CASE STUDY ON USAGE OF THERMAL PULVERISED FUEL ASH (FLY ...

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JOURNAL OF RESOURCE MANAGEMENT AND TECHNOLOGY

Page No:356 www.jrmat.com Vol 12, Issue4, 2021

CASE STUDY ON USAGE OF THERMAL PULVERISED FUEL ASH

(FLY ASH) IN UNDERGROUND EXCAVATIONS FILLING 1V NARASIMHAMURTY PAPPU, 2Mr.K.MANOJ, 3Mr. K.S. RAMBABU

1M.Tech Student, 23Assistant Professor

Department of Mechanical Engineering(MINING)

KAKINADA INSTITUTE OF TECHNOLOGICAL SCIENCES, Ramachandrapuram ABSTRACT

In order to prevent future ground stability issues in

the form of subsidence, it is necessary to back fill

mine voids. The use of mill tailings and river sand

as backfilling materials in mines has been

widespread for a long time. Nonetheless, tight

regulation and a scarcity of river sand in India have

posed a significant challenge to the country's

mining sector. The necessity to find alternative

engineering materials that can be used in place of

sand has risen to the forefront of public debate. A

significant issue arises from the large amounts of

fly ash emitted by coal-fired power plants, not only

because of the shortage of land accessible for its

disposal, but also because of its psychological and

environmental consequences. The amount of fly

ash accessible may be recycled, mostly via the

addition of fly ash to concrete. However, the

amount of fly ash that can be added to cement is

restricted since the pace at which cement is

produced is limited, as is the amount of fly ash that

can be contained inside the cement. Fly ash

composite materials (FCMs) were produced in this

research using fly ash collected from an adjacent

captive thermal unit and three different types of fly

ash. The primary constituents of the composite

were fly ash, lime, gypsum, and cement, with the

rest being minor components. The FCMs' physical

and engineering characteristics were established to

a fine level of detail. After 28 days of curing time,

significant improvements in compressive strength

were found, and it was determined that the fly-ash

composite produced had the potential to be utilised

as a replacement for sand for back filling mine

voids.

1. INTRODUCTION

During the 12th five-year plan period, India is

projected to generate more over 2,50,000

megawatts (MW). The coal output will increase

from 550 metric tonnes per year to 1000 metric

tonnes per year. Large subsurface holes that have

been created as a result of mining activities have

been causing a variety of ground stability issues in

several mining locations across India. Mine

subsidence is a manifestation of the gravitational

pressure on strata that have become unstable as a

result of the removal of their natural support across

a sufficiently wide region of land. Because of the

formation of opposing pressures at the lower level,

the strata sink slowly and at a varied pace from

seam to surface until there is a restoration of

equilibrium at the lower level. A fairly frequent

occurrence in many coal mining regions is

subsidence, which is caused by soil movement. As

soon as the excavation is completed, the states of

equilibrium of the strata are disrupted, and a variety

of pressures begin to exert their influence. The

weight of the strata above the roads is transmitted

to the coal pillars as the beds descend, momentarily

relieving them of the weight of the strata above

them. As the beds descend, the weight of the strata

above the roadways is temporarily relieved.

As the depth of mine is increased, the area

impacted by subsidence grows in proportion to the

depth of mining. Depending on the circumstances,

the maximum sinking may be as much as 90% of

the seam thickness on average and in certain cases

as much as the seam thickness itself [7, 8, 9].

Longwall mining involves nearly complete

extraction of the seam; as a result, subsidence

occurs on a regular basis and is usually greater than

in bord and pillar mining. It is impossible to

complete extraction in bord and pillar works, and

stocks or pillars are left in the goaf, which impedes

the settlement and sinking of the land. A significant

number of subsidence issues have been

documented to have happened unexpectedly and

beneath densely populated regions, resulting in the

collapse of structures and the deaths of people in

the vicinity. Subsidence issues often persist and

pose a significant danger to the future growth of

townships. Ground subsidence causes damage to

surface structures such as houses, railway tracks,

sewers, canals, and roadways, among other things,

because to the movement of the earth's crust. The

most significant damage happens where the

curvature is greatest, rather than necessarily where

the most significant subsidence occurs.

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When it comes to bulk fly ash use, mine backfilling

has proven to be an appealing alternative for those

coal-fired power stations situated near coal mines.

