Richard J. writes:
Is there anyone on utl who lives or works directly above an existing
tube (not sub-surface) tunnel? ... have you had any cracks and
subsidence?

Of course, if you want to talk about subsidence, there was the little
matter of November 27, 1923. But, on the other hand, that was during
an attempt to reconstruct the tunnel (then over 30 years old) while
maintaining a train service through it.

See http://www.davros.org/rail/culg/northern.html for a brief account.
--
Mark Brader, Toronto "These Millennia are like buses."
--Arwel Parry

My text in this article is in the public domain.