On Wed, 23 Jul 2008, Mr Thant wrote:

On 23 Jul, 18:27, Tom Anderson wrote:
As i mentioned then, there are cross-section diagrams of the CTRL which
also show station humps, although i imagine this is less about saving
energy and more about getting an otherwise very deep tunnel into shallow
cut-and-cover stations.

Crossrail vertical alignment diagram, showing tunnels dodged:
http://www.crossrail.co.uk/80256B090053AF4C/Files/centralareaverticalalignment/$FILE/vertical+alignment.jpg

There's an alternate version on the last page of this PDF, showing
geology:
http://billdocuments.crossrail.co.uk...ethodology.pdf

Oh, brilliant! The details on how the tunnels will be dug are fascinating.
IOW, boring is interesting.

London, it seems, sits on five layers of different materials. At the
bottom is chalk; on top of that, Thanet Sands, and then Lambeth Group
(which i read is a mixture of clays and sands of various kinds, with
pebble beds at the bottom in some places), on top of which is the famous
London Clay, and then a dusting of river terrace and superficial deposits
right at the surface (or, as laymen call it, 'earth').

Everything west of the junction between the eastern branches at Stepney
Green is going to be bored through London Clay, with a minor excursion
into the Lambeth Group beneath the Fleet valley. Most of the way, the
tunnel is near or at the base of the Clay - it's only west of Bond Street
that it's any distance above it, as that's where the Clay becomes much
deeper. Between Stepney Green Junction and Pudding Mill Lane, things are
much the same. Between the junction and the Victoria Dock portal, though,
the tunnel is deeper, and largely bored right through the Lambeth Group,
mostly at its base, where it rests on the Thanet Sands. For the hop across
the river, where the London basin ends and these layers fade away, the
tunnel is right through chalk.

Anyway, the upshot of all that is that, with the possible exception of the
far eastern end of the core tunnel, there isn't a geological constraint on
depth. It's clearly possible to tunnel through the Lambeth Group, as that
happens in the east, so i see no reason why that wouldn't be possible in
the west. I assume the real constraint is therefore the presence of
specific awkward things underground, which are not shown on those maps.

I note from another diagram that the core tunnel will be dug in three big
drives, and one little one. One comes from Royal Oak in to Farringdon, one
from the Limmo Peninsula in Docklands into Farringdon, one from the
Pudding Mill Lane portal to the Stepney Green junction, and then there's a
little one from Limmo to the Victoria Dock portal - don't know why. Now,
clearly, the branched tunnel has to be done with two drives, one starting
at either eastern portal, only one of which will continue to Farringdon.
But i find the choice of which that is interesting in comparison to the
geology: the central stretch is mostly through London Clay, as is the
Pudding Mill Lane stretch, whereas the Victoria Dock stretch is mostly
through Lambeth Group. I would naively have thought that you'd want to
customise your TBM for the kind of material you're digging through, and in
that case, it would make more sense for the Pudding Mill Lane drive to be
the one that carries on to Farringdon, so that you could have a
Clay-specific machine on that one, and a Group-specific machine on the
Victoria Dock drive. Evidently, though, i know nothing about this.

tom

--
Optical illusions are terrorism of the mind.