California Bearing Ratio (CBR) explained

The California Bearing Ratio, or CBR, is a simple index of subgrade strength used to judge how well ground will support construction traffic and pavements. In a lab or in situ test, a standard piston is pushed into a prepared soil at a constant rate and the force needed is compared with the force for a standard crushed stone. The result is expressed as a percentage. Higher CBR means a stronger subgrade.

CBR is widely used across UK highway and temporary works design to select foundation thicknesses and to plan haul roads and working platforms. UK design guidance stresses that a realistic assessment of subgrade CBR is critical to safe and economical solutions.

You will also see the term CBR used in geotextile data sheets as "CBR puncture resistance". That is a geotextile strength test defined in BS EN ISO 12236 and is not the same as soil CBR. Our catalogue and BS 8661 both reference this puncture test.

• Design thickness and stability - Subgrade CBR directly influences the thickness of sub-base or platform required to limit rutting and maintain access. Empirical and mechanistic methods alike depend on a sensible CBR value. 
• Temporary works planning - Haul roads and working platforms for plant depend on realistic CBR to avoid failures and costly remediation. 
• Value engineering with geogrids - Where subgrade CBR is low, reinforcement with geogrids can reduce aggregate thickness while maintaining performance. Our TerraGrid ranges provide this reinforcement by interlocking with aggregate, distributing load and reducing settlement.

Our Geoworks IQ design platform uses the recognised Giroud-Han methodology for unpaved roads and working platforms and highlights how sensitive outcomes are to the ground data you enter. Good inputs, including realistic subgrade strength, lead to better, safer designs.

Laboratory CBR - A compacted soil specimen is soaked where relevant, allowed to swell if specified, then penetrated by a 50 mm diameter piston at 1 mm per minute. The force at 2.5 mm and 5.0 mm penetration is recorded. The CBR is the higher of the two ratios compared with the standard crushed stone curve. 

In situ CBR - Field devices use the same principle directly on prepared ground, providing quick estimates for construction control and temporary works.

CBR is an index, not a soil classification. Two subgrades can have the same CBR but behave differently under traffic, particularly if moisture conditions change. As a rule of thumb used in UK practice, very low values indicate weak ground that typically requires improvement or significant thickness. Some highway authorities flag CBR below about 2% as requiring special treatment. Always follow the adopting authority’s current standard. 

When CBR indicates weak or variable subgrade, we typically design a reinforced build-up:

Reinforcement - A biaxial geogrid such as TerraGrid interlocks with aggregate to spread loads and limit deformation, enabling reduced thickness over low CBR soils.

Separation and filtration - A geotextile such as MultiTrack helps keep aggregate out of the subgrade and maintains drainage. Our product data provides CBR puncture resistance data for selection, but remember this is a geotextile strength metric, not soil CBR.

Geoworks IQ compares unreinforced and reinforced options side by side, showing potential savings in aggregate, CO₂e and vehicle movements once you input your site parameters, including subgrade condition.

1. Relying on a single-point test
   1. CBR varies across a site and with moisture. We recommend a test plan that reflects subgrade zones and seasonal water conditions, aligned to the responsible authority’s process. 
2. Using an unrealistic design CBR
   1. Temporary works guidance for working platforms and haul roads emphasises that sound design hinges on a realistic CBR. Overestimating can lead to rutting and failures.
3. Confusing soil CBR with CBR puncture resistance
   1. Soil CBR is a ground strength index. CBR puncture resistance is a geotextile property from BS EN ISO 12236. We select geotextiles by puncture data where relevant, but we never substitute that for soil CBR.