correlation between CBR and shear strength such as UCS 2

[tomo2666]

I'd like to know the correlation between CBR and sheer strength such as UCS. Is there any equation?

 

[Focht3]

I suggest you search this site using 'CBR unconfined' as your search terms. I'm fairly sure that your question has already been asked -



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?"

 

[jdmm]

A journal I read years ago published a relationship between CBR and undrained sheAr strength. Since there should be a relationship I haven't hesitated to use it.

CBR=Su/23 where Su is the undrained shear strength in kPa.

If you need the journal I could dig it up.

 

[BigH]

jdmm - you'll use this correlation and not the one for SPT and Su??? - just kidding. Is this a soaked CBR or dry CBR correlation? There are some other curves - one in Baker's Highway Engineering Handbook but not CBR to Su - I have seen another and will find reference. Again, as with all correlations - judgment is needed - as you all know anyway!!

 

[BigH]

Just another point - in TRRL's Overseas Research Note 31, there is a presentation of the CBR relationships vs %MDD Heavy Tamping for a couple of soil types. They do not give, though, a relationship of CBR vs UC (or Su).

 

[iandig]

I've just come accross this thread and I have a quick note to add to the relationship mentioned by jdmm. Quite often I will schedule a whole series of relationships on soils where I will do MC vs CBR and MC vs Shear strength on the same samples. After reading the relationship noted by jdmm, I did a qucik check on some of the values I had got recently, and found that typically the range for Glacial Lake Deposits in the north of the UK were in the range of CBR = Su/15 to CBR = Su/17 for a range of samples tested prior to the earthworks (SI stage) and during the actual earthworks.
So in light of this, I would suggest that it is always worth checking out the relationship, and not relying soley on historical values, the difference between CBR = Su/23 and Su/15 would be fairly dramatic to the final CBR.

 

[BigH]

Thanks for the case history ian. You make a good point with correlations - they really should be site or geologically specific to be anything more than guideline values.