From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
Date: Tue, 21 Aug 2001 23:46:52 -0400 (EDT)
While I am not as experienced as either Stan or Roger, I have known and
used "at rest" earth pressures for more than "a couple of years." It is
discussed in the edition of Bowles text book that I have in my grubby
little hand (fourth edition). From page 475:
"As previously noted, if there is not sufficient lateral displacement the
wall pressure is indeterminate between Ksub0 and Ksuba. Most walls are
designed fro resisting active earth pressure since any rotation for
failure is usually sufficient to develop the minimum (or active) earth
pressure case. Where soil-wall geometry is such that the active pressure
cannot develop it may be necessary to design the wall for the higher
lateral pressures. A flexible wall is very likely to always deform
sufficiently for the active pressure case prior to failure; however, a
very rigid wall might shear off suddenly without the active earth pressure
being allowed to develop."
Further, figure 11-3 on page 476 presents the same basic information that
Stan mentioned. It shows an "at rest" coefficient of being about 0.4 to
0.6 for cohesionless soils and 0.4 to 0.8 for cohesive soils.
Not that Bowles does not directly talk about "at rest" conditions (that is
use that terminology) except in figure 11-3. He talks about Ksub0 which
is the "at rest" coefficient as being a case between the lower bound
(active) and the upper bound (passive).
Bowles also offers these approximate amounts of necessary translation to
allow active pressures to be reached:
cohesionless, dense .001H to .002H
cohesionless, loose .002H to .004H
cohesive, firm .01H to .02H
cohesive, soft .02H to .05H
On Tue, 21 Aug 2001, Caldwell, Stan wrote:
> Roger Turk wrote:
> 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.
> Gosh, Roger:
> I'm almost as "experienced" as you are, and I've been using "earth pressure
> at rest" for basement wall design for as long as I can remember. Designing
> restrained walls for active earth pressure is clearly unconservative! My
> trusty old 1968 edition of "Foundation Analysis & Design" by Joseph E.
> Bowles clearly defines earth pressure at rest in Section 6-2, on Page 267:
> "Earth pressure at rest is the concept associated with the forces acting on
> the retaining structure before any movement takes place either into or away
> from the backfill mass."
> Bowles further defines the usual range of lateral earth pressure
> coefficients for "at rest" conditions as 0.4-0.6 for cohesionless soils and
> 0.4-0.8 for cohesive soils.
> I hope that this helps you catch up on those lost decades. Perhaps, next
> week, we can discuss life after DOS!
> Stan R. Caldwell, P.E.
> Dallas, Texas
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