Seismic stability of cohesive soil slopes

Abstract

This chapter presents a re-evaluation of the Fourth Avenue landslide in Anchorage that occurred during the 1964 Alaskan earthquake. This re-evaluation reveals that the slide was caused by a large undrained strength loss and the development of an undrained residual strength condition in the Bootlegger Cove clay. A constant volume ring shear apparatus was developed to measure the undrained peak and residual shear strength of cohesive soils. In this chapter, the results of constant volume ring shear tests on Bootlegger Cove clay are presented and compared to back-calculated shear strength values. The comparison shows that slide blocks, which moved less than one-half foot, mobilized the undrained peak shear strength. Slide blocks that moved more than one-half foot mobilized the undrained residual strength. The results of this study suggest that the peak undrained shear strength should be used to evaluate the seismic stability of cohesive soil slopes. If sliding is triggered due to earthquake shaking, an undrained residual strength can be mobilized and should be used to estimate the permanent lateral displacements.