SLOPE AND SEISMIC STABILITY OF THE CASTLE LAKE DEBRIS DAM, MT. ST. HELENS NATIONAL MONUMENT, WASHINGTON

The Castle Lake Debris Dam was created by a debris avalanche that flowed down the Toutle River Valley during the eruption of Mt. St. Helens on May 18, 1980. The dam is approximately 610m long by 425m wide and holds back a volume of 23,400,000 m3 of water. Sources for potential failure are seepage, erosion, slope failures and liquefaction. Using XSTABL, the dam is found to be stable from slope failure under climatic events. Earthquakes are the greatest threat to the long-term stability since they increase driving forces and decrease resisting forces that are not normally present, though ground accelerations and pore pressures increases. To determine the dam's earthquake susceptibility, the effects of amplification, liquefaction and earthquake-induced slope failures were analyzed for Mt. St. Helens Seismic Zone, Cascadia Subduction Zone, and intraslab earthquakes using SPT's, shear wave velocities, SHAKE91, and XSTABL. Results show that peak horizontal accelerations can be as high as 0.7g for crustal quakes and 0.5g for subduction and intraslab quakes. Based on shear wave velcocities and SPT's in the debris dam, a minimum M6.3 earthquake with a peak horizontal acceleration of 0.5g is needed to cause liquefaction on the dam, though the groundwater would have to be 2m higher than its present level. Using SHAKE91, liquefaction is susceptible for all the modeled earthquakes. Earthquake-induced slope failures start occurring with horizontal accelerations of 0.2g. Liquefaction and slope failures will likely occur during an earthquake but will not be large enough to initially release the lake. However, if a large enough portion of the dam is removed, it could change the groundwater system in such a way that piping, heave and internal erosion could retrogressively beach Castle Lake.