The surface resisting the turning is the cylindrical surface of the soil and the two end faces of the cylinder.
Therefore, the resisting moment =
=
where, r
= radius of the cylinder
= the undrained shear strength.
It is apparent at failure the resisting moment of the cylinder of the soil is equal to the turning moment applied at the torsion head.
Therefore,
he standard dimensions of the field vane are L = 11.25cm, r = 3.75cm.
Bjerrum(1922) back computed a number of embankment of embankment failures on soft clay and concluded that the vane shear strength tended to be too high.
A correlation is established as follows,
( field ) = ( vane ) where, is the correction factor.
( field ) = 
( vane ) where, 
is the correction factor. 
