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Stroud su from SPT Calculator

Estimate undrained shear strength (cu/su) from SPT N value using Stroud (1974) correlation. Select f1 factor by plasticity or enter a custom value.

Last updated 16 June 2026

About this tool

Estimate undrained shear strength (cu) from SPT N value using the Stroud (1974) correlation: cu = f1 × N. The f1 factor depends on soil plasticity — select from preset values based on PI range, or enter a custom value.

This is one of the most widely used correlations in UK geotechnical practice for preliminary assessment of clay strength from SPT data. It’s particularly useful when no undisturbed samples were taken for triaxial testing.

Reference: Stroud, M.A. (1974). The Standard Penetration Test in insensitive clays and soft rocks. Proceedings of the European Symposium on Penetration Testing, Stockholm.

How to use this tool

1. Enter the SPT N value — field N, not corrected.

2. Select f1 factor — based on soil plasticity. Use PI or LL to guide selection, or enter a custom value.

Technical information

cu = f1 × N (kPa)

f1 guidance: PI > 30 → f1 ≈ 4.0 | PI 15–30 → f1 ≈ 5.0 | PI < 15 → f1 ≈ 6.0

Consistency classification: <25 kPa Soft | 25–50 Firm | 50–100 Stiff | 100–200 Very stiff | >200 Hard

Limitations

The correlation is empirical and applies to insensitive clays and soft rocks. It is not reliable for sensitive clays, organic soils, or stiff fissured clays where SPT results are highly variable.

SPT N values in clay are affected by borehole disturbance, water table position, and sampler type. Use with caution in boreholes with poor flush return or base heave.

For design, always verify with undisturbed triaxial tests or in-situ vane shear. The Stroud correlation is for preliminary assessment only.

Revision history

16 June 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.

SPT N value

Integer blow count (0-100)

f1 factor

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