Conventional Laboratory Testing Methods & Issues
Transcript of Conventional Laboratory Testing Methods & Issues
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Conventional Laboratory Testing Methods & Issues
Ajanta Sachan Assistant Professor Civil Engineering IIT Gandhinagar
Geotechnical Engg Structures… Buried right Under your Feet…!!
Hiding World of Geotechnical Engg…!!
Foundations
Tunneling
Shoring
Soil Exploration
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Purpose of Geotechnical Testing?
ground
Can the soils Support the structure?
What is the impact of Excavation or Filling?
Are the earth and rock Slopes stable?
What type of Foundation is best suited for the structure?
How will the site respond to an Earthquake?
Is the site Contaminated?
Determine potential problems and Avoid surprises!!
Typical Geotechnical Project
construction site
Geo-Laboratory ~ for testing
Design Office ~ for design & analysis
soil properties
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Issue 1: Bearing Capacity Shear Strength of Soils
Issue 2: Settlement Compressibility parameters of Soils
The issues before designing the CE structure
Solution: 1. Ground Improvement 2. Choice of Foundation 3. Special cases/Problem soils: Specific analysis
If not satisfied…
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Soils generally fail in shear
strip footing
embankment
At failure, shear stress along the failure surface reaches the shear strength.
failure surface mobilised shear resistance
Issues 1: Bearing Capacity
… Blunders become monuments !
Classic Example: Settlement problem
Issues 2: Settlement
Other Information:
Resting on Shallow Foundation
Supported by soft soil underneath
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Leaning Tower of Pisa
Soil profile beneath the Tower
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Relationship between time, inclination and settlement
Non-uniform and overlapping pressure bulbs
Tower silos; Ontario, Canada
Leaning twin silos caused by non uniform settlement in zone of overlapping pressure bulbs
Settlement: Other example
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You pay for soil investigation whether you carry out or not. Infact you eventually pay more without a soil investigation.
Each and every project is unique for its geotechnical investigations…!!
Importance of Geotechnical Investigations…
Unique?
No Ready-made solution is available!!
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S : Solid Soil particle
W: Liquid Water (electrolytes)
A: Air Air
v
s
Ve
VVoid ratio,
Three Phases in Soils
Soil Sampling: Before Lab testing
Disturbed Samples: Natural soil structure is modified or destroyed during sampling Representative Samples:
Natural water content and mineral constituents of particular soil layer are preserved
Good for soil identification and water content
Non-representative Samples: Water content altered and soil layers mixed up
Of no use.
Undisturbed Samples: Soil structure and the other mineral properties are preserved to an extent. Some disturbance is always there, e.g. due to stress release.
However it should be minimized in order to have suitable sample for our analysis.
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Laboratory Test: Index Properties
Index Properties of soil:
Basic soil properties such as (a) Specific gravity (Gs) (b) Grain size distribution (dry/wet Sieve test, Hydrometer test), (c) Liquid Limit (LL), Plastic limit (PL) (d) OMC, Maximum Dry density(Compaction/Proctor test) (e) Permeability (Constant head/Falling head) (f) Relative Density (Minimum & Maximum density for cohesionless soils)
More tests for Problem soils: (a) Shrinkage Limit, Free swell, Swell pressure for Expansive soils (b) Pinhole test, Crumb test for Dispersive soils (c) Chemical Test (PH, Sulphite, Chloride, Iron etc) for soils (may affected with industrial waste or some other waste) (d) Furnace test for Organic Soils (peats etc)
“Representative Disturbed “soil samples are used to perform these tests.
Laboratory Test: Engineering Properties
Engineering Properties of soil:
Consolidation Properties (Oedometer setup) (i) Must to perform for Clayey soils; (ii) Soil parameters obtained: Cc,Cv,Cr, OCR, k
Shear Strength Properties (i) Direct Shear test (for cohesionless soil) (ii) Unconfined Compression test (for cohesive soil)
(iii) Triaxial test (for all soil types; cohesive, cohesionless)
Dynamic Properties (i) Cyclic Triaxial test (ii) Cyclic Simple Shear test (iii) Resonant Column test (iv) Bender Element test
“Undisturbed” soil samples are used to perform these tests.
