Passive pressure & retaining wall design 1

[MJC6125]

This question has been asked before on this website, and the consensus seems to be that you don't use passive pressure when checking retaining walls for overturning and soil bearing pressure. I'd like to pose a couple hypotheticals to get some opinions on the approach.

Let's say you have a building foundation wall that has negligible vertical loads on it, but it is acting as a retaining wall with exterior grade being lower than the interior floor by 3'-0". For case 1, you have you're bottom of footing at 2'-0" below the exterior grade (retaining 5'-0" on heel side). You neglect the passive pressure of the exterior soil, and that all seems to make sense. But how about for case 2, you instead have you're bottom of footing 4'-0" below the exterior grade (retaining 7'-0" on heel side). You will most likely end up needing a larger footing due to the overturning and soil bearing pressure checks even though the grading elevations haven't changed. Intuitively I feel like your footings should be the same size, but using a program like RetainPro that's not what you get.

One reason I've heard for neglecting passive pressure is because someone may excavate against the wall at a later date removing all of the passive pressure soil. If this is the reasoning why you neglect passive pressure, should you also be designing standard non-retaining foundation walls with neglecting the soil on one side which would create a retaining wall scenario? I assume most typical spread footing foundation walls would fail in that scenario. Why is it correct to design retaining walls as if there is no soil on the low side, but it is not correct to design standard stem walls that way?

For retaining walls, is there anything you can do to account for the fact that you have more or less soil on the toe side when doing overturning and soil bearing pressure checks(besides from the weight of the soil)?

 

[PEinc]

When you design a non-gravity retaining wall (soldier beams or SSP), you calculate how deep the embedment needs to be to get moment equilibrium. Then you increase it for factor of safety. If you install a deeper soldier beam or SSP than you need for moment equilibrium, the wall will not push backwards. It can't because, if the wall tried to push backwards, the retained soil would become the passive soil and prevent the wall from tipping backwards. Then, the new, passive pressure behind the wall would be greater than the former passive (now active) pressure in front of the wall. Then the cycle would keep repeating until the wall exploded! Walls do not push backwards due to passive pressure. Passive pressure is a reaction. It only increases to what is needed for overturning and sliding stability, or it doesn't and the wall fails by failing over - forward, not backwards.

http://www.PeirceEngineering.com

 

[Celt83]

As a structural guy I'm usually working with very limited information on the soil literally sometimes just those two equivalent pressure numbers.

We are saying the same thing I understand the passive pressure is a reaction of the wall sliding and engaging the soil cone. I am saying there is a point where the EFP provided to me by the geotech is no longer valid because at those bury depths the actual movement of the wall will not be anywhere near enough to engage that pressure. By a rational analysis if the movements are very small in either direction then it may be a safe assumption to use an EFP,passive = EFP,active in these cases or be overly conservative and ignore the passive contribution entirely.

There are site conditions that may drive the embedment well below what is required for equilibrium.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[BridgeSmith]

"By a rational analysis if the movements are very small in either direction then it may be a safe assumption to use an EFP,passive = EFP,active in these cases or be overly conservative and ignore the passive contribution entirely."

The problem with that approach is that it requires something more than what is normally considered "very small" movement in order to mobilize the passive resistance. It would likely be unconservative to count on the passive resistance to balance the active if substantial movement of the wall is unacceptable. There is no passive resistance without movement - the wall must press into and deform the soil for the passive resistance to be generated.

Active soil pressure is what you get when the soil moves to the wall. At-rest pressure is what you get when the soil is 'resting' against the wall. This is the condition we use as the maximum pressure condition for the sidewalls of a culvert where the backfill is compacted against the walls. Passive resistance occurs when the wall moves into the soil, compressing and deforming it.

 

[Celt83]

Picture may be better:

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[MTNClimber]

I get what you're saying Celt83. I believe this graph will add to your sketch:

The active "resistance" pressure is confusing to me (and probably others) because I've never seen anyone do that. I have (and seen others) ignored or discounted the passive pressure by a FOS but never say that it is equal to the active pressure.

It would be a designer's call to see if they wanted to include the at-rest (or less) to passive resistance in their calculations. As previously mentioned, there's a bunch of reasons to not include it. To add to the list, say you don't trust the contractor to actually backfill the wall evenly. I've been on a site where the 20 tall wall was pretty much fully backfilled on the active side and not backfilled at all on the passive side. The wall held up fine thanks to whatever FOS was used or resisting pressures were ignored. I still had the contractor install the fill on the passive side ASAP.