A controlled workflow for laboratory CBR calculation from test force and method-defined reference force.
Laboratory CBR Calculation
The California Bearing Ratio (CBR) compares the measured penetration resistance of a prepared material with the reference resistance defined by the selected test method. The result is empirical and depends on specimen preparation, density, moisture, surcharge, soaking and test procedure.
Note: This is draft calculation guidance. The current licensed laboratory method should be used for reference forces, curve correction, penetration selection and reporting rules.
General calculation
At a nominated penetration:
$$ CBR_p=\frac{R_{test,p}}{R_{reference,p}}\times100 $$where \(R_{test,p}\) is the corrected test resistance and \(R_{reference,p}\) is the method-defined reference resistance at penetration \(p\).
Proposed input record
- Test method and edition
- Sample, mould and material identifiers
- Preparation moisture and achieved density
- Compactive effort or preparation procedure
- Surcharge masses
- Soaked or unsoaked condition and conditioning record
- Penetration and force observations
- Swell observations where required
- Equipment and calibration identifiers
Load-penetration processing
The raw readings should be retained, with only the approved processing sequence applied on top. This may include seating treatment, origin correction, interpolation at nominated penetrations and calculation of more than one CBR value.
The correction must never overwrite the raw curve. Both raw and corrected points should be available in the audit record.
Illustrative arithmetic
Assume the approved method supplies a reference resistance of \(13.2\ \text{kN}\) at a nominated penetration and the corrected test resistance is \(4.0\ \text{kN}\):
$$ CBR=\frac{4.0}{13.2}\times100=30.3\% $$This example demonstrates the ratio only. The actual reference value, penetration and governing-result rule must come from the nominated method.
Validation controls
- Penetration values must increase monotonically.
- Corrected resistance should not become negative.
- Interpolation is allowed only between valid bracketing readings.
- Required target penetrations must be covered by the test record.
- Soaked and unsoaked results must not be combined.
- Density and moisture conditions must accompany every CBR result.
- A design CBR should never be selected automatically from a single laboratory result.