March 27, 2009

Outfall tunnel makes way for more housing in Mississauga and Peel Region

DAN O’REILLY

correspondent

MISSISSAUGA

A $23.5-million, 2.2-kilometre-long outfall tunnel is being installed deep below the waters of Lake Ontario at the Clarkson Wastewater Treatment Plant in Mississauga, so that houses and apartments can be built in the city and other parts of Peel Region.

A driving force behind the new tunnel is a planned $150-million expansion of the plant, scheduled to start near the end of this year. The expanded facility is needed to keep pace with Peel Region’s growth, but before expansion occurs the new tunnel has to be in place to handle the increased sewage, says William Fernandes, the region’s capital works manager for wastewater treatment.

PEEL REGION

Above, the concrete crew at work on the tunnels. They will stretch 2.2 kilometres and cost of $23.5 million.

Although the plant’s capacity will jump from 200 to 350 mega litres a day after the expansion project is completed in 2012, the tunnel has been designed to accommodate 500 mega litres a day to take into account future expansions, says Fernandes.

As might be expanded, building a lengthy tunnel deep below the cold waters of Lake Ontario is an incredibly complex undertaking for the region, consulting engineer R.V. Anderson Associates Limited and general contractor C&M McNally Engineering Corp.

At its end point, the tunnel is approximately 12 metres below the lakebed.

The project involved complex approval processes, including hydrologic dispersion modeling and fisheries approvals from the Department of Fisheries and Oceans, the Ministry of Natural Resources and the Credit Valley Conservation Authority.

Among the many construction and technical challenges has been the need to excavate a 47-metre deep, 12-metre diameter shaft to lower the tunnel boring machine, the physically demanding work associated with building the form work for the concrete liner, and the careful coordination required to link the tunnel with a diver’s access shaft and 18 14-metre-long vertical diffuser risers.

Then there were the logistics of transporting concrete to create a 45-metre deep, 12-metre diameter liner, says R.V. Anderson structural engineer Tom Richardson.

“Most of the tunnel is simply too far from the shaft to pump concrete.”

So, during the mining process, C&M McNally developed a unique concrete delivery system where concrete was poured into hopper cars traveling along a moveable ramp system and then remixed in a portable concrete remixing and pumping facility.

Superplasticizing chemicals were added, then the concrete remixed and pumped into the telescopic steel forms.

“It’s not an entirely new system, but it was a first for McNally,” says Richardson.

Preliminary construction work began in the spring of 2007, with the shaft excavation starting in September of that year. The actual mining of the tunnel began in February 2008.

“On some days the TBM would excavate 85 feet. That’s an incredible amount for an operation like this.”

About 26,000 cubic metres of Georgian Bay shale was excavated and sent to the nearby St. Lawrence Cement plant for recycling by the time mining of the 3.6-metre bore was completed last September. Meanwhile, the concrete pouring for the liner continues.

“Pouring of the liner is 40 per cent complete and will continue on a daily basis until the tunnel is fully lined,” says Richardson.

But building the tunnel was just one component of the overall project. About two thirds of the way is a diver access shaft, which divers will enter to inspect and maintain the tunnel.

Above, equipment gives divers access. The driving force behind the work is the planned expansion of the wastewater treatment facility, which should be completed by 2012.

It was installed over a four-month period in late 2007 by Dean Construction Company crews working from a jack up barge.

Last summer the firm returned to the site and installed 18 450-millimetre diffuser shafts, which were constructed off site, spaced at 12 metres near the end of the tunnel. They disperse the treated effluent.

“The bottom of the bored holes for the diver access shaft and diffusers extends into the location of the tunnel bore. When the tunnel was a few metres from the diver access, mining stopped and a small hole was drilled into the diver access shaft to allow the water in it to drain. Once the water was out, mining resumed.

“For the diffuser shaft not as much water was in the shafts and, therefore, the TBM just mined up to the location of the shaft and when we encountered the hole, the water drained out and mining resumed,” says Richardson.

The tunnel should be completely constructed and commissioned early this fall, he says.

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