California Bearing Ratio
From Pavement Interactive
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The California Bearing Ratio (CBR) test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high quality crushed stone material should have a CBR @ 100%). It is primarily intended for, but not limited to, evaluating the strength of cohesive materials having maximum particle sizes less than 19 mm (0.75 in.) (AASHTO, 2000). It was developed by the California Division of Highways around 1930 and was subsequently adopted by numerous states, counties, U.S. federal agencies and internationally. As a result, most agency and commercial geotechnical laboratories in the U.S. are equipped to perform CBR tests.
The basic CBR test involves applying load to a small penetration piston at a rate of 1.3 mm (0.05") per minute and recording the total load at penetrations ranging from 0.64 mm (0.025 in.) up to 7.62 mm (0.300 in.). Figure 1 is a sketch of a typical CBR sample.
Equation
Values obtained are inserted into the following equation to obtain a CBR value:
Typical Values
| General Soil Type | USC Soil Type | CBR Range |
| Coarse-grained soils | GW | 40 - 80 |
| GP | 30 - 60 | |
| GM | 20 - 60 | |
| GC | 20 - 40 | |
| SW | 20 - 40 | |
| SP | 10 - 40 | |
| SM | 10 - 40 | |
| SC | 5 - 20 | |
| Fine-grained soils | ML | 15 or less |
| CL LL < 50% | 15 or less | |
| OL | 5 or less | |
| MH | 10 or less | |
| CH LL > 50% | 15 or less | |
| OH | 5 or less |
Standard Test Methods
- AASHTO T 193: The California Bearing Ratio
- ASTM D 1883: Bearing Ratio of Laboratory Compacted Soils