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(Consolidation) Settlement of Piled Raft

(Consolidation) Settlement of Piled Raft

(OP)
Hello everyone,

This is a broad topic, but I just want to know what do you do in practice: I would like to know which theory do you use while estimating the settlement of piled raft? But please, assume that our piles are not rock socketed. For ease, you can assume piles are inside cohesive soils.

What do you use? I will list some methods. If you have any more method, can you please give some references?

*Imaginary footing (Equivalent Raft) method of Terzaghi (If this is the one you use, can you share your experience? I always noticed that settlements are much lower than anticipated by this method.)
*2D/3D finite element analysis. (If this is the one, can you please state which software do you prefer?)
*Randolph-Wroth Method (If you use that, can you explain how do you calculate G to reflect consolidation settlement.)

Thanks in advance.

RE: (Consolidation) Settlement of Piled Raft

"A"

f-d

ípapß gordo ainÆt no madre flaca!

RE: (Consolidation) Settlement of Piled Raft

(OP)
fd,

Thanks for answer. How was your experience with Imaginary footing method over the years? Are you comfortable with the results?

RE: (Consolidation) Settlement of Piled Raft

yes, and it's what Mike Duncan taught me and I was a good student three decades ago - not sure about today though!

f-d

ípapß gordo ainÆt no madre flaca!

RE: (Consolidation) Settlement of Piled Raft

(OP)
Wow. Do you recall how he stated that?

RE: (Consolidation) Settlement of Piled Raft

it's in my notes, but I don't recall how it was actually stated. I have citations on this matter, but don't really want to photocopy, scan and such to put it on the internet.

f-d

ípapß gordo ainÆt no madre flaca!

RE: (Consolidation) Settlement of Piled Raft

(OP)
Thanks fd.

I would really like to hear from other engineers here.

RE: (Consolidation) Settlement of Piled Raft

The modeling approach to settlement of the system depends on your soil type, and how much participation you expect from the pile (how much sharing between pile and mat underside).

The benefit of the raft system is shorter piles. For a raft system, you usually have a friction pile that moves more than the typical 1/2" and where the mat underside participates in bearing. Another way of looking at this is that the piles are engaged at service loads with FS near 1.0, and the mat engages for higher loads to provide added FS. Like a bilinear curve.
Familiar names regarding this subject are H. Poulos, Randolph, and B. Fellenius. For the simplified method, Poulos ISSMGE (2001) report suggests a mat analysis where the piles are replaced with higher localized bearing pressures, with lower bearing pressures (or equivalent springs) between the piles. Poulos case study papers can be found on the internet.
For a project of any importance, a non-linear FE analysis is in order. This will provide bending moments at the pile-mat interface and along the pile, which you will miss in a simplified approach.
So, to answer your question, start with method A hand calcs / spreadsheet, then move to method B.

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