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darogers (Structural) (OP)
17 Nov 09 10:43
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
ishvaaag (Structural)
17 Nov 09 10:57
If I understand well when the contractor has reached the wanted length, he starts to retire boulders and then leaves the sheetpile wall without bottom support. If required (you don't have a rigid inner structure supporting enough the sheet-pile walls, or any other way of support, and then bottom support is expected) I don't see this acceptable not even for temporary work.

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.
cntw1953 (Civil/Environmental)
17 Nov 09 10:59
I would treat it as there is 4' ring beam below the bottom of footing and see how it works.
PEinc (Geotechnical)
17 Nov 09 11:13
If you can't get sufficient embedment of the sheet piling below the expected subgrade, then you need at least two levels of braced wales, or you need at least one level of braced wales and the toe pins into rock.  Also, if the sheets will stop higher than your expected bottom of excavation, then you should set the sheeting walls back about 5 feet from the edge of excavation so that you can leave a rock bench or stable slope from bottom of sheets to subgrade.  This sheet pile setback means that the cofferdam will be significantly larger than if you could get sufficient toe embedment below subgrade.  If the sheet piles hang up on boulders or if the top of rock elevation varies significantly between adjacent sheet piles, you could have excessive leakage under the sheets and loss of soil which could fail the cofferdam.  

www.PeirceEngineering.com

darogers (Structural) (OP)
17 Nov 09 14:27
PEInc:
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.   
InDepth (Structural)
17 Nov 09 14:41
What is the issue? Check the bottom of the sheet pile as a cantilever without using the soil embed for resistance. i.e meaning that your wales are the restraint points for the system (as PEInc indicated...2 levels required...1 to resist the compression reaction, lower wale, and one to resist the tension reaction, upper wale).

If you have Civil Tech Shoring Suite 8 there is an option to turn off the toe resistance.
darogers (Structural) (OP)
17 Nov 09 15:07
InDepth:
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.
BigHarvey (Geotechnical)
18 Nov 09 3:40
Don't forget to check hydraulic conditions as well. I suppose you don't want to see materials running inside your excavated cofferdam.
PEinc (Geotechnical)
18 Nov 09 10:51
The sheet piling can be designed to cantilever below the lowest braced wale.  However, you need to check that there is enough temporary sheet embedment available in order to install the lower braced wale without the sheets kicking inward before the braced wale is in place.  If there isn't enough embedment, then you need to install both upper and lower braced wales at about the same time and then lower the lower braced wale as the excavation progresses.

www.PeirceEngineering.com

wedlmic (Structural)
18 Nov 09 14:06
You need to perform an analysis for each stage or condition of support. The struts,walers and sheeting would be design for the worst case of any intermediate stage. An apparent pressure envelope is not the usual method used in cofferdams - they typically use hydrostatic pressure distributions / net pressure diagrams. The net pressure diagram would change depending on the internal and external resitances and forces - an for no other reason. When you remove the soil inside the cofferdam, the net pressure diagram will change. The final condition would resemble a trench box. You want to try and keep the net resultant within the strut spacing to reduce the potential for tension in the struts / connections. Hanging the lower waler is highly recommended (slightly underdimensioned)and lowering the wale as excavation progresses - easy to shim later. Wale seats also recommended and not just for constructability. Depending on the base dimensions of the plug (slab) and uplift forces, you may want to install a stand pipe in the slab to relieve pressure.   

Wedlmic

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