Engineering Behavior of Slurry Consolidated Fly Ash Samples

Jalila Elfejji1, Youngjin Park2, and Miguel Pando3,

1Department of Civil Engineering, University of Wisconsin-Madison

2 co-Mentor, EPIC, UNC Charlotte 3Mentor, Department of Civil & Environmental Engineering, UNC Charlotte

Abstract

Fly ash is a residual product of burning coal and other fossil fuels. The vast volume in the United States

typically comes from coal-fired power plants. About 86 million tons of coal ash is produced annually in

the United States where 78% of that amount is fly ash. There are different methods to help mitigate the

potentially harmful effects of fly ash including reusing the ash for construction materials or using wet

storage of this material in “ash ponds.” This research focuses on the wet deposition of fly ash in coal ash

ponds. For this situation, the fly ash is mixed with water to form a slurry, in order to reduce the amount

of dust that is produced and to ease the pumping of the sluiced ash into the pond. Although ash ponds are

still commonly used because of its relatively low cost and ease of construction and management, in recent

years there has been major concerns related to the stability and possible failure of these ponds. An

example of such a failure is the Kingston ash spill of the Tennessee Valley Authority. Failures of ash

ponds can have serious environmental consequences and endanger livestock and humans located

downstream of the failure. Related to the environmental consequences besides surface damage, the

chemicals contained in the fly ash also have the potential to seep into soil and reach the groundwater

which can cause major health and environmental risks. In order to prevent future spills, there are several

research efforts to improve the stability of fly ash ponds. For this we need a good understanding of the

engineering properties and behavior of fly ash. This research focuses on characterizing the properties of a

fly ash from a power plant located in the southeastern region of the United States. Additional to the

characterization of the ash, we present slurry consolidation tests to understand the behavior of

consolidation expected at an ash pond. Consolidation tests were carried out using two approaches. The

first approach used gravity as the driving force to settle the fly ash particles. A 12” tall and 6” in diameter

transparent cylinder was filled with water and a specified amount of fly ash was spooned into the

container. Once thoroughly mixed, the falling elevation of the ash sample (mudline) was recorded over

time, and the normalized height of the ash vs. time as well as the total unit weight vs. time was plotted.

The second approach used a pressurized cell to densify the ash within the range of unit weights seen in

the field. Pressure ranging from 20-60 psi was applied to an 8 and 12 lb. sample to drain the excess water.

Similar to the first approach, the mudline was recorded over time. Based off of the final height of the ash

sample, the final unit weight was determined and checked to see if it fell within the target range. Samples

were extracted using small shelby tubes to be used for future static and dynamic Triaxial tests. The

layering and heterogeneity of the samples will be carefully studied by sampling the sediment ash at the

top, middle, and bottom layers of the sample. The ultimate goal is to use the measured engineering

properties of the slurried ash as a function of water content and density to help assess the stability of

typical ash pond facilities.

Introduction

Background

The burning of coal and other fossil fuels produces steam to power wind turbines for. A harmful result of

this process is the formation of particulate matter commonly known as fly ash. These fine spherical

particles contain dangerous chemicals such as carbon and arsenic, which pose a significant threat to the

environment and public health if not disposed of correctly. Although fly ash has several alternative civil

engineering applications, according to Electric Power Research Institute, approximately 86 million tons of

coal ash is produce annually in the U.S where fly ash accounts for 78% of the total ash (3). The main

concern of fly ash disposal is the potential of these chemicals seeping through underlying soil and

contaminating groundwater sources. Previous studies have found that the presence of fly ash affects both

the physical and chemical behavior of surrounding soil. With the addition of fly ash, the relative densities

of surrounding soil are reduced, hydraulic conductivity is increased, and rain easily dissolves the toxic

chemical compounds. (5)The fineness of fly ash particles makes them easily susceptible to wind activity.

For easy control of the material, the fly ash is mixed with water to form slurry, and is disposed of in large

containment areas known as ash ponds. According to the EPA, the Southeastern region of the United

States houses 40% of the nation’s ash ponds containing about 118 billion gallons of slurry, but

unfortunately there are few regulations that govern its safe disposal (2).

On December 22 2008, the TVA Kingston Plant in Harriman Tennessee collapsed and released

more than 5.4 million cubic yards of ash spreading across 400 acres. The spill had ravaged 12 homes,

caused a train accident, and contaminated the Emory River. Later investigations revealed that the

underlying layer of the slurry was unstable, and hadn’t been noticed in previous TVA inspections. The

ash underwent significant amount of liquefaction and creep (4). Figure 1 shows damaged homes and

Figure 2 shows the range of ash before and after the containment spill.

Figure1. Left: Kingston TVA plant before the spill. Right: Kingston TVA plant post spill.

