tremie seal concrete slab design

[morganjoe]

My question has to do with sheet pile cofferdams that are constructed in areas of high water table. Dewatering of the site is not allowed, so a tremie seal concrete slab will be placed at the bottom of the excavation. When designing the tremie seal slab, does the weight of the slab need to completely resist the hydrostatic pressure, or is a bond between the sheet piles and the concrete included. If a bond is used is it something like 5 psi, and then do you check theslab as an unreinforced slab. Is there any documentation, standards, or manuals that would outline the specific design of tremie seal slabs required thickness. These cofferdams are not for footing where concrete piles are present in the interior of the pit.

 

[PEinc]

I would use weight of concrete only. Assuming a bond between the sheets and tremie poured concrete is risky. Assuming you know how the concrete will bond and, therefore, bend is also risky.

 

[DRC1]

I agree with PEInc. I only use the weight of the concrete. Note that for submerged concrete the bouyant weight is about 80 pcf. Usually the tremmie is thick enough that stress in the concrete is not an issue.

 

[SlideRuleEra]

Concur with PEinc and DRC1. Here is some general information: "Design Loads and Construction of Tremie Sealed Cofferdams" at this link

http://www.pdhcenter.com/courses/g113/g113.htm

http://www.SlideRuleEra.net

 

[DRC1]

SlideRuleEra,

Could you post this to your website as itappers acess is limited, but the paper sounds very useful. Thanks

 

[PEinc]

I had no trouble opening and saving the document as a PDF.

 

[morganjoe]

Thank you for your responses, and I agree with you, but I have run across numerous designs that don't seem to use this design procedure.

Example:
Pit is 17'x25'
Water table at a depth of 8'.
Top of tremie seal slab at a depth of 27'.
They come up with a 7' foot thick tremie seal slab.
How exactly does that cals?

 

[PEinc]

I have seen many jobs where the thickness of the tremie pour shown on the contract drawings seems to be way too thin for the hydrostatic head. Unless the hydrostatic head is being significantly reduced as the water flows around the bottom of the sheet piling, you have 19' of head to resist. You would also need a safety factor in the calculation of your tremie thickness. If SF = 1.5, the tremie seal needs to resist 27' - 8' = 19' water head. 19' x 62.4 = 1186 psf uplift x 1.5 = 1778 psf / (145 - 62.4) = 21.5' thick tremie.

This is much thicker than 7'. Also, if the tremie is much thicker than 7', then the cofferdam will need to be much deeper with extra row(s) of bracing and more excavation.

Frequently, you can increase the sheet pile embedment below planned subgrade enough so that you can dewater without the bottom of the excavation heaving or piping (turning into a quick condition). Then, you won't need a tremie seal and you will need less excavation and bracing. I always try that before adding a tremie seal, especially if the new foundation need bearing piles.

 

[DRC1]

27-8 = 19 ft difference in elev.
62 x 19 = 1,178 psf net pressure
1,178 / 150 = 7.8 ft concrete round to 7 feet.

I have seen similar calculations. Not correct, but probably how it was done.

 

[PEinc]

DRC1, if the 19' head is to the TOP of the tremie concrete, then the concrete is submerged, buoyant, and weighs less than 150 pcf. Plain concrete weighs less than 150 pcf and you need a safety factor. Agree?

 

[DRC1]

I definately agree. Morganjoe was wondering how they got 7 feet. I'm betting that that is how they got it. I'm not saying that is the right way, but I have seen similar calculations and that is my guess as to the thought process.

 

[morganjoe]

Thanks for all the input. My calcs come out like yours PEinc. I always calc it like that. I wanted to make sure I was not missing something. The contractor always seems to be surprised at how thick the tremie seal slab needs to be, and I alway get the response of some other job they did with more hydrostatic head and a much thinner slab. I have seen some of those calcs, and whatever resistance that is needed over and above the wieght of the concrete, they put into a bond or some type of friction between the sheets and the tremie seal. They then check the shear of the concrete at this interface.

I am not feeling comfortable assuming some type of friction or bond between the sheet piles and the concrete without some widely accepted documentation or design procedures. I can't find any. I also don't want to be overly conservative with my designs. Any further discussion of this topic would be greatly appreciated.

Thanks.

 

[PEinc]

You need to decide if a tremie seal is really needed. See my previous post.