Laboratory Vane Shear Test of Soil

Aim

To determine shear strength of clay soil using laboratory vane shear apparatus.

Theory

In soil, shear strength is contributed by the two properties.

• Cohesion, and
• Angle of internal friction

In pure clays the shear resistance due to internal friction is negligible. Hence, the complete shear strength in clays is due to cohesion (C).

Apparatus

The vane shear test apparatus consists of a torque head mounted on a bracket. Four shear vanes are fixed on a shaft and the shaft is fixed in the lower end of a circular disk graduated in degrees. A torsion spring is fixed between torque head and the circular disk. A maximum pointer is provided to facilitate reading the angle of torque. As the strain indicating pointer rotates when the torque is applied, it moves the maximum pointer, leaving it in position when the torque gets released at failure and the vane returns to its initial position. Turning the torque applicator handles effects rotation of the vane.

Procedure

1. Clean the apparatus thoroughly. Apply grease to the lead screw.
2. Fill-up the sampling mould with remoulded soil at required density and moisture content or the undisturbed soil sample. Level the surface of the sample with the mould.
3. Mount the sampling tube with sample under the base of the unit and clamp it in position.
4. Bring the maximum pointer into contact with the strain indicating pointer. Note down the initial reading of these pointers on the circular graduated scale.
5. Lower the bracket until the shear vanes go into the soil sample to their full length.
6. Operate the torque applicator handle until the specimen fails, which is indicated by the return of the strain-indicating pointer or rotation of drum.
7. Note down the readings of the maximum pointer.
8. The difference between the two readings (initial & final) gives the angle of torque.
9. Repeat steps 3 to 8, on a number of test specimens to obtain the average shear strength of the sample.

Observations and Calculations

Calculate Torque

Calculate torque using following formula.

T = Ø * K/180

Where,

T = Torque

Ø = Difference of angle (angle of torque)

K = Spring factor

Calculate Shear Strength

Shear strength of the soil, C is computed using the following formula

T = Cπ*[(d²h/2) + (d³/6)]

Where,

D = Diameter of vane (cm)

H = Height of the vane (cm)

C = Shear strength (kg/cm²)

T = Torque applied (kg-cm)

Reference

IS: 2720 – Part-30 – Laboratory vane shear test of soil