Case study: Oso, Washington, landslide of march 22, 2014-material properties and failure mechanism

This paper describes investigation, testing, analysis, and slope history used to determine the two-phase failure mechanism involved in the 2014 landslide near Oso, Washington. The first phase involves a slide mass located above the frequent landslides in the lower portion of the slope and extends to near the slope crest. This slide mass had a large potential energy, which moved downslope, and pushed the water-filled colluvium that had accumulated along the slope toe across the valley, resulting in it flowing almost 1.5 km. Evacuation of the Phase I slide mass left the upper portion of the slope unbuttressed and oversteepened, causing a second landslide (Phase II) but it primarily remained on the source slope because the back edge of the Phase I slide mass prevented further movement and the dense and unsaturated upper soils did not undergo a significant strength loss like the water-filled colluvium.