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Equivalent fluid pressure

Equivalent fluid pressure

Is the EFP given in a geotech report for retaining wall design based on water saturating the soil behind the wall?  In other words, can a wall with completely dry soil retained actually reach an EFP of say 60 pcf?  

RE: Equivalent fluid pressure


The equivalent fluid pressure that is given in geotechnical reports does not mean that the soil is expected to become a fluid but rather that the distribution of lateral earth pressures is assumed to be triangular and that its rate of increase with depth can be modeled as a fictitous fluid with a unit weight given by the report.

There are various exceptions to this triangular lateral pressure distribution, such as the addition of a surcharge on the surface of the backfill behind the retaining wall.

I hope this helps.

Good luck.

RE: Equivalent fluid pressure

So saturation is not a factor in the EFP derivation.

RE: Equivalent fluid pressure

EFP is a difference between the effects of the active and passive earth pressures.  This way is a simpler way of accounting for the two.  I believe it assumes that the depth of soil on each side of the given vertical plane are equal, i.e. one side does not support more soil than the other, but I'm not sure.  You can check out in a book on geotechnical by Coduto, this is where I first read of EFP.

RE: Equivalent fluid pressure

The EFP given in a geotechnical report represents the pressure that the geotch believes will be exerted on the wall by the soil.  Wheather that pressure includes water pressure or not should be evident from the report, i.e. if the report says something to the effect "... drain holes should be installed every xx feet along the wall. OR ... a 4-inch diameter perforated drain tile should be installed behind the wall and sloped to daylight."  If the report did not address the issue specifically, then you should contact the geotechnical engineer and ask what forces are represented by the EFP.

As a side note, you indicate that the EFP is 60 pcf, that is about right for silty/clayey soils and NO water pressure.  It is very unlikely that the engineer intended for that pressure to include water pressure, but again the best would be to ask the engineer who signed the report.

RE: Equivalent fluid pressure

haynewp and UcfSE:

When a recommendation is made for an Equivalent Fluid Pressure, it is always accompanied by a statement about the expected movement of the retaining wall.  If the wall is assumed to be stationary, the EFP is derived from the Ko condition (at rest).  If the wall is free to rotate or slide outward (away from the backfill) the EFP is derived from the assumption of Active Earth Pressure.  If the wall is being forced back into the soil mass, the EFP is derived from the assumption of Passive Earth Pressure.

Generally, a geotechnical engineer will also make some statements about drainage behind the retaining wall and that it should be developed and maintained.  If the soils do not freely drain, the lateral earth pressures will be much higher and a different EFP will be applicable.  Hence, the water in the soil has a tremendous effect on the actual magnitude of the lateral forces.

What is somewhat confusing is that geotechnical engineers will use the term equivalent fluid pressure even when they specify that there should be fully drained conditions in the backfill.

As mentioned in my prior posting, the only reason to use the term equivalent fluid pressure is that from measurements, we have found that (to a first order approximation) the lateral earth pressures exerted on a retaining wall without a surcharge loading will increase linearly with depth (in the same manner as would occur in a fluid) and hence we can specify a fictitous fluid unit weight that would give the first order estimate of the lateral stress distribution acting on the wall.

The only time that a soil will behave like a fluid behind the wall is if the effective stresses go to zero (due to liquefaction or an upward flow of water).  In this case the lateral forces will be generated by a fluid with a unit weight on the order of 120 -135 pcf (the total wet unit weight of the soil).

I hope this helps.

RE: Equivalent fluid pressure

thanks for the responses

RE: Equivalent fluid pressure

ganderson has it right.  

I would only add that EFP can be calculated by the following for non-saturated soils:

EFP = total soil unit weight * Ka (or Ko or Kp)

If saturated conditions are anticipated then:

EFP = effective unit weight * Ka + unit weight of water

Therefore for saturate soils, the EFP will be greater than 62.4 pcf.  

For example, soil with total unit weight of 120 pcf and phi = 30 degrees has calculated EFP for the active case equal to 120 pcf * 0.33 = 40 pcf.  If this were saturated, then the EFP would equal (120-62.4)*0.33 + 62.4 =75 pcf.

The EFP for the passive case is usually given as about half of the calculated ultimate passive resistance due to the very large strains that are required to achieve ultimate.  Sometimes geotechnical engineers will provide the ultimate passive EFP, but with a cautionary that appropriate factors of safety should be used.

RE: Equivalent fluid pressure

The only time that a soil will behave like a fluid behind the wall is if the effective stresses go to zero (due to liquefaction or an upward flow of water).  In this case the lateral forces will be generated by a fluid with a unit weight on the order of 120 -135 pcf (the total wet unit weight of the soil).

Agreed - with the addition of one additional circumstance: the hydraulic placement of fill behind a bulkhead.  The use of the full unit weight as the equivalent fluid weight isn't precisely correct since some of the soil will probably settle out before all the fill is placed, but it's pretty close.

I haven't read Don Coduto's book.  I hope that he didn't use the term "equivalent fluid pressure" for the embedded portion of the wall, since this will tend to confuse the novices among us.  Could someone with the book kindly enlighten me about how Coduto addressed this?

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