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FW: soils pressure

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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.  





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