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Lecture-8

Shear Strength of Soils

Dr. Attaullah Shah

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Strength of different materials

Steel

Tensile strength

Concrete

Compressive strength

Soil

Shear strength

Presence of pore waterComplexbehavior

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What is Shear Strength?What is Shear Strength?

• Shear strength in soils is the resistance Shear strength in soils is the resistance to movement between particles due to to movement between particles due to physical bonds from:physical bonds from:

a.a. Particle interlockingParticle interlocking

b.b. Atoms sharing electrons at surface contact Atoms sharing electrons at surface contact pointspoints

c.c. Chemical bonds (cementation) such as Chemical bonds (cementation) such as crystallized calcium carbonate crystallized calcium carbonate

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Influencing Factors on Shear StrengthInfluencing Factors on Shear Strength• The shearing strength, is affected by:The shearing strength, is affected by:

– soil compositionsoil composition: mineralogy, grain size and grain size : mineralogy, grain size and grain size distribution, shape of particles, pore fluid type and distribution, shape of particles, pore fluid type and content, ions on grain and in pore fluid. content, ions on grain and in pore fluid.

– Initial stateInitial state: State can be describe by terms such as: : State can be describe by terms such as: loose, dense, over-consolidated, normally consolidated, loose, dense, over-consolidated, normally consolidated, stiff, soft, etc. stiff, soft, etc.

– StructureStructure: Refers to the arrangement of particles within : Refers to the arrangement of particles within the soil mass; the manner in which the particles are the soil mass; the manner in which the particles are packed or distributed. Features such as layers, voids, packed or distributed. Features such as layers, voids, pockets, cementation, etc, are part of the structure. pockets, cementation, etc, are part of the structure.

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Shear Strength of SoilShear Strength of Soil• In reality, a complete shear strength formulation In reality, a complete shear strength formulation

would account for all previously stated factors.would account for all previously stated factors.

• Soil behavior is quite complex due to the Soil behavior is quite complex due to the possible variables stated.possible variables stated.

• Laboratory tests commonly used:Laboratory tests commonly used:– Direct Shear TestDirect Shear Test– Unconfined Compression Testing.Unconfined Compression Testing.

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Soil Failure and shear strengthSoil Failure and shear strength.

• Soil failure usually occurs in the form of Soil failure usually occurs in the form of “shearing” along internal surface within the “shearing” along internal surface within the soil.soil.

• Thus, structural strength is primarily a Thus, structural strength is primarily a function of shear strength.function of shear strength.

• Shear strength is a soils’ ability to resist Shear strength is a soils’ ability to resist sliding along internal surfaces within the sliding along internal surfaces within the soil mass.soil mass.

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Slope Stability: Failure is an Slope Stability: Failure is an Example of Shearing Along Example of Shearing Along

Internal SurfaceInternal Surface

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Mass Wasting: Shear FailureMass Wasting: Shear Failure

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Shear Failure: Earth DamShear Failure: Earth Dam

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Shear Failure Under Foundation Shear Failure Under Foundation LoadLoad

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Shear failureSoils generally fail in shear

strip footing

embankment

At failure, shear stress along the failure surface reaches the shear strength.

failure surface

mobilized shear resistance

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Shear failure

The soil grains slide over each other along the failure surface.

No crushing of individual grains.

failure surface

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Shear failure mechanism

At failure, shear stress along the failure surface () reaches the shear strength (f).

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Retaining wall

Shear failure of soilsSoils generally fail in shear

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Retaining wall

Shear failure of soils

At failure, shear stress along the failure surface (mobilized shear resistance) reaches the shear strength.

Failure surface

Mobilized shear resistance

Soils generally fail in shear

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Mohr-Coulomb Failure Criterion

tancf

c

failure envelope

cohesion

friction angle

f is the maximum shear stress the soil can take without failure, under normal stress of .

f

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Mohr-Coulomb Failure Criterion(in terms of total stresses)

f is the maximum shear stress the soil can take without failure, under normal stress of .

tancf

c

failure envelope

Cohesion

Friction anglef

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Mohr-Coulomb Failure Criterion(in terms of effective stresses)

f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.

’

'tan'' cf

c’

’

failure envelope

Effective

cohesion

Effectivefriction angle

f

’

u '

u = pore water pressure

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Mohr-Coulomb Failure Criterion

'tan'' ff c

Shear strength consists of two components: cohesive and frictional.

’f

f

’

'

c’ c’cohesive

component

’f tan ’ frictional component

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Mohr-Coulomb Failure Criterion

tanff c

Shear strength consists of two components: cohesive and frictional.

f

f

c

f tan

c cohesive

component

frictional component

c and are measures of shear strength.

Higher the values, higher the shear strength.

