John Williamson wrote:
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.

Not necessarily.

Single tunnels have a lot more diameter than the train, but a double
tunnel will be a lot closer to 2 trains.

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.

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.

Not true. It is a very complex mix of machine sizes, cutting speeds
materaial and labour costs.

For sure when you go straight down with a cut and cover you get the
least material removed and the smallest footprint from a single tunnel.

So that will in general be CHEAPER than two.

When tunelling - well maybe the machines only do a single train sized
tunnel.