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TRL DCP Calculator

Convert DCP blow counts to CBR using the TRL correlation. Enter penetration and blows to get CBR and subgrade classification for pavement design.

Last updated 29 September 2026

About this tool

Convert Dynamic Cone Penetrometer (DCP) blow counts to estimated California Bearing Ratio (CBR) using the TRL correlation. Enter total penetration and number of blows to get the penetration rate (DN), estimated CBR, and subgrade classification.

The TRL DCP is the standard field tool for rapid subgrade assessment in pavement investigation. The correlation is one of the most commonly used formulae in highway engineering — and one of the most commonly looked up.

Results include a subgrade classification from very poor (<2% CBR) to very good (>30% CBR) with typical pavement design actions for each category.

How to use this tool

1. Enter total penetration (mm) — cumulative depth advanced for the test interval.

2. Enter number of blows — blows of the standard DCP hammer for that interval.

3. Read the CBR and classification — use for preliminary subgrade assessment and capping/sub-base design.

Technical information

log₁₀(CBR) = 2.48 - 1.057 × log₁₀(DN)

Where DN = penetration rate (mm/blow). Per TRL Road Note 8 / Overseas Road Note 31. Valid for DN range approximately 2–300 mm/blow.

Limitations

The TRL correlation is empirical and developed from specific soil types. It gives a reasonable estimate for most UK soils but should be calibrated against laboratory CBR tests for design use.

DCP results can be affected by gravel layers, cobbles, and cemented horizons which give artificially high blow counts. Check results against borehole logs.

The test measures strength at the time of testing. Subgrade CBR for pavement design should account for equilibrium moisture conditions.

Revision history

29 September 2026: Initial release

Disclaimer

This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.

Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.

All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.

By using this tool, you accept full responsibility for how you interpret and apply the information provided.

Spotted an error? Let us know.

Total penetration (mm)

Number of blows

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