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CPT Undrained Shear Strength Calculator

Estimate undrained shear strength (Su) from cone penetration test data using the Nkt cone factor method. BS 5930 consistency classification included.

Last updated 10 November 2026

About this tool

Estimate undrained shear strength (Su) from corrected CPT tip resistance. Su is a key parameter for cohesive soil design, used in bearing capacity, slope stability, and retaining wall analysis.

Uses the Nkt method from Lunne, Robertson & Powell (1997). Includes BS 5930 consistency classification (very soft to hard).

How to use this tool

Enter CPT data — cone resistance (qc), pore pressure (u2), and net area ratio.
Enter depth and GWL — required to compute total and effective vertical stress.
Set Nkt — default 14, typical range 10–18. Calibrate against vane shear or triaxial data where available.
Review Su result — with BS 5930 consistency classification.

Technical information

Su = (qt - σv0) / Nkt

where qt is corrected tip resistance, σv0 is total vertical stress, and Nkt is the empirical cone factor. Typical Nkt values: 10–15 for soft normally consolidated clays, 15–20 for stiff overconsolidated clays.

Limitations

Applicable to cohesive soils only — not valid for sands or gravels.
Nkt should be calibrated against vane shear or triaxial testing for the specific site geology.
Pore pressure correction (u2 and net area ratio) is significant in soft clays — ensure correct values are used.

Revision history

10 November 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.

Cone resistance, qc (MPa)

Pore pressure, u2 (MPa)

Depth (m)

Groundwater level (m)

Net area ratio (a)

Typical: 0.70–0.85 for modern cones

Cone factor, Nkt

Typical: 10–18, default 14

Unit weight above GWL (kN/m³)

Saturated unit weight (kN/m³)

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