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Equivalent Fluid Pressure

Equivalent Fluid Pressure

Equivalent Fluid Pressure

Hello all,

I need to compute the EFP for a foundation wall and am very new to EFPs. Using a soil density of 20 kn/m3 and a friction angle of 30 degrees I get a value of EFP=20*(tan^2(45-30/2)=6.67kn/m3. Is this correct or am I missing something? I compared with another EPF value computed for a nearby site and they computed a much higher EFP value.

RE: Equivalent Fluid Pressure

You calculated the active EFP. The other foundation walls may have been calculated for at-rest EFP which could be about 50% higher than active EFP.


RE: Equivalent Fluid Pressure

Yep Peinc, that's likely the source of the discrepancy. To answer the likely next question, for a foundation wall, you should probably be using the the at-rest pressure, since it is applicable to walls which cannot move. Active pressure is for unrestrained retaining walls and such, where the wall can move a little to mobilize the interlock of the backfill particles so that the soil has some internal restraint.

Although, I remember another thread here where I recently read that under at least some codes, there is an exception for short foundation walls (?) I don't quite understand that, unless it's an empirical thing.

RE: Equivalent Fluid Pressure

Keep in mind that if you backfill before you brace the basement walls they could move!

RE: Equivalent Fluid Pressure

BigH, they could, but my approach would be to not count on it in determining the soil pressure to use for design. I don't have any experience (or knowledge of the codes) regarding foundation walls, so I would go the conservative route.

RE: Equivalent Fluid Pressure

Another "NO NO" is jetting that backfill, as under an attached garage floor. I once confronted a contractor who's job had the basement wall cave into the basement due t0o jetting the garage fill. He had always done that before this accident. My sarcastic comment was: "Bet you don't do that again".

RE: Equivalent Fluid Pressure

Yeah OG, I can see that causing major problems.

I didn't know if BigH's comment would be taken as a caution or permission to use active soil pressure, so I thought I would give my 2 cents worth.

RE: Equivalent Fluid Pressure

Walls that are free to rotate at least 1" for each 10-ft height can be designed for active fluid pressure.
Walls that are not free to rotate should be designed for at-rest fluid pressure, which is a 1.5x factor, typically.
Then too, there's passive fluid pressure.

All this depends on knowing unit density and friction angle. Also, if you just report equivalent fluid density how will the structural engineer know to process interface wall friction? There's no friction angle reported to diminish along the interface?


ípapß gordo ainÆt no madre flaca!

RE: Equivalent Fluid Pressure

HotRod10 - if you backfill against unbraced walls, they act as cantilever walls. You need to look at both "design" pressures and "construction" pressures. The walls during construction will move and hence construction pressures would be active. I don't think I would ever backfill basement walls until the walls are up and braced.

RE: Equivalent Fluid Pressure

There are a few old ET threads I hunt for when I want a reminder of the way EFP works. I don't really understand why EFP continues to be used. I mostly see it used in "lay" terms for non-engineers (or for non-structural/geotechnical engineers on the EI/PE exam), I guess since it dumbs down the entire soil/wall interaction into a single number. I avoid it when possible.

Quote (gandersen)

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.

Also be aware that the stated EFP value might be higher due to the effects of hydrostatic pressure or surcharge or soil/wall friction or blah/blah/blah. Again it is mostly useless.

RE: Equivalent Fluid Pressure

EIT... why are you, instead of a Geotechnical, calculating an EFP?

That info is found in a Geotech Report.

Mike McCann, PE, SE (WA)

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