Shear

Strength

of Soil

Stability of Slopes Controlled by Shear Strength of Soils

Failure due to inadequate

strength at shear interface

Stability of Slopes Controlled by Shear Strength of Soils

Bearing Capacity Controlled by Shear Strength of Soils

Transcosna Grain ElevatorCanada (Oct. 18, 1913)

West side of foundation sank 24-ft

Shear Strength of SoilsCohesion and Friction

• Soil derives its shear strength from two

sources:

– Cohesion between particles (stress

independent component)

• Cementation between sand grains

• Electrostatic attraction between clay particles

– Frictional resistance between particles

(stress dependent component)

Shear Strength of Soils Cohesion

• Cohesion comes from

cementation or

electrostatic attraction

between particles

• This component of shear

strength is independent of

normal stress on shear

plane

• Apparent cohesion comes

from negative pore water

pressure

• Moist beach sand has apparent cohesion

• Negative pore water pressures

Apparent Cohesion

u

tancs

• Unsupported Cut

Shear Strength of Soils Cohesion

Shear Strength of Soils Internal Friction

• Friction is dependent of

normal stress on shear

plane

• Larger the normal stress

() larger the shear

strength

• Sand, gravel materials

derive their strength

almost entirely from

friction

• MSE Wall Backfill

Shear Strength of Soils Internal Friction

tancs

friction internal of angle

cohesionc

Shear Strength of Soils Mohr-Coulomb Failure Criterion

Shear

Strength,

S

Normal Stress,

C

Shear Strength of Soils Mohr-Coulomb Failure Criterion

Typical Values

Shear Strength of Soil

• Shear strength is the internal resistance to failure and sliding along any plane in a soil mass

• Soils fail because of a critical combination of normal stress and shear stress

• Mohr-Coulomb Failure Criteria

tf= c + tan

Mohr-Coulomb Failure Criteria

Mohr-Coulomb Failure Criteria

No Shear Failure

State of Failure

Not Possible

Mohr’s Circle and Failure

Envelope

t 2sin2

131 f

2cos2

1

2

13131 f

State of Failure

Inclination of Failure Plane

= 45 + /2

Mohr’s Circle and Failure

Envelope

’1= ’3 tan2(45+/2) + 2c tan(45+/2)

Measuring Shear Strength

In the Laboratory

Direct shear test

Unconfined compression test

Triaxial compression test

In the Field

Vane shear test

Standard Penetration Test (SPT)

Cone Penetration Test (CPT)

Direct Shear Test

Direct Shear Test Device

Direct Shear Test Device

Direct Shear Testers

Direct Shear Test

Example 1Direct Shear Test

Given:

A direct shear test conducted on a soil sample

yielded the following results:

Normal Stress,

(psi)

Max. Shear

Stress, S (psi)

10.0 6.5

25.0 11.0

40.0 17.5Required:

Determine shear strength parameters of the soil

0

5

10

15

20

0 10 20 30 40 50

Normal Stress (psi)

Max.

Sh

ear

Str

ess (

psi)

20)365.0(tan

365.048

)5.220(tan

5.2

1

psic

Example 1Direct Shear Test

Typical Test Results

Use of Test Results

Drained Shear Test Results

Overconsolidated Clay

Failure Envelopes for Clays

Foundations Behaviors

Foundation Model Tests

Typical Test Results

Triaxial Compression Test

• Unconfined compression

test is used when = 0

assumption is valid

• Triaxial compression is a

more generalized version

• Sample is first compressed

isotropically and then

sheared by axial loading

1

3

Triaxial Compression Test

Triaxial Compression Test

Triaxial Compression Test2 Stages of Loading

3

1= 3+

Apply Confining

Pressure Apply Axial Load

3

3

3

1

t

13

Vert. plane

Triaxial Compression TestInterpretation of Data

Triaxial Shear Test

Consolidated-Drained Tests

C-D Tests

Triaxial Test on Sand -

Figures

45 psi

30 psi

15 psi

c’

b’

a’

