Oso Landslide: Failure Mechanism and Runout Analyses

This paper describes and explains the large mobility of the 2014 Oso Landslide, which caused significant fatal consequences. This landslide occurred in two phases, characterized by significantly different material behaviour, strengths, and runout. A portion of the first phase underwent significant undrained strength loss (liquefaction) and travelled over 1.7 km over a nearly horizontal valley surface and devasted a residential community. The second phase underwent brittle failure with much less strength loss and runout than the first phase. The first phase slide mass is composed of insensitive, overconsolidated glaciolacustrine silt and clay, material not traditionally recognized as susceptible to a large undrained strength loss or liquefaction. A new rheology, appropriate for liquefied overconsolidated glaciolacustrine silt and clay, is presented and used in the runout analyses. The large runout occurred in two directions, which allows various runout models to be compared. Three numerical runout models were used to investigate their applicability to similar landslide configurations and future hazard and risk assessments. These runout analyses show the importance of: (1) using a digital terrain model in the runout analysis, (2) modeling field representative shear strength properties and failure mechanisms, and (3) predicting runout distance, splash height, and duration for risk assessments and to improve public safety for this and other slopes.