Backfilling, also known as sand stowing, has been

the method of choice for decades to combat the

issue of ground sinking as well as to enhance the

recovery of pillars. It has been shown that using

hydraulic sand stowing at a mine in the Jharia

coalfield, it is feasible to work a coal seam 7.5 m

thick below a railway line without experiencing

any detrimental subsidence consequences [7]. Mill

tailings, waste rock, quarried rock, sand and gravel

are some of the most frequent kinds of materials

used for backfilling, as are sand and gravel.

1.1 PROBLEMS ASSOCIATED WITH

FLY ASH GENERATION

India is the world's third-largest producer of coal,

and it also possesses the world's fourth-largest coal

reserve. The most significant issue connected with

fly ash is that its disposal not only necessitates the

use of vast amounts of land, water, and energy, but

also its fine particles, if not handled correctly, may

become airborne and pose an environmental threat

to nearby communities. India presently generates

160 metric tonnes (MT) of fly ash per year. Flyash

ponds cover about 300 km2 of land and are used

for many purposes. With such a large amount of

flyash, there are difficulties not only in terms of

land usage, but also in terms of health risks and

environmental deterioration. Due to the fact that

coal presently accounts for 70% of the country's

electricity production, there is an urgent need for

new and creative ways of decreasing the country's

environmental effect on coal. The following are

some of the issues that may arise as a result of fly

ash.

1. Fly ash is very difficult to manage in the dry

condition since it is so fine and easily dispersed

even in a little breeze.

2. It pollutes the air and water, as well as disrupting

the ecosystem of the surrounding environment.

3. Fine ash particles in the air cause difficulties for

individuals who live in close proximity to power

plants. It also has a negative impact on horticulture

since it corrodes structural surfaces.

4. The eventual settling of fly ash particles across

several hectares of land in the vicinity of a power

plant results in a noticeable degradation in the soil

properties of the surrounding area.

5. Inhalation of fly ash may result in silicosis,

fibrosis of the lungs, bronchitis, and pneumonitis,

among other diseases.

Table 1.1: Diseases due to the presence of heavy metals in fly ash

Metal Content (ppm) Possible Diseases

Chromium (Cr) 136 Cancer

Nickel (Ni) 77.6 Respiratory Problem, Lung Cancer

Lead (Pb) 56 Anemia

Arsenic (As) 43.4 Skin Cancer, Dermatitis

Antimony (Sb) 4.5 Gastroenteritis

Cadmium (Cd) 3.4 Anemia, Hepatic Disorder

1.2. NEED FOR UTILIZATION OF FLY ASH

According to the 12th five-year plan, 2,50,000

megawatts (MW) of energy would be generated,

necessitating the production of 1000 metric tonnes

of coal each year. As a result, fly ash production is

expected to reach 600 million tonnes per year by

the year 2030. Despite the fact that Orissa is not a

heavily industrialised state, the state generates

about 93 lakh tones of fly ash each year on average.

The use of fly ash in Orissa's thermal power plants

is only around 22.6 percent, according to the state's

energy department. If this pattern continues in the

future, it is possible that a significant quantity of

land will be required for the disposal of fly ash.

In India, there are about 86 large coal-fired thermal

power plants, according to the Central Electricity

Authority of India. There are more than 1800

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industrial units that have captive thermal power

plants that generate more than 1 MW of energy,

which have been chosen for this study.

The Current Situation Regarding Fly Ash in India

• Thermal power accounts for approximately 66

percent of total installed power generation; 350-400

million metric tonnes (MT) of coal are consumed

annually; high ash contents ranging from 30 to 50

percent; more than 180 million metric tonnes (MT)

of ash are generated annually; Ash generation is

expected to reach 600 million metric tonnes by

2030; According to Ministry of Environment and

Forest figures, 30 percent of ash is used in fillings,

embankments, and other construction projects;

Because of the high potential for use of fly ash

generated as a consequence of the combustion of

Indian coal, it is being explored for a variety of

purposes.

When compared to other nations such as Germany,

the Netherlands, and other European countries,

India's present percentage of fly ash usage is very

low when compared to other countries whose

utilisation is more than 90%. Due to the fact that

thermal energy accounts for approximately 66

percent of the country's total installed power

generating capacity, of which coal-based

generation accounts for 90 percent, About 86 main

thermal power plants and numerous captive power

plants, which utilise bituminous and subbituminous

coal and generate a significant quantity of fly ash,

are operational. The high ash content (30 percent -

50 percent ) of coal leads to the enormous

quantities of fly ash that accumulate. In addition,

the country's reliance on coal for electricity

production has remained constant. As a result, fly

ash management has emerged as a significant

source of worry for the foreseeable future.

