topic 1-shear strength (3).pptx

7/28/2019 TOPIC 1-SHEAR STRENGTH (3).pptx

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TOPIC 1 :

SHEAR STRENGTH OF SOIL

FACULTY OF CIVIL ENGINEERING & EARTH RESOURCES


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Mohr-Coulomb criterion*

Laboratory test for shear strength parameters *

- Direct Shear Test- Triaxial Test

- Vane Shear Test

Effective overburden pressure*

DAA3513 : GEOTECHNICAL ENGINEERING

TOPIC OUTLINE


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LEARNING OUTCOMES

Understand the concept of shearstrength

Briefly explain Mohr-Coulombcriterion and the relationship to the

soil shear strengthDetermine the soil shear strength

using various methods


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Soils derive their strength from contact between particlescapable of transmitting normal as well as shear forces. Thecontact between soil particles is mainly due to friction and thecorresponding stress between the soil grains is called theeffective (or inter-granular) stress .

Thus, the shear strength of a soil is mainly governed by theeffective stress. Besides the effective stress between soil grains,the pore water contained in the void spaces of the soil also exertspressure which is known as pore pressure, u.

The sum of the effective stress and pore pressure acting an anygiven surface within a compacted earth embankment is calledthe total stress .

GEOTECHNICAL ENGINEERING


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Shear strength of the soil is the internal frictionresistance per unit area that the soil mass can offer toresist failure and sliding along any plane inside.

The capability of the following comes from the soilshear strength :

Support loading from structure

Support its own overburden

Sustain slope in equilibrium



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WHY THE SHEAR STRENGTHPARAMETER IS IMPORTANT??

- As an important property to evaluate:

1) Bearing capacity of foundation

2) Stability of slope3) Design of a dam, embankment andect.

4) Lateral pressure onearth retaining

structure



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Soil derives its shear strength from two sources:

Cohesion between particles (stress independent

component) - c

Cementation between particle grains

Electrostatic attraction between clay particles

Predominant in clayey soils

Frictional resistance between particles (stressdependent component) -

Strength gained from internal frictional resistance

(interlocking action among soil particles)

Predominant in granular soils


COHESION vs FRICTION


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COHESION


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INTERNAL FRICTION


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Shear strength is not aunique property of a soil butdepends on many factors.

Factors ??



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1. Mineralogy of grains

2. Particle shape, size distribution and configuration

3. Void ratio and water content

4. Previous stress history

5. Existing stress in-situ6. Stress change imposed during sampling

7. Initial state of the sample

8. Stresses applied prior to test

9. Rate at which loading is applied10. whether drainage is allowed during test

11. Resulting pore pressure

12. Criterion adopted for determining the shear strength


FACTORS THAT INFLUENCE SHEAR STRENGTH


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Peak strength

Residual strength

Point of failurePoint of failure

Residual strength

Peak strength

** normal stress is constant DAA3513 : GEOTECHNICAL ENGINEERING


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tsn

The limiting shear stress (soil strength) is given by :

t = c + s tan where

c = cohesion

= angle of internal friction


MOHR COULOMB CRITERION


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Charles Mohr Charles Coulomb


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Shear

Strength, t

Normal Stress, s= s

C

=

Gradient of the line

Interception of y-axis


Mohr-Coulomb

envelope

REPRESENTATION OF SHEARSTRENGTH OF SOIL


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'tan' st c

c and are known as the effective (or drained) strength

parameters.

If the soil is at failure the effective stress failure criterion will

always be satisfied.

Soil behavior is controlled by effective stresses, and the effective

strength parameters are the fundamental strength parameters. But

they are not necessarily soil constants.


EFFECTIVE STRESS FAILURECRITERION

Where, s = s -u and u ispore water pressure


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t s cu n utan

If the soil is taken to failure at constant volume (undrained) then

the failure criterion can be written in terms of total stress as

cu and u are known as the undrained strength parameters

These parameters are not soil constants, they depend stronglyon the moisture content of the soil.


TOTAL STRESS FAILURE CRITERION


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LETS TRY THIS

t

t

s'

s'c

This soil would probably be..??

Sandy/course grain soil??

Clayey/ fine grained soil??


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How can wedetermine the soil

shear strength??



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Laboratory test for testing shear strength of soil are:

1. Direct shear test

2. Triaxial compression test

3. Unconfined compression test

Field test for testing shear strength of soil are:

1. Cone penetration test

2. Standard penetration test

3. Vane shear test

4. Dilatometer test

For cohesive &cohesionless soil

For cohesive soil only



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DIRECT SHEAR TEST (ASTM D3080)

Motor

drive

Load cell to

measure

Shear Force

Normal load

Rollers

Soil

Porous plates

Top platen

Measure relative horizontal displacement, dx

vertical displacement of top platen, dy



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PROCEDURES

Soil specimen (round or square) is placed in arelatively flat box and subjected to a verticalload

The box consists 2 parts (upper and lower)

If 1 part of the box held while another onebeing pushed, the specimen will experienceshear failure along the horizontal surface

Vertical load and shear force that induced thefailure of the specimen is recorded

The failed sample is discarded and anothersample is placed in the box

Experiment is repeated several time


T pical drained direct shear


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Typical drained direct shear

results

Horizontal displacement (dx)

ShearLoad(F)

Normal load

increasing


Shear load, FVS

Displacement, dx


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Shearstress



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1. Draw suitable axes (x and y) on a graph paper. The

same scale should be used for both axes

2. Using the axes, plot shear stress vs normal stress

3. Draw a best fit line connecting all the pointsplotted

4. c value obtained from the y-axis while value

obtained from the gradient of the strengthenvelope



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Based on the plot, the following parameters obtained :

1) Cohesion, c interception at y-

axis2) Angle of internal friction, =

gradient of the straight line



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Advantages of direct shear test

Most easiest and quickest test

Large samples may be tested in large shear boxes.Small samples may give misleading results due to

imperfections (fractures and fissures) or the lack ofthem.

Samples may be sheared along predeterminedplanes. This is useful when the shear strengthsalong fissures or other selected planes are required.



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The shear failure is forced to occur along or across apredetermined plane which is not necessarily the weakestplane of the soil specimen tested

Non-uniform deformations and stresses in the specimen. Thestress-strain behavior cannot be determined. The estimatedstresses may not be those acting on the shear plane.

In practice shear box tests are used to get quick and crudeestimates of failure parameters

Since development of the much better triaxial test, the usedof the direct shear test has decreased

Disadvantages of direct shear test



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EXAMPLE

A direct shear test conducted on a soil sampleyielded the following results:

Normal Stress, s

(psi)

Max. Shear Stress, S

(psi)

10.0 6.5

25.0 11.0

40.0 17.5

Determine shear strength parameters of

the soil



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0

5

10

15

20

0 10 20 30 40 50

Normal Stress (psi)

Max.

ShearStres

s(psi

6.20)375.0(tan

375.0)1050(

)0.520(tan

5.2

1

psic

Lets try examples given

A

NSWE

R:


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TRIAXIAL TEST (ASTM D2850)

Cell

pressure Pore pressure

and volume

change

Rubbermembrane

Cell water

O-ring

seals

Porous filter

disc

Confining

cylinder

Deviator load

Soil



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PROCEDURES

Cylindrical soil specimen (wrapped in a rubbermembrane) is subjected to a vertical load

This test require the present of the confining / lateralpressure

which achieved by introducing water or compressed airinto the chamber to surround the soil specimen

The vertical load is applied and steadily increased untilthe specimen fails

The vertical load and the lateral pressure that cause thespecimen failed is recorded

The test is repeated several times



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TYPES OF FAILUREIN TRIAXIAL TEST

