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Re[2]: Soil Bearing Pressure

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John Riley wrote:

>>Sounds like you're thinking of soil as a perfect liquid.  If it behaved as a
liquid, I'd agree with you.  But it doesn't.

Using your Rule of Thumb, what would be the bearing capacity of a footing
setting on grade?<<

and Tom Benson wrote:

>>Roger,

I would suggest a slight modification: replace "equal to or less than" with
"greater than or equal to (>)".  A basic geotechnical concept is "soil has
memory".  Overburden consolidates, densifies and strengthens soils with time. 
But soils also have intrinsic strength independent of the overburden
component. 
Almost always (except some Bay Mud sites, and other strange exceptions), your
basement level allowable bearing pressure should be greater than your at-grade
allowable bearing pressure for a given site.  As I always say, please consult
your project specific geotechnical engineer of record, in any case.

Tom Benson at Lowney Associates<<

I believe that the original question had to do with how do you calculate 
depth to take care of the permitted increases of Table 18-I-A, Allowable Soil 
Bearing Pressure.  Table 18-I-A gives an assumed allowable soil bearing value 
at 1-foot, then permits you to add the weight or effects of the soil to that 
for each foot below that level, doesn't it?  Therefore, wouldn't the 
allowable soil bearing values at a basement elevation be the assumed values 
at 1-foot, plus the weight or effects of the soil below 1-foot, as limited by 
Table 18-I-A.

If you have nothing else to rely on, and you want to be conservative, as you 
should be, you would use the allowable bearing pressure equal to or less than 
the weight of the soil above that level.  Granted, the soil is capable of 
taking more load, but if I said to use an allowable bearing pressure "equal 
to or greater than" the weight of the soil above that level, it would justify 
using 1.5 times, 2 times, 3 times, ... 10 times, the weight of the soil above 
that level, and that wouldn't be conservative, would it?

There is no substitution for a *good* geotechnical evaluation of a site, 
however, in some instances a geotechnical study is not available, and 
assumptions, on the conservative side, have to be made.

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

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