CBR value for base-course in roadworks

[Emilian]

I would like to find out if an unsoaked CBR value can exceed 100% for a base-course material: maximum size aggregate 37.5mmm and having a continuous grading curve, sourced from a borrow pit.
Technical specifications impose a value of minimum 80% for base-course.
Thanks for your help.

 

[dirtman85]

What standard are you working from. CBR tests are only suitable for material <20mm. I would suggest a value in excess of 100% would be easily achievable. As for it being unsoaked, there is little point soaking non-cohesive material anyway.

 

[Ron]

Emilian...yes, a CBR can exceed 100, particularly in an unsoaked condition. For the test, the lab can use a correction for the oversized material.

dirtman85....I disagree that only cohesive materials need to be soaked. Stability is greatly dependent on the state of saturation relative to the compaction at the time of saturation. A CBR or similar design curve is developed from the soaked condition, whether cohesive or non-cohesive.

 

[dirtman85]

Correction factors for oversized material are not the best idea Ron, they often entail extrapolation of a curve which is little more than a best guess, correction factors cannot take into account the destructive force on the larger particles as they are totally interdependant, eg; a lump of ash or clinker will obviously not behave in the same way as a lump of concrete when under stress, hence the true value and nature of the test is to remove fractions >20mm.

 

[Ron]

Dirtman...I'm not referring to a correction factor, but a volumetric replacement correction for the oversized material.

 

[Emilian]

Thanks for your help, your replies gave me a better image about CBR.

 

[Emilian]

I found this link over the internet and it confuses me...it says that a high quality crushed stone material should have a CBR approx equal to 100%. What about my natural base-course? Still possible to exceed 100%? See below....

"The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%)."

http://training.ce.washington.edu/WSDOT/Modules/04_design_parameters/04-2_body.htm

 

[BigH]

Crushed stone base courses have a CBR >100% - if I remember, offhand, this is "given" in AASHTO pavement design (else it is 80%). There is little difference between 80 and 100 or 120% as far as behaviour is concerned and I think that you can simply use the AASHTO recommendations. Most specifications I have seen for natural sand and gravel subbases require CBR >30 and this isn't hard to get. Ron is right - soaking is not just for clayey or silty soils but can be important in sands as well due to the transient buildup of porewater pressures in the pavement layers due to heavy (volume) traffic loading.