2020-02-20_REVISION - M1977493 - Laserfiche WebLink

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Modification to TR-21 Mayflower Flood Bypass Tunnel, Stage 2 Design Modification Submitted with TR-31 Background In 2012, Climax received approval for TR-21 - Mayflower Flood Bypass and Mayflower Tailings Storage Facility(TSF) Reclaim Water System Modification. Construction of Stage 1 of the Mayflower Flood Bypass Tunnel and Side Hill Riser was completed in 2014. Construction of Stage 2 of the tunnel began in the summer 2019 and was completed in November 2019. The proposed Stage 2 Tower Riser will connect to the Stage 2 Tunnel via a raise bore to bypass flood waters around the Mayflower TSF for the next 70 foot raise of the TSF. Description The plan developed in 2010 for the Mayflower tailing dam raise was to bypass flood flows around the dam by constructing a tunnel in the left abutment of the dam with a side hill riser structure upstream of the dam. The riser structure was designed to be raised as the crest of the tailing dam is raised. The structure would be capable of discharging the PMF (Probable Maximum Flood)which is 500 cfs with 5 feet of head and 1 foot of freeboard. The plan was to create 3 intake structures in phases to only store a maximum of 70 vertical feet of tailing on a structure before plugging and abandoning the riser structure. This will eliminate the need to build side hill risers for Stages 3 and 4 of the flood bypass tunnel as stated in TR- 21. The riser structures would be founded on competent rock to minimize seepage into the structure and to provide a solid foundation for the structure. During the winter of 2018/2019 conceptual level design began for the soil stabilization at the location of the Stage 2 side hill riser. Due to the large amount of soil over the bedrock, a side hill intake and a vertical tower were evaluated. On March 6, 2019, a Semi-Quantitative Risk Assessment(SQRA)was conducted to evaluate the risks associated with health and safety, dam safety, environment and legal and regulatory compliance for both options. A vertical tower was selected as the preferred alternative. A third party peer review was conducted with outside parties and their recommendations have been incorporated into the design as well. The proposed Stage 2 tower riser will be similar to the Stage 1 riser structure in that the tower will include 2, 8-foot wide bays to pass the PMF which is 500 cfs with 5 feet of head. The crest of the spillway will be maintained 6 vertical feet below the crest of the TSF to maintain 1 foot of freeboard for the PMF. During times when the TSF crest is being raised, Climax will install and weld I-beams (stop logs) in each bay to raise the crest of the spillway to maintain the 6 vertical feet of elevation difference. Flood water will pass over the crest of the tower spillway and will drop to the bottom of the tower. The flood water will flow under a concrete wall into the morning glory spillway chamber. The tower is connected to the Stage 2 tunnel via a morning glory spillway. The morning glory spillway will be constructed by constructing a 10-foot diameter raise bore and grouting into place a 7.5 foot diameter steel liner.A bell mouth steel liner will be welded onto the 7.5 foot diameter steel pipe for improved hydraulics. The flood water will flow over the crest of the morning glory spillway and drop approximately 40 feet into the Stage 2 tunnel. Three air vents will provide air flow into the morning glory spillway chamber. The air vents are connected to the top of the tower via a steel pipe. The Stage 2 tower riser will be constructed of reinforced concrete designed for the 10,000 year site specific seismic load as described in the Updated Site-Specific Seismic Hazard Analyses of the Climax Tailing Dams, Colorado dated June 23, 2017 by Lettis Consultants International, Inc. The concrete design is in accordance with ACI 350-06 and EM 1110-2-2104.