Helical piles vs. Poured piers on red clay

[jlbuildings]

Long before I was building for a living (partly), I was on here figuring out how to build my first little shed... so hoping some of yall can chime in to help out with this!

I'm designing a pier & beam foundation for a 28'x28' octagonal building southeast of Austin. Open crawlspace, so no continuous stem wall. Soil is hard, expansive red clay, with a fair amount of rocks. No live water. Building codes don't apply here, but I build way above code when I do pier & beam.

What's my best bet to make a foundation that doesn't move for 200 years, short of drilling 50' down to bedrock? I thought of 2 options:

(1) Concrete – Place each beam on 5'oc concrete piers, sitting on a 3' W x 18" D x 30' long strip footing running the length of the building. Tons of rebar and mud, spreading those pier-point-loads out real good. But the question is: how deep do you dig? 3 options:

(A) We could just go at it with the backhoe and see if we hit rock. But say we hit rock at one end of trench, but not the other. The ends are gonna settle very differently over the years, making us worse off than if we just...

(B) Go down ~3-4' and stop. Builders around here typically go 2' down.

(C) Hire a geotech. But the soil varies a lot here (4 acres away, neighbors hit solid rock 6' down, other neighbors didn't at 18' down), and I worry that a boring taken at Point A isn't really telling for Point B. Some geotechs acknowledge this, others rail against it. Some sources say "with expansive clay, you either hit rock or you're gambling."

(2) Helical/screw piles – consensus online is "Perfection! But 3 downsides..."

(A) They don't work so well in very hard or rocky soil (can cause them to spin out or give a false sense of load bearing) (e.g. https://www.twininginc.com/helical-piles-feasible-deep-foundation-alternative/). How hard? How rocky? This guy is no expert, but loves em – https://youtu.be/us6eXEHvahs?t=455 – and says "You can't use helicals in places where average size of aggregate is greater than 75% of space of helix... so if you have a 3" pitch on your screw, you need to stay with rock sizes under 2" for this to work." I've never seen this rule anywhere else. We got a ton of 2"+ rocks, but the "average" is just clay.

(B) Metal is gonna corrode eventually, period. HDG this, epoxy that...

(C) They can travel laterally, even shear 4-ply beams. It's also hard to get em in line in the first place.

Every installer you talk to brushes these problems off ("I can steer around rocks"). One told me that you can address the lateral movement concern by partly encasing the piles in concrete. Since they're installed to torque, I'd think there's no need for a geotech report... but US Helicals recommends getting one to ensure you go below the active zone (where moisture varies seasonally)... but I doubt the soil here gets much moisture 15' down...

Basically, helicals feel like you're putting a whole lot of trust into the installer. I prefer to do it all (except the concrete pour), but we're trying to do a forever foundation here...

(3) Feel free to throw out other options!

Thanks!

[Don_P]

I'm designing a pier & beam foundation for a 28'x28' octagonal building southeast of Austin. Open crawlspace, so no continuous stem wall. Soil is hard, expansive red clay, with a fair amount of rocks. No live water. Building codes don't apply here, but I build way above code when I do pier & beam.

What code are we referencing here? This would be news to me as the code is pretty clear about this on a couple of counts.

Have you ever had an engineer design a foundation for these conditions before?

What is typical of designed foundations, not owner or builder designed but engineered?

[akwoodchuck]

'Pier foundation' and 'forever foundation' are mutually exclusive....

[Don_P]

Since this is outside of prescriptive, a place I've been for the past several months, the next step is the IBC rather than the IRC codebook. Take a look in chapter 18, the foundation chapter and scroll down to expansive soils.

[jlbuildings]

What code are we referencing here? This would be news to me as the code is pretty clear about this on a couple of counts. Have you ever had an engineer design a foundation for these conditions before? What is typical of designed foundations, not owner or builder designed but engineered?

I apologize; I worded that sloppily. More precisely: I upsize my beams significantly (relative to IRC, etc. header/girder spans). As for piers which, as you say, require engineering, my approach (on the easier soils I'm used to) has been: well below frost line, extra piers, tied together with a 3'x3'x1.5' strip footing, #5 reinforced. This is significantly stronger than the engineered plans I've seen for P&B (which tend to resort to spread footings).

Thanks for the IBC reference, I just read that chapter at your suggestion The implication seems to be "have a geotech tell you how deep to drill piers, which should be 4000 psf concrete + other specs not difficult to achieve."

'Pier foundation' and 'forever foundation' are mutually exclusive....

Even with piers drilled past the active moisture zone, to bedrock if necessary (for bearing capacity)? "Forever" is a bit much of course, but.

[akwoodchuck]

Even with piers drilled past the active moisture zone, to bedrock if necessary (for bearing capacity)? "Forever" is a bit much of course, but.

A small building has no need to rest on bedrock, nor will bedrock do anything to laterally brace a pier foundation...

[jlbuildings]

A small building has no need to rest on bedrock, nor will bedrock do anything to laterally brace a pier foundation...

I can deal with lateral bracing, the depth would be based on trying to avoid uplift/settlement by getting past the level where moisture variation is a threat.