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RE: Earth pressures on retaining walls

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The book Foundation Analysis and Design by J.E. Bowles says that tests by
Terzaghi, Peck and several others that he lists tend to confirm the active
pressure concept reasonably well if the backfill is carefully placed so that
compaction effects do not created excessive stresses and if the wall rotates
or translates sufficiently to mobilize fully the "active" zone ....  For
cases which could include retaining walls, basement walls, or bridge
abutments, the lateral earth pressure will vary between active pressure,
where the wall can move, to at-rest or somewhat above at-rest values
depending on soil placement and whether all wall movement is restrained.  As
Brian Smith responded, Bowles does show the derivation of a factor of 2.00
instead of the ACI factor of 1.7 for very rigid walls.  He goes on to say
that the factor should be reduced to 1.7 since any wall should move somewhat
prior to failure.  He also points out that these factors are for design of
the concrete wall, not for the total wall stability.  He says that the
active pressure is adequate for wall stability since all walls must move
prior to failure.

The AREA Manual for Railway Engineering and my favorite geotechnical
engineer both recommend that "If the wall or abutment is prevented from
deflecting freely at its crest, as in a rigid frame bridge or some types of
U-abutments, the computed backfill pressure shall be increased 25 per cent."

All of this is subject to the backfill used, how it is compacted, how much
water can get trapped behind the wall and the potential for water freezing
behind the wall.

Clifton Lawson has done testing that shows if you do not keep even
lightweight mechanical compactors at least a foot from the wall backfill
pressures can develop that are far in excess of design equivalent fluid
pressures.

Roger Davis
SDS Architects, Inc
205 N. Dewey Street
Eau Claire, WI 54703
715-832-1605
rdavis(--nospam--at)sdsarch.com


-----Original Message-----
From:	Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent:	Tuesday, January 18, 2000 2:50 PM
To:	SeaInt Listserver (E-mail)
Subject:	Earth pressures on retaining walls

I'm curious if anyone has any data or applicable experience regarding long
term deflections of cantilever concrete retaining walls.  I have a deep wall
which we are concerned about limiting long term deflections but can accept
reasonable deflections during initial construction.  (We can adjust for
initial deflections but have an interface in final construction which
requires a more limited tolerance.)  The following questions have come up:
1.	If a retaining wall is designed for active lateral earth pressure, will
all of the deflection required to develop active pressure occur immediately
during backfilling or does it take time after backfilling to fully deflect
and develop active pressures?  It is understood that a concrete wall will
continue to deflect some due to creep of concrete but is there any ongoing
movement due to soil pressures?
2.	According to U.S. Army Corps of Engineers (COE) engineering manual EM
1110-2-2502 (1989) for Retaining and Flood Walls: "Designers have often
assumed active earth pressure on the driving side because movements required
to develop active pressures are small.  However, several reasons exist to
design walls for at-rest pressures.... Even for foundations capable of
yielding, certain experiments with granular backfill (Matsuo, Kenmochi, and
Yagi 1978) indicate that, following initial yield and development of active
pressures, horizontal pressures may in time return to at-rest values.
Another reference (Casagrande 1973) states that the gradual buildup of the
backfill in compacted lifts produces greater than-active pressures as do
long-term effects from vibrations, water level fluctuations, and temperature
changes."   The manual then concludes that walls should be designed assuming
at-rest earth pressures on the driving side - this is rather conservative
relative to common assumptions for retaining wall design.  Has anyone seen
these references or seen other data which supports the concept that lateral
earth pressures in granular fill may gradually return to at-rest values over
time?
3.	My own general impression is that retaining walls tend to deflect more
with time, which could be due to any of the above noted reasons.  If a wall
is founded on rock or on drilled piers, will the cantilevered stem wall
deflect more with time?  Will total deflection exceed that calculated solely
based on concrete creep for long-term effects?

Thanks in advance for any info you can provide.
William C. Sherman
Camp Dresser & McKee, Inc.
Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com