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Enhanced landslide mobility by basal liquefaction; the 2014 State Route 530 (Oso), Washington, landslide

Brian D. Collins and Mark E. Reid
Enhanced landslide mobility by basal liquefaction; the 2014 State Route 530 (Oso), Washington, landslide
Geological Society of America Bulletin (June 2019) 132 (3-4): 451-476


Landslide mobility can vastly amplify the consequences of slope failure. As a compelling example, the 22 March 2014 landslide near Oso, Washington (USA), was particularly devastating, traveling across a 1-km+-wide river valley, killing 43 people, destroying dozens of homes, and temporarily closing a well-traveled highway. To resolve causes for the landslide's behavior and mobility, we conducted detailed postevent field investigations and material testing. Geologic and structure mapping revealed a progression of geomorphological structures ranging from debris-flow lobes at the distal end through hummock fields, laterally continuous landslide blocks, back-rotated blocks, and finally colluvial slides and falls at the landslide headscarp. Primary structures, as well as stratigraphic and vegetation patterns, in the landslide deposit indicated rapid extensional motion of the approximately 9X10 (super 6) m (super 3) source volume in a closely timed sequence of events. We identified hundreds of transient sand boils in the landslide runout zone, representing evidence of widespread elevated pore-water pressures with consequent shear-strength reduction at the base of the slide. During the event, underlying wet alluvium liquefied and allowed quasi-intact slide hummocks to extend and translate long distances across the flat valley. Most of the slide material itself did not liquefy. Using geotechnical testing and numerical modeling, we examined rapid undrained loading, shear and collapse of loose saturated alluvium, and strong ground shaking as potential liquefaction mechanisms. Our analyses show that some layers in the alluvium can liquefy when sheared, as could occur with rapid undrained loading. Simultaneous ground shaking could have contributed to pore-pressure generation as well. Two key elements, a large and rapid failure overriding wet liquefiable sediments, enabled the landslide's high mobility. Basal liquefaction may enhance mobility of other landslides in similar settings.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 132
Serial Issue: 3-4
Title: Enhanced landslide mobility by basal liquefaction; the 2014 State Route 530 (Oso), Washington, landslide
Affiliation: U. S. Geological Survey, Landslide Hazards Program, Menlo Park, CA, United States
Pages: 451-476
Published: 20190625
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 133
Accession Number: 2019-081183
Categories: Engineering geologyEnvironmental geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 1 table, geol. sketch maps
N48°16'20" - N48°17'10", W121°51'10" - W121°49'50"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2020, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201943
Program Name: USGSOPNon-USGS publications with USGS authors
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