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Passive Earth Pressure: As Nonlinear Spring vs. As Applied Pressure?

hippo11 (Structural)
26 Apr 05 17:09
Hello--the concept of passive earth pressure is on my mind:

Seems like, in reality, passive earth pressure behaves like a spring with varying stiffness that is activated after, oh I don't know, a quarter inch of displacement or so...so how come people input passive pressure in computer programs such as Enercalc as an applied pressure, like 250 psf or something?  This isn't really what happens, and your moments, shears, etc. in the foundation itself is thrown off?  

Right?  It shouldn't be an applied pressure, but a support modeled as a nonlinear spring with the stiffness according the soil's modulus?  Thanks.
JAE (Structural)
26 Apr 05 17:19
I agree that it is not an applied pressure until it is engaged by the movement.  Then the pressure increases linearly up to a specified maximum limit (say 250 pcf) where it then "max's out" and no longer increases.
GeoPaveTraffic (Geotechnical)
27 Apr 05 9:25
Most soils are even more complicated than that, they behave in a non-linear manor.  So depending on the complexity of the problem, you may need to run the soil-structure interaction using a software program that models this non-linear behavior.

In my experinence this requires close work between the structural and geotechnical engineer, and may require several iterations to get to the final solution.
Panars (Geotechnical)
10 May 05 8:41
You are correct, passive soil resistance should be modeled as a series of non-linear springs.  But this is difficult to accomplish and not necessary for limit state analyses.  For load-displacement analyses (such as FEM of pile groups) modeling the passive resistance as a pressure does lead to erroneous results for moments and shears.  Robert Mokwa has examined the load-displacement behavior of pile caps in some detail.  His PhD dissertation may provide you with some information that might help you. The dissertation is available here:
http://scholar.lib.vt.edu/theses/available/etd-093099-180817/

I am also aware of the following articles on this subject:

Mokwa, R.L. and Duncan, J.M. (2001) "Experimental Evaluation of Lateral-Load Resistance of Pile Caps" J. Geot. and Geoenv. Engr. ASCE, Vol. 127, No. 2

Duncan, J.M. and Mokwa, R.L. (2001) "Passive Earth Pressures: Theories and Tests" J. Geot. and Geoenv. Engr. ASCE, Vol. 127, No. 3

Rollins, K.M. and Sparkes, A. (2002) "Lateral Resistance of Full-Scale Pile Cap with Gravel Backfill" J. Geot. and Geoenv. Engr. ASCE, Vol. 128, No. 9

mohanvenn (Geotechnical)
27 May 05 18:54
To achieve either Active or Passive presures sp,e amount of lateral movement is required. I remember seeing some reference in Soil Mechanics: Lambe & Wittman. So I refered it. According to the book,

For active pressures to get developed less than 0.5% strain is required. However, for passive pressures up to 2% strain is required to reach the max. passive resistance, and, 0.5% strain is reached at one-half the maximum passive resistance.

There are other references available, I have to dig through some of my stuff. They include field measurements on earth earth pressures versus wall movements.

Best Regards
Mohan




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