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Grain Size Distribution
In Coarse grained soils …... By Sieve analysis (Dry/Wet)
Sieve Analysis Hydrometer Analysis
soil/water suspension
hydrometer
stack of sieves
sieve shaker
In Fine grained soils …... By Hydrometer analysis
: Above 75 m particle size : Below 75 m particle size
Soil Groups Based on its Particle Size
Fine grain soils
Coarse grain soils
0.002 300 80 4.75 0.075
Grain size (mm) (IS code)
Boulder Clay Silt Sand Gravel Cobble
Granular soils or Cohesion less soils
Cohesive soils
Non-Clay minerals
Clay minerals
0.425 2.0
Fine Medium Coarse Fine Coarse
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Hydrometer (< 75 m size)
Sieve analysis (> 75 m size)
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Soil Texture
Particle size, shape and size distribution Coarse-textured (Gravel, Sand) Fine-textured (Silt, Clay) Visibility by the naked eye (0.05mm is the approx
limit)
Particle size distribution Sieve/Mechanical analysis or Gradation Test Hydrometer analysis for smaller than .05 to .075 mm
(#200 US Standard sieve)
Particle size distribution curves Well graded Poorly graded 60
10u
DC
D
230
60 10c
DC
D D
Grain Size Distribution
Poorly Graded
Well Graded
Gap Graded
60
30u
DC
D
230
60 10c
DC
D D
Coefficient of Uniformity
Coefficient of Curvature
For Gravel:
Cu < 4 Poorly graded Cu > 4 Well graded or Gap graded
1 < Cc <3 Well graded
For Sand:
Cu < 6 Poorly graded Cu > 6 Well graded or Gap graded
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Grain Size Distribution Curve
Gravel: Sand:
General Characteristics of Soils Soil Characteristics Gravel, Sand Silt Clay Grain size Granular, Coarse-grained,
particles can be seen through naked eyes
Fine-grained, can not see individual particles
Fine-grained, can not see individual particles
Plasticity and Cohesion Non-plastic, Cohesion less Slightly or no plasticity, Cohesion
Plastic, Cohesive
Effect of grain size distribution (Sieve analysis)
Important Less important Unimportant
Effect of water (Atterberg limits)
Unimportant (except for loose saturated soils under dynamic loadings)
Important Very important
Permeability and Drainage Pervious, Freely draining Less pervious Almost impervious
Compressibility Low Medium High
Shear Strength Depends on relative density (generally high)
Intermediate Depends on consistency (generally poor)
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Effect of Particle size
Relative Density
Void ratio (e)
1.0
0.8
0.6
0.4
0.2
0
emax
Dr = 0%
e
0%<Dr <100%
emin
Dr = 100%
max
max minr
e eD
e e
IS 2720 (Part XIV) 1983: emin (max density): Vibrating in mould under some surcharge load emax (min density): Pouring in a mould through funnel from ht of 2.5 cm.
(Lambe and Whitman, 1979)
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Atterberg Limits
Border line water contents, separating the different states of a fine grained soil
Liquid limit
Shrinkage limit
Plastic limit
0 water content
liquid semi-solid brittle-
solid
plastic
Atterberg Limits
(Holtz and Kovacs, 1981)
In percentage
The presence of water in fine-grained soils can significantly affect associated engineering behavior, so we need a reference index to clarify the effects.
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Group symbols: G - gravel S - sand M - silt C - clay O - organic silts and clay Pt - peat and highly organic soils H - high plasticity L - low plasticity W - well graded P - poorly graded
Soil Classification Systems
Plasticity Chart
Casagrande’s PI-LL Chart
0
10
20
30
40
50
60
0 10 20 30 40 50 60 70 80 90 100
Liquid Limit
Pla
stic
ity
Ind
ex
A-line
U-line
illite
kaolinite
chlorite
halloysite
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Typical Values of Atterberg Limits
(Mitchell, 1993)
Consolidation: Oedometer test
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Consolidation
When a saturated clay is loaded externally,
saturated clay
GL
the water is squeezed out of the clay over a long time (due to low permeability of the clay).
Consolidation Test
~ simulation of 1-D field consolidation in lab.
