The purpose of the project was to construct a flood bypass and tailing pond diversion structure to convey water around a tailing dam during a flood event.
Tailing deposition at Mayflower will raise the dam to a current life-of-mine crest elevation of approximately 12,820 feet, a 20-year increase of 210 feet. The flood bypass tunnel system will have inlets at approximately 70 feet intervals which will function in succession. As the dam crest is raised, the intakes for past phases will be abandoned and sealed.
The project is comprised of several components including the outlet portal (above), side-hill intake and drop structures, a seepage collection system, storm water conveyance structures from the outlet portal to the creek, and the tunnel itself.
The tunnel system is a 12 feet x 14 feet drill-and-blast excavation with a total length of approximately 5,760 feet. Various levels of ground support (rock bolts, welded wire mesh, fiber-reinforced shotcrete) were selected according to the RMR classification system.
Phase 2 construction included 1,030 feet of tunnel advance and an additional 101 feet between two muck bays.
From 2012 to 2013, AJAX engineers and geologists provided construction management support, quality assurance inspection and coordination of activities for all surface and underground construction activities during phase 1 (5,763 feet of tunnel). In 2018, AJAX engineers designed the final tunnel extension for phase 2. From 2019-2020, AJAX provided the same construction management and quality assurance oversight for phase 2 construction as we had during phase 1. Additionally, we provided construction coordination and bid support for the excavation and lining of a 60-feet raise bore from the tunnel to the ground surface, and as-built documents for compliance with the mine's permit with the state regulatory agency.