How Big a Hole Do We Need?

At its meeting on July 9, the TTC approved purchase of four tunnel boring machines from LOVAT Inc. for construction of the Spadina Subway extension at a cost of about $58-million.  There was considerable discussion about this expense from the point of view of whether any could be recouped after construction, or what commonality there might be with Transit City requirements.

Various tidbits came out during the questions to staff from Commissioners.

The Sheppard tunnels are 5200mm inside diameter, whereas the Spadina tunnels will be 5400mm.  The larger bore is required both to meet current fire code, and to allow trains to travel through curves with sufficient clearance.  (The Sheppard line is, pardon the pun, rather boringly straight.)  The larger tunnel size adds about $35-million to the cost of the 6km of bored tunnel on the Spadina line.

Transit City tunnel size will be determined by the dynamic envelope required for its cars and for the overhead power supply.  These tunnels may not be the same diameter as those on the Spadina subway, but more to the point, the construction period for both Spadina and Eglinton overlap and using the Spadina machines for Eglinton will delay that project.  It is conceivable that the Richmond Hill subway, if funded, might inherit the machines.  Otherwise, the TTC expects to be able to sell them for about 30% of their original value.

This question will also affect the Sheppard tunnel at Don Mills, a short but necessary piece of work to get under Highway 404.

The TTC has canvassed the world market for second hand tunnelling machinery, but none which has the required bore diameter and soil condition design is available.

In a conversation after the meeting, I learned that although the single large bore tunnel (13m) proposed for Eglinton might be feasible, this large tunnel greatly increases the cost of removing spoil (earth and rock) because the tunnel structure is much larger than would be the case for two single tunnels.  In turn, this begs the question of how much of the Eglinton line will be built cut-and-cover so that it is not dependent on the availability of tunnel boring equipment.  We shall see in the fall when the next set of community meetings come around for the Eglinton corridor study.

6 thoughts on “How Big a Hole Do We Need?

  1. Cut and cover is a fraction ot the cost, isn’t it? Makes you wish we’d thought of that before we let suburban malignancy spread… It makes me wonder what is the cheapest way to go (for this extension of no use to 90% of Toronto residents):

    – a drilled estension
    – cut and cover up Keele
    – sending the extension up the GO ROW

    Money is so hard to come by for these projects, from Liberal gov’ts much less Reform, that going cheap and less ideal is not a bad idea. At least we’ll get something like a passable subway system for Canada’s most populous region. That’s why I’d like to see the DRL follow the train lands, because if that needs to be drilled we’ll be waiting another two decades than the two decades we can now expect. It’s like waiting for the Leafs to get the cup!

    Steve: Actually the rail lands would be very difficult to do cut-and-cover because the rights-of-way are full and there is no place to temporarily relocate the train tracks while we build a rapid transit box structure underneath.


  2. Why doesn’t the TTC use Bombardier’s Primove technology in the tunnel? It’s a no-brainer. They are … STUPID STUPID STUPID STUPID.

    Steve: Because the power losses through an inductive coupling are very high. This system is intended for short sections of track in historically sensitive neighbourhoods, not for extended operation.


  3. If they decide to build the parts of the Jane LRT underground or parts of the Don Mills line, they can save the boring machines.

    There is hardly any news about what is happening to the Jane line.

    Steve: The Jane line is far off in the future. Don Mills gets some attention because there was already a Don Valley Transportation Corridor study underway, and because of its relationship with the Sheppard and Finch lines. Scarborough Malvern gets attention because UTSC would be a venue for the Pan American games, should Toronto be awarded them. If that falls through, UTSC will have only bus service for quite some time.


  4. I think you misunderstood my thoughts on using rail ROWs: you wouldn’t bury at all.

    Steve: The problem is that the Weston rail corridor will be full with the planned expansion for GO and ARL services, and even requires some expansion just for that, let alone for a surface subway/LRT. There are also issues with running equipment that is not designed to mainline railway standards in the same corridor as trains. All of this has been discussed before.

    If the DRL goes up the Weston corridor, it will be underground.


  5. You mentioned that “although the single large bore tunnel (13m) … might be feasible, this large tunnel greatly increases the cost of removing spoil … than would be the case for two single tunnels.”

    While true, this is only part of the cost of the tunnel section. A decision such as this (single or twin bore) should be based on the total cost of the construction.

    For single bore: two tunneling machines, two crews, two medium-sized shafts per station.

    For twin bore: four tunneling machines, four crews, one giant pit per station.

    Althought the single bore creates more spoil than twin bore, this needs to be compared with the amount of spoil from the cut-and-cover construction, not to mention the economic disruption caused by cut-and-cover.

    Steve: I agree. The TTC is still deciding which way to proceed, and I wouldn’t be surprised to see more than one approach as there are a few areas where the line would be very close to the surface and cut-and-cover may be a much simpler alternative.


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