Residential Foundation Practices 2

[JD P.E.]

I was talking to a home builder buddy of mine and had an interesting conversation. He showed me a soils report (sealed by a geotech engineer) and the soils are expansive (Texas clay). Geotech recommended drilled shafts for the grade beams (with suspended slab) or 8 feet of engineered fill to keep expansion to less than 1". They did list a table to reduce amount of fill in correlation to increased expansion (minimal fill shows 4" of expansion). I asked which route they were going to go out of curiosity and he said they always only do 1' of fill and use a post tensioned slab. In their mind that makes up for the issues here... I guess I am struggling to see this as a viable solution.

Am I missing something? Seems questionable to get a geotech study done to not really listen to any of it. I'm just curious if this is a normal school of thought?

 

[phamENG]

Most contractors seem to prefer ignoring engineers, so in that respect, yes, totally normal.

If I were designing the foundation, no. I would not ignore the geotech recommendation.

 

[XR250]

I think the PT slab is pretty common there. Basically just turn the house into a boat.

 

[lexpatrie]

Texas section of ASCE has recommendations on the PT slabs, so yeah, they seem to be common.

If you go to the ACI 318 forum there are several FAQ entries for those Texas documents.

 

[AskTooMuch]

I know this is common practice in Texas.
I have zero residential experience. How does one make sure plumbing don't leak when you turn a residential house into a "boat" that just move/rock with explansion/contraction of clay?

 

[JD P.E.]

Thanks everyone - after some targeted research it looks like XR250 is right about that school of thought. Interesting how the geotech never recommended that as an option then.

lexpatrie, which forum are you talking about? I'd like to do some more reading on those documents.

 

[lexpatrie]

Here

https://www.eng-tips.com/faq.cfm?pid=167

I guess it's globally ACI not specific to 318, anyhow.

Usually if they need to get under there they install piers that the slab sits on, as I understand it, a plumbing leak can do extensive damage to a foundation when there's an active zone of nasty soil under the foundation. Nasty meaning expansive, but with a plumbing leak it can be nasty soil in the other sense as well, I suppose.

 

[JD P.E.]

Thanks lexpatrie. To be honest, I didn't even know about the FAQ tab, I guess I haven't been paying attention to the forum closely enough. Thank you for that!

 

[lexpatrie]

Not to worry the FAQ isn't all that popular it's primarily me building it out way beyond where it was, not that it's a huge improvement but it's seeing some use.... plus I kind of like doing them even if they're just a mishmash of links. Most aren't all that detailed, because I'm just echoing existing research and articles from off site. Some are more detailed. Others not. Feedback is appreciated if there is anything anybody wants to add the FAQ is the best place to provide it.

Key limitation is you can't do images or attach documents.