Pouring under water
Pouring under water
(OP)
I'm about to take my first stab at pouring a footer about 2-1/2 ft below the water table. And as always, I don't really know what I'm doing. Can anybody offer helpful advice?
1) I understand that concrete will harden under water, but does that reduce its strength? would it help to line the trench with plastic sheeting? Or is that not necessary?
2) Does a footer need to be wider under water than if it were in dry ground (because the soil is not as compacted, I assume)?
3) Is it common practice to bring such a footer up above the water table before building on it (I will be putting a 10' high poured wall on it)? That would make it easier to do the wall, but I would have a REAL thick footer.
Thanks for any help--
-G
1) I understand that concrete will harden under water, but does that reduce its strength? would it help to line the trench with plastic sheeting? Or is that not necessary?
2) Does a footer need to be wider under water than if it were in dry ground (because the soil is not as compacted, I assume)?
3) Is it common practice to bring such a footer up above the water table before building on it (I will be putting a 10' high poured wall on it)? That would make it easier to do the wall, but I would have a REAL thick footer.
Thanks for any help--
-G





RE: Pouring under water
Even for bridges it is made so with the help of pile sheet wall plus temporary islands etc.
Another alternative that forfeits much of the problematic cases is the use of steel sheated in-filled or not piles. This way you can pass to soil the loads efficiently and in a far more controlled way.
If you still want concrete under true water, normally at least these things I would consider
first if possible do what above (forfeit concreting under water)
if not, see if the current will disrupt the placed concrete; you may need to build underwater some protective contraption to ensure that you won't be merely throwing concrete to one current able to wholly disperse it.
If it is a footer, it normally never will be one footing on the surface of the submerged ground. A cavity should be extant, and big enough, yes, to take unto account the detrimental effects of washing, the submerged ground strength and spread of the concrete.
Also, this operation if needs diving should be made by experts in the field and in compliance of the safety measures required.
Concreting underwater uses normally concreting funnels, to ensure that the concrete dumped in the funnel when it comes to the bottom will meet surrounding concrete around; this will diminish washing.
RE: Pouring under water
You can place the concrete under water but in the case you describe, you will dilute the concrete and lower its strength to some degree.
One thing you might do, which is relatively easy and inexpensive, it to dig a small trench off to the side but connected to your footing, and make it about a foot deeper than your footing. Just prior to placement, pump from this small trench so that the water in your footing runs into the trench and out the pump. If you have a relatively strong pump, you should be able to get the water out quickly and keep it out while your concrete sets up. Once the concrete has set up (6-8 hours, depending on weather), you can then stop pumping. At that point the water will not adversely affect the concrete, but will in fact, help the strength gain as wet curing.
You might want to over-excavate your footing by 4 to 6 inches and backfill with a layer of #57 stone to keep from having a "mushy" footing bottom prior to placing the concrete.
RE: Pouring under water
OR
You may be able able to place a geotextile fabric, usually a non-woven, light to medium weight 'filter fabric', along the footing base and sides. The fabric will be under water and should extend out to the sides, about the depth distance of the area under water. (Assume 18" thick gravel and 24" wide footing, the actual gravel with will be 18" + 24" + 18" = 60" wide). The fabric will probably have to be 'staked' in place. This fabric 'liner is then filled with a rock/gravel fill, which is rolled or worked into place. The concrete can then be placed on this rock/gravel fill. The actual design of the entire project should be carefully reviewed.
It is also possible to place the fabric under water, with the fabric extending up the sides of the excavation or within the footing forms and then carefully place the concrete within the fabric 'bag'. The concrete will have to be carefully placed, to avoid 'bubbles' of water within the concrete mass. Pumping the concrete will probably be best.
If the water is still or standing, the concrete is fairly stiff (low slump), the loss of fines and lowered concrete strength is not a significant problem. You would do well to obtain some local engineer to look at the project and to have plenty of help during the placement.
RE: Pouring under water
If you are doing this for a light foundation, I would suggest looking at the bags form the previous post or consider using 3/4 in. stone to fill the trench above the water. The stone will be much cheaper to buy and place than the concrete and aside from a little tamping at the top , compaction is not an issue. Then you can put your footer on top of the stone.
Good Luck
RE: Pouring under water
Also don’t forget that you will not see the foundation under water so there is a danger that the footing as placed will not be the size that you want it to be due to slumping of the excavation.
I’d recommend dewatering this and working in the dry.
Around here dewatering is usually accomplished by placing a series of sand points connected to a header. A sand point is a section of pipe, usually 30 in long 1 ¼ diameter with a point on one end for driving and the barrel is screened. It allows pumping from a water table in granular soil. The pump connected to this header must be capable of pumping air because you will quickly get the water table below the top of the point and suck air, which can cause you to loose your prime on the pump.
The water table can be dropped 4 ft or more easily by this method. With points spaced closer and longer pump times I've seen the water table taken down over 10 ft.
The points are typically placed 8 ft on center. Dewatering for sewer and water excavations often takes place in the road center so that the trenches on each side of the road are dry. (25 ft off center.) Setup for a couple hundred feet takes a day and the trench can be dry overnight ready for excavation.
Another possibility to consider is to use precast concrete, wooden or steel piles. The suitability of this will of course depend on your soil conditions.
Rick Kitson MBA P.Eng
Construction Project Management
From conception to completion
www.kitsonengineering.com
RE: Pouring under water
Many large bridges are sitting on foundations placed by fabric formwork bags. Intrusion Prepakt is currently placing 3000 cub meters around the foundation of a 13 century bridge in the Uk using this method. You can use bags as small as 1 cub meter or shaped to make a strip foundation. all you need is a bag an a small concrete pump. Depending on the use there are a no of different concrete mixes you can use.
Intrusion Prepakt
RE: Pouring under water