The California Bearing Ratio, was developed by The California State Highways Department. It is in essence a simple penetration test developed to evaluate the strength of road sub grades.

The Basic CBR Test

This consists of causing a plunger of standard area to penetrate a soil sample, (this can be in the laboratory or on site). The force (load) required to cause the penetration is plotted against measured penetration, the readings noted at regular time intervals. This information is plotted on a standard graph, and the plot of the test data will establish the CBR result of the soil tested.
The test is fully covered in :-
B.S.1377:Soils for civil engineering purposes: Part 4,Compaction related tests.

The Reason For The CBR Test

It sounds complicated, but the basis behind it is quite simple. We are determining the resistance of the sub grade, (i.e. the layer of naturally occurring material upon which the road is built), to deformation under the load from vehicle wheels.  Even more simply put,” How strong is the ground upon which we are going to build the road''.

The CBR test is a way of putting a figure on this inherent strength, the test is done in a standard manner so we are able to compare the strengths of different sub grade materials, and we are able to use these figures as a means of designing the road pavement depth required for a particular strength of sub grade.

The stronger the sub grade (the higher the CBR reading ) the less thick it is necessary to design and construct the road pavement, this gives a considerable cost saving. Conversely if CBR testing indicates the sub grade is weak (a low CBR reading) we must construct a suitable thicker road pavement to spread the wheel load over a greater area of the weak sub grade in order that  the weak sub grade material is not deformed, causing the road pavement to fail.

The CBR in spite of its limited accuracy still remains the most generally accepted method of determining sub grade strength, and as such this information, along with information on traffic flows and traffic growth is used to design road pavements.

CBR values in relation to site conditions at the time of construction

CBR values "on site" may not bear any relationship to the CBR values employed in the road design, due to softening from wet weather and trafficking from site vehicles.

This is of course true for any design method you employ if the soil conditions at the time of construction are different to the soil conditions upon which you based your design. It could be some time before the properties of the soil revert back to their original engineering condition, and by this time failure could have occurred.

"Capping layers" have been introduced to help solve the problem of sub-grades wetting up and losing strength during construction by protecting the sub grade from the worst of the damage caused by site traffic.

The opposite is also true, if CBR values are taken on site after the sub-grade has been exposed and dry weather has caused the moisture content of the soil to decrease, increasing soil stiffness, the CBR value will be higher than natural moisture content, this is an incorrect value for design purposes and if accepted will cause a serious under design of the road pavement.

Natural soil moisture content, after drainage, is the correct moisture content for determining CBR values for highway design purposes because in the course of time natural soil moisture conditions will be re-established.

Good drainage is an essential part of road construction to allow the optimum strength/CBR to be obtained from the soil foundation, whether it be insitu soil or imported fill.

It of course follows that the drainage must be kept operating efficiently during the life of the road to prevent the strength/CBR decreasing through weakening of the foundation by a rising water table. 

CBR on site testing equipment

Benkelman beam

Dynamic cone Penetrometer

Clegg Hammer