Trench Footing Placed in Water 1

[kennyb04]

I received pictures from one of our projects showing continuous trench footings being placed in water. The water is half way up the trench, rebar has already been places, and concrete is being placed in the trench as is. Can any one come up with a reference where this would not be allowed or is this common in areas with high water tables. I guess as the concrete is placed, it would push the water down the trench and out and would not mix much, if any, of the mixed concrete with the water in the trench. Any comments on this would be appreciated.

 

[jayrod12]

This is done from what I hear on some bridge pier type jobs however this is not ok.

Especially if the rebar has been sitting in this water for any length of time.

 

[SlideRuleEra]

Is this common in areas with high water tables?

No. A typical method is to pump the water from the excavation with a diaphragm pump (mud hog) then immediately place the concrete before the excavation refills with ground water. Winds up being a "best efforts" technique, but with skill and experience a Contractor can do good job.

Placing concrete directly in standing water will wash out too much cement from the mix, giving unpredictable results.

For larger placements, concrete can be successfully placed under water with a "tremie". Here is a short explanation of the principle:

http://en.wikipedia.org/wiki/Tremie

The concrete first placed with a tremie will go directly into water, thereby loosing cement. Once this weakened concrete is in place the tremie works successfully to place remaining concrete "from the bottom up." The initial weakened concrete is compensated for by making the under water structure (say, a cofferdam seal) of greater thickness than theoretically necessary.

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[Ron]

SRE is exactly right. This will dilute the concrete, increase the water cement ratio, decrease the strength and do so randomly....so you can't predict at any location how much strength you have in the footing. Not good!

This is a bad practice! Do not allow it on your projects. My only change to what SRE noted is that you pump from a sump adjacent to the footing, not in the footing. Pumping from the footing will loosen the footing bearing soils. Pumping from a sump adjacent to the footing will actually tighten the footing bearing soils, if granular material.

 

[oldestguy]

You must have specified a mix or concrete strength required. So get out there and core some footings and run some tests. In the mean time let it be known if tests fail that replacement will be required.

 

[SlideRuleEra]

Concur, using the sump Ron suggested is the way to go.

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[jike]

I agree with above recommendations but must add that footing excavation must also be mucked out (removal of disturbed, soft material) before placement of concrete. As the workers set and tie rebar, walking in the bottom of the excavation may disturb that layer, unless a stone or concrete mud slab was placed in advance. If there is water in the excavation, it can soften the bearing stratum and prevent you from observing whether the excavation is clean or not.

 

[BigH]

Can I inquire as to who approved the placement of the concrete in the water? Any contractor and inspector should know that this is unacceptable. To have done it is, in my opinion, very poor judgement. Exceptions can be made - as noted above, but only because all factors were considered and a best approach was made. In this case it is not. Who, now, is responsible for the non-conformance?

 

[darthsoilsguy2]

i would bet the inspector is the one who supplied the photos... i'm betting there was discussion or disregarded comments on-site.

 

[BUGGAR]

Yeah, I would say the photos are a dead giveaway that someone is telling you something. Time for SuperStructuralEngineer to take charge and do something! K some C's A.

 

[chicopee]

"This is done from what I hear on some bridge pier type jobs however this is not ok." The way we did these pier jobs over water, was first drive the sheet piles to make coffedams, suck out the river mud and sludge with air lift pumps, pour this concrete mud to the bottom of the flooded cofferdams, pump out the flooded cofferdams after the concrete mud had a chance to harden, then tie up your footing rebar to the steel piles that were driven in water, then pour the spcified concrete for the footing, build more rebar and pour more spec.concrete to build the pier.