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Re: Soil Bearing Pressure

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With regards to the "rule of thumb"  this is partly correct as the bearing
capacity of a foundation is proportional to the weight of the overburden
above the idealized failure plane (this is often a combination of a straight
line and a log-spiral curve which displays itself as a bulge in the ground
surface adjacent to the depression caused by the failing footing.  The other
factors not included in your rule are friction resistance a cohesion.
Friction angle  varies with soil type, grain-size, angularity of grains,
density, among other less critical factors; cohesion is usually a function
of clay content, type of clay, density, oversburden pressure,
stress-history and clay-water electrolytic forces.

There are two considerations for designing foundations: (1) Bearing
capacity; and (2) settlement.  These two are not synonomous.  Take the
example of a dense layer of sand and gravel overlying soft, compressible
peat.  The Bearing Capacity of the dense sand and gravel could be on the
order of 3 to 5 ksf depending on the footing width and depth below grade.
The foundation would not fail by general or punching shear but by
settlement. Conversely, if the soil layers were reversed in depth sequence,
the peat on the surface overlying dense sand and gravel, the footing would
fail by punching-in.  The lesson here is that there is no
one-solution-fits-all-cases approach to foundation design.  It is best left
to a competent Geotechnical Engineer.



----- Original Message -----
From: "John Riley" <jpriley485(--nospam--at)peoplepc.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, August 11, 2000 11:39 AM
Subject: Re: Soil Bearing Pressure


> 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?
>
> John P. Riley, SE
> Riley Engineering
> Blue Grass, Iowa
> -------------------------------------
> > Old "Rule of Thumb":
> >
> > The soil pressure that you can use has to be equal to or less than the
> weight
> > of the soil removed.  After all, wasn't the soil at that level
supporting
> > the weight of the soil above it before you removed it?
> >
> > A. Roger Turk, P.E.(Structural)
> > Tucson, Arizona
> >
> > Paul Reilly wrote:
> >
> > =-=-=-=-=-=-=-==-=-=-=-=-=-=-=----===-=-
> > > Ref Table 18-I-A, Allowable Soil Bearing Pressure, footnote 2...
> > "increase
> > > of 20% per each foot of embedment into natural grade greater than 1
> > foot".
> > > Is this provision for soil bearing applicable to sub-structures and
> > > basement footings
> >
> >
> > Overburden and skin friction were considered as candidates for the
> > increase, however, I was almost resolved to attribute the allowable
> bearing
> > increase to the probability of reduced deliterious materials insitu.
Any
> > geotechs out there?
> > =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
> >
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