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International Journal of Scientific & Engineering Research Volume 4, Issue 5, May-2013 ISSN 2229-5518

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Effect of Fly Ash on CBR and DCPT Results of Granular Sub Base Subjected to Heavy Compaction

Ratna Prasad, R., Darga Kumar, N., and Janardhana, M.

Abstract— The granular subbase compacted at 5% of fly ash is showed higher dry density compared to all other proportions of fly ash.

As the percentage of fly ash increases from 0 to 25%, the CBR values are decreasing. Up to 5 to 10% of fly ash addition to granular subbase

has not shown any large decrease in CBR values, but addition of fly ash content beyond 15% to the granular subbase showed about 50 to

60% reduction in CBR. The CBR values obtained from empirical formulae showed lower values than the laboratory CBR values.

Index Terms— CBR, DCPI60, fly ash,Gravelly sand, heavy compaction, MDD, OMC.

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1.0 INTRODUCTION

NDIA has about 70 thermal power plants and coal currently ac-counts for 70 per cent of power production in the country. The process of coal combustion results in fly ash. Various Indian

collieries supply the coal, which is known to have a very high ash content of almost 40 to 45 per cent. India's thermal power plants produce an estimated 100 million tonnes of fly ash per annum. The problem with fly ash lies in the fact that not only does its disposal require large quantities of land, water, and energy, its fine particles, if not managed well, by virtue of their weightlessness, can become airborne. Currently, huge amount of fly ash is being generated annu-ally in India, with 65 000 acres of land being occupied by ash ponds. Such a huge quantity does pose challenging problems, in the form of land usage, health hazards, and environmental dangers. Both in dis-posal, as well as in utilization, utmost care has to be taken, to safe-guard the interest of human life, wild life, and environment. The conventional method used to dispose of both fly ash and bottom ash is to convert them into slurry for impounding in ash ponds around the thermal plants. This method entails long-term problems. The severe problems that arise from such dumping are: (i). the construc-tion of ash ponds requires vast tracts of land and this depletes land available for agriculture over a period of time, (ii). when one ash pond fills up, another has to be built, at great cost and further loss of agricultural land, and (iii). huge quantities of water are required to convert ash into slurry. During rains, numerous salts and metallic content in the slurry can leach down to the groundwater and contam-inate it. Hence, it is necessitated to utilize the abundantly available fly ash for the civil engineering construction activities especially in the pavement construction so as to overcome the problems posed by the fly ash [12].

Coal-based thermal power stations have been operational for more than 50 years but the concept of developing environment friendly solutions for fly ash utilization is only about 20 years old. Overall fly ash utilization in India stands at a fairly low level of about 15 per

cent of the quantity generated.

Various possibilities for its use in different engineering applications are under research. Fly ash utilization towards road projects is rapid. The Planning, design and construction of ash disposal facility require the integration of geotechnical, environmental, hydrological engi-neering and other governing factors.

Field compaction technique type adopted plays a vital role in com-paction of fine grained soils [18]. Sheepsfoot or pad foot rollers are preferred because good compaction of the lift from the bottom up is achieved, while the kneading action helps to further mix the fly ash, soil, and water [3]. With the increased reactivity of self-cementing fly ash, however, a much shorter compaction delay time is typically specified. For self-cementing fly ash stabilized sections, compaction should commence as soon as possible after final mixing and be com-pleted within two hours, so the stabilized material will show less strength and density decrease “[18], [ 3]”. Generally, clay soils have soaked CBR values from 1.5% to 5%, which provides very little support to the pavement structure. Addition of 16% self-cementing fly ash increases the soaked CBR values of heavy clay soils, which is comparable to gravelly sands [15].

Long-term strength gain is expected for Class C fly ash stabilized soils [9]. A minimum of 3 to 5 per cent of lime stabiliser is necessary to gain a significant increase in the compressive strength of fly ash stabilized clayey soil [10]. Careful laboratory evaluation of different fly ash contents for a given soil is necessary to find the optimum ash addition rate [3]. Molded moisture content does not appear to affect CBR of fly ash mixed soil, but the level of compaction effort does [8]. Sub-standard compaction effort (~95% of standard Proctor) pro-duces unsoaked and soaked CBR values around 40%, while modified compaction effort yields values between 80% and 90% [8]. Very rapid stabilisation of water-logged sites has been achieved with the use of quicklime. Small quantities of quick lime typically 1 to 3 % are used to reduce the plasticity of the clay [19]. Optimum content of fly ash in decreasing the swell potential was found to be 20% [2].

