Expansive Soil - Residential Slab-on-Grade

[jpw2913]

I posted a version of this question in the foundation engineering forum, but go no response. I'm wondering how expansive underlying soil needs to be before removing a certain depth of it and replacing w/ non-expansive fill is warranted. In my area most homes are built on monolithically poured slabs-on-grade. The highest plasticity indicies we see are approximatley 29-32. I can go through the WRI method for slabs-on-grade and get a good design for the foundation with these PI's, but I wonder if removing and replacing soil should still be done anyway. If the soil moisture varies below the edge of the house, couldn't the house "tip" slightly, even if the foundation is strong enough to handle this. I guess my question is....is there a maximum soil PI or Potential Vertical Rise where soil removal and replacement would not be necessary? I have seen reports that state a potential vertical rise (PVR) of over 1/2" should be lowered by removing and replacing a certain depth of soil, but that can get extremely expensive for a typical house. Any advice or experience would be appreciated.

 

[dcarr82775]

In this area the depth of over excavation is specified by the Geotechincal Engineer. It normally only makes sense over several lots instead of a single lot. That is the only way the costs make sense compared to deep foundations or PT foundations.

 

[fegenbush]

Without a geotechnical report for your site or close by, it is difficult if not impossible to tell if expansive clay will be present and to what depth.

It may be possible to use other soil improvement methods other than cut and replace that may be advantageous to investigate.