Canadian Consulting Engineer. January/February 2010
Burloak Raw Water Intake
By combining marine pipeline with a tunnel, the design-build team for an intake from Lake Ontario to Oakville made construction easier and faster.
By: R.V. Anderson Associates
Originally, plans for a new water intake to feed the Burloak Water Purification Plant in Oakville, west of Toronto, called for a tunnel to be excavated for the entire distance between the intake site and the plant.
The intake is 1.4 kilometres out in Lake Ontario and the plant is 1.3 kilometres north of the lakeshore. However, the design-build team of C&M McNally (general contractor), R.V. Anderson Associates (design engineer) and Dean Construction (marine sub-contractor) proposed an alternative concept that saved $6.4 million over the base bid and enabled the work to be completed in the tight schedule of 30 months.
The design-build team's alternative solution combines a tunnel with a marine pipeline in the lake. The combination of tunnel and pipeline allowed for a shallower tunnel and reduced the depth of the access shafts. Instead of having to construct a riser shaft in 17-metre deep water, for example, the shaft connecting the tunnel to the marine pipeline was constructed in only 5 metres of water.
Tunnelling and riser shaft
A tunnel boring machine (TBM) was first launched from an entry shaft in Oakville's South Shell Park, near the shore of Lake Ontario. The Robbins TBM bored a 3-metre diameter tunnel north at a depth of up to 23 metres, reaching the site of a new water treatment plant in nine months. McNally used its own patented roof support system to stabilize the tunnel crown until the concrete liner was installed.
The TBM then tunnelled 320 metres south under the lakebed until it reached a riser shaft that connects the tunnel and the marine pipeline.
The riser shaft is 3.5 metres in diameter and was drilled 10 metres into the lake bed from the deck of a jack-up barge. The shaft consists of a custom-built precast concrete chamber section and a section of 1800-mm diameter concrete pressure pipe installed vertically into the bored shaft. The chamber uses standard concrete pressure pipe joints for sealing its plugs and cap and for making the connection between the vertical shaft and the pipeline. Duckbill valves (one-way rubber check valves) allow air bubbles released in the pipeline to escape into the lake at the riser shaft location.
When the TBM was within two metres of the riser shaft, the machine was halted and two small holes were drilled from the tunnel into the shaft to allow trapped water to drain from the shaft in a controlled manner and to check that the seals at the top of the shaft were holding. Once the water had drained from the shaft, the TBM advanced through the bottom of the shaft to complete the tunnel.
To accelerate the process of installing the marine pipeline under water, the team used a method that had not been used in Canada previously. The patented H&R Hydro-Pull system uses a pressure differential between the interior of the pipe and the surrounding water to connect the concrete units. The two units are manoeuvred into place, a pump is turned on to create a pressure differential that pulls the two sections together, and the jointing is completed in just 4 to 6 seconds. The diver needs only to direct the placement of the joint with the equipment supporting the pipe.
At the end of the pipeline is a 6.8- metre diameter precast concrete intake structure that looks like a mushroom cap, with stainless steel trash screens around the perimeter. Four 75-mm diameter PVC pipes along the pipeline are used to chlorinate the intake to discourage zebra mussels and to enable sampling of the quality of the raw water. Provision was provided along the pipeline for a second intake structure.
Looking after fish
Approvals from three different government agencies had to be met, and one of their environmental concerns was the 1,060-metre long trench that needed to be blasted in the lake bottom to hold the pipeline. To minimize the impacts, the team used precise surveying and drilling techniques, using a GPS receiver that could locate the blast holes with a 20-cm accuracy.
To frighten fish away before the blasting, a simple solution was used: a concrete vibrator inside a steel pipe. Also, an air bubble curtain in the lake reduced the shockwaves and the spread of turbidity during blasting.
Stone added over the pipeline trench created new habitat for the fish, while over 90% of the material excavated from the tunnel went to a nearby brick plant.
Clean water for the community
The 2.4-m diameter finished tunnel enabled the capacity of the intake to be double the required 220 megalitres a day, providing extra capacity for the region in future.
The project was completed on schedule and $850,000 under the final negotiated contract price. It recently won the 2009 Ontario Public Works Association Project of the Year Award in the structural category for projects worth more than $10 million.
Owner: Regional Municipality of Halton
Design engineer: R.V. Anderson Associates (Tom Richardson, P. Eng)
General contractor: C&M McNally Corp.
Other key players: Dean Construction (marine subcontractor), Associated Engineering (contract administration), Geo-Canada (geotechnical), Bowfin (environmental), Munro Concrete Products (concrete pressure pipe and Hydro-Pull)