Active earth pressure and sliding of a buried structure 2

[brotherson]

Hi

I've got a buried structure designed to BD31/01. It is currently designed for Ka as the driving earth force. BD31/01 clearly requires the use of ka in the sliding (ULS) check.

There is a discussion with the independent checker who has decided the structure is sliding. He is using k0 (at rest) as his driving earth pressure. I think it is pertinent to use k0 for structural design, but not for the sliding design. I've always used ka when, for example, designing retaining walls.

The checker's opinion is that if ka is used, the structure has already started to slide and this is unacceptable. He is paying no heed to the standard BD31/01.

I've looked though Bowles and struggled to find something but am looking for some excerpt or piece of literature that explains the rationale behind BD31/01 or just something similar that shows that we use ka in the sliding check because mobilisation of the friction strength in the soil is inherent in the sliding case. But more articulately than I have managed so far.

Thanks

 

[steellion]

Is your structure a buried concrete box? How big is it? If you have a 4-sided box that can be considered to act as one structure, you will have a passive pressure several times greater than the active pressure on the other side of the box to prevent sliding.

Choosing to use Ka or Ko depends on the restraint of the structure, not which failure mode you are checking. If your structure is restained by another structure at the top, use Ko. If it is theorhetically free to move, use Ka.

 

[brotherson]

It is theoretically free to move. It is not a box it is an integral portal frame on strip footings. Both footings are being used to resist sliding and the structure is designed to make sure the load distribution is catered for. There are other forces driving the sliding other than earth pressure, including a very large traffic surcharge. The FOS, taking the resistance of passive earth pressure into account is not meeting project specifications if ko is used.

I understand that restraint of the structure will determine the choice of earth pressure coefficient, if it had been restrained at the top I would have mentioned it. The important point is that it is 'theoretically' free to move, as you say. Because the earth pressure coefficient is called 'active' the checker is insisting that once ka is mobilised, the structure has moved and thus the game is lost. I disagree with this but am not having any luck turning them around. They are the ones that sign off the design so I need to turn them around.

 

[dcarr82775]

To get Ka you do not require sliding to occur, you only need movement at the top of the wall to occur. Depending on soil type anywhere from 0.001H to 0.02H movement at the top will get you active. How much does your frame deflect under load? if it deflects enough to get you to active you have your response.

If there is a design constraint that it shouldn't move you would need to use At-rest.

 

[KootK]

Unfortunately, I can’t think of any reference to arm you with other than Newton’s second law of motion. And that may not be well received by your reviewer. When the structure is at rest, at rest soil pressures apply and F=ma predicts movement. Once the structure slides, active soil pressures apply and F=ma predicts a cessation of movement. I realize that I’m not telling you anything that you don’t already know. I just can’t think of a non-condescending way to explain it.

I think that it’s still prudent to design your structure for at rest earth pressures even if you’re checking sliding against active pressures. After all, the structure still needs to be viable when it’s not sliding around.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

Is this thing buried on 3 sides? or 2? or 1?

Could you account for any friction on the side walls to help resist the at rest coefficients?

 

[Lomarandil]

I've always seen the movement required expressed in terms of rotation about the base, not lateral movement -- so I don't know whether that negates your argument about the mobilization of sliding friction.

If the level of movement required to develop active pressure isn't acceptable, will you really mobilize the passive pressure? I'd take a step back to determine what level of movement is really acceptable -- depending on the construction method, zero movement may not even be possible.

Depending on the source of your geotechnical information, it may be worth running a sensitivity analysis to determine whether eliminating potential conservatism in the reported soil properties would resolve the issue.

 

[KootK]

When we get a coefficient of friction from a soils report, are we generally getting a static value or a kinetic value?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dcarr82775]

What are you using to resist sliding? If you use any passive pressures at all, you must have already achieved active simply to mobilize any passive pressures. I think you can assume no movement has occurred for friction/cohesion but that is just MHO

 

[brotherson]

dcarr1: It's a portal frame and buried. Under dead load, the walls bulge out for the deflected shape. With the introduction of earth pressure, the deflected shape is still outward so I can't use it as an argument.
Kootk1: This is the way I think of it but unfortunately patronising them may be counter productive! The structure is designed for at rest pressures, it is only the sliding case where the active coefficient is considered, as per the standard being designed to and as per the logic as I see it.
jayord: It is buried on two sides only.
Lomarandil: lateral movement or otherwise, the way I see it is that the idea is still that you move away from the soil block, allowing the internal friction of the soil to be mobilised so the result should be the same. I agree that you need movement toward a soil block to get passive resistance just as you need movement away to get ka. I don't want to bring this up in case the checker gets confused and decides I can't use passive resistance either.
kootk2: We're using a kinetic value to avoid the argument.
dcarr2: we are using a combination of passive earth pressure and friction to resist. Again the standard BD31/01, that is our design criteria, clearly states that these are the coefficients to be used in the sliding check. It doesn't impose deflection criteria. For me I've never considered anything other than ka, for say a retaining wall design. Some of these retaining walls have been retaining highways. The movement is considered. The criteria for me has always been simply that it can. As kootk suggested: the equilibrium free body diagram when the structure is 'sliding' predicts a cessation of movement. Therefore it won't slide.

