Drained Instability in Loose Granular Material

Abstract

The constant shear drained (CSD) test is considered an appropriate test to simulate the instability behavior of soils in slopes under water infiltration conditions or lateral stress relief. However, contradiction of the drained instability in loose granular material exists in the reported test results, which is considered because of the condition of constant deviatoric stress (q) not strictly satisfied in the literature. Because the variation of q during the CSD test has significant effects on the test results, more data from tests under a true constant q condition are needed to clarify this subject. In this paper, CSD and conventional undrained triaxial tests in both compressive and extensive loading were performed to investigate the instability behavior of loose sand. All the CSD tests were conducted by advanced variable confining pressure triaxial apparatus, in which a true constant q was maintained during the test; thus, more reliable results were obtained. It is observed experimentally that when a specimen is sheared along a fully drained stress path with a constant q but decreasing effective mean normal stress, instability occurs before the effective stress path reaches the failure line. It is also found that the onset of instability for both undrained and CSD tests are initiated at almost the same stress ratio for a given void ratio of specimens. Hill’s stability conditions in terms of the sign of the second work increment are used to interpret the experimental results of instability. The high-quality test results obtained in this study may help to explain the contradiction reported in the literature and clarify the controversy in drained instability.