dynamic liquefaction of ponded fly ashflask to split mold vibration applied to flask to prevent...
TRANSCRIPT
Dynamic Liquefaction of
Ponded Fly Ash
Coal Combustion Products Program
http://ccp.osu.edu
2
Coal Combustion Products Program
http://ccp.osu.edu
Outline
• Background
• Implemented Work
• Material Properties
• Specimen Preparation
• Cyclic Triaxial Test
• Consolidated Undrained Triaxial Test
• Future Work
• Cyclic Torsional Testing
• Critical State Soil Mechanics
• Finite Element Method
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Coal Combustion Products Program
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Liquefaction
• Occurs due to contractive
nature of granular materials
• Increases pore water pressure
• Reduces shear strength &
stiffness of cohesionless
material
• Categories:
• Static
• Dynamic (Cyclic)
Image Source: Public Works Research Institute (2008)
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Coal Combustion Products Program
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Failure line
Dilative
Contractive
(liquefaction)
p, p'
qStatic Liquefaction
• Occurs in strain softening materials under
undrained conditions
• Material fails if static shear demand is larger
than capacity
• Can occur in loose cohesionless materials
Dynamic (Cyclic) Liquefaction
• Occurs due to shear stress reversals
• Requires sufficient undrained cyclic loadings
causing loss of effective confining stress
• Results in large deformations
p’
qFailure line
ESP
TSP
ESP TSP
Δu
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Coal Combustion Products Program
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Objective: Determine Geotechnical Properties of an Eastern US Power
Plant Fly Ash (Class F) for Liquefaction Potential Assessment By
Laboratory Testing
1. Cyclic Triaxial Test
2. Consolidated Undrained Triaxial Test
Isotopically Consolidated
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Coal Combustion Products Program
http://ccp.osu.edu
Why Not Use An In-situ Test Method
With A Proven Track Record?
1. Unlike Naturally Occurring Material, We Don’t Have Significant
Case History Data Available For Fly Ash Material
2. Fly Ash Is Silt Size But Behaves More Like Fine Sand
• Has Relatively High Permeability
• More Compressible Particles
• Internal Voids
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Basic Properties:
• Specific Gravity : 2.15
• Fines : 53%
• Coarse : 47%
• Minimum density* : 45.2 pcf
• Maximum density* : 61.2 pcf
• pH : 7.08
• CaO : 1995 mg/kg
Imaged at OSU-CEMAS
*Realizable dry density following EPRI (2012) suggested method
This Fly Ash
Uniformity (CU) : 7.17
Curvature (Cc) : 0.59
Coal Combustion Products Program
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Source: Bachus (2019)
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Coal Combustion Products Program
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Specimen Preparation
Reconstruction Objectives:
• Use Water Pluviation to Simulate In-Situ Deposition
• Target Relative Density* 60-65%
• Target Dry Density 54.8-55.6 pcf
• Saturated
∗𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐷𝑒𝑛𝑠𝑖𝑡𝑦, 𝐷𝑟 % =
𝛾𝑑𝑠𝑝𝑒𝑐𝑖𝑚𝑒𝑛 − 𝛾𝑑𝑚𝑖𝑛
𝛾𝑑𝑚𝑎𝑥 − 𝛾𝑑𝑚𝑖𝑛× 100
Reference: Bachus (2019)
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Split mold is
lined with
membrane
and filled with
water
Coal Combustion Products Program
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Premeasured
amount of fly
ash placed in
flask
Pluviation from
flask to split
mold
Vibration
applied to flask
to prevent
clogging
End of
pluviation
Triaxial Specimen Preparation By Pluviation
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Specimen after
removal of flask
Vacuum
application to
specimen
Specimen
after removing
split-mold
Coal Combustion Products Program
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Triaxial Specimen Preparation By Pluviation
Specimen in
triaxial chamber
for consolidation
Testing at the
end of
consolidation
11
Coal Combustion Products Program
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https://youtu.be/VHNFJIvXyuI
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Why
apply
vacuum?
