About this tool Calculate undrained shear strength (su) from field vane test torque. Enter the maximum torque, vane dimensions, and optionally the Plasticity Index for Bjerrum correction. If remoulded torque is provided, sensitivity is also calculated.
The field vane test is one of the most reliable in-situ methods for measuring undrained shear strength in soft to firm clays. The Bjerrum (1972) correction accounts for the known overestimation of su by the vane in high-plasticity clays.
Standards: BS EN ISO 22476-9 (field vane test), BS 1377-9. Reference: Bjerrum, L. (1972) Embankments on soft ground, ASCE Specialty Conference.
How to use this tool 1. Enter the maximum torque — peak torque from the vane test in N·m.
2. Select vane diameter and H/D ratio — standard field vanes are 63mm with H/D = 2.
3. Enter PI for Bjerrum correction (optional) — reduces su for high-plasticity clays.
4. Enter remoulded torque (optional) — for sensitivity calculation after 10+ rapid rotations.
Technical information su = T / K
K = (π × D³ / 2) × (H/D / 2 + 1/6)
μ = 1.7 - 0.54 × log₁₀(PI) — Bjerrum (1972)
Sensitivity classification: 1–2 Low | 2–4 Medium | 4–8 Sensitive | >8 Quick (extra-sensitive)
Limitations The vane test is only valid in soft to firm, intact clays. It should not be used in stiff fissured clays, sands, gravels, or organic soils with fibrous inclusions.
The Bjerrum correction is empirical and based on back-analysis of embankment failures. It may not apply to all clay types, particularly structured or cemented clays.
Rod friction can add to the measured torque if slip couplings are not used. Ensure the vane apparatus includes friction compensation per BS EN ISO 22476-9.
Revision history 1 September 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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