John Rowland wrote:
"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.

I'm not sure why you say it will never be used as a junction. When T5
is open, the trains that run via T4 will join the eastbound T5-T123
track at the new junction. There may also be a connection from the T4
loop to the westbound T123-T5 track, which I guess wil be rarely used.
Is that what you were referring to?
--
Richard J.
(to e-mail me, swap uk and yon in address)