Bearing Capacity of Multi-layered soil 1

[EireChch]

Would you be concerned with bearing capacity failure of a 1m clay layer directly beneath a 3 x 3 pad footing. Clay layer extends to 2.6m depth, footing founded at 1.4m. UU test in clay layer indicates Cu of 55kPa. Medium dense sand to depth N = 20

Q allowable of clay layer calculated at 130kPa.

Beneath clay layer Q allowable of medium dense sand is 370kPa plus. (based on phi 30)

I know settlement controls and that it would be easier to rip the 1m of clay out but entertain the discussion please.

 

[oldestguy]

Check the rules here. No design. I'd be very reluctant to take anyone's opinion of this log. Likely there are factors on this job that may or may not be important and thus the opinions may well be useful or not applicable. A common problem with newer users of this web site is they don't give enough info to start with and then many times after about 10 comments something is mentioned that negates all the earlier comments. You are not new, but this fits that situation

 

[EireChch]

OG - thanks for the response. Definitely not looking for any one to design this for me. It is a discussion that i was having with my manager (who has some 20 years experience on me). I have done a settlement calc and calculated settlement is approximately 25mm. This is marginally ok as 25mm is out limit. Being anywhere near 25mm is a pretty good given the tolerance of settlement calculations.

I raised the issue that technically we are applying a load that is greater than the allowable of the clay layer, however he indicated that as it is only a thin layer (1m approx) and that we are marginally over (150kPa applied, 130kPa allowable, i note i forgot the include this above. I had edited now) it shouldnt be a problem. With 130kPa allowable , our ultimate is 390kPa. Therefore 390kPa/150kPa gives a FoS of 2.6 instead of 3. Still a good chunk of FoS so i see his argument, plus he knows a lot more than me but i just said id raise the the point of discussion.

Cheers.

 

[BridgeSmith]

Through discussion with the guys in the geology department here, I've come to realize that calculated allowable bearing pressures are based on an assumption of tolerable settlement. If a larger settlement can be tolerated, the allowable bearing will be higher. Recently, we've started getting graphs from them that show allowable bearing capacities at several discrete values of settlement, so we can choose what to use based on the structure type it has to support (1/2" for a pier footing, but 2" for an MSE wall). So, marginally exceeding the allowable bearing capacity likely only means you have slightly more settlement than what was assumed to be the tolerable limit.

Because settlement under a particular load is actually a strain percentage calculation, the total settlement is also a function of the estimated thickness of the layers under the footing. If your footing elevation is different than what was assumed in the calculations, the allowable bearing will change because of that as well.

 

[fattdad]

[ul]
[li]Take all the borings and look at each one.[/li]
[li]Develop an average N-values within the zone of stress increase. If you have one foot of N=10, 4 ft of N=15 underlain by 5 ft of N=5, you'd return some average weighted N-value of 9.5, assuming the zone of stress change is 10 ft.[/li]
[li]Find the lowest average N-value and use that to obtain your rational bearing pressure terms.[/li]
[/ul]

Yes, settlement is the actual problem.

Notice that as the footing gets smaller or larger the thing that changes is the depth of study (and corresponding average N-value).

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

Eirech: Notice none of the responses so far answers the question you have.

 

[SlideRuleEra]

Would you be concerned with bearing capacity failure...technically we are applying a load that is greater than the allowable...FoS of 2.6 instead of 3.

I'll try to discuss this with you. To me, your question is "What's an acceptable factor of safety for the stated conditions?"
To work towards a meaningful answer, I'll ask you a question:

Where does the value of "3", as the accepted factor of safety, come from?

I don't mean something like "page xxx of code abc", but why is the value 3 instead of say, 2... or 4? There is probably an historical or experimental explanation (not just a "fudge factor") why FoS = 3 has been established; what is that reason?

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[oldestguy]

This old guy may be wrong, but without checking in my stuff it is the result of using a lab test for unconfined compression of a cylinder of the material against computing the failure by graphics for a footing on the surface, not embedded. Cohesion is used for the graphic shearing evaluation, being about 1/2 of the unconfined compressive strength. This factor 3 then is the bearing value with respect to the lab test for unconfined compression. Then for an embedded footing the actual numbers comparing the graphics method to the lab test will be greater than 3. Why stick with 3 for the safety factor? Probably for convenience (lazy way).

 

[SlideRuleEra]

OG - Thanks for an answer, but I really want ErieChch to think about my question... not just "get" an answer.

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[oldestguy]

OG again. While I admit I never have used the metric system for this work, I fail to see how one can compute settlement without so much as a simple few inexpensive tests, such as moisture content, Liquid and Plastic limits, let alone a consolidation test. Using the rough indicator "N" for any refined determination of settlement also is unrealistic. One reason for a large rupture safety factor, even if it really does not apply to settlement, is that it does give a form of "overall safety factor" better than none.

 

[EireChch]

Hot Rod, f-d, SRE and OG, thanks for taking the time to respond. I appreciate you taking the time.

