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.