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SHEET PILE RETAINING WALL - Design Considering Wall Displacments?

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I seem to have run into a mental ditch, and can't get out.

Maybe it's my age.

Anyway, please consider this along with me.

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.

And even more to the point, if I shore everything, then place the seal slab, 
 the program then 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.

What am I missing here?

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