The California Bearing Ratio (CBR) test is a penetration test used to evaluate the bearing strength of subgrade soils and base materials for road and pavement design. In Australia, CBR testing is conducted in accordance with AS 1289.6.1.1 and is a fundamental input to pavement thickness design using the Austroads pavement design method.
What Is CBR Testing?
CBR testing measures the resistance of a soil or pavement material to penetration by a standard plunger under controlled conditions. The result is expressed as a CBR value — a percentage comparing the material's resistance to that of a standard crushed rock.
CBR Value Scale
| CBR Value | Material Quality | Typical Use |
|---|---|---|
| > 80% | Excellent | High-quality base course |
| 40–80% | Good | Sub-base material |
| 15–40% | Fair | Working platforms, low-grade sub-base |
| 5–15% | Poor | Subgrade, requires thicker pavement |
| 2–5% | Very poor | Weak subgrade, needs stabilisation or capping layer |
| < 2% | Extremely poor | Unsuitable as subgrade without treatment |
Laboratory CBR Test (AS 1289.6.1.1)
Specimen Preparation
- Soil is compacted into a CBR mould at a specified moisture content and density
- Three specimens are typically prepared at different moisture contents or compaction levels
- A surcharge weight is placed on top to simulate overburden pressure
- Specimens may be soaked for 4 days to simulate worst-case moisture conditions (soaked CBR) or tested unsoaked
Testing Procedure
- The mould is placed in the CBR testing machine
- A standard plunger (49.6 mm diameter) is driven into the soil at a rate of 1 mm/min
- Penetration force is recorded at specified intervals (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5, 10.0, 12.5 mm)
- Force readings are plotted against penetration
Calculation
$$ \text{CBR} = \frac{\text{Test Load at 2.5 mm or 5.0 mm}}{\text{Standard Load}} \times 100 $$| Penetration | Standard Load (MPa) |
|---|---|
| 2.5 mm | 6.9 |
| 5.0 mm | 10.3 |
The higher CBR value at 2.5 mm or 5.0 mm penetration is reported.
Field CBR Testing
Field CBR testing can be performed using a DCP (Dynamic Cone Penetrometer) or in-situ CBR test, allowing correlation with laboratory CBR values without the need for sample transport and lab testing.
DCP-CBR Correlation
The DCP penetration rate (mm per blow) is empirically correlated to CBR:
$$ CBR = \frac{292}{(DPI)^{1.12}} $$Where DPI = Dynamic Penetration Index (mm per blow).
Factors Affecting CBR Values
| Factor | Effect on CBR |
|---|---|
| Moisture content | Higher moisture → lower CBR (especially for clays) |
| Compaction density | Higher density → higher CBR |
| Soil type | Granular soils → higher CBR; clays → lower CBR |
| Soaking | Soaked CBR is lower than unsoaked (worst-case design) |
| Surcharge | Higher surcharge → slightly higher CBR |
CBR in Pavement Design
CBR is the primary input for Austroads pavement design — the standard pavement design method used across Australia for roads, highways, and airport pavements.
Design CBR Selection
The design CBR is typically selected as the 80th percentile or 85th percentile of all test results, ensuring a conservative design for the majority of the pavement length.
Typical Design CBR Values
| Subgrade Type | Design CBR |
|---|---|
| High-quality granular subgrade | 15–20% |
| Sandy subgrade | 8–15% |
| Silty subgrade | 5–8% |
| Clay subgrade | 2–5% |
| Very soft clay (stabilised) | < 2% |
Australian Standards
| Standard | Title |
|---|---|
| AS 1289.6.1.1 | Determination of the California Bearing Ratio of a soil — Standard laboratory method |
| AS 1289.6.1.2 | Determination of the California Bearing Ratio of a soil — Standard field method |
| Austroads Guide to Pavement Technology | Pavement design methodology incorporating CBR |
| AGPT04A | Pavement structural design |
Frequently Asked Questions
How many CBR tests are needed?
For a typical road project, 1 CBR test per 500–1000 m² or per distinct soil type encountered is common. Larger projects require more tests.
What is the difference between soaked and unsoaked CBR?
Soaked CBR simulates worst-case moisture conditions (water table rise, rainfall infiltration). Soaked values are used for design in areas with high rainfall or high water tables. Unsoaked values reflect in-situ conditions at the time of testing.
Can CBR be estimated visually?
No. CBR must be determined by laboratory or field testing. Visual estimates are unreliable and should never substitute actual testing.
What CBR value is needed for pavement subgrade?
Typically a Design CBR of at least 3% for light traffic roads, and 5% or higher for heavy traffic / highway applications. Lower CBR values require a capping layer or ground improvement.