A possible causal relationship between creep and sliding on Rwaza Hill, southern Rwanda

Stability calculations on a strongly creeping 28o slope on Rwaza Hill in Southern Rwanda reveal an apparent contradiction: terracette formation within the kaolinitic soil mantle should only be possible during hydrological conditions of high phreatic surfaces, for which the entire soil mantle should already have slid down in a translational sliding movement. This apparent contradiction can only be explained if it is accepted that the soil shear resistance can fall temporarily below the residual soil shear strength. A creep experiment confirms the theoretical possibility of this hypothesis. Furthermore, the configuration of the sliding surfaces in the field, consisting in fact of lenses of crumbled earth, shows close affinities with the pattern of potential ruptures in a creeping soil as provided by rheological theory. These data strongly suggest that, on natural slopes, creep should be the ultimate soil mechanical resason for sudden failures.