Slope stability in unsaturated soils

Abstract

This thesis deals with the analysis of slope stability in unsaturated soils. The failure criterion (Mohr-Coulomb) is extended to take into account the effect of a second stress variable (i.e., matrix suction). Test results from the literature for unsaturated materials are used to illustrate and check the proposed concept.

Bishop's Simplified Method of slope stability analysis is modified to include the new failure criterion for unsaturated materials. The new slope stability equations are incorporated into a slope stability program which had been previously developed at the University of Saskatchewan.

A case history of a landslide that occurred in a compacted embankment of highly plastic clay is presented. Undisturbed block and tube samples from near the failure planes are used in a comprehensive testing program to determine the material properties. The testing program includes saturated triaxial tests, direct shear tests, consolidation tests, and suction tests. Hilf's Analysis is expanded and used to predict negative pore pressures.

The stability of the embankment slope is checked under conditions of soil suction (as constructed) and hydrostatic water pressure (saturation). The stability is also checked for the case of a tension crack forming and the tension crack filling with water. The effect of a frost line below the surface is also checked. Finally the failed conditions are analysed.

Conclusions reached concerning unsaturated materials and slope stability are presented.