THE ROLE OF CEMENTATION VARIABILITY IN COASTAL BLUFF-LANDSLIDE ANALYSIS

Steep cliff failures in coastal bluff (sea-cliff) areas of California continue to affect the built environment each year. Many of these cliffs are composed of variably cemented, weakly lithified sands such that the slope response can be dramatically different in adjacent areas of the coast. In a stretch of cliffs near San Francisco, California, wedge-shaped failures occur along parallel planes in weakly cemented sands, whereas failures occur as exfoliation fractures, typically of smaller extent and volume, in moderately cemented sands. The cementation variability is thought to be due to differential weathering of the sediments from groundwater seepage since their deposition during the late Pleistocene.

We present results from a field program and modeling effort that determined the predominant failure mechanisms and associated geotechnical analysis methods that correctly predict weakly lithified sand cliff-failures according to the degree of cementation. We show that whereas the traditional Culmann method can be modified to predict failure in weakly cemented sand cliffs, a limiting equilibrium approach does not correctly model the moderately cemented sand cliff behavior. Rather, in these materials, a tensile-strength and deformation-based analysis must be performed.