Direct Shear Test
The direct shear test is a test that uses the direct shear apparatus to determine the shearing strength of the soil and shear strength parameters for a given soil.
Especially in cohesionless soils, the direct shear test is used to predict the parameters such as the design of slopes, calculation of bearing capacity of any strata, calculation of consolidation parameters, etc.
Measurements of shear load, shear displacement, and normal displacement are recorded when the specimen, the normal load is applied and between the two halves of the shear box, the specimen is sheared across the pre-determined horizontal plane.
Planning and Organizing:
There are the following apparatus used in the direct shear test such as:
- Direct shear box apparatus.
- Loading frame and Dial gauge.
- Proving ring and Tamper.
- Weigh up to 200 mg Balance.
- Straight edge and Spatula.
- Aluminium container.
Preparation of Specimen:
From the undisturbed specimen collected from the field properly, specimens of the required size shall be prepared.
- To the required density and moisture content, cohesive soils may be compacted and to the required size, the sample extracted and then trimmed. After fixing the two halves of the shear box together the soil may be compacted to the required density and moisture content directly into the shear box.
- With the base plate and grid plate at the bottom of the box, cohesionless soils may be tamped in the shear box by itself.
- To obtain the moisture content the cut specimen shall be weighed and trimmings obtained during cutting shall be used.
There are the following steps for applying the direct shear test as given below;
- Assemble the shear box and after bringing it to optimum moisture condition compact the soil sample in the mould.
- Into the shear box carefully transfer the sample and on top of the upper porous plate, place the loading plate. Then place it on the loading cap after recording the weight of the loading carrier.
- Set the readings to zero after positioning all dial gauges and remove the alignment screws which hold together the two halves of the shear box.
- To reduce the frictional force tight the remaining two diagonally opposite screws until there is a small gap between the upper and lower boxes.
- If there is any vertical displacement when the desired normal load is applied then wait till the dial gauges indicate a constant reading and after that reset the reading of the dial gauge to zero.
- Then check that the screws have been removed and to produce the desired constant rate of shearing, start the motor.
- Then take the readings of the shear load from the proving ring, and shear displacement, and at every 10 division increment in the horizontal dial gauge take the vertical displacement reading.
- When the shear load starts to reduce or remains constant for at least three readings stop the test.
- With different normal loads at least for another two samples remove the soil and repeat the procedure.
Advantages of Direct Shear Test:
There are the following advantages of the direct shear test such as;
- Easy to prepare the sample.
- Convenient and simple test.
- The drainage is quick and the pore pressure dissipates very rapidly because the thickness of the sample is relatively small.
- Consolidated- undrained and drained tests take a small period of time.
- For conducting drained tests on cohesionless soils, the direct shear test is ideally suited for this.
- The apparatus used in this test is cheap.
Disadvantages of Direct Shear Test:
There are the following disadvantages of the direct shear test such as;
- Only at the failure, the stress condition is known.
- On the failure plane, the stress distribution is not uniform.
- As the test progress, the area under shear gradually decreases.
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