By Sophie Kneisel
Underwater tunnel to Toronto’s downtown airport coming soonTransportation Awards Bridges
A mere 80 years after the first attempt at building a tunnel to Toronto’s Island airport, travellers will be able to walk under the waters of the Western Gap to fly out of what is now Billy Bishop Toronto City Airport. Close to downtown, the airport is the ninth busiest in Canada, handling around two million passengers a year.
The tunnel was originally scheduled to open in spring of 2014, but is now slated for completion early next year — still on its $82.5-million budget, if not quite on time. The 244-metre long tunnel will house four moving sidewalks capable of transporting passengers to the country’s ninth busiest airport in six minutes in a relatively steady flow. The pedestrian access will be a major improvement over the existing 15-minute ferry rid that results in waves of 200 passengers arriving at each ferry terminal four times an hour.
Construction delays have been attributed to ice building up in the open shafts at either end of the tunnel during the long and extremely cold winter last year, as well as to obstacles encountered by the boring machines in April 2013. They came up against pilings from the politically scuppered attempt to build a $1-million vehicle-and-pedestrian tunnel in 1935.
In mid-November a construction milestone was reached when the first of two City of Toronto water mains were hooked up. The mains are built into two of seven overlapping drift tunnels that form the arch over the roof of the main pedestrian tunnel. Another of these tunnels houses a wastewater main.
The installation of these three mains during tunnel construction is saving the city an estimated $10 million in infrastructure costs. The remaining drift tunnels, three of which were fitted with pipes for pumping concrete to the island for construction of the tunnel pavilion (more efficient than transporting concrete by ferry), have now been filled with concrete.
The concept of securing the top of the tunnel by constructing a concrete arch above it is fairly innovative, says Stan Gonsalves, P.Eng., executive vice-president of exp Global, which is the firm responsible for designing the underwater excavation plan and managing the control of dewatering activities and contaminated overburden.
The tunnel site is difficult to work in, Gonsalves explains, because it’s under the lake and doesn’t have a lot of cover — approximately 10 metres of rock and lake-bottom “overburden” below 10 metres of water. Another complicating factor is that the shale the tunnel is built through is a saline environment, which swells when it comes in contact with fresh lake water. The resulting “time-dependent deformation” requires a design strategy to either restrain the swelling, or wait until the deformation is complete. The solution involves packing the area around the concrete structure with crushable material to allow for the squeezing effect of the swelling, while preventing damage to the rigid tunnel walls.
The Toronto Port Authority hired the consortium Forum Infrastructure Partners to design, construct and maintain the tunnel. The project has already won one award: in October the Tunneling Association of Canada named it “Tunnel of the Year” for its innovative construction and for working “within a constrained site, while keeping the airport fully operational, but also for overcoming the engineering challenges involved in the construction of a watertight 10-metre-wide tunnel, 40 metres below ground.” Receiving the award were the Toronto Port Authority, Technicore Underground, PCL Constructors, and engineering firms Hatch Mott MacDonald, exp, Arup and Isherwood & Associates.
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