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correlation between CBR and shear strength such as UCSHelpful Member!(2) 

tomo2666 (Geotechnical) (OP)
5 Apr 04 6:02
I'd like to know the correlation between CBR and sheer strength such as UCS. Is there any equation?
Focht3 (Geotechnical)
5 Apr 04 12:08
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?"

Helpful Member!(2)  jdmm (Geotechnical)
7 Apr 04 0:03
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 (Geotechnical)
7 Apr 04 1:48
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 (Geotechnical)
16 Jan 05 15:15
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 (Civil/Environmental)
17 Jan 05 8:43
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 (Geotechnical)
17 Jan 05 17:07
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.

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