After years of designing and planning, Hydro One’s Celtic Tiger Tunnel Boring Machine (TBM) was recently lowered into the ground, launching construction on a new underground tunnel that is expected to provide reliable electricity supply in downtown Toronto.
While construction activity on the $120 million infrastructure renewal project began in 2022, early engineering work began in 2018 and the environmental assessment was completed in 2020.
“This project has been in planning for over three years,” said Andrew Spencer, vice-president transmission and stations with Hydro One. “We’ve undertaken an extensive process…and through that we worked with environmental consultants, engineers, engaging with the community, business owners, residents, municipal staff to get their advice and input.
“So far everything is trending well from a cost and schedule perspective,” Spencer added.
Hydro One is working with the City of Toronto and Toronto Hydro to co-ordinate construction needs and the use of the tunnel boring machine as well as to minimize disruptions to residents and businesses.
The new tunnel, which will be about 2.5 kilometres long and located 85 feet below ground, will house new high voltage electrical cables to replace older ones.
“The depth is based on geotechnical and hydro geological studies that were conducted as part of the early engineering works,” Spencer said. “Underneath the downtown core is a myriad of infrastructure: electrical, telecom, water, existing and future. Some of the co-ordination on this was considering future transit projects in the area. We had to co-ordinate with Metrolinx as some of this crosses future locations for the Ontario Line.”
The tunnel boring is expected to be completed by early 2024 and the installation of the new cables is expected to be completed by 2026. Hydro One says the project will provide energy to “Hospital Row,” city hall and the financial district. It will run between Esplanade TS and Terauley TS which goes from The Esplanade and Lower Sherbourne Street to Bay and Dundas streets.
“This project is replacing existing transmission infrastructure that was installed 70 years ago,” Spencer said.
Part of the EA process included considering alternatives.
“There were multiple ways we could have constructed this project,” said Spencer. “We could have chosen what is commonly referred to as an open cut, when you would either cut in the roadway or the road allowance, like on the sidewalks and boulevards through the city. We would have been able to just excavate down six, eight, 10 feet…install all these cables, backfill it and then repave the road, refinish the sidewalks etc. The downside of doing that is it’s extremely invasive to traffic, residents and businesses.”
Using the tunnelling method versus the open cut method does cost a little more but minimizes the impact on the city.
“It’s also going to be a bit more resilient in the future, a bit more maintainable,” said Spencer. “This is generally highly reliable infrastructure. In the event that it does need some attention over its typical 50-year-plus life, having it in the tunnel allows us much easier access for our maintenance crews to conduct any inspections or repairs.”
One of the first construction activities undertaken was building the entry and exit shafts for the tunnel boring machine at both ends of the tunnel.
“The crane actually lowering the boring machine down into those shafts, that was an extensive piece of construction,” said Spencer. “Each end had to be excavated, shored up as it was being built and all the dewatering activities that go with the standard excavation work had to be undertaken. That work was completed in 2022…in preparation for the launch of the TBM.”
The boring of the tunnel itself is expected to take about a year. The machine moves about 40 to 50 feet a day depending on the soil conditions and runs 12 hours a day from 7 a.m. to 7 p.m.
“Then we do all the forming that will ultimately be backfilled with concrete and will form the liner of the tunnel,” said Spencer. “We’re drilling at 12-foot diameter hole and by the time you put the concrete lining around that tunnel, it’s going to shrink into a 10-foot-diameter so the finished concrete walls are about a foot thick on either side.”
After that the installation of drainage systems and electrical cables begins.
“These cables are probably about five-to-six-inch diameter,” said Spencer. “Those get installed throughout the tunnel. They get attached to the walls so they are safe and secure and maintainable in the future and then we install…lighting systems, ventilation systems. Because this is underground, we need to make sure there is a supply of fresh air for any future maintenance or inspection purposes.”
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