California Bearing Ratio (CBR) Test

The California Bearing Ratio (CBR) test is a penetration test developed by the California State Highway Department and is used to determine the mechanical strength of a soil. The test can be carried out on site or in the laboratory.

The VBR test is carried out using a cylindrical plunger which penetrates the soil at a given rate. The relationship between the force and penetration can then be used to calculate the CBR value of the soil being tested.

The test is carried out off the back of a 4WD vehicle or and excavator either at the ground surface or in pits up to around 1.0 m deep. The test area is levelled and any unrepresentative material is removed. Sufficient surcharge discs are then placed around the test area and plunger to correspond with the over burden pressure for the test. The test is carried out by forcing the plunger to penetrate the soil over a given time. The results of the penetration are used to provide a CBR value for the soild horizon.

However, the test is only appropriate to use on material with a particle size that does not exceed 20 mm.

An alternative technique to gain CBR values is to carry out a hand held Dynamic Cone Penetrometer (DCP) test.

Example CBR Log