GROUNDWATER AND SHEAR STRENGTH INFLUENCE ON FIRST-TIME LANDSLIDES IN VASHON ADVANCE GLACIOLACUSTRINE DEPOSITS, PUGET LOWLANDS, WASHINGTON

Advance glaciolacustrine deposits from the last Pleistocene glaciation into the Puget Lowlands (Qvgl) outcrop extensively in marine bluffs and valley slopes in the Cascade and Olympic foothills. Qvgl deposits are characteristically hard to very hard in consistency, laminated to thinly bedded, nonplastic to moderately plastic fine sands, silt and clay. Mass wasting is a widespread geomorphic process associated with these deposits. They commonly perch groundwater within the overlying advance outwash deposits and induce landsliding within the outwash deposits. The accumulated fine-grained colluvium on and at the base of these slopes is especially susceptible to precipitation-induced landsliding. Much less frequently in historic time, first-time landslides propagate deeply through intact Qvgl deposits. The 1997 Woodway landslide and the large retrogression associated with the 2014 SR 530 (Oso) landslide are examples of such failures; both have had post-failure investigations performed that included geotechnical drilling, instrumentation, and laboratory testing.

Post-failure piezometer data within intact Qvgl deposits adjacent to these landslides suggest that pre-failure conditions within these units may not always be fully saturated and that, where saturated, pore pressures may not be uniformly hydrostatic. Further, pore pressures within intact deposits may often be insensitive to storm and seasonal precipitation flux. Another large-volume, first-time landslide apparently initiating in intact Qvgl deposits, the 1979 Possession Bay Landslide on Whidbey Island, failed during the summer dry season. These data complicate interpretations about the role precipitation has on the initiation of failure within intact Qvgl deposits and, along with other available data, may indicate a progressive mode of failure.

Shear strength of intact Qvgl deposits is anisotropic, based on direct shear testing of samples parallel and perpendicular to bedding. Under presumed drained conditions at the onset of failure, limit-equilibrium and finite-element analyses revealed the strong influence that strength anisotropy and confining stress have on the shape, length, and location of the failure surface, and thus landslide volume.