### Field vane test

The test can be carried out either in the field, on the wall or at the bottom of an excavation, or even in a laboratory on a suitably contained specimen.

It consists of inserting a vane with four orthogonal blades into the soil, rotating

it at the prescribed speed, to measure the value of the torque required to

break the soil, by means of the calibrated torque wrench.

Afterwards the residual shear strength of the soil after significant deformation

can be measured by continuing to rotate the vane several turns until the soil

is completely mixed.

### Technical information

The vanes have a rectangular shape and a height double their diameter.

A thin enlarging ring can be installed above the blades of the vane so that most of the resistance due to the soil friction along the path of the rod inserted into the soil is eliminated from the measurements.

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Compatible vanes:

TG63-100: 60×30

TG63-150/200: 50×25

TG73-200: 38×19

Inner / Outer diameter of lining tubes: 32/48 mm

### Calculation of the

undrained shear resistanc

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.