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RE: Drilled Pier Minimum Diameter

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ACI 336.1-01 states that it applies to drilled piers of 30-inch diameter and
larger. This was based on potential physical entry into the shaft for
inspection. Smaller diameter piers are not specifically "disallowed", but
the document does not provide requirements for smaller piers. I questioned
this limitation of the document during the public review period and the
committee said that they would consider this as new business for future
revisions. I noted that "18-in and 24-in drilled piers are common in
practice". 

Section 1808.2.1 of the 1997 UBC has a limitation on the length to diameter
ratio but permits exceptions. I don't know of any specific "code"
limitations for absolute minimum drilled pier diameter, but an article I
have states "the smallest practical drilled shaft is 12-inches to 14-inches
in diameter, with a depth no more than about 30 feet. Typically, the ratio
of the length divided by the diameter should not exceed approximately 30." 

My personal opinion is that the minimum diameter should be 18-inches for
practicality in placing concrete, adequate concrete cover, and to minimize
effects on strength if soil encroaches on the diameter. 


William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com
 

> -----Original Message-----
> From: richard lewis [mailto:rlewistx(--nospam--at)juno.com] 
> Sent: Friday, October 15, 2004 8:32 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Drilled Pier Minimum Diameter
> 
> 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  My copy is outdated.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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