What is the California Bearing Ratio (CBR)?

What the test measures

The California Bearing Ratio (CBR) is a penetration test that compares the strength of a soil with the strength of standard crushed stone. The result is a percentage. The test pushes a plunger into the material at a uniform rate. The FAA gives an example: a CBR of 15 means 15 percent of the resistance of standard crushed limestone.

How the laboratory test works

The FAA requires laboratory CBR tests to ASTM D1883. The sample is material from the site, remolded to the moisture and density that construction will give. The sample is soaked before the test.

In ASTM D1883, the test drives a piston about 49.6 mm in diameter into the sample at 1.27 mm per minute. A surcharge of at least 4.54 kg rests on the sample, and the soak lasts 96 hours. The CBR is the ratio of the measured load to the standard load at 2.54 mm and at 5.08 mm of penetration. If the 5.08 mm value is higher, the laboratory repeats the test.

CBR tests are hard to interpret on gravelly materials. The mold confines the gravel, and the result is too high. For gravel and gravelly soils, the FAA limits the subgrade modulus E to 50,000 psi, which is a CBR of 33. Generally, three CBR tests on each major soil type are enough.

How CBR is used in airport pavement design

For flexible pavements, CBR is the usual way to describe subgrade strength. The FAA converts it to an elastic modulus with E = 1,500 x CBR (E in psi), which FAARFIELD uses for thickness design.

Some limits use CBR directly:

• Where the mean subgrade CBR is below 5, the FAA recommends that you improve the subgrade. Below 3, the FAA says to improve it.
• A flexible pavement needs a subbase on subgrades with a CBR below 20. The standard subbase (Item P-154) must have a CBR of at least 20.
• EASA lists four subgrade strength categories for flexible pavements in the ACN-PCN method. They use CBR 15 (A), 10 (B), 6 (C) and 3 (D).
• EASA guidance says that the soil 15 cm below a runway strip surface should have a CBR of 15 to 20. This prevents damage to a nose gear.

Origin

O. J. Porter of the California Highway Department developed the test. During World War II the Corps of Engineers adopted it for flexible airfield pavements. A 1942 Highway Research Board paper describes that adoption.

Field estimate

A dynamic cone penetrometer (DCP) gives a CBR estimate in the field. The FAA gives CBR = 292 / DCP^1.12 for most soils, with DCP in mm per blow.