Soak Hole

Hi there,

I have been lurking for too long, but now have a big question.

Building a new house and in the back garden, installed early this year, a 6500 liter rainwater collection tank for irrigating the garden. We have yet to connect the gutters to the tank, but all the pipework is in place around the house and to the tank.

Yesterday, I noticed a 'dip' just in front to the tank and stood on it. It collapsed, and below, is a bloody big hole. Seems that water has eroded the clay backfill, and the front and back of the tank plastic are visible.

There is also a hole at the front of the tank. I have poked around with a scaffold pole but am unable to find the concrete that the tank is embedded in.

So first question, could the concrete have split and pulled away from the tank at the front and back? If so, how on earth (no pun intended) do I fix it?

And what do I do now? I thought about filling with rubble but that would leave a lot of voids for more water to sit. A neighbour has suggested soil, but surely that will just erode away again?

Really grateful for any suggestions.

Many thanks,

Rex

 

Rex - I have no particular knowledge of these things - but a few questions have arisen in my mind. First what size is the tank in feet - am I right to assume it is the equivalent of a 6 feet cube? I can't picture 6500 litres - that's 6.5 cu metres isn't it? Second who installed it? and how - you say it was on a concrete base. A picture would be useful. And how was the hole dug - ie how much extra soil was removed from around it and had to be back filled?

There are two aspects that may have a bearing.

The first is that if you bury an empty tank and then get rain that trickles down the outside, it will form a pool under the tank, and the tank will try to float in that pool - ie move upwards. The upward force will be huge - your tank will have a buoyancy of 6.5 tonnes (ie 6500 litres of water) if it was totally under water. And remember - like a very fat person in the bath, it only takes a small amount of water to make the water level rise a long way! I suspect that the concrete base will not have split or moved - but the tank could have moved.

The second point is from a gardeners point of view. If you fill a pot with a plant and surrounded it with soil you should then water it in. In the process of watering it in, the water will lubricate the soil particles and they will compact and the soil level will drop. If you backfilled your hole and then it rained the soil will have compacted. If the tank is still sitting on its concrete base, you don't have a problem - you just fill the hole with more soil.

However if the tank has risen and there is an airgap under it and some soil has moved under into that airgap, you do have a problem. Because if you now fill the tank with water its base will not be properly supported.

Even if you had filled the tank with water first and then backfilled the hole with soil - there could still be a problem in the future if the tank was emptied and it rained. You need a certain amount of weight on top of the tank to stop it rising up. I suspect this is a standard problem with all sorts of underground tanks that are sometimes full and sometimes empty - such as petrol storage tanks at filling stations. There should be some guide lines on this.

 

Peter,

Many thanks for your reply.

The tank is about 2m in diameter and about 2 meters long. Basically a large barrel on its side. The hole was dug with an excavator and had a couple of feet extra room all around. The base was prepared with some pea shingle and a layer of concrete.

Before the holding concrete was poured, it rained and the hole collapsed. My ground worker, who reassured me that he had installed many rainwater tanks, told me it was OK as whatever the size of the hole was, it would just be filled with concrete.

That, as far as I can recall, is what happened. The tank was part filled with water and concrete poured. Next day, the tank was filled again and more concrete poured to over half the diameter. When set, the soil was replaced.

The garden is still a work in progress as the house is not complete yet. The inlet pipes are not connected to the gutter but they are visible at the tank. There is no visual evidence of the tank rising; it is still about half filled so hopefully, pressures are equalized.

But when I drop a scaffold pole in front or behind the tank, it does not hit concrete at about the 1m mark; it just keeps going until I hit soft soil. So where has the concrete gone?

Water is probably following the gutter pipework, but on the outside of the pipes, following in the pea shingle trench, which would be it's easiest route. Hopefully, when the gutters are connected, not some much water will be falling directly onto the ground, which may alleviate the problem somewhat.

The ground worker has been summoned, but in a phone call with him yesterday, he has not encountered this before, so I guess was are all guessing.

I hope you are correct and the issue is only soil compaction. That would be the best possible answer, but I can see the problem being a money eater.

Rex

 

I'm having trouble visualising this, but concrete does not just vanish, if there is no concrete around the tank now, then there never was, surely.

Did you actually see the concrete go in?

Or was the surrounding hole just filled with soil that had caved in?
And concrete poured on top.

Surely the caved in soil should have been removed before pouring the concrete

 

Rex - my main concern was that you had done this yourself and not been aware of the risk of the tank rising. But it clearly has been done by a professional in a professional manner. Though the collapsed side raises a bit of uncertainty. However a small collapse shouldn't be a problem.

