Footing Resting On Piles

[foxview11]

Our QC inspector requires us to remove the wet and loose soil which is present after excavation for footing to be supported by bored piles. As we can see in the site, the foundation level is below water table and it is not possible to compact the soil with required 95% compaction. Is it necessary to remove the soil and replace with granular fill for a footing resting on piles? I think, it may not be necessary since the footing is supported on piles.Please any expert on this, may I solicit your opinion. I am new on this kind of foundation..thanks a lot..

 

[EngineeringEric]

I think you already have a professional's opinion on the matter who is far more familiar with your exact situation.

 

[foxview11]

EngineeringEric,
I just want to seek second opinion, if any who can provide better idea on this situation

 

[DaveAtkins]

I wonder if the inspector is concerned about pouring the pile cap on wet and loose soil. You are correct--the soil won't matter once the concrete sets up, because all load is supported by the piles. But if you end up with a poorly constructed pile cap, that could affect load carrying capability.

DaveAtkins

 

[foxview11]

DaveAtkins, granting that all works are properly done and the pile cap is designed to transfer the load to the piles, there's no way for the pile cap to fail because it is soley supported by piles not by soil directly underneath. My question is, in what way that the pile cap is poorly constructed?

 

[EngineeringEric]

If the soil is loose and wet then you cannot place concrete in my opinion (think plastic settlement as the concrete cures it is slowly lowering).

I'd say remove the loose soil & place self compacting granular fill. place a mud slab. add some large pumps around the area to de-water.

 

[BUGGAR]

I think you have the basis for a construction claim. If they require you to build something that can't be done, like achieve 95 percent compaction under water, than you have a cause of action against the holder of the contract. Start compiling your delay costs immediately. And document the time you spent here trying to find a solution. We can submit invoices for our time which you then can pass on with a markup.

 

[SlideRuleEra]

foxview11 - I agree with the QC Inspector. If the wet uncompacted wet soil remained in place, it can be disturbed during concrete placement and mix with the concrete. This will compromise the quality of the concrete at the bottom of the footing, degrading that concrete (cover) protecting the bottom rebar from corrosion.

EngineeringEric's solution is correct, but you probably don't need a mud slab or large pumps. Excavate one foot deep, or so, below and to one side of the footing bottom. Backfill the excavated area with small gravel. Keep a small, shallow "pit" immediately adjacent to the gravel open. Use a modest size diaphragm pump (mud hog) to keep this pit (and the adjacent gravel under the footing) dewatered until concrete placement directly on the gravel has been completed. A nominal 2" or 3" size diaphragm pump should be fine.

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[cvg]

seems clear, plans and specs say 95% then you need to de-water and compact to 95%. if you want to change the design than submit an RFI for proposed change. if you did not plan on de-watering first than your estimator goofed. I dont see any changed conditions here that would warrant a change order.

 

[foxview11]

Thanks to all who contribute different opinions. For additional info, there is blinding concrete before construction of footing.

 

[hokie66]

If there is a layer of blinding concrete, and you are not placing concrete in standing water, I see no problem. Perhaps your QC inspector is just overly pedantic. There is no reason for soil compaction under pile supported footings.