1.3. AIM AND OBJECTIVES

The investigation's goal was to minimise the issue

of subsidence in order to preserve the surface

feature by mine void backfilling as well as the

efficient use of flyash, both of which were

investigated. The following were the particular

goals that needed to be met in order to achieve the

aim.

1. Determination of the characteristics of fly ash.

determining the geotechnical characteristics of a

composite material made of fly ash

3. At each separate OMC-MDD, the development

of composite materials made of fly ash, lime,

gypsum, and cement is underway.

Fourth, geotechnical characteristics such as

Unconfined Compressive Strength (UCS),

Brazilian Tensile Strength (BTS), and Ultrasonic P

wave velocity are determined.

5. Determination of stress and convergence across a

stowed region utilising a flyash composite material

via numerical modelling simulation and numerical

simulation

METHODOLOGY

Almost all thermal power stations are typically

situated close to coal mines in order to minimise

the cost of coal transportation to the power plant.

They generate a huge quantity of flyash, which

poses a significant issue in terms of disposal and

contributes to environmental degradation. Flyash is

used in a variety of geotechnical applications.

However, it is yet to be determined if it can be used

effectively in the field of mine backfilling. As a

result of the Indian government's decision to make

the use of flyash in mine backfilling a requirement,

this area has begun to get more attention.

The current investigation focuses on the usage of

fly ash-based composite materials for mine

backfilling, as well as the evaluation of their

effectiveness in terms of minimising convergence.

The findings of the study would be beneficial in

decreasing the subsidence issue in underground

mines as well as boosting the likelihood of flyash

being used in other applications. This study was an

effort to use fly ash in various compositions with

lime, gypsum, and cement in order to increase the

strength of the fly ash, and the results were

promising. The general strategy used to accomplish

the different goals necessary to reach the aim is

described in more detail below.

a. Review of existing literature in order to critically

acquire information on mine backfilling,

geotechnical characteristics of fly ash, strength

boosting materials, and other related topics.

b. Designing an experimental setup and

determining the composition of the components.

c. The development of composite materials

including fly ash and lime, fly ash and gypsum, and

fly ash and cement.

d. Testing and analysis such as moisture density

relationship, Slump cone test, Triaxial test,

Brazilian tensile strength, and ultrasound pulse

velocity are used to determine the geotechnical

characteristics of the composites that have been

created.

e. A simulation to examine the behaviour of

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stresses across a stowed region is being considered.

Figure 1.1: Flow chart of the methodology

2. Literature survey

When pulverised coal is burned at high

temperatures and pressures in power plants, coal

ash is formed as a mineral residue. Coal ash is

produced as a byproduct of the combustion process.

There are three kinds of byproducts produced

during coal combustion: fly ash, boiler slag, and

bottom fly ash. Fly ash is the most common form

of byproduct.

Fly ash accounts for 75-80% of the entire amount

of ash generated. Cement production, road

building, and brick manufacturing are all projected

to account for 25 percent of total fly ash use in

India.

FLYASH GENERATION AND COLLECTION

The fly ash generated by the combustion of

pulverised coal in a coal-fired boiler is referred to

as fly ash. Typically, coal is crushed and sucked

into the boiler's combustion chamber with the help

of air. Where it ignites instantly, creating heat and a

molten mineral residue as a result of the reaction.

Burning liquid mineral residue causes the boiler

tubes to remove heat from the boiler, chilling the

flue gas, causing the molten mineral residue to

solidify and produce ash (Figure 2.1).

Sphere-shaped particles make up the majority of

the material's fine texture. It is referred to as

‘Pozzolans' because it resembles volcanic ash and

is extremely similar in appearance. Fly ash is

formed by the combustion of different organic and

inorganic components contained in feed coals at

temperatures ranging from 1200 to 17000 degrees

Celsius. The ash percentage in Indian coal ranges

from 30 to 50%, depending on the type of coal.

Flyash is a word used to describe the finer ash

particles that are lighter in weight and stay floating

in the flue gas. While coarse ash particles, also

known as bottom ash or slag, fall to the bottom of

the combustion chamber, fine ash particles fall to

the top of the combustion chamber. Particulate

emission control equipment, such as electrostic

precipitators and filter cloth baghouses, are used to

remove flyash from the atmosphere.

It is important to note that the components of flyash

vary significantly depending on the source of the

coal being burnt, but that all flyash contains

significant quantities of silicon dioxide and calcium

oxide, both of which are naturally occurring

elements in many coal producing rock strata.

