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# Re: soils pressure

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: soils pressure
• From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>
• Date: Mon, 26 Apr 2004 08:45:58 -0500

```Initially, the at rest earth pressure is independent of the soil mass and
the Ko factor is as high as 1 same as for water it's not until the soil
moves that active pressures kicks in which reduces the lateral earth
pressure because of the soils own ability to remain erect.As the soil moves
the grains interlock restraining the available lateral force. It analogous
to removing the container from the water and it spreads out; remove the
container from soil and it slips but stays up.

In  Joe's case does he want his wall and floor to move enough to develop
active forces or does he want to provide the restraint required to keep the
soil from moving? Also if you consider the 2' column of soil is not very
effective in holding itself up. If the wall does move there's not enough
soil to provide support. It's dead weight, it (the soil mass) still wants to
fall down because there's not enough soil behind it to interlock with.

I'm considering the rock to be stable in order not to cloud the issue.

Rand

----- Original Message -----
From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 2:19 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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----- Original Message -----
From: "Robert Rogers" <RRogers(--nospam--at)lorwil.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 3:05 PM
Subject: RE: soils pressure

Man....I love this forum......

Believe it or not... I had this issues arise recently in a similare
situation......cutting a rock hillside.......wanting to leaving minimal
distance between the rock and the back wall of the building to install
formwork for the concrete wall & a drainage layer.......what pressure to
design the concrete wall for ?....others on the job were trying to say the
"width" of the cut greatly affected the load that the wall would see....I
disagreed.....the geotech settled the issue.... recommended an "at-rest"
coefficient (with use with the granular specific weight) due to potential
for "compaction" of the granular layer between the wall & the rock
cut....even though the rock was believed to be "good" (intact with little
potential for fracture).........the "width" of the granular layer was not a
factor  (4 ' or 7' no the geotech didn't make any distinction)........rock
cut varied from 8-10 ft. to 20 ft......I looked in several "references" for
an answer...couldn't find one....it's very difficult to find references that
address thing other than the "typical situations".....we all know that soil
& water behave a little differently...however we also realize that many
things happen not addressed in the "text books".....good'ole engineering
judgement....gotta love it.....

Robert Rogers, PE

-----Original Message-----
From: Rand Holtham, P.E. [mailto:rand(--nospam--at)sigmaengineers.com]
Sent: Friday, April 23, 2004 3:38 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: soils pressure

Please elaborate if you can. Doesn't a 12' tall 1'x1' column of soil not
weight say 1200# whether or not there is other soil around it? Isn't the
lateral force directly related to the weight of soil above it at a given
depth?

Rand
----- Original Message -----
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 2:10 PM
Subject: Re: soils pressure

Yes, no, no.

----- Original Message -----
From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 11:18 AM
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

----- Original Message -----
From: "Jesus Gomez" <jgomez(--nospam--at)schnabel-eng.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, April 23, 2004 8:04 AM
Subject: FW: soils pressure

Joe,

As indicated previously, the earth pressure is dependent on the width of
drainage material between the rock face and the wall. A small width (1 to 3
ft), will reduce the pressures significantly. Your assumption of full soil
pressure is very conservative (assuming there is no water and that there are
no rock pressures).

Jesús Gómez, Ph.D., P.E.
Associate
Schnabel Engineering
510 East Gay Street
West Chester, PA 19380
jgomez(--nospam--at)schnabel-eng.com

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