California Bearing Ratio and Brazilian Tensile Strength of Mine Overburden–Fly Ash–Lime Mixtures...

ORIGINAL PAPER

California Bearing Ratio and Brazilian Tensile Strengthof Mine Overburden–Fly Ash–Lime Mixtures for Mine HaulRoad Construction

Banita Behera • Manoj Kumar Mishra

Received: 17 May 2011 / Accepted: 7 November 2011 / Published online: 20 November 2011

� Springer Science+Business Media B.V. 2011

Abstract The production and utilization of coal is

based on well-proven and widely used technologies.

Fly ash, a coal combustion byproduct, has potential to

produce a composite material with controlled and

superior properties. The major challenges with the

production of fly ash are in its huge land coverage,

adverse impact on environment etc. It puts pressure on

the available land particularly in a densely populated

country like India. In India the ash utilization

percentage has not been very encouraging in spite of

many attempts. Stabilization of fly ash is one of the

methods to transfer the waste material into a safe

construction material. This investigation is a step in

that direction. This paper presents the results of an

investigation on compressive strength and bearing

ratio characteristics of surface coal mine overburden

material and fly ash mixes stabilized with lime for coal

mine haul road construction. Tests were performed

with different percentages of lime (2, 3, 6 and 9%).

The effects of lime content and curing period on the

bearing ratio and tensile strength characteristics of the

stabilized overburden and fly ash mixes are high-

lighted. Unconfined compressive strength test results

cured for 7, 28 and 56 days are presented to develop

correlation between different tensile strengths and

unconfined compressive strength. Empirical models

are developed to estimate bearing ratio and tensile

strength of mine overburden–fly ash–quick lime

mixtures from unconfined compressive strength test

results.

Keywords Compressive strength � Tensile strength �Bearing ratio � Fly ash � Overburden � Lime

1 Introduction

Huge quantities of coal combustion byproducts are

produced every year and only a small fraction of them

are utilized. The current annual production of coal ash

worldwide is estimated around 600 million tons, with

fly ash constituting about 500 million tons at 75–80%

of the total ash produced (Ahmaruzzaman 2010).

Disposal of ash in dry or slurry form in the proximity

of the thermal power plants not only occupies a large

area but also causes environmental pollution (Pandian

et al. 2000). Utilization of coal ash in construction

helps in saving of precious land area. Considering the

practical significance of the problem, experimental

investigations were carried out on fly ash to establish

its suitability for geotechnical applications.

Mine overburdens (O/B) are very important raw

materials which have been traditionally used in the

mines throughout the world. The annual coal produc-

tion from opencast coal mines in India was about 328

MT and overburden waste material of 535.08 Mm3

(Jamal and Sidharth 2008). The average stripping ratio

B. Behera � M. K. Mishra (&)

Department of Mining Engineering, National Institute of

Technology, Rourkela, India

e-mail: [email protected]

123

Geotech Geol Eng (2012) 30:449–459

DOI 10.1007/s10706-011-9479-9

of Indian coal mines (overburden to coal) is about 2:1

(Chaulya et al. 2000). The overburden is highly

heterogeneous. Gradation results suggest that fines

and coarse grains are approximately equally repre-

sented in the soil reported by Ulusay et al. (1995).

Fly ash, being very finer, is more reactive and

consequently more suitable for haul road construction

material as compared to other materials. Potential

application of fly ash alone or soil stabilized with fly

ash or fly ash and admixtures for road construction has

been reported by a number of researchers (Hobeda

1984; Bell 1996; Consoli et al. 2001; Acosta et al.

2003; Kumar 2005; Geiman 2005; Mohanty and

Chugh 2006; Goswami and Mahanta 2007; Mackos

et al. 2009). The enhancement of mechanical strength

of fly ash with addition of lime has been reported

elsewhere (Sivapullaiah et al. 2000; Beeghly 2003;