Generally the CBR/strength is contributed by the cohesion and fric-tion of fly ash [11]. Dynamic Cone Penetration (DCP) testing can be used widely in evaluation of pavement performance. DCP testing can be completed in five to ten minutes, depending on the test depth and strength of the material [20]. Maximum dry density of expansive soil mixed with fly ash occurred in the water content range of 12 to 14%

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R. Ratna Prasad, Research Scholar, JNTU Kakinada and Professor of Civil Engg., VVIT, Guntur, AP, India. E-mail: [email protected]

N. Darga Kumar, Professor of Civil Engg., ASTU, Adama, Ethiopia, E-mail:[email protected], or [email protected].

M. Janardhana, Professor of Civil Engg., JNTUH, Hyderabad, AP, India, E-mail: [email protected]

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[1]. Soft clay soil, asphaltic recycled pavement material (RPM), and road-surface gravel (RSG) were stabilized using class C and off-specification fly ashes to create working platforms or stabilized base course for construction of flexible and rigid pavements [7]. Studies were reported on expansive soil stabilization with fly ash and rice husk ash (RHA). RHA content of 12% and a fly ash content of 25% are very effective in strengthening the expansive subgrade soil [14]. The design thickness for a pavement is controlled by the subgrade stiffness, as measured by the CBR [21]. The maximum dry density (MDD) of the black cotton soil increased from 13.6 to 15.2 kN/m3 for addition of 40% fly ash obtained from Nyveli (NFA) and whereas for red earth MDD changed from 14.6 to 17.8 kN/m3 for same fly ash [13]. Though the works of numerous researchers in the past have helped in improving understanding of the beneficial use of fly ash in cement concrete, brick manufacturing and other applications, but a comprehensive idea about geotechnical aspects of fly ash –gravelly sand mixtures for various engineering applications especially for pavement construction have not yet understood clearly.

2.0 EXPERIMENTAL STUDY

2.1 Materials Used in the Study

2.1.1 Soil

The gravelly sand used in the present study was collected from the outer ring road area near Gandi Misamma in Hyderabad, AP state, India. The soil is a grayish to brown coloured gravelly sand and has no cohesion. The soil collected was kept in controlled conditions in the laboratory and was used for testing as per the Indian Standard specifications given in the respective test codes. For this soil, the basic tests were conducted in the laboratory for its characterization. As per the basic properties of soils are concerned, it indicates that the soil has soil proportions of gravel, sand and little fine fraction. In the soil the % slit and clay is around 7%, sand is 70% and gravel is around 23%. The grain size distribution curve of the soil is presented in Fig.1. The various basic properties of soil are presented in the Table 1. Fig.1. Grain size distribution curves for soil and fly ash.

2.1.2 Fly ash Used

The fly ash used in this investigation was collected from Vijayawada

Thermal Power Station (VTPS) Vijayawada. The fly ash sample col-lected was stored in the air tight containers. The grain size distribu-tion curve for fly ash is presented in Fig. 1. The various properties of the fly ash obtained from the VTPS are presented in the Table.2 and 3. The fly ash proportions adopted in the study by dry weight of soil are 0%, 5%, 10%, 15%, 20% and 25%.

TABLE 1 BASIC PROPERTIES OF SOIL

Property Value Specific gravity 2.68 Cohesion, c (kPa) at OMC 0 Angle of Internal Friction, (deg) at OMC 45 Optimum Moisture Content, OMC (%) 7.5 Maximum Dry Density, MDD (kN/m3) 20.90 Unsoaked CBR (%) 85 % Gravel 21.3 % Coarse Sand 14.6 % Medium Sand 38.3 % Fine Sand 18 % Silt & Clay 7.8 Soil Classification SW

TABLE 2

PHYSICAL PROPERTIES OF FLY ASH

Property Value

Specific gravity 1.97

Cohesion, c (kPa) at OMC 10

Angle of Internal Friction, (deg) st OMC 28

Optimum Moisture Content, OMC (%) 18

Maximum Dry Density, MDD (kN/m3) 13.80

Unsoaked CBR (%) 34

% Gravel 0

% e Fine Sand 97.5

% Silt & Clay 2.5

2.2 Tests Conducted

The following tests are planned and conducted on fly ash gravelly sand mixtures. The fly ash proportions adopted in the study along with the gravelly sand are 0%, 5%, 10%, 15%, 20% and 25% by weight of dry soil. The tests such as Modified Compaction test [5], California Bearing Ratio (CBR) test [6] and Dynamic Cone Penetration (DCP) tests are conducted. The tests such as CBR and DCPT are conducted on the specimens compacted at OMC as per the modified compac-tion. The modified compaction tests are adopted because; the majority highway pavements are designed for high traffic vol-umes.

