Shear Strength of soil and Tests on soil

Shear Strength of Soil

COSC 323: Soils in Construction

Shear Failure

Shear Strength due to Sliding Friction

Normal Force N

Shearing Force S

Failure Plane

Shear Strength due to sliding friction

SlidingForce

Normal Force

Shear Strength due to sliding friction

Normal Force

SlidingForce

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

Shear Strength due to Sliding Friction

Normal Stress (s)

Shea

r Stre

ss (s

)

Angle of internal friction (f)

tantan

tan

css

s

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

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

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

Unconfined compression test

2uqc

SoilSpecimen

uq

uq

cohesion :strength ecompressiv unconfined:

cqu

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?

Direct Shear Test

Normal Load

Sharing Force

Sharing Force

Normal Load

Sharing Force

Sharing Force

Soil Specimen

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

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

Triaxial Compression Test

Wrap the specimen with rubber membrane


Enclose the specimen in a chamber filled with water


s3

Apply a specific pressure using water

s3

s3

s3


s3

s3

p

Apply a vertical load and increase until the specimen fails

Use different lateral pressure, conduct the same experiments3

s3

p


s3

p

p s3 : Minor principal stress

Dp : Deviator stress at failure axial = (Load at failure / cross-sectional area)

s3

s3

s3


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


s3s3

s3

s3

s3s3

s1= s3+Dp

s1= s3+Dp

Before After


(s1)1

(s3)1 (Dp)1

Normal Stress

First TestShear Stress

A B


Shear Stress

Second Test

(s1)1

(s3)1 (Dp)1

(s3)2 (Dp)2

(s1)2

Normal Stress

A B C D


Shear Stress

Normal Stress

(s1)1

(s3)1 (Dp)1

(s3)2 (Dp)2

(s1)2

Strength Envelop

A B C D


c

F

Shear Stress

Normal Stress

(s1)1

(s3)1 (Dp)1

(s3)2 (Dp)2

(s1)2

Strength Envelop

A B C D

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

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

Example

0 2 4 6 8 10 12 14

2

4

6

8

Example

0 2 4 6 8 10 12 14

2

4

6

8

4

2

2

1

/9.0

2642tan

42tan

ftkipc

Variations in Shear Test Procedures

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

Shear Strength of soil and Tests on soil

Engineering

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