Mishra and Rao 2006; Ghosh and Subbarao 2007). But

a conservative estimate puts the unutilised fly ash

occupying about 65,000 acres of land (Das and

Yudhbir 2006) which demands increase the utilization

percentage. According to Pandian (2004), fly ash has

good utilization potential for geotechnical applica-

tions. Its low specific gravity, freely draining nature,

ease of compaction, insensitiveness to changes in

moisture content, good frictional properties etc. can be

gainfully exploited in the construction of roads,

embankments etc. Prabakar et al. (2004) studied fly

ash and soil mixes and concluded that the addition of

fly ash reduced the dry density of the soil due to low

specific gravity and unit weight. Sahu (2005) reported

that the plasticity index and linear shrinkage of all

types of soil decreased and CBR value increased with

the addition of fly ash which enhanced the suitability

of the soils for construction of base and subbase in

road works. There are many attempts to use fly ash in

raw stage in road construction, particularly in rural

road construction (Vittal and Mathur 2005; Singh and

Kumar 2005). The class-F fly ash after stabilizing with

10% lime and 1% gypsum achieved a compressive

strength of 6.308 MPa at 90 days curing and the CBR

value of 172 at 28 days curing reported by Ghosh and

Subbarao (2006).

Tannant and Kumar (2000) reported that mine spoil

or coal seam partings stabilized with fly ash suitable

for use in constructing haul road base and subbase

layers. There have been many successful instances of

fly ash being used as road construction material. Yet

its effectiveness in the surface coal mine haul road has

not been evaluated so as to establish it commercially.

Surface coal mine haul road undergoes more stress/

strain due to multiple reasons such as poor surface

course, traditional neglect in construction, high load

concentration at fixed locations, etc. A typical surface

coal mine has about 2–5 km of permanent haul road,

larger ones having longer lengths and various other

lumpy roads that are constructed from locally avail-

able material found near to the mine property. In the

past 30 years the carrying capacity of hauling equip-

ments e.g. dumpers/trucks has grown from a tiny

10–170 tons, 320 tons being envisioned at places,

requiring better haul roads to carry heavy loads. These

dumpers may achieve gross vehicle weight of more

than 1,000 KN and the tire pressure used to support

these tracks would be typically 600–690 kPa (Tannant

and Kumar 2000; Tannant and Regensburg 2001). But,

the vehicular traffic exhibit gross vehicle weight of

1.78 KN to about 200 KN and the tire pressure is about

249–550 kPa. The purpose of the present study is to

develop a better subbase/base material with fly ash,

mine overburden and lime as ingredients and to

determine their compressive strength, tensile strength

and California bearing ratio values. The paper also

develops empirical models to estimate bearing ratio

and tensile strength of stabilized overburden fly ash

mixes from unconfined compressive strength test

results.

2 Materials and Methods

The overburden used in this study was collected from

Bharatpur opencast coal mine, Talcher, Odisha. The

fly ash used in the present study was collected from

electrostatic precipitators of a thermal power unit of

Rourkela Steel Plant, Odisha. The additive selected

was commercially available superior grade quick lime.

The tests for specific gravity, consistency limits, grain

size distribution, free swell index, pH, loss on ignition

and compaction characteristics were carried out as per

the prescribed Indian Standards. The specific gravity

of the mine overburden and fly ash were determined

using volumetric flask method as per IS: 2720-Part 3.

The consistency limits of the mine overburden were

determined as per IS: 2720-Part 5 and Part 6. The

liquid limit of overburden was determined using

Casagrande liquid limit device. The liquid limit of

fly ash was determined by the cone penetration method

450 Geotech Geol Eng (2012) 30:449–459

123

as per BS 1377 due to difficulty in cutting a groove

using Casagrande device. The liquid limit is the

minimum moisture content at which the soil can flow

under a specified small disturbing force, the disturbing

force being defined by the method of testing. The

plastic limit is the minimum water content at which

soil ceases to behave as a plastic material. The

shrinkage limit is the maximum water content below

which the soil ceases to decrease in volume on further

drying. Free swell index was determined as per IS:

2720-Part 40. The pH value was determined as per IS:

2720-Part 26 to identify the acidic or alkaline char-

acteristic of overburden and fly ash. The measurement

of pH was carried out using Systronics scale pH meter

with accuracy up to ±0.02 units. The instrument was

standardized with three standard buffer solutions of

pH 7.00, 4.00 and 10.00 at 25�C as per the prescribed

procedure. Loss on ignition (LOI) of overburden and

fly ash were determined as per IS: 1760-Part 1. The

heavy compaction (modified Proctor compaction) test

was performed to determine the maximum dry density

and optimum moisture content of the overburden and

fly ash as per IS: 2720-Part 8. The chemical compo-

sitions of mine overburden and fly ash were obtained

using energy dispersive X-ray (EDX) technique.

