Estimate soil strength parameters from CPT
This free tool estimates the soil strength parameters most often needed for offshore and onshore design — cohesion (c), internal friction angle (φ), unit weight (γ) and elastic modulus (E) — from a Cone Penetration Test (CPT/CPTu) profile. Paste a depth / qc / fs / u2 profile and the calculator returns depth-wise parameters, a layered design summary, a depth profile chart, and a soil behaviour type classification for each layer.
Because the CPT records several quantities at once, both strength parameters can be resolved together and the soil can be classified without a separate borehole log.
The four parameters
- Cohesion — c (kPa)
- The shear-strength component independent of confining stress; important in clays and mixed soils.
- 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
Cohesion and friction angle
Cohesion and friction angle are solved simultaneously from cone resistance (qc), sleeve friction (fs) and pore pressure (u2) using the analytical approach of Motaghedi and Eslami (2013). Two coupled equations are formed — one for bearing-capacity failure at the cone tip and one for direct shear along the sleeve — and solved for c and φ together. Using all three CPT outputs, rather than cone resistance alone, reduces the influence of erroneous readings and yields both parameters even in mixed soils.
Soil behaviour type
Each layer is classified using Robertson's (2010) non-normalized soil behaviour type index, computed from qc and the friction ratio. This gives a soil description — from clay through silt mixtures to sand — alongside the strength parameters.
How to use the calculator
Enter one row per measurement as comma-separated values; a header row is skipped automatically. Columns are depth (m), qc (MPa), fs and u2 — the units of fs and u2 (MPa or kPa) are detected automatically. Click Analyze to see the layer summary and depth profile. The layering follows a fixed eight-layer scheme, and you can load a CSV 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 behaviour type uses the non-normalized index, which needs only basic cone measurements; a normalized index can be more reliable at greater depths. The behaviour-type index does not resolve certain special cases (sensitive fine-grained soils, or heavily overconsolidated / cemented soils), which require separate assessment.
Frequently asked questions
What input does the CPT calculator use?
One row per measurement: depth (m), qc (MPa), fs and u2 (units auto-detected). Outputs are cohesion c (kPa), friction angle φ (°), unit weight γ (kN/m³), elastic modulus E (kPa) and soil behaviour type.
How does CPT classify soil type?
Because the CPT measures cone resistance, sleeve friction and pore pressure together, soil behaviour type can be classified automatically. This tool uses Robertson's (2010) non-normalized soil behaviour type index.
How are cohesion and friction angle obtained?
Both are solved simultaneously from qc, fs and u2 using the analytical bearing-capacity approach of Motaghedi and Eslami (2013), which combines tip bearing-capacity failure with direct shear along the sleeve.
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.
References
- Motaghedi, H. & Eslami, A. (2013). Determining soil shear strength parameters from CPT and CPTu data. Scientia Iranica, Transactions A: Civil Engineering, 20(5), 1349–1360.
- Motaghedi, H., Eslami, A. & Shakeran, M. (2013). Analytical approach for determining soil shear strength parameters from CPT and CPTu data. Proc. 18th ICSMGE, Paris.
- Eslami, A. & Fellenius, B.H. (2004). CPT and CPTu data for soil profile interpretation. Iranian Journal of Science & Technology, 28(B1), 69–86.
- Robertson, P.K. (2010). Soil behaviour type from the CPT: an update. 2nd International Symposium on Cone Penetration Testing (CPT'10).