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Figure 2.1: Coal ash generations from a pulverized coal-fired boiler [4]

Flyash accounts for about 80% of the ash that exits

the furnace. Wet-bottom (or slag-tap) furnaces burn

pulverised coal at high temperatures, resulting in

50% of the ash being retained in the furnace and

50% being entrained in the flue gas during

combustion. When crushed coal is utilised in a

cyclone furnace, 70 to 80 percent of the ash is kept

as boiler slag, with just 20 to 30 percent of the ash

exiting the furnace as dry ash in the flue gas.

FLY ASH

Fly ash is the tiniest of coal ash particles, and it is

used to make fly ash. Due to the fact that it is

carried out from the combustion chamber by

exhaust gases, it is referred to as "fly ash." Fly ash

is made up of tiny particles that range in size from

0 to 50 microns.coal, comprising mostly of non-

combustible materials in coal and a little amount of

carbon that remains after partial combustion of the

coal, may be produced from the mineral content in

coal at various sizes, even 150 microns in certain

cases.

It is usually grey in colour, abrasive, typically

alkaline, and refractory in nature, and it is produced

by burning coal. Pozzolans, which are siliceous or

siliceous and aluminous minerals that, when mixed

with water and calcium hydroxide from

cementitious products at room temperature, are

referred to as admixtures in the construction

industry (Figure 2.2 ).

Figure 2.2: fly ash (http://geoinfo.nmt.edu/staff/hoffman/flyash.html)

http://geoinfo.nmt.edu/staff/hoffman/flyash.html)

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Its geotechnical features (such as specific gravity,

permeability, internal angular friction and

consolidation characteristics) make it an excellent

choice for use in the building of highways and

embankments, among other things.

Because of the pozzolanic characteristics of fly ash,

particularly its ability to bind lime, it is helpful in

the production of cement, construction materials,

and concrete admixed products, among other

applications.

Figure 2.3 shows a comparison of the diameters of

fly ash particles and the sizes of soil particles from

several kinds of soils.

Figure 2.3: Comparison of fly ash particles to those of several soils (Meyers et al, 1976)

CHARACTERIZATION OF FLY ASH

Thermal power stations that use coal to generate

electricity are experiencing significant difficulties

in managing and disposing of fly ash. It becomes

increasingly difficult to solve the issue since the

coal in India has a high ash percentage (between 30

and 50 percent). The number of thermal power

plants in operation now is about 86, with a total

annual output of roughly 180 million tonnes of coal

ash. The need for a big disposal area is a significant

source of worry, since it should not have a negative

impact on the surrounding ecosystem. As a result,

efforts are being undertaken to make use of the fly

ash in large quantities rather than just dumping it.

Coal ash is only used in bulk in geotechnical

engineering applications such as backfilling soil,

building embankments, and serving as a sub-base

material, among other things. It is necessary to

have a thorough knowledge of the physical,

chemical, engineering, and leaching behaviour of

the material. In order to do so, it is necessary to

characterise the fly ash in terms of its potential

geotechnical uses.

Table 2.1: Typical Chemical characteristics of fly ash [1]

Constituents Percentage

Carbon 2.10

Volatile matter 0.147

Fe2O3 8.83

MgO 0.84

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Al2O3 27.73

SiO2 58.9

P2O5 0.17

SO3 0.24

K2O 0.79

CaO 1.11

Na2O 0.14

TiO2 2.09

Table 2.2: Physical characteristics of fly ash

Parameters Value

Colour Light gray

Dry density, kg/m3 1.208 g/cm3

Optimum moisture content ,% 30

Permeability m/sec (3.5-3.7)* 10-6

Liquid limit % 40.89

Plastic limit % Non –plastic

Specific gravity 2.54

STATUS OF UTILIZATION OF FLY ASH

Fly ash has a variety of uses in a variety of

industries and is thus handled as a by-product

rather than as a waste. Because of its pozzolanic

properties, it is utilised as a raw ingredient in the

production of cement. Backfilling in opencast

mines, reclamation of low-lying regions, stowing in

underground mines, brick manufacture, road and

embankment building, and structural fills are some

of the other main applications for fly ash now being

explored. At the moment, fly ash is used to a

degree of about 50%. In addition to the use of fly

ash in mine backfilling and roadway/flyover

embankment construction, the production of

various building components such as bricks, tiles,

and blocks as well as its usage in agriculture are

also possible uses.

Table 2.3: Share of fly ash in various sectors

Area of Utilization Utilization(%) Utilization (MT)

Manufacture of Cement 44 35

Construction of Road Embankments 19 15

Substitution of cement 12 10

Back Filling in Mines 9 7

Reclamation of low lying Areas 7 6

Raising of Ash Dykes 4 3

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Brick Manufacturing 4 3

Agriculture 0.5 0.5

Others 0.5 0.5

Figure 2.4: Progressive utilization of flyash in cement industry during the period 1998-99 to

2010-11 (CEA REPORT, 2011)

3. METHODOLOGY

The investigation's main goal was to improve the

strength of fly ash composites, which were used for

backfilling an underground coal mine in the first

place. This chapter discusses the techniques and

resources that were utilised to achieve the objective

in question. The primary constituents were fly ash,

lime stone, gypsum, and cement, to name a few.

The production of samples, the numerous

techniques used for characterisation of components,

and the creation of diverse composite materials are

all covered in this chapter.

SAMPLE COLLECTION

It was necessary to collect fly ash, which is a

byproduct of coal combustion, from a neighbouring

power station. The facility is located in close

proximity to two large coalfields. It uses 2230

tonne of coal per year and generates about 600

tonne of fly ash per year, according to the

company. It is customary to dispose of flyash in a

neighbouring pond area.

The fly ash utilised in this research was collected in

dry form from the electrostatic precipitators at the

facility where it was generated. When pulverised

coal is burned, the volatile stuff is evaporated and

the bulk of the carbon is burned, resulting in a

cleaner burning product. In addition to coal

disintegration, the mineral stuff linked with coal

such as quartz, clay and feldspar disintegrates.

Electrostatic precipitators are used to capture the

finer particles that escape with the flue gases and

are stored in a hopper. The exits on the hoppers are

very tiny. In order to collect the dried flyash,

heavy-duty poly-coated cotton gunny bags with a

50 kg capacity were employed. Each bag was

sealed immediately after the collection of fly ash.

After that, the bags were transferred with care from

the factory to the laboratory, where they were

stored in a controlled atmosphere.

METHODS AND MATERIALS

This section describes the methods and materials

that were utilised in conducting the different

experiments on fly ash composites in order to

employ them in Geotechnical Applications of mine

filling, as well as the results of those tests. The

procedures in this part explain the techniques for

characterising fly ash, as well as the procedures for

producing specimens of lime-fly ash, gypsum-fly

ash, and cement-fly ash mixes. Fly ash takes up a

significant amount of space for disposal and poses

a risk of local ground water and surface water

contamination due to the leaching of metals. The

recycling of fly ash into a safe composite material

for use in mine backfilling and other applications

may help to alleviate the issue. Fly ash is utilised in

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a variety of civil engineering applications because

of the inherent self-hardening characteristics of the

material. However, it does not have sufficient

strength or durability. One of the ways for

increasing the strength of the fly ash is to use the

appropriate combination.

FLY ASH COMPOSITE

CHARACTERIZATION

Fly ash composite characterization test were

performed on the fly ashes that were investigated

in this project. Tests included compressive

strength, tensile strength. In addition to this

moisture content and true density of fly ash were

found.

COMPACTION TEST

This test is performed to determine the relationship

between the moisture content and the dry density of

a soil for a specified compactive effort. This test

will employ the tamping or impact compaction

method using the type of equipment and

methodology developed by R. R. Proctor in 1933,

therefore, the test is also known as the Proctor test.

ASTM D 698 - Standard Test Methods for

Laboratory Compaction Characteristics of Soil

Using Standard Effort (600 KN-m/m3) was used.

In Standard Proctor Test, the flyash is compacted

by making a 2.5 kg hammer fall a distance of one

foot into a flyash filled mold. The mold is filled

with three equal layers of flyash, and each layer is

subjected to 25 drops of the hammer.

Figure 3.1: Standard Proctor Mould and Hammer

The optimum water content is the water content

that results in the greatest density for a specified

compactive effort.

Water to add (ml) = flyash (gm)*0.16

1gm of water is equal to approximately one

milliliter of water.

The mutual relations is given by the following

equation.

𝜌

𝜌𝑑 = 1 + 𝑤

𝜌𝑑=dry density

𝜌= wet density

W= moisture content in percentage

SLUMP CONE TEST (ASTM METHOD C143)

Determining the consistency of concrete by filling

a conical mold with a sample of concrete, then

inverting it over a flat plate and removing the mold;

the amount by which the concrete drops below the

mold height is measured and this represents the

slump. This test method describes the procedure for

determining the slump of fresh concrete mixtures.

