Stability of Natural Slopes in Quick Clay

Abstract

Norway and Sweden have been subjected to landslides for several centuries, with some of the largest landslides involving a large block sliding out as a continuous flake on a deposit of soft quick clay. Attempts to calculate the stability conditions for this type of landslides by using a conventional effective stress analysis have shown to considerably overestimate the safety factor. The introduction of the concept of limiting or yielding stresses in the course of the last 3 to 5 decades resulted in the general acceptance that stability calculations, even for natural slopes, should be based on undrained shear strength. Early in the seventies the ADP-type of analysis was proposed (Bjerrum, 1973; Ladd and Foott, 1974), based on triaxial and direct simple shear tests on tests specimens reconsolidated to in situ stresses, and thus simulating the stress conditions along different parts of the failure surface. The present study describes recently developed relations between undrained shear strength parameters and effective shear strength parameters, friction and attraction. The new relationships make it possible to do an effective stress analysis, and thus take existing pore pressures into consideration in a better way than in an ADP-type analyses. These effective stress-strength parameters have been determined for several Norwegian and Swedish clays, and used for a recalculation of some of the older Scandinavian flake type landslides.