"Most walls are designed for resisting active earth pressure since any
rotation that tends to produce failure is usually large enough to allow the
active pressure to develope. If the wall is rigid or if top rotation may be
undesirable for aesthetic reasons, the wall is designed for higher(usually
K0)wall pressure. Even in this case if the wall starts into failure mode
some rotation/translation will take place and lateral pressure will start a
reduction toward the Ka state."
I would say this is quite reasonable specially when you have a cohesionless
soil since all you need to mobilize the active pressure is a deflection of
0.001 to 0.004H which is less than allowable deflection for a structural
member.(even a propped cantilever)
Having said that, there may be situations when you want to think twice to
avoid possible problems like sinking in the ground around a excavation or
when the soil is more of a cohesive type or if you suspect use of heavy
rollers to compact the backfill,etc.
Reza Dashti, P.Eng.
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.
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.
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