Geotechnical Engineering

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UNIVERSITI MALAYSIA PAHANG Department of Civil
and Environmental Engineering

• Geotechnical Engineering
• BAA 3513
• Chapter 1 Shear Strength Part 5

Muzamir bin Hasan, M.Eng. Lecturer
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Example 7.3 (page 188)

• Two similar clay soil specimens were
  preconsolidated in triaxial equipment under a
  chamber pressure of 600 kN/m2. Consolidated-draine
  d triaxial tests were conducted on these two
  specimens. Following are the results of the
  tests Specimen 1s3100kN/m2
• (?sd)f410.6kN/m2
• Specimen 2s350kN/m2
• (?sd)f384.37kN/m2
• Determine the shear strength parameters for the
  samples.

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Solution
Specimen 1
Specimen 2
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Solution
Specimens are overconsolidated, so
Specimen 1
Specimen 2
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Solution
By subtracting the equations
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Solution
Substituting ø112
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Unconsolidated-Undrained (UU) Test

• u uc ud but since uc B s3 and ud A sd so
  therefore, u B s3 A sd B s3 A (s1 - s3)
• This test is conducted on clay specimens, and
  depends on a very important strength
  characteristic if the soil is fully saturated.
• The failure envelope become horizontal, and hence
  f 0 and ?f cu where cu is the undrained shear
  strength and is equal to the radius of the Mohrs
  circles.

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Unconsolidated-Undrained (UU) Test
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Unconsolidated-Undrained (UU) Test

• The reason for obtaining the same added axial
  stress (sd)f regardless of the confining pressure
  can be explained as follows.
• If a sample (No. 1) is consolidated at a chamber
  pressure s3 and then sheared to failure without
  allowing drainage, the total stress conditions at
  failure can be represented by Mohrs circle P in
  the figure.
• The pore pressure developed in the sample at
  failure is equal to (ud)f. Thus, the major and
  minor principal effective stresses at failure
  are,
• s1 s3 (sd)f - (ud)f s1 - (ud) f and s3
  s3 - (ud) f

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Unconsolidated-Undrained (UU) Test
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The Unconfined Compression Test

• A special type of UU test that is commonly used
  for clay specimen.
• s3 0
• Axial load rapidly applied to the specimens to
  cause failure.
• At failure the total minor principal stress is
  zero and total major stress is s1

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mm
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The Unconfined compressive (UC) test on
saturated clays
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General Relationship of Consistency and
Unconfined Compression Strength of Clays
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The Stress Path Concept

• A convenient way of plotting triaxial test data
  is through diagrams called stress paths.
• A stress path is a line that connects a series
  of points, each of which represents a successive
  stress states experienced during a test.

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The Stress Path Concept

• Lamb (1964) suggested a new coordinate system of
  p versus q where,

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The Stress Path Concept

• Consider a normally consolidated clay subjected
  to an isotropically consolidated-drained triaxial
  test.
• At the beginning of the test, the deviatoric
  stress s1 s3 s3, so

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The Stress Path Concept

• and the p and q will plot as point I in the
  figure on the next slide. As the load is
  increased, the deviatoric stress s1 s3 ltsd
  and s3 s3, and the Mohr circle A corresponds
  to that state.

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The Stress Path Concept

• These two equations plot as point D at the top
  of the Mohr circle A.
• Each successive plot of p and q will continue
  this straight line that is at an angle of 45
  with respect to the p axis.
• Eventually we will reach the point D on the Mohr
  circle B, and the sample fails (it has reached
  the modified failure envelope).
• From this, q ' p ' tana

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Sensitivity and Thixotropy of Clays

• For naturally deposited clay soils, unconfined
  compression strength is reduced greatly when the
  soils are tested after remolding without any
  change in moisture content.
• This property is called sensitivity.

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Sensitivity and Thixotropy of Clays
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Sensitivity and Thixotropy of Clays

• The sensitivity ratio of most clays ranges from
  about 1 to 8 however, highly flocculent marine
  clay deposits may show sensitivity ratios ranging
  from about 10 to 80.
• There are some clays that turn to viscous fluids
  upon remolding.
• These are found mostly in the previously
  glaciated areas of North America and Scandanavia.
  
• These clays are referred to as quic clays.

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Sensitivity and Thixotropy of Clays

• Rosenqvist (1953) classified clays on the basis
  of their sensitivity.

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Sensitivity and Thixotropy of Clays

• The loss of strength of clay soils due to
  remolding is primarily due to the destruction of
  the clay particle structure that was developed
  during the original process of sedimentation.
• If, however, after remolding a soil specimen is
  kept in an undisturbed state (that is, without
  any change of moisture content), it will continue
  to gain strength with time.
• This phenomenon is referred to as thixotropy.

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Homework

• Problems 7.14
• Submit next class