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Suggested Citation format for this article:

Mallick, S.R., Mishra, M.K., 2013, Geotechnical Characterization of Clinker-Stabilized Fly Ash - Coal Mine Overburden Mixes for Subbase of Mine Haul Road. Coal Combustion and Gasification Products 5, 49-56, doi: 10.4177/CCGP-D-12-00011.1

http://www.coalcgp-journal.org/

I S S N 1 9 4 6 - 0 1 9 8

j o u r n a l h o m e p a g e : w w w . c o a l c g p - j o u r n a l . o r g

Geotechnical Characterization of Clinker-Stabilized Fly Ash–Coal Mine OverburdenMixes for Subbase of Mine Haul Road

Soumya Ranjan Mallick*, Manoj Kumar Mishra

Department of Mining Engineering, National Institute of Technology, Rourkela, India

A B S T R A C T

Fly ash is a major by-product of thermal power plants that adversely affects land, water, and air. Its gainful utilization in many

areas is being continuously explored. Use of fly ash in opencast coal mine haul road construction is one such option. This

article reports detailed laboratory investigations carried out on clinker-stabilized fly ash–mine overburden mixes to evaluate

their suitability for subbase of mine haul roads. Composite materials were prepared from fly ash, mine overburden, and clinker

at different proportions. Proctor compaction test, unconfined compressive strength (UCS), and ultrasonic pulse velocity tests

were carried out at different curing periods. UCS values were found to be strongly dependent on clinker content as well as

curing period. Ultrasonic pulse velocity tests confirmed the obtained UCS results. The composite with 62% fly ash and 8%

clinker content showed adequate mechanical strength suitable for the subbase of a mine haul road.

f 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association

All rights reserved.

A R T I C L E I N F O

Article history: Received 30 November 2012; Received in revised form 5 February 2013; Accepted 16 May 2013

Keywords: UCS; ultrasonic pulse velocity; clinker; coal mine overburden; fly ash

1. Introduction

Coal is the most abundant fossil fuel available in India to

produce thermal power, comprising about 75% of the total power

generation. Currently in India, about 83 thermal power stations use

bituminous and subbituminous coal, generating more than 180 Mt

of fly ash per year. This generation of fly ash will reach 225 Mt a

year by 2017. Previously, it was considered as an industrial waste

and was disposed in ash ponds near the power plants. Now it is

considered as a resource material and is being widely used in

various sectors. However, all those applications do not accommo-

date the huge quantities of fly ash being generated and, hence, new

avenues are continuously being explored.

There are about 170 surface coal mines in India, and many are

near thermal power plants. A typical surface coal mine has about

4–5 km of permanent haul road in addition to 10–12 km of branch

roads. Large-capacity haul trucks are used to run over these roads.

Haul roads with inadequate material adversely affect mining by

reducing productivity, as well as by increasing costs due to

repeated road and dumper maintenance. Typically, haulage cost is

about 30–50% of total cost incurred by a surface coal mine

(Thompson and Visser, 2003).

The haul road construction materials are sourced from

overburden dump. These materials are mudstone, sandstone,

siltstone, crushed gravel, and clay. These materials do not offer

any ground stability. Potholes, sinking, rutting, and uneven

surfaces are major symptoms observed in almost all mines

(Tannant and Regensburg, 2001). The grain sizes of coal mine

overburden material vary from fine to coarse particles, with

variable dimensions, and often create dump instability as well as

environmental problems.

Fly ash, being very fine and reactive, is more suitable for road

construction compared with other materials. Bulk utilization of fly

ash alone or fly ash stabilized with soil and additive has been

reported previously (Maser et al., 1975; Srivastava, 1995; Grice,* Corresponding author. Tel.: 91 661 246 4609, 91 9437545820(M). Fax: 91

661 2462999. E-mail: [email protected]

doi: 10.4177/CCGP-D-12-00011.1

f 2013 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association. All rights reserved.

