Shear Strength of Soil

By

Kamal Tawfiq

Fall 2007

Soil Shear Strength

Soil in weak in shear

To determine the shear strength of soil Mohr-Coulomb Yield Criterion is used

= c + n tan

Determination of soil strength parameters ( c and ) in the lab

1- Direct Shear Test2- Unconfined Compression Test3- Triaxial Compression Test

n

c

cntan

Yield Surface

Soil CohesionAngle of Friction

= Horizontal Displacement

= N/A

Shearing Force

Normal Force

Failure

Direct Shear Test

Failure

Failure

Failure

Test # Normal Stress (n) Shear at Failure ()1 √ √ 2 √ √3 √ √

n

c

cntan

Direct Shear Test

Unconfined Compression Test

Failure

Deviator Stress

n

cu

qu

Cuqu/2

qu = Unconfined compressive strengthCu = Unconfined shear strength

TRIAXIAL COMPRESSION TEST

Failure Surface

By Kamal Tawfiq, Ph.D., P.E.

Step 1 Step 2

Triaxial Compression Test1- Consolidated Undrained Test (CU)

u

n

cu

cntan

Failure

ConfiningPressure

Deviator Stress

Step 1 Step 2

Triaxial Compression Test2- Consolidated Drained Test (CD)

d

n

cd

cntan

ConfiningPressure

Failure

Deviator Stress

Triaxial Compression Test3- Unconsolidated Undrained Test (UU)

Step 1 Step 2

n

cu

c

ConfiningPressure

Failure

Deviator Stress

Cohesion = c

1 2 n 1 2 1

2

Triaxial Compression Test

Each Circle =

One Test

Failure Surface

By Kamal Tawfiq, Ph.D., P.E.

2

+ 2

2

+

2

2

2

2

2

2

+ 2

f

2

+

2

f

= 1

= 1

3- At Failure2- Loading1- Applying ConfiningStresses

1 = Major Principle Stress 2 = Minor Prencipal Stress = deviator stress

Confining stress =

FailureSurface

Deviator stress =

Axialstress

Ceff or C

2 n 1 2 1

2

+ 2

2

+

2

2

2

2

2

2

+ 2

f

+

2

f

= 1

= 1

2

3- At Failure(Undrained)

2- Loading(Undrained)

1- Applying ConfiningStresses

(Consolidation)

Triaxial Compression TestConsolidated Undrained Compression Test

1 = Major Principle Stress 2 = Minor Prencipal Stress = deviator stress

Confining stress =

Deviator stress =

Axialstress

EffectiveStresses= eff

PoreWaterPressure( u)

Pore WaterPressure= u

Cohesion = c

ufailure ufailure

TotalStressCircle

EffectiveStressCircle

total = eff + uTotalStresses= total

By Kamal Tawfiq, Ph.D., P.E.