Unconfined Compression Test And Vane shear Test - Determination of soil strength parameters

Unconfined Compression Test

Basically this is a test done using triaxial test apparatus, but for this we don't apply the cell pressure (sigma 3), that means no confining press ( sigma 3) apply because of that we call this as Unconfined test. this testing can only do for clayey samples (or rock samples) because such samples hold without breaking under unconfined conditions.

Samples with height to diameter ratio of   2:1 are loaded quickly and hence tested under undrained conditions.

In this test we measure the load applied to the specimen and the displacement occur due to that force.
  
To obtain the highest axial stress we need to plot axial stress Vs axial strain graph. we can find axial stress by dividing the axial force by corrected area.



C = sigma d / 2

At the failure point, by the applied ultimate load we can find the unconfined compression stress denoted by qu.

Vane Shear Test

Vane shear test is conducted by pushing a vane into the soil and rotating until failure occurs. ( A soil cylinder with the same dimensions as the vane is rotated against the soil resistance). This test is basically conducted on soft cohesive soils.

In this testing method we measure the rotation of the vane in degrees and the torque. From a graph between torque Vs rotation, the failure torque Tf is noted.

This is a undrained failure and hence by this test method we can find the undrained shear strength Su.

This specimen fail under undrained situation because fail under fast movement.


where,
T = applied torque
d = diameter of vane
h = length of vane
Su = shear strength


Note: Vane shear is very easy testing method to check the soil strength parameters in the field.