1998). The use of soil stabilized with fly ash in haul roads has

multiple benefits, including diverting fly ash from landfills to haul

road construction to increase mine productivity (Tannant and

Kumar, 2000; Mallick and Mishra, 2011).

There are many reports on bulk utilization of fly ash. Fly ash has

been extensively used for soil stabilization (Chu et al., 1955), as

embankment material (Raymond, 1961), structural fill (DiGioia

and Nuzzo, 1972), for injecting grouting (Joshi et al., 1981), as a

replacement to cement (Gopalan and Haque, 1986; Xu and Sarkar,

1994), and in roads and embankments (Singh and Kumar, 2005),

etc.

Class F fly ash mainly consists of siliceous (SiO2) and aluminous

(Al2O3) compounds that lack self-cementing nature, but in

presence of water, they react with calcium oxide to form

cementitious gels (Cockrell and Leonard, 1970). Its low specific

gravity, ease of compaction, frictional resistance, free draining

nature, and insensitivity toward change in moisture content can be

used for road and embankment construction (Pandian, 2004).

In previous studies, addition of fly ash decreased the plasticity

index and increased California bearing ratio (CBR) values of all

types of soil and improved their suitability for construction of road

base and subbase (Sahu, 2005). Fly ash, kiln dust, and mine spoil or

coal partings attained compressive strength of 1 MPa with elastic

moduli of 350 MPa at 14–28 days and were found to be suitable for

constructing haul road base and subbase layers (Tannant and

Kumar, 2000).

The addition of fly ash reduced the dry density of the soil due to

low specific gravity and unit weight (Prabakar et al., 2004). Class F

fly ash achieved a compressive strength of 6.3 MPa at 90 days of

curing and CBR of 172% at 28 days of curing when mixed with

10% lime and 1% gypsum (Ghosh and Subbarao, 2006).

Fly ash exhibited some cohesion when moist, which was

influenced by the size and number of void spaces, as well as by

degree of saturation (Ramasamy and Kaushik, 2001). Butalia (2007)

reported that fly ash filled voids in the granular pulverized

pavement mix, reducing permeability of the full-depth reclamation

stabilized base layer. CBR values of soil stabilized with 10% and

20% fly ash and 2.5% and 5% lime-kiln dust (LKD) were 69–142%

at 7 days of curing and .164% at 28 days of curing (Cetin et al.,

2010).

Ultrasonic pulse velocity measurement is a popular nondestruc-

tive method extensively used to test the properties of concrete

mixtures that is based on measurement of travel time of

longitudinal ultrasonic waves through the sample (Lin et al.,

2007). Limited studies have been carried out to evaluate its

application on stabilized mixes (Ferreira and Camarini, 2001;

Yesiller et al., 2001).

Overall, the higher the CBR values, the better is the suitability of

the material for haul road construction. This article reports on an

investigation conducted to develop an alternative subbase material

with mine overburden, fly ash, and clinker. The respective CBR

values of the developed composite materials have been confirmed

through ultrasonic pulse velocity test.

2. Materials and Methods

Class F type fly ash was collected by an electrostatic precipitator

from a thermal power unit of the Rourkela Steel Plant (RSP). The

mine overburden used in the study was collected from Basundhara

surface coal mine, Mahanadi Coalfields Limited, Orissa. The

additive as clinker selected for the study was collected from

nearby Cement Plant OCL India Limited, Rajgangpur. The Atterberg

limits, specific gravity, particle size distribution, pH, compaction

characteristics, CBR, and ultrasonic pulse velocity were tested as

per the prescribed Indian standards (IS).

The specific gravity of mine overburden and fly ash was

determined using volumetric flask method as per IS: 2720 part III.

Free swelling index was determined as per IS: 2720 part 40. Grain

size distributions were carried out through a standard set of sieves

as per IS: 2720 part IV. The material passing through the 75-mm

size was collected carefully, and grain size distribution analysis

was performed using the hydrometer test.

