By Jenny Jones
A design/build team uses an innovative tunneling method to create a pedestrian tunnel beneath a channel of Toronto Harbour.
Designed to be light, bright, and comfortable, the 9 m diameter Billy Bishop Pedestrian Tunnel will feature a central corridor with a moving walkway on each side. Courtesy of the Toronto Port Authority
November 19, 2013—A passenger who flies from Ottawa, Ontario, to the Billy Bishop Toronto City Airport, located on Centre Island, in Toronto, can expect the flight to take 45 minutes. But if that passenger lands at the airport during a particularly busy period, he or she may have to wait nearly as long just to catch the ferry that navigates the 122 m wide channel between the island airport and Toronto's mainland. To reduce those wait times and improve the passenger experience, a new pedestrian tunnel is being constructed beneath the channel, using innovative tunneling methods.
The Billy Bishop Pedestrian Tunnel will be a 9 m diameter climate-controlled tunnel that will extend beneath the channel of Toronto Harbour, a bay on the north shore of Lake Ontario, to connect the airport and the mainland. Currently, the only way to access the airport is by the ferry, which carries up to 200 passengers and makes the 2-minute trip between the mainland and the airport every 7 1/2 minutes. The Toronto Port Authority (TPA) operates both the ferry and the airport and has seen the airport's passenger count soar during the past eight years from 25,000 passengers a year to more than 2.2 million passengers annually.
As the airport's popularity has increased, the need for the tunnel has become evident. "If we have three or four full aircraft land at about the same time, you may have to wait for the second and possibly the third ferry to get across the channel," says Ken Lundy, P.E., the director of infrastructure, planning, and environment for the TPA. "With the tunnel, that will not be an issue, because we've designed the tunnel for the peak period arrivals of the aircraft, and we'll be able to manage the pedestrian flow from the airport to the mainland."
After developing a reference plan for the tunnel, the TPA advertised the project as a sort of private/public partnership—except that instead of using public funding, the $20 airport improvement fee that every passenger pays as part the airfare will help finance the project. The TPA received a great deal of interest in the project, and after narrowing the field and reviewing teams' design proposals and technical and financial plans, it selected a consortium known as Forum Infrastructure Partners to deliver the project and operate the tunnel for 20 years following its completion.
The tunnel will extend beneath the channel that separates the airport from Toronto’s mainland. Currently, the only way to access the airport is by the ferry. Courtesy of the Toronto Port Authority
Forum Infrastructure Partners comprises Forum Equity Partners, an infrastructure investment and development firm based in Toronto; PCL Construction, a contracting firm headquartered in Edmonton, Alberta, Canada; and Arup, an international engineering firm. The TPA selected the team on the basis of its innovative tunneling method, which involved creating seven smaller tunnels prior to excavating the larger primary tunnel, Lundy says.
The approach was driven by the fact that one of the greatest challenges of any underwater tunneling project is dealing with the unknowns of the site's geotechnical conditions. Although a limited geotechnical investigation had been completed at the Billy Bishop site, the schedule did not allow for a more detailed examination, leaving the team to wonder what it would find as it drilled beneath the lakebed. "The concern was there would be some feature in the rock, a fault or a shear, that would allow water to come into the tunnel," explains John Hurt, P.E., P.Eng, Ceng, MICE, M.ASCE, a principal of Arup and the firm's project manager for the tunnel.
To safeguard against such a mishap, the team developed seven 1.8 m diameter tunnels prior to excavating the 9 m diameter primary tunnel. Small boring machines were used to excavate each of the tunnels separately, and then each tunnel was filled with concrete to form an arch over the primary tunnel's crown. "There's a whole series of benefits to the method," says Robert Talby, Ph.D., P.E., M.ASCE, an associate principal and the deputy project manager for Arup. "First of all, [the smaller tunnels are essentially] large horizontal bore holes, so you can get an early idea of the ground conditions. Secondly, you're replacing any fractured, broken upper rock with concrete, and then thirdly, you're forming an arch to support the overburden above."
On the island side, the tunnel will be accessible by via escalators and elevators located within a new atrium that is also being constructed as part of the project. Courtesy of the Toronto Port Authority
Additionally, the smaller tunnels provided a passage for a set of water and sewer lines that the City of Toronto wanted to pass through the tunnel. "Within three of the small-diameter tunnels we built in a water main and two sanitary pipe systems," Lundy explains. "That saved the city over ten million dollars."
Protected by the arch, the primary tunnel excavation proceeded in three phases. First, a 6 m wide continuous excavation was made from the mainland to the airport to form the center of the tunnel. Then two 1 1/2 m wide excavations were made—one on either side of the center excavation—and finally a third excavation removed the remaining rock at the invert to form the tunnel's final profile. "That's pretty much the stage that we're at now," Hurt says. "The following stages include a whole sequence of operations to put the concrete lining in place."
Once the tunnel is smoothed, a fully sealed waterproof membrane will be added, and prefabricated reinforcement cages, which have been welded together to form large sections, will be inserted into the passage. Concrete will then be placed to form the invert slab, which will encase drainage pipes and electrical ducts. Following a second round of waterproofing at the crown, a 12 m long tunnel form purchased from the Devil's Slide tunnel project in San Francisco will be used to form the remainder of the concrete lining. (See "Taming the Devil's Slide," Civil Engineering, June 2013, pages 58-65.)
"We designed the tunnel to match the profile [of the tunnels at Devil's Slide] as much as possible," Hurt says. Throughout the process, the team will continue to monitor the rock for swelling, which is common following excavation.
Also as part of the project, the team is developing two small structures—one on the mainland and another on the island—that will house elevators and escalators for accessing the tunnel. The tunnel will reach a depth of 45 m near the mainland, rising slowly across the channel to a depth of 40 m at the island. It will feature a central corridor lined by two moving walkways and will be heated and air conditioned for passenger comfort. "It will be light and bright and very comfortable to go from one side to the other," Lundy says. "It's not an excessive walk; it's probably six minutes from the mainland to the island, including using the elevators and escalators to get up and down."
The design/build team used an innovative tunneling method that involved excavating seven 1.8 m diameter tunnels over the crown of the primary tunnel. Courtesy of the Toronto Port Authority
Completion of the project is anticipated in early fall 2014. In addition to improving efficiency at the airport, the tunnel will alleviate congestion in Toronto by better managing the number of people who arrive on the mainland at a given time. Rather than having 200 people departing the ferry and looking for rides at the same time, the flow will be more dispersed, Lundy says. "With the tunnel, it will be a much more even distribution of passengers, and we'll be able to manage them on the mainland much more effectively, with much less congestion and impact on the community," he says.