The purpose of doing a CBR test is to find out the California Bearing Ratio (CBR) of a material which has to be used for the subgrade of flexible pavements. The actual quantity that concerns the strength of the subgrade material is Resilient Modulus though. CBR is different from it but hugely corellated. Also, in literature there are curves plotted for the CBR against the expected traffic (million axles per year) for every subgrade thickness. We can read the required thickness for which we need to design the subgrade layer if we know the CBR of our material and the design traffic. So, it is preferred not to get into the complexity of resilient modulus calculation and use CBR instead which can be easily tested on site.
By definition, CBR is the ratio of the opposing bearing stress experienced by a standard penetrator when penetrated by a distance of 2.5 cm into the rock to the actual crushing strength of the rock in percentage.