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RE: Earth pressures on retaining walls

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Thanks to those who responded to my query.  The retaining wall I am
designing supports and is attached to a platform for a light rail passenger
station.  We can detail support for the platform such that the stem wall for
the retaining wall is not significantly restrained against deflection, i.e.
to permit active earth pressure to develop.  My concern was that there are
very tight tolerances for construction of the finished edge of platform
relative to the light rail vehicle, including limits on the maximum space
between the platform edge and the door edge for the vehicle.  However, I was
told today that long term deflections will not be a problem, as during
regular track maintenance they can adjust the track closer to the platform
if the wall deflects with time.  This resolves my main concerns with wall
deflections.

However in looking at the foundation text by Bowles, as well as the
references I noted my previous email, there is still some concern about
earth pressure to be used for wall design.  Bowles suggests that at-rest
earth pressures are likely on the lower one-half of the wall and various
sources say that compaction of soil can cause increased pressures above
active earth pressure.  Thus Bowles suggests designing the concrete elements
for at-rest pressures.  However,(as noted by Roger Davis) Bowles recommends
using active earth pressures for overall stability analysis, since the wall
must either tilt or translate before failure, which would again reduce
effective earth pressures to the active condition.  (Of course normal
factors of safety against sliding should be used with these values.)
 
My at-rest pressures are 1.7 times my active pressures (60 pcf vs 35 pcf).
(I also have a 3-ft surcharge due to the light rail vehicle.)  However, I am
considering developing my overall design using active earth pressures and
then increasing my concrete design values by an additional load factor of
1.3 (just a little more than Roger Davis' 25 percent).  This is roughly
equivalent to (0.75x1.7x60)/(1.7x35); i.e. as a compromise, this would bump
concrete design up to at-rest values but with an effective 1/3 allowable
stress increase for temporary or extreme conditions.  This would give the
concrete a greater margin of safety in case higher than active pressures
exist, but would minimize foundation size for economy - and if greater than
active pressures occur, deflection would increase and in turn reduce
effective soil pressure.