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RE: Soil stiffness

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Michael: I have never seen an active earth pressure quoted by a geotechnical
engineer that is more conservative than a comparable at-rest pressure. And a
strength level load factor is not justification to use unconservative design
loads. 

At-rest lateral soil pressure generally is the pressure which exists against
a wall which is "at-rest", i.e. it has not moved toward or away from the
soil. If the wall moves away from the soil, the lateral pressure decreases
until it reaches "active earth pressure"; this typically occurs rapidly,
i.e. with little movement. If the wall moves towards the soil, the lateral
pressure increases until it reaches "passive earth pressure"; this takes up
to 10 times the movement required to develop active earth pressure. For
granular soils, the amount of movement to develop approximately half of
maximum passive pressure is comparable to the movement required to develop
active earth pressure, thus I often use half passive values when I want to
limit movement.  

For structures which have a structural connection between the tops of two
opposing walls (top slab or tie beams), the walls cannot move inward since
the soil pressure is balanced on both sides. Thus at-rest soil conditions
will exist. Cantilevered walls typically can develop active earth pressures,
unless they are relatively stiff; however, there is still the question as to
whether long term creep will permit at-rest pressures to re-develop. Bowles
suggests that the structural elements could be designed for at-rest earth
pressures but the sliding stability could be designed for active earth
pressures; this prevents concrete cracking or failure due to the higher
earth pressures but accounts for the fact that ultimate sliding failure
cannot occur unless active earth pressure is first developed. For a
structure which may be sensitive to movement, the use of at rest pressures
for the complete design may be desirable to maintain a serviceable
structure. 


-----Original Message-----
From: Michael Bryson [mailto:MBryson(--nospam--at)mhpse.com]
Sent: Monday, August 20, 2001 10:14 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Soil stiffness


Of course, who knows how conservative the geotechnical engineer is in coming
up with their number. And the structural engineer uses a 1.7 load factor for
designing  retaining walls on top of all that.

-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Friday, August 17, 2001 12:26 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Soil stiffness

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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