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RE: soils pressure

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While water pressure is independent of the width of fluid behind a wall,
soil pressure acts differently due to "internal friction". Although soil
pressure is often quoted as an "equivalent fluid pressure", it does not act
the same as a fluid. (Please refer to your college soil's textbook for
Coulomb and Rankine earth pressure theory.) 

With a limited width of soil behind a wall, the lateral pressure on the wall
should be reduced due to the limited mass which can slide on a failure

If rock stands vertically, then at that point in time it is not exerting any
lateral forces. However, fractured rock can exert lateral forces due to
slipping of rock mass along a fractured surface. Thus the condition of the
rock mass will affect the magnitude of the lateral force. 

William C. Sherman, PE 
(Bill Sherman) 
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

> -----Original Message-----
> From: Rand Holtham, P.E. [mailto:rand(--nospam--at)] 
> Sent: Friday, April 23, 2004 12:19 PM
> To: seaint(--nospam--at)
> Subject: Re: soils pressure
> Doesn't a 1" diameter of water 12' tall exert the same 
> pressure as a 1 foot diameter column? Isn't it the same for 
> soil? doesn't a 12' tall column of soil exert the same 
> pressure if it's 2' thick or 10' thick column?
> Lateral pressures are usually based on the cubic foot weight 
> of soil never does the distance behind the wall get factored 
> in (except Caquat forces
> etc.) so I think so-called "full" soil pressure would be 
> present as long as loose (then compacted) fill is loaded 
> behind the wall. Use the rock as your wall attach a ledger 
> for your floor then you have an economic solution that your 
> "friends" will be happy with. Engineering is an art in that 
> when you can bring along the reluctant to do the right thing 
> it means so much more than just the design alone.
> My thoughts,
> Rand

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