Equivalent Fluid Pressure 1

[abih]

I am reviewing a geotechnical report for a cantilever retaining wall recommendation for a land job. The consultant recommends equivalent fluid pressures in conditions like "Above Water Table" and "Below Water Table". Since drainage like sump is always installed to prevent water accumulation, why does it go to the "Below Water Table"?

 

[fattdad]

If drainage is provided, then use the "above water table" value.

f-d

¡papá gordo ain’t no madre flaca!

 

[PEinc]

Consider the possiblility that the dewatering system may malfunction and shut down for some amount of time. The worst case for design would be when the ground water rises to subgrade and your passive resistance is cut in half due to the change from moist unit weight to buoyant unit weight. It may not hapen, but it could.

http://www.PeirceEngineering.com

 

[abih]

Here's his equivalent fluid pressures (active):
1. Lean Clays, 51 pcf (above) and 25 pcf (below)
2. Clayey Sands, 42 pcf (above) and 21 pcf (below).

So if there's water accumulated behind the wall, then I will use 25+62.4=87.4 pcf. correct me if I am wrong with the numbers. Thanks.

 

[Panars]

Yes, for the values given, it makes sense to add the water pressure to the equivalent fluid pressure for the soil below the water table.

 

[dgillette]

It doesn't take much more effort to do a proper effective-stress analysis by Coulomb or Rankine, so why use something as crude as equivalent fluid pressure? We all should have been taught how to do it properly in our junior or senior year of college, and it appears in many many textbooks, and in DM-7, as I recall.

 

[PEinc]

I would not get too hung up on the term "equivalent fluid pressure." Perhaps the project geotech already did an effective stress analysis and determined that the recommended values should be used for triangular, lateral, effective earth pressures. He may just be calling the pressures "equivalent earth pressure" because he believes the pressure distributions to be triangular. That being said, the designer should still do his own effective stress analysis to confirm or correct the recommended pressures. I would not blindly use any given earth pressures.

http://www.PeirceEngineering.com

 

[TXStructural]

Most cantilever retaining walls are designed by structural engineers, based on the soil parameters provided by a geotechnical consultant. The structural engineer needs whatever parameters are required to compute the loads and resisting forces. IBC (Table 1610.1) and other publications specify these load parameters in terms of equivalent active and at-rest/passive pressures. CRSI uses Rankine formulas to end up with a design based on, essentially, the equivalent fluid pressures. These may be excessive simplifications, but they have proven sufficient for a great many walls over several decades.

That said, I am in the middle of an update to the CRSI Design Handbook chapter 14 on cantilever retaining walls. If you have specific issues, I would really like to hear about them now (before the new edition is published), so we can be as useful as possible.

John Turner CSP PE
CRSI Greater Southwestern Regional Manager

 

[vulcanhammer]

Some textbooks teach equivalent fluid pressures to analyse lateral earth pressures, so that's where this may have infiltrated into the specification.

I used to teach with a book that did, but I quit both book and equivalent fluid pressures; I found it confusing and my students did also. So it's back to DM 7...

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