Augercast Pile Minimum Diameter for stability

[Boiler106]

Structural engineer here. Using IBC 2015 in the southeast USA, seismic design category, can someone please clarify the "the height does not exceed 12 times the least horizontal dimension" exception in 1810.10.2.2?

Specifically, i am looking to find the smallest diameter auger cast pile that can be used as a single-pile support for a building column. If the geotech's recommended embedment depths are 36 feet in stiff clay, would that make my minimum dimension 36 inches? Or is the "height" for a fully embedded pier considered approximately zero given that the element is not unbraced in air, water or fluid soils as in 18.10.2.1.?

 

[JAE]

I think that "Lateral Support" per 1810.2.1 is a different thing that bracing per 1810.2.2.
Exception 1 sort of shows this in its distinction that bracing isn't required as long as you have adequate lateral support and the 12:1 ratio is maintained.

So yes - a 36' deep pier would need to be 36" diameter unless you provide "bracing" for lateral stability at the top of the pier - some type of grade beam intersecting it and extending into some other foundation entity.

 

[dik]

I've not done foundations recently in Winnipeg, but with our stiff plastic clays, it was common to use 16" dia friction piles for a length of 35-40 feet.

 

[Boiler106]

Thanks guys. Shortly after posting this we came across an article by DFI and commentary in ibc 2024 that removes the provision. They pretty much say the ratio is not applicable for these single deep foundation elements.

DFI article
IBC excerpt

 

[ANE91]

Thanks guys. Shortly after posting this we came across an article by DFI and commentary in ibc 2024 that removes the provision. They pretty much say the ratio is not applicable for these single deep foundation elements.

DFI article
IBC excerpt

Good ol’ FB MultiPier. Looks like the L/D restriction is only relevant for second-order moments.