Effect of principal stress direction on the instability of sand under the constant shear drained stress path

Results of torsional shear hollow cylinder (TSHC) tests carried out on a reconstituted sand maintaining a constant direction of major principal stress relative to the vertical axis (alpha) and intermediate principal stress ratio (b) during shearing are presented. Tests were undertaken following the constant shear drained (CSD) stress path, which simulates stress conditions under a rising phreatic surface. The test programme was complemented by undrained TSHC tests to provide further insight regarding the behaviour of the sand when sheared under a range of alpha values. Various reconstitution methods (i.e. moist tamped (MT), dry pluviated (DP) and wet pluviated (WP)) were included to examine the effects of fabric on CSD triggering. Only MT and DP specimens could be prepared loose enough to exhibit liquefaction behaviour; hence the focus of this study was on these preparation methods. The programme indicated that the instability stress ratio (eta(IL)) under the CSD stress paths decreases as alpha increases, suggesting that cross-anisotropy strongly influences the shearing behaviour of sands under this trigger mechanism. In addition, the DP sand showed a greater decrease of eta(IL) with increasing alpha compared to the MT sand, consistent with other studies examining the effect of fabric and preparation methods on inherent anisotropy. The results of this study suggest that ignoring the effect of cross-anisotropy on the CSD trigger mechanism will lead to unconservative slope stability assessments.

Automated summary

The study investigates the shearing behavior of reconstituted sand under torsional shear hollow cylinder (TSHC) tests and finds that cross-anisotropy strongly influences the sand's shearing behavior. Ignoring the effect of cross-anisotropy leads to unconservative slope stability assessments.