September 21, 2009
Sewer and watermain
Fragile wetlands require less disruptive dig in Cambridge, Ontario
What might be considered old technology is being used in a new way as a 400-metre-long section of the 1.2-kilometre-long 675-mm-diameter Moffat Creek sanitary sewer line now under construction in Cambridge, Ont. will be tunnelled by hand later this fall.
Workers with subcontractor Jimmy Mack & Sons Construction will be using hand shovels to remove earth and, where needed, operating jackhammers to dislodge hard till and large boulders which geotechnical tests have indicated will be found along the route.
The labour-intensive operation was proposed by contractor Brantford Engineering and Construction in response to community, city and other regulatory agency concerns about the impact of open cut excavation to a provincially significant wetland.
With the exception of the hand tunnelled section, traditional cut and fill is being used to install the sewer which is an extension of an existing east to west sanitary trunk within the Moffat Creek Valley. It is needed to facilitate the planned growth of about 4,000 homes in Cambridge’s South East Galt community, say city officials.
Not only will the trenchless technology avoid disturbing that wetland, it will save the city approximately $2 million compared to other proposed methods including tunnel boring, which had been considered.
“What it means to the community is a less invasive process that preserves the environmental assets by utilizing primarily tunnelling techniques to dig. The trenchless technology is similar to the methods used in mining which means — that rather than open cutting the area — the majority of the route can be excavated or tunnelled underground,” says Jim Kirchin, the city’s director of planning operations.
The route of the trenchless section of the sewer alignment will run under several large trees, the wetland and will twice cross over Moffat Creek, says Brad Marin, contract administrator, Conestoga-Rovers and Associates, the consulting engineer.
A hydraulic jacking system will be used to install a 1,500-mm diameter steel liner. A shaft will be excavated to the proposed depth of the trunk sewer and a concrete thrust block will be installed at its base to commence the tunnelling. The jacking system will be prepared and installed and a hydraulic power unit will supply the thrust required to propel the liner through the ground.
An articulated mine shield will be attached to the first length of the liner. Miners at the shield will dig into the earth using hand shovels, and whenever hard till is hit, jack hammers will be utilized. As the liner sections are jacked in place, internal lasers will provide controlled guidance, says Marin. The excavated material will be removed in track-mounted cars.
While the hand-tunnelling will certainly by long and demanding, it’s not the only challenge of installing the sewer, which will run Franklin Boulevard to just east of Dundas Street.
“We’re working within the confines of the (Moffat) valley near the creek on one side and often near homes on the other side,” says Mathew Rich, vice-president of Brantford Engineering and Construction, a family-owned firm.
An approximately 50-metre-long support wall had to be erected near Dundas Street to protect the backyards of several homes overlooking the trench site. Considerable dewatering is also required. Two 10-man crews are currently working at different sections, says Rich.
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