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Re: Active, At-Rest, Passive Soil Pressure

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Rodger:

I have used at rest pressures for design for 25 years with no failures and
have investigated a few retaining wall failures where active pressures were
used for deisgn.  If any fine grained soil is present in the backfill I
think the soil has a tendency to creep on the long term and the lateral
earth pressures are equal to the earth pressure at rest.  If the backfill
materials are mostly clay, the lateral earth pressures can exceed the at
rest condition.

John Schenne, PE
Schenne & Associates


----- Original Message -----
From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, August 21, 2001 1:01 PM
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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