CBR Test Explained: California Bearing Ratio
A penetration test that rates a subgrade's strength against a crushed-stone standard
What the test measures
The California Bearing Ratio (CBR) test (commonly run per a standard such as ASTM D1883 for laboratory-compacted samples) measures the resistance of a compacted soil sample to penetration by a standard cylindrical plunger at a controlled rate, typically at penetrations of 0.1 inch and 0.2 inch.
Calculating the ratio
CBR (%) = (measured penetration load ÷ standard load for that penetration depth) × 100. The 'standard load' values are fixed reference figures representing a well-graded crushed-stone material, established as the benchmark the test compares against. The test is typically run at 0.1 inch and 0.2 inch penetration, and the higher of the two resulting ratios is usually reported (unless the 0.2-inch value is higher, in which case the test is often re-checked).
What the result is used for
CBR is a key input to flexible pavement thickness design methods — a low CBR (a weak subgrade) generally calls for a thicker pavement structure to spread traffic loads adequately, while a high CBR (a strong subgrade, closer to the crushed-stone reference) can support a thinner structure. Specific pavement thickness requirements come from the design method and governing specification being used, not from the CBR value alone.
Worked Example
At 0.1 inch penetration, a sample records a load requiring a ratio of 620 psi against a standard load of 1,000 psi.
CBR = 620 ÷ 1,000 × 100 = 62%
Assumptions & Limitations
- General ranges sometimes cited for subgrade quality (for example, low CBR for poor subgrades versus high CBR for excellent ones) vary by design method and agency — always check the specific pavement design method being used rather than a single rule of thumb.
Related Calculators
Related Guides
From the Blog
Sources
ASTM D1883
Standard Test Method for California Bearing Ratio (CBR) of Laboratory-Compacted Soils.