1. Complete shear failure2.Partial shear failure

3.Barrel or plastic failure



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Stresses in triaxial specimens

sr sr = Radial stress (cellpressure)

sa = Axial stress

F = Deviator load

sr

s sa rF

A

From equilibrium we have


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F/A is known as the deviator stress, and is given the

symbol q

q a r ( ) ( )s s s s1 3

where

s1 = axial pressure / major principal stress

s3 = lateral pressure / minor principal stress



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By plotting s1 and s3 in a graph, we can determine :

1. Cohesion, c

2. Angle of internal friction, HOW.??

Remember!!

s1 = axial pressure / major

principal stress

s3 = lateral pressure / minor

principal stress


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1. Draw suitable axes (x and y) in a graphpaper. The same scale should be used forboth axes

2. Using s1 and s3 values obtained fromseveral triaxial tests, draw a semicircle

3. Draw a straight tangent line (touch both

semicircles, if possible) to the semicircles

4. The straight line is called the StrengthEnvelope @ Failure Envelope @ MOHRS

Envelope5. c value obtained from the y-axis while

value obtained from the gradient of thestrength envelope



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Types of triaxial test


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There are many test variations. Those used most in practiceare:

UU (unconsolidated undrained) test.

Cell / lateral pressure is applied without allowingdrainage. Then keeping cell pressure constant, increasedeviator / axial load to failure without drainage.

CU (isotropically consolidated undrained) test.

Drainage allowed during cell pressure application. Thenwithout allowing further drainage increase deviator /

axial load, keeping cell pressure constant as for UU test.

CD (isotropically consolidated drained) test

Similar to CU except that as deviator stress is increased

drainage is permitted.

Types of triaxial test


T t t St 1 St 2


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Test type Step 1 Step 2

Consolidated

drained,

CD

Apply chamber pressure, s3.

Allow complete drainage, so

pore water pressure (u = u0)

developed is zero

Apply axial stress, s, slowly. Allow

drainage, so pore water pressure (u =

ud) developed through application of

s is zero. At failure, s = sf ;

total pore water pressure, uf= ua + ud

= 0

Consolidatedundrained,

CU

Apply chamber pressure, s3.Allow complete drainage, so

pore water pressure (u = u0)

developed is zero

Apply axial stress, s. Do not allowdrainage (u = ud 0). At failure, s

= sf; pore water pressure,

u = uf= ua + ud = 0 + ud(f)

Unconsolidatedundrained,

UU

Apply chamber pressure, s3.Do not allow drainage, so pore

water pressure (u = u0)

developed through application

ofs3 is not zero

Apply axial stress, s. Do not allowdrainage (u = ud 0). At failure, s

= sf ; pore water pressure, u = uf=

ua + ud(f)

ADVANTAGES OF TRIAXIAL TEST


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Specimens are subjected to (approximately) uniformstresses and strains

The complete stress-strain-strength behavior can beinvestigated

Drained and undrained tests can be performed

Pore water pressures can be measured in undrainedtests, allowing effective stresses to be determined

Different combinations of cell pressure and axialstress can be applied

ADVANTAGES OF TRIAXIAL TEST

DAA3513 : GEOTECHNICAL ENGINEERING** read more in the textbook


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Typical triaxial results

s

e

Increasing cell

pressure


Mohr Circles


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Mohr Circles

To relate strengths from different tests we need to use some results

from the Mohr circle transformation of stress.

t

s

s1s3

c

t s c tan

The Mohr-Coulomb failure locus is tangent to the Mohr

circles at failureDAA3513 : GEOTECHNICAL ENGINEERING


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EXAMPLE

The results of a trixial test are as follows :

No of test Chamber confining

pressure, kN/m

2

Deviator stress

at failure, kN/m

2

1 100 210

2 200 438

3 300 644

Draw the shear strength envelope anddetermine the shear strength parameters ofthe soil


Hint : s s + s


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Solution :

