California Bearing Ratio (CBR):
The most widely used test for determining the supporting power of a soil for designing the thickness of pavements is the California Bearing Ratio (CBR) test. The test is a penetration test in which a soil sample compacted to the specified density and soaked under water for 4 days in a mould is subjected to penetration by a plunger at a standard rate. The CBR value is expressed as a percentage of the actual load causing penetrations of 2.5 mm or 5 mm to the load required for a standard crushed stone sample.
Some typical values of the CBR of different types of soil are given below:
Table: CBR Values of Typical Soils
The use of CBR values in pavement design is discussed further.