Determination of shear strength of clay soils using laboratory plank shear equipment.


The shear strength in soil is contributed by two properties.

  • harmony, and
  • internal friction angle

Pure clay has negligible shear resistance due to internal friction. Therefore, the absolute shear force in the soil is due to cohesion (C,


Vane Shear Testing Equipment
Vane Shear Testing Equipment

The vane shear test apparatus consists of a torque head mounted on a bracket. Four shear vanes are mounted on a shaft and the shaft is fixed in the lower end of a circular disc graduated in degrees. A torsion spring is fixed between the torque head and the circular disc. A maximum indicator is provided to facilitate the torque angle reading. As the strain indicates that the pointer rotates when torque is applied, it moves the pointer maximum, releasing it when the torque fails, and returning the vane to its initial position. The turning torque applicator handles the impact rotation of the vane.

vane shear test
vane shear test


  1. Clean the equipment thoroughly. Apply grease to the lead screw.
  2. Fill the sampling mold with the required density and moisture content or the remolded soil on the undisturbed soil sample. Level the surface of the sample with the mold.
  3. Mount the sampling tube with the sample under the base of the unit and clamp it in position.
  4. Bring the maximum pointer in contact with the pointer indicating strain. Note the initial readings of these pointers on a circular graduation scale.
  5. Lower the bracket until the shear vanes run their full length into the soil sample.
  6. Operate the torque applicator handle until the sample fails, which is indicated by the return of the tension-indicating indicator or the rotation of the drum.
  7. Note down the reading of the maximum pointer.
  8. The difference between the two readings (initial and final) gives the angle of torque.
  9. Repeat steps 3 through 8 on several test samples to obtain the sample’s average shear power.

observation and calculation

calculate torque

Calculate torque using the following formula.

T = * K / 180


T = torque

= angle difference (angle of torque)

K = spring factor

calculate shear strength

The shear strength of the soil, C, is calculated using the following formula:

t = c,[(d2h/2) + (d3/6)]


D = diameter of the face (cm)

h = height of the vane (cm)

C = Shear Strength (kg/cm.)2,

T = applied torque (kg-cm)


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

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.