Residential Slab-on-grade w/ Expansive Soil

[jpw2913]

I have a question for experienced foundation engineers regarding SOG on expansive soil.
I need to design a residential SOG. The soil analysis gives Plasticity Indicies of 29 from the surface down to about 10', then 32 from 10' down to 15'.
I went through the WRI method for slab on grade design and feel good about grade beam spacing, depth, reinforcing, etc. But I'm wondering if a portion of the soil should be replaced w/ low PI fill as well.
Even if the foundation is designed to handle the potential differential movement, should I still remove and replace a portion of the in-situ soil to reduce the potential movement? Thanks for any advice.

 

[a2mfk]

I believe the WRI method you are talking about is a design guide for a SOIL SUPPORTED slab-on-grade? But you are designing grade beams with a suspended structural slab to span between these beams? And the beams are pile supported?

Or maybe I just misunderstood you...

 

[TXStructural]

In my area, with expansive soils (PVR of 6-9"), annual and periodic wetting and drying will cycle soil moisture down to at least 4 feet. If you want to minimize these cyclical changes, replacing or treating to 4-5 feet is useful. Be sure the fill is dense, cohesive soil - lean clay - so it doesn't become a bathtub (as will granular fill - sand or gravel).

If you are using grade beams to stiffen the slab against center-lift/edge-lift, then a downturned edge will also be useful as it reduces the movement of moisture and the associated soil shrinking and swelling.

That said, If I build a new house in this area, it will be a proper pier and beam, and will probably have a floating mud slab below the crawl space, sleeved around piers. I have repaired enough of these to know: 1) slabs on ground are not great in the long run, where utilities are inaccessible and soil movement cannot be overcome, 2) soil moisture changes happen despite the best laid plans, and 3) having a way to get under a house without digging is priceless over the life of a residence.

 

[oldestguy]

Try this link for some good information

http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-184.html