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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 10:51:24 -0500

```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,
>
>
> Jesus Gomez
> 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
>
>
> 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.
>
>
>
>
>
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