Quote:
Originally Posted by acottawa
Two points
A lot of people seem to make the assumption that shallower is cheaper. Is there any evidence of this? Deep tunnels seem to be the overwelming choice of almost every transit system built or planned in an urban area in the last 30 years (Washington, the recent Toronto lines, the gold line in south africa, Crossrail, the new rail tunnel under the Hudson, numerous lines in Asia, etc.). A shallow tunnels requires not only several years of hugely disruptive road closures but often relocation of all kinds of underground infrastructure that has been put under city streets over the decades.
I'm sure whatever engineers are hired for the project will be experts in this type of work and they that will know how to deal with any rock problems may exist (or change the project if the problems are insurmountable). There is nothing new or innovative about the planned line (unlike some of the zany schemes floating around) - it uses technologies that have been proven all over the world.
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There are two big reasons that a lot of the newest tunnels have been bored instead of using Cut & Cover technology: 1), there are already lots of other things closer to the surface – major infrastructure, other tunnels, etc. – which can’t be disturbed during the construction and so the tunnel must go under them; and 2), avoiding disruption to the surface is worth the extra cost of boring. Yes, boring does cost more for (relatively) shallow tunnels than Cut & Cover ones. If the tunnel is to be deep, then boring can be less expensive than a Cut & Cover equivalent. Reason number 1) above also usually provides the reason that a tunnel needs to be deep. I would imagine that through a green-field, no city (not even Ottawa) would opt for a deep tunnel. If there is to be a tunnel at all, it would be constructed using Cut & Cover.
So, once it has been determined that a tunnel should be bored, the cost of such boring can vary wildly based on the site conditions. If one is boring through a ‘solid’ mountain of competent rock, with no worries of water infiltration, then the cost can be (relatively) small. The ‘hard-rock’ TBM simply needs to bore through, leaving a tunnel in the rock. If there is the possibility of the rock not supporting itself, then extra costs are required to stabilize the tunnel’s roof. This can go to the extreme where a concrete liner must be constructed along the bore as the TBM passes. This is also the method used for tunnels through looser materials which could collapse after the TBM has passed. The possibility of water infiltration or very loose materials can add further expense because a more specialized TBM is needed. These Earth-pressure Balanced (EPB) TBMs create a higher pressure region in front of the TBM to hold back the water, etc. until the concrete liner is in place.
The condition through which the TBM must bore is obviously a factor in the cost of the tunnel. It might also be a factor in the tunnel depth since it might be less expensive to stay lower and bore in rock instead of needing a more expensive EPB TBM and boring closer to the surface. If a tunnel must be bored through two distinctly different zones of material, then there are machines which can do it, but they are not as efficient as dedicated TBMs and so bore more slowly, potentially increasing the tunneling costs.
Then there is the cost of out-fitting the stations: If the stations are close to the surface, then installing elevators/escalators/ and stairs will be less expensive. Not only because of the cost of the items themselves, but for the cost of cutting a connection to the surface. Ventilation is going to be more expensive for deeper stations since larger motors will be required for the blowers to lift the mass of air higher.
There are too many variables to state that for every tunnel it is cheaper to do ‘thus-and-such’. Currently the City’s estimates are based on what are considered to be ‘reasonable guesses’. More information is being gathered through the geotechnical studies to verify the conditions more accurately. This will allow for more precise estimates to be generated. Remember, like any big project, this is an iterative approach; City Council needs to have some idea of a cost so they know whether it is even worth-while to look into the possibility; then as more information is found, the estimates are refined.
Of course, these are only the costs of creating the tunnel in the first place; which is a one-time expense. The operation of the station is an on-going burden and can amount to many times the cost of the tunneling over the many year life of the tunnel.
On your second point: Absolutely, the work will be undertaken by professionals, and the technology of tunnel boring machines is fairly mature, but there are many variables, some of which are difficult, if not impossible to determine before the process begins. There are excellent examples of recent projects which could have been done without incident if other decisions had been made, but that didn’t stop problems from cropping up in those projects. Hind-sight is always 20/20. For example, if the Ontario Hydro water tunnel at Niagara Falls had been lined, then it would probably not have had the problems of cave-ins; but it would have been a much more expensive estimate originally. That project was also given to competent tunnelers who were using mature technology and it was planned based on the best available information.
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