There have been some good comments – from those who actually stayed on the topic of a Bank Street Subway.
• TBM don’t work well for shallow tunnels: A TBM needs about 1.5 X the tunnel diameter for overburden. If the bore is 5.5m, for a finished interior diameter of 4.5+m, then the top of the TBM should be at least 8.25m below grade.
• People get confused when they must choose the direction of travel when on the surface: Although I have the opposite opinion, since the trains would run in the same direction as the traffic above them. If you want to go north, you go down on the north-bound side of the street. However, given that the top of the tunnel will be at least 8.25m down, also that utilities are usually buried within the top 3m under a road, there would be about 5m of remaining height for an underground Concourse that can then lead down to a central platform. These would be built by driving piles at night, then using cut & cover to quickly build the top of the Concourse upon the pile caps (and perhaps the side walls), then the surface is restored, and then the chamber and station are excavated from below. The Concourses would be under intersections, with an entrance on each corner. Let’s put an estimated price of $25M for each of the ‘Intersection Stations’.
• The cost estimate: Let’s look at an Ottawa example, the CSST, that was bored with a 3m (finished inside diameter) TBM. Over 6 km of CSST tunnels cost $232M. That cost included other paraphernalia, specific to a sewer work, so let’s reduce the cost to $220M for 6 km of 3m tunnel. Assuming proportional costs, 4.5m bores would cost about 2.3 times what a 3m bore costs. Then, the added length increases the cost by 7/6 – and, of course, there will be two tunnels, so we double it. Therefore, the total cost for two finished 7 km X 4.5m tunnels should be about $1.2B. That cost is only for the tubes, the stations (as per the previous point) and all of the rail infrastructure is additional to that.
• Curve the Billings Bridge Station under the Transitway station or keep it under Bank Street: This is a matter of where the line could be extended in the future. Previously, I suggested that it could head south along the SE Transitway, to provide a connection with the airport. In order to do that, I imagine the line heading west from Billings Bridge, built into the slope behind the Data Centre Building, under Bronson, and curve to meet the existing Trillium Line Mooney’s Bay Station. From there, it would steal some of the SE Transitway corridor to get to South Keys. That is a relatively inexpensive extension that would better serve the Baseline BRT and the airport. That was my vision for the line, but others have equally valid ideas also. That will be part of the discussions in this thread.
• The elevation difference between a station above the Confederation Line and the bottom of the Ottawa River is too great: Quite true. Bank at Queen is at about 72m Above Sea Level (ASL) and the surface of the river is roughly 48m ASL. Assuming a depth of 3m-5m for the river, that would mean that the bottom of the tunnel would need to dive from 58m ASL to 30m ASL in about 400m – which is a grade of 7%! Too great. It seems that the station might need to be below the Confederation Line, if this train extended across the river. The bulk of the line could still be shallow, however, with this single station being deep. It would need to be mined out from the tunnel, up to the Lyon and Parliament/Parlement Stations. Because it makes use of the other stations for most of the vertical circulation, I am giving it an estimated cost of $40M.
• An alternative to the elevation change problem: It might also be possible to have the Bank Subway remain shallow, and above the Confederation Line, but exit the hillside and cross the river on a descending, elevated structure. It would seem from the REM construction that elevated structures are relatively inexpensive. This would offer another viewing option to see the river and Parliament Hill (from the north-west).
• Not including the Aylmer Tram with the Bank Subway: This is certainly an option. However, if the new line stays only under Bank Street, it will be less attractive for the Feds to pay a large portion of the cost. As an interprovincial line, it might get preferred funding – especially if it connects major Federal complexes.
• There is no good Business Case for a Bank Street Subway: Maybe not for the full length of Bank Street, but there might be good reasons to build enough of it, since it would provide superior public transit between the seat of Federal Government on the Ontario side and the large federal campuses on the Quebec side of the river. It also provides the ability to add a higher-order of public transit to a principal street that can then be narrowed to vehicles and made friendlier for active transport. (Then, maybe, some of the chatter about bike lanes on Bank Street would be relevant to this thread.) TOD could be encouraged along the section of Bank Street that is served by the subway. It can take some of the load off the Confederation Line as fewer south-end residents will be transferring from the Trillium Line at Bayview – to travel only two or three stations. This last point might be quite important as the Confederation and Trillium Lines are extended. More people will be transferring and there will need to be space in the trains that arrive – empty space that will need to travel all the way to Barrhaven/Kanata and back.