 

[dcarr82775]

I assume there is nothing like a slab on grade at the bottom to prevent movement?

I think you have everything you need. Both kinetic and passive require movement to occur. Movement means Ka. Sometimes you need to confront the reviewer bluntly and maybe even be a tad condescending. Explain to the owner the disconnect inherent in the comment and the consequences of Ka v Ko.

 

[KootK]

I would very much like to see a buried structure plowing uncontrollably through adjacent soil of infinite extent due to surcharge loads. It would be YouTube worthy at the least.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[brotherson]

haha kootk, I may use that in my next discussion!

thanks for the responses everyone.

 

[wannabeSE]

Brotherson,

If I understand correctly, the structure is similar to a box culvert without a bottom. If yes, then using the active pressure seems appropriate when checking the global stability of the entire structure for sliding (sloping backfill or non-uniform surcharge).

I am wondering about the structural design. When designing the structure, are the footings assumed to be restrained from sliding? Or, are the footing free to move laterally (bottom of walls move toward each other)?

 

[msquared48]

Why haven't you discussed this with your geotech for the project (I see no mention of it)?

You do have one I assume? The Geotech might be able to convince the plans examiner.

Mike McCann, PE, SE (WA)

 

[TXStructural]

Maybe I missed something, but you would use the active pressure to push the element and passive to resist. This changes if you have saturated soils, shrink and swell issues with clay, frost heave...
The pressure on a static structure is passive UNTIL it moves the first, very small increment, then it is active pressure (or in some soils, none at all - think about a footing against concrete - once the footing moves a millimeter away from the adjacent concrete, the adjacent material does not follow.)
In walls, the reasoning for use of active pressure when the top of the wall is unrestrained or free to rotate is that once it starts to move (virtually no movement in many cases), the passive pressure no longer is applied, only the active pressure is possible, and the rotation stops.
In your case, the frame and footings will sit still until a very small movement occurs (let's say 1/4 inch) after which it stops moving because the soil behind has insufficient force to overcome the resisting force which is at passive pressure. The soil behind only has the weight of itself and its repose as it settles to fill the old gap. The soil in front is in "compression" based on weight, cohesion, shear variables, etc.
You mention surcharge loads, and depending on the soil type and condition, traffic and other vibration could constantly rearrange the soil particles and after a first 1/4", the soil will shift and move, resulting in a new passive pressure force being applied, resulting in another 1/4" movement. See where I'm going?

That said, I think your checker is wrong if he reasons that "once the structure moves it has failed". Even for restrained walls this is seldom the case. This is a situation where you don't want ongoing movement, but if a trivial movement would have no deleterious effect it should be used to make the structure economical.

But if saving a $1000 on a $1,000,000 project will negatively impact performance, why do it? Designing too close to the margins is NOT usually good engineering. After all, anyone can design a structure that will stand up but it takes a structural engineer to design one that will barely stand up.

 

[Signious]

This thread reminds me a lot of this:

https://www.youtube.com/watch?v=BKorP55Aqvg

 

[DamsInc]

The pressure on a static structure is passive UNTIL it moves the first, very small increment, then it is active pressure

I thought it was "at-rest" until it moves?

My understanding is that the (relatively moderate) at-rest pressures are initially present. in order to mobilize (the relatively large) passive pressures, you require significantly more movement than to mobilize the (relatively small) active pressures. So once a wall starts moving, even though you develop the lower active pressure behind the wall due to small movements, you don't reach the full large passive pressure in front, then your resisting force is somewhere between at-rest and passive. So depending on the loading conditions, you might end up with more than a "very small" amount of movement until it stabilizes.

Another consideration: I have read that once the active pressure has occurred, it is likely that over time the larger at-rest pressure will reestablish itself.

In my mind, the plan checker is correct, and you would design for at-rest pressures if you don't want it to move, and for the internal strength calculations. But I have discussed the above with engineers who are much more experienced than I, and the majority don't agree. And it seems that most of the experienced people on this board don't agree either. But I'm not really sure where I am wrong on this.

 

[DamsInc]

In my mind, the plan checker is correct, and you would design for at-rest pressures if you don't want it to move

To clarify, I would agree with the plan checker only if you cannot tolerate any movement. If some movement is ok then the active pressure should be sufficient since the structure will presumably stabilize according to your calculations.

 

[Splitrings]

The SCS Technical Release #74 uses a wall thickness to height ratio as a guide as to whether the wall is flexible enough for active pressures to develop. It states a ratio of t/H<= 0.085 is flexible enough for Ka values to be used. This TR references a paper by Karl Terzaghi.

This same TR states that walls should only be designed for Ka if "they are certain to yield", "yielding is acceptable" and they are "backfilled with coarse cohesionless soil that can permanently maintain their mobilizes shear strength". The last is as DamsInc states, over time soils can creep due to vibration, etc and the soil forces redevelop causing the wall to deflect more. This cycle can continue until the wall fails.