August 24, 2007

DAYTON & KNIGHT ENGINEERING

The Fishtrap Island Collector Well and ancillary facilities supports positive change to the community and environment by providing a reliable source of water supply.

Environment

British Columbia's largest radial well earns project leader engineer award of merit

River-crossing added challenge to environment

JEAN SORENSEN

correspondent

PRINCE GEORGE, B.C.

Addressing stringent environmental constraints was the most challenging aspect of placing British Columbia’s largest radial collector well in place.

In addition, there was a pump station and a transmission pipe through the Nechako River to content with in order to improve the City of Prince George’s water supply, says John Boyle, P.Eng, project leader for Dayton & Knight Ltd.

Design included a year-long federal and provincial environmental assessment process following the development of a groundwater model, capture zone analysis and an aquifer protection plan. Approximately 100 commitments and assurances were required and agreed to during the environmental assessment process.

"With well defined limitations and allowances for cost, it made it easier for the contractors..."

John Boyle

Dayton & Knight Ltd.

The new well was under-taken to supply the Hart/Nechako area and to replace an existing well, which was located downstream from a landfill area. As well, with the ability to collect 93ML (20,500 million gallons) per day, it provided a means of augmenting service in the event that another well had a malfunction. The full capacity of the collector well is not presently being used; the unused portion also provides for increased population growth.

The well and pump station site had to be at least 50 metres from the river’s edge to meet fisheries requirements. Proactive measures were put in place to guarantee no site fuels or lubricants leached into the river.

“We had to also ensure that no more trees were removed and a survey of the plants that were in the area was carried out,” says Boyle. “Similar plants were installed afterwards for landscaping.”

In addition, the project team had to identify which trees surrounding the site were being used for nesting and the construction crews were not permitted to work during nesting season.

“This influenced the project schedule,” he says, adding design work carried on during the nesting season.

To ensure no materials fell into the river during construction of the river-crossing section of the transmission pipe, the contractor built a temporary floor between two of the bridge beams. The floor was further protected with polyethylene sheeting. It served as a working platform for crews and also captured debris or materials.

The well itself was a five metre diameter, cast-in-place caisson, installed by a pull-down method to a depth of 3.39 metres, with five metres of concrete poured underwater by divers at the bottom. This served as a large anti-buoyancy plug at the cylinder’s bottom preventing more water from entering.

The water in the caisson was then pumped out to provide a dry cylinder. Crews entered the dry well and started to drill out ten 300-mm diameter stainless steel well laterals projecting out.

“It is like spokes on a wheel,” says Boyle, adding work was done by specialized crews using equipment that would not allow water to seep back into the working area. Laterals were drilled out 48 metres from the well caisson. Each lateral can be monitored and activated by surface valves.

Boyle says while the project was challenging, the team’s achievement was the ability of the contractors to develop detailed environmental management plans before the project began.

“With well defined limitations and allowances for cost, it made it easier for the contractors to comply with these requirements,” says Boyle.

City personnel were closely involved in all phases of the work and Boyle credits their involvement as being a key factor to the success of the project. The project’s successful completion has earned Dayton & Knight Engineering an award of merit from the Consulting Engineers of B.C. in the municipal category.

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