Geotechnical Engineering

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UNIVERSITI MALAYSIA PAHANG Department of Civil
and Environmental Engineering

• Geotechnical Engineering
• BAA 3513
• Chapter 1 Shear Strength Part 4

Muzamir bin Hasan, M.Eng. Lecturer
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The Triaxial Shear Test

• One of the most reliable methods to determine
  shear strength parameters.
• Used widely for research and conventional
  testing.
• Soil specimen about 38 mm in diameter and 76 mm
  long.
• Specimen is subjected to confining pressure by
  compression of fluid in chamber (Note sometimes
  air is used as a compression medium)
• To cause shear failure in the specimen, one must
  apply axial stress through a vertical loading ram.

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The Triaxial Shear Test

• This stress can be applied in one of two ways
• Application of dead weights or hydraulic pressure
  in equal increments until the specimen falls.
• Application of axial deformation at a constant
  rate by means of a geared or hydraulic loading
  press. This is a strain-controlled test.

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The Triaxial Shear Test
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The Triaxial Shear Test
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The Triaxial Shear Test
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The Triaxial Shear Test
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The Triaxial Shear Test
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The Triaxial Shear Test
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The Triaxial Shear Test
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Consolidated-Drained (CD) Triaxial Test

• The CD test is started by increasing the
  confining pressure s3 in all the triaxial
  chamber.
• As s3 is increased, the pore water pressure u
  also increases (provided drainage is prevented).
• This increase in u is usually expressed as the
  Skempton parameter B,

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Consolidated-Drained (CD) Triaxial Test

• For saturated soft soils, B is approximately
  equal to 1 for saturated stiff soils B lt 1.
• As drainage is allowed, the excess pore pressure
  is dissipated and consolidation occurs.
• In time, uc becomes zero.

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Consolidated-Drained (CD) Triaxial Test
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Consolidated-Drained (CD) Triaxial Test
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Inclination of the Plane of Failure Due to Shear
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Inclination of the Plane of Failure Due to Shear
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Inclination of the Plane of Failure Due to Shear
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Example 7.1 (page 186)

• A consolidated-drained triaxial test was
  conducted on a normally consolidated clay. The
  results are as follows
• s3 276 kN/m2
• (?sd)f 276 kN/m2
• Determine
• Angle of friction, ø
• Angle ? that the failure plane makes with the
  major principal plane
• Normal stress, s, and shear stress, ?f, on the
  failure plane

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Solution

• a.

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Solution

• b.

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Solution

• c.

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Example 7.2 (page 188)

• For the triaxial test described in 7.1,
• Determine the effective normal stress on the
  plane of maximum shear stress.
• Explain why the shear failure occurred along a
  plane with ? 54.73º and not along the plane of
  maximum shear stress of maximum shear stress.

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Solution
a. Maximum shear stress occur on the plane with ?
45
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Solution
b. The shear stress that will cause failure along
the plane with ? 45 is
However, the shear stress induced on that plane is
Because ?138 kN/m2 lt 146.2 kN/m2 ?f, the
specimen did not fail along the plane of maximum
stress