shear strength of soil and tests on soil
TRANSCRIPT
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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
SlidingForce
Normal Force
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Shear Strength due to sliding friction
Normal Force
SlidingForce
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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
Normal Stress (s)
Shea
r Stre
ss (s
)
Angle of internal friction (f)
tantan
tan
css
s
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Coulomb Equation
: tan
:
:
:c:
σ
s
Effective intergranular normal pressure(perpendicular to the shear plane)
Shear strengthCohesion(Strength gained from the ionic bound between grain particles)
Angle of internal friction(Strength gained from internal friction resistance)
Coefficient of friction
tancs
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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 ( soil’s shear strength)• Shear Strength
– Soil’s 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
2uqc
SoilSpecimen
uq
uq
cohesion :strength ecompressiv unconfined:
cqu
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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
Sharing Force
Normal Load
Sharing Force
Sharing Force
Soil Specimen
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Direct Shear Test
Soil Specimen
Normal Load
Sharing Force
Normal Stress
Shear Stress
c
Normal Stress = Normal load / the specimen’s cross-sectional area
Shear stress = Shearing Force / the specimen’s cross- sectional area
The graph can be used to determine the given soil’s 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
angle of internal frictionc: 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 soil’s 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
s3
Apply a specific pressure using water
s3
s3
s3
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Triaxial Compression Test
s3
s3
p
Apply a vertical load and increase until the specimen fails
Use different lateral pressure, conduct the same experiments3
s3
p
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Triaxial Compression Test
s3
p
p s3 : Minor principal stress
Dp : Deviator stress at failure axial = (Load at failure / cross-sectional area)
s3
s3
s3
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Triaxial Compression Test
s3
p 31 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
p 31
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Triaxial Compression Test
s3s3
s3
s3
s3s3
s1= s3+Dp
s1= s3+Dp
Before After
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Triaxial Compression Test
(s1)1
(s3)1 (Dp)1
Normal Stress
First TestShear Stress
A B
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Triaxial Compression Test
Shear Stress
Second Test
(s1)1
(s3)1 (Dp)1
(s3)2 (Dp)2
(s1)2
Normal Stress
A B C D
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Triaxial Compression Test
Shear Stress
Normal Stress
(s1)1
(s3)1 (Dp)1
(s3)2 (Dp)2
(s1)2
Strength Envelop
A B C D
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Triaxial Compression Test
c
F
Shear Stress
Normal Stress
(s1)1
(s3)1 (Dp)1
(s3)2 (Dp)2
(s1)2
Strength Envelop
A B C D
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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 soil’s cohesion and angle of internal friction
SpecimenNumber
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
SpecimenNumber
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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2642tan
42tan
ftkipc
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Variations in Shear Test Procedures
• Unconsolidated Undrained (UU) – Q test• Consolidated Undrained (CU)• Consolidated Drained (CD) – S test