A vane shear tester is used to determine an approximate value of the shear strength of clay (undrained shear strength) in the field. The field measured shear strength values need to be corrected for plasticity of clay before using them in the design calculations. The tester has a blade diameter of 75 mm and a blade height of 150 mm. In a field test, the vane required a torque of 50 Nm to shear the clay, which has a liquid limit of 70% and a plastic limit of 23%. The undrained shear strength of clay to be used in foundation design is: Cu(Design) = Cu (Field) 4 F T Applied Torque, T T d'h d' + 2 Vane 2(correction factor) = 1.7 -0.54log(PI%) PI - Plasticity Index