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The long-awaited pedestrian tunnel to Billy Bishop airport is moving into the next phase of construction, with the goal of opening it to human traffic about a year from now. Starting in December, working off a launch platform set up 25 metres down a deep shaft, work crews have been boring seven smaller tunnels under the lake. This key stage is now finished and, with work beginning this week on the main tunnel, here is a snapshot of the project to date.

Each of the so-called drift tunnels is about two metres in diameter. They were designed to overlap slightly, making a continuous arch ((1)) over what will be the main tunnel. Laser-guided boring machines nicknamed Chip and Dale clawed their way through the 186-metre distance across the channel to dig the drifts, sending out approximately 3,400 cubic metres of rock on a temporary narrow-gauge railway. Each drift was then filled with concrete, together forming a structural crown that will offer crucial support for the roof of the walkway.

Before filling in the drifts, crews installed conduits. The three drifts on the right ((2)) have pipes that will contain sanitary and water mains. The Toronto Port Authority says this will save the city $10-million by allowing it to combine utility work with the tunnel dig. Over to the left ((3)), smaller tubes were put in place to allow concrete to be pumped over to the island throughout the rest of construction, instead of being trucked over on the small ferry.

The shaft is too small for a full-scale tunnel-boring machine to be used for the main dig, so Chip and Dale are getting another chance. The crew is sending the small TBMs through twice more, describing it as like drilling a small hole before switching to a larger bit. These two bores were started this week, the second of them on Thursday.

Crews will have to deepen the shaft after the TBMs have gone through the last two times – the base of the main tunnel will lie 7.5 metres beneath the base of the drifts ((4)) – and then a boom-mounted cutter on an excavator will start the main dig.

The 10.5-metre-wide main tunnel ((5)) excavation will slope up gradually, at about a 1-per-cent gradient, until roughly the midpoint of the way across the channel. From there the grade changes to 4 per cent. The tunnel will end at a depth of 30 metres.