Concrete micro piles with single vertical bar?

[jeffhed]

There are both expansive and collapsible soils in my area. All of the drilled concrete piers we have designed have been 10" or more recently 12" diameter with multiple vertical bars. The piers also have lateral ties spaced close together through the active zone and spacing out farther in the non active zone (usually below 6 to 10 ft.) There is one subdivision where there have been lots of problems with homes on collapsible soils, the foundations of several homes have been underpinned with concrete "micro piles". These micro piles are 6-8" in diameter and have a single #6 or #7 bar in the middle of the micro pile. The size is a little worrisome to me, as I thought I read somewhere that the code specifies that drilled concrete piers must be 12" in diameter (though I can't seem to find it now). I can't find anything in the code that allows only a single bar (although it does meet the minimum vertical reinforcing ratio), but more importantly can't find anything that would get you out of the horizontal ties. The ties in our drilled piers and others I have seen are spaced 6-8" o.c. Anyone know what parts of the code they are using if any? Maybe they are just designing the piers based on calculations and not taking into account any code requirements. Either way I would like to hear if others think this sounds as fishy as I do.

 

[Ron]

jeffhed...micropiles are not drilled piers. They are usually used in remediation applications where it would be impractical to use other methods. Don't mix the two or try to compare them for code purposes.

 

[bridgebuster]

We use mini/micro piles with a single bar in bridge work; sometimes we have to go to 8" diameter. However, we always use a permanent casing; so there aren't any ties.

At 6" diameter you would have to use a #3 bar for the ties to get to get a somewhat decent cover.

 

[jeffhed]

I know the two are not the same. I am trying to find code requirements for the concrete micropiles and have found very little. After having read more last night, IBC 1810.3.5.2.3 specifies micropiles must have an outside diameter of 12" or less. My area is typically a Seismic Design Category D, and IBC 1810.3.10 spcifies a permanent steel casing for SDC C and SDC D, E, or F is considered an alternative system design per IBC 104.11. Based on what I have read, there really isn't much as far as code requirements go. It seems to me that if you design the micropile per ACI 530 as far as axial and flexure go, you should be covered? Just trying to get pointed to more information if there is any.

@bridgebuster
there are no ties in these micropiles, a single vertical bar is the only reinforcing steel.

 

[born2drill]

In the absense of a local building code expressly dealing with micropiles, I would refer to the design procedures outlined in the FHWA Micropile Design and Construction Guidelines (updated by the Micropile Design and Construction Reference Manual, not electronically available on FHWA's website yet)

You can download the Guidelines here:

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

PS - micropiles have been in use in the US for some 30 years and are certainly not 'fishy' as long as they are designed and installed by competent and experienced engineers/contractors.

 

[jeffhed]

born2drill,
Thanks for that link. It has some good information.