appears, and the faster the shear rate of the same confining pressure is, the larger the peak value of the stress is, the faster the stress drop is, the larger the stress drop ratio is, the shear rate has a certain effect on the peak value of the stress and the residual shear strength, and the lower the confining pressure is, the greater the impact of the shear rate on the peak value of the deviator stress. The smaller the confining pressure, the larger the deviator stress ratio, and the larger the shear rate of the same confining pressure, the faster the deviator stress ratio increases. When the shear rate is lower than a certain rate, the change of the deviator stress ratio is not affected by the shear rate. Under the same confining pressure, because the confining pressure and pore pressure remain unchanged during drainage shear, the relationship between the deviatoric stress and the average effective stress is a straight line with slope of 3, and the trend of failure line is the same at different shear rates. The larger the shear rate is, the larger the failure-line angle is, but the difference is not large. The failure line is a straight line with slope of 1.75, and the internal friction angle of the calcareous sand is 42°. The average effective stress increases first and then decreases during the shearing process, which shows a change process from shear shrinkage to dilatancy. The pore ratio decreases under similar rates at different shear rates at the shrinking stage. The shearing rate has a greater influence on the pore ratio at the dilatancy stage. The larger the shear rate is, the larger the change of the pore ratio is.