2.3 Dynamic Cone Penetration Test

Dynamic cone Penetrometer (DCP) is portable equipment that can be used for evaluation of unbound granular base, sub base. Subgrade and also at pipe line congested narrow trenches where testing with

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other equipments is difficult and not feasible due to cost. Space and time constraints, therefore DCPT are now being used in many parts of the country for rapid characterization of unbound granular and sub grade material properties.

TABLE 3 CHEMICAL PROPERTIES OF FLY ASH

Content of the Component Value

% SiO2 60.5

% Al2O3 30.8

% Fe2O3 3.6

% CaO 1.4

% MgO 0.91

% SO3 0.14

% K2O+Na2O 1.1

Loss of ignition (LOI) 0.8

Specific surface area, m2/kg 338

Lime reactivity, N/mm2 6.2

The basic principle involved in operation of this apparatus is to measure resistance offered by the pavement layers to the penetration of a standard cone having 20mm diameter (with 60 deg apex angle) is driven by 8 kg hammer freely falling through a height of 575 mm [4]. The amount of penetration of cone is generally reported in terms of the average penetration per blow, DCPI60 (mm/blow). This indi-cates relative shear strength of the material across its depth tested. During Penetration of the cone, the shear strength of the material is mainly occurred due to resistance offered by the soil particles by shear displacement taking place. A greater value of DCPI60 indicates a softer material and vice versa in the pit over sub grade layer on the plain surface of granular base layer. It is noted that the DCP test can be conducted in laboratory on remolded soil or granular material compacted in a steel mould having internal minimum diameter of 304.8mm may be used which is significantly eliminate the effect of the confinement. As another method a test bed of pavement can also be constructed in an open area and such test can also be carried out proto type model. The typical sketch of DCP is presented in Fig. 2. The DCP test data can be represented in different formats for deter-mination of value of DCPI60 a plot is to be drawn with number of blows vs. cumulative penetration of the cone. All such plotted data points are to be connected by lines and slope changes of the lines are marked. With this demarcated slope along the plotted data points, the pavement layer thickness can be assessed. The obtained DCPI60 val-ue can be used for CBR determination. Several relationships between DCPI60 and CBR values were developed by different researchers in various countries across the globe. There is a applicable relationship between CBR and DCPI60 [17]. As per this, the relation between CBR and DCPI60 is given as:

Log10CBR=2.48-1.057Log10 (DCPI60) (1)

3.0 RESULTS AND DISCUSSION

3.1 Compaction Characteristics

Fig.3 presents the compaction curves of fly ash gravelly sand mix-tures subjected to heavy compaction. From this figure, it can be seen

that the gravelly sand compacted at 5% of fly ash is showing higher dry density compared to all other proportions of fly ash mixed. The compaction curves presented in this figure are showing the typical trend of compaction behavior of granular soil.

Fig. 2 Dynamic cone penetrometer test equipment

It can be understand that up to about optimum moisture content (OMC) the compacted behavior of soil is of rigid and from OMC onwards the behavior of the compacted soil is of flow able/soft na-ture. After OMC any small addition of water to the fly ash soil mix-ture is causing reduction in dry density. It is also noticed that as the percentage of fly ash increases from 5% to 25%, the behavior of the mixture shifting from granular soil to silt behavior. This behavior can be attributed as the fly ash replacing the granular soil and resulting in more silt fraction. Also due to modified compaction there can be some amount of generation of fine fraction in the fly ash soil mix-ture. The variation of OMC and MDD of soil with the percentage of fly ash is presented in Table 4. As the percentage fly ash increases from 0% to 15%, the OMC also increasing slightly. The OMC of fly ash gravelly sand mixtures is almost similar at 20% of fly ash and 0% of fly ash. This slight increase in OMC with the percentage of fly ash can be attributed to presence of silt fraction in the fly ash soil mixture. It can be seen that there is a increase in MDD at 5% of fly ash addition to soil and from 5% fly ash onwards the MDD is reduc-ing gradually. From this behavior, it can be attributed that the void spaces can be effectively filled up with the fly ash proportion up to about 5%, and beyond this further addition of fly ash may result in excess fine fraction.

Hence it is understand that the fly ash proportion beyond 5% addi-tion to soil leads to lesser weight and in turn cause reduced values of

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dry densities of fly ash gravel sand mixtures. It can be seen that the OMC of fly ash soil mixtures corresponding to modified compaction is varying from 7.5% to 8.5% and MDD of fly ash soil mixtures is varying from 21.2kN/m 3 to 20kN/m3.

