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The Ross Point landslide; an instrumental record of landslide reactivation

Lynn J. Moses
The Ross Point landslide; an instrumental record of landslide reactivation (in Landslides and engineering geology of the Seattle, Washington area, Rex L. Baum (editor), Jonathan W. Godt (editor) and Lynn M. Highland (editor))
Reviews in Engineering Geology (2008) 20: 167-181


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 (super 14) C date of 40,570+ or -700 yr B.P. and are considered to be the Olympia beds.

ISSN: 0080-2018
EISSN: 2169-799X
Coden: GAEGA4
Serial Title: Reviews in Engineering Geology
Serial Volume: 20
Title: The Ross Point landslide; an instrumental record of landslide reactivation
Title: Landslides and engineering geology of the Seattle, Washington area
Author(s): Moses, Lynn J.
Author(s): Baum, Rex L.editor
Author(s): Godt, Jonathan W.editor
Author(s): Highland, Lynn M.editor
Affiliation: Washington State Department of Transportation, Olympia, WA, United States
Affiliation: U. S. Geological Survey, Denver, CO, United States
Pages: 167-181
Published: 2008
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 17
Accession Number: 2009-038125
Categories: Engineering geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 8 tables, geol. sketch maps
N47°06'00" - N47°46'00", W122°32'60" - W121°04'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200921
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