CEE researchers visit site of Oso landslide

Timothy Stark collects samples at river level. Photo by Jordan Aaron.

Recently, two members of the University of Illinois research team studying the Oso, Wash., landslide traveled to Washington state to inspect the disaster site, obtain slide geometry and failure mechanism information, and collect samples for testing and analysis. CEE professors Timothy D. Stark and Gholamreza Mesri were joined there by local geologic consultant Daniel J. Miller and two graduate students from the University of British Columbia (UBC), Jordan Aaron and Andrew Mitchell.

The group investigated the slide mass and debris field, took soil samples along the length of the slide mass and at the old river bottom, i.e., potential slide surface, and measured splash height and run-out distance at the slide margins. They also observed some of the man-made changes that resulted from the disaster clean-up, including a new river channel that was excavated to reduce flooding initiated when the original river was dammed by the slide mass and a temporary highway built to skirt the debris field while the original highway is cleared. Piles of totaled vehicles and other personal items pulled from the debris sit slightly off to the sides of the slide mass.

One notable aspect of the powerful slide, which crossed a river and traveled almost a mile before coming to a stop, is visible evidence of a high splash zone which is evident from mud marks at various heights on trees along the sides, middle, and toe of the slide. Stark said that the extent and flowing pattern of the run-up indicates that the slide behaved like a fluid rather than as intact overconsolidated soil.

“That’s what I think makes this slide pretty unique,” he said. “You can see how high run-up or splash is. That gives you an idea of fluid nature of the slide mass and volume of the slide.”

Muddy tree trunks show splash height. Photo by Timothy StarkMuddy tree trunks show splash height. Photo by Timothy Stark.

Back in the lab, the collected soil samples are being tested to determine the geologic stress history of the lacustrine clay deposit and the shear strengths of the various layers in the stratigraphic section, including the lacustrine clay, for use in slope stability analyses. Precipitation data, seismic records and local land-use policies will further assist the team in understanding the cause, behavior and impact of the slide.  

The complete international and multi-disciplinary team includes five representatives from the University of Illinois at Urbana-Champaign (CEE Professors Timothy D. Stark and Gholamreza Mesri, geotechnical engineering; James M. LaFave, structural engineering; and Marcelo H. Garcia, hydrology; as well as Department of Urban and Regional Planning Professor and Head Robert B. Olshansky) and UBC Earth, Ocean & Atmospheric Sciences Professor Oldrich Hungr, who was represented at the site by Aaron and Mitchell. The Oso visit was facilitated by Dale Topham, Geotechnical Supervisor, and Jeffrey Jones, Geolologist, both of Snohomish County. The research is funded by a CEE Rapid Response Grant.