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?