Traffic congestion is about to get a lot worse on Eglinton Avenue as the city prepares to start digging a massive hole underneath the Allen Road intersection as soon as this fall.

The hole is just part of the process that crews will have to undertake to lift an 80-metre-long tunnel-boring machine up from deep underground during construction of the Eglinton Crosstown LRT.

While the city chose to use a tunnel-boring machine to dig the underground LRT shaft, minimizing above-ground disruptions to some extent, it has to be pulled out of the ground when it reaches the Spadina subway line and then placed back underground after the Yonge subway line, explained Jack Collins, Metrolinx vice-president of rapid transit implementation.

Using the tunnel-boring machine to go above or underneath the existing subway lines isn’t an option, due in part to a large storm sewer near the Spadina line.

“There really is not a lot of leeway to thread the needle,” Collins said.

So, a hole that is 20 metres long, 20 metres wide and 20 metres deep must be dug to pull the massive machine out in pieces, once it gets to a pre-determined point just west of Allen Road on Eglinton Avenue.

Metrolinx is hoping the city will approve a request to close north-bound lanes of the Allen Expessway for up to one year.

There are no dates for this closure yet, which still must be approved, but it would happen when the crew starts building the extraction shaft, which could be as soon as October or November of this year, after a contract is awarded in September, Collins said.

The Allen would remain open from Lawrence north, Collins said.

When the tunnel-boring machine goes back in the ground again, on the east side of the Yonge subway line, the hole required to get the machine back underground will be even bigger than the one needed to take it out.

The launch shaft, near Eglinton West Station, will need to be an estimated 90 metres long, 20 metres wide and 20 metres deeps, according to Metrolinx.

That’s just 10 metres shy of the length of a Canadian football field.

Luckily for drivers, the giant holes will be partially covered by pre-cast decking panels.

“It will be a reasonably smooth surface that you can cover up, so that traffic can run over it, but there will be some traffic impacts,” Collins said.

There will be delays on what is already a busy section of road, he warned.

“I know it’s going to be tough, because if you drive there now, it’s not the easiest place to drive in the peak hours,” Collins said.

Besides the holes to get the boring machine in and out of the ground, which Collins said will have the biggest effect on traffic, anyone who needs to travel along Eglinton Avenue will notice other disruptions as LRT construction proceeds.

Each time the tunnel-boring machine nears the site of a future station, crews will need to pour a concrete headwall, which will eventually become one end of the LRT platform.

After the tunnel is complete, stations will also have to be built, with each station taking approximately 2.5 years to complete, with activity happening both above and below ground.

And, because the tunnel-boring machine can’t be used near the Spadina and Yonge subway lines, this section of the track will have to be excavated using a cut-and-cover method.

In cut-and-cover, crews dig out the tunnel from above ground, but are able to place more decking panels down, which will allow traffic to run overtop of the work site.

Traffic, at times, will be reduced to a single lane in each direction along Eglinton Avenue, Collins said.

When the $8.2-billion project is completed in 2020, the 25-kilometre transit line will cut travel time in half for transit users on the east-west artery between Black Creek Drive in the west and Kennedy Station in the east.

It will also allow for easy connections to the Spadina and Yonge and subway lines.

In the past, Collins has said that the 2020 target is “aggressive” but is one that Metrolinx is committed to, despite a Toronto Transit Commission report suggesting that construction of the Eglinton LRT is more likely to be complete by 2022.

Want to learn more? Metrolinx released this YouTube video showing the tunnel-boring process: