DIRECT SHEAR TEST
Objective
To determine the shearing strength of the sample, using direct shear apparatus.
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| Direct shear Apparatus |
Need and Scope of the Experiment
In many engineering problems such as design of foundation, retaining walls, slab bridges,pipes, sheet piling, the value of the angle of internal friction and cohesion of the soil involved are required for the design. Direct shear test is used to predict these parameters quickly.
Apparatus Required
1. Direct shear box apparatus
2. Loading frame (motor attached).
3. Dial gauge.
4. Proving ring.
5. Tamper.
6. Straight edge.
7. Balance to weigh upto 200 mg.
8. Aluminum container.
9. Spatula.
Knowledge of Equipment:
Strain controlled direct shear machine consists of shear box, soil container, loading unit,proving ring, dial gauge to measure shear deformation and volume changes.
A two piece square shear box is one type of soil container used. A proving ring is used to indicate the shear load taken by the soil initiated in the shearing plane.
Procedure
1. Check the inner dimension of the soil container.
2. Put the parts of the soil container together.
3. Calculate the volume of the container. Weigh the container.
4. Place the soil in smooth layers (approximately 10 mm thick). If a dense sample is desired tamp the soil.
5. Weigh the soil container, the difference of these two is the weight of the soil.
Calculate the density of the soil.
6. Make the surface of the soil plane.
7. Put the upper grating on stone and loading block on top of soil.
8. Measure the thickness of soil specimen.
9. Apply the desired normal load.
10. Remove the shear pin.
11. Attach the dial gauge which measures the change of volume.
12. Record the initial reading of the dial gauge and calibration values.
13. Before proceeding to test check all adjustments to see that there is no connection
between two parts except sand/soil.
14. Start the motor. Take the reading of the shear force and record the reading.
15. Take volume change readings till failure.
16. Add 5 kg normal stress 0.5 kg/cm2
and continue the experiment till failure.
17. Record carefully all the readings. Set the dial gauges zero, before starting the experiment.
Observation and Calculation
The values of shear stresses are noted down for a particular normal stress provided to the specimen compacted into the shear box. A graph is then plotted between the normal and the shear stresses involved in the procedure and from the graph the cohesive force and the friction angle of the soil (or fly ash) is calculated.
The representation of the relation between the shear stress, cohesive force, the normal stress and the angle of internal friction is given below.
S = C + σ tanθ
Where,
S = shear strength of the sample
C = cohesion of the sample
Φ= Angle of internal friction
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| Direct shear strength |