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|Title:||Geophysical assessment of foundation conditions : right abutment Mill Creek Dam|
|Authors:||United States. Army. Corps of Engineers. Walla Walla District.|
Butler, Dwain K.
Yule, Donald E.
Llopis, José L.
Sharp, Michael K.
Mill Creek Dam, Washington
|Publisher:||Geotechnical Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical report (U.S. Army Engineer Waterways Experiment Station) ; GL-89-12.|
Abstract: Results of a comprehensive , integrated geophysical investigation of the right abutment area of Mill Creek Dam near Walla Walla, Washington, are presented. The damsite has experienced anomalous seepage since the first reservoir filling in 1941, despite remedial measures, including a concrete cutoff wall completed in 1983. Self potential surveys and other geotechnical investigations conducted in 1984 concluded chat the major anomalous seepage paths were through the right abutment area. Sinkholes and cracks in the reservoir floor and right abutment and the presence of fines in drain discharges indicate that silt, which forms the floor of the reservoir and the abutments , is being piped through the foundation of the dam. A potential is also considered to exist for piping of dam materials into the foundation. The geophysical investigations and a concurrent drilling program were designed to detect anomalous conditions, e.g. cavities and low density zones, in the dam and dam foundation in the right abutment area. The geophysical program included shallow, high resolution seismic reflection, ground penetrating radar, electrical resistivity, electromagnetic conductivity, and microgravity surveys. Interpretations of the results of the geophysical surveys consistently indicate two anomalous trends leading from the reservoir and apparently passing under the dam. The anomalous zones are interpreted to have low density and high water content and are located within a conglomerate formation, which lies below the Palouse Silt (loess) on which the dam is founded. The overall interpretation is only marginally confirmed by the exploratory drilling program -- drilling stopped at the top of the conglomerate. A recommended confirmatory drilling program is presented. Note: This file is large. Allow your browser several minutes to download the file.
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