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Re: augered piles

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Jim,

        Structurally, the floating slab design works very well.  I've
done this several times using a thickened edge to the slab for pre
engineered metal buildings.  I usually use two feet thick overall, one
foot wide at the bottom, and slope the inside face of the "grade beam"
at 45 degrees to avoid formwork on the inside.  Reinforcement might
include two 15M or 20M bars (#5 or #6) top and bottom, with stirrups or
ties at 18" and say 0.002 reinforcing for the slab.  I have never been
advised of any structural problems with this design.

        There may be a problem with differential movement between the
slab and underground services, however.  Water, sewer, gas, and
electrical services will have to be detailed to accommodate this
possible movement; but it can be done.

Regards,

H. Daryl Richardson

jim jensen wrote:

> Mike , what would happen, let's say, you pour an 8"
> thick slab, rebar mat of 5/8", 3000 lbs psi concrete,
> and let the home "float", hand in hand with a
> comprehensive landscape design ie positive site
> drainage. Just a thought, Jim Jensen, an unlicensed
> builder in Arizona
> --- Roger Turk <73527.1356(--nospam--at)compuserve.com> wrote:
> > Mike,
> >
> > 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
> > force?)
> >
> > HTH
> >
> > 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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