New Construction Helical Piers

[XR250]

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!!

 

[XR250]

Found this on AB Chance's website....


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).

 

[dik]

Don't know why the helix is embedded into the grade beam; there is little to gain by this and the beam capacity is compromised. I'm not sure what is causing the rotation. Is there a slab in the crawlspace? or just PEVB and sand? If a slab and main floor, there should be little rotation.

Dik

 

[XR250]

Thanks DIK. I guess you are right about the embedment. As far as rotation goes, I figure

​

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.

 

[dik]

fair comment... I made the oversight of using a 'sloped cut for HSS tubing used for the shaft on my first helical pile project. I should have used an end plate and round bar to make a spike. The bevelled cut allowed the end of the pile to 'displace'.

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

 

[EngineeringEric]

We have taken our helicals and stagger them 3" from centerline at a closer spacing. this stagger helps with some of the torque you are referring to (torsion of the grade beam). Also on concentrated loads we put in 2-3 helicals and for long spans of grade beam that do no have a return grade beam i may through in something to help that (reduce that torsion span in a sense)

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....)

 

[JAE]

You might check out the IBC requirement for deep foundations supporting walls - section 1810.2.2 - which mandates a 1 ft. stagger along the wall.
There are some exceptions to this in the section that might apply to your situation.



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[XR250]

Thank you dik, EE and JAE.

 

[KootK]

In my neck of the woods, this would be done with:

- 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.

 

[KootK]

How tall is this crawl space and what do things look like where the main floor framing abuts the grade beams?

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.

 

[XR250]

I do feel that money would be better spent on a wider grade beam than on the pile caps.

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!

 

[XR250]

How tall is this crawl space and what do things look like where the main floor framing abuts the grade beams?

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.

 

[KootK]

Are you saying a wider grade beam would be more stable but still use the single line of 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.

Seems like my approach does not cost significantly more than a single line (in this particular case) but is more stable.

If your way is really cost neutral then, of course, go for it. It is the more robust system.

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.

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.

 

[XR250]

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 have used that in the past. That will require some rebar in the wall. I am not sure I want to count on the helicals resisting any lateral force as they are pretty flimsy. I guess the grade beam will resist it thru active soil pressure as it is not going to be a significant load.
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.

 

[KootK]

My pile cap would be the same pour as the footing so really the only additional cost is extra concrete and rebar.

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.

 

[XR250]

The footing is the grade beam - just 24" wide as that is the only bucket he has. The pile cap is the same elevation as the footing (just wings coming out of the side of the footing). I am simply using the dug trench as the formwork.

Thamks!

 

[dik]

Engineering Eric... that's why I intend to use an end plate and a 'spike' to minimise the amount of 'walking'.

Dik