Long time lurker, learned a lot here. I’ve built buildings that might fall cause of you guys!

 

I’m designing a pier-and-beam foundation for a 28’x28’ stick-framed building southeast of Austin. Open crawlspace, so no continuous stem wall. Soil is hard, expansive red clay, with a fair amount of rocks. Building codes don’t apply here, but I build way above code.

 

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!