On Fri, 26 Mar 2021 10:22:45 +0000, nightjar
wrote:

On 25/03/2021 21:58, T i m wrote:
I started to hear a regular 'clonk-donk' as cars were driving over
something in the road, like a bit of chipboard shelf but it then
persisted for around a couple of weeks.

Yesterday I investigated when we took the dog for a walk last thing
and it appears at least one manhole cover has been lifted ... or has
worked loose(?) and is now rocking about 5mm when you stand on it?

This is just one of the single piece, all metal (cast iron), slightly
rectangular ones with just a BS number on it and no clues to what
utility it might serve but from it's location I'm thinking it might be
the sewer?
...

That can be caused by the brickwork of the hole itself starting to fail,
causing the support frame to move.

I'll have another look and check but from memory in this case I think
it's 'just' the manhole rocking in it's frame and the external /
visible parts (at least) of the frame looking sound. I think this has
resulted from some sort of works / inspections as there are also
several manhole covers on the pavement I now notice aren't fully flush
or rock as well (but don't tend to attract vehicular traffic so not as
noisy).

Repairing that requires major works
of the sort that normally has to be planned months in advance.

;-(

It can also be due to part of the cover or frame breaking off and
allowing the cover to rock.

I am amazed what size (and so weight) of vehicle we sometimes get down
here (arctic drivers lost looking for a trading estate) so I guess one
of those might have been the 'final straw'? I'm more amazed they seem
to find their way out though some pretty tight (and often car lined)
twists and turns (although that said, we have had a car hit and
another written off by them).

That can sometimes be alleviated by pouring
sand in, but only if it stays in place and doesn't trickle down the hole
because it is part of the frame that has broken.

Assuming nothing has broken and noting that all the others that don't
look to have been disturbed have a good layer of 'stuff' in the gap, I
don't know if this one being devoid of such has been caused by it
being lifted or (as you say), something giving, allowing it to start
to move and then clearing the infill (and so move more).

I think if I knew the interfaces between cover and frame better it
might help me understand how I might be able to (safely) try to
silence it (and hope that would then be further supported by std road
dirt / rain etc), assuming the relevant agency doesn't get to it soon
etc.

Cheers, T i m