Ashton House

67 Compton Road



California Bearing Ratio Testing

As an aid to flexible pavement design in road and car park construction, the California Bearing Ratio (CBR) test is carried out to determine the strength of the subgrade on which the sub-base is to be constructed.


In simple terms, the aim of the test is to push a cylindrical plunger into the soil at a given rate and compare the relationship between the force required and penetration into the soil to that of a standard material. At certain values of penetration the CBR is defined in terms of a percentage, as the ratio of the force exerted on the soil to a standard force that would be exerted on a specified crushed rock compacted and confined in a given manner.


GIS carry out the test in-situ in accordance with clause 4.3 of BS 1377: Part 9: 1990, using a frame attached to the rear of a 4 wheel drive vehicle ballasted to carry a stable dead weight capable of providing the reaction load required for the test.


On account of the plunger size being c.50mm in diameter, the test is most suited to fine soils having a maximum particle size not exceeding 20mm. Where coarser material needs to be tested, a rigid plate (normally of 150mm diameter) is used to penetrate the soil (clause 4.1 of BS 1377: Part 9: 1990). In such cases, because the force to achieve the penetration is much greater, then the reaction load comes from kentledge provided by the frame and deadweight of a mechanical excavator.


Whatever the material to be tested, GIS will ensure the appropriate CBR test is carried out.




For more information about CBR testing. Call us today at:

01902 717 653