To determine the shear strength of a clayey soil by laboratory vane shear test.

OBJECT: To determine the shear strength of a clayey soil by the Laboratory Vane Shear Test.

THEORY

The vane shear test is a useful method of measuring the shear strength of clay. It is one of the cheaper and quicker methods. The laboratory vane shear test for the measurement of shear strength of cohesive soils is useful for soils of low shear strength (less than 0.3 kg/cm²) for which triaxial or unconfined tests cannot be conducted. The test gives the undrained shear strength of fully saturated clays without disturbance. The undisturbed strength obtained is useful for evaluating the sensitivity of soil.

STANDARDS:

AASHTO T236-90

ASTM D3080-90

BIS1377:part 4

EQUIPMENTS

Vane shear apparatus, Specimen, Specimen container, Calipers

THEORY:

The vane shear test apparatus consists of a four-blade stainless steel vane attached to a steel rod that will be pushed into the specimen. The height of the vane is usually twice its overall width and is often equal to 10 cm or 15 cm, whereas the dimensions for the machine are D = 2.5mm and the length of the rod = 60mm.

PROCEDURE:

1. Prepare two or three specimens of the soil sample of dimensions of at least 37.5mm diameter and 75mm length in specimen (L/D ratio 2 or 3).
   
2. Mount the specimen container with the specimen on the base of the vane shear apparatus.
   
3. Gently lower the shear veins into the specimen to their full length without disturbing the soil specimen. The top of the vanes should be at least 10 mm below the top of the specimen. Note the readings of the angle of twist.
   
4. Rotate the vanes at a uniform rate, say 0.1″/s, by suitable operating the torque application handle until the specimen fails.
   
5. Note the final reading of the angle of twist.
   
6. Find the value of blade height in cm.
   
7. Find the value of the blade width in cm.
   

OBSERVATION:

Calculations:

Shear strength=S=T/x(D²H/2+D³)=_________kg/cm^2.. (Average)

Where,

T=torque in kg/cm,

D=overall diameter of vane blades in cm.

Result:

The average shear strength of soil comes out to be _________ kg/cm².

Review Questions

1. What is an undisturbed soil sample?
2. What is the main difference between laboratory and field vane shear test equipment?

How does shear failure of the soil specimen occur in this test?