The Vane Shear Test is carried out in accordance with IS : 2720 (part XXX) – 1980. This test is useful for determining the undrained shear strength (S) of clay and can be performed both in the lab and in the field.
A four-bladed vane on the end of a rod makes up the apparatus. The vane’s height is usually twice its width. A boring is made to the depth at which the test will be carried out. The vane is inserted into the soil at the bottom of the hole and slowly rotated at a rate of 6° per minute while the torque is measured using an instrument.
The torque (T) needed to shear the soil is determined assuming shear strength (S) is constant over the cylinder of soil sheared by the vane.
Formula Used in Vane Shear Test
The torque is applied until failure equals the amount of the resisting torque at the sides (T1), as well as the resisting torque at the top and bottom (T2). As a result,
….(eqn 1)
The resisting torque on the sides equals the resisting force developed on the cylindrical surface multiplied by the radial distance.
….(eqn 2)
By integrating the torque produced on the circular ring of radius (r) and width (dr), the resisting torque (T2) due to the resisting forces at the top and bottom of the sheared cylinder can be determined.
….(eqn 3)
But,
….(form eqn 1,2 and 3)
or
Here,
D = vane diameter
S = shear strength
H = vane height
- 24 x 12 mm for soft clay (lab vane)
- 100 x 50 mm for firm clays.
- 150 x 75 m for soft clays.
The remoulded strength can be measured after calculating the maximum torque in an undisturbed state. To remould the soil in the sheared field, the vane is rotated quickly through a few revolutions. To obtain the remoulded power, the vane is rotated at a rate of 6° per minute without further delay.
Generally if vane top is above soil surface depth inside sample is (H1), then shear strength becomes,
Shear strength of soil under undrained condition equals to apparent cohesion (Cu).
Also Read : California Bearing Ratio Test【CBR Test】of Soil
Also Read : Effective Stress Concept in Soil Mechanics
Also Read : Soil Structure and Their Types
Characteristics of Vane Shear Test
- It’s a fast and easy test.
- When the failure envelope is not horizontal, the test does not produce correct results.
- The test cannot be performed on clay that contains sand, silt, or is fissured.
- It’s perfect for evaluating the undrained shear strength of completely saturated clay that isn’t fissured.
- The test can be used to quickly assess the soil’s sensitivity.
Advantage and Disadvantage of Vane Shear Test
| Advantages | Disadvantages |
|---|---|
| Vane shear test results of undisturbed and remoulded soil samples can also be used to assess soil sensitivity. | Clay that includes sand, silt, or is fissured is ineligible for the this test. |
| The vane shear test can also be used to determine the shear strength of soft clays at greater depths. | Vane shear testing is not appropriate for clays with sand or silt laminations. |
IS 2720 (Part XXX) – 1980 PDF (Methods of Test for Soil)
The protocol for performing laboratory vane shear tests on cohesive soils with low shear strength to determine their undrained shear strength is covered in this norm IS 2720 (Part XXX) – 1980.
In this IS code detailed procedure of the calculations along with the apparatus used for performing laboratory vane shear test on cohesive soil is given.
You could refer the below IS code for better understanding of the practical procedure.