Coal-Fired Plants

According to the Electric Power Research Institute, 37% of electrical energy produced in the United

States comes from the burning of coal. There are 4 basic steps that turn coal into power: first, coal is

finely milled to the consistency of talcum powder, mixed with hot hair and blown into a firebox. The coal

and air combust, producing heat. Water is then pumped by pipes through the boiler and is turned into

steam by the heat that was produced in the firebox. From there, the steam turns a series of turbine blades

that are connected to the generator which produces electricity. Lastly, excess steam is drawn into the

condenser where cool water from nearby water sources is pumped through tubes in order to convert the

steam back into water which can be used again to repeat the cycle.

Figure 2. Left: Coal fire plant process Right: Location of Belews Creek coal ash plant.

Literature Review

Due to the TVA Kingston Plant disaster and other problems concerning the stability of ash ponds,

research carried out by the Electric Power Research Institute has been done to characterize the

engineering properties of ponded fly ash. These properties included gradation, specific gravity of solids,

consolidation, permeability, and strength tests. Ash samples were also prepared to analyze the potential

for static liquefaction by performing drained and undrained Triaxial tests. Results from this study have

shown that ponded fly ash has dilative properties when undergoing shear which hinders its potential for

static liquefaction (7)

Methods

Sedimentation Test

In order to densify the ash samples to the correct unit weights seen in the field (Figure ), two different

approaches were carried out. Because there are no standard testing procedures for sedimentation tests, the

first approach used gravity as the driving force to settle the fly ash particles. The materials needed for this

experiment included: transparent cylinder 6” in diameter and 12” tall, 6” diameter perforated bottom cap,

and a 6” diameter cloth mesh. First, the cloth mesh was taped to the perforated bottom cap and placed at

the bottom of the cylinder. The cylinder was than filled with 7” of water from the base of the cylinder. For

the first sample, 8 lbs of dry fly ash was weighed followed by 6 lbs, 4lbs, and 2lbs for the remaining 3

tests. The 8 lbs of fly ash as gradually spooned into the water, making sure to spoon in all remaining dust.

The slurry was stirred for a couple minutes until evenly mixed and a cap was placed on the cylinder to

prevent evaporation. Once the cap was placed, the time it took for the ash sample to drop a quarter inch

was recorded until it reached equilibrium. The same process was repeated for the remaining fly ash

samples.

Figure 3. Left: Transparent cylinder, perforated bottom cap, and cloth mesh. Right: Experimental set-up.

Consolidation Test

The second approach for the consolidation tests was to use a batch consolidator in order to drain the

excess water from the slurry mixture. For the experiment, the cell was filled with 5” of water from the

bottom porous stone. 12 lbs of fly ash was spooned into the water and mixed for a couple minutes until

combined. The top cap of the cell was then attached, and the pressure valve was opened with a pressure of

40 psi applied. After the excess water was drained, or until the top porous stone touched the top of the ash

layer, the pressure valve was closed. The height of the top rod and the height of the ash sample were then

recorded. The pressure valve was then opened, and a pressure of 40 psi was applied for 48 hours followed

by 60 psi for three days. Calculations for determining the targeted unit weight is shown in the appendix.

6”

7” 12”

Perforated Bottom Cap

Sampling Methods

After extracting the ash from the batch consolidator, a small amount of ash was taken from the top,

middle, and bottom layers to determine the respective water contents. Each amount was placed in a

weighed tin, and the mass of the wet ash plus the tin were recorded for each. After the samples were

placed in an oven for 24 hours, the mass of the dry ash plus tins were recorded. Equation 2 was used to

obtain the water content from these measured values. Also, a1.5” diameter shelby tube was pressed into

the ash to obtain a sample for Triaxial testing. Another way that an ash sample could be prepped for

Triaxial testing is my using a sample trimmer. For this experiment however, the sample trimmer molded a

sample that was too large, so it started to slump before it could be placed in the cell.

Consolidated Undrained Triaxial Test

The Consolidated Undrained (CU) Test is one of the most common tests performed on soil samples. The

benefits of performing this test is the ability to determine the undrained shear strength, compression

index, stiffness, permeability, and many more properties of the sample. There are three phases of a CU

test which include saturation, consolidation, and shearing. The purpose of the saturation phase is to ensure

that all voids are filled with water, which is done by increasing the pore and confining pressure of the

sample. The purpose of the consolidation phase is to bring the ash sample to the effective stress required

Pressure Valve Top Rod

Top Porous Stone

Stone

Bottom Porous Stone Drainage

Tube

Figure 4 Left: Batch consolidator. Middle: Pressure Control Panel. Right: Drainage tube.

connection.

1 2

Figure 5. Left: Ash sample after consolidation test. Middle: Sample taken from shelby tube. Right: Sample trimmer.

0

0.2

0.4

0.6

0.8

1

1.2

0 5000 10000 15000 20000

H(t

)/H

0 (

in)

Time (s)

Normalized Height vs Time of Duke Belews Creek Fly Ash

Fluid Unit Weight= 132.5 pcf

Fluid Unit Weight=97.4 pcf

Fluid Unit Weight= 80 pcf

Fluid Unit Weight= 114.9 pcf

for shearing which was done by increasing the confining and back pressure. Lastly, a deviator stress was

added vertically to the ash sample until it reached failure.

