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III. DAM EMBANKMENT ENLARGEMENT <br />We have drilled four deep test holes through the existing embankment <br />into the foundation, to determine the present condition of the dam. Measure- <br />ments of the embankment and foundation properties were made in the test holes <br />and samples were taken. In addition, five holes were drilled just downstream <br />of both the dam and dike to determine foundation requirements for enlarge- <br />ments on the downstream side. Shallow depths of peat were found. <br />Technical results of these investigations are summarized on Drawing No. 3 <br />attached. They show the existing embankment to be constructed of excellent <br />material, clayey glacial till, moderately compacted, resting on medium dense <br />sandy soils. Water levels in the holes show that water is passing through <br />localized areas of the embankment, at a high elevation. <br />Samples were taken of proposed borrow soils for the enlarged embankment, <br />from exposures in the old borrow areas. Laboratory tests show the materials <br />to be ideal for the purpose. <br />The dam height can most efficiently be increased by addition of new em- <br />bankment on the downstream slope, as shown on the drawings. Excellent <br />materials are available at short haul from portions of the glacial moraine <br />humps between the left abutment of the main embankment and the Forest Service <br />road. The materials are basically granular, with high shearing strength, and <br />contain sufficient clay to be quite impervious after proper compaction. <br />There has been ravelling of the downstream slope of the main embankment, <br />and ''seep'' areas have developed which would be dangerous if the reservoir <br />were continuously full, or under higher heads. Blanketing with pervious sand <br />and gravel with the blanket being carried to the downstream toe, will provide <br />adequate drainage to this area as well as to the foundation area for the new <br />embankment. A source of suitable, natural sand - gravel has been located about <br />two miles upstream along the new road. Water from the underdrain blanket will <br />be collected in a small rock zone at the downstream toe, constructed primarily <br />of rock oversize raked from the embankment layers during construction. <br />An 18 -inch layer of dumped riprap, on a 6 -inch thickness of unprocessed <br />sand - gravel from the upstream source mentioned will adequately protect the <br />upstream slope of the new embankment. Ample quantities of good rock are <br />available nearby along the road north of the dam. <br />It will be necessary to ex-cavate the peat and other soft or organic soils <br />from the foundation areas of the new embankments. The depth will range from <br />about 4 or 5 feet below the main dam to 6 inches higher on the abutments. <br />Borrow areas for embankment soils, sand and gravel, and rock will re- <br />quire Forest Service approval. <br />Design Flood <br />A preliminary design inflow flood was determined for feasibility study <br />purposes on the basis of a maximum probable occurrence. While such an <br />3 <br />