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Re: Drilled Pier Minimum Diameter

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Some of the communities north of here have modestly expansive soil. The local buildings departments provide a "standard" design involving 10" to 12" dia. piers in the range of 6' to 7' with a single #4 bar. The sites are flat & with piers at 6' or 7' o.c. the foundations seem to perform O.K. for them.
Chuck Utzman, P.E.

Scott Maxwell wrote:
Richard:

I don't know if anything that specifies a minimum diameter.  I will note
that the ACI 336 documents (at least 336.1 and 336.3R) state that they
generally only apply to piers that are 30" in diameter or greater.  336.3R
states that it is "...difficult to detect sidewall collapse in small
diameter piers during concrete placement and casing extraction."  Thus,
it states that small piers are generally only used where there are
consistantly stable soils or the casing are left in place.

I would say that minimum diameter would be largely a function of how the
pier will be used (i.e. end bearing or skin friction) and what the soils
are at the site in question.  Thus, it would likely be something that you
would need to get a recommendation from the geotech on.  If they are going
to be short, end bearing piers, then usually a recommendation from the
geotech on minimum size would be given.  I know that usually most geotech
reports that I have gotten recommend that strip footings be no less than
16" in width.  Thus, it would not suprise me that a geotech might
recommend against a 12" diameter pier.  But, it will depend on loading,
how the pier will resist the load, and the soil at the site.

HTH,

Scott
Adrian, MI

On Fri, 15 Oct 2004, richard lewis wrote:

  
I am looking for minimum diameter of a drilled pier.  I was told by
another engineer that is was 18" by either the ACI or the AASHTO
specifications.  I looked in my copy of ACI 336 .3 "Suggested Design and
Construction Procedure for Pier Foundation" and didn't find any minimum
diameter requirements.  My copy is outdated.  I don't have any AASHTO
specifications.

The reason I ask is I was wondering if 12" diameter was standard
practice.  I have never specified that small before.  It seems to me the
rebar would be too close in a 12" diameter shaft.

I would appreciate any insight on this.

Thanks.

Rich

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