May 09, 2010
CITY OF TORONTO
Imagine constructing a tunnel 42 metres beneath the surface — tunneling 510 metres into the ravine and through wet soil, with the constant fear of hitting the unexpected.
It isn’t, said Lou Di Gironimo, general manager of Toronto Water. “The tunneling is straightforward . . . it’ll be complex right at the end when we are doing the connection and the transfer from the old sewer to the new. The two tie-ins are the most challenging parts.”
In Toronto, the words “sewer” and “challenging” in the same sentence mean the Coxwell tunnel: the giant pipe buried under the Don Valley with a series of cracks that can’t be repaired because the sewer can’t be shut down and emptied. The city is now constructing a bypass at the cost of almost $29 million — construction starts this week and the new bypass will be functional by January.
McNally Construction Inc. was awarded the contract in March.
The 4.8-kilometre Coxwell sewer handles sewage of 750,000 people living in the centre of the city. The cracks, beneath Barbara Cres., north of Coxwell Ave. and O’Connor Dr., were discovered by a robotic camera in late 2008.
Built more than 50 years ago, the pipe is steel plate on the outside, with 56 centimetres of concrete inside. If it were to break or collapse, its contents would spill into the Don River. That would be catastrophic.
It’s not going to happen, said Di Gironimo. “But because it’s such a critical sewer, we have to get in there and repair just so we don’t have any problems.”
The permanent bypass will start adjacent to the parking lot of Taylor Creek Park and will be tunneled through the ravine, reconnecting to the current sewer at the intersection of Coxwell Blvd. and O’Connor Dr., downstream from the damaged section.
This route — in an arc west of the damaged area — was chosen because it is the shortest and will involve the least property encroachment or traffic disruption.
At Taylor Creek Park, tunneling will begin at 8.5 metres. The deepest point will be 42 metres at Coxwell and O’Connor. But before tunneling begins, two shafts will be dug at the connection points.
The tunneling machine will be inserted at the entry shaft, and as it excavates, it will remove debris through the shaft. The tunnel boring machine moves approximately 5 metres a day. Construction crews will build the pipe in sections as the tunneling continues.
Most people won’t feel any vibration because the tunneling is so deep, but “some very sensitive people can detect something,” said Di Gironimo. The construction of one shaft will have a greater impact on some homes because it’s close by, he added.
Once the new pipe is ready, structures will be built around the existing pipes where the bypass will connect. “That gives it structural strength and rigidity underneath the pipe and supports through the section.”
Diverting the sewage is the trickiest part. Isolation gates will be used to control the flow, starting from the top. “There will be some coordination to be done at low-flow times,” Di Gironimo said.
“We’ve built larger tunnels, subways, but that’s all clear of sewage,” he said. “Here, the challenge is the tie-in and because you are dealing with a very large sewer, it becomes all the more complicated.”
The connection will be made over a few days.
Once the old pipe is isolated, the city will decide its fate. Even with the new bypass, it will still be the only pipe carrying a full third of the city’s sewage. That’s why the city is considering twinning the Coxwell line, Di Gironimo said. A twin pipe would cost about $100 million and its routing is still under public consultation.
Meanwhile, some residents of Barbara Cres., who live close to where the exit shaft will be, are confident the construction will go smoothly. “The city seems to be on top of things — no, I’m not really concerned,” said Martha Minchopoulos, who has lived at Barbara Cres. for 21 years.
But Helen Zoubaniotis, who lives three houses from the exit shaft, is slightly concerned. “It worries me because the existing pipe is also below my house. I’m just glad the city has taken measures to fix the problem before it becomes bigger.”