"Tom Anderson" wrote in message
h.li...
On Tue, 2 Dec 2008, John B wrote:

On Dec 2, 3:39 pm, Tom Anderson wrote:
My gut feeling is that adding a third bore to a two-bore setup is a lot
more expensive than adding a footbridge to a railway bridge. In fact, i
can't see how there being two bores there already makes the third any
cheaper, so it would be as expensive as building it as a standalone.

The DLR Woolwich Arsenal twin tunnels were built by sending one boring
machine from King George V to Woolwich, turning it round, and sending it
back again. So in theory you could give it another twirl (probably works
OK time-wise too, as fitting out a rail tunnel takes a lot longer than
fitting out a foot/cycle tunnel) for at least *some* cost savings.

Yebbut then you've got to pack it up at the far end, rather than at the
near end, where it started, and where you thus already have all the gear
and access. Logically, you should bore a *fourth* tunnel, to minimise
costs.


You'd have to bore a fourth tunnel, to separate the cyclists and the
pedestrians.

c.f. the various Tyne tunnels, where they are just completing the fourth
(sunken prefabricated) tunnel, having done everything in the wrong order
over the years... :-)

Paul S