Sheet Pile Cofferdam to Rock
Sheet Pile Cofferdam to Rock
(OP)
I have a design for a client for a sheet pile cofferdam to provide support during pouring of a pier footing. The borings indicated boulders and varying top of rock elevations. Due to this variation, the contractor is required to overexcavate down to rock and backfill with Class C concrete to the bottom of footing elevation.
So the contractor plans to drive the sheeting to refusal. This could be anywhere from 4' to 12' below the bottom of footing. The contractor does not want to have to pin the bottom of the sheeting to the rock. Instead, they would like a design that allows them to maximize the cantilever length of the sheeting below the lowest waler/strut and rely on the flexural strength of the sheeting. This would only be a temporary condition then, since as soon as top of rock is found everywhere, they will pour up to the bottom of footing with Class C.
In all previous instances where rock was expected, I have had the contractor pin the bottom to the rock. My main question with what the contractor is asking is whether this changes the soil pressure diagram since there is no longer an embedment or pin at the base. Are there any other concerns that need to be addressed?
Any references or any advice from someone who has done a similar design would be appreciated.
Thanks,
Dan
So the contractor plans to drive the sheeting to refusal. This could be anywhere from 4' to 12' below the bottom of footing. The contractor does not want to have to pin the bottom of the sheeting to the rock. Instead, they would like a design that allows them to maximize the cantilever length of the sheeting below the lowest waler/strut and rely on the flexural strength of the sheeting. This would only be a temporary condition then, since as soon as top of rock is found everywhere, they will pour up to the bottom of footing with Class C.
In all previous instances where rock was expected, I have had the contractor pin the bottom to the rock. My main question with what the contractor is asking is whether this changes the soil pressure diagram since there is no longer an embedment or pin at the base. Are there any other concerns that need to be addressed?
Any references or any advice from someone who has done a similar design would be appreciated.
Thanks,
Dan





RE: Sheet Pile Cofferdam to Rock
Said otherwise, if that is his plan, a rigid inner body needs be planned and maybe even some anchoring downwards to avoid surprises. Anchored, rigid and in place you may start underwater concrete operations, likely with the same level of water inside than outside since the boulders and the construction gap may not leave another alternative.
RE: Sheet Pile Cofferdam to Rock
RE: Sheet Pile Cofferdam to Rock
www.PeirceEngineering.com
RE: Sheet Pile Cofferdam to Rock
There will be at least 2 levels of braced walers. The first level will be a few feet from the top of the sheet piling and the second level will be about 9'-10' below the top of the sheet piling.
As an example of what the contractor is asking, at one location the sheeting may extend down 6' below the second level of walers/struts at which point it hits rock and he can't get any embedment. He is required to excavate down hto this point, but He doesn't want to pin it to rock, instead he is asking whether the flexural strength of the sheeting can act as a cantilever and resist the bottom 6' of soil/water pressure.
I have always pinned it, but this contractor is indicating that he has done this before. I'm just not sure how to analyze it.
If I understand what you are recommending, a better solution would be to push the perimeter of cofferdam out and use that additional clearance to slope up from the rock and provide an embedment that way? With a sloped embedment, that woudl probably require a significant offset to get the required embedment depth, so I'm assuming the contractor won't be thrilled with this option.
cntw1953: I'm not sure I understand what you mean by treating it as a 4' ring.
RE: Sheet Pile Cofferdam to Rock
If you have Civil Tech Shoring Suite 8 there is an option to turn off the toe resistance.
RE: Sheet Pile Cofferdam to Rock
That is exactly what I am doing. However, if you review my original question, you'll see that I wanted to clarify whether the soil pressure diagram changes in any way since there no longer is any base resistance.
Otherwise I agree, the top braced waler will resist tension and the lower braced waler will resist compression. The sheeting will be analyzed as a cantilever.
RE: Sheet Pile Cofferdam to Rock
RE: Sheet Pile Cofferdam to Rock
www.PeirceEngineering.com
RE: Sheet Pile Cofferdam to Rock
Wedlmic