diminutive
Active Member
http://stevemunro.ca/?p=2372
"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."
In short, dirt disposal is a non-trivial cost.
Yet other TTC reports have specifically cited single tunnel bores in Madrid and elsewhere as part of the cost difference to Toronto... It's not that I don't understand the geometry of why a single tunnel will have proportionately more waste. A 13m tunnel would have roughly twice the cross sectional surface area as 2x 6.7m tunnels.
But it is odd that the TTC would specifically cite single bores as a cost saving measure then go on to say it would be more expensive.
Also, 13m seems quite large...
Anyways, as far as DRL goes, it still seems like simplest would be above ground through the rail-corridor.
Going underground really should be the absolute last resort.