kuat geser tanah class 2
TRANSCRIPT
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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