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You may be anchoring the piles to rock, but the expansive soils may blow out 
your slab!

IMO, the expansion index is meaningless for structural engineers.  The 
information that *I* need is the confinement pressure that is necessary to 
*prevent* expansive soils from expanding.  If the confinement pressure is 
5 psf, I am certain that I can handle that.  However, if the confinement 
pressure is 5 TONS per square foot, there is nothing that I can reasonably do 
to accommodate that.

(If your expansive soils have a 1 percent expansion potential, but you have 
20 feet of expansive soils, the 1 percent results in a 2.4" expansion.  If 
the confinement pressure is 1,000 psf, can the slab resist that kind of 


A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Michael Hemstad wrote:

. > I have a project in Texas with a large concrete slab
. > supported on auger-cast piling, lots of them, going
. > down about 25 feet to limestone.  Due to expansive
. > clays, I have potential uplift.  I am resisting this,
. > or trying to, using a high-strength (Dywidag or
. > Williams) threadbar drilled and grouted into the
. > underlying rock.  To install these, a plugged pipe
. > (about 4 inch diameter) will be pushed down the middle
. > of the pile before the grout sets.  Then, a few days
. > later, the driller will drop a rock drill down the
. > pipe and drill down into the rock.  The bar will be
. > dropped in and grouted full length.

. > My question is, should I take the opportunity to
. > pretension this bar?  The potential uplift is a good
. > deal more than the dead load, and probably greater
. > than the tension capacity of the grout.  It means
. > two-stage grouting of the bar, and the cost of the
. > jacking, so it's not free, but not too expensive.  Any
. > opinions?

. > Thanks,
. > Mike Hemstad
. > TKDA
. > St. Paul, Minnesota

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