What Does Vane Shear Test (VST) Mean?
The vane shear test is used to measure the undrained shear strength of cohesive soil especially soft clays. The test can be conducted in-situ or in the laboratory. It is simple, quick, and cost-effective to carry out.
The laboratory test of shear strength using the vane shear test is useful for soils with a low shear strength of less than 0.5 kg/cm2. The test can be used to find the undrained strength of soil in the undisturbed and also the remolded state.
This test is a cheaper option compared to the other methods of testing shear strength of clay such as triaxial shear test or direct shear test. The standard method for field vane shear test in cohesive soil is described in ASTM D 2573-72.
The vane shear test is also known as the borehole vane test.
Trenchlesspedia Explains Vane Shear Test (VST)
The apparatus for the vane shear test consists of a four-bladed stainless steel vane attached to a steel rod that is pushed (not rotated) into the ground. Other items used include a torque wrench, drive head, extension rods, spanner, and a few different vane sizes.
The procedure can be carried out from the ground surface or from the bottom of a borehole or test pit. To reach undisturbed soil, it is generally recommended to go five borehole diameters deep. Once inserted the vane is rotated at a slow rate of 6 degrees to 12 degrees per minute.
The torque is measured at regular intervals and the test is continued till the maximum torque is reached and the vane rotates rapidly for a number of revolutions. This is when the soil fails in shear on a cylindrical surface around the vane.
The rotation can be continued after shearing has taken place and the torque is measured to find the remolded shear strength.