Figure 6. Consolidated Undrained Triaxial test set-up.

Results

Sedimentation Tests:

Deviator Stress (q)

Confining Stress (σc)

Pore

Pressure

Figure 7. Normalized height vs. time plot.

35

45

55

65

75

85

95

105

115

125

0 5000 10000 15000 20000

Satu

rate

d U

nit

Wei

ght

(pcf

)

Time (s)

Fluid Unit Weight=132.5 pcfFluid Unit Weight=97.4 pcfFluid Unit Weight= 80pcfFluid Unit Weight=114.9 pcf

Field Saturated Unit Weight

Field Saturated Unit Weight

Figure 9. Saturated unit weight vs. time plot.

Consolidation Tests:

Figure 9. Soil properties after consolidation test.

Figure 10. Water contents of ash layers.

Final Height of Ash (in)

Total Volume (VT) (in

2)

Volume of water (Vw) (in3)

Weight of water (Ww) (lb)

Water

content (s)

Dry Density

(d) (pcf)

Wet Density

(m) (pcf)

8.83 249.52 101.49 3.66 0.31 83.14 108.53

Ash Layer Water Content (%)

Top 0.334135442

Middle 0.337677725

Bottom 0.338436078

CU Triaxial Tests:

Figure 11. Top: Axial strain vs. deviator plot. Bottom: Axial strain vs. pore pressure plot.

0

10

20

30

40

50

60

70

80

90

0 5 10 15 20 25

q (

psi

)

Axial Strain (%)

-25

-20

-15

-10

-5

0

5

10

15

0 5 10 15 20 25

Po

re P

ress

ure

(p

si)

Axial Strain (%)

Figure 11. P vs deviator stress plot.

Discussion

For the sedimentation tests, gravity was not enough to densify the ash sample to the target saturated unit

weight range of 84.pcf-120.5 pcf. Figure 9 shows that for the four different ash tests, the final saturated

unit weights were around 65 pcf or lower. It can also be determined that the higher the slurry unit weight

of the ash sample, the slower the sedimentation rate. Because of this, the ash samples for the first

approach were not suitable for Triaxial testing. For this type of set-up, it cannot be assumed that the

density is uniform throughout the sample, the top layer of the sample was soft and soupy while the bottom

layer was denser. The consolidation tests yielded much different results. For a sample with 0.082 ft3 of

water and 12 lbs of dry ash, the final slurry unit weight came out to be 108.53 pcf which is within the

target range. The ash particles were uniform throughout the top, middle, and bottom layers with water of

contents of 33.3%, 33.6%, and 33.7% respectively. The sedimentation tests (Method A) did not produce

samples dense enough for Triaxial testing, but it did give us a good representation of how the slurry will

settle overtime in an ash pond. Method B, involving a batch consolidometer, was successful in replicating

field densities. A CU Triaxial test on a slurried ash confirmed wet pond ash is very soft and weak (as per

stiffness and strength obtained). Additionally the fly ash specimen exhibited a dilative behavior under

undrained shear.

y = 0.5749x

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100 120 140 160

q (

psi

)

p' (psi)

Appendix

Equation 1: 𝛾𝑡=

62.4 ∗𝑉𝑤+𝑊𝑠

𝑉𝑤+𝑉𝑠

Where:

𝐺𝑠 =𝛾𝑠

𝛾𝑤=

𝑊𝑠𝑉𝑠

𝛾𝑤

𝛾𝑤 = 62.4 pcf

𝑉𝑠 =𝑊𝑠

𝐺𝑠 ∙ 𝛾𝑤

𝑉𝑇 = 𝑉𝑠 + 𝑉𝑤

𝑉𝑤 = 𝑉𝑇 − 𝑉𝑠

𝑊𝑤 = 𝛾𝑤 ∙ 𝑉𝑤

Equation 2: 𝑀1−𝑀2

𝑀2−𝑀𝑐

Where:

M1= Mass of tin+ wet ash

M2= Mass of tin + dry soil

Mc= Mass of tin

Literature Cited

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energy.com/pdfs/Coal_Ash_Human_Health.pdf TVA Kingston Fossil Plant coal ash spill. (August 2012). Source Watch. Guiseppe Ferrailo, Mario Zilli, Attilio Converti. (2007). Journal of Chemical Technology and

Biotechnology. Volume 47, Issue 4, 281-305. Catawba Riverkeeper. Duke Energy Dan River Coal Ash Spill Updates. netCorps. Accessed at

http://www.catawbariverkeeper.org/issues/coal-ash-1/duke-energy-dan-river-coal-ash-spill-what-do-we-

currently-know-what-do-we-need-to-know K. Ladwig. Geotechnical Properties of Fly Ash and Potential for Static Liquefaction. (2012). Electric

Power Research Institute. Volume 1, 1.1-5.7 Geosyntec consultants. (2012). Geotechnical Properties of Fly Ash and Potential for Static Liquefaction.

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