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RE: Active, At-Rest, Passive Soil Pressure

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I believe the book "Foundation Analysis and Design" by Joseph E. Bowles
discusses the issue of "at rest" soil pressure and where it is
recommended for use.....I don't recall the exact location where it
discusses this, but it is an excellent reference...I dont' think your
statement "for soil to exert a pressure against an object it must be
mobilized" is not entirely true.... refer to the effective stress
concept when it comes to just plain soils engineering.....Rankine
active, I believe (by no means am I a soils expert) means you have a
granular material throughout the angle of internal friction (phi) which
is really seldom true..... most of the time behind a retaining wall you
have a layer of granular and then your back into a cohesive soil type
(non-granular)...........at rest pressure is just more conservative
where the coefficient is a little higher (active Rankine in the .3 range
whereas at-rest is more in the .4 range)...... again, just my
thoughts....... hope it helps...

Robert C. Rogers, PE
rogersr(--nospam--at)amkinney.com <mailto:rogersr(--nospam--at)amkinney.com> 



-----Original Message-----
From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
Sent: Tuesday, August 21, 2001 1:01 PM
To: seaint(--nospam--at)seaint.org
Subject: Active, At-Rest, Passive Soil Pressure


It has been only in the last couple of years that I have heard the term,

"at-rest" pressure.  None of my soils books (Spangler, and Peck, Hanson
& 
Thornburn in particular) have the term.

One former local soils engineer had recommended/advocated passive
pressure be 
used on all restrained walls, such as basement walls restrained by 
diaphragms.  I have had a difficult time justifying that in my mind as 
passive pressure requires the element to be "pushed" against the soil,
which 
a restrained basement wall does not do.

I also have a difficult time visualizing "at-rest" soils pressures.  For
soil 
to exert pressure against an object, it must be mobilized, i.e., unless 
the soil particles move, there can be no pressure.  Likewise, the soil 
particles cannot move until the object deflects, and if it deflects,
then 
isn't that the criteria for "active pressure?"  And, if the object 
deflects, and the soil particles are not mobilized, isn't the pressure 
released?  If the "at-rest" pressure exists without a failure plane in
the 
soil, what affect does the soil bridging between the restrained top of
the 
wall and the restrained footing have on the "at-rest" pressure?
(Similar to 
material in a hopper bridging.)

Likewise, compacting behind a retaining wall has also been curious to me
as 
it seems that it could cause pressures greater than the active soil 
pressures, yet, I have never seen or heard of a retaining wall failing
when 
the soil behind it was compacted using hand equipment.  (I did see a
12-ft to 
15-ft cmu retaining wall that collapsed when the contractor was using a 
sheepsfoot roller and dozer to compact the backfill.  And, of course,
all of 
the reinforcing was spliced at the top of the footing.)

I would appreciate hearing others thoughts on this.  And to be able to 
compare apples to apples, let us limit our discussion to non-expansive
soils 
above the water table.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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