Triaxial Shear Test

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Triaxial Shear Test
Chapter 11

• CE 317
• Geotechnical Engineering
• Dr. Tae-Hyuk Kwon

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Determination of shear strength parameters of
soils (c, f or c, f)
Other laboratory tests include, Direct simple
shear test, torsional ring shear test, plane
strain triaxial test, laboratory vane shear test,
laboratory fall cone test
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Laboratory tests
Field conditions
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Laboratory tests
Simulating field conditions in the laboratory
Step 2 Apply the corresponding field stress
conditions
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Triaxial Shear Test
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Triaxial Shear Test
Specimen preparation (undisturbed sample)
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Triaxial Shear Test
Specimen preparation (undisturbed sample)
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Triaxial Shear Test
Specimen preparation (undisturbed sample)
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Triaxial Shear Test
Specimen preparation (undisturbed sample)
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Types of Triaxial Tests
Is the drainage valve open?
Is the drainage valve open?
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Types of Triaxial Tests
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Consolidated- drained test (CD Test)
Step 1 At the end of consolidation
Step 2 During axial stress increase
Step 3 At failure
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Consolidated- drained test (CD Test)
Deviator stress (q or Dsd) s1 s3
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Consolidated- drained test (CD Test)
Volume change of sample during consolidation
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Consolidated- drained test (CD Test)
Stress-strain relationship during shearing
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CD tests
How to determine strength parameters c and f
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CD tests
Therefore, c c and f f
Since u 0 in CD tests, s s
cd and fd are used to denote them
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CD tests
Failure envelopes
For sand and NC Clay, cd 0
Therefore, one CD test would be sufficient to
determine fd of sand or NC clay
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CD tests
Failure envelopes
For OC Clay, cd ? 0
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Some practical applications of CD analysis for
clays
1. Embankment constructed very slowly, in layers
over a soft clay deposit
t in situ drained shear strength
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Some practical applications of CD analysis for
clays
2. Earth dam with steady state seepage
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Some practical applications of CD analysis for
clays
3. Excavation or natural slope in clay
t In situ drained shear strength
Note CD test simulates the long term condition
in the field. Thus, cd and fd should be used to
evaluate the long term behavior of soils
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Consolidated- Undrained test (CU Test)
Step 1 At the end of consolidation
Step 2 During axial stress increase
Step 3 At failure
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Consolidated- Undrained test (CU Test)
Volume change of sample during consolidation
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Consolidated- Undrained test (CU Test)
Stress-strain relationship during shearing
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CU tests
How to determine strength parameters c and f
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CU tests
How to determine strength parameters c and f
uf
Effective stresses at failure
Mohr Coulomb failure envelope in terms of total
stresses
fcu
ccu
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CU tests
Shear strength parameters in terms of effective
stresses are c and f
Shear strength parameters in terms of total
stresses are ccu and fcu
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
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Some practical applications of CU analysis for
clays
1. Embankment constructed rapidly over a soft
clay deposit
t in situ undrained shear strength
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Some practical applications of CU analysis for
clays
2. Rapid drawdown behind an earth dam
Core
t Undrained shear strength of clay core
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Some practical applications of CU analysis for
clays
3. Rapid construction of an embankment on a
natural slope
Note Total stress parameters from CU test (ccu
and fcu) can be used for stability problems
where, Soil have become fully
consolidated and are at equilibrium with the
existing stress state Then for some reason
additional stresses are applied quickly with no
drainage occurring
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Unconsolidated- Undrained test (UU Test)
Step 1 Immediately after sampling
Step 2 After application of hydrostatic cell
pressure


Duc B Ds3
Note If soil is fully saturated, then B 1
(hence, Duc Ds3)
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Unconsolidated- Undrained test (UU Test)
Step 1 Immediately after sampling
Step 2 After application of hydrostatic cell
pressure
Step 3 During application of axial load
Step 3 At failure
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Unconsolidated- Undrained test (UU Test)
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 different cell pressures
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Unconsolidated- Undrained test (UU Test)
Mohr circles in terms of total stresses
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Unconsolidated- Undrained test (UU Test)
Effect of degree of saturation on failure envelope
S lt 100
S gt 100
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Some practical applications of UU analysis for
clays
1. Embankment constructed rapidly over a soft
clay deposit
t in situ undrained shear strength
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Some practical applications of UU analysis for
clays
2. Large earth dam constructed rapidly with no
change in water content of soft clay
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Some practical applications of UU analysis for
clays
3. Footing placed rapidly on clay deposit
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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Unconfined Compression Test (UC Test)
s1 sVC Ds
s3 0
Confining pressure is zero in the UC test
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Unconfined Compression Test (UC Test)
tf s1/2 qu/2 cu
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Thank You