puddled clay

Nineteen years ago friend dug a 400m2 1.5 m deep pond on the slope of a chalky hillside and puddled clay as a liner. After two years the water started disappearing during the summer and now it is dry.
In the winter it partially refills from the bottom which suggests after heavy rain when the soil is saturated the water table is higher than the bottom of the pond.
I know that dewponds on chalk were filled by rainwater and retained water because the evaporation was less than the rain input. Obviously the clay seal has failed but I wonder if the hydrostatic pressure from the water table could displace the clay and rupture it?
A plastic liner will be rather expensive and I have witness one of those being blistered from water rising underneath in a pond belonging to a denizen of this group.
I'm looking for pointers to solve the leak, attempts have been unsuccessfully made to puddle the clay again and an exploratory dig appears to show the clay is good and moist.
AJH
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On Sunday, 16 June 2019 12:29:22 UTC+1, AJH wrote:

The problem with puddled clay is if it dries out, cracks appear and it leaks. The cracks are not "self healing" either, it has to be reconsolidated.
Also,if it is too thing, it gets washed into crevices/porous ground beneath and than leaks.
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I suspect that was me:-)
PVC liner is much cheaper than the proper stuff but degrades in sunlight.
The annual river valley water table variation caused the problem here: with trapped water ejecting the pond fill through the overflow.
Strategic sand bag ballast plus raising the pond sides was a partial solution initially. I think the pond lily roots and 20 years of leaf mould are holding it down now.
Would Canal people elsewhere know the answer?
--
Tim Lamb

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wrote:

Chalk in general is free draining. If the pond is on the side of a chalky hill, it seems to me unlikely that it's below the water table, even in winter. Does an un-puddled pit alongside fill with water in winter?
Clay is not totally impermeable; water can diffuse through it, albeit very slowly in some cases. The quality of the puddling clay is important. It should have a high clay content, i.e. not much sand or silt mixed in with it.
You need a layer about 20cm thick, and it should be well compacted while being put in place, either gets loads of people to tramp to and fro over it, repeatedly, wearing their welly boots or more fun, bare feet if the weather's fine and war (make it a party, provide inhibition-removing refreshment etc.) or use a whacker plate or drive a quad bike or similar back and forth over it to compact it.
As Harry says, if it's allowed to dry out it may crack, requiring serious repairs.
Roots of plants or trees nearby can grow into and through the clay layer and damage it.
Lots more here https://tinyurl.com/y2ykc5fz and here https://tinyurl.com/y2j5zmsb but they broadly say the same things.
--

Chris

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On Sunday, 16 June 2019 12:29:22 UTC+1, AJH wrote:

yes
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snipped-for-privacy@gmail.com writes

Is the pond on level ground?
I inherited a bit of arable land in Cambridgeshire. From time to time, it *springs* according to the tenant at rent review time:-)
Rising land over some nice chalk. I suppose most years with normal rainfall the problem is not seen but every 20 years or so water appears part way up the hill and heads for the river via the roadside gardens.
--
Tim Lamb

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On 16/06/2019 19:06, Tim Lamb wrote:

No it's half way up a fairly steep slope

Chris Hogg said the chalk would be free draining but I suspect much the same happens here in that the rainfall from the hill above percolates through the soil to the chalk but in winter some travels down hill near the surface because the chalk underneath is saturated and this fills the pond.
Either way if the pond didn't leak it would gradually fill anyway.
I suppose if the plastic liner is buried 6" under a covering of soil the blistering wouldn't be an issue.
AJH
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Soil won't weigh that much below water.
Could your digger move the clay on top of the liner as you unroll it? I managed to find a use for lots of woven fertiliser bags protecting our liner:-)
--
Tim Lamb

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