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USGS1948

Preliminary report on the geology along the route of a proposed tunnel to develop hydroelectric power from Eklutna Lake, Alaska

A preliminary investigation of the geology of the ridge north- west of the lower end of Eklutna Lake was made in the period June 11-18, 1947, by the writer, assisted by L. A. Hale. The purpose of this study was to obtain geological information bearing on the feasibility of constructing a tunnel through this ridge as part of a hydroelectric power development under consideration by the city of Anchorage. Several traverses were made up each side of the ridge in a belt judged to include the most logical routes for the proposed tunnel. For the purpose of localizing the study two tentative routes were selected, of which one would require the shortest possible length of tunnel, and tho other would be somewhat longer but would place the lower portal of the tunnel at a location considered to be the most favorable for the construction of the penstock and power plant. The first route extends northwestward from tho most westerly embayment on the north shore of the lake to the canyon about half a wile east of Pioneer Creek (see map) . The second route extends from tho same point in the lake to the nearest point on the steep slope southwest of the mouth of Goat Creek. The distance along the first route from the lake shore (elevation 862 feet) to the 800-foot contour on the north side of the ridge is 4.3 miles. The distance between corresponding points along the second route is 4.5 miles. The length of the tunnel would exceed these distances by amounts depending on the elevation and grade at which the tunnel is constructed.

Alaska↗
USGS1947

Reconnaissance report on geology of Eklutna Lake dam site and conduit route near Anchorage, Alaska

Summary and Recommendations 1. Eklutna Lake and Eklutna Creek lie in a wide, deep, glaciated, trough-like valley. Downstream from the lake, this valley is partially filled with unconsolidated glacial and alluvial deposits. 2. Eklutna Lake dam site, located about 400 feet below the lake outlet, is suitable for a low dam of flexible, earth-embankment type. Adequate control of the stream can be obtained by raising the lake level about 50 feet to altitude 910 feet, which will provide hold-over storage from wet years to dry years. Such a structure will have a crest length of about 1,950 feet. A. Bedrock is probably 200 to 475 feet below stream bed at the proposed axis. The dam will rest on glacial deposits of till, clay, sand and gravel, and on deposits of lake-shore and alluvial fan gravels. B. Geologic conditions in the area of the right abutment, as yet imperfectly known, may make necessary a long, deep cutoff extending for an unknown distance beyond the north end of the dam. C. A foundation exploration program is recommended that includes deepening test pit No. 1 and drill hole No. 2, and drilling 11 new holes. It is suggested that one drill hold near the center of the valley be taken to bedrock to give a complete picture of the fill materials underlying the foundation. 3. Delivery of water from the forebay of the reservoir to the powerhouse eight miles downvalley by means of a conduit is regarded as infeasible because: difficult terrain of the route will require earthwork more extensive than the volume of the dam; the route is subject to land slides, and will require expensive maintenance; it is more or less completely exposed to adverse winter conditions that may engender icing conditions; and it is easily subject to sabotage. It is recommended that the water be taken to the powerhouse through a rock tunnel.

Alaska↗