Figure 3.2: Slump Test Apparatus

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BRAZILIAN TEST (ASTMD 3967)

The tensile strength is determined by indirect

method called Brazilian test.

Procedures: Core specimens with length-to-

diameter ratios (L/D) of 0.5 are placed in a

compression loading machine. The maximum load

(P) to fracture the specimen is recorded and used to

calculate the split tensile strength.

Figure 3.3: Setup for Brazilian tensile test in standard loading machine

The samples were placed diametrically during test.

The sample fails diametrically in tension by

application of load. The indirect tensile strength is

calculated as:

2𝑃

𝜎𝑡 = 𝜋𝐷𝑡

4. Result and discussion

The study's main goal was to create a composite

material made of flyash, lime, gypsum, and cement

that may be used as an alternative to sand

backfilling in mines in the future. Backfilling is

done in order to alleviate the surface sinking issue

caused by voids produced by underground mining

operations. The fly ash was combined with cement,

gypsum, and lime to improve the pozzolanic

characteristics of the finished product. The findings

of the test that was performed are presented in this

section.

COMPACTION TEST

The optimum moisture content was 30% and the

dry density was found out to be 1.208 g/cm3

Density determination Mold volume = 996.95

cm3 Mass of the mold =3.694 kg

Table 4.1: Water content determination

Water added 16% 20% 24% 28% 32% 36%

Mass of empty container(gm) 20 20 20 11 13 11

Mass of container+ flyash (moist) 67 88 85 73 52 61

Mass of container+ dry flyash(gm) 61 77 73 60 43 48

Mass of dry flyash (gm) 41 57 53 49 30 37

Mass of pore water(gm) 6 11 12 13 9 13

%water content 14.6 19.3 22.6 26.5 30.0 35.1

Table 4.2: Dry density determination

Sample No 1 2 3 4 5 6

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Assumed water % 16 20 24 28 32 36

Actual avg water content % 14.6 19.3 22.6 26.5 30 35

Mass of fly ash+mold (kg) 4.985 5.056 5.128 5.212 5.222 5.180

Mass of wet flyash(kg) 1.291 1.362 1.434 1.518 1.528 1.486

Wet density(gm/cm3) 1.295 1.366 1.438 1.523 1.533 1.490

Dry density(gm/cm3) 1.130 1.145 1.170 1.204 1.180 1.103

Figure 4.1: Graph between Dry density and Water content

It shows as the moisture content increases,

Maximum dry density increases. The optimum

moisture content was found out to be 30 % at

maximum dry density of 1.208 gm/cm3 Figure 4.1).

SLUMP CONE TEST

The slump test is a low-cost technique for

predicting the flow behaviour of a flyash-based

composite material. It is from this that the spread of

the material is determined. The highest slump

height of flyash composite material was discovered

to be 110 m when using flyash-gypsum composite

material, while the lowest slump height of flyash

composite material was shown to be 70mm when

using flyash-cement composite material (figure

4.3). As a consequence, the cement-based

composite produced a superior outcome since it is

not only a stronger material but also binds fly ash

particles more effectively than the gypsum and

lime composites.

Figure 4.2: Slump height vs. fly ash composite material

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Figure 4.3: slump height of flyash-cement composite material.

BRAZILIAN TENSILE STRENGTH

Tensile strength is a measure of a composite's

resistance to tensile forces applied from outside the

composite. In order to measure the tensile strength

of the fly ash composites, Brazilian indirect tensile

strength tests were performed in the same testing

equipment that was used to evaluate the

compressive strength of the composite.

The samples for the test were cut from a specimen

that had been prepared for ultrasonic pulse velocity

tests and measured 60 mm in diameter and 30 mm

in thickness. The samples were put into the

apparatus along the diametrical axis, in accordance

with the procedure. It was discovered that the

tensile strength of samples containing 4 percent

lime, 4 percent gypsum, and 4 percent cement was

0.10 MPa, 0.15 MPa, and 0.60 MPa, respectively,

when the samples included 4 percent lime, 4

percent gypsum, and 4 percent cement (Figure 4.4).

All of the samples failed in less than 110 seconds,

on average.

