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~•~ <br />Mr. Ronald J. Valentine, PE <br />Climax Molybdenum Company <br />Februazy 17, 2004 <br />Page 2 <br />A new decant structure (Decant Structure 2) was constructed near Decant Structure 1 to provide <br />additional system flow capacity. The new decant structure was a new design and also constructed of <br />acid resistant materials including stainless steel, corrosion resistant fiberglass reinforcement, and <br />high-strength sulfate resistant concrete. <br />DECANT STRUCTURE 2 DESCRIPTION <br />Decant Structure 2 consists of a lazger diameter-lined vertical drilled shaft, with a vented stainless <br />steel intake, and surrounded by a cover (building) consttucted of reinforced concrete. A vertical <br />shaft was drilled about 165 feet intersecting the Tenmile Tunnel and lined with a welded stainless <br />steel casing grouted in place. The intake was encased in concrete and a reinforced concrete circulaz <br />intake weir was constructed. A reinforced concrete building covers the intake and flow to the inlet is <br />controlled by a slide gate located in a building wall. <br />The decant structure is sized to pass a flow of 39,350 gallons per minute (gpm) at about three feet of <br />head and has an operating elevation range of about 11,076 feet to about 11,080 feet above mean sea <br />level. A copy of the design drawings are provided in Attachment A. <br />SUMMARY OF CONSTRUCTION <br />A dewatering berm (coffer dam) was constructed around the proposed structure location to facilitate <br />construction in the dry. The structure construction required overexcavating subgrade into competent <br />bedrock. The bedrock surface was located within a few feet of the existing ground surface, and the <br />majority of excavation was made by blasting. The excavated material was replaced with fill to allow <br />drilling of the borehole. <br />A level drilling platform was placed at the vertical borehole in the general location shown on the <br />Drawings. A vertical borehole was advanced through the platform and foundation bedrock <br />approximately 165 feet to the existing Tenmile Tunnel. The borehole was advanced by drilling a <br />pilot hole using air rotary drilling technique. The pilot hole was then enlarged to 38 inches using a <br />raised bore drilling technique. <br />A 30-inch diameter welded 316 stainless steel casing was installed and centered in the completed <br />borehole. The annular space between the casing and the borehole was grouted. A temporary <br />bulkhead was installed at the casing outlet to support the initial grout lift. Subsequent grout lifts <br />were placed in stages. <br />The fill around the casing was removed, the stainless steel casing cut and a flange welded to the <br />exposed casing at the excavation base. A prefabricated stainless steel intake structure, consisting of <br />a conical inlet and vent pipes, was attached to the steel casing with a flanged connection. The entire <br />~ AEVi.ooc ovnroory:io vrar <br />