Effective Stress Failure

Envelope (Sand, NC Clay)

Effective Stress Failure

Envelope (OC Clay)

Consolidated-Undrained Tests

(C-U Test)• Drainage allowed during initial

consolidation due to 3

• Drainage port closed during application of

d

• Pore pressures monitored during testing

Consolidated-Undrained Tests

(C-U Test)

Unconsolidated-Undrained

Tests (U-U Test)• Drainage not allowed during initial loading

to 3

• Drainage port closed during application of

d

• Pore pressures not monitored during

testing

Unconsolidated-Undrained

Tests (U-U)

Unconfined Compression Test

Unconfined Compression Test

Equipment

Unconfined Compression

Test ASTM D-2166; AASHTO 208

• For clay soils

• Cylindrical Test

specimen

• No confining stress

(i.e. 3 = 0)

• Axial stress = 1

3 = 0

1

Unconfined Compression TestFailure Occurs by Shear!

3=0

1

Failed test

sample

Unconfined Compression TestInterpretation of Data

3=0

1 t

1

Horizontal

plane

3

Vertical

plane

Unconfined Compression Test

Data

c

c

A

P

AA

l

l

1

0

0

2

uu

u

qStrngthShearUndrainedS

StrengthnCompressioUnconfinedq

Unconfined Compression Test

qu= at failure

C=Su

Unconfined Compression

Test Example 2

Given:

An unconfined compression test conducted on a

soil sample yielded the results shown in the

table.

Required:

Determine undrained shear strength, Su of the

soil

psiA

P

inA

A

l

l

c

c

45.437376.1

48.75

7376.1)1143.01(

539.1

1

1143.0

20

0

Unconfined Compression

Test Example 2

qu= 43.45psi=6257 psfSu= 21.7psi = 3128 psf

Unconfined Compression

Test Example 2

Sensitivity of Clays

Vane Shear Test

Lab Vane Shear Device

Other Lab Devices

PocketPenetrometer Torvane

Device

In Situ Undrained Shear

Strength (cu / ’o)

Soil Response under LoadingSands and Gravels

• Excess pore pressure dissipates immediately

• Pore pressure remains at hydrostatic value

• changes but can be calculated; = -u

• Therefore, use S = c’+ tan

Drained Conditions!

Soil Response under LoadingClayey Soils

• Excess pore pressure builds up as soil is loaded

• Pore pressure cannot be determined; u = u0+ue

• remains at initial value (S=100%, no drainage)

• Therefore, use S = Su ; c = Su and = 0

Undrained Conditions!

• Drained conditions occur when rate at

which loads are applied are slow

compared to rates at which soil material

can drain

• Sands drain fast; therefore under most

loading conditions drained conditions

exist in sands

• Exceptions: pile driving, earthquake

loading in fine sands

Soil Shear Strength under Drained

and Undrained Conditions ….

• In clays, drainage does not occur quickly;

therefore excess pore water pressure

does not dissipate quickly

• Therefore, in clays the short-term shear

strength may correspond to undrained

conditions

• Even in clays, long-term shear strength is

estimated assuming drained conditions

Soil Shear Strength under Drained

and Undrained Conditions ….

Shear Strength in terms of

Total and Effective Stresses• Shear Strength in terms of effective stress

• Shear strength in terms of total stress

u

tancs

tan cs

u at hydrostatic value

Shear Strength in terms of Total

Stress; = 0 condition• Shear strength in terms of total stress

• For cohesive soils under saturated

conditions, = 0.

tan cs

csu

Normal Stress,

Shear

Strength,

S

C=Su

= 0

Fully Undrained ConditionsMohr-Coulomb Failure Criterion

Triaxial Compression TestDetermining C and

• Consolidated Undrained Test (CU-

Test)

• Consolidated Drained (CD-Test); Also

called “Drained Test”

Triaxial Compression TestDetermining C and

Triaxial Compression TestDetermining C and