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Specimen Summary
Specimen
Relative
Density, Dr
(%)
Dry
Density
(lb/ft3)
Moisture
Content
(%)
B-valueCSR
(𝛕cyc/σc’)
S2-P8 60% 54.7 80% 0.96 0.10
S3-P9 55% 54.0 79% 0.99 0.15
S1-P10 61% 55.0 79% 0.98 0.25
S2-P11 61% 55.0 85% 0.96 0.30
S3-P12 58% 54.5 81% 0.96 0.35
S2-P14 60% 54.8 77% 0.96 0.40
S1-P15 57% 54.4 83% 0.99 0.35
S3-P17 64% 55.4 81% 0.99 0.20
S3-P20 59% 54.7 76% 0.98 0.15
S2-P21 60% 54.9 73% 0.98 0.35
Average 60% 54.7 79% - -
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Coal Combustion Products Program
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Cyclic Triaxial Test Protocol
1. Consolidate specimen to 20 psi
2. Apply predetermined stress cycles until :
• εda < 20 %,
• εc/e < 20 %,
• Ncycles < 500, or
• Deteriorated waveform
Evaluation of Liquefaction
• Initial liquefaction (Δu=σ’c)
• No liquefaction beyond 500 cycles
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Coal Combustion Products Program
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0 2 4 6 8 10 12
Time (sec)
0
20
40
60
80
100
120
Excess P
ore
Pre
ssu
re R
atio (
%)
0
5
10
15
20
25
30
35
40
45
50
Exce
ss P
ore
Pre
ssu
re (
psi)
Cyclic Triaxial Test
Pressure Ratio
Excess Pore Pressure
Residual Pore Pressure
Liquefaction
0 2 4 6 8 10 12
Time (sec)
-20
-15
-10
-5
0
5
10
15
20
Str
ess (
psi)
-5
-4
-3
-2
-1
0
1
2
3
4
5
Str
ain
(%
)
ID:S2-P14, Dr:60%, B-value:0.96, CSR:0.4, nLiq:7
Deviator Stress
Liquefaction
Strain
0 2 4 6 8 10 12
Time (sec)
0
20
40
60
80
100
120
Excess P
ore
Pre
ssu
re R
atio (
%)
0
5
10
15
20
25
30
35
40
45
50
Exce
ss P
ore
Pre
ssu
re (
psi)
Cyclic Triaxial Test
Pressure Ratio
Excess Pore Pressure
Residual Pore Pressure
Liquefaction
0 2 4 6 8 10 12
Time (sec)
-20
-15
-10
-5
0
5
10
15
20
Devia
tor
Str
ess (
psi)
-5
-4
-3
-2
-1
0
1
2
3
4
5
Axia
l S
tra
in (
%)
ID:S2-P14, Dr:60%, B-value:0.96, CSR:0.4, nLiq:7
Deviator Stress
Liquefaction
Strain
Initial Stiffness
Reduced Stiffness
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Coal Combustion Products Program
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0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
1 10 100
Cyclic
Str
ess R
atio (
CS
R)
Cycles to Liquefaction (Nliq)
LIQUEFACTION TEST OF PONDED FLY ASH AT OHIO STATE
AMO
BIG
CLI
GAV
MUS
SPO
TAN
CAR
GLE
No Liquefaction
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Coal Combustion Products Program
http://ccp.osu.edu
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
1 10 100
Cyclic
Str
ess R
atio (
CS
R)
Cycles to Liquefaction (Nliq)
Liquefaction Test of Ponded Fly Ash done at Ohio State
AMO
BIG
CLI
GAV
MUS
SPO
TAN
CAR
GLE
No Liquefaction
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Coal Combustion Products Program
http://ccp.osu.edu
500, 0.098
175, 0.15
25, 0.25
19, 0.3
10, 0.35
7, 0.4
11, 0.35
43, 0.2
181, 0.15
8, 0.35
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
1 10 100
Cyclic
Str
ess R
atio (
CS
R)
Cycles to Liquefaction
19
Coal Combustion Products Program
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Consolidated Undrained Triaxial
1. Consolidate specimen to 20 psi
2. Apply strain at a constant rate
Source: Yamamuro, Covert (2001)
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Future Work
Coal Combustion Products Program
http://ccp.osu.edu
Cyclic Torsional Testing
Advantages
• Better simulates earthquake induced
stress path
• Drainage and confinement are
controllable
Challenges
• Complicated loading mechanism
(torsion)
• Uncommon load frame
p
q
Cyclic Triaxial
Earthquake
Tension + Torsion Frame at Ohio State
Source: http://www.eng.hokudai.ac.jp/labo/soilmech/facilities_en.html
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Future Work
Coal Combustion Products Program
http://ccp.osu.edu
Critical State Soil Mechanics
Advantages• Based on state-independent critical state
concept
• Well-suited for numerical modeling
Disadvantages
• Limited amount of research and data
available for fly ash
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Future Work
Coal Combustion Products Program
http://ccp.osu.edu
Finite Element Modelling
Implement CSSM material model in Plaxis 2D
• Allows custom material model implementation (material non-
linearity)
• Allows large deformation analysis (geometric non-linearity)
Numerical simulation items
• Elastoplastic load-deformation
• Pore water pressure
• Stress path
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Thank You!
Coal Combustion Products Program
http://ccp.osu.edu