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RE: Soil Bearing Pressure

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C'mon guys!  

Roger's rule of thumb was just intended as a reality check in the absence of
other information.  It was not a treatise on the best way to develop bearing
pressures.  Certainly there are other factors one considers if the
information is available.  But sometimes, a conservative estimate serves
just fine!  And sometimes it lends a check on the over-conservative
recommendations of a geotechnical engineer.  I know I've questioned numerous
recommendations that appeared way too conservative using a reality check
similar to Roger's and they almost always change.   

The important thing to recognize is what is important to the behavior of
your structure and the attendant cost of the conservatism.  For most
residential work, conservative assumptions provide no cost penalty.

Regards,
Bill Cain, S.E.
Oakland CA


	-----Original Message-----
	From:	L. Thomas Bayne [SMTP:tom(--nospam--at)soilsengineering.com]
	Sent:	Monday, August 14, 2000 11:01 AM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Re: Soil Bearing Pressure

	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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