Undrained shear resistance at failure (Su(FV)) is calculated by the maximum torque (Tmax) required to cut the soil between the vane blades.
The general formula for rectangular vanes with height (H) and diameter (D), is:
Su(FV) = T /((p D3/2) (H/D + a/2)) (1)
Where:
T = maximum applied torque (net of friction).
a = factor which depends on the assumed shear stress distribution at the ends of the cylinder obtained rotating the vane blades and amounting to 2/3 for uniform shear stress.
For rectangular vanes with H/D = 2, the equation (1) simplifies to:
Su(FV) = 6T / 7p D3 = 0.273T / D3 (2)
The residual shear strength value is calculated using the formula above (2) introducing the value of the torque, net of friction, measured after a few rotations of the vane, that is when the soil offers an essentially constant resistance.