Helical piers and rebar placement

[YoungGunner]

We've seen a lot of helical pier foundations required by the geotechs lately and I'm getting ahead of how to properly design for them. Most common we see out here is designing the footing as a grade beam spanning between piles which are spaced anywhere between 8ft to 10ft. I saw this picture on the manufacturer's website and it begs a few questions:
1) Does that helical pier embedded in the concrete impact the strength at all? Especially shear where that is supposed to be the support?
2) Is the flexure rebar fine to be below the helical pier cap or should the grade beam be designed entirely above the cap?
3) If I put a stem wall (let's say 30" tall) on top of this footing designed as a grade beam, but it's poured at a different time than the footing, do I still need to consider two-way shear on the grade beam?

 

[KootK]

Yeah, I've not been enamored with the concrete detailing that comes typical in this space.

1) Does that helical pier embedded in the concrete impact the strength at all? Especially shear where that is supposed to be the support?

I think that you need to view the stem wall as the spanning element and the footing thing as, effectively, a pile cap. The strip footing could probably just be a 2' square cap for all the difference that it makes. Moreover, sometimes they just omit the footing altogether and embed the pile into the wall. They are capable of the rather astonishing tolerances required to manage that it seems.

2) Is the flexure rebar fine to be below the helical pier cap or should the grade beam be designed entirely above the cap?

Same as above. That said, no, if the footing were the spanning element it would not be okay to have the bars below the cap. Your instincts are good there. Having the bars low would compromise the beam shear capacity and make it impossible to satisfy the requirement for the anchorage of positive moment reinforcement (at least without some fancy stirrup detailing etc).

) If I put a stem wall (let's say 30" tall) on top of this footing designed as a grade beam, but it's poured at a different time than the footing, do I still need to consider two-way shear on the grade beam?

No, assuming that the wall winds up more or less directly over the pile. In my opinion, the footing only requires transvers reinforcement locally, right over top of the pile. That, to give some homage to lateral bursting stresses that may develop there.

 

[TLHS]

Yeah, for downward vertical load I'd be assuming this is bearing on the plate and pushing up in to the wall. I would argue, however, that with better detailing there's potentially a load path to engage the bottom steel as a grade beam in a unified way with the wall. That being said, I don't think it's necessarily a big deal to drop some stirrups around there locally to try to hold things together.

My thought is that the stem wall reinforcement should be terminating above the pile in maybe one location because of the interference but it's otherwise drawn incorrectly. It should be heading under the lower steel. You might want to put a couple of extra vertical bars right beside the pile cap. If you've got that in place and you size it right, you could take the pile load up into the wall, spread it through the wall (or just develop the two adjacent bars into the pile load's cone of influence) and then basically hang the load back down to the bottom face of the footing using the wall starter bars.

This might just get you back to stirrups, though, because is a single hook really developed quickly enough to make that work well?

Side note: I personally wouldn't try to get the pile into the wall unless you have a backup plan ready. I've done enough projects with helical piles to know that on a regular project you're not even close to guaranteed to hit the tolerances they promise. They are huge in Alberta and I've touched thousands of the things at this point. A good contractor in good soil going pure vertical can likely hit the tolerances most of the time, but even the most experienced contractors end up handing me enough out of tolerance piles that I know I need to either have a repair strategy ready, or I have to design with a larger tolerance. That's not really a knock on the screw pile guys. I wouldn't be trusting driven pile tolerances all that much either if I were trying to get them into walls. Piles are great if you've got something between the pile and your structure to eat up tolerance. As soon as you start trying to perform surgery with pile placement to avoid that in between piece it tends to become a shit show a not-insignificant amount of time. Maybe once you've worked with certain contractors for a while you can have a bit more trust in their ability to hit things, but detailing fixes for piles that are three times out of tolerance is a pain if you don't have something to work with.

 

[TLHS]

Also, Koot, you've apparently had this discussion and sketched it all up before, so for the benefit of people:

https://www.eng-tips.com/viewthread.cfm?qid=456574

 

[KootK]

The tolerance part of it kind of depends on the kind of screw pile you're talking about. In Alberta, it might be a 4" round and I'd not try and embed that into a common foundation. In Colorado, the screw piles that I worked with were 1.5" square. That's a bit easier to get into a wall. Granted, much comes down to the cap plate detail as well.

 

[dik]

Why the footing? I usually cut off the pile at length required and weld a plate with headed studs to cast into the concrete at the bottom. There is no embedment into the 'footing'.

That bevelled tip can be a problem. On the first screw pile project I did, a transformer pad and a couple of buildings, I used a bevelled tip; I'm not sure if the shafts were 8, 10, or 12" dia. I kept the same for all screw piles; some were very heavily loaded. I also used them for buildings adjacent to the transformer pad. The bevel caused the pile to wander during installation, by several inches. Future jobs, I used a a 'spike' welded to a plate for starting the screw pile.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TLHS]

I don't love using the smaller shaft anchors as compression elements. I have used them extensively for utility structures in tension, but compression has always felt a little icky. I don't do small scale residential, though, which I think is where they might shine. Helical piles are definitely still pretty local in how they're used and what styles are common.

 

[dik]

Forgot to add that the screw piles had an 8' thick Sorbweb oil containment basin below them and in the event of a failure the pit would have to be excavated, the piles were unsupported for a length greater than that. The 3'+ transformer pad supported six 400,000lb transformers. I asked the constructor if he wanted a different design for the buildings and he didn't; that's why the building piles were so large... the number of spirals was reduced.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

I don't love using the smaller shaft anchors as compression elements.

Me neither, I think that they are altogether sketchy, even in residential. Any consideration of eccentricities pretty much kills the setup. I went out of my way to make sure that everything supported by those piles ran over top of them in a continuous fashion. My situation was a tough one for me. The geotechnical engineer recommended these piles, the contractor intended to charge extra for anything other than these piles, and 20+ homes in the same development had been built with theses piles over the last decade. I decided to accept that the problem must really have been with my lack of familiarity with the system and did my best to learn how to work with it.