October 27, 2010

Contractor rises above challenges on Moffat Creek sewer-line job

DAN O’REILLY

correspondent

Sewer main contractors aren’t normally trail builders, but in Cambridge the recent opening of a new section of a hiking trail capped off an incredibly complex sanitary sewer project marked by more than a few environmental constraints.

Incorporating 90 metres of boardwalk, the 0.7-kilometre extension of the existing Moffat Creek trail was built over a section of the 1.2-kilometre-long 675-mm-diameter Moffat Creek sanitary sewer line.

On hand for the city-sponsored official ribbon cutting were representatives of Conestoga-Rovers and Associates and Brantford Engineering & Construction Ltd., the respective designer and builder of both the trail and the sewer line.

Designed to meet the demands of planned new growth of about 4,000 homes in Cambridge’s South East Galt community, the sewer line was installed in the environmentally sensitive Moffat Creek Valley, home of a provincially significant wetland.

Protecting the wetland and the creek, which meanders through the valley, required a whole myriad of protective measures, says Brad Marin, contract administrator with Conestoga-Rovers and Associates.

Apart from standard construction practices such as controlling silt and sand, special precautions were needed to limit the damage from the dewatering operation.

To construct the dewatering system, the contractor had to gain access to an area beside the tunnel alignment and that required a controlled and selective removal of some trees and shrubs, he says.

Before that was done, several test sections for the access way were examined by his firm working in concert with the contractor and the Grand River Conservation Authority, says Marin.

When it came to the construction, Brantford Engineering built a small log road to support the drilling machine and other heavy equipment used in the dewatering.

“It was like an old-fashioned corduroy road,” Marin said.

The alignment for that road later became the route of the Moffat Creek Trail extension, he points out.

But the corduroy road wasn’t the only constructiontechnique used that might be considered old fashioned.

While open-cut construction was the primary method, approximately 400 metres of the sewer line had to be hand-tunnelled under the wetland and one creek crossing at depths ranging from three to six metres. The contractor proposed it as a less expensive and less environmentally intrusive option compared to tunnel boring.

“It took longer, but cost less,” says Marin of the operation which was conducted over a five-month period by Jimmy Mack & Son Construction, one of the few firms specializing in this type of work.

A hydraulic jacking system and concrete thrust block was used to install a 1,500-mm diameter steel liner.

As the steel liner was inserted, workers in the reinforced section removed soil and often large rocks using shovels and jack hammers, says Marin.

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