On Mar 15, 11:11*am, Roland Perry wrote:
In message , at 10:55:45 on Thu, 15 Mar
2012, remarked:

http://www.bbc.co.uk/news/uk-17365934

There doesn't seem to be much distance between the 2 bores. Surely they won't
be that close the entire route? Looks like it would collapse.

The tunnel is concrete-lined as the machine moves forward.

I'm more interested in them apparently using GPS 38m underground!


There distance between the tunnels may not be constant.

Does GPS penetrate that far? One is intrigued as to how this works.
ISTR earlier tunnels being aligned by laser.

The Twenty-first Century is starting with this much needed and really
exciting rail project. When the economic recovery comes we see more
projects like this. London badly needs them.