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/cm^{2}). 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.
Fig. 1: Lab Vane Shear Apparatus | |
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.
IS 2720(Part 30):1980 Methods of test for soils: Laboratory vane shear test (first revision). Reaffirmed- Dec 2016.
S.No. | Initial Reading(Deg) | Final Reading(Deg.) | Difference(Deg.) | T=Spring Constant/180x Difference (Kg-cm) | G = 1/π(D^{2}h/2 + D^{3}/6) | S=TxG (Kg/cm^{2}) | Average 'S' (Kg/cm^{2}) | Spring Constant (Kg-cm) |
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05 |
Table 1 : Recordings during Lab Vane Shear Test
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
G = 1/{π*[(d^{2}h/2) + (d^{3}/6)]}
Where,
d = diameter of vane (cm)
h = height of the vane (cm)
Shear Strength, S = T * G
OR
S = (3/19)*T
Where,
S = Shear Strength in kgf/cm^{2}
T = Torque applied in cm kgf
The formula for shear strength is based on following assumptions: