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Re: Expansive Soil Found

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Expansive soils will lift the least lightly loaded part of a structure first, 
notably the floor slab and non-bearing partitions.  First indications of 
movement will either be in the floor slab (cracking), buckling of edge beads 
on dry wall, or cracking of the ceiling adjacent to the non-bearing 
partitions.

I have a problem with the expansion index test that is usually required to 
determine whether a soil is expansive or not.  I feel that this test does not 
give the structural engineer enough information on which to base a design.  
If a soil has "low," "moderate," or "high" expansion potential, what 
information does that give to a structural engineer?  I am trying to get 
local soils labs to provide confinement pressure tests, i.e., give the 
pressure that is required to *prevent* expansion.  That would be a number 
structural engineers could work with.

I also don't believe that a soil has to be "plastic" to have expansion 
problems although expansive clay has to be present.  The analogy that I use 
is if we had a jar of marbles, representing non-cohesive soil, and balloons 
interspersed throughout the marbles, representing expansive clay, and air 
was introduced into the balloons, the marbles would move.  I can see the same 
thing occurring in non-plastic sandy-clay, clayey-sand or silty-sand soils.

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