What is the California Bearing Ratio?

California Bearing Ratio (CBR) BS 1377-9 is a load bearing strength measurement test. It is used for determining the thickness of materials needed for road construction. It is used as part of the process in the design and construction of foundations, pavements, roads, car parks and runways. The ratio is measured using a standardised penetration test and was originally developed by the California State Highway Department in 1990.

PSL performs California Bearing Ratio Test with two types of frame:

  • Vehicle mounted
  • JCB attachment
In Situ Density Testing
Projects requiring a CBR Test

Roads & Pavements

If your project involves road or pavement construction, you will need a California Bearing Ratio Test. The test procedure determines the strength or the load bearing capacity of the highway sub-base and subgrade soil. The appropriate thickness of materials can then be determined. The test is designed to replicate the stresses placed on subgrade soils once they are subjected to vehicular traffic. It will also enable you to select the appropriate pavement thickness.


In civil engineering the stability, strength, and characteristics of the ground you are building on must be considered. This is for both for safety and for regulation compliance. Undertaking a California Bearing Ratio Test is the easiest way to determine if the soil is suitable for your planned project. It will determine if additional elements need to be factored into the design to increase stability of your planned construction.

Expert Technicians

The California Bearing Ratio Test will be carried out on-site by one of our expert technicians. They will arrive on site with a 4-wheel drive or JCB. This will provide the force and transport the specialist equipment required for the tests.

Once set up a cylindrical plunger will be inserted into the previously prepared ground at regular intervals. This will measure the pressure required to penetrate a standard area of soil. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The harder the surface, the higher the CBR value will be.

Typically, a value of 2% equates to clay material, whilst some sands could have a CBR value of 10%. High quality sub-base will typically have a value of between 80-100%. A minimum of three tests will be performed to enable accurate data analysis and results. On a typical site, and assuming surfaces are prepared in advance, up to 8-10 tests can be carried out per day. Provisional results will available on site following the testing procedures.

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