Calculating coeff of friction from a CPT

[Gellis]

I am using a coeff of friction of 0.2 for a soft clay, but have CPT data with Cone resistance and sleeve friction info. can i calculate the coeff direct from this info?
i so hope this is not a simple question and i look like a dunce!

 

[jdonville]

Gellis,

What exactly is coefficient of friction? Is it the same as tan(angle of internal friction)?

I don't want to give you a bad answer.

Jeff

 

[dgillette]

Expanding on jdonville's question, is the 0.2 intended as a drained friction angle, phi', an undrained strength ratio, c/p a.k.a. Su/sigma'vc, or???

If you are looking for undrained shear strength of the soft clay, you can try Su = qc/Nc, where Nc varies from 9 to 20, depending on what test the undrained strength is referenced to, anisotropy, etc. (Nothing is simple.) Then, you get the strength ratio from Su and your estimate of sigma'vc, if that's what you want.

I've also seen Su = (qc-ut)/Nc' where Nc' is like 6 to 12.

I don't think you can get a drained friction angle from the cone in soft clay, but I'm not positive about that.

Regards,
DRG