On Wed, 18 Nov 2009 23:18:36 GMT, "Richard J."
wrote:

You're talking as though the problem is insufficient brickwork at the
top of the arch. But it's clear from the photo that the failure occurred
lower down, where a great mass of brickwork has moved. Looks like a
gross failure of the foundations.


Indeed it does. It is probably a result of scour under the
foundations as a result of the extreme flows of water.


I wonder why it was necessary to renew so much of the brickwork at the
top of the arch (different colour bricks obvious in photo). This is
pure speculation, but I'm wondering whether there was some earlier
instability of the foundations that resulted in some movement higher up,
which was just patched up rather than properly investigated.


More likely, a combination of gradual long term settlement of the
structure (it appears to be very old) and frost damage to the
brickwork. Problems can then be caused by using hard modern bricks
and hard cement mortar, rather than the hand made clay bricks and
slightly flexible lime mortar that would have been used in the
original structure.