Cofferdams Seal Slabs - Your opinion
Cofferdams Seal Slabs - Your opinion
(OP)
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.
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.





RE: Cofferdams Seal Slabs - Your opinion
RE: Cofferdams Seal Slabs - Your opinion
RE: Cofferdams Seal Slabs - Your opinion
RE: Cofferdams Seal Slabs - Your opinion
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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RE: Cofferdams Seal Slabs - Your opinion
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,
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