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Direct Shear Test

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Direct Shear Test CEP 701 PG Lab Mohr-Coulomb Failure Criterion (in terms of total stresses) f is the maximum shear stress the soil can take without failure, under ... – PowerPoint PPT presentation

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Title: Direct Shear Test


1
Direct Shear Test
  • CEP 701 PG Lab

2
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 ?.
3
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
?.
4
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
13
Mohr Circle of stress
14
Mohr Circles Failure Envelope
?
?
15
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.

16
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.

17
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.
18
  • 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.

19
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
20
Direct shear test
Preparation of a sand specimen
21
Direct shear test
Test procedure
22
Direct shear test
Step 2 Lower box is subjected to a horizontal
displacement at a constant rate
23
  • 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

24
Direct shear test
25
Direct shear test
Analysis of test results
Note Cross-sectional area of the sample changes
with the horizontal displacement
26
Direct shear tests on sands
Stress-strain relationship
27
Direct shear tests on sands
How to determine strength parameters c and f
28
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
29
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
30
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)
31
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
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