Pouring a thick concrete slab into a hole full of water

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I want to lay a thick concrete slab to act as a base to a telescope. The slab is to be about 700*700*600 deep. I've dug the hole to the correct dimensions and now ,because of all the rain, I have a fish pond!
Is there any way I can pour the concrete without waiting for the water level to drop. Pumping out is no use as it fills up straight away. Is lining the hole with polythene and then pouring an option?
 
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Why does it fill up again? Could be something that will compromise it once you pour it.
 
You can pour dry mixed cement, sand and ballast into a wet pit until all the water has been soaked up. Then continue with a wet mix, We did this ( with the building inspector's approval ) for some of the pad foundations for our house.

As mentioned above, do you know why the hole fills up straight away ?

How far below ground level is the top of the water ?
 
I dug most of the hole a few months ago and it stayed dry. With all the recent rain, the ground is saturated. I have been pumping it out so I can get the hole to its final level, but after a few minutes the water is ankle deep again and overnight it fills to about 500mm deep. The top is loamy soil and underneath is slate and clay, very hard to dig through. It used to be farm land.
 
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If you can mix and pour slightly wet mix faster than the water rises then pump the hole almost dry and start pouring. The liquid concrete in the bottom of the hole will then stop more water coming into the hole. Then keep pouring wet mix until the hole is filled.

Concrete needs water to set so some excess water is not a problem unless it is either (a) washing material away or (b) allowing the mix to separate out.
 
The hole is currently about 350mm deep with water so it has gone down a bit (picture) I think that pouring even dry mix in could cause it to separate out.
 

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petej42, good evening.

I have many years ago placed a load of concrete under water, the job was a "Dolphin" to which a MoD LCT would be moored, we had to place several hundred M3 of Concrete into the sea as a "Toe" for the ramp.

OK we were moving a load more concrete than you are contemplating and the Concrete was direct out of several mixer trucks

We use a plastic ball of suitable dimension to stop the sea water entering the tube and giving the concrete a better chance.

As I recall, the trick is to keep the discharge end of the pipe below the level of the Concrete, in effect you are "Bottom Feeding" the concrete being placed.

And yes it worked very well, it followed a week or more of a diver removing sand with an "Air Lift" suction system.

What is Tremie method of concreting?
The tremie concrete placement method uses a vertical or nearly vertical pipe, through which concrete is placed by gravity feed below water level. ... Concrete must be poured at a rate which avoids setting in the tremie. Admixtures may be used to control setting time, slump and workability.
 
yes as Ken mentions that's also how they do some types of piling, to avoid the bore collapsing they keep it full of water and then displace with concrete from the bottom up, to displace the water without mixing excess with the concrete, which would affect the strength.
Your polythene idea seems alright as long as you can avoid it overflowing water inside the concrete area. The only think I wouldn't really just go chucking the concrete in with the water, by the time it hits the bottom the cement may well be floating around separately in the water and the aggregate may be fairly clean.
 
Another method, used in at times deep piling, but not individual piles more a "Wall" built before the earth is removed.

Diaphragm walls using not water but bentonite [a form of fluid clay] the clay is displaced by Concrete forcing the bentonite out,

To be clear a deep trench is dug, filled with Bentonite [which is fluid] as the dig proceeds down, these things go down up to as far as the arm on the digger can reach. We then have a Bentonite filled trench, the walls do not collapse the Concrete is Tremmied in and the Bentonite is expelled the Bentonite is filtered and re-used, in all a complex procedure, requiring some Pre-Planning.

Did one small one, only once, as previous post it works very well. OK the surface finish is crap but when you are trying to drain the swamp the last thing you need is to be up to your wast in Crocodiles.

Ken.
 
I came across the Tremie method whilst looking on the web for a solution, but I had not come across the Bentonite one. Both seem a bit complex to achieve for a comparatively small hole, especially seeing as I am mixing and pouring the stuff myself.
I think I am going to have to go with the polythene method if no one thinks it is a bad move. I have the ballast and cement ready to go and have cut some wooden formers. I have a small mixer so will probably take 10 to 15 loads. Hopefully, if I can pump most of the water out I should get enough mixes in before the water rises up to much, especially if the water level drops a bit more. I could probably dig a bit of an "expansion" trench thinking about it.
 
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Important thing is get your concrete flat on the ground underneath and straight vertical sides, to keep the forces symmetrical. So make sure no large void underneath from water that didn't escape
 
This is a common problem when building swimming pools. You dig your hole, the next day it rains raising the water table and you have an instant pool. You can't just pump it out and hope to get the concrete and walls in quickly because of the water pressure. The solution is to dig a small but deep sump hole next to the main hole that you can keep pumping, with a float switch if required, until the job is done. A smaller version of this could be used in this case, keep pumping until the concrete is in.
 

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