nfitz
Superstar
Deep level tunnels do indeed work where the geology is appropriate; it's extremely effective in London, where the deep level tubes are constructed in an excellent and consistent unit that's only disadvantage is that it's a bit deep; whether it's chalk or clay I can't quite remember right now.Deep level tunnels, depending on the ground conditions, can be really cheap and the only disruption on the surface ... Out in Kensington Market and Cabbagetown, beneath a layer of sand, it is solid basalt.
But in Cabbagetown? I've drilled numerous boreholes there, and most what I have seen has been sand. Sure there's bedrock eventually, but not at any depth that would be feasible. These old Lake Iroquois beach deposits can be quite deep.
As for basalt ... I can assure you that you won't find basalt underneath Toronto! Well not for a few miles at least. It's an igneous rock, and underneath the sedimentary bedrock that you'd eventually find underneath the sands and other unconsolidated material. The first bedrock unit you'd hit should be the Georgian Bay Formation, which is a shale. Though further west, at least in Mississauga, you'd hit the Queenston shale ... sometimes it's quite shallow. But in Toronto ... well at Gerrard and Parliament I'd guess you'd be looking at 60 metres to bedrock! Typically a deep London tube is about 20 metres.
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