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Geologic Features at McNary Dam, Oregon-Washington

Charles J. Monahan
Charles J. Monahan
(U. S. Corps of Engineers, Walla Walla, Washington)
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January 01, 1957


The McNary dam is a combination embankment-concrete spillway dam on the Columbia River in the Umatilla Basin, 300 miles from the ocean. The concrete part of the dam rests upon a nearly flat-lying massive basalt which in turn lies above a sedimentary layer 40-60 feet thick. The embankment rests upon two terrace deposits which overlie the lava. These sediments are protected by a blanket of impervious material. The underlying lava was scoured to a depth of 5 feet by water flowing 25 miles an hour while the water was being diverted in the course of construction and up to 50 feet by water moving with a maximum velocity of 32.4 miles an hour. These velocities are greater than the anticipated maximum velocity after the dam is completed. The underlying lava was broken by two faults. One, a small fault, was cleaned out and backfilled with a few feet of concrete. The other, a large fault with gentle dip and gouge several feet thick, was cleaned out and back filled with cement to a point sufficiently far below the surface of the overlying bedrock to make a firm foundation.

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GSA Engineering Geology Case Histories

Engineering Geology Case Histories Number 1

Parker D. Trask
Parker D. Trask
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Geological Society of America
ISBN electronic:
Publication date:
January 01, 1957




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