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FW: soils pressure

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Rand,

imagine your car in standing on a sloping road without brakes or gears
engaged and tends to rol downslope. You are pushing on it to keep it from
moving. You may be able to keep it steady. However, if the car has a 20-ft
boat in tow, you'll need to exert a larger force. 

When a backfill fills a narrow gap, the volume of soil you are trying to
support is much smaller than for the condition where no rock face exists and
a full edge of soil is tending to mobilize. Therefore, the support pressure
has to be smaller.

If you remove the wall, the backfill will collapse for sure. However, the
volume (and weight) of the collapsed backfill will be much smaller than if
there was no rock face behind to start with. 

The wall does need to move to develop frictional forces. However, this
movement will be very small for a 12-ft wall (0.1 inch). Even if you line
the back of the wall and the rock face with frictionless sheeting, the
pressures would still be less due to the simple fact that you are retaining
less soil within the gap than if there was no rock face.



-----Original Message-----
From: Rand Holtham, P.E. [mailto:rand(--nospam--at)sigmaengineers.com]
Sent: Monday, April 26, 2004 11:51 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: soils pressure


Jesus,

You present an interesting point! Sort of like how water sticks to the wall
of a
test tube. If the basement wall were removed would you expect the backfill
to remain erect? Doesn't the wall have to move to develop the frictional
forces? If the backfill is not compacted wouldn't that lower the friction?

Rand

----- Original Message ----- 
From: "Jesus Gomez" <jgomez(--nospam--at)schnabel-eng.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Saturday, April 24, 2004 12:48 AM
Subject: FW: soils pressure


> Rand,
>
> Please read below..
>
> Jesus Gomez
> Head of Geostructural Design
> Schnabel Engineering
> West Chester, PA 19380
>
> -----Original Message-----
> From: Rand Holtham, P.E.
> To: seaint(--nospam--at)seaint.org
> Sent: 4/23/04 3:38 PM
> 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?
> =============
> The soil does weight 1200 # regardless. However, the narrower the soil
> column, the more the relative importance of the friction along the
> boundaries (vertical rock face, slip plane, and wall). In fact, when
> calculating the earth pressure generated by a 2-ft thick column of soil
> between a concrete wall and a rock face, it is not unusual to obtain
values
> that are really low (40 - 50 psf).
>
>
> Isn't the
> lateral force directly related to the weight of soil above it at a given
> depth?
> ============
> Yes, in a free-field condition, i.e. no nearby bondaries along which
> relative displacement and friction can take place.
>
>
>
> Doesn't a 1" diameter of water 12' tall exert the same pressure as a 1
> foot
> diameter column?
> ===========
> Yes because the water has no internal friction or cohesion
>
> 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?
> =================
> No as indicated above.
>
> 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.)
> =================
> This is a conservative approach which has costed many a lot of money. It
is
> OK for small jobs
>
> Another couple of comments:
>
> Liquefaction cannot likely occur within a 2-ft wide gap of soil between a
> concrete wall and a rock face, and could never occur in a compacted
backfill
> under these conditions.
>
> Compaction of the backfill will increase the stresses on the wall.
However,
> it is very hard to compact a 2-ft wide column of soil behind a retaining
> wall and you don't really need to.
>
>
>
>
>
> This e-mail including attached files is confidential.  Its transmission is
> solely as an accommodation for the benefit of the recipient.  The
recipient
> bears the responsibility for checking its accuracy against corresponding
> originally signed documents provided by Schnabel Engineering.  If you
> received this e-mail in error, its use is prohibited.  Please destroy it
and
> immediately notify postmaster(--nospam--at)schnabel-eng.com
>
>
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This e-mail including attached files is confidential.  Its transmission is
solely as an accommodation for the benefit of the recipient.  The recipient
bears the responsibility for checking its accuracy against corresponding
originally signed documents provided by Schnabel Engineering.  If you
received this e-mail in error, its use is prohibited.  Please destroy it and
immediately notify postmaster(--nospam--at)schnabel-eng.com


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