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Re: Expansive Soils

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GSKWY(--nospam--at) wrote:

"a high plasticity index is an indicator of an expansive clay."

This seems to imply that if you have a high PI, your soil is expansive. Although expansive clays will have a high plastic index, soils with high plasticity are not necessarily expansive. It would sound as odd as "having four straight sides is an indicator that a closed polygon is a square." It might be a rhomus, parallelogram, or rectangle (or none of the above). You can have high plasticity silts with no clay content (and no expansive properties). You can have medium to high plasticity clays with little or no expansive properties. We tend to view anything higher than a PI of 15 as suspect, but thats mostly conservatism relative to the extreme damage that can be done by these soil types.

Disclaimer: Most of what I know comes from a local soil scientist (who happened to do her thesis on expansive clays) and a recent undergrad geotech class.

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