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%). 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.

 

Figure 1. 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



Footnotes    (↵ returns to text)
  1. Standard Specifications for Transportation Materials and Methods of Sampling and Testing, Twentieth Edition.  American Association of State Highway and Transportation Officials.  Washington, D.C.