huangyhg

individual helical anchors as column supports
any body have experience with individual helical anchors as bearing support for independent columns?
i've been a little reluctant in the past to consider helical anchors, i guess its the engineer/inertia issue.
i'm working on a small project that has limited space and where a lot of excavation causes problems. the idea is to drill in single helical anchors under each of 10 columns supporting a braced elevated deck and pour a cap on top of the anchor with some sort of void under thae cap to accomodate frost expansion. maximum column loads are 35k. multiple anchors per column area nice solution for me but raise the eyebrow of the arch/owner.
thoughts?
the column should be concentric or very near so installed, or you'll have an eccentric load and induced moment and so forth on the anchors. i don't know how well they would take that.
if you know of a lateral and moment capacity of the anchors, you can probably back calculate a tolerance for the eccentricity on the anchor once it and the column are installed. otherwise it sounds like you would need multiple anchors, probably at least three to make field tolerance more reasonable.
most helical anchors are only 1 1/2" sq. bars of steel so eccentricity is a concern. you can use triple anchors under major columns with concrete caps. or, use double anchors with a cap and add a grade beam to tie the cap to another cap or structural element to take out any eccentricity across the pair of piles.
or - if you use one pile, use grade beams in two directions to take out the eccentricity. it probably wouldn't take much grade beam to do the job unless your column load is large...but then you'd probably have multiple piles anyway. i think for most uses, the helical piling carries capacities in the 10,000 lb. to 20,000 lb. range.
concur with jae. multiple anchors will be required.
another possible solution is geopiers.
geopier is a rammed aggregate pier below the footing.
the geopier increases the allowable soil bearing and reduces settlement.
all this of course depends on soil conditions.
cost may be an issue. i have never compared geopiers to helical anchors.
i think i've "heard" that geopiers are quite expensive compared to helical piles.
i agree with jae. i have used the helical piers on some projects and almost all the times i used them with grade beams. i also like to specify higher capacity for the helix than i need to be on the safe side.
keep in mind that if you have any appreciable lateral load, the helical anchors become much less attractive in single place configurations. with a proper footing, three anchors splayed in a battered configuration can give you some lateral resistance, but not much.
there is also a technique to grout the helical pier as it is installed that provides additional lateral support. this is often used in soft soils.
some other things to consider might be mini-piles or auger-cast piles if low overhead is a concern. however, these technologies would most likely be more expensive.
yukon- agree with other posters that multiple helicals will be required under your cols to help provide stability and reduce effects of likely eccentricity. are you sure you will be able to get 35k allowable capy for one helical pile, anyway? of course actual capy will depend on local geotech conditions.

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