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Modification to TR-21 Mayflower Flood Bypass Tunnel, Stage 2 Design Modification <br /> Submitted with TR-29 <br /> Background <br /> In 2012, Climax received approval for TR-21 - Mayflower Flood Bypass Tunnel and Mayflower TSF <br /> Reclaim Water System Modification. Construction of Stage 1 of the Mayflower Flood Bypass Tunnel was <br /> completed in 2014. Construction of Stage 2 of the tunnel will start in the summer 2019. During final <br /> engineering of Stage 2, the conceptual design that was originally approved in TR-21 was modified and <br /> those modifications are described below. <br /> Description <br /> Stage 2 of the Mayflower Flood Bypass Tunnel was redesigned ahead of planned construction in 2019. <br /> During the engineering process, the decision was made to not construct the Conceptual Stage 2 Tunnel <br /> and instead extend the Conceptual Stage 3 Tunnel to include Inlet Portal No. 2. The original arrangement <br /> can be seen in drawing C-02 and the modified arrangement can be seen in drawings MFT-C-03 and <br /> MFT-C-07. In drawing C-02, what is designated as Conceptual Tunnel Stage 3 is now referenced as <br /> Stage 2 Tunnel in drawings MFT-C-03 and MFT-C-07. The change was made to reduce the linear <br /> footage of tunnel to be constructed, therefore reducing costs. Under this new arrangement the Mayflower <br /> Flood Bypass Tunnel will still perform as it was originally intended. This next stage of tunnel will be <br /> constructed in the same manner as the previous stage and will be adequate to pass a PMP event. The <br /> muck generated from the Stage 2 construction will be placed in the existing spoil/muck disposal areas <br /> used for Stage 1. <br /> Additional changes to the design are that Inlet Portals No.2 and 3 will no longer have portals, but instead <br /> will be raised bores. During the geotechnical investigation of the construction site for these inlets it was <br /> discovered that the ground and rock condition was of poor quality and raised bores would be safer. In <br /> addition, a side-hill riser design will no longer be utilized for Inlet No.2 and 3 and a tower riser design will <br /> be utilized instead (see drawings: Tower Intake Plan Option 1 and Tower Intake Profile Option 1). A risk <br /> assessment of the tower riser versus the side hill riser was conducted. From this assessment, there were <br /> no critical risks that made one option better than the other, so the decision was made to construct the <br /> tower riser after evaluating other criteria like cost and ease of maintenance. <br /> As indicated in the adequacy review for TR-21, Climax will maintain PE certified design drawings for the <br /> new Stage 2 tunnel and will provide DRMS with certification once construction is complete. Lastly, this <br /> design change will not affect the final closure design for this facility and the closure cost will remain the <br /> same. <br />