Though Toronto is covered in snow, it’s immediately apparent how confined all of the work sites are for the Midtown tunnel project. The largest of the three has a footprint measuring just 30m by 30m. Located within residential and commercial areas, and near busy roads, finding the best way to support the excavation of the project’s three shafts gave contractor Technicore an opportunity to stretch its muscles and, for the first time, do its own groundfreezing.

Sizing Up
The client, Hydro One, which provides electricity across Ontario, first built transmission lines along Toronto’s midtown power corridor in the 1920s and the first underground cable was laid in 1954, meaning it’s now reaching the end of its useful life. In addition to replacing existing cable, the midtown power corridor also needs to increase its capacity.

Hydro One references a prolonged outage in January 2009, which left more than 20,000 households without power, stating: "The power corridor is already operating beyond its reliable limit at times of peak demand. With no reserve capacity at peak times when the probability of cable failure is highest, the prospect of power interruptions is a distinct possibility should existing circuits be disabled for any reason."

To house six new high voltage cables, the Midtown tunnel project concerns a 2.4km-long tunnel between Bayview Avenue and Yonge Street and Birch Avenue, of 3m finished diameter. The alignment is approximately 60m deep in shale bedrock, with three vertical shafts through glacial overburden.

While the alignment does pass under several roads, at least one water crossing and beneath one subway line, Mike MacFarlane, project manager, with Technicore, describes the tunnelling work on the project as pretty straightforward, even with two tight radius curves. The project’s real challenge was excavating the three shafts.

Small Spaces
Located on Mount Pleasant Road the main shaft is adjacent to The City of Toronto’s Rosehill Pumping Station. From here Technicore launched and relaunched its 3.9m diameter shielded TBM (Figure 1).

While the depths of each shaft varied (see Table 1) all three shafts are constructed through approximately 120ft (36m) of glacial overburden before reaching the shale where Technicore excavated the tunnel.

"The till is primarily comprised of cohesionless silts and sand, which would be flowing materials under the hydraulic pressures of the high water table," MacFarlane explains. "120ft deep is beyond the limit of a reliable, interlocked caisson wall shaft, and a slurry wall would be way too messy and disruptive for the neighbourhood. Dewatering would have been possible but the environmental permitting requirements and potenetial of causing settlements and structural damage to dwellings withing 15m of the shafts ruled that out.

"That left ground freezing as the logical, least intrusive ground stabilization method."

Standing at the Birch work site he points out where the freeze plant had been previously installed in the cramped space, and scraping away several inches of snow, the decking from which they monitored the freeze ring.

Technicore used a small drill rig to install the freeze pipes, each spaced 5ft (1.5m) apart, and 5ft from the excavation limit of the shaft, through which they pumped a brine mixture 120ft deep. "We went about 10ft [3m] into the shale to give security that the overburden would be frozen as well as the weathered shale interface," MacFarlane explains.

Each pipe was capped and housed a second pipe that brought the brine back to the surface to the freeze plant. The vertical pipes connected to horizontal pipes on the surface to create the freeze ring. This ensured a nearly constant temperature throughout the entire system around the shaft’s circumference. To gauge the temperature of the ground they installed two temperature probes another 5ft away from the freeze pipes, on each side.

"The hard part was getting all these freeze holes in this small footprint," MacFarlane says. "Can you imagine the impossibility of the large rigs required for a slurry wall working on this site? rigs that would have had to come into these sites? It just wouldn’t have happened."

In November 2011 Technicore started the ground freeze operations for the Mount Pleasant shaft. The Bayview shaft followed in April 2013. Once the plant was up and running it took about six weeks to freeze the ground to the point where it was ready for excavation.

Learning curve
For the freeze plant Technicore subcontracted design help from a contact at Thyssen Mining, based in Saskatchewan, which works mainly on potash mines. During the Midtown tunnel tender the company only had one freeze plant, but now has two, and has also frozen a ground on another tunnel in Toronto.

"There was a significant learning process to it," MacFarlane says. "We found out how differing soils freeze to different degrees, we determined how frozen we needed to make the ring around the shaft to keep it safe yet allow for productive excavation, and we learned how to cycle the plant to maintain the freeze yet not have to run the plant 24 hours a day to appease the local residents, even though the plant runs very quietly."

Technicore lined the shafts with concrete — first with a drop form, working down the shaft pouring as they excavated — later switching to ribs and lagging, adding the 16in thick concrete lining on the way up.

The original design in the contract called for conventional reinforced concrete walls – using steel rebar – for the entire shaft depth.

"While the design as-constructed keeps the original steel rebar configuration near the ground surface, for the great majority of it’s depth the steel rebar was replaced with steel fibers," says Andrew Cushing, tunnel design specialist, with Arup, which provided design services on the project.

The top 5m and bottom 4m have rebar because that’s where there are actual bending movements, MacFarlane explains. But otherwise, the shafts are mainly lined with fiber reinforced concrete, with a Greenstreak strip in each joint. On completion of the concreting, the shafts were relatively dry with very little leak sealing required.

Technicore explains, there is no provision in the Occupational Health and Safety Act in Ontario to consider frozen soils. "Even though they have physical characteristics of a soft rock, they still must be considered as soft ground in terms of the maximum unsupported height in an excavation – 4ft. This does not seem realistic, but ground freezing has never been used in civil tunnel works in the Province and there is no track record to support a modification to the legislation."

Technicore has been working with Isherwood and Associates, Geostructural Engineers, to oversee and document the freeze operation, soil movement characteristics and pressures exerted on the shaft linings during the freeze process to try to bring more science to the operation.

Mining Midtown
Technicore built the 154in diameter mixed face TBM for the Midtown tunnel drive. First launching the machine from Mount Pleasant in January 2013, the 1.5km drive to the Bayview shaft is Technicore’s longest single tunnel drive to date.

The TBM was then brought back for maintenance and relaunched in November 2013 for the final 750m drive to the Birch shaft.

The tunnel’s alignment follows the existing Hydro One underground cable route, for the most part, except where it had to deviate to avoid a bridge at Yonge Street. This meant exiting onto Yonge Street a block north, turning 90 degrees to go south on Yonge Street and then another 75 degree turn to hit the Birch shaft. Essentially the TBM had to manage two tight 32m curves, which actually proved impossible to do.

"We didn’t make the 32m radius but we did make 44m," MacFarlane reports. Standing at the bottom of the Birch shaft he explains, "We got to a point where we could have made the curve to come into this shaft but we would have gone onto private property which wasn’t allowed."

Technicore installed a rib and board lagging to support the excavation, and then did a cast-in-place concrete liner, 16in thick, using 80ft (24.3m) and 100ft (30.5m) forms to give the tunnel its finished diameter of 10ft (3m).

At the time of publication all concrete lining work will be completed on the tunnel, and the project is on track for completion this year.

Beneath Mount Pleasant Road Technicore built a breakout for Hydro One if in the future another utility tunnel is needed to connect to the Mid- Town tunnel. Each terminal shaft has provisions for a tunnel stub, as well.