Underground water tank

[JimG]

I'm looking to get some guidance from fellow members of the forum in an are a where I don't have much experience viz the DIY construction of an undergr ound concrete water tank. The tank is for collection of water from land dra ins - for subsequent usage as irrigation when required.

The soil in heavy clay and the water table is quite high. The proposed int ernal dimensions of the tank are 2.85M x 1.5M x 1.5M deep and the top surfa ce of the tank will be 0.25M below ground.

The research I have done so far suggests the best way to go is to use 100mm dense hollow cement blocks with 6mm rebar every 100mm vertically and filli ng each block with concrete mix. 6mm rebar would be placed between every co urse. Base to be 4" with A142 mesh.

Is this a reasonably sensible approach, given that I'm retired and have lot s of time available.

And also how should I tie in the sides to the base?

Any guidance would be much appreciated.

[Rick Hughes]

Need to consider 'Clay Heave' which can be significant, that will depend on local land, slope, ground etc.

From the way you are thinking of going you need to make sure that you have vertical rebar well formed to be part of the floor ... to ensure floor & walls are adequately locked ...

90 degree bends and at least 450mmm in the horizontal well tied to the mesh. You can have these as 300 upstands and then tie vertical rods to them as you 'grow' but need to get them cast in.

Remember you need minimum of 50mm cover on all the rebar.

[Bob Minchin]

Not something I've done but I'd lay a single course of the blocks first. Fit the rebar hammering the ends into the soil. Lay the concrete base to your desired thickness on top of the blocks then build the walls as you suggest.

[Andrew Mawson]

I would lay a base slab reinforced with mesh, and plant strategically placed uprights of rebar with a hook in the lower end round the periphery. Then when the base concrete has set build the walls with 200 mm 'hollow concrete blocks' of the sort that have two rectangular openings with them threaded onto the re-bar. Fill the voids with dry mix as you go up. Then cover with concrete lintels or 'block and beam' incorporating a suitably sized access man hole. No real need to worry about the walls being tanked and waterproof as it is land drainage anyway :)

Andrew

[JimG]

As for the "clay heave", what I can tell you is that in winter, the water t able rises right up to the surface so the lawn is totally water-logged. I g uess this is because we are sitting on clay soil at the bottom of a slope. I reckon heave is a potential problem also.

To avoid the risk of the tank "floating" upwards, should I drive/extend som e half inch rebars horizontally from the floor into the clay outside the pe rimeter of the tank? Or go one stage further and undercut the vertical side s and extend the base with rebars set in concrete so as to anchor the tank and also prevent leverage from pressure in the soil trying to crush the tan k?

I am reminded of this as when we were out in Zambia, a few people who pumpe d out their swimming pools in the rainy season, to clean them, found that t hey had popped out of the earth because of the hydraulic pressure.

Just love these engineering challenges!

[Tim Lamb]

In message , Andrew Mawson writes

Why not just bury a second hand unbunded fuel tank? The one I currently use for farm Diesel is roughly the dimensions proposed. These tanks are being replaced because the current legislation requires them to be bunded. Find the local installer and ask what he does with the old ones.

Clay heave can be countered with compressible foam lining.

[Rick Hughes]

you are right pools can float.

Might be worth a chat with local BCO ... he may be aware of how much 'care' is needed in local conditions.

[JimG]

The reason is that the lawn is at the rear of the house and the only access is via a side passage that is only wide enough for a wheel barrow.

[Tim Lamb]

In message , JimG writes

Afternoon crane hire £150?

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[Andrew Mawson]

Oil tanks come in all shapes and sizes precisely because some unfortunate people do have access problems. I've seen really slim ones

Andrew

[tony sayer]

In article , Tim Lamb scribeth thus

150 quid?. Where and what sort of crane?, seems very cheap best quote we had for 20 metres height under the boom was a tad short of a grand;!...

[Dave Liquorice]

But I doubt 6,000 litre ones which is less than the capacity of the proposed constructed tank.

[Tim Lamb]

In message , tony sayer writes

A guess frankly:-)

But Tonout? at Welwyn Garden City were prepared to negotiate price for a part day job. 150 was my recollection from some years back. Clearly a long front garden wouldn't help.

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[Rick Hughes]

I have 2 x 5,000L plastic water tanks which take all my surface water .... my annual water bill is now peanuts.

[Rick Hughes]

As mentioned I have 2 x 5000L plastic rainwater storage tanks .... movable by hand ..(or crane) Fully buried, just access hatch visible.

A messy way (but workable, is several bulk juice containers connected together.

[Dave Liquorice]

grand;!...

The size of the crane required will be governed by how heavy the object being lifted is (plastic tank, 4/5ths of bugger all)and how far the maximum horizontal reach is.

[Muddymike]

In message , tony sayer writes

I arranged the replacement of MILs oil tank about 20 years ago. The same problem of narrow access to back garden was overcome by using a round plastic replacement oil tank, and rolling it over the roof of her bungalow. Four blokes, two long ropes, two ladders, and it only took half an hour or so.

Mike

[tony sayer]

In article , Muddymike scribeth thus

Theres a vid on Youtube of that going wrong somewhere;)...

[tony sayer]

In article , Tim Lamb scribeth thus

I remember the time it was that "sort of" money but not anymore, seems you have to arrange all the usual H&S stuff and Insurance yourself, or get them to do a contract lift..

One we had this year for a lift under the jib of 35 metres was £1300 in mid Suffolk;!..

[tony sayer]

I suspect it thats off a small vehicle with a crane attachment that will be a different entity to a proper purpose built crane but the few like that I know off are OK up to 3 ton's IIRC but their reach is sod all..