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RE: Active, At-Rest, Passive Soil Pressure

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Roger-

Although you might have a few years on me, the young thing at the movie
theater gave me the senior citizen rate the other day without asking me if I
was one :<)   My better half hasn't let me live that one down! 

The explanation (based on tests by Terzaghi) in Taylor, "Fundamentals of
Soil Mechanics" (1948), one of my college soil texts, is that there is a
pressure on a wall which has been rigidly held while sand backfill is placed
behind it. "If the wall undergoes movement in the direction away from the
backfill, the force decreases and after a small movement reaches a minimum
value..." For visulaization purposes, remember that there is a Poisson ratio
effect for a body subject to vertical normal stresses. The Terzaghi tests
are often presented as a plot of wall pressure vs distance each way from the
"at rest" position with a movement away from the soil decreasing the
pressure on the wall (to active pressure) because the soil is able to be
mobilized to resist part of the load thus preventing movement of the wedge
behind the wall and movement toward the soil increasing as the soil provides
additional resistance to act with the weight of the passive wedge as it is
mobilized by movement toward the soil.

Taylor goes on to cite the Terzaghi tests as producing lateral at-rest
pressure coefficients of 0.40 to 0.50 times the vertical stress for sands
with clays having about twice those values. In my experience, expansive
clays will continue to exert high pressure year after year resulting in
progressive collapses.  If the wall is backed with granular backfill which
is compacted, a movement as small as 1/16" in an 8' high wall is sufficient
to mobilize the soil capacity and reduce pressures to the active level. That
is probably why restrained walls have not caused problems (providing they
are backfilled with drain rock). A movement of almost 2 inches might be
required for a soft cohesive soil to reduce pressures to the active level.

Movements required are reported in NAVFAC DM7-2 as follows:

Soil Type and Condition        ROTATION (Y/H)
                             ACTIVE      PASSIVE
Dense cohesionless            0.0005      0.002
Loose cohesionless            0.002       0.006
Stiff cohesive                0.01        0.02
Soft cohesive                 0.02        0.04

where Y = deflection at top of wall
      H = Height of wall

Regards,
Bill Cain, S.E.
Oakland CA

-----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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