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RE: Soil stiffness

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Conrad Guymon wrote:

> Soil pressures can increase over time, especially if you have a clay or
other plastic 
> soil that is subject to creep.  Soil pressure starts out being active, and
then over 
> time increases to at-rest.   

I've suggested this to other engineers before and they question my sanity,
so I've finally located a reference which backs this statement up. 

Corps of Engineers Publication EM 1110-2-2502 (1989):

"Even for foundations capable of yielding, certain experiments with granular
backfill indicate that, following initial yield and development of active
pressures, horizontal pressures may in time return to at-rest values.
Another reference 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."

And Sowers textbook says:

"Soft cohesive soils do not remain in either the passive or active condition
for long. Slow yield of the soil (often termed creep) tends to return the
soil mass to the "at rest" state. In the case of walls supporting a soft
clay backfill, this means that there will be a continual slow, outward
movement of the wall if the wall is designed to support only active
pressure." 


I've seen walls that appear to confirm this statement. I've often argued
that "active earth pressure" is over used in an unconservative manner by
engineers, and that "at-rest earth pressure" should be used in far more
applications. 




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