SRE - yes I have never known where the FoS for bearing capacity came from. I imagine the guys like Terzahgi, Hansen etc. developed their bearing capacity methods and acknowledged that soils can be that variable and unpredictable so lets throw a large FoS of 3 on it. People have used these for a long time (since Terzaghi 1943?)and its worked pretty well so, if its not broke dont fix it?

And yes - my question is really "what is the bearing capacity for the stated conditions", I am not looking for anyone to calculate it, i have done that above so lets just assume they are right. My question boils down to engineering judgement really i think. Fos of the clay layer is 2.6. At what point to you say "ah its close enough to 3, should be fine", when it becomes 2.8, when it becomes 2.9? The clay layer is only 1m below the pad, with medium dense sand beneath. So should this clay layer be dictating the design/assessment.

f-d : you propose a good approach. However I have a clay over sand. I am using UU results in my clay layer. Cu of 55kPa. We have conservatively adopted a phi of 30 for the sand/gravel layer below. How do i use a weighted average then? If it was a loose sand layer instead of the clay layer then i could apply it OR if it was soft clay over stiffer clay i could take a weighted average of Cu?

Also, correct me if i am wrong, does bearing capacity failure mobilises shallower soils than settlement (based on standad bearing capacity failure envelopes). Would you use the depth of where 10% increase in applied stress occurs as your depth of study or a shallower depth.

O-G - "I fail to see how one can compute settlement without so much as a simple few inexpensive tests, such as moisture content, Liquid and Plastic limits, let alone a consolidation test" I have all that information. 1-d consolidation data will be used for settlement of the clay layer. E' values will be used for the granular material. I just want to discuss the issue of a thinner soft layer "controlling" bearing capacity when it sits over a much denser material.

Thanks again all.

 

[SlideRuleEra]

My question boils down to engineering judgement really i think.

IMHO, you nailed it! And your (and I'll bet your manager's) reasoning is exactly inline with what I call "Engineering Judgement".
You hear the terms "intuition" or "gut feel"; to me, that is NOT "engineering judgement".

Using "intuition" the Engineer mentally sums up the facts and makes a best guess on what they mean and how to use them. A big improvement over a wild guess but there is a better way.

"Engineering judgement" is really a form of fuzzy logic. The Engineer makes the same summary as above... but then does more. Look at the variables involved and considers several things, such as:

1. Does the set of fact resemble anything I have come across in the past? What was the outcome?
2. In your example, does the clay happen to have (what I'll call) "better than average" soil properties... for clay?
3. What would be the consequences of having, say, 0.3 meters... or 3 meters of clay below the foundation instead of 1 meter?
4. The question I asked you, just how well founded is the accepted FoS?
5. Perhaps make some some very simple pencil and paper calcs based on simplifying assumptions.

Then you have the basis to apply "engineering judgement" with a certain level of confidence. The nice thing is that by using engineering judgment all the time, even when it is clearly not needed, the Engineer's judgement improves.

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[BridgeSmith]

I tend to try to assess situations like this in terms of what constitutes failure, and what the consequences of that failure would be. In this case, (geotechs please correct me if I'm wrong) the failure that is of concern is greater settlement than expected. The consequences of that failure depend on the structure the footing supports. How detrimental would it be if the settlement is more than the assumption used to calculate the capacity.

 

[oldestguy]

OG here again. HotRod hit the nail on the head. In answer to the first post question, bearing capacity failure is nowhere in the picture. Reflecting on the various posts and which is the way to go, the boss obviously is right..

 

[EireChch]

Thank you all for the responses. All valuable input.

SRE - as you highlighted, Engineering judgement is a skill acquired over time. Jobs/discussions like this definitely help me develop it.

HR/OG - again i agree with you both. Settlement is controlling. This disucssion started in the office as one of our strucutral engineers was pushing the pad size out to 3.8m square or something to reduce the bearing pressure to that of the allowable. This is where we explaining how the settlment is teh controlling factor. He wasnt listening to me much and then my manager had to back me up!

 

[Okiryu]

EireChch, looking at your log, there is not groundwater, so I would consider that you are calculating elastic settlements. I would expect that most of the settlement is occurring in the clay layer rather than in the granular soils. So, my grade of confidence about increasing the allowable bearing pressure from 130 kPa to 150 kPa will depend on how accurate the modulus of the clay was calculated. If you are using N-value correlations, I would be more careful.

BTW, about bearing capacity, if your UU test is giving you a c of 55 kPa, I think that 150 kPa is high. Normally, I would consider 2c=qu as the bearing capacity considering a FOS of 3. However, 150 kPa may still okay if you are comfortable with lower FOS. Again, I think that this does not matter since as you mentioned, settlement controls your design. If the settlement for an allowable bearing pressure of 150 kPa is acceptable for the design team, I think that it can be used. However, again, I think that it will be depend on how you have calculated the clay modulus and also as f-d mentioned, you have also to look at all the borings and try to look at the thickest clay layer for your settlement calcs.