Estimate soil strength parameters from SPT
This free tool estimates the soil strength parameters most often needed for onshore foundation and earth-structure design — cohesion (c), internal friction angle (φ), unit weight (γ) and elastic modulus (E) — directly from a Standard Penetration Test (SPT) N-value profile. Paste a depth / N-value / soil-type profile and the calculator returns depth-wise parameters, a layered design summary, and a depth profile chart.
The strength parameter is back-calculated with an energy-based inversion consistent with the driving resistance recorded by the N-value, rather than a single empirical correlation.
The four parameters
- Cohesion — c (kPa)
- The shear-strength component independent of confining stress; governs the undrained behaviour of clays.
- Internal friction angle — φ (°)
- The stress-dependent shear-strength component; the primary strength parameter for sands.
- Unit weight — γ (kN/m³)
- The soil's weight per unit volume, used for overburden and self-weight calculations.
- Elastic modulus — E (kPa)
- A measure of soil stiffness, used to estimate settlement and deformation.
How the parameters are derived
The tool applies an energy-based inversion: the work delivered by the SPT hammer is balanced against the penetration resistance mobilised by the sampler, so the strength parameter consistent with the measured N-value can be recovered. Sands are treated as fully drained, giving the internal friction angle (φ); clays are treated as undrained, giving cohesion (c). Unit weight and elastic modulus are estimated from the N-value.
Because an N-value on its own cannot identify soil type — a given N can correspond to either dense sand or stiff clay — you supply clay (1) or sand (0) for each point, typically read from the borehole log. This keeps soil classification an explicit input rather than an assumption hidden inside the calculation.
How to use the calculator
Enter one row per measurement as comma-separated values; a header row is skipped automatically. Columns are depth (m), N-value, and type (1 = clay, 0 = sand). Set the number of design layers, then click Analyze to see the layer summary and depth profile. You can also load a CSV file exported from a spreadsheet.
Methods, assumptions and limitations
The parameters are engineering estimates intended to support — not replace — the judgment of a qualified geotechnical engineer, who remains responsible for the values adopted in design. Soil type must be supplied by the user; the accuracy of the clay/sand assignment directly affects the result. The drained/undrained idealisation (sand fully drained, clay undrained) is a simplification appropriate for preliminary estimates.
Frequently asked questions
Can SPT determine soil type automatically?
No. An SPT N-value alone cannot distinguish dense sand from stiff clay, so you supply the soil type (clay or sand) for each layer, usually read from the borehole log.
What input does the SPT calculator use?
One row per measurement: depth (m), N-value, type (1 = clay, 0 = sand). Outputs are cohesion c (kPa), friction angle φ (°), unit weight γ (kN/m³) and elastic modulus E (kPa).
How are the strength parameters obtained?
They are back-calculated from an energy-based inversion consistent with the SPT penetration resistance: friction angle for sands (fully drained) and cohesion for clays (undrained).
Can I use the results directly for design?
The values are reference estimates that support engineering judgment. A qualified geotechnical engineer should verify them against site-specific data before use in design.
Method & status
The SPT strength parameters are produced by an energy-based inversion developed in-house; a formal publication of the method is in preparation. For CPT-based estimation, which follows a published analytical approach, see the CPT calculator.