Cofferdams Seal Slabs - Your opinion

[bridgebuster]

My office is in a battle with a contractor over a cofferdam seal slab thickness. On projects that follow the NYSDOT specs designers are only required to provide a workable concept for the cofferdam. The plans show a 4' seal for about 26' of head, assuming uplift resistance from the piles.

The contractor came up with a 14' seal - which would result in a claim since the workable concept is wrong. The designers are trying to justify 4' minimum. As a casual observer, I've noticed there's a disparity in seal slab design.

I've found a chart from a DOT out west that would justify about 4' but I also found literature that justify 15' and somewhere in between.

What is acceptable practice? To assume there's no leakage thus no buoyancy; to assume leakage and assume a byoyant condition? Assume resistance from the sheeting?

I'm curious how others design them.

 

[Ron]

You have to check the "leakage" based on existing, in-place soil parameters. "Boiling" at the bottom of the cofferdam will require a thicker seal slab. Check the flow net for the soil conditions.

 

[bridgebuster]

Ron - As I said, I'm just a casual observer in this matter. It's not my project. I'm just curious about the approach others use for design of the seal.

 

[DRC1]

If you are pumping from the top of the slab, submerged unit wt of concrete is about 85 pcf. An alternative is to use drilled in anchors to stabilized the slab if it is too thin. However for a dead wt seal the 14 ft sounds closer to the truth than the 4 ft. 21*62/85=15. Note that although it exists, adhesion to the sheets is highly variable, especially in a tremie, so it is generally disregarded.

 

[SlideRuleEra]

bridgebuster - IMHO, thick seals are necessary for safe cofferdams. I believe this though is based on Contractors' long term experience for several reasons:

1. Quality of in-place concrete placed by tremie is difficult to verify.

2. Extreme tidal variation (storms, spring tides, etc.) or flood waters can result in deeper water and more uplift than theoretically planned.

3. The design loads (hydrostatic) are real, and large, not based on probability like wind, seismic, etc.

4. The consequences of cofferdam failure can be sudden and catastrophic.

To me, this combination of conditions suggests a cautious, conservative design. A thick seal, that uses simple dead weigh to resist buoyancy, is a good first step.

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[Qshake]

As a casual observer and leaning on experience, 4' of seal concrete is not adequate for 26' of head with a sound safey factor.

I would have expected at least twice that value or even 10'.

There are numerous texts and other publications for the design of seal courses. Flownets as Ron suggests are included for consideration. Design of Temporary Structures by Robert Ratay is one reference.

good luck.

Regards,
Qshake

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