Vane Shear Test

Vane Shear Test (VST) is a field test for undrained shear strength of soft cohesive soils (clays, silty clays). Per IS 4434:1978, a four-bladed vane (typically 50 mm dia × 100 mm height for shallow tests, 75 × 150 mm for deeper) is pushed into the soil at the bottom of a borehole. The vane is then rotated until the soil shears; the maximum torque is recorded and converted to undrained shear strength (cu).

For a standard vane (50 × 100 mm) in soft clay: typical torque 5-50 N·m; shear strength cu = T × π × d² × h / 6 where T is torque, d is vane diameter, h is vane height. Result: cu = 25-200 kPa range depending on soil. The test is rapid (5-15 minutes per location), provides direct measurement (less correlation uncertainty than laboratory tests on disturbed samples), and is suitable for soft clays where laboratory tests on extracted samples are unreliable.

Limitations: (a) Soil disturbance during vane insertion — affects strength measurement; corrected via Aas correction factor. (b) Soft clay only — VST not applicable to sandy or stiff clay soils (high resistance, vane breaks). (c) Strain-rate dependence — laboratory test rates vs field rotation rates differ. (d) Anisotropy — VST measures in horizontal-vertical plane; deeper investigation may have different anisotropy. Indian use: typical for major foundation projects in soft clay regions (Mumbai reclaimed land, Kolkata, Indo-Gangetic plain). Cost ₹15,000-50,000 per test; multiple tests for important structures.