Figure 4.4: Failure profile of different fly ash composite material

5. Numerical Investigation

Florida is a two-dimensional explicit finite

difference programme that is used for engineering

mechanics computation. It is possible to model the

behaviour of buildings made of soil, rock, or other

materials with this software. When their production

limitations have been reached, they may experience

plastic flow. A zone or element represents a

material, and together they create a grid that may

be customised to suit the geometry of the item that

is being modelled by the user. Each element

responds to applied forces or boundary restraints by

behaving in accordance with a prescribed linear or

nonlinear stress/strain law, depending on the type

of law. The material may yield and flow, and the

grid can deform and move in sync with the material

that is being shown on the screen. FLAC's explicit,

Lagrangian calculation scheme, combined with the

mixed-discretization zoning technique, ensures that

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plastic collapse and flow are accurately

represented. Because no matrices are created,

massive two-dimensional computations may be

performed without placing an undue strain on the

system's memory. By using automatic inertia

scaling and automatic damping, which do not affect

the mode of failure, the explicit formulation's

drawbacks can be mitigated to some extent.

In spite of the fact that FLAC was originally

designed for geotechnical and mining engineers,

this software programme has a wide range of

capabilities for solving complex mechanical

problems. There are a number of built-in

constitutive models available that allow the

modelling of extremely nonlinear, irreversible

responses that are typical of geology or comparable

materials to be performed. Aside from that, FLAC

has a number of unique features, including as:

Figure 5.1: Flow chat of modeling procedure [15]

The geo-mining conditions of a neighbouring

underground coal mine were taken into

consideration while determining the impact of

backfilling. This study examined the geological

mining conditions of the mine using various

mixtures of flyash with limestone, gypsum, and

cement in order to evaluate the strata's structural

integrity. Table 5.1 lists the parameters that were

taken into consideration for the numerical analyses.

The geo-mining condition that has been modelled

is provided. in Table 5.2.

Table 5.1: Properties of coal

Property Coal Sandstone

Bulk Modulus 3.67 GPa 6.67 GPa

Shear Modulus 2.2 GPa 4.0 GPa

Density 1427 kg/m3 2300 kg/m3

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Tensile Strength 1.86 MPa 9.0 MPa

Cohesion 1.85 MPa 12.0 MPa

Friction Angle 300 450

Table 5.2: Geo-mining condition of the mine

Parameters Values

Thickness of seam 11 m

Width of the development gallery 4.2 m

Height of the development gallery 3.0 m

Size of the Panel 150m x 120m

Area of the Panel 16000 m2

No. of Pillars 6

No. of Rooms 9

Depth 323 m

Average size of pillars (m2) 60 X 50

Gradient of the seam 1 in 5.5

Total Coal in the Panel 283000 T

Table 5.3: Engineering properties of flyash composite material (FCM)

Parameters FA+lime(4%) FA +gypsum(4%) FA +cement (4%)

Density(kg/m3) 1365 1277 1534

Young’s modulus (MPa) 157.6 185.2 220.3

Bulk modulus (MPa) 175.1 220.5 306.0

Shear modulus (MPa) 58.4 68.0 79.8

Poisson ratio 0.35 0.36 0.38

Cohesion (MPa) 0.890 0.574 1.17

Friction angle 26.0 27.7 30.2

Tensile strength (MPa) 0.10 0.15 0.60

The model anlyses were carried out for various combination and situations with and without fly ash filling.

Maximum vertical deformation observed in the gallery next to goaf edge without backfilling was 100 mm

(Figure 5.2).

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Figure 5.2: Vertical Deformation in the gallery next to goaf edge without backfilling.

Maximum vertical deformation observed in the gallery next to goaf edge after backfilling with mixture of

flyash with 4% limestone was 12.5 mm (Figure 5.3).

Figure 5.3: Vertical Deformation in the gallery next to goaf edge after backfilling with mixture of flyash with

4% limestone.

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flyash with 4% gypsum was 12.5 mm (Figure 5.4).

Figure 5.4: Vertical Deformation in the gallery next to goaf edge after backfilling with mixture of flyash

with 4% gypsum.


flyash with 4% cement was 10 mm (Figure 5.5).

Figure 5.5: Vertical Deformation in the gallery next to goaf edge after backfilling with mixture of flyash

with 4% cement.

6. Conclusion

As an alternative to sand as a backfilling material,

the current research sought to investigate the

engineering characteristics of newly created fly-ash

composite materials. Because of the inclusion of

lime, gypsum, and cement, the strength of the fly

ash composite that was produced has been

improved significantly.

