Examples of multiple rock-slope collapses from Köfels (Ötz valley, Austria) and western Norway

Abstract

This paper describes the effects of large massive rock-slope failures on subsequent slope stability. Three examples of large rockslope failures from the Austrian Alps and Norway demonstrate that failure increases the probability of further collapses. At Köfels, Austria, a Holocene rock-slope failure several km 3 in size filled the Ötz valley. The morphology of the deposits indicates that at least one subsequent failure occurred along the head scarp of the first failure, most likely a slide of similar size. Debris of the second landslide slid over the older deposits, forming the famous Köfels frictionite. At least three rock-slope failures, all of them in excess of 10 6 m 3 , occurred from the same mountainside within the last 6000-8000 years at Tafjord in western Norway. The most recent of these failures in 1934, triggered a destructive tsunami. Five large failures with volumes z50,000m 3 occurred at Ramnefjell, Norway within 50 years; two of them caused considerable damage and a large number of death due to the formation of destructive tsunamis. Two-dimensional finite element models of rock-slope stability before and after the Köfels and Tafjord landslides show that massive rock-slope failures produce: (a) irregular slopes, parts of which are as steep or steeper than the slope before failure and which represent new zones of instability; and (b) zones of weakness related to slow slope deformation and related cracking.

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