The undrained shear strength of soft clays can be determined in a laboratory by a vane shear test. The test can also be conducted in the field on the soil at the bottom of a bore hole. The field test can be performed even without drilling a bore hole by direct penetration of the vane from the ground surface if it is provided with a strong shoe to protect it.
The vane shear test instrument consist of a vertical steel rod with four thin stainless steel blades (or vanes) fixed at its bottom end. The IS: 2720-1980 code recommends that the height, H of the vane should be equal to twice the overall diameter, D. The diameter and the length of the rod are recommended as 2.6 mm and 60 mm respectively.