Overview
The Strontia Springs Dam Reservoir’s Emergency Reservoir Drawdown System (ERDS) rapidly releases water in an emergency scenario through a fifty-foot-long tunnel that runs through the base of the dam. The thrust nut on the ERDS control gate was damaged and required replacement, 250 feet below the reservoir’s surface. Denver Water selected Ballard as its Construction Management at Risk (CMAR) partner to perform the necessary repairs.

Scope of Work
Ballard determined that the implementation of an effective dredging plan and careful execution of a massive bulkhead installation was pivotal to the success of the project. Ballard employed a marine barge spread which housed an on-board crane and supported diver-assisted dredging and mixed gas diving operations. Dredging material from in front of the existing trash rack was necessary to gain access to the upstream face of the ERDS. Ballard engineered and fabricated an in-water cofferdam to reduce the volume of material that required removal. Sediment was removed using a hydraulic pump and disposed of in a topside decanting system.

Innovative Solutions
Ballard removed the trash rack, as well as four seventeen-ton fixed wheel gates and their support chains. Ballard worked with Denver Water to design, fabricate and install temporary covers to minimize the opening left by the removal of the four fixed wheel gates. After removing the trash rack, divers guided the 44,000-pound bulkhead into place. Cold weather and high altitude posed operational challenges when lowering and positioning the bulkhead in front of the ERDS. Working 250 feet beneath the surface placed limitations on working time and visibility. To correct the effects of high altitude, Ballard used mixed gas diving techniques to maximize time in the water. Once the bulkhead was fully installed, Ballard coordinated with Denver Water to open the downstream gates which created sufficient differential pressure to seal the bulkhead.

Results
In collaboration with Denver Water, Ballard removed the damaged control and maintenance gates, and oversaw the in-place machining of the gate sills and frames. Ballard installed new discs, stems, and thrust nuts for the maintenance and control gates, and reconnected both cylinders to the existing hydraulic power unit. Ballard then removed the temporary bulkhead, reinstalled the trash rack, and removed the in-water cofferdam.