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[1]). 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
- Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Twentieth Edition. American Association of State Highway and Transportation Officials. Washington, D.C.↵