COMBINED GEOLOGIC CONDITIONS RESULT IN SLOPE INSTABILITY AND FAILURE: TWO CASE STUDY EXAMPLES

A number of geologic factors influence the stability of both soil and rock slopes. Included are material strength, slope angle and height, fractures and their orientation, groundwater, and external loads such as earthquake shaking. Slope stability is dependent on the relationship among these geologic factors. As examples, two vastly different case histories are considered: Liquefaction of Alluvium and Glacial Outwash in Evansville, Indiana and Rock Slope Failure in Valdez, Alaska. Soil conditions in alluvium and glacial outwash north of the Ohio River in Evansville, IN would be a major concern during earthquake shaking. An earthquake similar to the New Madrid events of 1811 and 1812 could cause liquefaction in fine grain sands and silts. Studies by Kayabali and West (1994, 1995) concluded that a high groundwater table during intense earthquake shaking would yield extensive liquefaction failures. By contrast, a study by West and Cho (2007, 2008) in Valdez, Alaska indicated that high piezometric levels in fractured metasedimentary rocks, in combination with moderate earthquake shaking could lead to slope failure of rock blocks from areas between rock bolts. In both cases, a combination of two critical occurrences, high water table and earthquake shaking, would cause significant slope failures.