"Nicholas Walker" wrote:

Hello Nicholas

NW I've recently scooped out an old silted up pond in my garden
NW which filled up and overflowed every winter but dried out in
NW the summer. It's a fairly large triangle of about 20 yards
NW each side. It takes four days to fill with a hose and then
NW two days to drain. The clay base (of unknown
NW thickness) seems watertight, but I reckon it's leeching out
NW where the clay meets the topsoil sides.

If it's leaking there, then why is it emptying completely?

Clay is great, until it's allowed to dry out. Then it cracks and needs
to be repuddled to make it proof. Chances are you have several leaks
at all depths.

NW What are my options? It's a bit big for a liner but I

Yers, it is. Cost me 500ukp for butyl for a 8Mx5M liner, so would be
very expensive for your pond.

NW understand that the clay could be extended up the sides.
NW What is the technique for doing this? How thick does the

Google around for "clay puddling" - there's probably a few sites
detailing how it was done. From what I've read (not done), you get rid
of all the topsoil and gunk in the pond. Soak the clay. Jump in
barefoot and squelch around for ages to work it together and make it
workable. Then you smear it around with hands and feet, polishing off
the top layer. And don't let it dry out - keep working the layer just
above the waterline as it fills. Although if it takes four days, you
might want to point a sprinkler at it to give you more time.

It's a very messy job.

NW layer of clay need to be?

Check, but I think it's around 4-6", maybe thicker at the base.
Obviously keep the layer up to the very top.

NW How do I test that the clay to be used is up to the job?

When it stops leaking.

--
Simon Avery, Dartmoor, UK Ý http://www.digdilem.org/