The field vane test enables the direct measurement of the undrained shear resistance of saturated cohesive soils. It 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.
The test consists of forcing a vane with four orthogonal blades into the soil, and then rotating it until soil failure, measuring the maximum torque value required to generate this. 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.
The vanes have a rectangular shape and a height double their diameter, as specified in the recommendations included in EUROCODE 7 (1977) and ASTM Standard Code (D 2573); the latter also allows vanes with a tapered end.
These standards specify that the rotation must be carried out at a rate of 0.1-0.2 degrees/sec. (6-12 degrees/min).
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.
The vanes which can be used with the various types of lining tubes are as follows:
|Penetrometers||Inner / Outer diameter of lining tubes||Compatible vanes|
|20 / 33 mm||38×19|
|32 / 48 mm||60×30
Tests can be carried out at the bottom of a borehole with stable walls. The vane must penetrate at least five times the hole diameter into undisturbed soil (ASTM D 2573).
The test is carried out as follows:
- drive the rod with the vane to the specified test depth;
- rotate the vane from the surface at the specified speed using the dedicated torque wrench;
- measure the maximum torque required to obtain soil failure and continue regularly rotating the vane for at least 3 complete turns, measuring and recording the residual torque.
N.B. To reach the required depth, the lining tubes supplied with Pagani Geotechnical Equipment penetrometers can be used, following the procedure shown in the following screen.
- Driving the penetrometer bit and corresponding lining tubes until a depth around that of the test depth is reached.
- Extraction of the penetrometer bit, leaving the linings in place.
- Lowering of the vane connected to the corresponding rods and driving the blade under pressure into the undisturbed soil at the bottom of the hole.
- Arrangement of the centring ring, fitted with a thrust bearing, between the rod and lining.
- Arrangement of the sleeve to support the extension rods.
- Tightening of the screw to fasten the extension rods to the support sleeve.
- Application of the torque wrench.
- Running the test.
Calculation of the undrained shear resistance
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)
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.