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A/ternative No. 3— Sand/gravel chimney and toe drains. This is the prefened <br />altemative since it has a proven track record on other projects. This alternative was the lowest in <br />cost of the alternatives acceptable to the State Engineers Office. <br />The preferred alternative (Altemative No. 3) will consist of construction of sand/gravel chimney <br />and toe drains on the downsiream side of the dam. The chimney drain will be placed on the <br />downstream face of the embanlnnent approximately 20 feet above the downstream toe to <br />intercept seepage currently exiting the downstream face of the dam. This element will be <br />approximately 18 inches wide and 10 to 15 feet deep. The chimney drain will pazallel the crest <br />of the dam at an elevation that is approximately 20 feet above the toe of the dam in the maacimum <br />dam section and that will extend in both directions longitudinally along the dam at a bottom <br />slope of approximately 2 percent. The chimney drain will be filled with sand/gravel drain <br />material, which will tie into the outfall described below. The outfall for the chimney drain will <br />be located at the masimum section of the dam, which, as cited above, is also the low point of the <br />drain. The outfall will consist of a 6-inch solid PVC outfall pipe. The outfall pipe will be placed <br />in an 18-inch-wide trench and backfilled with native materials. <br />The chimney drain will be installed using an access bench which will cut into the downstream <br />slope of the dam at the appropriate elevation. This bench will be used as a construction platform <br />for excavation of the chimney drain and placement of sand/gravel drain material. Once the <br />chimney drain is installed, the bench will be backfilled with excavated materials and compacted <br />to 95 percent of the maa�imum proctor density as determined by ASTM D-698. <br />A toe drain will be installed along the downstream toe of the dam to intercept seepage flows, <br />which currently exit the toe of the dam, and direct them to an engineered outfall. The toe drain <br />will consist of a perforated PVC drainpipe surrounded by sand/gravel drain material. The drain, <br />which will be 18 inches wide and 3 feeY deep, will be directed to an outfall which will consist of <br />a solid PVC pipe. The outfall trench will be backfilled with excavated materials. It should be <br />noted that the toe drain and chimney drain outfalls will be separate. A map of the project site <br />and the surrounding area is shown on Figure C110. Generic sections of the dam and proposed <br />rehabilitation features are shown on Figure C 120. <br />The estimated cost of the preferred altemative is summarized below. <br />G.lwpwpk11800s178�5A1CetlarMes3FeasiGU�yRepM.00C 3 Ju1y2001 <br />