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Failure behaviour of two landslides in silt and clay Fletcher, Lara Alexandria

Abstract

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.

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