On Wed, 26 May 2010 20:41:10 +0100, Tom Anderson
wrote:
On Wed, 26 May 2010, d wrote:
Tunnels arn't dug with picks and shovels any more - a TBM won't care if
it has to dig the whole tunnel itself or theres a small tunnel already
there
Yes it will - TBMs are built for digging through clay, gravel, and the
like. Sticking a bloody great cast-iron pipe like the PO railway end-on in
its way will completely bugger it.
But you wouldn't do that. Tunnel miners would dismantle the segmental
lining of the smaller diameter tunnel, ring by ring, a short distance
in front of the main tunneling shield. The shield would then move
forward, excavating as it went, and the new, larger diameter lining
would be installed at the rear of the shield.
This is all routine tunnelling work. The only complication is the
need to stop any excavation of the larger diameter tunnel while miners
dismantle the lining of the smaller diameter tunnel. But that would
happen simultaneously with the building of a ring of new lining, so it
wouldn't actually need to stop progress.
However, what would kill the idea stone dead is that the PO railway
tunnel is on the wrong alignment and is far too shallow for Crossrail.
Also, it is only one tunnel and Crossrail needs two. You cannot build
one alongside because the ground at that depth is almost certainly now
peppered with bearing piles for buildings that have been built since
the PO railway.
So, a complete non-starter.