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Compaction Characteristics and Permeability Of Tanjung Bin Coal Ash Mixtures

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Compaction Characteristics and Permeability Of Tanjung Bin Coal Ash Mixtures
2011 International Conference on Environment Science and Engineering
IPCBEE vol.8 (2011) © (2011) IACSIT Press, Singapore
Compaction Characteristics and Permeability Of Tanjung Bin Coal Ash Mixtures
Marto A., Awang A.R.
Makhtar A.M.
Faculty of Civil Engineering,
Universiti Teknologi Malaysia,
81310 UTM Skudai, Johor
[email protected]
[email protected]
Faculty of Civil Engineering,
Universiti Teknologi Malaysia,
81310 UTM Skudai, Johor
[email protected]
Abstract— Fly ash and bottom ash are coal ashes which are the
residual by-products of coal burning to produce electricity.
They are typically treated as waste materials and usually
disposed in ponds. Reutilization of these materials in civil
engineering applications that require large volumes of fill or
backfill materials, such as embankments and retaining
structures, is beneficial. There are limited information on the
use of mixtures of fly and bottom ash in these types of
applications. However there is a suggestion that one may use
the ash mixtures in the proportions in which they exist in the
disposal ponds whereby there will be no costs associated with
mixing the ashes in situ at the time of construction. In addition,
the use of ash preserves traditional materials and allows
savings on both land acquisition and disposal operations. This
paper presents the result of laboratory investigation on the
compaction characteristics and permeability of Tanjung Bin
coal ash mixtures. Standard compaction tests to obtain
maximum dry density (γdmax) and optimum water content (wopt),
and permeability tests had been conducted on ash mixtures
with four mixture ratios: 30%, 50%, 70%, and 90% fly ash
content by weight. The compaction test results showed that,
with increasing fly ash content, the maximum dry density
(γdmax) decreased and the corresponding optimum moisture
contents (wopt) increased. The coefficient of permeability (k) of
ash mixtures compacted to 95% relative density, were found to
decrease slightly with increasing fly ash content. This is
primarily due to the increase in specific surface with the
increase in fines content, generating more resistance to water
flow through voids between particles. The overall range of the
coefficient of permeability values is similar to that of a fine
sand/silt mixture or silt. The results presented in this paper
show that coal ash mixtures from Tanjung Bin had the
properties that allow their use to replace the use of traditional
materials in the construction of embankment or as
replacement of unsuitable soils.
Keywords-Waste materials;
Compaction; Permeability
I.
Fly
ash;
Bottom
ash disposal sites and the associated rising costs to acquire
more disposal space [1]. Use of coal ash in construction
projects requiring large volumes of materials, such as
highway construction, is a promising solution to the disposal
problem. It also helps to preserve the environment, as there is
no consumption of traditional materials. In common disposal
practice, fly ash and bottom ash are either ponded or
landfilled together in mixtures. In general, both the
production and disposal ratios of fly ash and bottom ash are
approximately 80:20 by weight [2]. To maximize coal ash
use, direct use of the disposed ash, with its high fly ash
content, would be desirable. From the standpoint of highway
engineering applications, compaction of ash materials is a
primary concern. In highway embankment construction,
stable slopes and settlements that do not exceed certain limits
are basic requirements. Fill materials should be designed
such that strength and stiffness requirements are satisfied.
This paper focuses on the compaction characteristics and
permeability of fly ash and bottom ash mixtures. Both the
characteristics are important parameters affecting the
behavior of coal ash in various engineering applications.
Tanjung Bin Power Plant
Pontian Johor
ash;
INTRODUCTION
Every year a large fraction of coal ash is disposed of as a
waste at utility disposal sites. Typically, fly ash refers to fine
ash particles suspended in the boiler furnace during coal
combustion, while bottom ash consists of coarse particles
that settle at the bottom of the boiler furnace. The disposal of
coal ash has been a source of significant concern to electric
utility companies due to the need for continuing expansion of
Figure 1. Location of Tanjung Bin power plants in Malaysia (Mahmud,
2003)
134
II.
