Quote:
Originally Posted by lrt's friend
OK, the reason why we need such a deep tunnel is because of the Rideau Canal. Why not convert the MacKenzie King Bridge into transit only and build a shallow cut and cover tunnel east and west of the bridge and run the trains over the bridge? Because the tunnel will be more shallow (you can rise to the surface faster) and will not run under the canal, the overall tunnel length will be shorter and therefore cheaper. Is this feasible? This would also make for shorter escalator rides, saving travel time.
Portals for the west and east tunnels could be placed just east of Elgin Street and just west of Waller Street.
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Well, I’m afraid that your query begins with an invalid assumption; that the depth of the tunnel has been dictated by the Rideau Canal.
In fact the main reasons the tunnel needs to be so deep are its alignment, including the station locations, and its method of construction.
Take a look at the latest profile Staff has provided for the Rideau area:
This image includes an additional option to what the original image provided
The dark blue lines outline the Staff proposed tunnel and station locations. The red line is the level of the ‘Top of Rail’. The green line is the current ground level and the black lines are the start of weathered rock (upper line) and bedrock (lower line).There are two purple ovals marked in about the center of the profile. These are two large diameter sewers which run roughly north-south. Just left of half way, you can see in the profile that the Rideau Canal is nowhere near as deep as the two sewers.
The sewers are actually the limiting factor; and the method of construction and the pre-determined position of the station have made things even worse.
Although the tunnels themselves will be bored using a Tunnel Boring Machine (TBM) which leaves a concrete liner behind it, the station volumes will be mined out using conventional mining techniques. This means drilling and blasting to remove all the rock between the two tunnel bores and to bring down the overburden to the required height. Because there is blasting, the rock will become fractured, leading to the very real possibility of small cave-ins leading to ‘chimneys’ which extend higher than the desired height. If there is not a sufficient thickness of buffer rock between the blasting and anything above it, then the chimney could actually rise up to cause damage to whatever is above it. Worse still is that the existing foundations, sewers, etc. above the station locations have generally been dug down using blasting which has probably already fractured the rock.
So, in short, the method of creating the stations is to blast UP, into the rock above. If that rock has already been fractured by blasting DOWN from above for other reasons, then there is a good possibility that the rock will not hold itself up long enough to add supports. The tunnel sections (between the stations) are not under the same constraints because of the concrete liner which is built immediately as the TBM bores the tunnel.
The light blue lines in the above profile are my musing about moving the station west far enough to have the actual station not directly under the sewers. In this case, only the tunnel would need to pass under the sewers so the tunnel and station could be raised about 10 metres. This, however would put the station under the Confederation Square so the City will not accept it; it doesn’t service The Market and there is no station east of the canal, i.e., along Rideau. (And, heaven forbid we add an extra station under Waller.)
Anyway, my point (if you have stuck with me this far) is that the Rideau Canal is not a controlling factor in the depth of the tunnel system. In fact, if the Rideau Station were pushed east a bit, it might be possible that a shallow (Cut & Cover) tunnel could pass under the canal (dig it in the winter when it won’t drain the canal), and over the sewers. I do not know what other infrastructure is under Rideau Road to verify such an option, though.
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