Clay mineralogy and construction problems in volcanic soils near Mount St. Helens: Sediment Retention Structure, southwestern Washington, USA

摘要

During the late 1980's, a 60 m earth-cored dam was constructed across the North Fork of the Toutle River in southwestern Washington to impound sediment released during the 1980 eruption of Mount St. Helens volcano. Unanticipated soil conditions during excavation, handling, stockpiling, and compaction were encountered in saprolite developed from flow-top breccia and volcanic debris flows and in less intensely weathered bedded sediment. Saprolite developed from glassy flow-top breccia and 35 m below the surface contains 100 percent low-charge smectite. In saprolite developed from dacite and andesite clasts and matrix in volcanic debris flow, the percentage of halloysite increases upward while the percentage of smectite and expandable mixed layer clays decreases. Increase in the percentage of fines, decrease in bearing strength and stability, and decrease in optimum moisture content from in place to remolded soils characterize saprolite used in the impervious core of the structure. The bedded sediment was disposed as waste due to high natural water content relative to optimum moisture content and loss of stability and bearing capacity upon handling.