Upload Now
ADVERTISEMENTS:
In this article we will discuss about:- 1. Apparatus for Vane Shear Test 2. Procedure for Vane Shear Test 3. Observations 4. Calculations 5. Field Vane Shear Test.
Apparatus for Vane Shear Test:
A typical vane shear apparatus consists of the following components:
1. Vane-The vane consists of four blades each fixed at 90° to the adjacent blades. The vane forms a circle of diameter 10 mm when rotated about the central axis. The height of the vanes is 24 mm.
ADVERTISEMENTS:
2. The vane blades are welded to a central rod, of diameter 2.5 mm.
3. Arrangement for lowering the vane into the soil specimen gradually and vertically.
4. Arrangement for rotating the vane steadily at a rate of approximately 6° per minute (0.1° per s) and for measuring the rotation of the vane.
5. A torque applicator to rotate the vane in the soil and a device for measuring the torque applied to an accuracy of 0.05 kgf.cm
ADVERTISEMENTS:
6. A set of springs capable of measuring shear strength of 0.5 kgf/cm2.
7. A cylindrical mold in which the soil specimen is placed.
Procedure for Vane Shear Test:
1. The specimen in the mold should be at least 30 mm in diameter and 75 mm long. Mount the specimen container with specimen on the base of the vane shear apparatus and fix it securely to the base.
2. If the specimen container is closed at one end, it should be provided at the bottom with a hole of about 1 mm diameter. Lower the vanes into the specimen to their full length gradually with minimum disturbance to the soil specimen so that the top of the vane is at least 10 mm below the top of the specimen. Note the readings of the strain and torque indicators. Switch on the motor and allow the vane to rotate at a uniform rate approximately 0.1° per second (6° per minute) until the specimen fails.
ADVERTISEMENTS:
3. Note the final reading of the torque indicator. Torque readings and the corresponding strain readings may also be noted at desired intervals of time as the test proceeds.
Observations Recorded during Vane Shear Test:
Following observations are recorded during the test:
1. Diameter of the vane, D (mm).
2. Height of the vane, H (mm).
ADVERTISEMENTS:
3. Water content of the soil specimen, ω (%).
4. Bulk density of soil specimen, γ(kN/m3).
5. Dry density of soil specimen, γ(kN/m3).
6. Initial dial reading, D1 (degrees).
ADVERTISEMENTS:
7. Final dial reading, D2 (degrees).
8. Spring constant, k (kgf.cm).
Calculations made in Vane Shear Test:
1. Calculate the Torque applied using –
where T is the torque at failure, D2 is the final dial reading and D1 is the initial dial reading.
2. Calculate the shear strength of the soil using –
where T is the torque at failure, D the overall width or diameter of the vane, and H is the height of the vane. Here τu is the shear strength in kgf/cm and T the torque in kgf. cm.
Note:
Equation (13.52) is based on the following assumptions:
1. Shearing strengths in the horizontal and vertical directions are the same.
2. At the peak value, shear strength is equally mobilized at the end surface as well as at the centre.
3. The shear surface is cylindrical and has a diameter equal to the diameter of the vane.
Field Vane Shear Test:
The vane shear test is more commonly conducted in the field to determine the in situ undrained shear strength of soft clays. The field vane is 5-10 cm in diameter and 10-20 cm in height. To conduct the test, the vane is fixed to the drill rod of the boring equipment in the field and lowered into the bore hole to the required level to determine the in situ strength. The vane is pushed vertically down into the soil and then a torque (T) is applied to the rod to cause shear failure of the soil over the surface and ends. The undrained shear strength is obtained from the expression –
where T is the torque at failure; D the overall width of the vane; H the height of the vane; and a = 2/3 for uniform end shear, 2/5 for parabolic end shear, and 1/2 for triangular end shear.
The remolded shear strength may be determined after remolding the soil by rotating the vane several revolutions. From the undisturbed and remolded shear strength, the sensitivity of the soil can be determined.
Comments are closed.