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RE: Active, At-Rest, Passive Soil Pressure

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Roger:

The reason, I use at rest pressures for walls supported top & bottom, is 
because the wall does not deflect enough (in my opinion) to develop active 
pressure.  Cantilever retaining walls deflect alot more and therefore 
active pressure is appropriate. I do not remember the name of my soils book 
anymore but I do remember a graph that showed how much a wall has to 
deflect in order to realize active pressure. It was quite enlightening!

With the added unknown regarding compaction increasing the lateral forces, 
I try to play it conservative. We require the contractor to use small hand 
compactors against walls.

Jim K.



-----Original Message-----
From:	Roger Turk [SMTP:73527.1356(--nospam--at)compuserve.com]
Sent:	Tuesday, August 21, 2001 12:01 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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