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augered piles

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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

Mike Hemstad
St. Paul, Minnesota

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