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Re: Soil Bearing Pressure

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there are other considerations that can enter into the picture
very well said. although i can see a lot of confusion and misconceptions on the
list.
i have been following the thread for a while.
chris lillback

"L. Thomas Bayne" wrote:

> 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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