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Re: soils pressure

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Bill's response matches my understanding.  What originally brought this
issue to my attention was looking at different widths of granular granular
backfill specified for retaining walls.  Why were different widths called
out? Was I designing for the backfill or the material behind the backfill
(not really a question)?  It seemed (not exactly) that 35 pcf with a
granular backfill was being specified regardless of the type of soils
encountered at the site (okay clay-cohesion had an impact) but really: does
the width of granular material matter if it can be placed and if it drains.
Backfill with native material setting the EFP makes sense to me.  An EFP of
35 with granular backfill regardless of native noncohesive soil type does
not without further clarification.

 Enough of this rambling, what does everybody use for the density of the
soil resisting sliding - 120 pcf for the compacted fill or 100 pcf for the
native material?


----- Original Message -----
From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 12:19 PM
Subject: RE: soils pressure


> 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
> plane.
>
> 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)cdm.com
>
>
> > -----Original Message-----
> > From: Rand Holtham, P.E. [mailto:rand(--nospam--at)sigmaengineers.com]
> > Sent: Friday, April 23, 2004 12:19 PM
> > To: seaint(--nospam--at)seaint.org
> > 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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