Dynamic Liquefaction of Ponded Fly Ashflask to split mold Vibration applied to flask to prevent clogging End of pluviation Triaxial Specimen Preparation By Pluviation 10 Specimen after

Dynamic Liquefaction of

Ponded Fly Ash

Coal Combustion Products Program

http://ccp.osu.edu

2


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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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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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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

5


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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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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

7

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


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Source: Bachus (2019)

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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)

9

Split mold is

lined with

membrane

and filled with

water


http://ccp.osu.edu

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

10

Specimen after

removal of flask

Vacuum

application to

specimen

Specimen

after removing

split-mold


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Triaxial Specimen Preparation By Pluviation

Specimen in

triaxial chamber

for consolidation

Testing at the

end of

consolidation

11


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https://youtu.be/VHNFJIvXyuI

https://youtu.be/VHNFJIvXyuI

12

Why

apply

vacuum?

13

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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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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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

16


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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

17


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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 done at Ohio State

AMO

BIG

CLI

GAV

MUS

SPO

TAN

CAR

GLE

No Liquefaction

18


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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


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Consolidated Undrained Triaxial

1. Consolidate specimen to 20 psi

2. Apply strain at a constant rate

Source: Yamamuro, Covert (2001)

20

Future Work


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


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


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

23

Thank You!


http://ccp.osu.edu

Documents

Dynamic Liquefaction of Ponded Fly Ashflask to split mold Vibration applied to flask to prevent clogging End of pluviation Triaxial Specimen Preparation By Pluviation 10 Specimen after