Design Piled Raft Foundation 1

[dr. rifat]

hello engineers,
Please excuse my ignorance on the subject. This is the first time I am designing piled rafts. I have a few questions for which I would appreciate some direction/references.

Soil safe bearing capacity(F.S.) 1.43 ksf for borehole 1 in 5' depth.
Soil safe bearing capacity(F.S.) 0.07 ksf for borehole 2 in 5' depth

I want to rest foundation below 5' from EGL.

For simplicity assume Soil safe bearing capacity 1 ksf(Assume)

which I obtain from a soil test report.

Now how to design this piled raft foundation in this type of soil.

Pile allowable load capacity 46 ton in 60' depth.Ultimate skin friction force 232.9 kip & ultimate end bearing force 106.03 kip.
Assume column load 800 kip.

If I want to restrict 4 piles footing,how is raft thickness required?How to calculate its settlement?

Sorry for my bad English.

RIFAT

 

[DaveAtkins]

If you are putting the raft on piles, soil bearing pressure has nothing to do with the design.

This type of project is best solved using finite element analysis software (such as RISA-3D). This is modeled as a plate on four pinned supports.

You need to check bending, punching shear, one way shear, and rebar development. There will be essentially no deflection, since the raft is supported on piles.

DaveAtkins

 

[dr. rifat]

Dear Daveatkins,
That means here pile bearing capacity is our main subject?
Then,I hear form expert that "If we provide mat over pile,then mat can take over 20% of total load"is it true?

RIFAT

 

[SlideRuleEra]

dr. rifat - Two years ago, right here on Engineering Tips I learned that there is such a thing as a "piled raft" foundation. The piles take some of the load, the mat take the remainder.

See BigH's comments in this thread.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[DaveAtkins]

I did not know a raft could take part of the load if supported by piles. It doesn't make sense, since the piles allow virtually no displacement, while you need displacement to engage bearing pressure in the soil under the raft.

But I suppose assuming something like 20% of the load is taken as bearing pressure could be possible...

DaveAtkins

 

[Retrograde]

since the piles allow virtually no displacement

In a piled raft the piles are not founded in rock so the assumption of no displacement does not hold. Settlements in the range of 50-100mm are common.