Initiation of 2014 Oso Landslide Using 3D Slope Stability Analyses: Effect of Infiltration

March 2014 Oso landslide, which demolished Steelhead Haven Community near Oso, Washington, is the deadliest in US history. Several factors have been suggested for the initiation of this landslide by various researchers including, the heavy rainfall before the landslide, timber harvesting on the upper plateau, river erosion at the slope toe, groundwater rise due to rainfall infiltration, and irregular 3D topography of the slope. This study utilizes 3D limit equilibrium and 3D finite element seepage analyses to investigate the effect of rainfall infiltration in the 7.5 acres timber harvest area at the top of the slope as well as potential changes of groundwater elevation on the initiation of 2014 Oso landslide. To achieve these objectives, the infiltration rate was varied in the harvested area to investigate its effect on the first phase initiation. Also, two potential groundwater levels were examined to assess the sensitivity of the first phase slide mass to changes in groundwater conditions. The 3D factor of safety for the critical slip surfaces was obtained for the two groundwater levels using two conventional limit equilibrium methods. The safety factor for the critical slip surface is slightly different for the first and second groundwater levels. The difference is such small that it can be concluded that a reasonable rise in groundwater level would not have been the triggering factor for the 2014 Oso landslide. The impacts of other potential factors such as shear strength reductions of upper layers due to the rainfall infiltration, timber harvesting, and/or 3D slope geometry effects appear to have been more influential in the initiation of the landslide.