smallspy
Senior Member
A single bored tunnel needs to be located quite a bit deeper than a pair of bored tunnels. The basic rule of thumb is that the tunnel needs to be covered by at least its own diameter.Is there a reason Metrolinx is going with the twin bored tunnels again? I thought they would be going forward in the future with single bore (as in the case with the Scarborough subway extension) in order to reduce costs amongst other reasons.
As well, as the tunnel alignment is not strictly underneath the road, there may be some concern about any subsidence affecting neighbouring properties. It's generally easier to control it by using two smaller tunnels.
Dan