The California Bearing Ratio, or CBR, is an empirical penetration test used to characterise subgrade and pavement materials. It compares the resistance of a prepared or in-situ material with a defined reference resistance.
CBR is widely used as an input to pavement design, but it is not a fundamental soil property. The result depends on how the specimen is prepared and conditioned, including moisture content, density, compactive effort, surcharge and soaking.
What does a CBR result mean?
The test reports resistance as a percentage:
$$ \mathrm{CBR} = \frac{R_{\mathrm{test}}}{R_{\mathrm{reference}}} \times 100 $$Here, R test is the measured resistance at the specified penetration and R reference is the corresponding reference resistance.
A higher result generally indicates greater penetration resistance under the test conditions. It does not create a universal good-or-bad rating. Suitability depends on the pavement design, material layer, traffic, drainage, authority requirements and project specification.
Values above 100 can occur for some high-strength materials because the result is a comparison with a reference material, not a percentage composition.
Laboratory CBR testing
For a laboratory test, material is prepared in a mould to specified conditions. The required conditions should come from the project specification or test request.
Key variables include:
- target dry density or compaction condition
- moulding moisture content
- compactive effort
- surcharge during conditioning and penetration
- soaking period, if any
- any required curing for treated material
- sample preparation and oversize particle treatment
A plunger is then driven into the specimen at the specified rate while force and penetration are measured. The force-penetration curve is processed and the CBR is calculated according to the applicable test method.
Australian laboratory CBR testing is commonly specified under the relevant method in the AS 1289.6.1 series. The standard should be consulted for the complete procedure, calculation and reporting rules.
Soaked and unsoaked CBR
A soaked CBR is measured after conditioning the specimen in water for the specified period. It is often used where the design needs to represent a wet or weakened condition.
An unsoaked CBR is measured without that soaking stage. It can be relevant where the nominated design or specification requires a different moisture condition.
Soaked results are not always lower, and the required conditioning should not be assumed. The test request needs to define conditions that represent the design case and comply with the applicable road authority or project specification.
Swelling and changes in moisture condition during soaking can provide additional useful information.
Field CBR and DCP are not the same test
A field CBR test measures penetration resistance directly using a field CBR procedure. It may be useful where the in-situ condition needs to be assessed and the test setup is suitable.
A Dynamic Cone Penetrometer, or DCP, records penetration per blow with depth. DCP data can be used to estimate CBR through an empirical correlation, but a DCP test is not a CBR test.
The relationship between DCP penetration and CBR varies with material type, moisture, density, particle size and the correlation used. A single generic equation should not be applied without confirming that it is valid for the material and project. Direct laboratory or field CBR testing may be required where CBR is a specified acceptance or design parameter.
Read the Dynamic Cone Penetrometer guide for the purpose and limitations of DCP testing.
Factors that influence the result
| Factor | Why it matters |
|---|---|
| Moisture condition | Can significantly change penetration resistance, especially in moisture-sensitive soils |
| Density and compaction | Changes the soil structure and resistance |
| Soaking and drainage | Can weaken some materials and reveal swelling |
| Grading and large particles | Affect specimen preparation, repeatability and representativeness |
| Surcharge | Represents confinement under the nominated test condition |
| Sample disturbance | Can make the specimen less representative of field material |
| Cementation or treatment | Curing time and preparation can strongly influence results |
Because these variables matter, two CBR values are only directly comparable when the test conditions are also understood.
Using CBR in pavement design
CBR can support:
- subgrade characterisation
- empirical pavement thickness design
- assessment of selected fill or pavement material
- evaluation of potential treatment or stabilisation
- comparison of material under nominated moisture and density conditions
The design CBR should be selected using the applicable pavement design method, road authority requirements, investigation coverage and engineering judgement. It should not be chosen from a universal percentile rule or a generic soil-type table.
A low result does not automatically prescribe one solution. Options may include additional investigation, drainage improvement, undercut and replacement, capping, stabilisation or a pavement design that accommodates the material. The preferred response depends on constructability, variability, environment and whole-of-life performance.
Frequently asked questions
How many CBR tests are required?
There is no universal test frequency. The number and location of tests should reflect the project specification, geological variability, material sources, risk and the purpose of the results. A sampling and testing plan should define representative coverage.
Can CBR be estimated from soil classification?
Classification may indicate broad behaviour, but it cannot reliably determine a design CBR. Testing under specified conditions is needed where CBR affects design or acceptance.
Which result is used for design?
That decision depends on the applicable design method and specification, including how variability and moisture conditions are addressed. The geotechnical or pavement designer should document the adopted value and its basis.
Is DCP testing a faster replacement for laboratory CBR?
DCP provides rapid depth profiling and can be valuable for identifying variability. It only estimates CBR when a suitable correlation is used. It does not automatically replace specified laboratory or direct field CBR testing.
Related resources
- Pavement Design Using Austroads
- Dynamic Cone Penetrometer Testing
- Laboratory CBR Calculation
- Compaction Testing
- Soil Classification
- Understanding Chainage