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Sheet Pile Wall Design Considering Wall Displacements During Construction

Sheet Pile Wall Design Considering Wall Displacements During Construction

Sheet Pile Wall Design Considering Wall Displacements During Construction

(OP)
I've got a 20-ft deep underground vault, about 30 ft by 40 ft in plan, open at the top, formed by driving sheet piles in a box shape. After excavation, a 2-ft min. thick slab will be poured at the bottom to seal the vault off from groundwater. There is estimated to be about a 16-ft hydrostatic head causing pressure at the bottom of the slab, which must be resisted.

I've computed the necessity of having shoring (waler) frames at about 3 feet and 16 feet from the top of the vault respectively. After the seal slab is poured, the lower waler frame could be removed (I'd use something like a hydraulically-operated shoring system for the lower waler in that case).

The typical design examples I see in textbooks, and the sheet pile design software I am using, do not seem to take into account the changing deflected shape of the sheet pile during construction. For instance, it will always set the deflection value at the shoring points at ZERO deflection, even though at the time of the shoring installation, the deflected position of the point at which the shoring is placed may be quite significant.

I realize this can be considered relative deflection, but I cannot tell if the REACTIONS that it's giving me for the shoring take this into account. I've tried checking it by running several cases with different excavated depths, and using a spreadsheet to try to determine the actual deflected shape when all is said and done, but it doesn't seem to correlate with the reaction forces the program is giving me.

And even more to the point, if I shore everything, then place the seal slab, the program considers the seal slab as a "shore point," and gives me a value for the compressive force per unit length along the edge of the slab.

If I use this force to compute the resistance of the slab against hydrostatic uplift (assuming a coefficient of friction between the concrete and the steel of the sheet pile, established by the Florida DOT design manual - which seems to have become a sort of de facto standard for this purpose), the slab works to resist the uplift.

But I don't think that force is really there. Unless I remove the lower shore, and allow the sheet to deflect and "clamp" the slab, the actual "shoring force" at the slab appears to me to be ZERO, in which case I might have to resort to e.g. welded studs for anchoring the seal slab.

What am I missing here? Is it that the method is so approximate there's an assumed factor of safety?

RE: Sheet Pile Wall Design Considering Wall Displacements During Construction

I would not rely on friction between the sheet piles and the concrete slab to resist uplift. You either need to increase the thickness of the slab (or add other weight) or you need to relieve the water pressure.

Mike Lambert

RE: Sheet Pile Wall Design Considering Wall Displacements During Construction

Twenty years ago, I designed a cofferdam like this for a refinery tank. I used welded shear studs to attach the concrete slab and walls to the SSP and used the SSP embedment below the bottom of the slab to provide additional uplift resistance. It hasn't floated yet.

www.PeirceEngineering.com

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