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RE: Soil Pressure

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First of all, if this is a basement wall it's not free to rotate about the
bottom, so the soil lateral pressure will be *at-rest* pressure, not active.
The pressure distribution is approximately linear downward from the ground
surface, increasing at a rate of Ko times the soil density lbs/sq ft/ft
depth.  Ko depends on the soil strength parameters friction and cohesion,
but might typically be 0.5 for a good, cohesionless granular backfill.  

The uniform pressure would be appropriate if there is a surcharge loading on
the soil, and again it would be Ko times the vertical surcharge.  

Seismic soil pressure is often characterized by an "upside-down" triangular
pressure distribution, with the peak pressure at or near the ground surface,
and the resultant force at 0.6H above the base of the wall.

The geotechs should probably be more complete in their explanation, but it
sounds as if you need to review the soil mechanics you learned in college to
be better able to understand the explanation.

	-----Original Message-----
	From:	Francisco Duarte [SMTP:fduarte(--nospam--at)leasung.com]
	Sent:	Tuesday, January 26, 1999 4:03 PM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Soil Pressure

	Hi,

	I am working on a project for which I am designing a retaining wall.
The
	report describe the loading as fallow:

	The basement wall should be design to resist a lateral earth
pressure
	equivalent to a fluid weight of 35 pound per cubic foot , and an
	additional uniform pressure of 15H pound per square foot, where H=
back
	fill height above the base of the basement wall footing in feet....
	Additionally the basement wall should be designed to resist lateral
	pressure induced by earthquake. An additional earthquake -induce
	pressure equivalent to a fluid weight of 20 pounds per cubic foot
may be
	used in design. However, distribution of the lateral pressure should
be
	taken as an "invert triangle " with its resultant force acting at a
	point two third of the wall height above the base of the wall.
	 Now I have tried to calculate the active overall presure and I came
up
	with 75 psf(no including the 15H), which I think is to high and the
	soils engineer says there is something wrong in my calculations.
When I
	ask him/her to give me his calculated number, the only thing I
recieved
	was a diagram in which he/she shows the 15H psf rectangle , 20H psf
	rectangle and a 15H psf triangle
	I guess I am missing something here.
	1- Why can he/she give me a straight foward numerical answer?
	2- What will the answer be for a  8 feet wall ?

	--
	Francisco Duarte
	Lea and Sung Engineering, Inc.
	Tel: 1-510-887-4086
	Fax: 1-510-887-3019