Field
GL
Lab
undisturbed soil specimen
Dia = 50-75 mm
Height = 20-30 mm
metal ring
(oedometer)
porous stone
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Consolidation Test: Oedometer Test
Input: Vertical Load, Vertical Displacement
Output: Consolidation parameters (Cv, Cc & Cs); void ratio versus overburden pressure curve; (e-logp); permeability (k)
H -e Relation
saturated clay
GL
q kPa
saturated clay
GL
q kPa
Ho
Time = 0+
e = eo
H
Time =
e = eo - e
average vertical strain =
oH
H
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H -e Relation
Consider an element where Vs = 1 initially.
e
1
eo
Time = 0+ Time =
average vertical strain =
oe
e
1
H -e Relation
Equating the two expressions for average vertical strain,
oe
e
1
oH
H
consolidation settlement
initial thickness of clay layer
initial void ratio
change in void ratio
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Overconsolidation ratio (OCR)
original state
log v’
void
ratio
virgin consolidation line
p’ vo’
eo
Field
vo’
'
'
vo
pOCR
Example 1: Oedometer test
e-logp curve (Void ratio versus pressure curve): Incremental Loading & Unloading
0.40
0.45
0.50
0.55
0.60
0.1 1 10
Vo
id R
atio
, e
Log Effective Stress in kg/cm2
Loading: 0.1, 0.2, 0.5, 1.0, 2.0, 4.0, 8.0 Kg/cm2
Unloading: 8.0, 4.0, 2.0, 1.0, 0.5, 0.2, 0.1 Kg/cm2
Compression index (Cc) = 0.1
Re-compression index (Cr) = 0.01
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Compressibility parameters (Cc & Cr):
log log1 1
s c c c c o avc
o o o c
C H C HS
e e
log1
c c o avc
o o
C HS
e
Settlement for NC soil
Settlement for OC soil
Compressibility parameters Cc & Cr are used in settlement calculations. Cc is the slope of loading curve and Cr or Cs is the slope of unloading curve.
Oedometer test: Coff of consolidation (Cv)
Casagrande Method Taylor Method
Time-settlement analysis at given load in Oedometer test (consolidation test)
1. Casagrande Method (logt method) 2. Taylor Method (√t method)
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Coefficient of Consolidation (Cv)
t i cS S US 2v
t
c tT
H
Settlement curve (oedometer test at each load):
Casagrade method : t50 (U = 50%)
Tayor method : t90 (U = 90%)
U = Degree of consolidation
T = Time factor
Find coefficient of consolidation (Cv) ?
2v
t
c tT
H
Permeability: k = Cv mv gw
Oedometer test: Time-settlement curve
9.6
9.62
9.64
9.66
9.68
9.7
9.72
9.74
9.76
9.78
9.8
9.82
0 10 20 30 40
Dia
l G
auge
Re
adin
g (m
m)
Square Root of Time
100 kPa vertical stress
Taylor Method
2v
t
c tT
H
Tayor method: t90 (U = 90%)
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Void ratio versus pressure curve
Void ratio versus Stress (e-p) relationship
mv = av/(1+e0)
Compaction: Proctor test
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Compaction
A simple ground improvement technique, where the soil is densified through external compactive effort.
+ water =
Compactive effort
Laboratory Compaction Test
- to obtain the compaction curve and define the optimum water content and maximum dry density for a specific compactive effort.
hammer Standard Proctor: Modified Proctor:
• 3 layers
• 25 blows per layer
• 5 layers
• 25 blows per layer
• 2.7 kg hammer
• 300 mm drop
• 4.9 kg hammer
• 450 mm drop
compaction mould
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Example 2: Standard Proctor test
1.791.801.811.821.831.841.851.861.871.881.891.901.911.921.931.941.951.961.97
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Dry
de
nsi
ty (
gm/c
c)
water content (%)
OMC 11.0 %
MDD 1.96 gm/cc
OMC = Optimum Moisture Content
MDD = Maximum Dry Density
Compaction Curve
Water content
Dry
den
sity
(
d)
optimum water content
d, max
- Soil grains densely packed
- Good strength and stiffness
- Low permeability
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Compaction Curve
What happens to the relative quantities of the three phases with addition of water?
Water content
Dry
den
sity
(
d)
soil
water
air
difficult to expel all air
lowest void ratio and highest
dry density at optimum w
Effect of Compactive Effort
Increasing compactive effort results in:
Lower optimum water content
Higher maximum dry density
E1
E2 (>E1)
Water content
Dry
den
sity
(
d)
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Compaction Control Test
compacted ground
d,field = ? wfield = ?