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Other laboratory tests include,Direct simple shear test, torsional ring shear test, plane strain triaxial test, laboratory vane shear test, laboratory fall cone test

Determination of shear strength parameters of soils (c, orc’’

Laboratory tests on specimens taken from representative undisturbed samples

Field tests

Most common laboratory tests to determine the shear strength parameters are,

1.Direct shear test2.Triaxial shear test

1. Vane shear test2. Torvane3. Pocket penetrometer4. Fall cone5. Pressuremeter6. Static cone penetrometer7. Standard penetration test

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Laboratory tests

Field conditions

z vc

vc

hchc

Before construction

A representative soil sample

z vc +

hchc

After and during construction

vc +

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Laboratory tests

Simulating field conditions in the laboratory

Step 1

Set the specimen in the apparatus and apply the initial stress condition

vc

vc

hchc

Representative soil sample taken from the site

0

00

0

Step 2

Apply the corresponding field stress conditions

vc +

hchc

vc + Traxial t

est

vc

vc

Direct shear test

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Direct shear testSchematic diagram of the direct shear apparatus

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Direct shear test

Preparation of a sand specimen

Components of the shear box Preparation of a sand specimen

Porous plates

Direct shear test is most suitable for consolidated drained tests specially on granular soils (e.g.: sand) or stiff clays

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Direct shear test

Leveling the top surface of specimen

Preparation of a sand specimen

Specimen preparation completed

Pressure plate

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Direct shear test

Test procedure

Porous plates

Pressure plate

Steel ball

Step 1: Apply a vertical load to the specimen and wait for consolidation

P

Proving ring to measure shear force

S

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Direct shear test

Step 2: Lower box is subjected to a horizontal displacement at a constant rate

Step 1: Apply a vertical load to the specimen and wait for consolidation

PTest procedure

Pressure plate

Steel ball

Proving ring to measure shear force

S

Porous plates

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Direct shear test

Shear box

Loading frame to apply vertical load

Dial gauge to measure vertical displacement

Dial gauge to measure horizontal displacement

Proving ring to measure shear force

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Direct shear testAnalysis of test results

sample theofsection cross of Area

(P) force Normal stress Normal

sample theofsection cross of Area

(S) surface sliding at the developed resistanceShear stressShear

Note: Cross-sectional area of the sample changes with the horizontal displacement

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Direct shear tests on sands

Sh

ear

str

ess

,

Shear displacement

Dense sand/ OC clayfLoose sand/ NC clayf

Dense sand/OC Clay

Loose sand/NC Clay

Ch

ang

e in

hei

gh

t o

f th

e sa

mp

le Exp

ansi

on

Co

mp

ress

ion Shear displacement

Stress-strain relationship

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f1

Normal stress = 1

Direct shear tests on sandsHow to determine strength parameters c and

Sh

ear

stre

ss,

Shear displacement

f2

Normal stress = 2

f3

Normal stress = 3

Sh

ear

stre

ss a

t fa

ilu

re,

f

Normal stress,

Mohr – Coulomb failure envelope

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Direct shear tests on sandsSome important facts on strength parameters c and of sand

Sand is cohesionless hence c = 0

Direct shear tests are drained and pore water pressures are dissipated, hence u = 0

Therefore,

’ = and c’ = c = 0

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Direct shear tests on clays

Failure envelopes for clay from drained direct shear tests

Sh

ear

stre

ss a

t fa

ilu

re,

f

Normal force,

’

Normally consolidated clay (c’ = 0)

In case of clay, horizontal displacement should be applied at a very slow rate to allow dissipation of pore water pressure (therefore, one test would take several days to finish)

Overconsolidated clay (c’ ≠ 0)

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Interface tests on direct shear apparatusIn many foundation design problems and retaining wall problems, it is required to determine the angle of internal friction between soil and the structural material (concrete, steel or wood)

tan' af c Where,

ca = adhesion,

= angle of internal friction

Foundation material

Soil

P

S

Foundation material

Soil

P

S

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Triaxial Shear Test

Soil sample at failure

Failure plane

Porous stone

impervious membrane

Piston (to apply deviatoric stress)

O-ring

pedestal

Perspex cell

Cell pressureBack pressure Pore pressure or

volume change

Water

Soil sample

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Triaxial Shear Test

Specimen preparation (undisturbed sample)

Sampling tubes

Sample extruder

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Triaxial Shear Test

Specimen preparation (undisturbed sample)

Edges of the sample are carefully trimmed

Setting up the sample in the triaxial cell

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Triaxial Shear Test

Sample is covered with a rubber membrane and sealed

Cell is completely filled with water

Specimen preparation (undisturbed sample)

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Triaxial Shear Test

Specimen preparation (undisturbed sample)

Proving ring to measure the deviator load

Dial gauge to measure vertical displacement

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Unconfined Compression Test (UC Test)

1 = VC +

3 = 0

Confining pressure is zero in the UC test

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Unconfined Compression Test (UC Test)

1 = VC + f

3 = 0

Sh

ear

stre

ss,

Normal stress,

qu

τf = σ1/2 = qu/2 = cu

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The EndThe End