Landslide Mobility enhanced by Basal Liquefaction: Evidence from the 2014 Oso Landslide, Washington State, USA
Abstract
Landslide mobility greatly affects hazard and risk as more mobile slides can spread destruction across broad areas. Assessing this threat for a given location can be challenging, as the underlying causes of mobility can vary. The March 2014 Oso (SR530) landslide provides an exceptional opportunity to identify specific causes of enhanced mobility. The slide was more mobile than a typical debris avalanche, traveling >1 km to the opposite side of the North Fork Stillaguamish River valley in western Washington State and killing 43 people as it swept across a flat alluvial plain. Prior historical landslides originating from the same location did not travel far past the toe of the slope. Following the 2014 event, we performed detailed field investigations aimed at illuminating the event sequence and the mechanisms that enhanced mobility.
Field studies revealed that the slide created a variety of deposits, including an extensive debris-avalanche hummock field spreading across much of the river valley. Throughout this hummock field, we mapped >350 sand boils, definitive indicators of liquefaction, as both isolated vents and fields of multiple vents. Sand boil evidence was transient; most of those we identified during the summer of 2014 were destroyed by the following year. Sand boils erupted through the glacial sediments, including lacustrine clays. Boil grain-size characteristics closely matched underlying alluvial sands. The hummocks themselves were not liquefied and typically contained intact pieces of glacial stratigraphy as well as rafted forest floor on their surfaces. We used coupled fluid-sediment deformation analysis, triaxial geotechnical testing, and standard cyclic-mobility liquefaction analysis to assess the viability of basal liquefaction. Our analyses indicate that mobility of the Oso landslide was enhanced by a large slope failure traveling quickly onto alluvial sediments saturated by rainfall prior to the landslide. This rapid loading liquefied underlying loose sediments, greatly reducing shear strength, and promoting swift passage of the slide mass on a liquefied base. When factors align to promote basal liquefaction, mobility can be greatly enhanced.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMNH14A..04R
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGYDE: 4302 Geological;
- NATURAL HAZARDSDE: 4303 Hydrological;
- NATURAL HAZARDSDE: 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDS