# California Bearing Ratio

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%).

- Canonical URL: https://pavementinteractive.org/reference-desk/design/design-parameters/california-bearing-ratio/
- Last updated: 2018-09-07
- Review: reviewed by licensed professional engineers on the previous Pavement Interactive site, where reviewers were not recorded by name
- Standards: AASHTO T 193, ASTM D1883

The California Bearing Ratio ([CBR](https://pavementinteractive.org/glossary/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.

![CBR Sample](https://pavementinteractive.org/wp-content/uploads/2007/08/Cbr.jpg)

*Figure 1.* CBR Sample

## Equation

Values obtained are inserted into the following equation to obtain a CBR value:

$$\mathit{CBR}\ (\%) = 100 \left( \frac{x}{y} \right)$$

where:

- $$x$$ = material resistance or the unit load on the piston (pressure) for 2.54 mm (0.1") or 5.08 mm (0.2") of penetration
- $$y$$ = standard unit load (pressure) for well graded crushed stone = for 2.54 mm (0.1") penetration = 6.9 MPa (1000 psi); = for 5.08 mm (0.2") penetration = 10.3 MPa (1500 psi)

Calculator: CBR to resilient modulus — https://pavementinteractive.org/tools/cbr-mr/

## Typical Values

**Table 1.**

| 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

## Sources

[^1]: AASHTO (2000a). Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Twentieth Edition. American Association of State Highway and Transportation Officials. Washington, D.C.