LITERATURE REVIEW
In the state of Indiana, USA more than 66% of the coals
burning by-products are disposed as class-F fly ash and
bottom ash [4]. According to [2], fly ash and bottom ash are
typically produced and disposed in the ratio of 80:20 (fly ash:
bottom ash ratio by weight). However, current Indiana
Department of Transportation (INDOT) specifications for
coal ash utilization in highway construction allow only
mixtures with fly ash content less than 40%. Even though it
is advantageous to use the as-disposed ash mixtures in
construction, there are very limited studies on the properties
of fly and bottom ash mixtures [16]. Most research studies
focused on the engineering and environmental behavior of
either fly ash or bottom ash [3].
In order to investigate the mechanical behavior of
mixtures with various fly ash contents, a research study
consisting of two phases had been carried out by [17],
funded by INDOT. The first phase involved a comprehensive
laboratory testing programmed to characterize the behavior
of fly and bottom ash mixtures (with fly ash contents of 50,
75, and 100% by weight). The second phase of the research
focused on the construction, instrumentation, and monitoring
of the field performance of a demonstration embankment
constructed using a fly and bottom ash mixture. The
laboratory study indicated that fly and bottom ash mixtures
exhibit relatively higher peak-friction angle (φpeak values
ranging from 30° to 47°) from direct shear test and lower
maximum dry unit weight (ranging from 14.1 kN/m3 to 17.7
kN/m3) from compaction test, than typical fill materials of
comparable particle size. In addition, the hydraulic
conductivity of fly and bottom ash mixtures is similar to that
of a fine sandy silt or silt (ranging from 2x10−5 to 1x10−4
mm/s). These laboratory test results showed that fly and
bottom ash mixtures can perform satisfactorily when used as
fill materials in the construction of embankments and
retaining structures [17].
III.
of the particles occurred within the size in the range between
0.001 mm and 0.6 mm. The bottom ash gradations also
exhibited well graded size distribution. However, bottom ash
particle sizes range from fine gravel to fine sand sizes and
the majority of the particles occurred in the range between
0.075 mm and 20 mm. The coefficient of uniformity (Cu) for
bottom ash was about 16.56 while the coefficient of
curvature (Cc) was 1.01. The work of [19] also showed that
according to the Unified Soil Classification System (USCS),
Tanjung Bin bottom ash could be classified as well graded
sand and according to the classification by AASHTO system,
it was A-1-a. The specific gravity (Gs) of fly ash was
obtained as 2.3 while the bottom ash was 1.99. As for the
coefficient of permeability (k), [19] found that the value was
4.87 x 10-9 m/s for fly ash and 1.72 x 10-4 m/s for bottom ash.
IV.
TESTING PROGRAMME
Sample preparation was carried out initially to obtain
coal ash mixtures with different mixture ratio, that were 30%,
50%, 70%, and 90% of fly ash content. This was carried out
by weighing the coal ash in dry condition to obtain the
mixture ratio by weight.
The compaction and permeability tests had been carried
out in accordance with BS1377:1990. In compaction test, at
least seven tests had been carried out at each specific ash
mixture ratio, using various range of water content.
In determining the coefficient of permeability, each coal
ash mixtures were compacted to 95% of maximum dry
density obtained from the compaction test, but using the
water content at the dry side. Upon completion of the
compaction, the respective samples were saturated first in the
permeameter, before the falling head permeability test were
carried out.
Figure 3 shows the Standard proctor compaction test and
permeability test equipments.
THE MATERIALS
Figure 2 shows the coal ash samples used in this study,
obtained from Tanjung Bin Power Plant, Pontian, Johor. This
plant uses pulverized coal burning units and produce class F
fly ash and bottom ash with a general production ratio of
80% fly ash and 20% bottom ash.
(a) Permeability test
(b) Compaction test
Figure 3. Equipments in Standard proctor compaction test and
permeability test
V.
