Kuat Geser Tanah Class 2

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    Kuat Geser Tanah (Shear Strength)

    - Triaxial Test-(Courtesy of COSC 323: Soils in Construction)

    oleh:

    A. Adhe Noor PSH, ST., MTStaf Pengajar Program Studi Teknik Sipil

    Jurusan Teknik Fakultas Sains dan TeknikUniversitas Jenderal Soedirman

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    Triaxial Shear Test

    Soil sample atfailure

    Failure plane

    Porousstone

    impervious

    membrane

    Piston (to apply deviatoric stress)

    O-ring

    pedestal

    Perspexcell

    Cell pressure

    Back pressure Pore pressure orvolume change

    Water

    Soilsample

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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 arecarefully trimmed

    Setting up the sample inthe triaxial cell

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    Triaxial Shear Test

    Sample is covered with arubber membrane andsealed

    Cell is completelyfilled with water

    Specimen preparation (undisturbed sample)

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    Triaxial Shear TestSpecimen preparation (undisturbed sample)

    Proving ring tomeasure thedeviator load

    Dial gauge to

    measure verticaldisplacement

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    Types of Triaxial Tests

    Is the drainage valve open?

    yes no

    Consolidatedsample

    Unconsolidated

    sample

    Is the drainage valve open?

    yes no

    Drained

    loading

    Undrained

    loading

    Under all-around cell pressure c

    cc

    c

    cStep 1

    deviatoric stress( = q)

    Shearing (loading)

    Step 2

    c c

    c+ q

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    Types of Triaxial Tests

    Is the drainage valve open?

    yes no

    Consolidatedsample

    Unconsolidatedsample

    Under all-around cell pressure c

    Step 1

    Is the drainage valve open?

    yes no

    Drainedloading

    Undrainedloading

    Shearing (loading)

    Step 2

    CD test

    CU test

    UU test

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    Consolidated- drained test (CD Test)

    Step 1: At the end of consolidation

    VC

    hC

    Total, = Neutral, u Effective, +

    0

    Step 2: During axial stress increase

    VC =VC

    hC =hC

    VC +

    hC 0

    V =VC + = 1

    h =hC = 3

    Drainage

    Drainage

    Step 3: At failure

    VC + f

    hC 0

    Vf =VC +f= 1f

    hf =hC = 3fDrainage

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    Deviator stress (q or d) = 13

    Consolidated- drained test (CD Test)

    1 = VC +

    3 = hC

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    Volumecha

    ngeofthe

    sample

    Expansion

    Compression

    Time

    Volume change of sample during consolidation

    Consolidated- drained test (CD Test)

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    Deviatorstress,

    d

    Axial strain

    Dense sand or

    OC clay

    (d)f

    Dense sand orOC clay

    Loose sand or

    NC clay

    Volumechang

    eof

    thesample

    Expansion

    Com

    pression

    Axial strain

    Stress-strain relationship during shearing

    Consolidated- drained test (CD Test)

    Loose sand orNC Clay(d)f

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    CD tests How to determine strength parameters c and f

    Deviatorstres

    s,

    d

    Axial strain

    Shear

    stress,t

    or

    fMohr Coulombfailure envelope

    (d)fa

    Confining stress = 3a(d)fb

    Confining stress = 3b

    (d)fc

    Confining stress = 3c

    3c 1c3a 1a

    (d)fa

    3b 1b

    (d)fb

    1 = 3 + (d)f

    3

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    CD tests

    Strength parameters c and f obtained from CD tests

    Since u = 0 in CDtests, =

    Therefore, c = c

    and f = f

    cd and fd are usedto denote them

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    CD tests Failure envelopes

    Shearstress,t

    or

    fdMohr Coulombfailure envelope

    3a 1a(d)fa

    For sand and NC Clay, cd = 0

    Therefore, one CD test would be sufficient to determine fd ofsand or NC clay

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    CD tests Failure envelopes

    For OC Clay, cd 0

    t

    or

    f

    3 1(d)f

    c

    c

    OC NC

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    Some practical applications of CD analysis forclays

    t t = in situ drained shearstrength

    Soft clay

    1. Embankment constructed very slowly, in layers over a soft clay deposit

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    Some practical applications of CD analysis forclays

    2. Earth dam with steady state seepage

    t = drained shear strength ofclay core

    t

    Core

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    Some practical applications of CD analysis forclays

