Conventional Foundation design for expansive soil...

[carsonwjohnson]

Anyone know where the code specifies foundation design for expansive soil? I've been looking everywhere and all I can seem to find is post-tension design stuff. My client wants a conventional foundation. I'm in California, so CBC 2013 (IBC 2012). Any info would be greatly appreciated. Thanks.

 

[jdgengineer]

Yes.

It specifies it in Section 1803.5.3

You need a geotech.

 

[carsonwjohnson]

Yeah. Geotech is on board. He's just a derelict. He's not giving me much to go on. It was designed using the old method the first time around since that's all I could find, but the plan check bounced it back. Thanks for the info. I'll check it out.

 

[jdgengineer]

If he's not giving you much to go on you need a new geotech.
We work on expansive soil commonly.

For high-end residences it varies between.

-Rigid interconnected shallow footings.
-Mat slabs / PT slabs
-Drilled Piers & Gradebeams with voidforms & sonotube sleeves, etc.

There is no "conventional" approach. Depends how nasty the stuff is. If the geotech isn't helping time to get a new geotech...

 

[JedClampett]

We've used overexcavation and replacement, concrete piers with cardboard forms and other subgrade improvements. If the Geotechnical Engineer won't help you, go to a Foundation Contractor (Hayward Baker, Layne, Schnabel, etc.) and explain the issue. They'll steer you to what they do, but it's better than a problem later on.
I'd still follow on with the Geotechnical Engineer and get his concurrence. You don't want to be left completely holding the bag on foundation design. Make sure you document your discussion if he won't revise his report.

 

[CELinOttawa]

An unhelpful Geotech should be an unpaid Geotech, either this time or next.

Don't keep paying for dead weight. Sub-consultants are not simply line items to be marked up, they need to be the foundation of your work.

 

[manstrom]

There is a method for conventional based on the PTI method. Basically, design it as an uncracked PT slab, then solve for the I. Convert the I to a cracked conventional slab with some kind of reduction for a cracked section.

"Design of Post Tension slabs on ground" is the publication

To save you some time. It's a lot more concrete using conventional over PT. The whole thing is basically based on rigidity. I did a whole neighborhood on PT waffle slabs. It was not awesome.

If you build in a basement, you get out of the bad soils usually.

Another way to do it is what they do in Texas - cardboard forms running between deep foundations.

 

[jayrod12]

cardboard void form below grade beams or walls between piles is the way we deal with highly expansive soils.