Tunnelling in shale poses many challenges at Billy Bishop Airport’

Sometimes the smaller jobs can be the toughest and the most challenging. Take, for example, the pedestrian tunnel currently under construction that will link the mainland with the Billy Bishop Toronto City Airport, located on the western tip of the Toronto Islands.

The elevator shaft on the mainland is a mere 100 feet deep—small change by mining industry standards—and the tunnel beneath the Western Channel is just over 600 feet long—which may not raise many eyebrows among underground miners.

“It’s not a long tunnel,”  concedes Gary Benner, Vice-president of Newmarket-based Technicore Underground Inc., the prime contractor on the project, “but it’s complicated because it’s in shale, not hard rock. That has a big impact on how you build it.”

Work on the $82.5 million project—to be funded largely through airport user fees—began April 2012 and Technicore crews have toiled round the clock since then except for a few days over Christmas 2013 when a devastating ice storm hit the city.

The project is slated for completion later this year, at which point the nearly 2.4 million passengers who use the island airport annually will be whisked back and forth on fast-moving pedestrian walkways rather than a ferry named Marilyn Bell I.

Technicore crews began by excavating the elevator shaft and that’s where they encountered their first challenge. They had to dig through 30 feet of soft, wet sandy overburden before hitting bedrock and they had to install interlocking secant piles to secure and seal this segment of the shaft. The piles are cylindrical holes—one metre in diameter in this case. Once drilled, they are filled with concrete and they are overlapping, or interlocking, which adds strength and keeps out water.

Technicore crews drilled the primary or odd numbered piles first; say, for example, one, three, five and seven, and then poured concrete into them. After the concrete had set, the crews drilled the intermediate or even-numbered piles (two, four, six and so on) and they cut into the edges of the primary piles to create the overlap or interlock. By December 2012, the shaft had been excavated to depth and the tunneling began and that’s where things became very complicated.

As Benner explains, the tunnel passes through soft, horizontally layered shale that is prone to collapsing. Before they could begin to excavate the tunnel, they had to create an arched crown, or secant wall above it in order to distribute the loads and eliminate the possibility of a collapse. They used two laser-guided, manually operated tunnel boring machines (TBMs), nicknamed Chip and Dale, designed and manufactured by a Technicore affiliate.

The cylindrical TBMs drilled seven piles, each of which were 1.9 metres in diameter, and they drilled from the mainland shaft to the island, initially at a grade of one per cent. However, about three-fifths of the way through, the grade changed abruptly to four per cent and the TBMs broke through on the island nearly 50 feet above where they had started.

Technicore crews first drilled the primary piles; one, three, five and seven, and these had to be at different elevations to create an arched crown. Then they filled each of them with some 500 cubic metres of concrete before they drilled the intermediate piles; two, four and six.

“It was a huge challenge to get that concrete in place,” Benner says, “because we pumped from the low end on the mainland and had to fill 600 metres of tunnel and we had to go uphill 50 feet to the island. Everybody said you couldn’t do, but we did it.”

Another affiliated company mixes, supplies and delivers the concrete for all of Technicore’s tunnelling projects and the affiliate found the solution. “We came up with a proprietary mix,” says Benner.

“It would remain fluid for up to 10 hours. It would flow for the length of the drifts and it was the right strength to allow us to drill the intermediate drifts and create the interlock. The interlock had to be almost perfect to achieve the structural integrity of the arch. It worked out to within an inch all the way around, which is pretty amazing.”

The excavation of the actual tunnel, which measures 36 feet wide by 26 high, began in June and Technicore used the TBMs to drill two pilot holes. That made it easier for German-manufactured Liebherr mining backhoes, equipped with hydraulic breakers, to chip away at the rock until the tunnel was excavated to its full size. As well, the pilot holes served as ventilation shafts and fans installed on the island side sucked out the enormous amounts of dust created while the backhoes were at work.

The breakthrough to the island occurred at 11:30 a.m. on August 23, 2013 and was cause for a celebration and a major media event.

“This is a watershed moment for our passengers,” said Mark McQueen, Chair of the Toronto Port Authority board. “The pedestrian tunnel is well on its way to being complete, now that the most technically difficult element of the project is behind us.”

Since then,  Technicore has sprayed the base, the sides and the roof with a thin layer of concrete to stabilize the shale. Next a sub-contractor waterproofed it by installing sheets of rubberized material similar to swimming pool liner. Over the past few weeks, Technicore has been pouring in stages a steel re-enforced concrete base that is four feet thick to resist upward pressures that might cause cracking.

By the end of March, Benner says, Technicore should be finished pouring the concrete for the walls and the roof and will make way for other contractors who will install electrical and mechanical systems, lighting, the mobile walkways and a bank of 10 elevators.