NotATrainspott
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- 2 Feb 2013
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not on SSC and don't want to dig into their mammoth threads, but taking this off-topic- has the issue of the vast amounts of extra spoil this would create been raised? Crossrail's platforms are 5m wide at floor level. That's about 17-18m diameter tunnel plus ~1m of tunnel lining. Lets go with 18m TBM then. That's a 56.5m^2 cross section. By comparison two 7.1m bores (Crossrail) is 44.6m^2. Creating 25% more spoil is not trivial
The real world use of this idea is on Barcelona's Line 9/10, which uses a ~12m diameter tunnel boring machine to provide space for both running lines, on two separate levels within the circular tunnel. Normally, the running line is in the middle of the floor, but at stations it moves to the side and there's then enough space left on the other side for the platform. The rest of the station then just consists of the escalator/lift shafts.
My suggestion is that each running line would still have its own separate tunnel and TBM, but instead of using a TBM only wide enough for the running tunnel and then having to manually carve out the wider platform tunnel, the entire tunnel would be built to the same diameter as one of the single platform tunnels. The Crossrail running tunnels are approximately 6m in diameter and I think the platform tunnels are around 9m. Although more spoil would have to be removed from the tunnel during construction, the flipside is that the amount of intense construction work in the city centre itself is reduced. Given that any Edinburgh tunnel would be relatively short, the extra waste of additional tunnelling wouldn't be as great as it would be for a longer line without many more stations.