Unless you are over an old mineshaft or the equivalent, I can't see that your concrete can have disappeared or even moved. It must be there, as long as you saw it poured in. If the tank hasn't risen - and I don't see why it should have done from your description, I don't see that you have a problem. I would think that you could continue to fill in with soil.

However, being me, I would be tempted to dig a small pilot hole down by the side of the tank to see when you do hit concrete. If you were to find it in about the right place, it would set your mind at rest. I would make a hole in a place where I had seen the concrete - ie not where the collapse was.

 

Thanks for the replies.

I have to admit that I do not recall seeing concrete being poured, but as I regularly visit the site, I must have seen it before the soil was replaced. And yes, I know it cannot vanish.

However, on one photo of the ring beams being poured, there is a line of concrete leading to the tank area. The specs included concrete around the tank and that is what (I think) happened.

The tank is not attempting to pop out of the ground (yet) so a small hole to one side will tell me if there is concrete down there.

I don't see how it could have happened but I think the soil has been undercut by the water erosion and the concrete has split (like one would break a Mars bar or similar) and pulled away from the front and back. This would explain why I can see the tank but not the concrete.

Rex

 

I still cant visualize, whats going on, I see a tank in a hole, with concrete poured around it.
If the concrete has split, as you suspect, then surely it was not thick enough and needs to be removed and the hole around the tank re- excavating and new concrete poured.

If its been washed away, (concrete that is), I certainly hope your foundations/ footings to the house were layed by someone different.

 

I can't see why the concrete should split. When concrete is poured round the sides it will fill up any spaces that there are, and will self level. Even if the sides of the hole have collapsed in one or more places and the hole is partly filled with earth, the concrete will still flow round, so that the bottom of the hole is completely filled. Once the bottom of the hole if filled with concrete there are no forces on it making it want to split and no space for it to split into (if you see what I mean).

The concrete can't have washed away - there's nowhere for it to go to. The soil under the tank, should not have been disturbed and is very unlikely to move

I think you have got to dig a trench round part or all of the tank to see exactly where the top of the concrete is. My biggest fear is that when the sides collapsed. the soil might have filled up the majority of the hole, and as a result they didn't bother to pour much concrete. But even so I would have thought they should have poured it on top of the collapsed soil, in which case the top of the concrete would be even closer to the surface.

 

Thanks for the various replies and suggestions.

I have the groundworker who installed that tank coming to have a look-see.

It is the same guy who did the foundations, but since the house is sitting on 36 x 10m deep piles and a 40 x 50 ring beam, I think it should be OK. All the concrete was brought in from the local concrete mixing plant and conformed to the required specs.

I agree that I cannot see how the concrete collar could split but that is the only explanation as to why when I drop the scaffold pole in front of the tank, I do not hit concrete at about 1.5 meters.

I'll keep you posted!

Rex

 

Is the ground a bit unstable? Boggy?

Just seems so, if piling is necessary.

 

Piling was necessary because it site is clay and that was the requirements. The previous bungalow was just sitting on a solid slab.

Have sound out what is happening and it will be put into the hands of the insurance company today. The tank is barrel shaped, lying on its side. The ends were convex, but I guess due to ground pressure, they are now concave. It's like the tank is imploding.

Since the bottom half is sitting in a concrete mold, I guess the ground pressure is from the top and pressing on the ends. Don't understand how, as the tank was installed about one year ago, so the ground at that time was wet. If anything, the ground should have pulled away as it dried during the summer.

Will see what the insurance company have to say as it was all done under the build insurance.

Rex

 

Rex - I am glad you have got to the bottom of it, even though the problem is far from solved. Like you I am having difficulty in understanding how this came about. I assuming that the concrete around the bottom is intact and unbroken, but that the top half of the tank, which was unsuported and hence very weak had been pushed in by the surrounding ground. If the top half had been pushed in it would cause the bottom half to follow and to pull away from the concrete mould.

I suspect this could be related to all the rain we had. Water will lubricate the soil. Think of slag heaps that start to move in the rain. And if there was heavy equiptment moving nearby or some sort of pressure downwards, the ground would squish out sideways. Being convex the tank should have had some strength to resist, but obviously not enough. Its a shame the tank had not been completely full of water when the hole was backfilled. The hydrostatic pressure inside the tank would have been able to withstand the pressure from outside if it was just from the soil itself. However if they had parked a bulldozer very close on the soft ground it would have greatly added to the pressure. Had it been dry, I don't think this would have happened.

 

I tend to go along with Peter, surely when encasing the tank in concrete it should be full of water, aleast to the level of the concrete, which in my thinking should go all the way to the top and maybe even covered with concrete lintels or similar, to take any heavy traffic in the future.
Of course that would depend on the positioning of the tank.