Curvature of Failure Envelopes for Normally Consolidated Clays

Shear strength parameters play an important role in the stability of slopes. Accuracy of the analysis result depends on the nature of the failure envelopes used, especially in the case of shallow landslides, where the shear strength is mobilized at low effective vertical stresses (σ _n ′). Ten normally consolidated samples prepared from mixtures of montmorillonite, kaolinite and quartz were sheared in a constant volume simple shear device at the normal stresses ranging from 25 to 800 kPa. From the results, it was noted that correlations for undrained shear strength should be selected with respect to the anticipated failure mechanism. The power function can be used to represent the curvature in failure envelopes. Direct shear results suggests a lack of curvature in the effective failure envelopes for normally consolidated clays. However, results from the simple shear testing show that failure envelopes are curved for low effective normal stresses and thus, the use of a linear c′−ϕ′ envelope can be unconservative for shallow landslides.