The Phoenix Mine site supported a significant population and utilized Marshall Lake as its water supply. When more water was needed, a dam was constructed in 1956, and later rebuilt in 1961 and again in 1966. The volume of water stored was 327,180 m3 (266 acre feet). Since its original construction, the dam has experienced significant seepage. On file were conflicting design documents surrounding the inclusion of a cut-off wall and outlet construction.
The licensee and owner, the Ministry of Forests, Lands and Natural Resource Operations, decided to remove the dam because the benefits of upgrading and maintaining the dam were no longer viable. When the conditions at the dam worsened, the Ministry drew down the reservoir and lowered the static forces on the dam.
Kerr Wood Leidal was retained to provide detailed design, support public and regulatory processes, and oversee construction of the dam’s decommissioning. The project entailed a pump drawdown of approximately 3.3 m to return the lake to the original Marshall Lake level, removal of approximately 8,500 m3 of dam fill, disposal of the dam fill on the immediate downstream area, re-establishment of the lake outlet, and rehabilitation of the disturbed areas.
The challenges to this project included:
- A well used Forest Recreation Site and local cross country ski trails are located adjacent to Marshall Lake. The remainder of the lake was situated on Crown and private land.
- The dam was situated above the community of Greenwood, Highway 3, and private woodlots; therefore, large releases of water from the reservoir would not be acceptable.
- The dam was in a remote location, communication was challenging, cell serivce was unavailable and two-way radio’s had limited distance. Advnced planning included the development of enhanced communicaion protcols should any issues occur at the dam and notificaton to downstream areas be requied.
- The reservoir volume had rebounded and the lake was higher than anticipated for the start of construction.
- The volume of dam fill that required disposal was significant, and determining a suitable location was essential.
The pump drawdown of the lake began on August 20, 2013, and the construction was completed on September 3, 2013, on budget and ahead of schedule.