The California Bearing Ratio test, or CBR test, is really an in-situ testing solution that is used to calculate roughly the bearing amount and the physical strength of highway sub-bases and subgrades. In the test, a cylindrical plunger which has an area of 1935mm2 is forced into the soil at a uniform rate of 1 mm per minute. This requires the use of a reaction load, (usually a 4WD drive vehicle), to provide the force to the plunger and CBR press. The CBR is the ratio of the force required to cause a standard penetration divided by the force required to cause the same penetration in a standard material, expressed as a relative amount.
The California Bearing Ratio (CBR) is a gauge of the supporting value of the subgrade. It is not unique and other tests such as the R-Value test and the Triaxial are used occasionally. It is however by far the most commonly used in Pavement Design. The CBR test should be used with soil at the calculated equilibrium precipitation content.