Larger footing-larger settlement
Larger footing-larger settlement
(OP)
I have gotten lower allowable bearing pressure values for mats before which I understand that through activating soils deeper in the ground that are poor would give higher settlements than a smaller footing that affects the upper soils only, assuming they are better quality.
But I was talking with another engineer that suggested you would always get larger settlement with a larger footing. For example, 500 kips on a 10 ft square footing would give less settlement than 500 kips on a 20 ft square mat. Assume the load is distributed over the same area in each case (say 2 square feet at the center).
I don't see how this is possible if the soil that is affected by the pressure bulb is the same quality in each case. I can sort of understand how it could happen if the deeper soils that were affected in the larger footing case were of poorer quality than the upper soils, and assuming a rigid footing. But often I see better blow counts with deeper soils.
Thoughts?
But I was talking with another engineer that suggested you would always get larger settlement with a larger footing. For example, 500 kips on a 10 ft square footing would give less settlement than 500 kips on a 20 ft square mat. Assume the load is distributed over the same area in each case (say 2 square feet at the center).
I don't see how this is possible if the soil that is affected by the pressure bulb is the same quality in each case. I can sort of understand how it could happen if the deeper soils that were affected in the larger footing case were of poorer quality than the upper soils, and assuming a rigid footing. But often I see better blow counts with deeper soils.
Thoughts?





RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
This said is generally for most soil situations, uniform or not uniform in tendency to settle.
Back to your situation with equal loads between larger footing and small footing. Change the dimensions from your 10 or 20 ft. dimensions and consider yourself out on a muddy construction site. You sink down pretty easily. However, throw down a 4 x 8 sheet of plywood and no sinking down. Then use the simplifying rule that only the zone of subsoil affected is where the added pressure from footings is 10 percent or more of existing pressure. You will soon see that that depth for the larger the added soil pressure there is much less. Means less settlement generally. That's why we enlarge footings where settlement of smaller may turn out to be excessive.
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
Haynewp's question is also similar to ones that I've had geotechnical engineers ask. While you might think that such is intuitively obvious to any geotechnical engineer, that is not necessarily so. Add that to the fact that many cannot agree on geotechnical theory (such as....what effect does significant stratification close to the footing bearing depth have on the bearing capacity/settlement relationship?).
I think you should remove your last sentence using the edit feature. A legitimate question asked by a long-time member and contributor such as haynewp deserves consideration.
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
Hokie, the other engineer (a PE but isn't a geotech) I am working with insisted on this idea which I have never heard before, which is why I brought the question here.
RE: Larger footing-larger settlement
hokie66.....I've heard the same before so I think haynewp didn't misunderstand.....yes, the other engineer was speaking from whence the sun doesn't shine. (I'll assume the grammar is correct since I'm from the southeastern US where grammar is sometimes not a strong point!)
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
Year's back when I was in the Navy those Rebels really were not using a secret language after all.
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
What the heck is Poke Salad?
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
Have to say that I've not come across "Poke Salad". Must be an Upcounty (SC) dish.
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RE: Larger footing-larger settlement
My parents grew up in depression era Alabama and they spoke many times of gathering the leaves, washing them and cooking them in hard times on the farm. I have tried it and it tastes a bit like a slightly bitter spinach. Since most of us in the south who like greens (collards, mustard, turnips, etc.) use copious amounts of hot sauce (pepper sauce in the south), the bitterness of some greens doesn't register. Mustard greens can be bitter as well.
RE: Larger footing-larger settlement
As a kid, we gathered sassafras roots to make sassafras tea.
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RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
My family is from Alabama too. My fav is collard greens, black eyed peas, fried chicken and corn bread. I never thought I would be admitting that on THIS site.
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
Mike McCann, PE, SE (WA)
RE: Larger footing-larger settlement
haynewp.....and you didn't marry her??!!
RE: Larger footing-larger settlement
This Google search explains a lot.
https://en.wikipedia.org/wiki/Phytolacca_americana
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
As for the original subject, if consolidation is not an issue, the formula for elastic settlement of a square footing is S = 0.95 qb(1-u^2)/E, where q is the ground pressure, b is the footing width, u is Poisson's ratio, and E is Young's modulus. For a constant load, if b is doubled, q will be reduced by a factor of 4, so the settlement is reduced by a factor of 2.
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
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RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
RE: Larger footing-larger settlement
In the context of this thread, I think "all else being equal" means the force on the footing, not the pressure.
RE: Larger footing-larger settlement