The Atterberg limits of mine overburden and fly ash were

determined as per IS: 2720 part V and part VI. Liquid limit was

determined using standard liquid limit apparatus designed by

Casagrande. Liquid limit is the minimum water content at which

a part of the material cut by a standard groove will flow together

for a distance of 12 mm under an impact of 25 blows. Plastic

limit is the minimum water content at which soil will begin to

crumble when rolled into a ,3-mm-diameter thread. Shrinkage

limit is the maximum water content at which reduction in water

content will not decrease the volume of soil. Plasticity index is

the difference between the liquid limit and plastic limit.

The pH value was determined as per IS: 2720 part 26 to identify

acidic or alkaline behavior of mine overburden and fly ash. The

measurement of pH was carried out (Systronics scale pH meter,

India) with an accuracy up to 60.02 units. The instrument was

Table 1

Various proportions of fly ash, mine overburden, and clinker

Fly

ash (%)

Overburden

(%)

Clinker

(%)

Fly ash

(%)

Overburden

(%)

Clinker

(%)

90 10 0 70 30 0

88 10 2 68 30 2

86 10 4 66 30 4

84 10 6 64 30 6

82 10 8 62 30 8

80 20 0 60 40 0

78 20 2 58 40 2

76 20 4 56 40 4

74 20 6 54 40 6

72 20 8 52 40 8

Fig. 1. Grain size distribution curve of fly ash and mine overburden.

50 Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013)

standardized with three standard buffer solutions of pH 7.0, 4.0,

and 10.0 at 25uC. Loss on ignition (LOI) of mine overburden, fly

ash, and clinker was determined as per IS: 1760 part I. Chemical

compositions of mine overburden, fly ash, and clinker were

determined using the energy dispersive X-ray (EDX) technique.

The optimum moisture content (OMC) and maximum dry density

(MDD) of different compositions of fly ash–overburden–cement

clinker were determined by modified Proctor test as per IS: 2720

part VIII. The prepared samples were compacted in five layers in

the Proctor mold. A rammer of weight 4.5 kg dropped from a

height of 0.45 m was used for compaction. Each layer was given 25

blows.

2.1. Sample preparation

The fly ash–overburden–clinker composite materials were

prepared at their respective OMC and MDD obtained from the

modified Proctor compaction test for determination of unconfined

compressive strength (UCS) and P-wave velocity. The raw materials

were blended in the required proportions in a dry state. The

required amount of water was added to respective mixtures and

mixed thoroughly. Wet mixtures were then compacted in the mold

as per guidelines. The aim of the investigation was to develop and

evaluate performance of the fly ash major composite materials for

haul road application. Hence, the fly ash amount was kept more

than 50% (Table 1).

2.2. Unconfined compressive strength

A split mold 38 mm in diameter and 76 mm in length was used

for preparation of the UCS test samples as per IS: 2720 part 10

(1991). Samples were prepared with uniform tamping. Two circular

metal spacer discs 5 mm in height and 37.5 mm in diameter, each

with a base 7 mm in height and 50 mm in diameter, were used at

the top and bottom ends of the mold to compact the sample such

that the length of the specimen was maintained at 76 mm. Then,

the discs were removed, and another spacer disc 100 mm in height

and 37.5 mm in diameter, with a base 7 mm in height and 50 mm

in diameter, was used to remove the sample from the mold. The

final prepared specimen had a length-to-diameter ratio of 2.

2.3. Ultrasonic pulse velocity test

The ultrasonic P-wave velocity test was carried out as per ASTM

D2845-08. All pulse velocity measurements were determined using an

ultrasonic velocity measurement system (GCTS, Tempe, AZ, USA).

This system includes a 10-MHz bandwidth receiver pulse with a raise

time of ,5 ns, 20-MHz acquisition rate with 12-bit resolution

digitizing board, transducer platens with 200-kHz compression mode,

and 200-kHz shears mode. The test was conducted by placing two

sensors on opposite surfaces of the prepared sample. Honey was used

to increase the surface contact between two sensors and the sample.

