A study of delayed failure in a cut slope in stiff clay

Abstract

A first-time, delayed failure occurred in a 7 m deep cut in overconsolidated glacial lake sediments. Remedial work on this landslide allowed a detailed examination of a large part of the slip surface. The backscarp section was readily divisible into two major zones on the basis of macrostructure of the soil. The upper zone was highly fractured and displayed a blocky or nugget structure. The lower zone, while not totally intact, did not display the nugget structure. A third zone in the toe area was subsequently defined. The three zones were sampled and each was described in detail with regard to textural, compositional, and structural features.The samples were tested in triaxial and direct shear tests and the results indicated that relatively distinct strength parameters existed for each zone. The upper and toe zone results were interpreted as having a bimodal failure envelope on a Mohr diagram. Stability analyses were carried out using the strength parameters determined from the testing program. The slope failure may be best accounted for by a slip plane mobilizing virtually zero cohesion with a high angle of shearing resistance in the toe and upper zones and partially softened parameters in the lower or central zone. Key words: delayed failure, shear strength, slope stability, weathering.