Potential for Seepage Erosion of Landslide Dam

The failure potential of the debris-avalanche dam at Castle Lake near Mount St. Helens, Washington, by three processes of seepage erosion (1) Heave; (2) piping; and (3) internal erosion, is examined. A three-dimensional ground-water model of the dam was constructed and used in conjunction with properties of the blockage material to examine the dam’s stability against failure by heave and piping. Results indicated that the dam is stable against piping but potentially locally unstable against heave. Comparison of physical properties of the blockage materials with critical physical properties for embankment dams that have experienced distress because of internal erosion shows the blockage to be marginally stable against internal erosion. A ground-water mound as much as 10 m (35 ft) above lake level formed in the dam and is responsible for creating vertical hydraulic gradients capable of inducing heave. Similar mounds may exist in embankment dams of comparable geometry and hydraulic conductivity.