Larger footing-larger settlement 1

[haynewp]

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?

 

[oldestguy]

You forgot to calculate the unit pressure from the footings themselves. That so called "rule" applies if the unit footing pressures are the same. This comes about if an architect is sizing all footings for the same bearing pressure. In your two cases even though the bulb for larger may be larger, the intensity of increase in soil pressure above the non loaded pressure is less for that larger footing at a lower unit footing pressure.

 

[haynewp]

Ok, I am not quite following what you're saying. Assume the footings are both 3 ft thick just for this discussion. The pressure under the larger ftg will of course be less when using the same 500 kip applied load. All else being equal including the quality of soil in the ground at whatever depth, how can there ever be greater settlement in the larger ftg case?

 

[oldestguy]

Larger footing at same footing pressure (#/sf) will be carrying a higher overall load. Thus the 0.1 times the footing pressure, the depth of this effect is significantly deeper for the larger footing, thus affecting more soil to greater depth resulting in settlements affecting deeper soil, with total settlement of larger being more.


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.

 

[haynewp]

Sorry for the inconvenience. The thread can be deleted if it is not worth discussing and using too much space.

 

[Ron]

OG....notice that haynewp's area of practice is "structural", not "geotechnical". The info you gave is good and would be commonplace for practicing geotechs; however, as a structural engineer one only gets to deal with bits and pieces of geotechnical theory and practice, thus having such explained is not only relevant but useful info here on the site for reference of all.

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.

 

[oldestguy]

Ron: Ok OK Hope I got the question answered. You're more patient than I. On occasion some here have made reference to various texts, but not everyone has access to them. So an attempt here is without that assist. It appears that his advice from a geotech may not have been explained very well. I find my wife is in that same boat, ha.

 

[hokie66]

I think that all that needed to be said is that either haynewp misunderstood, or the other engineer was silly. Same force over larger area causes less settlement, but same pressure over larger area causes more settlement.

 

[haynewp]

Ron, I really appreciate that.

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.

 

[Ron]

OG...you answered the question nicely! Thanks.

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!)

 

[hokie66]

Ron, I'm from the South as well, so I won't be arguing with you about grammar.

 

[haynewp]

Makes three of us. We could carry on entire conversations and nobody else here would know what we tawkin bout.

 

[oldestguy]

Now it is all clear. That other engineer was a Yankee I'll bet.

Year's back when I was in the Navy those Rebels really were not using a secret language after all.

 

[Ron]

OG...that's it!

 

[oldestguy]

Two digression items. What happened to the SlideRuleEra post?

What the heck is Poke Salad?

 

[hokie66]

SRE must have deleted his own post, for some reason. But you can still see it by clicking on the "Post Deleted" line. As to Poke Salad, never had it. Think they eat it in GA and SC.

 

[SlideRuleEra]

I deleted my own post... seemed kind of corny.
Have to say that I've not come across "Poke Salad". Must be an Upcounty (SC) dish.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ron]

Polk Salad or Poke Salad.....made from the leaves of a wild plant that grows in the southeast, mostly in a belt from southern Louisiana to South Carolina, but also found in other areas. Most common use areas are Mississippi, Alabama and Georgia. Berries of the plant are supposedly poisonous to humans, but the leaves are commonly harvested and cooked in a variety of ways, usually stir fried (with bacon or other pork fat, of course) or just boiled down like collard greens. Usually only the leaves are cooked.

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.

 

[SlideRuleEra]

Interesting, Ron. I just checked with my wife (a Lowcountry native, I'm from the Sandhills). She has heard of Poke Salad, but known only people who eat a variation - dandelion greens.
As a kid, we gathered sassafras roots to make sassafras tea.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

The question comes from Morse code conversations I have had with fellow hams from Memphis area about 1950's. They would jab fun at me for not knowing the ways of the the south. However, I've had my share of dandelion greens.