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RE: Expansive Soil

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Eric Green wrote:

. > Depends on the specifics of the soil. In general, in our local highly
. > expansive soils (Texas gulf coast), a two story building does not have
. > enough weight to stop expansion. OTOH, if you building is very heavy, it
. > might.

This reinforces a point that I have been trying to make for some time now.  
The geotechs do not give us the confinement pressure, i.e., the pressure that 
is necessary to prevent the soil from expanding.  Once geotechs start giving 
us confinement pressure, I think that we will find that non-plastic soil, 
e.g., clayey sand, that has expansive clay particles will also expand and 
will exert about the same pressure as a highly plastic soil containing the 
same expansive clay.

The nebulous expansion index and "potential for expansion" quantification 
does not give us, as structural engineers, any information on which we can 
base our structural design.

It is my understanding that confinement pressure can easily run several tons 
per square foot.  While the weight of a building may exert similar pressures 
on the footings, not too many floor slabs are designed to resist these force 
levels.

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

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