2.1 Sample Preparation

The following proportions of overburden fly ash

mixtures were considered as presented in Table 1.

The aim of the study was to evaluate fly ash utilization

prospects to replace a part of the traditional sub base

material i.e. the overburden. Hence experiments were

carried out to maximum of 50% fly ash addition in

overburden as reported in the Table 1. The optimum

lime content was determined to be 3% for the above

combinations from pH test. Lime was added in the

above combinations of overburden–fly ash in varying

percentage of 2, 3, 6 and 9% to determine respective

geotechnical parameters. Heavy compaction (modi-

fied Proctor compaction) test were performed to

determine the maximum dry density and optimum

moisture content of all the mixes. Then samples were

prepared at their respective optimum moisture content

and maximum dry density. The raw materials such as

fly ash, mine overburden and lime were blended in the

required proportion in dry condition. After dry mixing

of the ingredients, respective water content was added

to the mixes and mixed thoroughly. Then the mixture

was left in a closed container for uniform mixing and

prevents loss of moisture to atmosphere. The wet

mixture of amount corresponding to the required dry

density was compacted in the mould. Split mould of

38 mm diameter and 86 mm length was used for

preparation of the unconfined compressive strength

(UCS) test samples. Samples were prepared with

uniform tamping. Two circular metal spacer discs of

height 5 mm and diameter 37.5 mm each with base

(7 mm height, 50 mm diameter) were used at top and

bottom ends of the mould to compact the sample such

that the length of the specimen was maintained at

76 mm. Then the discs were removed and an another

spacer disc of height 100 mm and diameter 37.5 mm

with a base (height 7 mm, 50 mm diameter) was used

to remove the sample from mould. The sample for

Brazilian tensile strength test was prepared using the

same mould of UCS test samples. For this purpose,

two circular metal spacer discs of 5 and 62 mm heights

and 37.5 mm diameters with base (height 7 mm,

50 mm diameter) were used. The California bearing

ratio (CBR) test samples were prepared using standard

CBR mould of 150 mm diameter and 175 mm height.

The sample was statically compacted in the mould,

such that the height was maintained at 127 mm. A

circular metal spacer disc of 148 mm diameter and

47.7 mm height was used to compact the sample.

2.2 Unconfined Compressive Strength Test

Unconfined compressive strength is a common crite-

rion to determine the strength of stabilized materials.

The specimens prepared for compressive strength test

were of 38 mm diameter and 76 mm long. The

specimens were then cured in a humidity chamber

Table 1 Various proportions of FA ? O/B

Fly ash (%) Overburden (%)

15 85

20 80

25 75

30 70

35 65

40 60

45 55

50 50

FA fly ash, O/B overburden

Geotech Geol Eng (2012) 30:449–459 451

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(relative humidity [ 95%) for 7, 28 and 56 days at

30 ± 2�C. The unconfined compressive strength tests

were conducted as per IS: 2720-Part-10 at a strain rate

of 1.2 mm/min. Load and deformation data was

recorded till failure of the specimen.

2.3 Brazilian Tensile Strength Test

Brazilian test make the sample fail under tension

though the loading pattern is compressive in nature.

The tensile strength was determined as per ASTM

D3967. The length (thickness) to diameter ratio of

specimen is 0.5. The specimens prepared for tensile

strength test were of 38 mm diameter and 19 mm

thick. The specimens were placed diametrically during

test. The sample fails diametrically in tension by

application of load. The indirect tensile strength is

calculated as follows:

rt ¼2P

pDL

where, rt = Brazilian tensile strength; P = failure

load, D = diameter of the specimen; L = length of

the specimen.

2.4 California Bearing Ratio Test

California bearing ratio (CBR) tests were performed in

accordance with IS: 2720-Part 16. The samples were

statically compacted to 95% of maximum dry density

in the mould for CBR test. The samples were soaked

for 4 days in water and were allowed to drain for

15 min before test to obtain soaked condition results.