Fly ash
RESULTS AND DISCUSSION
A. Compaction Behaviour
Four (4) ash mixtures were prepared and tested by the
standard proctor compaction procedure. The compacted dry
unit weight versus the water content curves of the ash
mixtures are displayed in Figure 3. The values of maximum
dry unit weight and corresponding optimum water content
and air content are tabulated in Table 1. The test results show
Bottom ash
Figure 2. Samples of Tanjung Bin coal ash
From the work of [18], it was found that the grain size
distributions of Tanjung Bin fly ash showed a well graded
curve, ranging from mostly fine silt to fine sand sizes. Most
135
TABLE I.
that, as the fly ash content increases from 30% to 90%, the
maximum dry unit weight (γd,max) decreases from 16.74
kN/m3 to 15.50 kN/m3, while the optimum water content
(wopt) increases from 14 % to 18.5 %. The air content (A) at
the maximum dry density decrease from 5.62% to 1.00%
with the increase of the fly ash content. The gradations of the
ash mixtures explain the change in dry unit weight. The
addition of fly ash to bottom ash leads to increasingly more
well-graded size distributions, which allows the fly and
bottom ash particles to pack more closely, resulting in the
increase in γd,max. The increase in wopt with increasing fly ash
(in compaction test) follows from the need to release the
capillary tension from the greater exposed surface of the fine
fly ash particles. Compared with the γd,max of compacted
soils, the γd,max values of ash mixtures tends to be lower than
those of soils, which range typically from 17 kN/m3 to 20
kN/m3 [19].
Tests on silty sands by [13] revealed that in low silt
contents ranging from zero to about 25 %, both the γd,max and
γd,min of a silty sand increase with increasing fines content
due to the fines occupy the voids between sand particles.
However, further increase in the fines, exceeding about 25%,
causes the fines to begin to separate adjacent sand particles,
resulting in a decrease in γd,max and γd,min. Similarly, in the
ash mixtures with high fly ash content (i.e. Fly ash > 50 %),
bottom ash particles can be separated by fly ash particles and
are not, on average, in contact. At a certain level of fly ash
content, the bottom ash particles may be completely
separated, floating in a fly ash matrix. As a result, further
increase of fly ash content up to 90% causes the decrease in
the γd,max. The behavior of a material with a floating fabric
may be quite different from one in which the bottom ash
particles are in contact.
90% FA
2.00
Optimum
Water
Content (%)
Max. Dry
Density
(kN/m3)
90
70
50
30
18.50
16.20
15.00
14.00
15.50
16.20
16.37
16.74
Air content
(A)
at Max. Dry
Density (%)
1.00
1.10
5.26
5.62
B. Permeability
The permeability of the coal ash mixtures was
determined for ash mixtures compacted at 95% relative
density through by the falling head permeability tests
(BS1377-5:1990). Table 2 and Figure 4 display the values of
coefficient of permeability for compacted ash mixtures.
Measured coefficient of permeability (k), decreases as the fly
ash contents increases from 30% to 90%. However, the range
of variation on the coefficient of permeability are relatively
small. The values of coefficient of permeability range from
2.09 ×10-7 mm/s to 14.7 ×10-7 mm/s. The compacted ash
mixtures with high fly ash contents exhibit the permeability
approximately corresponding to that of the fine sand/silt
mixture or silt.
It appears that the fineness of fly ash in the mixture
caused the coefficient of permeability of the ash mixtures to
decrease with increasing fly ash content. The coefficient of
permeability is primarily influenced by the nature of the
voids in between the particles. Fine fly ash particles have
voids much smaller than the bottom ash particles. Larger
specific surfaces of fly ash would cause more resistance to
the flow of water through the voids. [15] did a series of
permeability tests on Indiana bottom ashes. He observed that
the fines included in bottom ash had a predominant effect on
the permeability and thus the coefficient of permeability
decreased as the fine (fly ash) contents increased.
70% FA
1.90
COMPACTION PROPERTIES OF ASH MIXTURES.
Fly ash
content
(%)
1.80
0%
1.70
TABLE II.
