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RE: Foundation on clay

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While the expansion index test is a "nice" test, it doesn't give me, as a 
structural engineer, any pertinent information on which to base the design of 
a structure on expansive soils.  Classifying an expansive soil as having a 
"low," "medium," "high," or "very high" potential for expansion has no 
meaning to me (but it sounds nice).

The information that I need is the confinement pressure, i.e., the pressure 
that it would take to keep the soil from expanding.  *This* is information 
that a structural engineer can use.  From information that I have gleaned 
from literature, the confinement pressure can be from a couple of tons per 
square foot to 10-15 tons per square foot.  And, this would be the pressure 
whether or not the soil exhibits 1% or 100% expansion.  This is also the 
pressure that a single particle of clay would exert when expanding.  Since an 
expansive soil will first manifest damage on the lightest element of a 
building, the floor slab, weighing approximately 50 psf, will be the first to 
see damage.

It is virtually impossible to keep the soil moisture constant under slab on 
grade construction.  Any cutoff wall would have to be carried down to below 
where geologic, climatic and meteorological moisture changes occur.  And 
then, the wall would have to be waterproofed as is a basement wall, and I 
think that we all are aware how unsuccessful that frequently is.

A monolithic slab on grade needs to be designed like a ship; for a hogging 
condition and for a sagging condition as we don't know where the expansion 
will take place and where it won't.

Underground water lines should be avoided.  Leaks can be detected much 
earlier, and repaired quicker, if the water lines are installed in the attic 
or ceiling crawl space.  While some people will complain about the "mess" an 
overhead leak makes, I assure you, it would be nowhere near the damage an 
underground water line leak would cause in expansive soils.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Gary Gill wrote:

> I'm in the early design phase of developing a foundation plan for a
> rectangular shaped one-story house. My initial design concept was
> pouring a
> continuos perimeter footing with interior piers ( the house will have a
> crawl space - floor joist). However, a preliminary soil report
> indicates a 4
> ft thick layer of pipe clay about 6 ft below grade. I'm re-thinking my
> initial foundation concept. A monolithic slab with turndowns might be more
> appropriate. That way the entire house can float. Any other suggestions?