1. Complete the table

2. Using the graph paper (same scale on both axis), drawthe mohr envelope and sketch the tangent line.

3. Check the interception on y-axis and the gradient ofthe line

No of test s3, kN/m2 s, kN/m2 s1, kN/m

2

1 100 210

2 200 438

3 300 644

Hint : s1 = s3 + s

Lets try examples given


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VANE SHEAR TEST (ASTM D2573)

Vane shear apparatus : consists of four thin,equal sized steel plates welded to a steel

torque rod DAA3513 : GEOTECHNICAL ENGINEERING

hi i d d i h h f h i
http://www.envi.se/geotechnical/pictures/memovane.jpg


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This test is used to determine shear strength of cohesive

soils

Normally done in-situ ( field test) on a soft clayey soil

(cohesive soil)

The vane is pushed into the soil

Torque,T is applied at the top of the torque rod to rotate

the vane at a uniform speed A cylinder of soil (height h & diameter d) will resist the

torque until the soil fails

42

32dhd

Tcu

where = coefficient of shear strength

mobilization at the end of soil

cylinder.



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LEARNING OUTCOMES

Calculate the effective

overburden pressure of soil


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STRESS PATH

Results of triaxial tests can be represented by stresspath

Stress path = line that connects a series of point, eachof which represents a successive stress stateexperienced by a soil specimen during the process of atest

Stress path = diagrams/graphs of stress changes


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Types of stress path

1. Stress path in s/e space

2. Stress path in s1/s3 space

3. Stress path in t/s space

4. Stress path in q/p space

Advance

level


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SENSITIVITY CLAY

Sensitivity = the reduction in unconfinedcompression strength of clay soils due toremolding although no changes in the moisture

content

Degree of sensitivity, St =

)(

)(

remoldedu

undistubedu

q

q



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may range from 1 to 8 or might be reached as high as80 (flocculent marine clay deposits)

1 8 16 32 642 4

Slightly

sensitive

Medium

sensitive

Very

sensitive

Slightly

quick

Medium

quick

Very

quick

Extra

quick

Sensitivity, St (log scale)



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THIXOTROPHYCLAY

Thixotrophy = is a condition whereby a soil specimenwhich is kept in undisturbed condition (afterremolding) will experienced a continuous increased

in strength with time



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The loss of strength is gradually regained with timewhen the materials are allowed to rest

However most soils are partially thixotropic(part of the strength loss caused by remolding is neverregained with time)

The soils strength will never achieved the initialstrength although the specimen were preparedundisturbed

*** Refer to the handout given



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Effective Overburden Pressure

Factors affecting overburden pressure

i) Depth of the point of interest

deeper the point, larger the pressure

ii) Groundwater level same soil profile will give different pressure value

due to the water level

iii) Types of soil

soil with higher unit weight value (dense soil) give

larger overburden pressure

Lets try examples given DAA3513 : GEOTECHNICAL ENGINEERING


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Factors affecting overburden pressure

i) Depth of the point of interest

deeper the point, larger the pressure

ii) Groundwater level same soil profile will give different pressure value

due to the water level

iii) Types of soil

soil with higher unit weight value (dense soil) give

larger overburden pressure



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Calculate the effective stress at point A, B, C and D.

Take w = 9.81kN/m3

Lets try examples given DAA3513 : GEOTECHNICAL ENGINEERING

Claysat = 19.25kN/m313m

3m

Dry Sand

dry= 16.5kN/m3

Impermeable layer

A

B

C

D

3m

Q i


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Calculate the effective stress at 5m below the river bed

(Point A) which consist of sand. The depth of water in

the river was 2m. Take w = 9.8kN/m3

5 marks. DAA3513 : GEOTECHNICAL ENGINEERING

River bed

Sandsat = 20kN/m35m

-2m

A

River

w = 9.80kN/m3

Quiz.


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topic 1-shear strength (3).pptx

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