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goldilocks (Computer) (OP)
9 Oct 01 10:52
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
ishvaaag (Structural)
9 Oct 01 13:52
Concreting under water is really an specialty. If it is just the watertable, the only case I have personally found, the alternative I have always seen here, even for riverside locations that need hube pumping, is to lower the watertable and then proceed in dry.

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.
Ron (Structural)
9 Oct 01 17:03
Goldilocks...
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.
emmgjld (Geotechnical)
10 Oct 01 1:00
Placing the open graded stone on the soil surface is usually a good idea. I usually recommend the stone be worked into the underlying soil  
  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.
DRC1 (Civil/Environmental)
21 Mar 04 10:33
Pouring concrete under water is commonly done and is refered to as Tremie Concrete. However it does require a little different technique. Some engineers will alow structural tremies (i.e. including reinforcing steel) others simply use the tremie to get out of the water and place the structural concrete on top of that. In my experience it is probaly about 95% of the later. Concrete underwater works well unless it is exposed to water prior to by covered with more concrete. When the exposed face is in contact with water, a whitish powder forms on the edge which is known as "laitance".( I hope I spelled that right) This prevents bonding of concrete placed above it. It is not a problem for the exterior edges. Tremie concrete is placed using a pump truck. A small inflatable ball is placed in the end of the hose from the pump. The hose is placed agaist the botto fo the excavation.The ball is forced out and the pump starts pumping concrete. The conrete flows out and into the trench. The trick is to make sure the end of the hose never comes out of the concrete, otherwise a joint with laitance will develop. If the hose does come out or has to be moved, the ball is reinserted into the hose and the pump hose is inserted into the fresh concrete. It is not technically difficult, but does require a little practice to get the hang of it. Also it is hard to do in 2 1/2 ft of water, 4 ft is usually the minimum we like to see when doing this.
   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
RDK (Civil/Environmental)
13 Apr 04 9:11
Just remember that the soil bearing strength below the water table can be roughly one half of that of dry soil.

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

Prepakt1 (Coastal)
13 Apr 04 13:19
You can pour concrete underwater using fabric formwork.
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
facca (Civil/Environmental)
14 Apr 04 17:21
I would go a least a foot wider than the wall and higher than the water.If your trench holds up then you are fine if it doesn't you will need to find a method to retain the excavated hole- steel plate or some short sheet pile.If this is your own project just pour the concrete starting at one end and try to displace the water as you work your way to the other side.Use a water pump to remove the water if you don't have a place for it to go.Watch out for the concrete as it gets close to the pump or isolate the pump with a sleel pale with cut slots.Stick rebar in the concrete at 1' centre's  and about 4' high.

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