"John Williamson" wrote in message
...
polygonum wrote:
On Sun, 01 Jan 2012 21:00:46 -0000, The Natural Philosopher
wrote:

polygonum wrote:
On Sun, 01 Jan 2012 18:30:22 -0000, dennis@home
wrote:


Its cheaper to bore one tunnel these days.

How much would it cost to bore, says, one mile of dual tunnel capable
of carrying two tracks, and how much for one tunnel large enough for
both tracks? Make any assumption you like about the specification of
the trains - e.g. Circle line standard.

essentially about 50% more material at a wet finger guess has to be
removed if the tunnel is circular.

I guessed around twice as much using simplistic pi r squared.

Four times, as doubling the bore diameter increases the bore area by four
times. So, boring a double track tunnel costs twice as much (roughly) per
route mile as two single track tunnels. The tunnel walls also have to be
built stronger, increasing the costs further.

That isn't always true.
To start with you don't need twice the diameter to run two tracks.
The boring machine would be bigger and will be capable of boring faster as
you can get more works into the machine than a small one.


There is also the disadvantage, if it's a new scheme under a city, that
the larger tunnel has far more restricted routing options, as it is harder
to avoid obstacles like sewers, pipes and cables, not to mention the fact
that any building it goes under will need much more expensive underpinning
work than with two single track tunnels.

This is a tunnel, you normally go under the services.
Its cut and cover where you hit big problems with sewers, traffic, etc. It
also disturbs a lot of surrounding ground as it buggers up the water table.
You really wouldn't want to do a 100 foot deep cut and cover in London.

You don't need to underpin if the tunnel is done properly at a sensible
depth.



The *only* time a double track tunnel is cheaper to build than two singles
is when you use a cut and cover system, as used for much of the Paris
Metro, and most of that follows the surface street layout, so dodging the
problems of access under buildings and supporting them after you dig the
hole.