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Passive pressure & retaining wall design

Passive pressure & retaining wall design

Passive pressure & retaining wall design

(OP)
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)?

RE: Passive pressure & retaining wall design

I would always consider passive resistance in retaining wall design. I would discount the upper 2 ft (frost depth, etc.).

I also consider how much wall movement is actually reasonable. It does take more movement to mobilize passive resistance then it takes to mobilize active pressure.

I'll also share that I worked with a designer that always used at-rest earth pressures on both sides of the wall. That's the surest way to (practically) eliminate movement!

f-d

ípapß gordo ainÆt no madre flaca!

RE: Passive pressure & retaining wall design

Passive pressure is not applicable to a bearing pressure check. The weight of the soil as an overburden may confine the foundation material, increasing the bearing capacity, but that is not passive soil resistance.

For the overturning check, I would presume that passive resistance is generally ignored due to the movement required to mobilize the resistance being excessive. I think you would find that the rotation of the wall would be unacceptable in most instances before the passive resistance is substantially engaged.

I've only used passive soil resistance in a few instances to mitigate sliding. The criteria for doing so are generally that there is assurance that the material in front of the toe will stay in place during the service life of the wall and that the movement required to mobilize the passive resistance will not cause serviceability problems.

RE: Passive pressure & retaining wall design

I think Meyerhoff's consideration for depth of burial relate to log-spiral passive resistance. So, technically, the depth of burial is consistent with passive theory - just not Rankine passive theory?

Then again, it's been a few decades since I was in graduate school!

f-d

ípapß gordo ainÆt no madre flaca!

RE: Passive pressure & retaining wall design

yes, you can use passive pressure and it is documented in Corps of Engineers "EM 1110-2-2502 RETAINING AND FLOOD WALLS", section 3-8. this reference limits it to 1/2 the passive in order to limit the amount of rotation.

"Development of the maximum possible horizontal stress, or passive pressure, requires much larger wall rotations than for the active case, as much as 0.02 to 0.2 radian. It should be noted that the deformation required to mobilize one-half of the passive pressure is significantly smaller than that required for full mobilization."

RE: Passive pressure & retaining wall design

If you can accommodate the 0.02 to 0.2 radians rotation (from 1-5/8" up to 16" at the top of a 7' high wall), then sure, you can utilize some of the passive soil pressure to resist overturning. Seems as if it would be fairly impractical for most foundation walls.

RE: Passive pressure & retaining wall design

(OP)
Thank you for the Corps of Engineers reference. I will look into that.

HotRod10, how much a grade difference do you need on the two sides of a wall before you would consider it a retaining wall? Let's say I have a 4' tall wall that has 4' of soil on one side and 3'-6" on the other side (6" grade difference). Is this a retaining wall where you need to check it for overturning neglecting the passive pressure? What about 1'-0" grade difference? or 1'-6" or 2'-0"?

What I'm getting at is that in all of those scenarios the only difference on the overturning check would be varying weight of toe soil acting as a resisting moment. Assuming you have a small toe on your wall, this shouldn't make much of a difference and the overturning check on all cases should be the same. However, I imagine the greater the grade difference the greater there is a chance of overturning.

Do you know why all of those scenarios should have the same overturning check, but at some point when the grade difference is a negligible amount we can ignore lateral soils pressures all together?

RE: Passive pressure & retaining wall design

however, half the passive requires significantly less rotation. (than 0.02 to 0.2 radians)
according to the figure below, the rotation is essentially nil at Kp=0.5



RE: Passive pressure & retaining wall design


Quote (mjc6125)

how much a grade difference do you need on the two sides of a wall before you would consider it a retaining wall?

I work in Ontario and the Ontario Building Code says less than 1 metre is not a retaining wall and includes a whole bunch of qualifying statements. This height is probably determined arbitrarily and other jurisdictions also probably have something similar. I recommend you ask the local building department in the area of the wall.

The first stage of site investigation is desktop and it informs the engineer of the anticipated subsurface conditions. By precluding the site investigation the design engineer cannot accept any responsibility for providing a safe and economical design.

RE: Passive pressure & retaining wall design

"HotRod10, how much a grade difference do you need on the two sides of a wall before you would consider it a retaining wall? Let's say I have a 4' tall wall that has 4' of soil on one side and 3'-6" on the other side (6" grade difference). Is this a retaining wall where you need to check it for overturning neglecting the passive pressure? What about 1'-0" grade difference? or 1'-6" or 2'-0"?"

What you 'consider' it is irrelevant. Before there is movement, you either have active or at-rest earth pressure (for loose or compacted backfill, respectively) acting on each side of the wall. The net force and moment on the wall is based on the difference in depth of the backfill. If the force is unbalanced, the wall moves; how much movement occurs is a function of how the resistance increases as the resisting soil deforms. How much of a height differential is tolerable is a function of the soil force-deformation characteristics (commonly referred to as a P-y curve) and how much deflection of the top of the wall is acceptable or tolerable.

