CBR Calculator: California Bearing Ratio and Subgrade Rating

The California Bearing Ratio is the odd survivor of geotechnical testing: a purely empirical index invented by the California Division of Highways in the 1930s that has outlived generations of more theoretically respectable competitors, simply because an enormous body of pavement design experience is calibrated to it. The idea is almost naive in its directness: push a standard 49.6 mm piston into the soil at a fixed rate, measure the load at standard penetrations, and express it as a percentage of the load that well-graded crushed stone (the best road base material California could find) requires for the same penetration.

The arithmetic

The piston load converts to stress over the 1,935 mm2 piston area, and the ratio is taken against the crushed-stone standards: 6.9 MPa at 2.54 mm penetration and 10.3 MPa at 5.08 mm. By convention the 2.54 mm value is the CBR, unless the 5.08 mm value comes out higher, in which case the standard calls for a retest and, if it repeats, the 5.08 mm value governs. This calculator computes both, applies that rule, and rates the result on the customary subgrade scale.

Worked example

• A compacted subgrade sample carries 4.0 kN at 2.54 mm and 5.8 kN at 5.08 mm penetration
• Stresses: 2.07 and 3.00 MPa; CBR values: 30.0% and 29.1%
• The 2.54 mm value governs: design CBR = 30, a good subgrade in granular subbase territory
• On a typical design chart, that CBR supports a far thinner pavement section than a CBR 5 clay would

What the number is worth, and what it is not

The CBR earns its keep as the input to empirical pavement thickness design and as a quick, cheap comparator between borrow sources and treatment options; lime or cement stabilization proposals are routinely justified by before-and-after CBR. Its honest limitations are equally well documented. It is an index, not a fundamental property: two soils with equal CBR can have different resilient moduli, which is why mechanistic pavement design has been migrating away from CBR-based correlations. The soaked-versus-unsoaked distinction is the biggest practical trap, since a clay that tests at CBR 15 dry can drop below 3 after the standard four-day soak, and the soaked value is what a wet winter will actually deliver. And the widely used correlation to stiffness, resilient modulus of roughly 10 x CBR in MPa, is a scatter-heavy rule of thumb that deserves the words “first estimate” attached every time it is written down.

Related Calculators and Reference

Subgrade preparation quality that CBR depends on is set by compaction, covered in the Proctor compaction calculator, the soil classification that predicts CBR behavior comes from the Atterberg limits calculator, and deeper bearing questions belong to the bearing capacity calculator. The test procedure is standardized as ASTM D1883; the California bearing ratio overview covers its history and use.