The CBR test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. 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. Typically, a value of 2% equates to clay, while some sands may have a CBR value of 10%. High-quality sub-base will have a value of between 80-100% (maximum).
The CBR test is carried out on soils with a maximum particle size of 20mm. (Note: For material greater than 20mm please see Plate Bearing Tests). The technique involves driving a small cylindrical plunger (approx 50mm) into the ground at a uniform rate, using a four-wheel-drive vehicle as the reaction load to provide the force.
Tests are normally carried out at surface level or at depths of between 500-1000mm in 20-30m intervals along the proposed construction centreline. A minimum of three tests are usually carried out at each site.
On a typical site, and assuming surfaces are prepared, up to 8-10 tests can be carried out in a day, by a single operator, with provisional results available on site to learn more about bearing testing.