Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
(OP)
Is it true, that the absolute settlements under the 100m² are larger than under 10m² spread foundations? I assume that the bearing pressure is equal under both foundations! What about relative settlements? I think they are smaller under 100m² spread foundations. Am I right?
PS: I hope that I use the right words (large-big, small-less than, etc.). I’ll be glad if someone will correct me.
Drile007





RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
Hope this helps.
f-d
¡papá gordo ain’t no madre flaca!
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
We have a building which total weight is F and we have an option to transfer this weight either on two foundation systems:
A) Number of pad foundation (np=10...number of pad foundation, a=10m² area of one pad foundation)
B) One large raft foundation (nr=1...number of raft foundation, A=100m² area of raft foundation).
If we calculate the stresses under foundations, we see that they are 10 times smaller under the raft than under each pad foundation. OK, that’s clear, but which system will have smaller settlements? Stresses under the raft are 10 times smaller, but as Fattdad said, there is a big influence of stress increase with depth? I think this influence is not of factor 10? How big is it?
Drile007
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
10 x 10 = 100, same area, same stress, same settlement.
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
Hope someone will reply to my previous doubt: the influence factor is...
Thanks in advance
Drile007
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
Read the fattdad reply!
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
If I understand fattdad right, there is a big difference between ten 10m² and one 100m² footing. It's different if you put those ten 10m² footings one by one or you separate them far away from each other. If they are closed together each one has impact to his neighbour. And this impact is summing! If they are far away this impact will disappear.
And the consequence is:
Maybe I'm wrong? That's my doubt: the influence is not of factor 10? How big is it?
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
If you then attempt to support that by one 33 ft square footing you'd have the same reaction. Bearing in mind that you'd have a lot of building spilling over the edge of the footing.
Bousinesq would show that the stress distribution immediately below each footing will be 3,000 psf. However, the stress distrubution at depths of 10, 20, 30, 40, and 50 ft will be very different.
If you have similar soil conditions, you will experience more settlement below the larger footing. That's a big IF! More typically, you will have hyperbolic modulus relationships where the elastic modulus will increase with depth. This may all equal out - don't know. Then again, you may have bedrock at the depth of 20 ft. If so, 100 percent of the seat of settlement for the smaller footings will be in the soil zone as opposed to the larger footing.
Unfortunatly, sometimes these answers require calculation and there's no general answer. . . .
Hope this helps.
f-d
¡papá gordo ain’t no madre flaca!
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
f-d
¡papá gordo ain’t no madre flaca!
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
The above is "good" for elastic settlements (granular - or immediate settlements of clay) - but, there is also a problem of consolidation settlement - again, you need, even more so, to know the actual stress distribution under your footing (large or small) and also the overconsolidation behaviour of the stratum with depth - to determine settlements in these cases.
There are many variables - and I think that if you sit down one evening and do a few scenarios like indicated above, using a calculator, bousinesque charts, graph paper, engineers scale (hopefully you know what it is) and pencil, you will gain a lot better appreciation of what has been explained above.
RE: Absolute-Relative settlements under 10m¦ vs. 100m¦ spread foundation?
Thanks (again) guys, but I'm wondering now what are the Pros/Cons between pad and raft foundation (if the soil conditions are uniform and the distances between pad foundations are big):
Pad:
Pros: absolute settlements are smaller; the stress/capacity ratio is higher; their area is smaller, so they are cheaper
Cons: relative settlements are bigger (between pads) and the consequence is aditional "load" on the structure
Raft:
Pros: relative settlements are smaller, so there is no danger for the structure above the foundations
Cons: absolute settlements are bigger; the stress/capacity ratio is small; they are huge and consequently more expensive
Am I right?