Foundation walls, by definition, support a structure, so they generally can accommodate very little movement, whereas a retaining wall is generally not restrained or limited to small movements. If a retaining wall moves an inch or 2, it usually isn't a problem, but a foundation wall that moves (or tries to move) even half an inch would often cause issues for the supported structure.

RE: Passive pressure & retaining wall design

A 1/2-ft tall retaining wall on a 1.5H:1V overconsolidated clay site would be a problem that warranted stability evaluations. We can look at extremes, but in the end it's what you'd put your stamp on that really matters.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Passive pressure & retaining wall design

(OP)
Is it acceptable to consider the at-rest or active pressure on the low side of a retaining wall to resist overturning when you do you're overturning and footing bearing pressure checks?

RE: Passive pressure & retaining wall design

You would use the soil pressure appropriate to the level of compaction. The geotech guys can correct me if I'm wrong here, but my understanding is that if the backfill is not compacted, active pressure would be most appropriate, while at-rest pressure would be more appropriate for compacted fill.

Again, active, at-rest, or passive are all lateral soil pressure conditions, which do not affect bearing capacity. Overturning is only marginally affected by the resisting soil pressure. Usually, resisting soil pressure is only used in the sliding check, if it is used at all.

RE: Passive pressure & retaining wall design

(OP)
I understand that lateral soils pressures don't affect bearing capacity. I'm talking about the soil bearing pressure check that must include the moment that the lateral soil pressures cause (creates a trapezoidal or triangular bearing pressure distribution on the bottom of the footing).

I'm pretty sure the program I use for retaining wall design (RetainPro) does not include include resisting lateral soil pressures for the bearing pressure and the overturning checks. It sounds to me like it would be acceptable to include an active lateral pressure on the low side of the wall as a resisting moment for these two checks. But including it won't necessarily have much of an effect.

RE: Passive pressure & retaining wall design

If you want to see how passive soil pressure is typically utilized, look at the design of cantilevered retaining walls (sheet pile and soldier pile walls).

RE: Passive pressure & retaining wall design

MJC6125:
At a sufficient buried depth it makes sense to consider an active pressure on the un-retained side of the wall, I believe there is actually a check box in one of the option menus for RetainPro to use the passive pressure for the stability checks.

As others noted to engage full passive takes some decent movement which you aren't likely to achieve in some of the scenarios you suggested where the footing is much lower than the retained height. In these instances you can either conservatively ignore the restoring soil and get a larger heel or conservatively use active pressure ignoring a reasonable depth from the top surface or use 0.5*passive and again ignore a reasonable depth from the top surface.


Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering

RE: Passive pressure & retaining wall design

Active earth pressure is a driving force and not a resisting force. I've never seen active earth pressure used to provide wall stability at the front face of a wall or wall footing. Passive pressure can be used for overturning and sliding resistance but is often ignored for various reasons, as discussed above.

www.PeirceEngineering.com

RE: Passive pressure & retaining wall design

PEinc:
With large bury depths due to various reasons either bad soil layer, MEP coordination, influence zone etc. you get to a condition of competing driving forces rather than a driving and resisting. There comes a point where if you consider passive it actually fully overpowers the active pressure.

EDIT: In reality it becomes more of a pile analysis

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering

RE: Passive pressure & retaining wall design

Celt83, how can passive pressure "overpower" active pressure? Passive pressure is a resistance or reaction, not an applied driving force. It builds up only as needed or as capable; or it is insufficient to stabilize the wall and the wall moves toward the low side and fails. Passive pressure cannot push a wall backward. If it could, it would be active pressure (a driving force) and the pressure on the other side of the wall would become the passive resistance.

www.PeirceEngineering.com

RE: Passive pressure & retaining wall design

Usually given equivalent fluid pressures (EFP)
with:
EFP,active = 45*H
EFP,passive = 300*H

Retained height = H
Passive height = xH

Passive Moment: 300*xH*xH*0.5*xH/3 = 50*H^3*x^3
Active Moment: 45*H*H*0.5*H/3 = 7.5*H^3

Passive = Active => x = 0.531
so in any condition where the passive soil side has an effective height greater than 0.531*retained height the passive pressure would by the numbers overpower the active pressure, per the chart posted by CVG this condition would result in very small movements and you would get to a condition where Kp approx. = Ka.

Edit: PEinc I think we are saying the same thing but as I usually do I am fumbling terminology, it is still a passive pressure but the movement is such that you are only mobilizing a fraction of the full passive value which may be nearly equivalent to the active pressure.

Open Source Structural Applications: https://github.com/buddyd16/Structural-Engineering

RE: Passive pressure & retaining wall design

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.

www.PeirceEngineering.com

RE: Passive pressure & retaining wall design

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

RE: Passive pressure & retaining wall design

"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.

RE: Passive pressure & retaining wall design

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.

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