CBR Value Calculator

The California Bearing Ratio is a comparative index, not a fundamental soil property. A cylindrical plunger of 49.6 mm diameter is driven into a prepared soil specimen at a constant 1.27 mm per minute, and the load reached at a given penetration is divided by the load a reference crushed stone required at the same penetration. Those reference values are 13.24 kN at 2.54 mm and 19.96 kN at 5.08 mm, corresponding to 1,000 and 1,500 psi on the plunger face. The 2.54 mm value normally governs; when the 5.08 mm ratio comes out higher the standard calls for repeating the test, and only if the result repeats does the deeper value control. Specimens are compacted at optimum moisture, loaded with annular surcharge weights representing the pavement above, and usually soaked 96 hours to reproduce worst-case field moisture. Typical soaked values place heavy clay at 2 to 5 percent, silt at 5 to 10, clean sand at 10 to 40, sandy gravel at 20 to 60, and crushed stone base at 80 to 100. Agency specifications commonly demand at least 80 for base course and 30 for subbase, accepting whatever the natural subgrade offers. Two correlations carry CBR into pavement design: resilient modulus in psi approximated as 1,500 times CBR, which is reasonable only for weaker fine-grained soils below about CBR 10, and modulus of subgrade reaction near 5.5 times CBR plus 20 in pci. The main limitation is conceptual: CBR is a large-strain bearing failure measurement while pavements respond to small-strain repeated loading, so the correlation to stiffness is loose. Results are highly sensitive to compaction and moisture, and values much above 100 fall outside the range the test was designed to resolve.

The test was developed by the California Division of Highways in the late 1920s, with O. J. Porter generally credited as its principal author, as a rapid empirical way to compare subgrade soils and set flexible pavement thickness for the state road program. Porter published the method and the accompanying design curves in Highway Research Board proceedings during the late 1930s and early 1940s, and the California highway agency gave the ratio its lasting name. Its global spread was a product of the Second World War. The US Army Corps of Engineers needed to design and build heavy bomber airfields quickly, in unfamiliar terrain, with minimal laboratory support, and the CBR test offered exactly that trade of theoretical rigor for speed and simplicity. The Corps extended Porter's curves to aircraft wheel loads and carried the method wherever it built. After the war it entered British and Commonwealth practice through the Road Research Laboratory, whose Road Note 29 codified CBR-based pavement thickness design, and it was eventually standardized as ASTM D1883 and AASHTO T 193 in the United States and BS 1377 Part 4 in the United Kingdom. Since the 1990s the shift toward mechanistic-empirical design, culminating in the AASHTO Mechanistic-Empirical Pavement Design Guide, has demoted CBR from design parameter to screening index feeding resilient modulus correlations. It nonetheless remains the most widely performed subgrade strength test in the world, because the apparatus is inexpensive and the procedure can be taught in an afternoon.