"Richard J." wrote in message
...
Tom Anderson wrote:
I can see why it might be greater - there's
more mass to shift, more
surface to line - but not massively greater.
I think that nowadays these seem to be a relatively small part of the cost
of building a railway.
I think you may not have seen the actual figures. AFAIK these are
external tunnel dimensions (from Crossrail documents):
Victoria Line: 3.81m (older tube tunnels are slightly smaller)
Jubilee Line extension: 4.35m diameter
Crossrail: 6m diameter
Volume to excavate per metre of tunnel:
Victoria: 11.4m³ Jubilee: 14.9m³ Crossrail: 28.3m³
Surface area to line per metre of tunnel:
Victoria: 12.0m² Jubilee: 13.7m² Crossrail: 18.9m²
Something else worth considering is that a crossover cavern has to have more
than twice the radius of the running tunnels, and four times the
cross-sectional area. For Crossrail in particular the crossover caverns
would be, well, cavernous, and the potential for surface disruption above
such a large void is significant. This is why there were going to be no
crossovers or sidings in the tunnel section at all, although apparently TPTB
have now changed their minds about this.
Another thing worth mentioning is that most of the UndergrounD was
deliberately built beneath public highways in order to avoid wayleave
payments to landowners. This meant putting the two tunnels on top of each
other in many places, especially around corners. The impact of tunnel size
should be obvious.
Further to one of Mark Brader's points, when Crossrail 2 (aka Chelsea
Hackney) was planned to be tube gauge it was planned to have a station at
Piccadilly Circus. When the plan changed to mainline gauge, this station was
deleted from the plan because there is not enough room in that area for the
larger platform tunnels that a mainline gauge line would need.
--
John Rowland - Spamtrapped
Transport Plans for the London Area, updated 2001
http://www.geocities.com/Athens/Acro...69/tpftla.html
A man's vehicle is a symbol of his manhood.
That's why my vehicle's the Piccadilly Line -
It's the size of a county and it comes every two and a half minutes