3. Results and Discussion

The aim of the investigation was to develop and evaluate fly

ash–based composite material to replace the common subbase

Table 2

Physical properties of fly ash and mine overburden (O/B)

Property Fly ash O/B

Specific gravity 2.10 2.63

Atterberg limits

Liquid limit (%) 31.57 26.90

Plastic limit (%) Nonplastic 17.10

Shrinkage limit (%) – 16.02

Plasticity index (%) – 9.80

Sieve analysis (%)

Gravel (.4.75 mm) – 8

Sand (4.75–0.075 mm) 18 27

(A) Coarse sand 0 13

(B) Medium sand 0 6

(C) Fine sand 18 8

Silt (0.075–0.002 mm) 79.8 57

(A) Coarse silt 52 46

(B) Medium silt 16 10

(C) Fine silt 11.8 1

Clay (,0.002 mm) 2.2 8

Coefficient of uniformity (Cu 5 D60/D10) 4.47 4.25

Coefficient of curvature (Cc 5 [D30]2/D10 3 D60) 1.82 0.94

pH value 7.10 5.5

Free swell index Negligible 18.18

Table 3

Chemical composition (% by weight) of fly ash, mine overburden (O/B), and clinker

Constituents SiO2 Al2O3 Fe2O3 CaO K2O MgO TiO2 Na2O SO3 LOI

Mine O/B 48.24 29.18 8.36 1.10 0.40 1.30 0.69 – – 10.73

Fly ash 53.11 33.64 6.44 0.55 1.45 0.83 2.05 0.13 – 1.8

Clinker 20.46 4.52 3.57 66.38 0.68 2.01 – 0.16 1.39 0.75

Note: LOI 5 loss on ignition.

Fig. 2. Moisture content–dry density relationship of fly ash and mine

overburden.

Mallick and Mishra / Coal Combustion and Gasification Products 5 (2013) 51

material typically used in the haul road of a surface coal mine. The

experiments and their results are reported next.

3.1. Physical and chemical properties

Particle size distribution reflects whether the material is poorly,

medium, or well graded (Figure 1). Fly ashes are predominantly silt

sized, with some sand-sized fractions. The fly ash contains more

than 50% coarse-grained silts (0.020 mm , particle size ,

0.075 mm) and so belongs to the nonplastic inorganic coarse-sized

fraction, i.e., MLN group according to the geotechnical classifica-

tion system developed by Sridharan and Prakash (2007) (Table 2).

Mine overburden contains a mixture of poorly graded sand and silt

and hence belongs to the SM group (Sridharan and Prakash, 2007).

Coefficient of uniformity (Cu) and the coefficient of curvature (Cc)

values of fly ash and mine overburden represent that both fly ash

and mine overburden are poorly graded (Pandian, 2004).

The specific gravity of fly ash is less than that of mine

overburden, as it contains a large number of cenospheres and less

iron content (Sridharan and Prakash, 2007). The pH values indicate

that fly ash is slightly alkaline and mine overburden is acidic in

nature due to the presence of free lime content and alkaline oxide

content. Carbon content is typically assessed by measuring LOI

as 90% of LOI value (Sear, 2001). The carbon content of the

overburden material and fly ash is 9.65% and 1.6%, respectively.

High carbon content adversely affects material properties. Chem-

ical composition suggests the possible applications of fly ash. The

amount of SiO2 or (SiO2+Al2O3) in fly ash influences the pozzolanic

activity (Throne and Watt, 1965). EDX analysis confirms that fly

ash satisfies the chemical requirements for use as a pozzolana

(Table 3).

3.2. Compaction characteristics

Modified Proctor compaction was carried out to consider higher

standards of compaction. MDD of the composites decreased with

an increase in fly ash percentage. The OMC of all the composites

was between 14% and 20%, with highest OMC being 22.3% for fly

ash only. The highest MDD obtained was 1941 kg/m3 for mine

overburden only, whereas lowest MDD was 1296 kg/m3 for fly ash

only due to its noncohesive nature (Figure 2).

