Landslides and Engineering Geology of the Seattle, Washington, Area
This volume brings together case studies and summary papers describing the application of state-of-the-art engineering geologic methods to landslide hazard analysis for the Seattle, Washington, area. An introductory chapter provides a thorough description of the Quaternary and bedrock geology of Seattle. Nine additional chapters review the history of landslide mapping in Seattle, present case studies of individual landslides, describe the results of spatial assessments of landslide hazard, discuss hydrologic controls on landsliding, and outline an early warning system for rainfall-induced landslides.
The Ross Point landslide: An instrumental record of landslide reactivation
Published:January 01, 2008
Lynn J. Moses, 2008. "The Ross Point landslide: An instrumental record of landslide reactivation", Landslides and Engineering Geology of the Seattle, Washington, Area, Rex L. Baum, Jonathan W. Godt, Lynn M. Highland
Download citation file:
In September 1998, a geotechnical investigation was initiated by the Washington State Department of Transportation (WSDOT) to assess stability of a slope in the vicinity of Ross Point, located along the State Route 166 (SR 166) corridor. The roadway lies at the base of a marine bluff composed of Pleistocene glacial and interglacial deposits, and it has a history of roadway closures associated with landslides and embankment failures. As part of the investigation, geotechnical test borings were drilled to define subsurface conditions, and inclinometer casing and open-standpipe piezometers were installed to monitor slope movement and groundwater levels. All drilling was completed and inclinometer and piezometer installations initialized by 30 December 1998.
The Ross Point landslide occurred on 29 January 1999. This landslide is unique in that the subsurface conditions had been thoroughly characterized in a geotechnical investigation prior to failure, and rainfall and groundwater levels were being frequently monitored at the time of the failure. Although slope failures are common in the landslide sequence of Esperance Sand overlying Lawton Clay, monitoring of rainfall and groundwater levels prior to and during landslide failure is unique. The groundwater levels monitored within the Ross Point landslide mass showed a rapid response to intense rainfall events, whereas the groundwater response measured in boreholes located outside of the failure was subdued. Excessive pore-water pressures rapidly developed within the landslide mass following significant rainfall events. This rapid increase in pore-water pressure, coupled with high antecedent groundwater levels, is considered to be the critical factor in triggering the Ross Point landslide. Wood sampled from the lowest nonglacial sediments exposed at Ross Point yielded a 14C date of 40,570 ± 700 yr B.P. and are considered to be the Olympia beds.