The curing periods adopted were immediate, 7 days

(3 days moist curing ? 4 days soaking) and 28 days

(24 days moist curing ? 4 days soaking). CBR tests

were carried out at the end of respective curing period.

Two surcharge disks, each weighing 2.5 kg, were

placed over the sample and a plunger, 50 mm in

diameter, was used to penetrate the sample at a rate of

1.25 mm/min during CBR test.

3 Results and Discussion

The physico-chemical properties of fly ash and mine

overburden are reported in Table 2. The specific

gravity of fly ash was found to be less than that of mine

overburden, due to the presence of cenospheres and

less iron content. The materials with higher iron

content have relatively high specific gravity (Sridha-

ran and Prakash 2007). The fly ash tested was

non-plastic and hence plastic limit could not be

determined. It was also not possible to carry out

shrinkage limit tests since the ash pats crumbled upon

drying. Since the amount of shrinkage was very less,

the shrinkage limit would be high. Hence shrinkage

would not be a constraint. Free swell index of the fly

ash was found to be negligible due to flocculation

(Pandian 2004). The pH values indicate that fly ash

was alkaline and overburden was acidic depending on

alkaline oxide content and free lime content. The

chemical compositions of mine overburden and fly ash

are presented in Table 3. The chemical composition of

fly ash indicates that it has less calcium content. Thus,

it is classified as ‘‘Class F’’ fly ash as per ASTM 618

specifications.

3.1 Compaction Characteristics

The maximum dry density (MDD) and optimum

moisture content (OMC) achieved by the heavy

compaction tests, made on fly ash, mine overburden

individually and also on their aggregate mixture with

varying percentages of lime are shown in Figs. 1, 2, 3

and 4. The maximum dry density of fly ash was lower

than that obtained for mine overburden as fly ash is

non-cohesive in nature.

3.2 Unconfined Compressive Strength

The unconfined compression test is one of the widely

used laboratory tests in pavement and soil stabilization

Table 2 Physico-chemical properties of fly ash and mine

overburden

Property Fly ash Overburden

Specific gravity 2.16 2.6

Consistency limits

Liquid limit (%) 30.75 25.7

Plastic limit (%) Non-plastic 15.04

Shrinkage limit (%) – 13.44

Plasticity index (%) – 10.66

Free swell index (%) Negligible 20

pH value 7.2 4.85

Loss on ignition (%) 2 10

452 Geotech Geol Eng (2012) 30:449–459

123

applications. The unconfined compressive strength

(UCS) of overburden material is considerably one of

the most important designing parameters. It is often

used as an index to quantify the improvement of

materials due to treatment. Compressive strengths of

mine overburden stabilized with 15, 20, 25, 30, 35, 40,

45, and 50% fly ash were 0.71–3.14 MPa after 7, 28

and 56 days of curing (Figs. 5, 6, 7). The mine

overburden mixed with 30% fly ash and 9% lime

exhibited maximum compressive strength as com-

pared to that for other mixes at 7, 28 and 56 days of

curing. Addition of lime improved the strength of fly

ash–overburden mixes. Tannant and Kumar (2000)

reported that the stresses at the base layer of mine haul

road are about 300–650 kPa with 23–170 tonne

dumpers. The strength achieved in all the mixes in

this study are above these values after a period of

curing and hence would be useful for mine haul road

construction.

Table 3 Chemical composition (% by weight) of O/B and fly ash

Constituents SiO2 Al2O3 Fe2O3 CaO K2O MgO TiO2 Na2O

O/B 55.33 31.65 9.24 1.21 0.43 1.37 0.77 –

Fly ash 52.13 35.64 6.47 0.53 1.46 0.52 3.02 0.2

Fig. 1 Compaction curves of fly ash and overburden

Fig. 2 Compaction curves of the mixes containing 15, 20 and

25% fly ash

Fig. 3 Compaction curves of the mixes containing 30, 35 and

40% fly ash

Fig. 4 Compaction curves of the mixes containing 45 and 50%

fly ash

Geotech Geol Eng (2012) 30:449–459 453

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3.3 Brazilian Tensile Strength