Compaction
specifications
Compare!
w
d
Shear Strength Testing (Laboratory)
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Shear Strength Testing (Lab methods)
Shear Strength Lab testing methods: 1. Direct Shear test:
Cohesionless soil (sands, silts) 2. Unconfined Compression test: Cohesive soil (sample can stand by itself) 3. Triaxial test: Mostly compression test a. Unconsolidated Undrained (UU) b. Consolidated Undrained (CU) c. Consolidated Drained (CD)
Unconfined Compression Test (UC test) (Recommended for Cohesive soils)
Input: Vertical Load, Vertical Displacement
Output: Shear Strength under Undrained Conditions (Su)
Platen
Platen
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Unconfined Compression (UC) Test on Soils
-
0
50
100
150
200
250
300
0 2 4 6 8 10 12 14 16 18
Axi
al S
tres
s (k
Pa)
Axial Strain (%)
Test1
Test2
Test3
qu = Unconfined compressive strength c= cohesion Deformation rate =1.25mm/min Sample size = 38 mm dia & 76 mm ht
qu 267 kPa
c 133 kPa
Example 3: Unconfined Compression (UC) test
- Recommended for Cohesive soils
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Direct Shear Test (Recommended for Cohesionless soils)
Input: Vertical Load, Vertical Displacement, Lateral Load Lateral Displacement
Output: shear strength, friction angle (f)
Direct Shear Test, contd…
Measured Quantities: 1. Vertical Load 2. Vertical Displacement 3. Lateral/Shearing Load 4. Lateral/Shearing Displacement
Interpretations : 1. Shear Strength under Wet
Conditions
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0
20
40
60
80
100
120
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
She
ar s
tre
ss (k
Pa)
Shear deformation (mm)
Test1 Test2 Test3
Test 1: at 0.5 Kg/cm2 Test 2: at 1.0 Kg/cm2 Test 3: at 1.5 Kg/cm2
Example 4: Direct Shear test
- Recommended for Cohesionless soils
Example 4: Direct Shear test
0.0
0.4
0.8
1.2
0.0 0.5 1.0 1.5 2.0
She
ar s
tre
ss (
kg/c
m2)
Normal stress (kg/cm2)
Test Normal stress
Shear stress at
failure
Shear stress
at failure
(kg/cm2) (kPa) (kg/cm2)
Direct shear test 1 0.5 39.6 0.396
Direct shear test 2 1.0 64.8 0.648
Direct shear test 3 1.5 100.7 1.007
Deformation rate =0.25mm/min Sample size = 60mmx60mmx25mm
f 34 deg
c 0 kPa
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Shear Stress-shear displacement curves of soils (Direct Shear test)
Triaxial Test
porous stone
impervious membrane
piston (to apply deviatoric stress)
O-ring
pedestal
perspex cell
cell pressure back pressure
pore pressure or
volume change
water
soil sample at failure
failure plane
Loading conditions: Static/Monotoinc loading (compression is common)
Measures shear strength parameters of soil: cohesion & friction angle
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Triaxial Testing Setup
Soil specimen
Triaxial setup
Control Panel
Input: Vertical Load, Vertical Displacement, Pore pressure, Cell pressure
Output: Shear Strength properties of soil under UU, CU, CD Conditions: friction angle (f), cohesion (c)
Triaxial Test, contd…
Interpretations : 1. Shear Stress-Strain Relationship under triaxial
compression/extension Conditions 2. Volumetric Response or Void ratio change 3. Shear Strength under Undrained/Drained triaxial shearing Conditions
Measured Quantities: 1. Vertical Load 2. Vertical Displacement 3. Confining Pressure 4. Back Pressure/Excess
Pore Pressure 5. Volume change by
measuring expelled water volume
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Types of Triaxial Tests
Under all-around cell pressure c
Shearing (loading)
Is the drainage valve open? Is the drainage valve open?
deviatoric stress ()
yes no yes no
Consolidated sample
Unconsolidated sample
Drained loading
Undrained loading
Shear failure
At failure, shear stress along the failure surface () reaches the shear strength (f).
