VANE SHEAR TEST ON SOIL
The vane shear test apparatus consists of four stainless steel blades fixed at right angle to each other and firmly attached to a high tensile steel rod. The length of the vane is usually kept equal to twice its overall width. The diameters and length of the stainless steel rod were limited to 2.5mm and 60mm respectively.
Let = unit shear strength of the soil
H = height of vanes in cm
d = diameter of vane tester in cm
Shear strength at failure along the cylindrical surface
Consider the sheared surface to be composed of a number of elementary rings of thickness dr.
Shearing resistance on an elementary ring of radius ‘r’ =
Total shear resistance on the top and bottom of the surface of the cylinder
Total shear strength = (1) + (2)
Equating the torque T to the total shear resistance, we get
If the top of the vane is above the soil surface and depth of the vane inside the sample is , then
PROCEDURE FOR VANE SHEAR TEST
The following steps are involved:
1. Push into the clay, the vane and rod below the bottom of the bore hole, ensuring the verticality of the central rod.
2. Rotate the vanes at a constant speed of per minute by suitable equipment.
3. The test can be conveniently used to determine the sensitivity of the soil.
The shear strength of the soil under undrained conditions is equal to the apparent cohesion.
The vane shear test can be used to determine the sensitivity of the soil. After the initial test, the vane is rotated rapidly through several revolutions such that the soil becomes remoulded. The test is repeated on the remoulded soils and the shear strength in remoulded condition is determined. Thus,
ADVANTAGE OF VANE SHEAR TEST
i. The test is simple and quick.
ii. It is ideally suited for the determination of the undrained shear strength of non-fissured fully saturated clay.
iii. The test can be conveniently used to determine the sensitivity of the soil.
iv. The test can be conducted in soft clays situated at a great depth, samples of which are difficult to obtain.
DISADVANTAGE OF VANE SHEAR TEST
i. The test can not be conducted on the clay containing sand or silt laminations or the fissured clay.
ii. The test does not give accurate results when the failure envelope is not horizontal.