1.70
A = 1.00%
5%
Dry density (Mg/m3)
Dry density (Mg/m3)
1.80
1.60
10%
1.50
1.40
A = 1.10%
Fly ash content
(%)
90
70
50
30
1.60
1.50
1.30
1.40
1.20
10
12
14
16
18
20
7
22
9
11
13
15
17
19
Moisture content (%)
Moisture content (%)
50% FA
1.90
1.80
Dry density (Mg/m3)
1.90
Dry density (Mg/m3)
1.80
1.70
A = 5.26%
16
1.70
A = 5.62%
14
1.60
1.50
12
1.50
1.40
10
1.40
1.30
1.30
1.20
1.60
Coefficient of permeability, k
(mm/s)
2.10 x 10-7
2.65 x 10-7
5.82 x 10-7
1.47 x 10-6
30% FA
2.00
2.00
COEFFICIENT OF PERMEABILITY OF ASH MIXTURES
8
6
11
16
6
21
1.20
6
11
16
Moisture content (%)
21
4
Moisture content (%)
2
0
Figure 3.
0
Compaction Curves of Fly Ash (FA) - Bottom Ash (BA)
Mixtures for Various Fly Ash Content
20
40
60
80
100
Fly ash content (%)
Figure 4.
136
The relationship between coefficient of permeability with
fly ash content in the ash mixtures
VI.
[12] Sikes, P.G and Kolbeck, H.J.“Disposal and uses of power plant ash in
urban areas.”J of the power division, vol 99, 1993, pp 217-234.
[13] Hough, B.K.“Basic Soils Engineering.”The Ronald Press Company,
NewYork, 1957.
[14] Marto, A., Mahir, A. M., Lee, F. W., Yap, S. L. and Muhardi (2009).
Morphology, Mineralogy and Physical Characteristics of Tanjung Bin
Coal Ash. Proceedings of 4th International Conference on Recent
Advanced in Materials, Minerals and Environment (RAMM) & 2nd
Asian Symposium on Material & Processing (ASMP), 1-3 June 2009,
Pulau Penang, Malaysia.
[15] Karim, A., Lovell, C., and Salgado, R. (2007). Building
Embankments of Fly/Bottom Ash Mixtures. Joint Transportation
Research Program, Purdue University.
[16] Kim, B.J., Yoon, S.M., and Balunaini, U. (2006). Determination of
Ash Mixture Properties and Construction of Test Embankment –Part
A. Joint Transportation Research Program, Final Report,
FHWA/IN/JTRP-2006/24! Purdue University, W. Lafayette, Indiana.
[17] Kim, B., Prezzi, M., and Salgado, R. (2005). “Geotechnical properties
of fly and bottom ash mixtures for use in highwayembankments.” J.
Geotech. Geoenviron. Eng., 131(7), 914–924.
[18] Muhardi, Kasim, K.A, Makhtar, A.M, Lee, F.W, and Yap, S.L
Engineeering Characteristic of Tanjung Bin Cola Ash. Vol. 15
[2010], Bunk. K.
[19] U.S. Navy. ~1986!. “Design manual—Soil mechanics, foundations,
and earth structures.” NAVFAC DM-7, Dept. of the Navy,
Washington, D.C.
CONCLUSIONS
Fly-bottom ash mixtures (with mixture ratios ranging
from 30% to 90% fly ash content) were found to exhibit
relatively well-defined moisture-density relationships, and
the relationships varied with the mixture ratio. The addition
of fly ash to bottom ash leads to increasingly more wellgraded size distributions, which allows the fly and bottom
ash particles to pack more closely
The coefficient of permeability of compacted ash
mixtures were found to decrease slightly with increasing fly
ash content. This decrease is due to the increasing specific
surface with increasing fines content, which generates more
resistance to water flow through voids between particles.
Based on the results obtained in this study, it appears that
coal ash mixtures from Tanjung Bin had the properties that
allow their use to replace the use of traditional materials in
the construction of embankment or as replacement of
unsuitable soils.
ACKNOWLEDGEMENT
The authors would like to thank the Management of
Tanjung Bin Power Plant, Pontian, Johor, for supplying the
coal ash samples throughout this research works and also to
Universiti Teknologi Malaysia (UTM) for providing the
facilities for laboratory testings.
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137
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