Aggressive dewatering aids concrete rehab at Abitibi
An enormous volume of water passes through the Abitibi Canyon Generating Station every hour of every day. In order to complete critical concrete repairs on the hydro power station, general contractor The Barclay Construction Group needed to find a way to pump and discharge almost 30 million gallons of water in a 500-metre section of the river upstream from the station in under 30 hours.
Aquatech Dewatering Co. of Concord, Ontario was sub-contracted to provide aggressive dewatering services for the contract. Adding to the challenges were a remote location, and significant environmental sensitivity of the marine habitat. The facility, located about a 45-minute drive from the small town of Smooth Rock Falls, Ontario was built by the Dominion Construction Company in 1933. Recently, the concrete tailrace pier began to show its age, prompting owner Ontario Power Generation to tender a remediation contract.
“The Barclay Construction Group already had a considerable fleet of equipment, including cranes, on site,” says Russell Thomas, senior project manager with Aquatech. “The site had a very tight staging area, but Barclay had already constructed an access road from grade at the top of the dam to the Abitibi River — it was a thing of beauty.” As with all such efforts, project design is paramount. Dewatering a section of river and discharging it at another part of the stream requires water taking and discharge permit approval from the Ontario Ministry of the Environment. Protective measures for river species must be approved by Fisheries and Oceans Canada (DFO).
Barclay Construction began the dewatering process by constructing a cofferdam 500 metres downstream and a second smaller dam at the worksite. “It’s very difficult to install these cofferdams with a high degree of success,” says Thomas. “This project demonstrates the competence of Barclay’s cofferdam design.” The scope of the Aquatech contract involved draining the half-kilometre tailrace section of the dam, in addition to pumping 2,000 gallons per minute in recharge leakage from the primary cofferdam. Total estimated pumping volume: 28 million gallons over 30 hours. “The water was about five metres deep, and we were required to drain it pretty well to the riverbed,” notes Thomas.
Initial mobilization for the contract required about a week, as Aquatech moved its pumps into place. The company selected three 12-inch diesel pumping units each with 18-inch suction lines and 300 feet of 12-inch discharge lines. To account for recharge leakage through the primary cofferdam, Aquatech employed two eight-inch diesel pumping units — one primary, one back-up — throughout the duration of the primary drainage and the concrete restoration operation. The pumps were fuelled by five 500-gallon doublewalled diesel tanks, set back 30 metres from the water behind an engineered berm.
Protection of marine wildlife was critical. “The river contains quite a few fish including pickerel and smaller fish,” says Thomas. “When you’re pumping water at a rate of 4,000 gallons per minute per pump, it’s essential to protect aquatic life. “To comply with DFO requirements and given the environmentally sensitive setting of the work we installed a floating fish protection system on the suction side, about 100 feet away from the pumps.” Pumping operations could not begin until fish were removed from the Abitibi River. The fish were collected and re-introduced into the river downstream. Any fish that evaded the operation and might be trapped against the screens were released using a fourinch-high head pump that kicked in at intervals to backwash the screens at 50 to 60 psi.
Discharge water was directed to the downstream side of the primary cofferdam towards the rock wall face of the dam to reduce the velocity of the discharge. The contractor used additional methods to further reduce the velocity and energy of the water, which re-entered the river at a deep pool, preventing scouring or erosion of the river bottom. During the dewatering, Aquatech crews collected water samples for independent analysis of discharge water quality in accordance with the governing permits. The crews also aided Barclay in monitoring the secondary pumping system for the length of the concrete remediation operation.