The loose sand used as a backfill material just

serves to fill in the subterranean area produced by

the mining operation, and tests conducted in the

past have shown that it does not generate any

lateral stresses that would help to maintain the

integrity of the aperture. The inquiry resulted in the

following findings, which are listed below.

1. It is of class F and does not have any self-

cementing characteristics, but it does have

pozzolanic properties, as seen in Figure 1.

2. Its optimal moisture content was 30%, and its

dry density was 1.208 gm/cm3 (grammes per cubic

centimetre).

3. It was discovered that the maximum slump

height in fly ash-gypsum was 110 mm and the

lowest slump height in fly ash-cement was 70 mm.

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4. The maximum tensile strength of fly ash-cement

was found to be 0.6 MPa, whereas the lowest

tensile strength was determined to be 0.1 MPa in

fly ash-lime.

5. In addition, an ultrasonic pulse velocity

test revealed that the strength of the

composite improves when the curing time

is increased.

6. Backfilling with fly ash composite resulted in a

reduction in vertical strains, horizontal stresses, and

vertical deformation when compared to the control.

7. Use of composite materials made of fly ash

lowers vertical stress to 5 MPa from 15 MPa, as

well as horizontal stress to 2 MPa from 7.5 MPa.

Vertical distortion is reduced to 10mm from

100mm in a similar way.

8. Comparing fly ash cement composite with lime

and gypsum, it has superior characteristics. 9. The

backfilling characteristics of all three composites

are good.

The study's main finding was that fly ash from a

neighbouring thermal power station had more

potential to be turned into a strong engineering

material with the addition of lime, gypsum, and

cement than previously thought.

RECOMMENDATION FOR FUTURE WORK

It was decided that the present investigation's

purpose and goals would be confined to laboratory

analysis. It is suggested that the following be

looked into further in order to get a better

knowledge of subsidence management and stress

distribution in mines.

1. The geo-technical characteristics of fly

ash composites containing different

percentages of lime, gypsum, and cement

should be investigated in order to

determine their composition.

2. Secondly, further tests should be

conducted in order to change the settling

characteristics of fly ash composite

materials, using either traditional or novel

alternatives.

3. It is necessary to investigate the long-term

impacts of fly ash composites under field

conditions.

REFERENCES

1. Mishra M.K. and Rao K.U.M.,

Geotechnical Characterisation of Fly ash

Composites for Backfilling Mine Voids.

Journal of Geotech and Geol Eng, 24

(2006): pp. 1749- 1765.

2. Ahmaruzzaman M., “A review on the

utilization of fly ash”, Journal of Progress

in Energy Combust. Sci., 36 (2010): pp.

327-363.

3. Mishra M.K., “Experimental and

Numerical analysis of behaviour of model

pillars trapped with reinforced fly ash

composites”, Ph.D thesis, Indian Institute

of Technology, Kharagpur, India, 2003.

4. Behera R.K, Characterization of Fly ash

for their Effective Management and

Utilization, B.Tech thesis, National

Institute of Technology, Rourkela, India

,2010

5. Yamatami J. and Kotake Y., “Pillar

control and effects of backfilling support

at Osaka mine”, International journal of

rock mechanics, mining science and

geomechanics,1986, Vol. 23, No.2, pp.

44-53

6. Chugh, Y.P., D. Biswas, D. Deb and G.

Deaton (2001) Underground placement of

coal processing waste and coal

combustion byproducts based paste

backfill to enhance mining economics.

ICCI No. 97, US1, p 52

7. Singh R.D . Principle and Practices of

Modern Coal Mining . New Delhi : New

Age International, 2011

8. Mallick S.R, Development and Evaluation

of Clinker Stabilized Fly Ash Based

Composite Material for Haul Road

Application, M.Tech thesis, National

Institute of Technology, Rourkela, India,

2012

9. DiGioia, A. M. and Nuzzo, W. L. (1972)

Fly ash as structural fill, Journal of Power

Div.,ASCE, vol. 98 (1), 77-92

10. Das A, Strength Characterization of Fly

ash Composite Material, B.Tech thesis,

National Institute of Technology,

Rourkela, India ,2009

11. Fawconnier, C.J. and Korsten, R.W.O.

(1982) Ash fill in pillar design- Increased

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Underground Extraction of Coal, The

SAIMM Monograph Series 4, pp 277 –

361

12. Galvin, J.M. and Wagner, H. (1982) Use

of ash to improve strata control in bord

and pillar working, in Proceed.

Symposium on Strata Mechanics, Univ. of

Newcastle upon Tyne, pp264-269

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