Passive Pressure and Shallow Octagon Foundation

[hitch22]

Hi fellow engineers,
[ul]
I’m evaluating an existing shallow foundation, please see the attached diagram, and I would like to know if it’s logical to account for passive soil resistance in the “empty + wind” load case. Here is the background:
[ul]
[li]Octagonal foundation (7’ feet, 7’-10” deep) has stood for 46 years[/li]
[li]Foundation is for a vessel. The vessel will be replaced in kind. Therefore, I’m assuming the weight will be the same.[/li]
[li]Using an Ultimate Limit State load combination the soil underneath the foundation fails by a large margin (13,000 psf>>6200 psf) by bearing.[/li]
[li]Soil is clay.[/li]
[/ul]I think my results are unrealistic/over conservative because this structure has stood in place for 46 years without any problems. I checked the original calculations (allowable stress) and the original calculations are correct.
I would agree to using passive pressure when checking the sliding case and the overturning case. However, I’m not sure about the “empty+ wind” case.
[ul]
Can someone please tell me if I can or can’t use the passive pressure?
Thank you very much in advance.

 

[KootK]

I'm pretty hesitant to use passive pressure in these situations.

1) It takes quite a bit of movement to mobilize it.
2) You may not be pushing against undisturbed, soils report soil. It might just be backfill that may or may not be compacted.
3) Depending on your location, the top few feet are probably all loosened up from frost action.

I would agree to using passive pressure when checking the sliding case and the overturning case. However, I’m not sure about the “empty+ wind” case.

My preferences aside, I don't see why the availability of passive pressure would vary from load case to load case. Either you've got it or you don't.

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.

 

[hitch22]

Thanks for the reply KootK.

Would you consider any lateral soil resistance even if not passive? For example at-rest?

 

[KootK]

I'd be willing to take active pressure for sure. Maybe at-rest if I were feeling desperate.

The passive pressure takes movement to mobilize it and that's why I said it makes sense to use it in the sliding case for example.

Isn't sliding/OT under the "empty + wind" case what we're dealing with here?

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.

 

[hitch22]

The soil is failing under bearing at the toe due to a high eccentricity. For example, e/D=0.36.

 

[BigH]

Where did you get 6200? presuming you are using that as the "safe" bearing pressure. I would be interested in any soils information you have - you say clay - but is it "firm" or "stiff" or "hard" . . .

 

[FixedEarth]

In addition to soil consistency, we need to know proximity to descending slope, compressibility & allowable bearing capacity for the 10 ft or so beneath the foundation.

The wind design & seismic codes have changed drastically since the 60's and this could be the reason why you have eccentricity issue? When you are embedded 8 ft, you can certainly use passive resistance. May be you can ignore the frost depth or minimum of 3 ft in warm climate. You can also limit your cohesion and just rely on frictional component of the passive resistance. But the question becomes, what friction angle should you use to compute your passive resistance? And do you use wall friction? and how much sliding friction coefficient is applicable? That is why you need a soils report, to get all these answers and design safely for another 50 years. The last process tower I had, the seismic overturning moment was too much and we could not use ma foundation. We specified drilled pier supported mat foundation. It was about 60 ft tall & 15 ft in diameter in highly seismic region.

http://www.soilstructure.com/

 

[fattdad]

I would calculate the passive pressure and show the triangle as depicted in the sketch (I'd omit any passive contribution in the upper 2 ft or frost-zone interval). I would also calculate base shear using some interface friction angle. I'd realize that mobilization of passive pressure requires movement - just like the mobilization of base shear also requires movement. Considering active earth pressure in a passive mode doesn't make sense, so I would not do that.

I'd know that my safety factor will result in using less then the ultimate values for either base shear or passive. So, I'd never stress the soil to the passive failure movement criteria.

f-d

ípapß gordo ainÆt no madre flaca!

 

[KootK]

@fattdad: you've helped me out with a number of geotechincal issues in the past and I have a ton of respect for your geo-chops. That being said, I disagree with you quite strongly on a few points here. Granted, I'm no geotechnical engineer. However, I've designed a lot of structures for overturning and, if you don't fight above your weight class from time to time, learning comes slow.

I would calculate the passive pressure and show the triangle as depicted in the sketch

It surprises me that proponents of using passive pressure rarely have anything to say about the fact that back fill, of unknown compaction, is involved in that load path. See sketch #1 below. It seems to me that the passive soil pressure of interest would be that associated with the back fill rather than the native soil. And it's a pretty safe bet that the Kp in the soils report, should one be forthcoming, will be the value appropriate for the native soil. On a few occasions, I've asked my geotechical consultant for Kp values for back fill. In all cases, the answer has been something like "uh.. maybe it would best to just not count on that unless you're back filling with lean mix concrete".

