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.
Apparatus
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.
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 soil.
5. Weigh the soil container, the difference of these two is the weight of the
soil. Calculate the density of 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
Proving Ring
constant....... Least count of the dial........
|
S.No |
Normal load |
Normal |
Normal |
Remark |
|
1 |
REMARKS
1. In the shear box test, the specimen is not failing along its weakest plane but along a predetermined or induced failure plane i.e. horizontal plane separating the two halves of the shear box. This is the main draw back of this test.
Moreover, during loading, the state of stress cannot be evaluated. It can be evaluated only at failure condition i.e Mohrs circle can be drawn at the failure condition only. Also failure is progressive.
2. Direct shear test is simple and faster to operate. As thinner specimens are used in shear box, they facilitate drainage of pore water from a saturated sample in less time. This test is also useful to study friction between two materials one material in lower half of box and another material in the upper half of box.
3. The angle of shearing resistance of sands depends on state of compaction, coarseness of grains, particle shape and roughness of grain surface and grading. It varies between 28o(uniformly graded sands with round grains in very loose state) to 46o(well graded sand with angular grains in dense state).
4. The volume change in sandy soil is a complex phenomenon depending on gradation, particle shape, state and type of packing, orientation of principal planes, principal stress ratio, stress history, magnitude of minor principal stress, type of apparatus, test procedure, method of preparing specimen etc. In general loose sands expand and dense sands contract in volume on shearing. There is a void ratio at which either expansion contraction in volume takes place. This
void ratio is called critical void ratio. Expansion or contraction can be inferred from the movement of vertical dial gauge during shearing.
5. The friction between sand particle is due to sliding and rolling friction and interlocking action.
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