Building an underground water tank... - How would you go about it?

[Ted]

In Africa, underground water tanks are pretty common as the utitlity companies do not supply many areas and reliance is made on privately owned tanker trucks doing water rounds.

A basic water tank will be made in the following way:

i) Excavate suitably sized rectangular shaped pit;
ii) Dig a hole in each corner and pour a footing with rebar sticking up vertically out of it;
iii) Shutter the rebar and pour four reinforced concrete columns, typically 20cmX20cm by whatever height the tank is to be;
iv) Pour a reinforced floor slab for the bottom of the tank and finish it with a steel trowel to assisit with its waterproof qualities;
v) Build a single skin of block work off the floor slab in between the four columns to the height of the columns;
vi) Render the inside of the tank producing a cement plaster finish like the floor slab;
vii) Shutter the proposed underside of the top slab and pour a reinforced concrete slab off the walls and columns to provide the top cover at natural ground level, not forgetting to position a manhole cover in place for access.

The ground here is typically sand or laterite. Some tanks will be built to a higher quality with rebar well tied together and some with hollow concrete blocks and reinforcement. Te quality of workmnship varies a lot.

Last year a civil engineer gave me plans for a tank like this which I said were a load of crap and I wouldn't build it like that. He subsequently got someone else to build it and it has now collapsed. I have not seen it in its collapsed state so don't know where it failed, but I made it clear I thought it idiotic not to reinforce the walls.

If you were going to follow the above approach in what areas would you consider improving it, if any.

Or if you think the above idea is dreadful, how would you go about building an underground tank?

[lutonlagerlout]

i think i would shutter the whole thing, doing the floor and walls in 1 pour to aid water retention,the roof could be cast last
block walls in general are weaker from lateral force than downward force,as the tank empties and gets filled the forces on the walls changes and creates a lot of stresses ,e.g. swimming pools should not be left empty too long
in fact a swimming pool spec would be ideal IMHO
LLL

[seanandruby]

excavate to dig level, blind with concrete. put in spacers 40 /50 ml and place steel on top of these. build the steel mat, cage or what is specified. Around the edges ( allowing 30/40 ml cover for concrete) place L bars to come up through the kicker for use as starter bars to splice to. Build edge shutter and hang kicker for base. Pour slab and kickers in one. Make sure kickers are fairly level in case you have to use hydrotite strip. next day strike shutter scabble and clean top of kicker. Fix steelwork to walls topping with U bars to carry cover slab, they need to be higher than finished wall height. Erect shutter to walls and pour. strike next day and put false-work up to hold the ply for the cover slab, place steel mat , side shutter. Box out for opening 600 x600 plus 100 ml rebate around the top allowing sufficient depth to bed cover and frame then pour concrete. hope this makes sense. good luck.

[Tony McC]

I think Sean has got it pretty much spot on - when he writes that fast he makes it seem easy!

Incidentally, Sean, where would be the best place to get me paws on those hi-vis end caps that are supposed to be fitted to protruding kickers? The price quoted by my local rebar supplier is urine extraction of the highest quality!

[lutonlagerlout]

i reckon sean has done a bit of shuttering
LLL

[flowjoe]

excavate to dig level, blind with concrete. put in spacers 40 /50 ml and place steel on top of these. build the steel mat, cage or what is specified. Around the edges ( allowing 30/40 ml cover for concrete) place L bars to come up through the kicker for use as starter bars to splice to. Build edge shutter and hang kicker for base. Pour slab and kickers in one. Make sure kickers are fairly level in case you have to use hydrotite strip. next day strike shutter scabble and clean top of kicker. Fix steelwork to walls topping with U bars to carry cover slab, they need to be higher than finished wall height. Erect shutter to walls and pour. strike next day and put false-work up to hold the ply for the cover slab, place steel mat , side shutter. Box out for opening 600 x600 plus 100 ml rebate around the top allowing sufficient depth to bed cover and frame then pour concrete. hope this makes sense. good luck.

Took the words right out of my mouth :laugh:

[seanandruby]

( Incidentally, Sean, where would be the best place to get me paws on those hi-vis end caps that are supposed to be fitted to protruding kickers? ) AKA mushrooms. ....Buidspan, or hy-ten. think buidspan are pretty much everywhere and hy-ten are in surrey.
In my game you have to be hands on with everything. at the moment my manholes are constructed up to kicker height then i lift them in by crane, or 360 put in the stubs and rockers shutter the walls and pore in with the ground slab. then go back and add channels etc and bench them. a lot of hard work for me but its the only way to keep ahead of the steel fixers, chippies and of course the concrete

[Tony McC]

There's a Hy-ten in Bootle. They were always slightly more expensive for rebar, but you could be sure they'd always have the right bars/mesh in stock, whereas my local supplier could never guarantee to have anything other than the usual suspects available off-the-shelf, even if he was around 5% cheaper.

I have to go to the HSE Offices in Bootle tomorrow, so I'll nip in to Hy-Ten, if possible. Thanks!

[matt h]

sean has got this one nailed, did three underground storage tanks for Bowaters some ten years ago used preformed scaffold tube formers with curved steel plate in three interlocking sections so the formers could be reused in each pour. Craned in and out shutter dismantled and re-erected same day , cleaned and oiled and pour worked out at three days between each tank. There was a problem with the qs wanting to pour when the shutter wasnt complete on the first one ...nobber anyway we got round the problem after removing the distorted shutter and rebuilding later. Cost them three day delay heyho. Curved shutter gives added strength to the tank, just like victorian wells..

[Dave_L]

There was a problem with the qs wanting to pour when the shutter wasnt complete on the first one ...nobber

Aren't 99% of the QS's you encounter! Biggest PITA's ever!

[seanandruby]

used that system matt. on cold icy days we had to cover the whole structure in polythene and put blow heaters on to keep a constant warm temp. otherwise the heat of the concrete curing against the cold shutter would blow the lot apart. if you get 5 minutes look at the "Doka formwork" site, also "gass formwork" SGB. theres a video on there about a cement mixer and noise. hope it doesnt become compulsory :laugh: had to do a moving shutter to line the inside of a 100ft deep shaft once, f**king nightmare, concrete went off in minutes. imagine chippies, steelfixers.plus the weight of the materials. and hanging below in a compartment where 4 indian concrete finishers. because the rebar centres where to small we couldnt get the pokers in to vibrate, so the first bit had to be broke out and started again. there was six lorries with 2.5 metres in each ( thats all they could carry as it went off to fast. ) had to be sent away because it was going off in the drums. i jacked in the end.

[matt h]

Thats what happens when the designers and the production teams dont know what the process actually involves work wise... some people never learn...Did some work with Balfours and three teams had jacked before we got there as they all couldnt erect a preformed shutter... half an hour with a tape measure and a scale drawing revealed that the shutter kicker was 15 degrees out...checked with theodolite and hey presto my calculations were proved correct.. Two days of jack hammering and rebaring later we erected the shuter no probs... got laid off after they finished the pour cos they were too embarrassed to admit they were at fault:( Two weeks later asked if we'd go back and sort the other side of the bridge as they couldnt erect that one either... I was very polite with my refusal considering:p

[matt h]

looked at both those sites sean.. when it becomes must have, I suppose we will all have to have such equipment on standby... can you imagine the cost of pouring a concrete base in a back garden if you had to erect that stuff everytime...it will undoubtedly become law some time but until then its plod on...