    3. Excavation or natural slope in clay

    t = In situ drained shear strength

    t

    Note: CD test simulates the long term condition in the field.Thus, cd and fd should be used to evaluate the longterm behavior of soils

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    Consolidated- Undrained test (CU Test)

    Step 1: At the end of consolidation

    VC

    hC

    Total, = Neutral, u Effective, +

    0

    Step 2: During axial stress increase

    VC =VC

    hC =hC

    VC +

    hC u

    Drainage

    Step 3: At failure

    VC + f

    hC

    Nodrainage

    No

    drainage

    uf

    V =VC + u = 1

    h =hC u= 3

    Vf =VC +f uf = 1f

    hf =hC uf = 3f

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    Volumecha

    ngeofthe

    sample

    Expansion

    Compression

    Time

    Volume change of sample during consolidation

    Consolidated- Undrained test (CU Test)

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    De

    viatorstress,

    d

    Axial strain

    Dense sand or

    OC clay

    (d)f

    Dense sand orOC clay

    Loose sand

    /NC Clay

    u

    +

    -

    Axial strain

    Stress-strain relationship during shearing

    Consolidated- Undrained test (CU Test)

    Loose sand orNC Clay(d)f

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    CU tests How to determine strength parameters c and f

    Deviatorstress,

    d

    Axial strain

    Shearstress,t

    or

    (d)fb

    Confining stress = 3b

    3b 1b3a 1a

    (d)fa

    fcuMohr Coulomb failureenvelope in terms oftotal stresses

    ccu

    1 = 3 + (d)f

    3

    Total stresses at failure

    (d)fa

    Confining stress = 3a

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    (d)fa

    CU tests How to determine strength parameters c and f

    S

    hearstress,

    t

    or 3b 1b3a 1a

    (d)fa

    fcuMohr Coulomb failureenvelope in terms oftotal stresses

    ccu 3b 1b3a 1a

    Mohr Coulomb failureenvelope in terms ofeffective stresses

    f

    C ufa

    ufb

    1 = 3 + (d)f -uf

    3= 3 -uf

    Effective stresses at failure

    uf

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    CU tests

    Strength parameters c and f obtained from CD tests

    Shear strengthparameters in terms oftotal stresses are ccu and

    fcu

    Shear strengthparameters in terms ofeffective stresses are cand f

    c = cd and f = fd

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    CU tests Failure envelopes

    For sand and NC Clay, ccu and c = 0

    Therefore, one CU test would be sufficient to determine fcu

    and f(= fd) of sand or NC clay

    Shearstress,t

    or

    fcuMohr Coulomb failureenvelope in terms oftotal stresses

    3a 1a

    (d)fa

    3a 1a

    f

    Mohr Coulomb failureenvelope in terms ofeffective stresses

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    Some practical applications of CU analysis forclays

    t t = in situ undrainedshear strength

    Soft clay

    1. Embankment constructed rapidly over a soft clay deposit

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    Some practical applications of CU analysis forclays

    2. Rapid drawdown behind an earth dam

    t = Undrained shear strengthof clay core

    Coret

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    Some practical applications of CU analysis forclays

    3. Rapid construction of an embankment on a natural slope

    Note: Total stress parameters from CU test (ccu and fcu) can be used for stabilityproblems where,

    Soil have become fully consolidated and are at equilibrium with theexisting stress state; Then for some reason additional stresses areapplied quickly with no drainage occurring

    t = In situ undrained shear strength

    t

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    Unconsolidated- Undrained test (UU Test)

    Data analysis

    C = 3

    C = 3Nodrainage

    Initial specimen condition

    3 + d

    3

    Nodrainage

    Specimen conditionduring shearing

    Initial volume of the sample = A0 H0

    Volume of the sample during shearing = A H

    Since the test is conducted under undrained condition,

    A H = A0 H0

    A (H0H) = A0 H0

    A (1

    H/H0

    ) = A0

    z

    AA

    1

    0

    U lid t d U d i d t t (UU T t)

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    Unconsolidated- Undrained test (UU Test)

    Step 1: Immediately after sampling

    0

    0

    = +

    Step 2: After application of hydrostatic cell pressure

    uc =B 3

    C = 3

    C = 3 uc

    3 =3 - uc

    3 =3 - uc

    Nodrainage

    Increase of pwp due toincrease of cell pressure

    Increase of cell pressure

    Skemptons pore water

    pressure parameter, B

    Note: If soil is fully saturated, then B = 1 (hence, uc

    = 3

    )

