Effect of principal stress angle on the critical stress ratio of a silty sand tailings

A series of drained torsional shear hollow cylinder (HC) and triaxial compression (TC) tests were conducted on a silty sand iron ore tailings to investigate the effect of the principal stress angle on the critical stress ratio M. Two different combinations of principal stress angle α and intermediate principal stress ratio b were tested in the HC (α = 23° and b = 0·3 or α = 45° and b = 0·3) and then compared with the axisymmetric condition from the TC tests. The volumetric coupling parameter N (Nova’s flow rule) and the state-dilatancy coefficient χ were additionally estimated for each loading condition. The results showed that M is not unique for the same b, being also affected by α, which may be attributed to the effects of anisotropy. An increase in M of 9% was found when α was decreased from 45° to 23° while keeping the same b of 0·3. The results further show that χ does not depend on α when tested at a fixed value of b; however, it exhibits substantial variation relative to the TC condition when tested at b = 0·3.