Critical State Soil Mechanics Issues

[MRM]

Hopefully, someone has some comments on the following:

On the subject of critical state soil mechanics, CSSM, if I obtain an NC slope in e-ln p' space (lambda) for a clay or sand, could I obtain the compression index, Cc, by using the following relationship?

Lambda=Cc/2.3

If this is indeed true, it seems apparent to me that the Cc obtained would represent the stress conditions at critical state for the soil considered. Therefore, the Cc obtained would represent the soil at a specific depth and specific void ratio where critical state conditions under loading would be met. Is that reasonable to say? Could this Cc be used as an estimate in the absence of other data and be considered somewhat valid? (just for "interest of the subject" purposes)

A paper by Prof. Santamarina and Gye Chun Cho has sparked my interest entitled, "Determination of Critical State Parameters in Sandy Soils-Simple Procedure," Geotechnical Testing Journal, Vol 24, No. 2, June 2001. This paper describes a simple method to determine critical state parameters and I'm thinking of trying it out. Has anyone reviewed this paper that would care to comment on the method? If I can get critical state parameters AND some compressibility information for some of my sands that would be very interesting.

Of course I'm aware of the limitations of actually using compressibility information for sands in calculations based on a lab test like this compared with the field conditions, but I've run several 1-D tests on certain sand samples and I'd like to see how they would compare to those found with this test. By the way, the main reason I did some 1-D tests on prepared, NC, unaged sand samples was to establish an upper limit for compressibility characteristics for some of the sands I frequently encounter. The reason I’m interested in getting CS parameters is just to expand my knowledge base of the soils I’ve been working on.

To me, CSSM is useful and very important to consider qualitatively for projects, but I haven’t had the need to use CSSM quantitatively yet. Does anyone have examples of projects where you’ve used CCSM in the quantitative sense? That would be interesting to hear too.

Thanks for your comments!

 

[BigH]

I'll have to ponder this a bit - its been a long time but I would suggest that you get a book by Malcolm D. Bolton "A Guide to Soil Mechanics", Macmillan Education Ltd (Publisher) - ISBN 81 7371 245 8. This is a highly readable book and goes into Critical State in detail. In India, this costs about $5.50 US.

 

[BigH]

MRM - check out

http://www.geo-slope.com/pub/articles/a351.pdf

http://www.civil.usyd.edu.au/publications/r814.pdf

They confirm - my looks at a couple of books too. Try a search of Cam Clay Consolidation or Cam Clay Strength on a search engine such as mywebsearch.com and you will get a few nice hits.

 

[MRM]

BigH,
I appreciate your help! Thanks for the links. It would appear that the "Cc/2.3" relationship is valid for preliminary design purposes as of now. I too will need to think about my own question more...

I've noticed that most CSSM figures or charts use the natural log (ln) rather than the base 10 Log. However in the paper I mentioned by Prof. Santamarina, he uses the base 10 log version. Example: e-logp' instead of e-lnp' for the critical state line. Is this a matter of preference? I've seen the natural log used in most other references. Any thoughts?

 

[Focht3]

I think you're right - it's personal preference. While both can be used, I personally prefer log₁₀ to log_e. The math is "simpler", and I have a much better feel for the values.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[BigH]

Thought I might make a note - years ago in Geotechnique I saw a paper where they plotted logp vs log e - gives a sharper peak and perhaps better estimate of preconsolidation pressure - not really Critical state stuff but something to look into as you are seemingly trying to get "into" the consolidation behaviour.