I keep reading/hearing comments about the depth of the tunnel so let’s take a serious look at that:
A general rule of thumb is that your TBM should be at least 1 to 1.5 tunnel diameters below any obstacles or grade. It looks as if Staff is planning for a 6 metre bore so that is the first constraint – the tunnel should be at least 6-9 m below grade or any existing underground utilities or obstacles. But the tunnel is probably not the prime constraint; the stations are since they will be mined out to a radius of 10-12 metres. If the same rule of thumb held, then there should be 20-24 metres between the top of the stations and anything we don’t want falling into those stations.
So what is determining the depth of the tunnel and stations?
Because the Downtown-West Station is of central-platform configuration, with a high arch over the center, the station is fairly wide and needs to be sensitive to the depth of the buildings on either side of it. I don’t know how many levels of underground parking exist under the Constitution Square Building, but there are five underground parking levels under the Place de Ville. If I assume three metres per level, then the building foundation would descend 15 metres. This constraint is shown on the diagram in fuchsia.
And now the bad news: The building’s depth was likely achieved by blasting. Blasting fractures rock and, unfortunately, it does not just act in an upward direction. The rock under the lowest level of the building will also be fractured. Given that the stations will be mined out from inside the tunnel, up and between the two tunnels, there will be times when there will be very little support of the overburden material. If it is fractured, the rock might start to fall. Leaving a larger buffer is a safe thing to do. This is much less of an issue for the actual tunnel since, I believe, Staff has decided to use a TBM which builds a tunnel lining out of precast segments. This means that the tunnel overburden is always supported so it doesn’t really matter how loose the overburden is. (The lining became a requirement when the tunnel was extended through the alluvial deposits of Sandy Hill.)
So the steep descent (5%) and the depth of the Downtown-West Station are needed because the station extends beside the Place de Ville with its five levels of underground parking.
The Downtown-East Station has the same kind of constraint under the Sunlife Building since there are four levels of parking under the east tower. (The west tower sits over only three levels of parking.)
Since there is extra buffer room above this station, it might be possible to raise this station, but if the next station, Rideau, has a lower constraint, then this station would be kept low to avoid the ‘roller-coaster’ effect.
You can see from the Rideau Station picture that the building foundations are not the main constraint for this station’s depth; it is the two sewer pipes. These two pipes are nearly 20 metres underground and there would have been lots of blasting involved in laying them that deep. Notice that the station is directly under the pipes.
Thus the depth of the tunnel is really being dictated by the positioning of the stations. Had the station locations been chosen differently (say, based on what was under ground) then the tunnel could be less deep. As it happens, the east end of the Downtown-East Station will be roughly 43 metres deep; or
approximately 12 stories underground.
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