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Re: Expansive Soil Found

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My comments are within:

Roger Turk wrote:

> 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 also look for cracks at the corners of doors and windows.  Doors will be
unlevel in their jambs and will also stick.  Seasonal variation in water content
(dry season to wet season) will also cause the edge of the slab to move up and
down.  The center of the slab will slowly gain moisture until equilibrium.  Water
intrusion and control of moisture near the building are necessary to control

> 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.

Rightly so Roger, you should ask the geotech consultant to either perform a
pressure swell test or to use one of published models to estimate the potential
vertical rise and swell pressure.

> 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.

These soils can expand only if they at a very low moisrure content (arid
conditions) and the clay content is highly active, such as illites.  I would
normally be more potential collapse upon indunation.

Arvel L. Williams ,P.E

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

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