Shear Strength of Soil

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Shear Strength of Soil

• COSC 323 Soils in Construction

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Shear Failure
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Shear Failure
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Shear Failure
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Shear Strength due to Sliding Friction
Normal Force N
Shearing Force S
Failure Plane
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Shear Strength due to sliding friction
Normal Force
Sliding Force
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Shear Strength due to sliding friction
Normal Force
Sliding Force
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Shear Strength due to Sliding FrictionNormal
Stress vs. Shear Stress
Normal Stress (s) Shear Stress (f)
10 kPa (kN/m2) 6 kPa
20 kPa 12 kPa
30 kPa 18 kPa
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Shear Strength due to Sliding Friction
Shear Stress (s)
Angle of internal friction (f)
Normal Stress (s)
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Coulomb Equation
Shear strength
Cohesion (Strength gained from the ionic bound
between grain particles)
Effective intergranular normal pressure (perpendic
ular to the shear plane)
Angle of internal friction (Strength gained from
internal friction resistance)
Coefficient of friction
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Shear strength of soil

• Soil cannot resist tension
• Soil can resist compression.
• For excessive compression failure occurs in the
  form of shearing along the internal surface
  within the soil
• Structural Strength of soil f ( soils shear
  strength)
• Shear Strength
• Soils ability to resist sliding
• Important for
• foundation design
• Lateral earth pressure calculations
• Slope stability analysis

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Methods of investigating shear strength

• Unconfined compression test (for cohesive soil)
• Direct shear test
• Triaxial compression test
• Vane test (for soft clay)
• Standard penetration test (for cohesionless soil)
• Penetrometer test

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Unconfined compression test
SoilSpecimen
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Example

• A clayey soil subjected to an unconfined
  compression test fails at a pressure of 2540
  lb/ft2 (i.e., qu 2540 lb/ft2). What is cohesion
  of this clayey soil?

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Direct Shear Test
Normal Load
Sharing Force
Soil Specimen
Sharing Force
Normal Load
Sharing Force
Sharing Force
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Direct Shear Test
Normal Load
Sharing Force
Soil Specimen
Normal Stress Normal load / the specimens
cross-sectional area Shear stress Shearing
Force / the specimens cross- sectional area The
graph can be used to determine the given soils
shear strength for any load Problem Shear
failure is forced to occur along a predetermined
plane, which is not necessarily the weakest plane
of the soil specimen tested.
Test with different Normal Load
Shear Stress
f
c
Normal Stress
f angle of internal friction c cohesion
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Example

• A series of direct shear tests was performed on a
  soil sample. Each test was carried out until the
  soil specimen experienced shear failure. The test
  data are listed next. What is soils cohesion and
  angle of internal friction?

Specimen Number Normal Stress (lb/ft2) Shearing Stress (lb/ft2)
1 604 1522
2 926 1605
3 1248 1720
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Triaxial Compression Test
Wrap the specimen with rubber membrane
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Triaxial Compression Test
Enclose the specimen in a chamber filled with
water
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Triaxial Compression Test
Apply a specific pressure using water
s3
s3
s3
s3
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Triaxial Compression Test
Apply a vertical load and increase until the
specimen fails Use different lateral pressure,
conduct the same experiment
s3
s3
s3
s3
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Triaxial Compression Test
s3 Minor principal stress Dp Deviator
stress at failure axial (Load at failure /
cross-sectional area)
s3
s3
s3
s3
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Triaxial Compression Test
s3 Minor principal stress Dp Deviator
stress at failure axial (Load at failure /
cross-sectional area) s1 s3 Dp major
principal stress How to obtain cohesion and
angle of internal friction?
s3
s3
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Triaxial Compression Test
s3
s1 s3Dp
s3
s3
s3
s3
s3
s1 s3Dp
After
Before
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Triaxial Compression Test
First Test
Shear Stress
A
B
Normal Stress
(s3)1
(Dp)1
(s1)1
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Triaxial Compression Test
Second Test
Shear Stress
A
B
C
D
Normal Stress
(s3)1
(Dp)1
(s1)1
(s3)2
(Dp)2
(s1)2
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Triaxial Compression Test
Shear Stress
Strength Envelop
A
B
C
D
Normal Stress
(s3)1
(Dp)1
(s1)1
(s3)2
(Dp)2
(s1)2
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Triaxial Compression Test
Shear Stress
F
Strength Envelop
c
A
B
C
D
Normal Stress
(s3)1
(Dp)1
(s1)1
(s3)2
(Dp)2
(s1)2
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Example

• Gien
• Triaxial compression tests on three specimens of
  a soil sample were performed. Each test was
  carried out until the specimen experienced shear
  failure. The test data are tabulated as follows
• Required
• The soils cohesion and angle of internal friction

Specimen Number Minor Principal Stress (kips/ft2) Deviator Stress at Failure (kips/ft2)
1 1.44 5.76
2 2.88 6.85
3 4.32 7.50
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Example
Specimen Number Minor Principal Stress (kips/ft2) Deviator Stress at Failure (kips/ft2) Major Principal Stress (kips/ft2)
1 1.44 5.76 7.2
2 2.88 6.85 9.73
3 4.32 7.50 11.82
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Example
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Variations in Shear Test Procedures

• Unconsolidated Undrained (UU) Q test
• Consolidated Undrained (CU)
• Consolidated Drained (CD) S test