| Summary: | Two large landslides in overconsolidated glacio-lacustrine clay and silt deposits of British
Columbia river valleys have been examined: the Attachie landslide, Peace River and the Slesse
Park landslide, Chilliwack River. Both cases are quite similar in their main aspects. However,
their failure behaviour was markedly different. One exhibited intermittent ductile deformations
typical of compound landslides in stiff clay. The other developed suddenly into a catastrophic,
extremely rapid flow-slide of 6.4 Mm³ , following tens of years of slow, probably episodic
movement. A comparison of the two cases is presented, with the purpose of explaining the
difference in their failure behaviour. Comparison includes physiography, ground water
characteristics, stratigraphy, material properties of the dominant units, and geometry. The most
significant difference between the two cases, apart from scale (volume) is the presence at
Attachie of a considerable content of non-plastic, cemented, unsaturated silt units.
Three possible mechanisms for brittle flow slide formation are proposed. The first, undrained
brittleness, is the usual cause of flow slides in what are generally described as cohesionless,
weakly bound or cemented, high porosity materials (Hutchinson, 1992). At small strains the soil
structure collapses resulting in a sudden reduction in shear strength and allowing a flow slide to
develop. The second is a process that leads to what can be described as "macroscopic"
brittleness. In short, a substantial portion of the previously intact soil volume is transformed into
a mass of blocks of intact clay and till separated by joints filled with a disturbed, loosely
deposited matrix. Liquefaction of the loose material then occurs, exerting heavy fluid pressure
inside the cracks. The third was proposed by Hutchinson (1987) to explain the 1963 Vaiont
Slide in Italy. The factors which lead to this behaviour include: 1) a compound (markedly noncircular)
shape of the bounding slip surface, so that the slide forms initially a kinematically
inadmissible mechanism; 2) a strong contrast between a low shear strength, non-brittle bounding
slip surface and a high strength, brittle slide mass. These conditions may produce a stored deficit
in the overall factor of safety on the boundary shear surfaces, which can be suddenly released by
a brittle failure along internal shears in the slide mass. Results of stability analyses suggest that a
combination of the latter two mechanisms likely led to the development of the flow slide. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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