Breaking Through to Billy Bishop
Technicore completes largest tunnel ever built in Georgian Bay shale
It is only 120 metres across the Western Gap of Toronto Harbour from the Toronto waterfront to the Billy Bishop Toronto City Airport (BBTCA) but it still requires a ferry boat ride for passengers going to and returning from the airport on Toronto Island. The ever-increasing popularity of this airport with close proximity to downtown Toronto has pushed the ferry capacity to its limits.
In 2010, the Toronto Port Authority which owns and operates the BBTCA, committed to build a pedestrian tunnel to the airport to alleviate the current crowding and provide for future growth in air passenger travel. In 2011, Requests for Proposals were issued and three consortia submitted design-build proposals to build a new tunnel. The successful proponent was a team consisting of Forum Equity Partners, PCL Constructors Canada Inc., Technicore Underground Inc., ARUP, ZAS and Exp. Work started in early 2012.
There are many components to the final result of this project including terminal buildings, elevators, people movers etc., but it was the challenges involved in the construction of the shafts and tunnels from an impossibly crowded site without interrupting airport operations, that had everyone’s attention. Added to that, was the building of the tunnel itself in shale bedrock under the active shipping channel. The 8- by 11-metre concrete arched tunnel would be be the largest tunnel ever mined in Toronto shale. It was also going to be excavated with only about 15 metres of rock cover between the bottom of the channel and the roof of the tunnel. The risk of shale collapse in the tunnel or Lake Ontario, finding its way in through unknown rock fractures was very high.
Interlocking secant piles used as pre-support
The burden of dealing with these risks fell to tunnel contractor Technicore Underground Inc. (Technicore) of Newmarket, Ontario working in close collaboration with world-renowned tunnel-design engineers, ARUP. Several construction techniques were considered but it was Technicore owner Tony DiMillo’s concept for interlocking horizontal concrete-filled tunnels (called secants) forming a structural arch above the tunnel, that was decided on and everyone held their breath as ground was first broken in April of 2012.
The Mainland Shaft was 15 metres by 30 metres and had to be excavated to a depth of 32 metres, of which the top 10 metres was wet sandy fill. In order to support the unstable ground and keep the adjacent lake out of the excavation, Technicore used vertical concrete interlocking piles around the entire circumference of the shaft, socketed three metres into the shale bedrock to hold back the overburden. One of the secant piles encountered steel sheet piling, causing a delay and foreshadowing more difficulties to come. As the overburden was excavated, old wooden pilings were uncovered as well as a sheet pile wall that had been buried. On testing of the soil, it was found to be contaminated, causing a further delay as an appropriate dump site was located. Research by the Toronto Port Authority determined that the sheet piling was from a prior attempt to construct a tunnel under the channel in 1935, which was abruptly cancelled following a change of government in Ottawa.
Technicore overcame all of the challenges in the Mainland Shaft and continued the excavation in the shale to a depth of about 25 metres required to construct the seven 1.9-metre-diameter interlocking arch tunnels. At the same time, the Island Shaft was being sunk in a similar manner to a depth of 27 metres. The Island Shaft was a long dog-legged, sloping shaft in which a bank of three escalators would be installed to carry passengers up into the terminal on the island. These arch tunnels were mined with twin Technicore-designed and built rock Tunnel Boring Machines (TBMs) with the first one reaching the island in late January 2013. The TBM was then extracted from the Island Shaft, transported back to the Mainland Shaft and set up for another drive. With the tunnels at differing invert elevations around the arch, varying combinations of steel frames and sea can containers were employed to get the launch pads at the correct elevations.
Getting the interlocking arch right was critical. The tunnels had to be mined to overlap a minimum of 300 millimetres, so that when filled with concrete they formed a structural arch which would hold up the shale above while allowing the large excavation to proceed beneath. Had the alignment of the tunnel deviated, the arch would have been ineffective. To make things more interesting, the profile of the tunnels included a three percent grade break near the middle that all the tunnel drives had to follow precisely to maintain the overlap.
The tunnels were filled with concrete shortly after each was mined. This was another technical challenge as the 550 cubic metres required had to be pumped from the surface down into the Mainland Shaft and into the bulkheaded tunnels. The concrete had to flow from its low end at the Mainland Shaft a full 185 metres, all the way to the Island Shaft with a fivemetre vertical rise. The concrete had to maintain its flowability for the expected seven-hour duration of the pour. Normally, a high cement content mix is used to achieve the flowability, but for three of the tunnels, the TBMs had to cut through the edges of the already-concrete-filled tunnels while excavating the shale still remaining in between. If a high cement content mix had been used the concrete would have achieved a strength such that mining through it would be very difficult. Technicore developed a proprietary concrete mix working with BASF to achieve the long-duration flowability without the strength. The tunnel arch was successfully completed in May of 2013.
Installation of city water and sewer mains
One of the innovations of the project was incorporating new City of Toronto sewage forcemains and watermain into the pedestrian tunnel project. Before the arch tunnels were filled with concrete, steel casings were fixed in the tunnel to provide a protected conduit into which the pipes could be threaded. Using the tunnels for the utilities saved the city $10 million from having to build a separate utility tunnel under the channel. The Mainland Shaft was then excavated to full depth and Technicore drove two more 1.9-metre tunnels across to the Island below the arch to facilitate the rock breaking, to allow the noise and dust to be ventilated to the island. The centre cut of the excavation was made with a Liebherr mining backhoe and reached the Island on August 23, 2013 with the final breakout viewed by a throng of engineers, media and VIPs.
In the fall of 2013, the tunnel excavation was widened and lowered to its final dimensions with the use of a Dosco roadheader and a scoop tram. The excavation of the tunnel arch shape was controlled by the use of large templates hung from a track bolted to the roof on perfect grade. As a result there was almost no overbreak and very few “tights” that had to be trimmed. The concrete arch performed perfectly in holding up the shale and the tunnel was surprisingly dry considering how close Lake Ontario was. As part of the design-build process, ARUP monitored the movement of the shale which is renowned for generating substantial horizontal pressure. Shale movement proved less than predicted, allowing the finish tunnel work to be accelerated.
The next step was the pouring of a concrete mud mat on the invert and spraying shotcrete on the entire tunnel arch to facilitate the placing of the waterproofing by Wisko America. Pouring the thick concrete base designed to resist the expected hydraulic uplift pressure over top of the waterproofing without puncturing it was the next challenge. Technicore called on its affiliate, Ewing Fabricators to build all the heavy reinforcing steel in prefabricated mats that could be placed on top of the waterproofing with a Manulift mobile crane reaching over a 12-metre length of base pour still setting up. This allowed the tunnel base construction to be much faster than if the steel had to be tied in place.
The pouring of the finished tunnel arch was almost anticlimactic as the 18 pours were completed with 12-metre hydraulic forms very routinely. The final measurements showed the concrete was poured within millimetres of the design crosssection.
Technicore was off the site in early June 2014, leaving PCL and the other trades to complete the remaining components of the project, which is planned to open in February 2015.