Let’s get out the tape measures – the really big tape measures.

How big does a subway tunnel need to be? Should you dig two tunnels to fit one train each or one tunnel to hold both? And, should the tunnel be big enough to contain the stations, or should they be dug out later?

Tunnel size is among the most important decisions that have to be made before subways are built.

Metrolinx planners have been working through a list of possibilities to deliver a massive subway expansion in the Greater Toronto Area. The $28.5 billion subway program includes the Ontario Line, the Scarborough Subway Extension, the Eglinton Crosstown West Extension, and the Yonge Subway Extension.

As technology evolves for boring and mining, subway planners have more choices to make about how big tunnels should be. Tunnel types can vary from line to line. A twin tunneling approach, with two separate caverns for trains going opposite directions, is the most prevalent, but it is not the only option.

Most of the current TTC subway system consists of parallel twin tunnels, where each track has a hole to itself, six metres in diameter.

Richard Tucker, project director for the Ontario Line and Scarborough Subway Extension at Metrolinx, explains that groundwater in the soil surrounding the tunnel is an important consideration in determining its size.  

“Imagine that under the surface, going through groundwater and soil, the subway tunnel is basically a big hollow straw,” Tucker said. “It is going to float, so the weight of soil above it must be heavy enough to keep it down.”

To keep the tunnel stable, the general rule is that the metres of soil above should be at least equal to the diameter of the tunnel. So, each six-metre tunnel needs six metres of earth on top to keep it in place.

This is important for planners who are trying to minimize depth, because the further down the tracks are, the longer it will take for passengers to get out of the station.

Bigger tunnels need to be laid deeper. A tunnel built to fit two tracks inside a single bore would typically be 10 metres in diameter and need at least 10 metres of ground cover.

“That may or may not be a problem, because sometimes as you go below utility lines and building foundations you have to go deeper anyway,” Tucker said.

Another consideration when looking at twin bore tunneling is that it removes less soil from the ground than one larger tunnel that is, big enough for two trains.

On the other hand, once twin bores are drilled, they require extra digging for special structures.

“With a twin tunnel route, you have to add cross passages every so often for safety features, such as emergency exits,” Tucker said. “Then, we have to add special structures – mined or by open cut – for the crossovers that move trains from one track to another.

“That extra work is very specialized.”

Tunnels must also accommodate pocket tracks that give trains a place to sit during off-peak hours before going back online for rush hour. They can also be used to store service cars for tunnel and track maintenance.

“When you have two tunnels, with one track each, and then you need a third in some places, you have to mine caverns or dig from the surface to create those structures,” Tucker said.

As for cost, drilling the smaller twin tunnels starts out less expensive than one larger single hole, but there is no easy rule for determining if the cost of adding these structures outweighs that advantage, or if it is even feasible to build them, until you look at the geography of the route and the position of stations.

“Depending where you are in the city, you may not even be able to dig a hole for these special structures,” Tucker said. “The more of them and the more cross passages you need, the more expensive the twin tunnels become.”

“But, if you have stations that are close together you don’t need as many cross passages.”

The larger, single bore tunnels simplify these additions by removing the need for them to be dug out from above.

“Putting both tracks within one tunnel allows us to put crossovers anywhere without disrupting life at street level because we don’t need to dig down to build cross passages.”

Conditions below the ground can also influence the choice of tunnels. Metrolinx looks very closely at the stratigraphy of the route – which is the layering of soils in the ground – before deciding on a tunneling approach.

“You may look at the stratigraphy and find you can fit a six-metre tunnel in an area that’s favourable for boring, advancing with good efficiency, productivity, and not a lot of risks,” Tucker said. “But then you might find that as you go to a larger diameter, you would start to encounter things you would prefer to stay away from.

“Generally, an area with interface between bedrock and soil above, while workable, is one you would prefer to avoid. Aquifers and sandy soil may cause excess water to enter the tunnel boring machine’s working face.

“And, under certain ground water conditions, soils like silt and clay may tend to clog the machine.

“So, that’s another consideration that goes into thinking about what you want to do.”

Curvature of the route can also factor into the decision, if the tracks have sharp turns.

“A small diameter tunnel can go around tighter curves,” Tucker said. “When comparing a smaller tunnel boring machine to a larger one, it’s like driving a passenger car versus an 18-wheeler.

“Smaller is more manoeuvrable – both in the vertical and horizontal lines – though the latter is limited by the slope that the train can manage for acceleration and braking.”

The six-metre twin tunnels also have an advantage in that the boring machines and expertise are widely available.

“The bigger the machine gets, the more specialized a manufacturer you need, and specialized operators too,” Tucker said. “The number of people with experience with six-metre machines is bigger than for larger machines, just because there are more tunnels of that size.”

Those are a lot of considerations, but after detailed studies, the pros and cons of each approach come into focus.

“When you start to think about availability of machines, the type of ground you’re going through, how much surface disruption areas along the line can accommodate, how deep the stations have to be, depth of the creek beds that the tunnel alignment crosses, how many crossovers and special tracks you need and how many stations are on your line, you might get an idea of your ideal tunnel and arrangement for your tunnels,” Tucker said.

While the typical choice has been between twin six-metre tunnels and larger 10-metre single bores, there is a new trend towards larger megabores.

“With a bigger bore, you can actually start to put the station right in the tunnel,” Tucker said. “There is enough room for both the tracks and the station.”

“This gives us a lot more flexibility into where to put the station and it minimizes construction impacts on the surface.

“It can be more cost effective, but the bigger and deeper tunnel is more expensive. You save on the station and pay more for the tunnel, so you have to look at all the benefits and drawbacks.”

In some cases – most notably along the Barcelona Metro’s Line 9 – megabore tracks run side-by-side and then, when approaching the station, one line goes on top of the other. This makes better use of the tunnel space, as the platforms are not side-by-side.

“Passengers don’t really notice the vertical change because the lines are designed so that all the comfort criteria are met,” Tucker said.

Barcelona set the standard for a megabore at 12 metres when Line opened in 2009, and the size keeps growing.

“It’s like a space race or the way cell phones have gone from 3G to 4G then LTG and now 5G,” Tucker said. “They keep pushing the envelope on what a megabore is.”

By 2015 the largest diameter on record was 17.6 metres and there is talk that they could reach 19 metres, Tucker said.

While there are some real benefits to the megabore approach, there are also more risks. The larger tunnel boring machines are relatively new technology, after all, so the chances of encountering unexpected issues is somewhat higher.

All of the pros and cons must be weighed.

To ensure that the best decisions are made for each individual project, Metrolinx uses a separate request for proposal process for every line, with prospective bidders submitting their plans.

“Each line has different considerations, so it’s not a matter of one size fits all – we expect bidders to customize designs that suit the characteristics of each line,” Tucker said.

“There’s a whole bunch of balancing factors that go into choosing the best tunnel and each situation is different, which is why across the current subway network you don’t see the same tunnel for every line.”

As Metrolinx moves forward with four priority subway projects, different local factors will affect the respective tunnelling decisions.