Estimate the undrained shear strength (Su) of cohesive soils from corrected Standard Penetration Test (SPT) N60 values using the Stroud (1974) correlation.
The tool provides two approaches: the full Stroud method using plasticity index (Ip) to determine the f1 factor, or a simplified rule of thumb (f1 = 4.4) for quick estimation when Atterberg limits are unavailable.
Results include BS 5930 consistency classification (Very Soft to Hard) for immediate practical interpretation.
Enter corrected N60 - input the SPT N60 value from field testing or your energy correction calculator.Select approach - choose Stroud (1974) with plasticity index for the most accurate result, or the rule of thumb (f1 = 4.4) for a quick estimate.Enter Plasticity Index - if using the Stroud method, enter the soil's Ip (%) from Atterberg limit testing.Review the result - the tool outputs Su in kPa, the f1 factor used, and the BS 5930 consistency classification.What is undrained shear strength? Undrained shear strength (Su or cu) is the maximum shear stress a cohesive soil can sustain under rapid loading without drainage. It is the primary strength parameter for clays and silts under short-term (undrained) conditions.
Su is used extensively in geotechnical design for:
Short-term bearing capacity of foundations Slope stability during and after construction Retaining wall and excavation support design Pile capacity in cohesive soils Stroud (1974) established the relationship:
Su = f1 x N60 (kPa)
Where f1 varies with the plasticity index (Ip) of the clay:
Ip = 10%: f1 = 6.5 Ip = 15%: f1 = 5.5 Ip = 20%: f1 = 5.0 Ip = 25%: f1 = 4.5 Ip = 30%: f1 = 4.2 Ip = 40%: f1 = 4.0 Ip = 50%: f1 = 3.8 Ip >= 60%: f1 = 3.5 Intermediate values are linearly interpolated. The rule of thumb (f1 = 4.4) is a commonly used approximation when Ip is unknown.
BS 5930 consistency classification is applied to the calculated Su:
< 20 kPa: Very Soft 20-40 kPa: Soft 40-75 kPa: Firm 75-150 kPa: Stiff 150-300 kPa: Very Stiff > 300 kPa: Hard Reference: Stroud, M.A. (1974). The Standard Penetration Test in insensitive clays and soft rocks. Proceedings of the European Symposium on Penetration Testing , Stockholm, Vol. 2.2, pp. 367-375.
Limitations Applies to cohesive soils only - not valid for sands, gravels, or cemented materials. The f1 factor is sensitive to plasticity index - using the rule of thumb when Ip is available may reduce accuracy. SPT blow counts in very soft clays (N < 5) are unreliable - vane shear or other in-situ tests are preferred. Energy and overburden corrections must be applied before using N60. Laboratory testing (e.g. triaxial UU or hand vane) should confirm design values for critical structures. Revision history 23 March 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.
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