"Richard J." wrote in message
k...

There are some details in this month's Modern Railways
(page 52). The ground at the junction site consists of
water-bearing gravel, and conventional techniques to
build a step-plate junction would risk a collapse of the
existing tunnel. "At the site of the existing junction,
the ground is being opened up for the construction
of a big coffer dam extending 25 metres down. ...
Once complete,this box will then hold the junction."
TBMs will drill towards the site from the T5 end,
but the final breakthrough will be done outwards from
the box. Then they have the task of restoring the
concrete aircraft taxiway over the box. In
the circumstances, 20 months seems reasonable.

Did I fall asleep at LURS in November, or was this
important information omitted from the talk on the T5 extension?

Omitted, I believe.

If I'm understanding this correctly, this info seems to significantly reduce
the wastage involved in building a step plate junction which will never be
used as a junction, but doesn't seem to completely eliminate it.

--
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