Matt Wheeler wrote:

I'm sure they do understand, hence the replacement bus service, and
the publicity of the works. Since Terminal 5 is happening, how would
you propose extending the Picadilly line to that terminal without an
enforced closure of parts of the line. Or would you rather the
Terminal 5 passengers (when complete) are inconvenienced by poor rail
links when it opens, just so people wanting Terminal 4 aren't
inconvenienced now.

What would you propose to do ?

As was done during the building and extending of every deep tube line on the
underground since the 1920s. Build the junction round the existing tunnel
whilst allowing the service to continue running.

Brimstone wrote:

As was done during the building and extending of every deep
tube line on the underground since the 1920s. Build the
junction round the existing tunnel whilst allowing the service
to continue running.

That is not possible at Heathrow, as Richard J. has indicated
elsewhere.

Basically, the ground is so unstable and poor that if a traditional
step-plate junction were built, the tunnels could collapse. Thus a huge
cofferdam is needed to enclose the site of the junction.

I highly doubt that TfL wanted to do this in the first place, but they
would rather close the loop than risk a collapse and entrapment of a
service train with hundreds of passengers.

Which is preferred?

TheOneKEA wrote:
Brimstone wrote:

As was done during the building and extending of every deep
tube line on the underground since the 1920s. Build the
junction round the existing tunnel whilst allowing the service
to continue running.

That is not possible at Heathrow, as Richard J. has indicated
elsewhere.

Basically, the ground is so unstable and poor that if a traditional
step-plate junction were built, the tunnels could collapse. Thus a
huge cofferdam is needed to enclose the site of the junction.

I highly doubt that TfL wanted to do this in the first place, but they
would rather close the loop than risk a collapse and entrapment of a
service train with hundreds of passengers.

Which is preferred?

My response was made before Richard J's appeared on my server. He has
answered the question that I've been trying to find the answer to.

TheOneKEA wrote:

Brimstone wrote:

As was done during the building and extending of every deep
tube line on the underground since the 1920s. Build the
junction round the existing tunnel whilst allowing the service
to continue running.

That is not possible at Heathrow, as Richard J. has indicated
elsewhere.

Basically, the ground is so unstable and poor that if a traditional
step-plate junction were built, the tunnels could collapse. Thus a huge
cofferdam is needed to enclose the site of the junction.

I highly doubt that TfL wanted to do this in the first place, but they
would rather close the loop than risk a collapse and entrapment of a
service train with hundreds of passengers.

Which is preferred?

The huge cofferdam, obviously - but that doesn't explain why such a long
closure is required. Aren't cofferdams relatively easy to construct?

Wasn't there a big tunnel collapse some years back, during work by Balfour
Beatty? Fortunately it was section not yet in use, or a disater might have
occurred. I seem to remember tunneling methods used by Austrian engineers were
tried, but these had worked in granite, not London Clay.
Perhaps better informed readers can recall details of the event?
I do remember a minor incident blocking the lines to Heathrow one Sunday
morning - a tower crane had come down across it - fortunately it came down in
the night, though one family living nearby had a narrow escape when the top of
the crane crashed through their roof.
Next morning I witnessed some passengers clamouring at the Acton Town ticket
office, apparently unable to comprehend that the besieged clerk could not get
the obstruction cleared at once and trains running again just for them.

In message ,
CharlesPottins writes

Wasn't there a big tunnel collapse some years back, during work by Balfour
Beatty?

Not on the Piccadilly. It was during the building of the Heathrow
Express tunnel (although it nearly affected the tube tunnel as well):

http://news.bbc.co.uk/2/hi/business/280107.stm
--
Paul Terry