We need help to overcome a clay lined pond problem before 22nd July.

Work has now stopped on a two pond project because we cannot seal pond 1,which is clay-lined. We have spent countless hours with teams of committed students and staff puddling close to 20 tons of Essex Blue Clay – using hand tools and barrows. We have put in £3,500 worth of hard-earned sponsorship and we took expert advice on pond location. The results are

• 2 ponds excavated by hand and machine - total dimensions 80ft x 30ft - maximum depth 4ft. The bedrock is heavy clay with flints.
• the clay pond is not holding water - it holds some but not enough.
• last week's hot weather has cracked the exposed banks of the pond
• we have £300+ of plants sitting in our poly-tunnel slowly cooking - they should be growing around the pond.

We need help and advice on site. I suspect, if we get to 22nd July, without help, 8 months of commitment will have been in vain. This area will wither and die in the summer holiday period. The sadness is that the hard work has actually been done.

It was our intention to use this pond project to create an educational

Resource for our students and to provide learning experiences for hundreds of local primary school students. There are plans to film underwater life as it develops, to record research data on ph levels and developing eco-systems and the students involved were to provide primary schools with environmental science learning materials linked to on-line support. Our hope is that this project might be an interesting departure from the norm for you and it would be of reader interest. If completed the site could yield further information on ecological development in the future. We live in hope.

Pete asked

How was the excavation consolidated?

Essentially a JCB cut into heavy ?orange ? clay with flints ? the clayhold its shape ? vertical sides. Tapered depth 4 ft to 1 ft.

Pete asked

How steep are the sides?

Steepness of sides ? deepest area it is vertical ? shallower 20 degrees slope.

Did a geotextile go down on top under the clay?

No geotextile fabric is under the clay.

Pete asked

How was the Essex blue clay prepared?

The Essex clay arrived late September in dry form By October it as wet.

Clay was dug out and barrowed to the pond and puddled by feet ? pushed onto the walls by feet from spades.

Pete asked

How thick was it when it went down? Did you manage to get it all down in one day? If not did any of it dry out during the process?

No ? we did not get it done in one day. We tried to create an 8 inch thick layer ? layers were added weekly, It did begin to dry out in March/April.

We simply added more clay over the cracks as another layer.

Pete asked How many layers?

I should guess there were 4 layers applied October ? April.

Pete asked

How was it puddled?

It was puddled by foot with the same methodical team.

Pete asked

Did a geotextile go down on top?

No geotextile went on top.

Pete asked

Is there a loam or gravel layer over the clay?

Have you engineered a 'capillary barrier' around the top edge?

There is no loam or gravel over the clay and no capillary barrier around the top edge.

We may need to raise part of the down slope side of the pond as the upslope side exposes 1-2 ft of dry wall. This would increase the volume. It takes 2 days with a garden hose to pill the pond.

Sadly Peter we began this journey with honest endeavour and learned as we went along. We are encouraged to develop projects like this but you quickly discover that you are very much on your own from design through to problem solving to completion.

We will reluctantly replace the clay with an artificial skin if need be? But it is not what we wanted to do. We wanted this pond to be a natural pond.

Pete Answers

Here is a list of guidelines that relate to the construction of clay ponds that will help to ensure that you have a watertight liner at the end of it.

• As with all pond excavations, ensure that you have a level area cut into the ground that will be the pool and its edging.
  Drive in stakes that mark the water level at their top and that are also mark with the level to which the excavation at that point needs to go to.
• Around the top edge and right down through the pool you will need to allow for at least 15cm of clay, 30cm around the top edge, plus down through the pool 15cm of subsoil or gravel.
• The final pool need not be more that 1metre deep at the maximum, so refrain the digger operator from going more than 1.3 metres down.
• The slope of the sides should not be more than 30%. That is a fall of more than 30cm in one metre. This will help prevent the gravel, subsoil or clay making its way down to the bottom of the pool and will also aid the puddling process.
• The substrata in the excavation should be consolidated with a good ‘whacker plate’ (plate vibrator).
• The clay used should be uniform in consistency and pliable. Some suppliers, particularly on the continent, supply it in 10cm thick tile shapes that have had already kneaded, with all the air squeezed out of them, and are notched in a way that they convenient overlap.
• If it doesn’t arrive in ready made the clay should be stamped out on a flat surface and cut into squares and laid almost as though you were laying a lawn. There must be no hollow paces between the units of clay. If in doubt, lay another layer of tiles over the existing ones, overlapping the previous joins, but the lower layer must never be allowed to dry out. Large sheets of plastic are worth their weight in gold here. If you cant get it all done in one day, do one section completely, to get all the subsoil and gravel( geotextile if you are going to use it) in place before starting another section.
• Navvies used to puddle with their feet, Capability Brown with herds of cows. It is a job that requires conviction beyond the normal call of duty. There is no room for the notion of ‘that’ll do’. You must go way beyond that.
• Modern contractors just puddle with a ‘Whacker’ plate or power tamper (‘Paddy’s motorcycle’). With ‘home made’ clay, I’d take the precaution of puddling and then finish off with a ‘Whacker plate’.
• Once the clay is in place I would put down a layer of ‘pool underlay’ or geotextile to protect the clay liner from intrusion from plants and other damage. Then in on top goes a subsoil layer or gravel 10-15cm thick. This is your plant medium.
• Where the clay meets the soil, it is inevitable that capillary seepages will set up and leach water from the pool to some depth.
• To prevent this you can set up a ‘capillary barrier’. It will also function as insurance fro keeping the water level high enough above the clay liner to keep moist. This can be set up before you start by excavating a shallow 15cm deep and wide and 30cm below water level around the top of the pool and filling it with a kneaded ring of clay.
• In your case you would have to excavate this trench excavating around the top edge of the pool below the layer of clay. With the trench full of clay, lay a strip of cheap pvc 45cm wide on top of the clay. 15cm of its inside width lies on top of the clay and the other 30cm is used to face the inside face of the soil abutting the pool.
• On top of the liner on top of the clay will go a 15cm thickness of clay that will link in with clay lining the pool and then the a layer of subsoil with gravel on top as the water level laps at its outer reaches and is prevented from travelling beyond the clay to the outlying soil. An over flow can ensure that the water is never allowed to go above the trapped piece of liner and thus defeating the object of it being there.

PHEW! Is there no wonder the world and his dog have resort to the convenience of a pvc and rubber pool liners.

As a remedy for your particular problem at the moment, I would let the water go down for as long as dare. Where the clay is exposed, I would moisten it an keep it covered with plastic. Keep checking it.

Then get a ‘whacker plate’ on a rope and let it vibrate down as far as you dare. Get the two strongest folks on the team to do this, because they are heavy tools. It may take plenty of wetting an several runs of the ‘whacker’ plate to get the clay elastic again. Once you have done this all round, fill it up whilst ensuring the clay is covered still. If the slope is shallow enough in places to have some soil or gravel, then cover it with this. (Don’t use geomembrane with out the capillary barrier, because this will act as a wick.)

If there are any nearby buildings, sheds would it be possible to divert the runoff into the pool to keep the water level up? Your exposed bank of clay may best be converted to a rock face. Raising one side at this late stage sounds like asking for trouble. But this I cannot truly tell without seeing it.

Pete

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