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Shear Strength of Soil by Vane Shear Test

Vane shear test is used to determine the undrained shear strength of soils especially soft clays. This test can be done in laboratory or in the field directly on the ground. Vane shear test gives accurate results for soils of low shear strength (less than 0.3 kg/cm2).

Table of Contents

• Shear Strength of Soil by Vane Shear Test
  • Apparatus
• Procedure of Vane Shear Test
• Observations and Calculations of Vane Shear Test
• Result of Vane Shear Test
• Advantages of Vane Shear Test
• Drawbacks of Vane Shear Test

Shear Strength of Soil by Vane Shear Test

Apparatus

Apparatus required for vane shear test are:

1. Vane shear apparatus
2. Soil specimen container
3. Vernier callipers

Fig 1: Vane Shear Apparatus

Vane shear apparatus consists high tensile steel rod to which four steel blades (vanes) are fixed at right angles to each other at the bottom of rod.

Fig 2: Steel Rod with Vanes

Procedure of Vane Shear Test

Test procedure of vane shear test contains following steps:

1. Clean the vane shear apparatus and apply grease to the lead screw for better movement of handles.
2. Take the soil specimen in container which is generally 75 mm in height and 37.5 mm in diameter.
3. Level the soil surface on the top and mount the container on the base of vane shear test apparatus using screws provided.
4. Lower the vane gradually into the soil specimen until the top of vane is at a depth of 10 to 20 mm below the top of soil specimen.

   

   Fig 3: Lowering Vane into the Soil Specimen

5. Note down the reading of pointer on circular graduated scale which is initial reading.
6. Rotate the vane inside the soil specimen using torque applying handle at a rate of 0.1oper second.
7. When the specimen fails, the strain indicator pointer will move backwards on the circular graduated scale and at this point stop the test and note down the final reading of pointer.
8. The difference between Initial and final readings is nothing but the angle of torque.
9. Repeat the procedure on two more soil specimens and calculate the average shear strength value.
10. Measure the diameter and height of vane using Vernier callipers.
11. Sensitivity of given soil sample is determined by repeating the above test procedure on remolded soil which is nothing but soil obtained after rapid stirring of vane in the above test.

Sensitivity of soil = undisturbed shear strength/ remolded shear strength.

Observations and Calculations of Vane Shear Test

Shear strength of given soil sample is calculated from below observations.

• Diameter of vane, D = ___ cm
• Height of vane, H = ____ cm
• Torque, T = (Spring constant /180)*(initial reading-final reading)

Shear strength of soil (S) is calculated from below formula.

S.No	Initial Reading	Final Reading	(Initial Reading – final reading)	Torque (T)	Shear strength (S)
1
2
3

Result of Vane Shear Test

Shear strength of soil specimen = ________ kg/cm2.

Advantages of Vane Shear Test

Advantages of vane shear test are as follows:

• Vane shear test is easy and quick.
• This test can be performed either in laboratory or in the field directly on the ground.
• In-situ vane shear test ideal for the determination of undrained shear strength of non-fissured, fully saturated clay.
• Shear strength of soft clays at greater depths can also be found by vane shear test.
• Sensitivity of soil can also be determined using vane shear test results of undisturbed and remolded soil samples.

Drawbacks of Vane Shear Test

Drawbacks of vane shear test are as follows:

• Vane shear test is not suitable for clays which contain sand or silt laminations in it.
• It cannot be conducted on the fissured clay.
• If the failure envelope is not horizontal, vane shear test does not give accurate results.