California bearing ratio

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The California bearing ratio (CBR) is a penetration test for evaluation of the mechanical strength of road subgrades. It was developed by the California Department of Transportation.

The test is performed by measuring the pressure required to penetrate a soil sample with a plunger of standard area. The measured pressure is then divided by the pressure required to achieve an equal penetration on a standard crushed rock material. The test can be performed on laboratory-prepared samples or in the field. The CBR test is described in ASTM Standards D1883 and D4429, and AASHTO T193.

The CBR rating was developed for measuring the load-bearing capacity of soils used for building roads. The CBR can also be used for measuring the load-bearing capacity of unimproved airstrips or for soils under paved airstrips. The harder the surface, the higher the CBR rating. A CBR of 3 equates to tilled farmland, a CBR of 4.75 equates to turf or moist clay, while moist sand may have a CBR of 10. High quality crushed rock has a CBR of 100.

[edit] Origin of the CBR

The CBR test was developed in World War 2 by american military engineers. As US forces captured pacific islands held by the Japenese new airfields were constructed hurriedly to bring american airpower closer to the main Japenese islands. The CBR test gave engineers instantaneous results allowing the amount of aggregate used in runway construction to be calculated to the minimum requirements, thus ensuring rapid construction. The rapid advancement of US airpower was one of the factors that lead to the surrender of the Japanese.