Despite the millions of people travelling through them each day, subway tunnels are among the least visible parts of a transit system.

While some more obvious differences about tunnels might be spotted while waiting on a platform, few riders ever know the backstory that makes each subway tunnel different.

As technical teams map out the requirements of each new or expanded subway line, they make choices about the tunnels based on geography they will go through.

Even the answer to the question of whether to go above or below ground often depends on the surroundings. For example, it includes a stretch of the Ontario Line that will be built in the GO Transit corridor, alongside tracks that are currently used for the Stouffville and Lakeshore East lines.

“If you use the infrastructure that’s available, you can go a lot further before you run out of money,” said Duncan Law, head sponsor for four priority subway projects at Metrolinx. “The benefits are greater with added distance, as more people can access transit for employment or school.”

The evolution of plans for the Scarborough Subway Extension and the Eglinton Crosstown West Extension reached an important milestone today (August 5), when the Government of Ontario released shortlists of three bidders for each project that have been shortlisted to advance tunnelling work. They are invited to respond to a Request for Proposals and detail how they will deliver the projects. Upon evaluating the proposals received, Infrastructure Ontario and Metrolinx expect to award these tunnelling contracts in mid-2021.

As teams prepare their proposals, selecting the right technique for digging each tunnel section is a key decision.

Some tunnels are drilled with tunnel boring machines (TBMs), while others are mined out in a progressive excavation technique called sequential excavation method (SEM). Also known as the Austrian method, SEM is used to mine out specific sections of the tunnel while keeping the ground stable during the mining.

There are advantages and disadvantages to both methods.

As a general rule, the speed advantage of a TBM, once it is in the ground and digging through 10 to 15 metres per day, makes up for the quicker staging time and lower cost of mining tunnels of two kilometres or more.

“A TBM should have a higher daily production rate than SEM once it gets going, but the cost is the year or so to get the machine, set up the launch shaft and launch the machine,” said Richard Tucker, a project director for the Ontario Line and Scarborough Subway Extension at Metrolinx.

Each TBM is custom built for a particular job. The size, type of cutter and engineering details are selected based on each project’s need. This adds time to the construction schedule, as does the need to build launch shafts and get sufficient power to the site.

“It takes you about a year to get one, but once it’s started and it’s going, it tends to be an efficient and safe method of work,” Tucker said.

There are several different types of TBMs, all of which are customized for the ground conditions they are anticipated to encounter over the length of the tunnel drive.

Some machines have faces designed to go through rock, while others go through soil saturated with ground water. Others have mixed-face models that can be made for tunnels that will go through different underground conditions.

TBMs can be as long as a football field, making them essentially moving tunnel factories. Each has a rotating cutter at the front end, chewing through the ground, an auger to pick up the soil, and a conveyor belt at the back end to clear out the debris.

“They also have arms built in that simultaneously put concrete liner segments in place to form the tunnel walls,” Tucker said. ”The TBM has a big suction cup that will just place the liner segments like LEGO blocks, stick them on the wall and bolt them in place, placing them repeatedly in rings.

“It cuts, fills, jacks itself forward a couple of meters in a repetitive and highly efficient cycle,” Tucker said.

Boring below Toronto is nothing new, including the work on the Eglinton Crosstown light rail transit (LRT) project, the technology continues to fascinate the public. In the Crosstown project, crowds turned out when the four TBMs – nicknamed during a contest as Dennis, Lea, Don and Humber – were put into the ground between 2013 and 2015. After digging 5.75 meter in diameter tunnels, at a pace of about 10 meters a day, they completed their underground journeys in 2016.

To see a video as they were brought back out of the ground, just go here.

Roadheaders – machines originally designed for the coal industry and now used for SEM mining – can be ordered more quickly than TBMs because they are not so customized.

“One advantage of the sequential approach is that you don’t have to wait for a TBM,” Tucker said. “You can get a roadheader that doesn’t need to be custom built for the application.

“You may get specific teeth and a motor for certain ground conditions, but they are generally available, so you can order one and put it in the tunnel. They give you a little more flexibility.”

Successful mining with a roadheader depends on a skilled operator.

“A roadheader looks like a big arm, with a big spikey rotating ball, like a hedgehog, on the end with cutter teeth,” Tucker said. “An operator moves the arm around and chips off the rock or soil with the rotating ball”

Mining techniques require support to create a canopy above the tunnel as it is dug. Supports can include rock bolting, steel arches, metal mesh and pipe canopies.

“In SEM they don’t take the whole tunnel cut-out at once. Instead they take this part and that part in a certain sequence, so they can guarantee stability of the ground they are going through,” Tucker said.

After the tunnel is dug, the liners are installed in big arched segments that can be steel or concrete.

Picking the type of tunnel lining is another key decision. Some thicker linings are designed with less reinforcement and some are thinner with more reinforcement. This decision impacts the size of the external tunnel diameter.

In determining the size of the tunnel there is one essential rule.

“The bigger the diameter, the more earth you remove, and nobody’s interested in making it any bigger than it has to be,” Tucker said.

Tunnels all have to be big enough to accommodate the trains, a flat concrete surface at the bottom for the tracks, a safety walkway that can also be used for maintenance access and emergency evacuations, and systems for power, signalling, lighting and communications.

As the ground conditions and urban environment change from place to place along the line, different tunnelling techniques, alignments and tunnel dimensions need to be considered to optimize the design and benefits of each tunnel.

When going through glacial till, which is a mix of boulders, silt, sand and clay, planners have to think through differing conditions, including ground water and soil stability.

That’s a lot to consider, but things can be simpler when drilling through solid rock.

“Even though rock is harder, it has some benefits in that it stands up around the tunnel and it is likely more consistent than soil, where conditions vary as you go from sand to silt to clay to gravel,” Tucker said.

“Generally, the rock in Toronto is more consistent from one area to another, even though there can be fractures or valleys, and that can change the type of boring machine you consider using, the size of tunnels, and the means and methods for advancing the tunnels smoothly and efficiently,” Tucker said.

There many variables to consider, and to the decision-making process, Law said the fundamentals are the same whether planning a tunnel or a bridge.

“When you build a bridge, you look at various options,” Law said. “There are many structures that can give you the same benefit of going from A to B, but they all have different elements, like cost, appearance and engineering process.

”In evaluating options, for every $100 we spend, we have to ask if we would get substantially more benefit by spending $110 or if we could get the same outcome for $90.”

Law and his team ensure that all choices are properly reviewed. They seek out best practices that can both mitigate risk and capture the benefits of all new and proven technologies that are available.

“Saying ‘I built it this way before so I’ll just build it this way again’ isn’t the right way to go,” he said. “You start by asking ‘what are the options?’ and defining the cost of doing it properly, then explaining your decisions, alongside the benefits you generate.”

The time and care taken to get the early planning decisions right can pay off as the projects evolve. It is why sponsors are so active in the early stage of a project lifecycle.

By building flexibility into the planning process and concentrating on the technical details and characteristics of each of the four priority subway projects, people can expect different decisions to be made for each line, including different tunnel diameters, alignments and digging techniques.

In fact, given geographic changes from neighbourhood to neighbourhood, you can’t necessarily expect to see the same type of tunnel used for all sections of the same line.

Planners have an array of options to choose from including single, twin, medium, large, and mega bores.

Look for an article that explores these options and the factors that go into making the right choice for each stretch of a subway line in an upcoming post on Metrolinx News.