March 28, 2008

Early May is the target for crews to be done the mining operation for the $76-million interceptor sewer construction in the town of Richmond Hill.

Innovation

York Region interceptor sewer project employs most current tunnel technology

DAN O’REILLY

correspondent

A year after construction started, a $76-million interceptor sewer employing the most recent advances in tunnel technology is nearing completion in Richmond Hill.

“We should be finished the mining by early May and substantially finished by the end of June,” says Steve Skelhorn, project manager for McNally/Aecon, the joint-venture tunneling contractor.

Designed to relieve pressure on the close-to-capacity existing Yonge Street sewer without damaging the sensitive Oak Ridges Moraine, the 4.4-kilometre long 2,100-mm one-pass reinforced concrete segmented pipe is being installed by a Lovatt-manufactured earth pressure balance tunnel-boring machine.

The route is along 19th Avenue between Yonge Street and Leslie Street and a small portion along Leslie Street that was completed last July. A 1.6-kilometre sewer installed along that street by open-cut methods last year is also considered part of the project.

“We’re mining about 24 metres a day, which is the best production we’ve had and that’s been consistent for about 2 weeks,” says Skelhorn.

Tunneling at an approximate rate of 1.7 metres per hour, the tunnel machine is just west of Bayview Avenue heading west towards Yonge Street.

“The tricky part is well behind us,” says Skelhorn, noting the most complicated aspect of the project was actually at the start when two machines had to operate from a single shaft at Leslie Street and the 19^th Sideroad. The second machine was for the excavation work along Leslie Street.

“We hope to commission the sewer sometime early this summer,” says Laura McDowell, manager of the York Durham Sewage System.

The commissioning will mark the end of a long planning and regulatory approval process for York Region that included considering nine alternative routes. Planning began several years ago, an environmental assessment was conducted in 2003 and a special dewatering permit was required from the Ministry of the Environment.

As the route is on the Oak Ridges Moraine there are several sections with artesian well-like conditions where the water table is above the level of the sewer.

“But we never had to withdraw any water,” says McDowell, citing the use of an earth pressure balance machine. The excavation technology provides continuous support to the tunnel face by balancing the earth pressure against the forward pressure of the machine.

What makes the technology so significant is the ability to work through almost all soil conditions, most notably water bearing soil, she explains.

“Conventional tunneling techniques require dry soil conditions and therefore dewatering is necessary in order to complete construction. We learned that lesson from another project.”

The earth pressure balance excavation wasn’t the only safeguard employed by the region and Earth Tech, its consulting engineering firm. Before work got under way, construction crews attended a special training session that dealt with issues such as sediment control and other measures for protecting the moraine. Workers were also required to place stickers on their hard hats indicating they had taken the course.

Construction was intensely monitored by regional and Earth Tech officials, says McDowell. “A lot of eyes were on this project.”

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