Oso Landslide Geotechnical Construction Observation

(Snohomish County, WA)  Landau Associates provided geotechnical construction observation services to Snohomish County (County) during the removal of soil and debris that originated from the Oso Landslide from the State Route 530 (SR 530) right-of-way. The overall size of the Oso Landslide was approximately 270 million cubic feet (USGS 2014), placing it among the upper tier of mass movements that have occurred in Washington over recent decades. The mud, soil and rock debris left from the slide covered an area 1,500 ft long, 4,400 ft wide, and deposited debris 30 to 70 ft deep. The soil and debris that was removed to allow the reopening of SR 530 was placed at various locations along and near the County’s right-of-way that was formerly occupied by the Whitehorse Trail.

Landau Associates engineers and geologists worked two 12-hour shifts per day, seven days a week and monitored and documented that the soil and debris was removed and placed in a manner that would likely reduce the potential for impacts (i.e., unstable embankment or cut slopes, large settlement where it cannot be tolerated, erosion and/or drainage issues, etc.) to the County’s right-of-way. Landau Associates engineers and geologists also acted as Snohomish County’s onsite representatives, and coordinated with Washington State Department of Transportation (WSDOT) staff and WSDOT’s design/build contractor. Daily field reports prepared by Landau Associates staff documented highway excavation activities, grading, debris placement, characteristics of excavated materials, and other operation details.