The tensile strength is a vital parameter to evaluate

the suitability of the stabilized soil or fly ash as road

base material. This investigation reports the Brazil-

ian tensile strengths of mine overburden and fly ash

(15, 20, 25, 30, 35, 40, 45 and 50%) mixes stabilized

with 2, 3 6 and 9% of lime. Brazilian tensile strength

test results were 55.7–291 kPa and 73–357 kPa at 28

and 56 days of curing respectively (Figs. 8, 9). The

mine overburden mixed with 30% fly ash and 9%

lime exhibited maximum tensile strength as com-

pared to that of other mixes at 28 and 56 days of

curing respectively. The Brazilian tensile strength as

a percentage of unconfined compressive strength

against curing period are plotted, as shown in

Figs. 10 and 11. The average value of Brazilian

tensile strength as a percentage of unconfined

Fig. 5 Effect of lime on compressive strength of overburden–

fly ash mixes at 7 days curing





Fig. 8 Effect of lime on tensile strength of overburden–fly ash

mixes at 28 days curing

Fig. 9 Effect of lime on tensile strength of overburden–fly ash


454 Geotech Geol Eng (2012) 30:449–459

123

compressive strength was 8.4% at 28 days and 10%

at 56 days. It is clearly observed that the tensile

strength was a function of the amount of cementi-

tious compounds formed, which increased with

increase in curing period. This compares favourably

with that observed for the fly ash stabilized with

10% lime only (Ghosh and Subbarao 2006). Consoli

et al. (2001) observed that the average ratio of

Brazilian tensile strength and unconfined compres-

sive strength increased from 4% at 7 days to about

15% at 180 days for the mixes containing soil

and 25% fly ash stabilized with 4, 7 and 10% of

lime. The current investigation confirms these

observations.

3.4 California Bearing Ratio Behavior

The CBR values of fly ash, mine overburden and the

mixes are reported in Table 4. In unsoaked condi-

tion, the bearing ratio of overburden stabilized with

35% fly ash was more as compared to that of other

mixes by 50%. But, in case of soaked condition, the

CBR value of 15% fly ash was higher than that for

other mixes by 78%. The CBR value of overburden

Fig. 10 Effect of curing period on tensile strength as percent-

age of unconfined compressive strength of the mixes containing

15, 20, 25 and 30% fly ash

Fig. 11 Effect of curing period on tensile strength as percent-

age of unconfined compressive strength of the mixes containing

35, 40, 45 and 50% fly ash

Table 4 CBR values of FA, O/B and mixes

Sample CBR (%)

Unsoaked

condition

Soaked

condition

Fly ash only 22.42 0.72

Mine overburden

only

23.65 2.95

15% FA ? 85% O/B 32.07 2.34

20% FA ? 80% O/B 27.09 1.87

25% FA ? 75% O/B 26.16 1.4

30% FA ? 70% O/B 26.47 1.31

35% FA ? 65% O/B 45.31 1.64

40% FA ? 60% O/B 34.57 1.12

45% FA ? 55% O/B 32.7 1.03

50% FA ? 50% O/B 32.23 0.82

Geotech Geol Eng (2012) 30:449–459 455

123

increased with the addition fly ash in unsoaked

condition and decreased in soaked condition due to

saturation. The CBR values under soaked condition

were less than that of the unsoaked condition due to

decrease in effective stresses and loss of surface

tension forces upon soaking (Toth et al. 1988).The

increase in CBR value of overburden upon addition

of fly ash is attributed mainly due to the relative

contribution of frictional resistance between them

(Pandian and Krishna 2002).

Generally, additives like cement or lime improve

the strength of the soil or fly ash. A series of CBR

tests were conducted with 2, 3, 6 and 9% of lime

content in overburden–fly ash mixtures to observe the

effect of lime. The effect of lime content on CBR

behavior of overburden–fly ash mixes are shown in

Figs. 12 and 13. It is observed from the results that

the CBR values increased from 42 to 149% and 85 to

195% at 7 and 28 days of curing respectively. The

mine overburden mixed with 30% fly ash and 9%

lime resulted in maximum bearing ratio as compared

to other mixes at 7 and 28 days curing respectively. It

can be compared with the CBR value of the soil

stabilized with fly ash (10 and 20%) and lime kiln

Fig. 12 Effect of lime on CBR behavior of overburden–fly ash


Fig. 13 Effect of lime on CBR behaviour of overburden–fly

ash mixes at 28 days curing

Fig. 14 Relationship between Brazilian tensile strength and

unconfined compressive strength for mine overburden–fly ash–

lime mixtures at 28 days of curing

Fig. 15 Relationship between Brazilian tensile strength and

unconfined compressive strength for mine overburden–fly ash–

lime mixtures at 56 days of curing

456 Geotech Geol Eng (2012) 30:449–459

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dust (2.5 and 5%) obtained by Cetin et al. (2010) to

be 69–142% at 7 days curing and greater than 164%

at 28 days curing.

