Slope Failure in Weathered Claystone and Siltstone

This paper describes the failure of an 8.5-m-high fill slope constructed over weathered claystone and siltstone in Martinez, Contra Costa County, in northern California. The failure occurred about 5 months after construction. Several piezometers and inclinometers were installed to measure the pore pressures and the location of the failure surface(s), respectively. The failure surface was observed just below the interface between the fill and the natural materials. Factors of safety of the failed slope calculated using laboratory-derived effective shear strength parameters were on the order of 1.58–1.95. Reduction factors were necessary to reduce the shear strength values and to obtain reasonable agreements with the field observations. A class “C” prediction of the location failure surface and the factor of safety using the reduced shear strength values indicated results that were consistent with the field observations. This paper highlights the importance of exercising extreme caution when evaluating the stability of slopes in plastic natural clays.