Critical State Study of Natural Silty Sand Instability under Undrained and Constant Shear Drained Path

The prefailure instability of the granular soil slope due to lateral stress relief under the drained condition has been investigated experimentally under the constant shear drained (CSD) path in the literature. However, consensus on the drained instability and associated instability stress ratio, ${\eta }_{\mathrm{I}\mathrm{S}}$, has not yet been reached due to the differences in experimental setup and interpretation techniques. Earlier studies applied inconsistent real-time specimen-area correction and used three different interpretation techniques for ${\eta }_{\mathrm{I}\mathrm{S}}$: (1) rapid generation of axial strain, (2) change of volumetric dilation to contraction, and (3) a modified second-order work criterion. In this study, an advanced triaxial testing system was used for the CSD path to apply real-time area correction, and it was found that the aforementioned three techniques estimated slightly different ${\eta }_{\mathrm{I}\mathrm{S}}$. The rate of reduction of confining stress ($\Delta {\sigma }_3/\mathrm{d}t$) and initial state were found to have a significant effect on ${\eta }_{\mathrm{I}\mathrm{S}}$. The intriguing observation is that unlike undrained instability, drained instability can be recovered to a stable state by changing $\Delta {\sigma }_3/\mathrm{d}t$. The drained stress path used in this study can be applied in analyzing and designing tailing dams (e.g., Fundão tailing dam). Further, to compare these findings, a critical state line (CSL) and instability curve were developed through a series of undrained and drained triaxial tests. The results show that ${\eta }_{\mathrm{I}\mathrm{S}}$ and the state parameter at the onset of drained instability exhibit a narrow trend, which also matches with the instability curve from undrained tests.