On 30/06/2020 22:01, Vir Campestris wrote:
On 30/06/2020 18:31, Jeff Layman wrote:
Once the lined pond is full of water, it won't make any difference if
rainwater gets behind the liner as the pressure inside and out will be
equal.

Our rather badly made pond gets water behind the liner. On occasion
there have been big bulges, which is a PITA because once the ground
water has dropped - which doesn't usually take more than a few weeks -
the pond is only half full. Despite all the rain.

Andy

If the subsoil is heavy clay then the trick is to 'wipe' some of
the excess clay up the sides to cover the topsoil and don't use
a manmade liner. This is how reservoirs and canals were lined
by the Victorians. Just keep it topped up in hot weather to
prevent exposed clay drying out and shrinking. Planting the
usual bog plants around the edges helps to keep sun off the
clay just above water line.

Putting a liner on top of heavy clay runs the risk of water
getting betwen the liner and the clay.