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Written Paper

Construction Foundation Report. Mud Mountain Dam Seepage Control Cutoff Wall, White River, Washington  [1991]


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Mud Mountain Dam is located on the White River, near Seattle, Washington. The dam is used solely as a flood control facility for Puyallup, Summer, Tacoma and the lower White and Puyallup river basin. The construction of the dam was authorized by the Flood Control Act of 1936. The flood Control Act of 1938 provided for the operation and maintenance (O and M) of the project. This foundation report documents construction procedures and foundation conditions. In 1980 a single open piezometer tube (P-40) was installed in the core of the dam at its deepest point. The piezometric surface was monitored through 1984 with unsettling results. Through the 4 year period, the piezometer responded progressively faster to fluctuations in seasonal pools. This was disturbing in light of the fact that design core permeability was on the order of 0.000001 cm/sec. Seventeen additional piezometers were installed in 1985-86 to verify and characterize the problem. These borings encountered loose zones, heaving, clean sands and gravels and suspected voids. Gradation tests on samples taken from these borings suggested they were core materials from which the fine sand sizes had been removed. Construction of cutoff walls by the slurry trench method has been proven reliable and effective in dam remediation projects.