Direct Shear Test

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Direct Shear Test

• CEP 701 PG Lab

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Mohr-Coulomb Failure Criterion(in terms of total
stresses)
?
?f is the maximum shear stress the soil can take
without failure, under normal stress of ?.
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Mohr-Coulomb Failure Criterion(in terms of
effective stresses)
u pore water pressure
?f is the maximum shear stress the soil can take
without failure, under normal effective stress of
?.
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Mohr-Coulomb Failure Criterion
Shear strength consists of two components
cohesive and frictional.
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Normal stresses and shear stresses on any plane
can be obtained with the following equations
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Principal stresses
or
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Mohr Circle of stress
Resolving forces in s and t directions,
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Mohr Circle of stress
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Mohr Circle of stress
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Mohr Circles Failure Envelope
?
?
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Direct shear test

• NEED AND SCOPE
• 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.

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Direct shear test

1. This test is performed to determine the
   consolidated - drained shear strength of a sandy
   to silty soil.
2. The shear strength is one of the most important
   engineering properties of a soil, because it is
   required whenever a structure is dependent on the
   soils shearing resistance.
3. The shear strength is needed for engineering
   situations such as determining the stability of
   slopes or cuts, finding the bearing capacity for
   foundations, and calculating the pressure exerted
   by a soil on a retaining wall.

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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.
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• PROCEDURE
• Check the inner dimension of the soil container.
• Put the parts of the soil container together.
• Calculate the volume of the container. Weigh the
  container.
• Place the soil in smooth layers (approximately 10
  mm thick). If a dense sample is desired tamp the
  soil.
• Weigh the soil container, the difference of these
  two is the weight of the soil. Calculate the
  density of the soil.
• Make the surface of the soil plane.
• Put the upper grating on stone and loading block
  on top of soil.

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Direct shear test
Direct shear test is most suitable for
consolidated drained tests specially on granular
soils (e.g. sand) or stiff clays
Preparation of a sand specimen
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Direct shear test
Preparation of a sand specimen
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Direct shear test
Test procedure
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Direct shear test
Step 2 Lower box is subjected to a horizontal
displacement at a constant rate
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• PROCEDURE
• Measure the thickness of soil specimen.
• Apply the desired normal load.
• Remove the shear pin.
• Attach the dial gauge which measures the change
  of volume.
• Record the initial reading of the dial gauge and
  calibration values.
• Before proceeding to test check all adjustments
  to see that there is no connection between two
  parts except sand/soil.
• Start the motor. Take the reading of the shear
  force and record the reading.
• Take volume change readings till failure.
• Add 5 kg normal stress 0.5 kg/cm2 and continue
  the experiment till failure
• Record carefully all the readings. Set the dial
  gauges zero, before starting the experiment

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Direct shear test
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Direct shear test
Analysis of test results
Note Cross-sectional area of the sample changes
with the horizontal displacement
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Direct shear tests on sands
Stress-strain relationship
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Direct shear tests on sands
How to determine strength parameters c and f
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Direct shear tests on sands
Direct shear tests are drained and pore water
pressures are dissipated, hence u 0
Sand is cohesionless hence c 0
Therefore, f f and c c 0
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Direct shear tests on clays
In case of clay, horizontal displacement should
be applied at a very slow rate to allow
dissipation of pore water pressure (therefore,
one test would take several days to finish)
Failure envelopes for clay from drained direct
shear tests
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Interface tests on direct shear apparatus
In many foundation design problems and retaining
wall problems, it is required to determine the
angle of internal friction between soil and the
structural material (concrete, steel or wood)
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Advantages of direct shear apparatus

• Due to the smaller thickness of the sample, rapid
  drainage can be achieved

• Can be used to determine interface strength
  parameters

• Clay samples can be oriented along the plane of
  weakness or an identified failure plane

Disadvantages of direct shear apparatus

• Failure occurs along a predetermined failure plane

• Area of the sliding surface changes as the test
  progresses

• Non-uniform distribution of shear stress along
  the failure surface

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THE END