California Bearing Ratio

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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, 2000¹). 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:

\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

1993 AASHTO Guide · MEPDG

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

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APA

Pavement Interactive. (2018, September 7). California bearing ratio. HeadLight. Retrieved [the date you read it], from https://pavementinteractive.org/reference-desk/design/design-parameters/california-bearing-ratio/

ASCE

Pavement Interactive. (2018). “California Bearing Ratio.” HeadLight, <https://pavementinteractive.org/reference-desk/design/design-parameters/california-bearing-ratio/> ([the date you read it]).

BibTeX

@misc{pavint-california-bearing-ratio,
  author = {{Pavement Interactive}},
  title = {California Bearing Ratio},
  year = {2018},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/design/design-parameters/california-bearing-ratio/}},
  note = {Accessed [date]}
}

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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. ↩

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