    U lid t d U d i d t t (UU T t)

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    Unconsolidated- Undrained test (UU Test)

    Step 3: During application of axial load

    3 + d

    3

    Nodrainage

    1 =3 +d- uc ud

    3 =3 - uc ud

    ud =ABd

    uc ud

    = +

    Increase of pwp due to increase

    of deviator stress

    Increase of deviator stress

    Skemptons pore water

    pressure parameter, A

    U lid t d U d i d t t (UU T t)

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    Unconsolidated- Undrained test (UU Test)

    Combining steps 2 and 3,

    uc =B 3 ud =ABd

    u=uc + ud

    Total pore water pressure increment at any stage, u

    u=B [3 + Ad]Skemptons porewater pressureequation

    u=B [3 + A(13]

    Unconsolidated Undrained test (UU Test)

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    Unconsolidated- Undrained test (UU Test)

    Step 1: Immediately after sampling

    00

    Total, = Neutral, u Effective, +

    -ur

    Step 2: After application of hydrostatic cell pressure

    V0 =ur

    h0 =ur

    C

    C-u

    r

    + uc

    = -ur

    + c(Sr = 100%;B = 1)

    Step 3: During application of axial load

    C +

    C

    No

    drainage

    Nodrainage

    -ur + c u

    VC =C +ur - C=ur

    h =ur

    Step 3: At failure

    V =C + + ur - c u

    h =C +ur - c u

    hf =C +ur - c uf =

    3f

    Vf =C +f+ ur - c uf = 1f

    -ur+ c uf

    C

    C + fNodrainage

    Unconsolidated Undrained test (UU Test)

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    Unconsolidated- Undrained test (UU Test)

    Total, = Neutral, u Effective, +Step 3: At failure

    hf =C +ur - c uf =3f

    Vf =C +f+ ur - c uf = 1f

    -ur+ c ufC

    C + fNodrainage

    Mohr circle in terms of effective stresses do not depend on the cell pressure.

    Therefore, we get only one Mohr circle in terms of effective stress for differentcell pressures

    t

    3

    1f

    Unconsolidated Undrained test (UU Test)

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    3b

    1b3a

    1af

    3

    1

    Unconsolidated- Undrained test (UU Test)

    Total, = Neutral, u Effective, +Step 3: At failure

    hf =C +ur - c uf =3f

    Vf =C +f+ ur - c uf = 1f

    -ur+ c ufC

    C + fNodrainage

    t

    or

    Mohr circles in terms of total stresses

    uaub

    Failure envelope, fu = 0

    cu

    Unconsolidated Undrained test (UU Test)

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    3b 1b

    Unconsolidated- Undrained test (UU Test)

    Effect of degree of saturation on failure envelope

    3a 1a3c 1c

    t

    or

    S < 100% S > 100%

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    Some practical applications of UU analysis forclays

    t t = in situ undrainedshear strength

    Soft clay

    1. Embankment constructed rapidly over a soft clay deposit

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    Some practical applications of UU analysis forclays

    2. Large earth dam constructed rapidly with nochange in water content of soft clay

    Core

    t= Undrained shear strength

    of clay core

    t

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    Some practical applications of UU analysis forclays

    3. Footing placed rapidly on clay deposit

    t = In situ undrained shear strength

    Note: UU test simulates the short term condition in the field.Thus, cu can be used to analyze the short term

    behavior of soils

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    Example

    Given

    Triaxial compression tests on three specimens of a soil samplewere performed. Each test was carried out until the specimenexperienced shear failure. The test data are tabulated as follows:

    Required

    The soils cohesion and angle of internal friction

    Specimen

    Number

    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

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    Example

    Specimen

    Number

    Minor PrincipalStress

    (kips/ft2)

    Deviator Stressat 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

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    Example

    0 2 4 6 8 10 12 14

    2

    4

    6

    8

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    Example

    0 2 4 6 8 10 12 14

    2

    4

    6

    8

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    Example

    0 2 4 6 8 10 12 14

    2

    4

    6

    8

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    Example

    0 2 4 6 8 10 12 14

    2

    4

    6

    8

    4

    2

    2

    1

    /9.0

    26

    4

    2tan

    4

    2tan

    ftkipc

    f

    f

    f

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    THE END