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

"AJH" wrote in message
...
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

you can't beat puddle clay......

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

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.

In message , AJH
writes
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.

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 he
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

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

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?

yes

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

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?

yes

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

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

Is the pond on level ground?

No it's half way up a fairly steep slope

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.


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

In message , AJH
writes
On 16/06/2019 19:06, Tim Lamb wrote:

Is the pond on level ground?

No it's half way up a fairly steep slope
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.


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.

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