I'd realize that mobilization of passive pressure requires movement - just like the mobilization of base shear also requires movement.

All load resistance comes at the expense of deformation. No argument there. The question here is that of compatibility. You can develop base shear resistance with relatively little movement whereas the development of passive resistance requires quite a bit.

Considering active earth pressure in a passive mode doesn't make sense, so I would not do that.

You've assumed that this is a passive mode. As I mentioned above, merely relying on/developing base shear resistance doesn't necessarily imply that. Really, most structural engineers use Ko or Ka in these situations simply to introduce some conservatism into situations where they don't feel confident in developing passive pressures. We don't mean to imply that Ko or Ka are theoretically consistent with a structure plowing through soil.

I'd know that my safety factor will result in using less then the ultimate values for either base shear or passive. So, I'd never stress the soil to the passive failure movement criteria.

We choose our safety factors in an attempt to produce uniform structural reliability in our infrastructure. Relying upon, or encroaching into that "wiggle room" to establish safety invalidates the entire design philosophy.

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.

 

[KootK]

The soil is failing under bearing at the toe due to a high eccentricity. For example, e/D=0.36.

In my opinion, stiffness incompatibility makes it is unreasonable to use the passive soil resistance to shield the soil from vertical bearing over-stress. See sketch #2 below. The springs representing vertical bearing stiffness be pretty stiff, particularly given your allowable stress. The springs representing lateral soil stiffness will be quite flexible, initially, until enough movement has taken place to mobilize passive resistance. I'd be willing to wager that you'll over stress your soil in vertical bearing long before passive resistance is fully mobilized.

Oftentimes, allowable soil stresses provided by geotechnical engineers are established to mitigate differential settlement. As such, two things may be possible with the help of a geotechnical engineer:

1) You may be able to use higher allowable for transient loads such as wind and/or;
2) You may be able to use a non-triangular stress distribution as shown in sketch #3 below.

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.

 

[fattdad]

to your first quote: o.k. I mean if you don't get the appropriate passive pressure for the actual site conditons, I have no opinion on that!

to your second quote: o.k. Strain compatibility at failure mode is important. That said, who is running the strengths up to failure mode?

i don't get your third point. there are active earth pressures, at rest and passive. Any and all of these can be in play. You have asked a question about passive pressures. That's all I'm responding to in my first reply.

to your fourth point. I've worked with structural engineers for decades. I've never been sued, never been to court for my work and never had a geo-structural failure. Not sure what point you are trying to make, but o.k. carry on in your analyses. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[KootK]

You have asked a question about passive pressures. That's all I'm responding to in my first reply...but o.k. carry on in your analyses.

I didn't ask a question about passive pressures. Have you perhaps mistaken me for the original poster here? I'm just a responder on this one, same as you.

i don't get your third point.

The intent of my third point was simply to explain why structural engineers often make assumptions regarding lateral soil pressures that may not be strictly consistent with the nature of the movement being considered.

to your second quote: o.k. Strain compatibility at failure mode is important. That said, who is running the strengths up to failure mode?

to your fourth point. I've worked with structural engineers for decades. I've never been sued, never been to court for my work and never had a geo-structural failure. Not sure what point you are trying to make

I'd know that my safety factor will result in using less then the ultimate values for either base shear or passive.

I believe that all three of these issues are rooted in structural reliability and how structural reliability is often viewed differently by structural and geotechnical engineers. I'll present this in LRFD terminology as that is inherently more rational.

[Reliable Capacity < Ultimate Load] or, restated..
[phi x capacity < gamma x service load]

I understand the above statements to describe the fundamental premise of strength design. Both the phi and gamma factors are determined probabilistically and in keeping with what society has determined the level of risk ought to be. And that may result in a large effective factor of safety.

Just because the factor of safety is large, however, does not mean that the ultimate state can be disregarded. We are designing for an ultimate condition that may have a low probability of occurring but, nonetheless, must be assumed to actually occur. If we fail to do this, then we violate the fundamental premise of strength design. Consequently, I find the last three quoted statements to be misleading as they imply that safety factors somehow prevent the occurrence of ultimate limit states.

Lastly, there's these two statements considered together:

So, I'd never stress the soil to the passive failure movement criteria.

I would calculate the passive pressure and show the triangle as depicted in the sketch

If, by your own admission, the full passive pressure will never be developed, is it not a contradiction to recommend that passive pressures be used to stabilize the structure?

Educate me fattdad. Frankly, I often have difficulty understanding the unique perspectives of my geotechnical engineer friends. And I suspect that they feel the same way about me.

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.