1. objective
The laboratory vane shear test for the measurement of shear strength of cohesive soils,
is useful for soils of low shear strength of less than about 0.5 kgf/cm2.
This test gives the undrained strength of the soil, in undisturbed as well as remoulded
conditions both. Vane shear test is a cheaper and quicker method of measuring the shear
strength of clay, as compared to very elaborate tri-axial shear test or
direct shear test.
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Fig. 1: Lab Vane Shear Apparatus
The apparatus may be either of hand-operated type or motorized. It should have arrangement for rotating
the Vane steadily at a rate of approximately 1/60 rev/min (0.1 degree/S) and for measuring the rotation of the Vane.
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The vane shall consist of four blades each fixed at 90° to the adjacent blades. The width of vane, from edge of one vane
to edge of another vane, is 12±0.15 mm. The height of vane is
24±0.25 mm. The thickness of vane is 0.5±0.05 mm. The vane
should not deform under the maximum torque for which it is
designed. The penetrating edge of the vane blades shall be
sharpened having an included angle of 90°. The vane blades
shall be welded together suitably to a central rod, the
maximum diameter of which should preferably not exceed 2.5
mm in the portion of the rod which goes into the specimen
during the test. The height of rod, above the vanes, is 60mm.
The vane should be properly treated to prevent rusting and
corrosion.
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3. reference
IS 2720(Part 30):1980 Methods of test for soils: Laboratory vane shear test
(first revision). Reaffirmed- Dec 2021.
4. procedure
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Prepare two or three specimens of the soil sample of dimensions of at least 30.0
mm diameter and 75 mm long specimen.
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Mount the specimen container with the specimen on the base of the vane shear
apparatus and fix it securely to the base. If the specimen container is closed at one end, it should be provided
with a hole of about 1 mm diameter at the bottom.
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Gently lower the shear vanes into the specimen to their full length without
disturbing the soil specimen. The top of the vanes should be atleast 10 mm
below the top of the specimen. Note the readings of the torque indicator.
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Rotate the vanes at an uniform rate say 0.10/s by suitably operating the torque
applicator handle until the specimen fails.
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Note the final reading of the torque indicator (T).
5. observation and recording
S.No. |
Initial Reading(Deg) |
Final Reading(Deg.) |
Difference(Deg.) |
T=Spring Constant/180x Difference (Kg-cm) |
G = 1/π(D2h/2 + D3/6) |
S=TxG (Kg/cm2) |
Average 'S' (Kg/cm2) |
Spring Constant (Kg-cm) |
01 |
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02 |
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03 |
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04 |
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05 |
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Table 1 : Recordings during Lab Vane Shear Test
6. calculation
For vane testing instruments that do not read the torque directly, a
calibration curve to convert the readings to cm kgf of torque shall be
provided. These calibration curves shall be checked periodically.
Torque, T = Ø * K/180
Where,
T = Torque applied in cm kgf
Ø = Difference of angle (angle of torque)
K = Spring factor
Shear Strength, S = (3/19)*T
Where,
S = Shear Strength in kgf/cm2
T = Torque applied in cm*kgf
7. General Remarks
The formula for shear strength is based on following assumptions:
- Shearing Strength in the Horizontal and Vertical directions are the same.
- At the peak value, Shear Strength is equally mobilized at the end surface as well as at the centre, and
- The shear surface is cylindrical and has a diameter equal to the diameter of the vane.