choice of Nk factor for soft clay

[stolt]

I am trying to find references that are related to appropriate choice of Nk factor for use in interpreting undrained shear strength of soft (marine)clay. Can anyone help me out?
Thanks

 

[rgrobinson]

The best reference/book I have come across is:

"Cone penetration testing in geotechnical practice by Lunne et al. (1997). Blackie academic and professional, An imprint of Chapman & Hall."

Hope this helps

Robinson

 

[coneboy]

To Stolt

The previously mentioned manual by Lunne, Robertson and Powell has a considerable amount of info regarding Nk and Nkt cone factors. The bad news is.....to the best of my knowledge this book/manual is out of print.

Another source that I have found to be quite usefull is by Jim Greig, Dr. Campanella and Robertson and is called "Comparison of Field Vane Results with other In-situ tests. It is a paper/thesis published by the UBC Soil Mechanics group.

A few other items that might help you:

*** Nk and Nkt factors are site specific. Be carefull applying a Nkt factor from a site in British Columbia to a distant area. ***

To the best of my knowledge the best?? or most common method I encounter of determining soil Su values involves the use of a Nkt factor. (Not Nk) Since you are examining soft silts and clays then including the pore pressure measurements only seems appropriate. The Nk factor uses the cone tip (Qc) reading while the Nkt factor uses the Qt reading.

A few vane tests will of course enable you to back calculate a site specific Nkt factor.

On some sites an examination of the Su/E.O.S ratio might reveal where the soil becomes N.Consolidated.

Hope this helps

Coneboy

 

[BigH]

Look up "Mayne, Paul" on the internet search. He is a prof at Georgia Tech and has been interested in cone testing ever since I've known him. He has a 'ell of a lot of papers out there discussing in situ testing of soils.

 

[lhe]

Yes, I agree with coneboy that Nkt is more appropriate than Nk because Nkt has been corrected for the "unequal area effect". Besides Nkt is a site-specific value. Therefore, it is suggested to refer to any publication regarding the Nkt that relevant to your site. In addition, please take note on the type of undrained shear test that been used for the correlation. Hope this will help you.
lhe

 

[jenesaispas]

hope this doesn't come a bit too late. I do agree that Nk/Nkt factor is site specific. From my experience, Nk of 15 to 20 is the safest bet, though it can be higher for heavily overconsolidated clay. What I normally do is, whenever possible, get the average ratio of qc over shear strength from lab tests and that will pretty much give me a rough estimate of the nk value. Hope this helps.