Geotechnical Engineering

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UNIVERSITI MALAYSIA PAHANG Department of Civil
and Environmental Engineering

• Geotechnical Engineering
• BAA 3513
• Chapter 1 Shear Strength Part 3

Muzamir bin Hasan, M.Eng. Lecturer
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The Shear Failure Law in Saturated Soils
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The Shear Failure Law in Saturated Soils

• For sand and inorganic silt, c 0
• N.C. clays, c 0
• O.C. clays, c gt 0

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The Shear Failure Law in Saturated Soils
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Example 1.2

• Determine the maximum shear stress the soil can
  take without failure by using the Shear Failure
  Law in saturated soil, if the properties for the
  sample are as given below
• friction of angle (based on effective stress)
  20º
• cohesion (based on effective stress) 5 kN/m2
• total stress 22 kN/m2
• pore water pressure 10 kN/m2

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Solution Example 1.2
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The Direct Shear Test
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The Direct Shear Test

• Sample may be square or circular in plan.
• The box is split horizontally in to two halves.
• Normal force applied from top of shear box by
  dead weights.
• Shear force applied to the side of the top half
  of the box to cause failure.

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The Direct Shear Test
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The Direct Shear Test

• In dense sand, the resisting shear stress
  increases with shear displacement until it
  reaches a failure stress ?f.
• ?f is called the peak shear strength.
• After failure, stress is attained, the resisting
  shear stress gradually decreases as shear
  displacement increases until it finally reaches a
  constant value called the ultimate shear strength.

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The Direct Shear Test

• Direct shear are repeated on similar samples at
  various normal stresses.
• The normal stresses and the corresponding values
  of ?f obtained from a number of strength
  parameters are determined.
• The following figure shows such a plot for a test
  on a dry sand.
• The equation for the average line obtained from
  experimental result is

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The Direct Shear Test
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The Direct Shear Test
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Drained Direct Shear Test on Saturated Sand and
Clay

• Shear box that contains the soil sample is kept
  inside container that filled with water.
• Drained test -- rate of loading slow enough --
  excess pore water pressure generated in soil is
  dissipated by drainage.
• F from drained direct shear test of saturated
  clay is will be the same as that for similar
  sample for dry sand.

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The Direct Shear Test
Results of a drained direct shear test on an O.C.
clay
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The Direct Shear Test
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Summary Direct Shear Test

• Advantages
• Simple to perform.
• Economical.
• Suitable for both dry or saturated sandy soil.

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Summary Direct Shear Test

• Disadvantages
• Soil is not allowed to fail along the weakest
  plane but is forced to fail along the plane of
  split of the shear box.
• The shear stress distribution over the shear
  surface of the specimen is not uniform.