What ho one and all,

When we built the house and before the foundations where constructed, we installed a 6000 litre roto-moulded ploy tank to collect rainwater.

A few months later, there was a problem in that since it was yet to be connected to the downpipes and since the groundworkers had been using the water to clean their machinery, hydro-static pressure got between the tank and the concrete, causing one convex end to 'pop' inward to become concave. I had an engineer report but insurance wanted someone to be guilty. The ground workers had long since gone and had more or less, installed it correctly; it has not 'floated' out of the ground. There was no way to prove that the tank itself was faulty; result was that we have managed to live with a reduced volume, probably around 5000 litres.

However, due to this prolonged period of no rain, the tank is empty until a few weeks ago, when we had a bit of rain in the south-east. The tank is now around 1/3 full, but when I was checking, I think the concave is increasing, which means there is hydro-static pressure behind the tank. That in itself is strange as we have not had sufficient rain to raise the water table.

So my question is, what can I do? Removal is pretty much out of the question. Grateful for any ideas.

The photo is of the interior with the red line 'kinda' showing the concave end, which sold be 'popped' the other way.

Im guessing the concrete youre talking about is mass fill between the tank and the surrounding hole?? If thats the case was the tank braced internally when the concrete was poured? If it wasnt braced it seems theres a fair chance that the weight of the concrete could have collapsed the end of the tank.

If its just ground water youd be able to slowly jack the collapsed end back out, allowing time for the ground water to disperse into the ground . If its concrete behind the collapsed end itll not budge and would be solid to a tap with a rubber mallet

I am at a loss to understand why you would not just sand in the tank rather than use concrete. I am also somewhat sceptical that even if water did get between the concrete and the skin of the tank that it would have sufficient pressure to deform the tank unless the tank was very flimsy.

I guess the tank was encased in concrete, due to the water table and stopping it floating up (which is an alternative to a concrete base and strapping down). Hopefully it was filled with water at the same rate as the concrete was poured around it, to balance out the forces.

Assuming the above to be true, then the concrete should hopefully still be in place, with the form of the tank originally, with the ground water filling the void, as Rex suggests.

It does seem strange that a tank which could regularly be empty for periods of time (unlike a septic tank which would only be empty for a few hours as it should be filled after de-sludging), would fail this way. Probably a dead end fighting the tank manufacturer, but you could ask?

Personally, I would drill the bulged end, and let the water pressure out, then try to force it back into place (maybe some gentle warming would help). Then patch the hole(s).

Maybe the water has brought silt in with it, and so there are solids behind the bulge, but I wouldn't have thought so, since with would have been seeping through concrete.

Gentlemen,

Thanks for the comments. The tank was installed correctly (as per the supplied instructions but with a small omission as the ground workers did not consider it necessary) with concrete to half way and filled with water to prevent collapse. The some more concrete to get it past the max diameter. The ground is London Clay and floating out of the ground can be an issue, but it is not.

The 'omission' was that the instructions said to line the excavation with a plastic sheet prior to pouring the concrete, apparently to reduce the chance of this happening. But the ground workers said they had never done this before and there was no need, so I believed them. The engineer report also said this would be a waste of time as it is unlikely to get a large enough single sheet or if it was two sheets, water would get in in time. Additionally, he suggested that a plastic liner could cause more problems as it would prevent any water from leeching back into the concrete and soil.

Following the installation, the build-up was another layer of concrete, topped off with gravel and then soil around the access turret. I only discovered the issue during the build when I saw some crazing of the soil and stomped on it, only to discover there was a 'sink-hole' into cavity, into which plenty of gravel had obviously fallen.

That is when I found the tank was empty as the ground workers (long sine gone) had been using the water to wash their machinery. I tried to push the 'pop' back with a couple of acros but it was not possible. It was also suggested that the best way would be to seal all access and pressurize with air to 'pop' it back. That is not an option, then or now!

So we have lived with the the reduced volume for the past twelve years; not a problem. But It looks to me as if the position of the curve is different, which makes me wonder if it is getting worse?

I know there is a pile of 'stuff' in the cavity so pressing it back to its original shape will probably not be possible. But there should be a nice concrete mould that should retain water but I suspect that it is somewhat porous so would imagine that any collected water would be lost.

Impossible to know if it was a manufacturing flaw as it is designed for underground installation. Next time, I will go for the concrete (https://www.marshalls.co.uk/commercial/product/basic-rainwater-harvesting) option.

Could that bulge really have displaced 1000L? Maybe the pic. doesn't show the true size.

Admittedly, the reduced volume is guesswork.

Certainly an internal support would be a good idea, but as with many things in life, easier said than done. When I was trying to pop it back with an acro, as pressure was applied, the acro just slid off the curve. Don't know how I would support a brace without drilling into the tank.

Then there is the issue of actually getting inside. The access hatch is OK but the inlet, filter and overflow are across the middle of the access. Would have to remove the inlet, outlet and filter unit, which although normal underground fittings, was installed when all the pipework was in trenches. Now it is covered, there is probably no chance to move anything. And unfortunately, it would have to be removed to get a ladder inside.

Back filled with concrete does seem to be the norm and that was certainly on the installation instructions.

However, with the overnight rain in leafy Surrey, the problem is resolved as there is no way to get inside the tank without scuba gear.