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Mohr-Coulomb Failure Criterion
f tan cf
c
f
cohesion friction angle
f is the maximum shear stress the soil can take without failure, under normal stress of .
f
Mohr-Coulomb Failure Criterion
f tanff c
Shear strength consists of two components: cohesive and frictional.
f
f
f
c
f tan f
c
frictional component
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Mohr Circles & Failure Envelope
Y
Initially, Mohr circle is a point
c
c
c
c+
The soil element does not fail if the Mohr circle is contained within the envelope
GL
Mohr Circles & Failure Envelope
Y
c
c
c
GL
As loading progresses, Mohr circle becomes larger…
.. and finally failure occurs when Mohr circle touches the envelope
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Orientation of Failure Plane
Y
c
c
c
GL
c+
90+f
f
45 + f/2
Failure plane oriented at 45 + f/2 to horizontal
45 + f/2
Y
Envelopes in terms of & ’
Identical specimens initially subjected to different isotropic stresses (c) and then loaded axially to failure
c
c
c
c
f
Initially… Failure
uf
At failure,
3 = c; 1 = c+f
3’ = 3 – uf ; 1’ = 1 - uf
c, f
c’, f’
in terms of
in terms of ’
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1-
3 Relation at Failure
X
soil element at failure
3 1
X 3
1
)2/45tan(2)2/45(tan231 ff c
)2/45tan(2)2/45(tan213 ff c
UU: Unconsolidated Undrained Test
-
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Test 1: at 100 kPa Test 2: at 200 kPa Test 3: at 300 kPa
Example 5: UU Triaxial test
Deformation rate =0.4 mm/min Sample size = 38 mm dia & 76 mm ht
0
200
400
600
800
1000
1200
1400
0 5 10 15 20 25
Axi
al s
tra
a (k
Pa
)
Axial strain (%)
Test1
Test2
Test3
Example 5: UU Triaxial test
ccosf = asinf = tanx
- a is intercept of q-p curve - x is slope angle of q-p curve
3 d q p
(kPa) (kPa) (kPa) (kPa)
Test 1 100 531.1 265.55 365.55
Test 2 200 957.8 478.9 678.9
Test 3 300 1163.3 581.65 881.65
q = (1-3)/2
p = (1+3)/2
0
100
200
300
400
500
600
0 200 400 600 800 1000
q (k
Pa)
p (kPa)
f 38 deg
c 57 kPa
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Thank You
Time Rate of Settlement
For open clay layer with two way drainage use curve for V=1
Assumption of pore pressure distribution under the given stress conditions
IS 8009 (Fig 13)
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Isotropic Compression Test Increase in Cell Pressure
o
Z
Y
X
o
o
o
o
o
Measured Quantities: 1. Cell Pressure 2. Volume Change & Pore Pressure 3. Axial Displacement
Interpretations : 1. Consolidation parameters, Transient Flow, T50, T100
2. Stress-strain relationship, Cc and Cs
3. Sense of Anisotropy by ea-ev relationship
CU: Consolidated undrained Test
-
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Output: - Axial stress- axial strain curve
- Pore pressure-axial strain curve
A typical CU test
Example 6: CU Triaxial test
f'= 27 deg
c' = 0 kPa
Example 6: CU Triaxial test
-100
-50
0
50
100
150
200
250
-15 -10 -5 0 5 10 15 20ea (%)
Exc
ess
po
re p
ress
ure
(kP
a)
Extension, OCR=1
Extension, OCR=10
Compression, OCR=1
Compression, OCR=10
0
20
40
60
80
100
120
140
160
180
-15 -10 -5 0 5 10 15 20ea (%)
Dev
iato
ric
stre
ss (
kPa)
Extension, OCR=1
Extension, OCR=10
Compression, OCR=1
Compression, OCR=10
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CD: Consolidated Drained Test
Output: - Axial stress- axial strain curve
- Volumetric strain -axial strain curve
A typical CD test
Example 7: CD Triaxial test
0
100
200
300
400
500
0 5 10 15 20 25 30
d (K
Pa)
ea (%)
0
1
2
3
4
5
6
7
8
9
0 5 10 15 20 25 30
Vo
lum
etri
c st
rain
(%)
ea (%)
f = 26.6 deg
c = 0 kPa
Confining pressure = 276 kPa
Total three triaxial tests at three different confining pressures need to be performed to obtain shear strength parameters of soil under consolidated drained (CD) conditions.
