September 9, 2010

FOCUS | Sewer and Watermain

Second watermain across Fraser River will cost Metro Vancouver $230 million

PETER KENTER

correspondent

All systems are go on a second watermain designed to cross the Fraser River in conjunction with the Port Mann Bridge construction.

The second main will increase water supply to Metro Vancouver at an estimated capital cost of $230 million.

“In May, four firms were advanced to the RFP stage for submissions which will be required by Oct. 1,” says Bill Morell, media relations and issues management division manager for Metro Vancouver.

The existing Port Mann Main Fraser River Crossing was built in 1974. It consists of a 1,200 mm (48-inch) diameter welded steel pipe about one kilometre long that was installed in a trench in the riverbed, downstream of the Port Mann Bridge.

The main supplies water to communities south of the Fraser River, but was damaged in 1997 by a riverbed scouring operation. The damage temporarily disrupted the water supply to several communities.

“The repair, completed in 1998, consisted of replacing only the damaged section of the watermain along the original alignment and providing a protective apron to protect against future scour and undermining,” notes Frank Huber, division manager of engineering support and technical services for Metro Vancouver, in an update on the project.

But the repair was a temporary fix, with the full knowledge that construction of a second crossing would soon prove prudent.

A seismic vulnerability assessment study conducted in 2001 concluded that the main would fail at several locations during a moderate or a major earthquake.

“The subsoil would liquefy, causing the ground, particularly along the shorelines, to move towards the river,” says Huber.

The solution was to install a steel pipe in a tunnel constructed using a tunnel boring machine (TBM) under the Fraser River.

The tunnel crossing will replace the existing watermain and will be located immediately downstream of the new Port Mann Bridge. The design phase for the new, larger main project began in 2006. Design consultants Sandwell Engineering, Jacobs Associates and Golder Associates drilled 18 boreholes for precise geotechnical data to design the tunnel project.

The tunnel will be excavated between two vertical shafts, one in Maquabeak Park in Coquitlam and another near the CN Rail yard in Surrey. Each shaft will be about 60 metres deep. The new water main will be constructed inside the tunnel and shafts, and connect to the existing Port Mann Main on both the south and north banks of the river.

The tunnel will be about one kilometre long, and will be located about 30 metres below the river bottom. The tunnel will accommodate a 2,100-mm (84-inch) diameter welded steel watermain.

“The sequence of construction will generally involve excavating a 15-metre diameter vertical shaft on the south side of the river to allow launching of the TBM, tunnelling in soil below the river using a pressurized-face TBM, and excavating a 10-metre diameter vertical shaft on the north side of the river to allow retrieval of the TBM,” says Huber.

A technical review board will continue to review all aspects of the construction phase. Tunnelling specialist consultant Hatch Mott MacDonald will provide construction management services during construction.

Once the new main is in place, the Fraser River Estuary Management Program (FREMP) will require the removal of the old main, a project that could cost several million dollars.

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