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RE: Earth friction on sonotubes

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Some of the reason for disregarding the upper layers of soil can be
that it's loose sand and maybe saturated, therefore it will have minimal
friction.  Also the testing of piles have shown that the friction values
change quite a bit in the upper part of the pile.  Basically from the
surface down 15D (where D is the pile diameter).  So with a
1 ft. diameter footing, the friction value will go from zero to
the uniform friction value for that soil type in 15 ft.  It greatly 
simplifies the calculations to omit the variable friction section and just 
use the sections where the friction is uniform, especially when the pile
is very deep.

In the case that Drew presented, I think that the friction will be very
poor.  It would be easy to build one on site and do a pull test to
measure what it can develop.  I suspect that it will be much less than
what they think it will be based on the IBC friction values.

BTW, I think that those friction values are for a retaining wall's sliding
friction.  This makes certain assumptions about the weight on the soil
and the direction of load.  I wouldn't try to use those values for skin
friction of a pile.

Take Care,
Lloyd

On 24 Feb 2014 at 17:17, Polhemus, William wrote:

> I doubt seriously you will find any geotech who would agree to his
> assumption.
> 
> IBC aside, in the typical geotech report I see where uplift is
> addressed regarding deep foundations, the geotechnical engineer will
> disregard from 5
> to 10 feet of the soil from grade as "disturbed" or what-have-you.
> 
> A geotech report done late last year for a location near the Houston
> Ship Channel that I'm using right now for foundation design, disallows
> the upper
> FIFTEEN FEET of soils for uplift resistance.
> 
> -----Original Message-----

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