4 Empirical Model to Estimate Tensile Strength,

Bearing Ratio From UCS Test Results

The variation of tensile strength and bearing ratio with

unconfined compressive strength are shown in

Figs. 14, 15, 16 and 17. The data are analyzed using

linear regression model. The parameters in linear

regression model are estimated by the method of least

squares. There are many attempts have been made to

correlate compressive strength of fly ash/lime and fly

ash/lime and gypsum mixes with chemical composi-

tion, loss on ignition, CBR and tensile strength using

power model (Das and Yudhbir 2006; Ghosh and

Subbarao 2006). In the present investigation, it is

observed that linear regression model suits for the fly

ash and overburden mixes stabilized with lime. The

empirical models along with the value of correlation

coefficient (R) are presented in the scatter plots

(Figs. 14, 15, 16, 17). It is observed from the

relationship between tensile strength and compressive

strength that R value is more at 56 days than that at

28 days curing. It confirms that the relationship

between tensile strength and compressive strength

become stronger with increasing curing period. Sim-

ilar results were also observed between bearing ratio

and unconfined compressive strength. The results of

linear regression model between California bearing

ratio values, unconfined compressive strength and

tensile strength at different curing period are reported

in Table 5.

5 Conclusions

The following conclusions are drawn from the above

study:

1. The unconfined compressive strength increased

with increase in curing period. The unconfined

compressive strength of mine overburden mixed

Fig. 16 Relationship between bearing ratio and unconfined

compressive strength for mine overburden–fly ash–lime mix-

tures at 7 days of curing

Fig. 17 Relationship between bearing ratio and unconfined

compressive strength for mine overburden–fly ash–lime mix-

tures at 28 days of curing

Table 5 Results of linear regression model between California bearing ratio values, unconfined compressive strength and tensile

strength at different curing period

Dependent Best fit equation R value No. of observation Curing period (days)

Brazilian tensile strength, rt rt = 121.2rc - 60.84 0.899 32 28

Brazilian tensile strength, rt rt = 130.36rc - 61.763 0.952 32 56

California bearing ratio (CBR) CBR = 108.8rc - 14.14 0.847 32 7

California bearing ratio (CBR) CBR = 56.45rc ? 39.12 0.873 32 28

Geotech Geol Eng (2012) 30:449–459 457

123

with 30% fly ash and 9% lime achieved was

3.14 MPa at 56 days curing as compared to that of

other mixes.

2. The Brazilian tensile strength and California

bearing ratio of all the mixes increased with

increase in lime content and curing period.

3. The mine overburden mixed with 30% fly ash and

9% lime produced highest compressive strength,

tensile strength and bearing ratio value as com-

pared to that of other mixes at 7, 28 and 56 days of

curing respectively.

4. Linear regression modeling showed that there

exists a strong relationship (high R value)

between Brazilian tensile strength and unconfined

compressive strength and California bearing

ration and unconfined compressive strength.

5. A very strong correlation (R = 0.95) between

Brazilian tensile strength and unconfined com-

pressive strength was observed for mine overbur-

den–fly ash–lime mixtures at 56 days of curing.

Similarly, strong correlation was observed with

increase in curing period from 7 to 28 days in case

of bearing ratio.

6. The results reported were developed for the coal

mine materials and fly ash from specific sites in

India and they should not be used for other sites

without any verification.

Acknowledgments The authors would like to acknowledge

the fund provided by Fly Ash Unit, Department of Science and

Technology, Govt. of India under R&D Scheme vide approval

No: FAU/DST/600(17)/2008-09 dated 12.09.2008.

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California Bearing Ratio and Brazilian Tensile Strength of Mine Overburden–Fly Ash–Lime Mixtures...

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