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Example 7: CD Triaxial test
-4
-2
0
2
4
6
8
10
-14 -10 -6 -2 2 6 10 14 18 22 26ea (%)
Vo
lum
etri
c st
rain
(%
)
Extension, OCR=1
Extension, OCR=10
Compression, OCR=1
Compression, OCR=10
0
50
100
150
200
250
300
350
400
450
500
-14 -10 -6 -2 2 6 10 14 18 22 26
ea (%)
Dev
iato
ric
stre
ss (
kPa)
Extension, OCR=1
Extension, OCR=10
Compression, OCR=1
Compression, OCR=10
Other Soil Properties: Dynamic Properties
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Soil Properties
Monotonic Loading (Shear strength properties of soil)
Angle of Internal Friction (f)
Cohesion (c)
Dynamic Loading (Dynamic properties of soil)
Shear Modulus (G)
Damping Ratio (D)
Dynamic properties of Soil
Shear Modulus, G = .VS2
Shear wave velocity = VS (m/sec)
Mass density = (g/g) (Kg/m3)
Unit weight of soil = g (KN/m3)
Acceleration of gravity = g (m/sec2)
Damping, D = decay in energy
Shear Modulus (G) is measured in KN/m2 & Damping (D) in %
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Dynamic properties of soil
Low Strain Amplitude test
For strains (10-6% to 10-4%)
Frequency range: 10 Hz to 200Hz
Vibratory loading (Rotating Machinery etc)
High Strain Amplitude test
For strains (10-4% to 10-2%)
Frequency range: 0.1 Hz to 2 Hz (in general)
Blast loading, Earthquake
Dynamic properties (Lab test)
High Strain Amplitude test
Cyclic Triaxial Test
Cyclic Direct Simple Shear Test
Low Strain Amplitude test
Resonant Column Test
Bender Element Test
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Cyclic Triaxial Test (High strain amplitude test)
Dynamic properties of soil using Cyclic Triaxial system: 1. Shear Modulus (G) 2. Damping ratio (D)
Cyclic Triaxial Test
DDamping EModulus Young Dynamic
dStress Dynamic aeStrain Axial
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Cyclic Simple Shear Test (High strain amplitude test)
Digitally controlled Electro-mechanical actuators are used to apply the stress or strain controlled loading
Output: Shear modulus (G), Damping (D)
Cyclic Simple Shear Test
DDamping GusShearModul
gnShearStraisShearStres
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Resonant Column Test (Low strain amplitude test)
The basic principle of the resonant column device is to excite one end of a confined cylindrical soil specimen in a fundamental mode of vibration by means of torsional or longitudinal excitation. Once the fundamental mode of resonance frequency is established, measurements are made of the resonance frequency and amplitude of vibration from which wave propagation velocities and strain amplitudes are calculated using the theory of elasticity.
The Resonant Column Test provides laboratory values of Shear modulus (G) and Damping ratio (D).
Resonant Column Test (Low strain amplitude test)
(a) Specimen is excited at the bottom and the response is picked up at the top (velocity or acceleration) (b) Driving force is applied on the top. The response pickup is also placed on the top
With known value of the resonant frequency it is possible to back-calculate the velocity (vs or vl) of the wave propagation and thereby G or E After measuring the resonant condition, the drive system is cut of and the specimen is brought to a state of free vibration. Damping is determined by observing the decay pattern
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tiCt e)(
Acc.
ff
Resonant freq. f1+
Sample Geometry+
End restraint+
Wave equation (torsion)
( 2
1220 2
Ts F
fHvG
Resonant Column Test: Determination of Shear Modulus of soil (G)
Resonant Column Test: Damping properties of soil (D
D = 1/2·1
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Bender Element Test (Low strain amplitude test)
Bender Elements (made by Piezoelectric material)
Bender Element Test (Low strain amplitude test)
Piezo-ceramic elements distort or bend when subjected to a change in voltage. Two Piezoelectric bender elements are placed opposite one another and inserted a small distance into a soil sample. One bender element work as source and other as receiver. The voltage in one element is varied creating shear waves through the sample, which are received by the opposite element. The input voltage, (created using a function generator) and the received signal are recorded continuously using an oscilloscope, allowing the travel time of the shear waves to be measured from which the dynamic elastic shear modulus (G) can be determined. Bender elements provide a reliable, cost effective alternative to undertaking locally instrumented stress path triaxial tests and can be readily performed on unconfined samples in the laboratory.