Factor k in the correlation of OCR with CPT data

[jackzhong]

Normally soil overconsolidation ratio (OCR) is related to CPT data as: OCR = k (qc - rD)/(r'D)
Where qc = cone resistance
r = soil unit weight
r' = effective soil unit weight
D = penetration
k = constant
Several references mentioned that the range of k is 0.2 to 0.5 with an average value of 0.3; higher values of k are used in aged, heavily overconsolidated calys.

My questions:
1. Is there any formula or curve available to get k?
2. Can anybody who has the following reference tell me if I get help from it,I have difficulty obtaining this paper.
Proceedings of the Geotechnology Conference on
Penetration Testing in the U.K., Birmingham, July 6-
8,1988,pp.151-156 Powell, J.J.M: Interpretation and use
of piezocone test in UK clays
Any help is appreciated.

Jack

 

[coneboy]

Hello Jack

Sorry Jack, I couldn't find the paper you are refering to, nor could I find the formula/calculation which you mentioned.

But on a positive note I did find some newer research articles and a few alternative methods of calculating the soil OCR and k using CPTU data.

One method of determining k involves the use one of a CPTU data interpretation programs. Interp programs for CPT data are quick, cheap and easy to use. One that comes to mind, called CPTINTERP, is sold by University of British Columbia in Vancouver, Canada. I don't know for sure but I bet FUGRO (CPT Testing Co) also has interpretation program of this nature.

In the book "CPT in Geotechnical Practice", by Robertson, Lunne and Powell I did see a chart correlating Ko, fs/effective stress to OCR. The correlation chart is by Masood and Mitchell in 1993. This book is published by Blackie Academic.

In the book mentioned above I also noticed a chart by Kulhawy and Mayne that can be used to estimate a k value from qt, eff stress and total stress. I don't believe OCR was used in the equation. Prof Mayne has devoted considerable time to the problem of determining soil OCR for a variety of silts/clays.

Best of luck in tracking down the CPT book or a CPT interp program.

Coneboy