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RE: Geotech

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Harold Sprague wrote:

. > You get one value for spread footings and the caveat of 1" of differential
. > settlement for any soil ...

Now ask the Geotech about the distance in which that 1" of differential 
settlement will occur.  Is it within 1 foot, 10 feet, 100 feet, or what?  Do 
they really mean 1" of differential settlement or do they mean that their 
settlement estimate is accurate to within +/- 1"?

Here in Arizona we have collapsable soils, soils that will collapse under 
their own weight when they get wet, yet borings are typically only 10 feet 
deep for a lightly loaded structure when the collapsable soils may extend 
several times that depth.  The explanation that I have gotten is that the 
stress due to the loading is insignificant at that and further depths.  While 
that may be true, collapsable soils *don't* require loading to collapse, yet 
the geotech does not want to bore thru the collapsable soil.

I can't recall ever seeing a *good* geotechnical report.  I was lamenting 
this in an architect's office one day and he handed me a report and asked me 
what I thought of it.  Thumbing thru the report and getting past the 
boilerplate, I saw the comment that there was 2 feet of collapsable soil and 
that a 10 percent settlement should be expected.  When I expressed my 
surprise to the architect and finally explained to him that a 10 percent 
settlement represented 2.4 inches, he said that he had better put out an 
addendum.

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