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RE: Active, At-Rest, Passive Soil Pressure

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Roger:

I don't have Spangler, but my copy of Peck, Hanson, and Thornburn (2nd
edition, 1974) DOES discuss "earth pressure at rest" on p. 417.  It
goes on (pp. 424, 425) to describe why checking stability of
cantilever retaining wall for active pressure works despite the fact
that higher pressures have been measured in real applications.  The
fact that stability failure would require movement (which would
produce the active state) clearly doesn't apply if the movement is
restrained.  Soil can't fail in the at-rest condition, but the
structures that resist the corresponding pressures can.

-Mike

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Michael Valley, P.E., S.E.                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.              Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699      Fax:        -1201

-----Original Message-----
From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
Sent: Tuesday, August 21, 2001 10:01 AM
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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