Estimate the effective friction angle (φ′) of cohesionless soils using corrected Standard Penetration Test (SPT) N₆₀ values.
This calculator applies well-known empirical correlations to relate N₆₀ to φ′ for sands, silty sands, and gravels - allowing rapid estimation of shear strength for geotechnical analysis and design.
The tool is based on published research including Kulhawy & Mayne (1990) for general sands, and Hatanaka & Uchida (1996) for clean sands (Japan).
It is intended for preliminary design and comparison where laboratory testing is unavailable.
Enter corrected N₆₀ - input the SPT N₆₀ value from field testing or your energy correction calculator.Select soil type - choose the predominant material (e.g. clean sand, silty sand).Review the result - the tool outputs the estimated friction angle (φ′) in degrees, along with the applied correlation.What is φ′? φ′ (phi prime ) represents the effective angle of shearing resistance - a measure of how soil resists sliding or failure along internal surfaces.
It depends on soil type, particle arrangement, density, and effective stress.
For sands and gravels, φ′ typically ranges between 28° and 45°, increasing with relative density and decreasing with fines content.
Estimating φ′ from SPT data allows quick assessment of soil strength for:
Foundation bearing capacity Earth pressure calculations Slope stability analysis Retaining wall design Limitations Correlations are empirical and based on typical soil behaviour; site-specific conditions may differ. Valid mainly for cohesionless soils (sands, silts, gravels). Should not be used for clays or cemented soils. Energy and overburden corrections must be applied before using N₆₀. Laboratory testing (e.g. triaxial or direct shear) should confirm design values for critical structures. This calculator estimates the effective friction angle (φ′) from corrected SPT N₆₀ values using established correlations from published literature.
Correlation by soil type
Clean sand - Hatanaka & Uchida (1996)
φ′ = 27.1 + 0.3 N₆₀ − 0.00054 N₆₀²
Silty sand - Peck, Hanson & Thornburn (1974)
φ′ = 27 + 0.28 N₆₀
Gravelly sand - Japanese Road Association (2006)
φ′ = 28 + 0.35 N₆₀
Non-plastic silt - Sowers & Hedges (1966)
φ′ = 25 + 0.25 N₆₀
Mixed or unknown sands - Kulhawy & Mayne (1990)
φ′ = 20 + 0.5 N₆₀ (limited to 25°–45°)
Assumptions
N₆₀ is corrected for energy ratio = 60 %. Applicable to cohesionless soils under drained conditions. φ′ values are given in degrees. Correlations are empirical and should be verified by laboratory shear testing for critical projects. Revision history 11 February 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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