which depth to take for a basement footing for bearing capacity

[ketanco]

Hello,
I am preparing the civil PE exam in geotech and in the NCEES Geotechnical afternoon sample questions, question 520, for the depth while calculating the D, why do they take 2 feet and not 10, which is (2+8) feet up to the overall ground surface?

for the ones that do not have the book, the question asks bearing capacity of a footing in the basement of a building. the footing is 2 feet below the basement ground level, but it is 10 feet below the overall ground level around the building. i calculated using 10 feet, but in the solution it calculates by taking 2 feet only and i dont know why.

Thanks

 

[GeoPaveTraffic]

The question you need to ask is why would using 2 feet of embedment make sense? The answer, relative to bearing the footing can fail one of two directions. Toward the inside of the basement or toward the out side of the basement. In the first direction there is 2 feet of soil cover, in the second there is 12 feet of soil cover. You need to ask yourself which direction is going to control the design.

Mike Lambert

 

[Phidelt218]

So it sounds like you have a building that has a basement that is 8 feet below the ground level. The footing is 2 feet below the basement. Why would you count the additional 8 feet when bearing capacity is resisting the vertical forces above it? In this case the building/basement is bearing down on the footing and only 2 feet of soil separates.

FYI for these kids of problems the best thing to do is draw it out. I do it for all my actual problems and for the PE.

 

[msquared48]

If there is not a concrete slab present in the basement, then I would agree, but if there is one present that could prevent a rotational failure of the soil matrix, I would rethink the problem here... and use a value of something less than the 10 foot burial depth.

Mike McCann
MMC Engineering

 

[BigH]

This is an interesting quention . . . 10 ft on the outside of the footing; 2 ft on the inside. Use the 2 ft has been put forth above but that is only "half" of the bearing capacity failure surface - half is exposed to 2 ft; half to 10 ft . . . Mmmmm - do we ignore that "outside" the basement? This situation doesn't agree with the classical bearing capacity failure surfaces - and I don't ever remember any text or manual that has ever really addressed this situation. See Lambe and Whitman (page 485) with a "tank" sitting on level ground - they actually do a rotational failure - one direction - rather than a "Prandtl" style 2 direction failure surface.

 

[ketanco]

thanks for all answers
in response to above questions:
mssq: yes there is concrete slab
phidelt: my reasoning was, that depth was subject to all 10 feet of soil loading in the past. thats why i took 10 feet. so a building that has footing 2 feet below natural ground and a building that has footing 2 feet below the ground which is 8 feet below original ground must be different - i thought

 

[Ron]

Agree with msquared. The slab confines the soil in the same manner as overburden on the outside, and as long as you don't fail the slab or uplift the slab, classical bearing capacity failure can't occur.

The only reason that the solution would use 2 feet is using the assumption that the slab will offer no confinement. As a practical matter, I would check it both ways and design the slab to achieve confinement.

 

[Phidelt218]

I think to remember for the PE test, is that a lot of questions on the PE will try to cause you to over-think a problem and make you think it is more complicated then it really is. This happens easily with problems giving more information then what is needed. I took it a year ago, i can not stress how many problems were "concept" questions. If you know the concept of how it works then the problem is easy. I have 20% of the problems were not problem solving. An example: what will cause a cantilever beam's deflection to reduce, a) increase beam size, b) move point load, c) ect.

 

[ketanco]

ron & ms squared: thanks for answers but even if there was no slab, that soil below footing was carrying 10' of soil weight in the past. so still shouldn't we use 10' in our calculation even if no slab with that logic? or did i not study enough of bearing theory yet and that depth component is there by thinking the rotation of soil and that's why slab makes a difference?

 

[Ron]

ketanco...NO! Bearing capacity variations from overburden can only consider the existing overburden, not past.

 

[Phidelt218]

Ron is quicker then I am lol.

 

[ketanco]

ok i see, and thanks... but now here is my next question based on this info... if past is not considered at all, then 10' should not even be considered at all. and we should use 2' right away.. why is the discussion of slab present vs not present then? or the other users answers about using something in between etc... after all, 10' is a past condition. it was there before the basement was excavated. if past is not considered at all, then the answer is using 2' right away...

 

[Phidelt218]

ketanco
I think when it comes to the lab they are referring to more in depth analysis of the soil and horizontal forces acting on the slab rather then the basic bearing capacity. If there was no slab then this would not really be a bearing capacity question it would be more of a pile question. Such as in alaska we have houses elevated on helical anchors rather then on slab and grade foundation.

 

[Ron]

ketanco...bearing capacity failures can occur in multiple directions, but usually only two directions...perpendicular to the footing length. In your case, failure could occur to the outside or to the inside. The 10 feet is to the outside....that is a present condition, not a past condition. The 2 feet is to the inside with no consideration of concrete slab offering any resistance.

 

[wannabeSE]

Ketanco,
Maybe some sketches will help. Look at this webpage:

http://wwweng.uwyo.edu/classes/fa2013/ce3600/11_foundations/11_foundations.htm

 

[BigH]

wannabeSE - there is nothing in the sketches showing the situation in the OR - that of 2 ft of "overburden" on the right side of the footing and 10 ft on the left side. The failure surface if using the Prandtl failure configuration would have to be taken up the left side and the unbalanced pressures are not addressed either in the formulation or the bearing capacity factors. One might want to consider taking the wall and footing as a retaining wall problem - presuming that there would be a "near" active pressure on the left side - check for sliding, overturning and the kicker - the allowable bearing pressure - which is the problem. What would the global stability look like? This is why I initially referred earlier to the problem given in Lambe and Whitman who use a single sided failure surface for a tank foundation.

 

[wannabeSE]

BigH,

Several people mentioned that the failure surface occurs on either side of the footing. Perhaps I misunderstood kentanco's replies. But, I thought that kentanco may not remember the mechanism of the failure for a typical case with soil balanced on both sides. That is why I posted the link.

 

[Phidelt218]

The problem is about a basement with footing 2' below the basement and to the top of the soil is 10' from the footing. Everyone is making this much more complicated then it is asking. The basement has 8' of fill on all sides and footing 2' below that. The bearing capacity is the soil resisting the vertical force of the building and the basement. In the real world we would have to make sure that the basement walls can withstand the soil pressures and all those forces. But for bearing capacity there is the the entire house+building acting on the footing which is 2' below basement.

 

[oldestguy]

I'll jump in. Everyone has missed the point. The point at which the foundation is considered as having failed is when it has settled too much. That's before any shearing failure. 99% of the time it is allowable settlement that controls the design.

 

[BigH]

You are very correct oldestguy - seldom have I done any bearing capacity calculation other then 2(Su). Still, the OP was related to the PE exam that he was studying for . . .