Fig. 4. Moisture content–dry density relationship of fly ash (FA)–mine

overburden (O/B)–clinker (CL) mixes containing 10% and 20% mine O/B.


overburden (O/B) mixes without additive.


overburden (O/B)–clinker (CL) mixes containing 30% and 40% mine O/B.

Fig. 6. Variation of maximum dry density with clinker content. O/B 5 mine

overburden.


The variation of dry density and moisture content of composites

without clinker is shown in Figure 3. Addition of clinker to fly

ash–overburden mixes resulted in an increase in MDD and decrease

in OMC. The variation of dry density and moisture content of

composite with clinker is shown in Figures 4 and 5. As specific

gravity of clinker is higher than that of fly ash and mine

overburden, replacement of fly ash or mine overburden by clinker

resulted in increased MDD (Figure 6).

3.3. Unconfined compressive strength

The UCS test is one of the common laboratory tests in pavement

design and soil stabilization applications and is often used as an

index to quantify the strength enhancement of materials due

to treatment. The UCS values of untreated fly ash and overburden

composites immediately after preparation could not be obtained as

they failed immediately after loading. Marginal increase in UCS

values was observed at different curing periods (Figure 7).

The compressive strength values changed significantly with the

addition of clinker. The composites achieved UCS values between

0.15 to 1.1 MPa, which were significantly dependent on clinker

Fig. 7. Unconfined compressive strength (UCS) values of fly ash (FA)–mine

overburden (O/B) mixes without additive at 7, 14, and 28 days of curing.

Fig. 8. (A–D) Unconfined compressive strength (UCS) values of fly ash (FA)–mine overburden (O/B)–clinker (CL) mixes.


content as well as on curing period. The composite 70% fly ash +30% overburden with 2–8% clinker showed the highest strength

(0.32–1.09 MPa) as compared with other composites at 7 days of

curing (Figure 8A). The composite 62% fly ash + 30% overburden

stabilized with 8% clinker achieved a UCS value of 1.4 MPa at

28 days of curing (Figure 8C).

The availability of additional clinker produced enhanced

bonding between reactive elements. Each composition exhibited

higher strength values with an increase in clinker content and

curing period. These values are far above the minimum values

suggested for subgrade (Das, 1994).

Table 4

Unconfined compressive strength gain of fly ash–mine overburden–clinker

composites

FA

(%)

O/B

(%)

CL

(%)

Curing

period (days)

UCS

(MPa)

UCS

gain

90 10 0 28 0.2 1

88 10 2 28 0.35 1.75

86 10 4 28 0.48 2.4

84 10 6 28 0.63 3.15

82 10 8 28 0.99 4.95

80 20 0 28 0.22 1

78 20 2 28 0.41 1.86

76 20 4 28 0.55 2.5

74 20 6 28 0.7 3.18

72 20 8 28 1.12 5.09

70 30 0 28 0.27 1

68 30 2 28 0.52 1.92

66 30 4 28 0.71 2.62

64 30 6 28 0.9 3.33

62 30 8 28 1.4 5.18

60 40 0 28 0.25 1

58 40 2 28 0.47 1.88

56 40 4 28 0.62 2.52

54 40 6 28 0.81 3.24

52 40 8 28 1.29 5.16

Note: FA 5 fly ash; O/B 5 overburden; CL 5 clinker; UCS 5 unconfined compressive

strength.

Fig. 9. Failure of a few unconfined compressive strength samples.

Fig. 10. (A–C) Young’s modulus values of fly ash–mine overburden (O/B)–

clinker mixes.


The composite containing 62% fly ash and 30% mine overburden

with 8% clinker exhibited maximum compressive strength as

compared with other composites at 7 and 28 days of curing.

