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CPT Robertson Classifier

Classify soil behaviour type from CPT data using Robertson (2009) normalised SBTn framework. Single-point, CSV, or AGS file import. Free, browser-based.

Last updated 3 November 2026

About this tool

Classify soil behaviour type from cone penetration test (CPT) data using Robertson's normalised SBTn framework (2009).

Computes corrected tip resistance (qt), normalised cone resistance (Qtn), normalised friction ratio (Fr), and soil behaviour type index (Ic). Results are classified into Robertson's 9 SBT zones with an interactive chart and depth profile visualisation.

Supports three input modes: single-point entry, CSV paste, and AGS file import that auto-detects SCPG and SCPT groups.

How to use this tool

Choose input mode — single point, CSV paste, or AGS file import.
Enter or upload CPT data — qc, fs, u2, and depth. AGS files auto-populate from the SCPT group.
Set site parameters — groundwater level, net area ratio, and unit weights.
Review results — Robertson SBTn chart, depth profiles, and zone summary.

Technical information

Implements Robertson (2009) normalised SBTn with iterative stress exponent:

Ic = √[(3.47 − log Qtn)² + (1.22 + log Fr)²]

Stress exponent n = 0.381 × Ic + 0.05 × (σ'v0/pa) − 0.15, capped at 1.0, converges in 3–5 iterations.

Limitations

Classifies soil behaviour type, not soil type — behaviour may differ from physical classification.
Net area ratio must match the cone used. Incorrect values significantly affect qt in soft clays.
Unit weight assumptions affect normalisation. Use site-specific values where possible.

Revision history

3 November 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.

Input mode

Cone resistance, qc (MPa)

Sleeve friction, fs (MPa)

Pore pressure, u2 (MPa)

Depth (m)

Groundwater level (m)

Net area ratio (a)

Unit weight above GWL (kN/m³)

Saturated unit weight (kN/m³)

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