LBR vs. CBR

[jgailla]

Does anyone know of any correlation between the Limerock Bearing Ratio and the California Bearing Ratio? We use LBR here and most textbooks, research, etc. only discuss CBR, I assume because LBRs are only used locally. I have been using a rough correlation of 100% LBR=80% CBR. As far as I know, the methods of testing are identical but performed on different materials.

 

[iandig]

I have not come accros the LBR test before, but as a quick breakdown of the CBR (in metric units) I have summarised below some of the key points of the test(based on BS 1377-4:1990 Method 7)

The CBR is a measure of the force required to penetrate a soil with a plunger of end area 1935mm² at a rate of 1mm per minute, expressed as a percentage of the following forces:
At 2.50mm, Force reqd/13.2kN as a pecentage
At 5.00mm, Force reqd/20kN as a percentage
The CBR mould is 152mm in diameter and 127mm high. No material greater than 20mm should be included.
The CBR value should be reported for both 2.5mm and 5.0mm and if they are within 10% of the mean, then average CBR can also be reported. Corrections are required pending the shape of the graph of load vs penetration. Seating loads are required at differing levels pending predicted strength. Test can be undertaken in-situ but the sample is then not confined and as such this WILL effect the result.
Based on the above information, it may be possible to carry out a simple calculation/graph of the results of both methods, but without being familiar with the LBR not sure how this will work out.

I am sure that other contributors out there have slightly different numbers from above, as the test was 'converted' from imperial measurements, and I look forward in anticipation to other replies.

 

[cbosy]

I worked in Florida and we ran LBR tests. Maybe talk to somebody from the FLDOT.

 

[jgailla]

thanks iandig and cbosy

I've talked to several people, and they all say it's comparing apples and oranges. It seems to me there should be a correlation.

 

[Ron]

jgailla...the LBR is unique to Florida...It is a Florida DOT method. It is based on the CBR and is very similar but the baseline of load correction calculation is 800 psi for the penetration correction whereas the baseline for the CBR is 1000. Therefore, the CBR value would generally be 80 percent or so of the LBR value, though direct correlation is not necessarily good to do beyond a gross comparison, because in plotting the curves, the penetration calculation defines the inflection of the curve, thus potentially giving a larger (or smaller) difference than the 80 percent comparison.