Typically, the stress values at the base/subbase layers of a mine haul

road for 35–170 t dumpers are 300–650 kPa, respectively (Tannant

and Regensburg, 2001). The strength achieved by almost all the

mixes in this study is above these values after curing and hence

suitable for mine haul road construction.

UCS gain is the ratio of UCS value of clinker-treated composite

to untreated composite. The UCS gain values were between 1.75

and 5.18 for 28-day cured composites (Table 4). Optimum

quantities of CaO, Al2O3, and SiO2 react among themselves and

exhibit maximum UCS values. More availability of CaO, Al2O3, and

SiO2 do not add to strength gain (Sivapullaiah et al., 1995).

3.4. Young’s modulus

Young’s modulus values were obtained from the UCS test. All

the samples in unconfined compressive loading conditions exhib-

ited shear type failure (Figure 9). All but a few samples failed by

shear, reflecting the combined influence of sample and machine

characteristics (Singh and Ghosh, 2006). Load-bearing capacity

and longitudinal-displacement recording were done until failure,

i.e., peak strength of all the samples. The axial strain values could

not be recorded for postfailure investigation, because the weakened

sample disintegrated soon after its peak strength. The Young’s

modulus values (stress/strain) were calculated for every sample.

Maximum Young’s modulus value was achieved by 64% fly ash +30% overburden + 6% clinker, i.e., around 150 MPa at 28 days of

curing (Figure 10C). Young’s modulus values of different compo-

sitions are shown in Figures 10A, 10B, and 10C.

3.5. Ultrasonic pulse velocity

The ultrasonic pulse velocity method, a nondestructive method

to evaluate the quality of the composite materials (Yesiller et al.,

2001), is influenced by factors such as direction, travel distance,

diameter of sensors, and material properties. Laboratory ultrasonic

velocity measurements have been used to study the elastic

behavior of geologic materials (Dimter et al., 2011).

P-wave velocities varied between 550 and 1650 m/s, with the

maximum being with 62% fly ash and 8% clinker content at 7 days

of curing (Figure 11A). The trend is the same at 14- and 28-day

curing periods with an increase in P-wave velocities (Figures 11B

and 11C). P-wave velocities increased with clinker content as well

as curing periods. The results compare favorably to those for

material with fly ash and cement binder (Lav et al., 2006). At initial

stages of curing, the composites had high moisture content before

the onset of hydration, which caused instability in the specimen

and allowed pulse velocity to pass through the shortest possible

path. As hydration progressed with higher clinker content, the P-

wave velocity value increased, resulting in higher UCS and

Young’s modulus values.

4. Conclusions

Our investigation evaluated the geotechnical characteristics of

20 different composite materials with fly ash in major percentages as

a replacement for conventional material in the subbase of a surface

coal mine haul road. The results obtained were encouraging, and the

following conclusions are drawn from the investigation.

Fig. 11. (A–C) P-wave velocity values of fly ash-mine overburden (O/B)-clinker

mixes.


1. Mine overburden material does not exhibit suitable strength

values for haul road application.

2. Different mixtures of fly ash–mine overburden without additives

do not have sufficient strength to be used as subbase material.

3. Addition of clinker improves strength values significantly.

4. The curing period, as well as the clinker percentage, has a strong

influence on the strength behavior of composites.

5. At 2% clinker content, the curing period is the dominant factor

for suitability of the composite material.

6. At 8% clinker content, most of the composites achieved the

desired UCS strength values.

7. The UCS values of the optimum composite exceed the minimum

required values for use in subbase of a haul road. The P-wave

velocity results confirmed the observations.

8. The composite with 62% fly ash and 8% clinker content exhibits

the best result for haul road application as a subbase material.

9. The fly ash–based composite materials would facilitate the use

of a high percentage of fly ash in haul road construction.

Acknowledgments

The authors acknowledge the financial assistance provided by

the Council of Scientific and Industrial Research (CSIR)–New Delhi

under EMR-II Scheme Vide Letter No. 22/0474/09/EMR-II dated

12-02-2009.

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