Pond liners

[Algernon Goss-Custard]

We have a low-lying, boggy garden with a stream running through it. Come the lockdown and Lady Goss-Custard decided to have a pond. So we got a guy in with a mini JCB and he dug a hole about 5.5m by 3.3m by 1m deep. There is about nine inches depth of topsoil, below that it is all solid yellow clay.

Having dug the pond, we ordered the butyl liner, only to find it would take a couple of weeks to arrive. A few days after the digging, and before the liner arrived, the rain absolutely chucked it down for a couple of days, so that the surface run-off filled the pond almost to the top. The level has stayed steady ever since, two weeks now.

In the meantime the liner has arrived. The dilemma is now: Do we leave the pond as it is, without a liner, on the assumption that it will stay full of water and the walls will not collapse? Or should we pump all the water out, put the liner in and pump water back in to fill it up again - which we foresee will be a very dirty and unpleasant job now that everything is sodden? If we do put the liner in and refill, what will happen in the next downpour - will rainwater get in behind the liner and balloon it inwards into the pond?

[Jeff Layman]

You don't just put the butyl liner in the excavated earth. You need make sure you've removed any sharp objects - particularly stones - which could pierce the butyl liner. Then you need to line the "hole" with sand or layers of something else several cm thick. That guards against any sharp objects you have missed. More information here:

So you will have to pump out the pond anyway before fitting the liner. 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.

[Brian Gaff (Sofa]

Yes indeed, you use sand cos it helps with the passage of water as well of course. I think maybe the pond is a little shallow, but I guess it depends on what you are going to do with it. Brian

[David]

Old carpet is a good way to line a pond and protect the butyl liner.

As Brian has already said, 1 metre isn't that deep but with luck it will be deep enough to prevent the water freezing all the way to the bottom in a harsh winter.

Cheers

Dave R

[Vir Campestris]

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

[tabbypurr]

Many ponds are clay lined. The use of plastic/rubber liners is a modernish phenomenon

NT

[harry]

If there is ground water, it may need to be drained away ie a permanent drain under the liner. Water under the liner causes it to lift away from the ground.

You need to make sure there are no sharp object to pierce the liner plus you need a "woolly" subliner to protect the liner. Some people use old carpets.

It's a good ie to position any off cuts round the edge above the water level to protect the liner from UV. Esp the South facing edge ie the bit that catches the sun.

Our liner is about 20 yrs old but there is now a slight leak somewhere close to the waterline. Can't find it.

[Tim Lamb]

My *no cross posts* rule has wiped most of this thread...

We created a pond in similar circumstances some 25 years back. Dry, low lying drainage ditch, Summer.

Deepened to around 1m, it had collected about 100mm of water. Cheap PVC liner on top of old carpets for *softening* and pumped full of water.

All was fine until the Autumn. Curious bulges appeared in the liner as the water table rose and pond water was displaced through the overflow.

Mitigation attempts with bags of sand were only partially successful. A more permanent cure was to raise the pond fill height by a further

150mm. Still not 100% but gradually the pond lily roots created a rigid base and accumulated silt helps.

Basic rules... don't use a lightweight liner unless the planned surface level is well above the Winter water table, don't allow PVC liners to be exposed to sunshine, don't allow *water soldier* plants anywhere near, if planting pond lilies...expect they will fill the pond in a few years...

[Algernon Goss-Custard]

Chris Hogg [e-mail] posted

We have pretty much decided to leave it as is without the liner. The level still hasn't gone down - it drops by maybe half an inch after a few sunny days and then rises back up to the topsoil level after rain. We get quite a lot of rain here in Devon so it should be ok.

The main drawback is that the clay makes the water cloudy. It is clearing gradually, but I expect the next heavy downpour will stir it up again. At least the dragonflies are now attending regularly.

So we now have an unused liner in the shed - too late to return it now.

[T i m]

<snip>

My Uncle used to work for a water authority and specifically used to be responsible for the maintenance of the New River.

They would regularly use a long chain with interlinked blades and a handle both ends and 'walk' the river (working upstream I believe) cutting any vegetation that was growing from the bottom. This would float downstream and get caught in the grills and they would have to clear that as well before the water level got too high upstream.

I understand that's lined with clay and carries millions of gallons of drinking water from various sources in Hertfordshire down to Stoke Newington.

Walking it with the dog it's amazing to think they were able to engineer something like that in the early 1600's and for it to have a fall of only 5" / mile.

Uncle would turn in his grave if he saw the lack of upkeep on bits of it now. I think the biggest issue for him were water voles, digging though the clay and that allowing the water to run out, washing away the supporting soil with it. ;-(

Cheers, T i m

[Andrew]

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.

[Andrew]

Red clay + straw = cob. Many Devon houses built with it, including a new one, as seen on Grand Designs about 10 years ago.

[David Hill]

You may find this of interest

[Vir Campestris]

On my job list is to remove the liner and dig it deeper.

As we have four species of amphibian breeding in it timing is a bit interesting...

Andy