Coffer Dam Removal – Hebgen Dam
Located near West Yellowstone, MT, and at over 6,500ft (1982m) in elevation and tucked into mountains, conditions at the dam are harsh. Weather is among the coldest and snowiest of any location in the lower 48 states; winter temperatures have recently been below -30F (-34.4C).
As part of a comprehensive seismic rehabilitation of the Hebgen Dam intake, the Ballard team was involved in the removal of an emergency closed cell cofferdam that was used to isolate the deep intake structure. It had been designed as a free standing sheet pile structures with three cells containing fill material.
The task involved removal of the three coffer cells. Initial fine clam dredging 13,000 square feet of fill material was strategically removed and placed on dump trucks. Underwater concrete demolition using a variety of tooling including the use of a 185 ton diver held hydraulic rock splitter. The team extracted over 400 sheet pile with an average length of 75ft (23m). The project utilized a 220 ton crane barge that along with support vessels was the work platform. Commercial Diving at altitude was performed with Nitrox to allow for longer working time in the high altitude.
Full environmental containment was in place during all phases of the project. Over 6000 man-hours of work were completed with no work injuries.