New Construction Helical Piers
New Construction Helical Piers
(OP)
The foundation is being dug for a one story, lightly loaded, wood framed building with a crawlspace (34' x 70'). Turns out it is sited on 12 ft. of backfill. The contractor wants to construct the entire foundation on about 50 helical piers. I am not a big fan of constructing a grade beam on a single line of helical piers as I worry about rotation. I have come up with an alternate strategy of using a pair of piers every 11 ft. (see attached).
I still end up with 50 piers but it would seem that this is a more stable system. Any thoughts on this? Also, currently, the piers in each pair are designed to be 24" apart but can be increased a few feet if needed. Is the capacity of the piers reduced due to their closeness? Should I worry about them hitting each other if they are not installed straight?
Thanks!!
I still end up with 50 piers but it would seem that this is a more stable system. Any thoughts on this? Also, currently, the piers in each pair are designed to be 24" apart but can be increased a few feet if needed. Is the capacity of the piers reduced due to their closeness? Should I worry about them hitting each other if they are not installed straight?
Thanks!!






RE: New Construction Helical Piers
Helical Anchor/Pile Spacing
Once the capacity of the helical anchor/pile is determined, concern may turn to location of the foundation
element with respect to the structure and to other helical anchors/piles. It is recommended that the
center-to-center spacing between adjacent anchors/piles be no less than five times the diameter of the
largest helix. The minimum spacing is three feet (0.91 m). This latter spacing should be used only when
the job can be accomplished no other way and should involve special care during installation to ensure
that the spacing does not decrease with depth. Minimum spacing requirements apply only to the helix
bearing plate(s), i.e., the anchor/pile shaft can be battered to achieve minimum spacing. Spacing
between the helical anchors/piles and other foundation elements, either existing or future, requires
special consideration and is beyond the scope of this section.
Group effect, or the reduction of capacity due to close spacing, has never been accurately measured with
helical piles. However, bearing capacity theory would indicate that capacity reduction due to group effect
is possible, so it’s considered good practice to install helical piles into dense bearing stratum when
center-to center spacing is less than 4 feet (1.2 m).
RE: New Construction Helical Piers
Dik
RE: New Construction Helical Piers
1) The resultant of the loads on the foundation is not extremely accurate.
2) They are not likely to get the piers in the exact spot I want them.
There is no slab. The only thing keeping the strip footing from accidental rotation is the bearing capacity of the soil - which I am not sure I can count on.
RE: New Construction Helical Piers
Also torque values for helical piles should be recorded. There is often a 'direct' relationship to torque and load resistance. Check with your geotekkie.
Dik
RE: New Construction Helical Piers
Also worth noting, the helicals walk as they go in so give your calcs a lateral tolerance. And then we normally embed them 8" or so which i believe is what they reccomend for their new construction pile caps (verify embedded length....)
RE: New Construction Helical Piers
There are some exceptions to this in the section that might apply to your situation.
Check out Eng-Tips Forum's Policies here:
FAQ731-376: Eng-Tips.com Forum Policies
RE: New Construction Helical Piers
RE: New Construction Helical Piers
- one line of screw piles, no stagger.
- grade beam in the 16" width range.
- no embed of the pile as dik mentioned.
- deformed bar anchors on top of the cap plates to get the piles nominally flexurally continuous with the grade beam.
- cap plates are field welded to field cut pile ends
I don't disagree that this seems like a tolerance trap and, perhaps, an IBC vioation. But it's how it works here and it seems to. I do feel that money would be better spent on a wider grade beam than on the pile caps.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: New Construction Helical Piers
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: New Construction Helical Piers
I am not sure I understand this comment. FWIW, the excavation contractor is also the helical installer. He only uses 2 7/8" OD helicals and has no access to welding and only has a 24" bucket The caps are sleeved onto the shafts. Are you saying a wider grade beam would be more stable but still use the single line of piers?
I am pretty sure the other engineers in my area just use a single line as well. I do the same when it is only 20 or so feet of foundation. This is 210 ft. so I feel my exposure is alot higher and if I ended up in court, i am not sure how I could defend a single line other than by saying it is standard practice.
Seems like my approach does not cost significantly more than a single line (in this particular case) but is more stable.
Thanks!
RE: New Construction Helical Piers
I think it will be about 32" clear. Gonna have CMU walls on top of the grade beam with 2x10's and stud wall on top.
RE: New Construction Helical Piers
I'm saying that, with suitable diameter screw piles, a wider grade beam may be stable enough and more economical than the grade beam + pile cap + strip footing arrangement. Simpler formwork and only one pour.
If your way is really cost neutral then, of course, go for it. It is the more robust system.
Seems to me that you could consider the grade beam + CMU to be one unit from a rotational perspective. The bottom of that unit would be laterally restrained by the screw piles and the top of that unit would be laterally restrained, presumably, by some kind of connection to the floor diaphragm. If all that's true, then you've got a couple in play that prevents rotation without resorting to the double pile scheme.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: New Construction Helical Piers
My pile cap would be the same pour as the footing so really the only additional cost is extra concrete and rebar.
Maybe I will go back to your idea.
RE: New Construction Helical Piers
I don't see that you need the footing. And there is also a formwork cost associated with the caps.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: New Construction Helical Piers
Thamks!
RE: New Construction Helical Piers
Dik