geotech help.. skin friction, shear strength, compressive strength of drilled pier 1

[delagina]

I have a 24" diameter x 15 feet drilled pier
I only have boring log with compressive strength of 820 psf, qu

this is where I need help if I'm using correct formula.
The objective is to get overall drilled pier capacity both skin friction + end bearing

Su = qu / 2 = 820/2 = 410psf

skin friction = alpha x Su / 2 = 0.6 x 410 / 2 = 123 psf

end bearing = 9Su/3 = 9*410/3 = 1230 psf


is my Su, SF, end bearing formula correct?

 

[GeoPaveTraffic]

Your math is correct.

Whether your logic is correct is more than anyone here is going to be able to say based on what you have provided. You need to have an experienced geotechnical engineer look at the boring log, geology, proposed structure, allowable settlements, and a few dozen other generally less important details to be able to determine the allowable load for a drilled shaft.

Mike Lambert

 

[delagina]

geotech report usually have skin friction and soil bearing capacity for drilled pier. this one doesn't have it and I'm trying to figure out based on the boring log.

 

[GeoPaveTraffic]

You really need to have a geotechnical engineer look at it. There are too many things that should be considered that either aren't on the boring log or that aren't obvious from a boring log.

Mike Lambert

 

[delagina]

@geopave, when is Su = qu? I was told sometimes Su = qu, sometimes Su= qu/2.

 

[delagina]

I was told if the test type used in undrained condition (UC) then qu = Su?

 

[Brad805]

Why not ask geotech to finish his job?

 

[GeoPaveTraffic]

Su is always equal to qu/2.

qu is the unconfined compression test.

If an undrained triaxial test, UU, was performed, then the test result is Su.

But again, undrained strength is just part of the problem. You need to have someone with experience determine an appropriate allowable foundation design.

Mike Lambert

 

[delagina]

@geopave, if I choose to use a belled-bottom drilled pier as opposed to straight drilled pier

can I use this formula for end bearing as the soil bearing capacity?

end bearing = 9Su/F.S. = 9*410/3 = 1230 psf

 

[GeoPaveTraffic]

Again, it depends on many factors.

The 9Su part of the equation assumes that the bottom of the bell is more than 4 diameters below the ground surface.

You also need to confirm that the Su value is appropriate for several diameters below the bottom of the bell.

Then you need to look at settlement, constructability, and many more factors.

Mike Lambert