consolidation settlement of deep foundation

[bensarengat]

Hi there,

I`m actually a structural engineering, trying to understand how to get a proper model of 10 storey building on analyzing settlement.
i`m use pile caps and tie beams as the foundation, the top of pile caps is just the same as the foundation slabs.

i`m getting confuse about the proper countour settlement and the way to model the load. should it be uniformly distributed over the foundation slab? or i have to put each load in every pile caps.??

each option caused diffrent result. i attached the ilustration of my case.


which one you guys think the most reasonable result?

this is the case :

option one :
this is what happened when i put the load in each pile caps

option two :
for distributed load on the foundation slabs

thank you,
best regards

 

[bensarengat]

i tried using settle 3D software.
usually, the geotech engineer gives the second result (the distributed one) from previous projects.
is there any suggestion how to make a proper model?

 

[BigH]

Looks to me like you have two issues - and you have not provided us anything about the stratigraphy that you are in - i.e., are the piles resting on rock? Driven through soft clays to firmer clays or granular?

The first problem as I see it is the settlement of a single pile group. This can be estimated using the equivalent raft method as found in Tomlinson's Pile Foundation and Construction book. I have various editions but you can look at the 5th for instance. This translates the group to an equivalent foundation and then you determine the settlement under the equivalent foundation. One could argue if it is rigid or flexible - try both to get a handle on what you will expect.

The second problem is overlapping of the pile groups with each other - again on your sketch you didn't advise what the column spacings were. I'd look at the depth of the equivalent rafts of each of the groups and then using Boussinesque or perhaps the 1H:2V approach to see if the groups overlap - if they don't, then the first problem would be in effect. if they do intersect you could look at the effects of interacting footings - this is relatively standard practice although I find it a laugh when all these new-fangled codes (LRFD, etc) that are in the various FHWA or state DOTs always spend copious pages on the settlement of a single footing but scant mention other than a paragraph on interacting foundations which makes a lot more sense of consideration than they cover. The settlements can then be determined . . .

Yes, this is not dandy-fancy computer programmes - 2D FE or 3D FE etc. but seems that you might want to do this as a "hands-on" exercise so that you can get a feel of what is happening - then if anything is critical - go to a computer model.

 

[oldestguy]

The question brings up another question. As a structural engineer and with a significantly important structure, and apparently not having had experience with hands on soil engineering computations to double check answers, why even try? The risk is serious. I'd pass the responsibility on to an experienced geotechnical engineer.

 

[geot88]

Hi. I agree with oldestguy. All structures are important, but a 10-story building has a special importance.
And I also agree with BigH. Loads, soil profile and compressibility of each layer is needed in order to perform a settlement analysis since the type of settlement depends on the soil you have. Then you can choose a method for load distribution -i.e. 2:1 approach for the pile group.

 

[Ron]

I would use option 1 to model the load on the piles. That is the more conservative approach. This assumes that the soil support for the slabs is irrelevant and the slabs are designed to transfer loads to the grade beams and the grade beams get no help from soil support!

 

[bensarengat]

hi guys,

i`m using the boussiqnesq method for stress distribution on both case.
i`m familiar with hand calculation of settlement analysis of shallow foundation, but for the deep foundation, it`s different story.


thank you for all your feedbacks,
i`m really appreciate it

 

[L0k]

Hi Ben
Make it easy,say: 1. do the soil investigation to find out the soil strata either hit a hard strata or to reach a suitable layer based on your local experience to be able to carry the pile group of estimated 1 MN.
2. Design the foundation pile either floating pile or end bearing pile depending on your strata.
3. Calculate the settlement for each column if you are using floating pile.
4. Find out the differential settlement, if this exceed the allowance, the pile must go deeper.
5. Good luck!

 

[Okiryu]

I found these figures about distribution of stresses for pile groups that may help...

 

[BigH]

Okiryu - nice reference - basically the same one that I put forth as found in Tomlinson's book which gives an "equivalent raft" for the pile group.