Contrasting failure behaviour of two large landslides in clay and silt

Two large landslides in overconsolidated glaciolacustrine clay and silt, located in river valleys in British Columbia, were found to be quite similar in their main aspects. However, their failure behaviour was very different. One continues to exhibit intermittent, mainly ductile deformations with limited mobility which are typical of compound landslides in stiff clay. The other suddenly developed into an extremely rapid flow slide of 6.4 Mm(3), damming a large river in the course of a few minutes and projecting a wave onto the opposite bank. The flow slide followed a period of strong infiltration. A comparison of the two cases explains the difference in their failure behaviour. Three possible mechanisms for brittle flow slide formation are proposed, the first based on the undrained brittleness of soil at the rupture surface, the second on the high internal strength of the sliding body, and the third on macroscopic brittleness developed by pre-failure ductile deformation and softening. Both latter mechanisms are facilitated by the presence of nonplastic units in the sliding mass.