3.2 CBR Results

The load penetration curves of fly ash gravelly sand mixture samples prepared at respective OMCs and obtained in un-soaked test condi-tion, are presented in Fig 4.

Fig. 3. Compaction curves for fly ash mixed gravelly sand

TABLE 4 OMC, MDD VALUES WITH % FLY ASH

% Fly Ash % OMC MDD (kN/m3)

0 7.5 20.9

5 7.5 21.2

10 8 20.9

15 8.5 20.82

20 7.5 20.3

25 8 20.05

The curves corresponding to 0% and 5% fly ash are moving parallel and at higher level compared to the curves concerning to 10%, 15%, 20% and 25% fly ash. In all these curves, initially it is observed that there is a concave upwards and beyond certain penetration levels such as from 2 to 4 mm onwards, these curves are moving towards linear and curve linear pattern. From these curves, the CBR values were recorded after applying the suitable correction for the initial

concave upward potion. The CBR values obtained for various per-centages of fly ash under un-soaked condition are presented in Table 5. From this table, it can be seen that as the percentage of fly ash increases the CBR values are decreasing. Up to 5% of fly ash addi-tion to soil has not shown any large decrease in CBR values. For the fly ash content beyond 15% addition to soil is causing about 50% to 60% reduction in the CBR values. As the percentage fly ash increas-es the granular soil behaving as sandy silty soil and hence causing reduced values of CBR. From this CBR behavior with the percentage of fly ash, it can be suggested that 10% fly ash can be effectively utilized along with the granular material for road payment construc-tions. In majority times, though the CBR value of a subgrade soil is high, its value may be limited to 20%. From the results it can be seen that at 25% fly ash, the CBR of gravelly sand is 43%. Hence, even addition of 25% fly ash to gravelly sand can perform better in the pavement construction.

Fig. 4. Load – penetration curves for fly ash mixed gravelly sand tested at OMC

3.3 DCP Test Results

To understand the penetration aspects of compacted gravelly sand sub grade soil prepared in the CBR mould at OMC at different pro-portions of fly ash, the standard test such as DCPT was conducted on the compacted un-soaked samples. The number of blows verses DCP total penetration curves are presented in Fig 5 for various percent-ages of fly ash such as 0%, 5%, 10%,15%, 20% and 25%. From this figure, it can be noticed that as the number of blows increases, the DCP total penetration is also increasing. Up to about 10% of fly ash, the penetration values observed are about 10mm/two blows, whereas for fly ash content of 15% and above, and for the same number of blows two, the penetration values observed are 18 to 30 mm. From this penetration curves, it is further noticed that for the same number blows, the penetration levels are increasing as the percentage of fly

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ash increases from 0% to 25%. This increase in penetration levels is more for the fly ash content of more than 10%. From these curves the DCPI60 values are derived for the linear portion of the curves. The DCPI60 is defined as the ratio of total penetration in a particular stretch of liner portion of the curve to the number blows in that stretch of penetration. The CBR values from the DCP tests are ob-tained by substituting DCPI60 (Dynamic Cone Penetrometer Index value corresponding to cone angle of 600) in the formulae given in Eqn.1. Since the penetration is almost linear for the first two blows, and hence, the DCPI60 has been calculated for the penetrations oc-curred in the first two blows only for all the fly ash gravelly sand mixtures tested on samples prepared at respective OMC. The DCPI60 and CBR values obtained from DCP tests conducted on un-soaked samples are presented in Fig 6. Fig. 5 Dynamic cone penetration curves for fly ash mixed gravelly sand

Fig. 6 DCPI60 and %CBR curves for fly ash mixed gravelly sand

From this curve, it is clearly understand that as the percentage of fly ash increases, the DCPI60 values are increasing and whereas the CBR values are reducing. From this plot, it can be further seen that in be-tween 10% and 15% of fly ash the DCPI60 and CBR curves are meet-ing at a point. The variation of CBR values obtained from DCP test and from the laboratory CBR test along with the percentage of fly ash is presented in Fig. 7. From this figure, it can be noticed that the CBR values obtained from DCPI60 and laboratory CBR test are de-creasing as the percentage of fly ash increases from 0% to 25%. Also it is noticed that the CBR values obtained from DCP test are decreas-ing linearly with percentage of fly ash and whereas the CBR values obtained from CBR test are decreasing in a curve linear fashion. And from these two comparisons, it is noticed that CBR values obtained from DCP test are so low as compared to laboratory CBR values. The CBR values obtained from laboratory CBR test and DCP test are presented with the percentage of fly ash in the Table 5.

Fig. 7 Comparison of CBR values obtained from CBR Test and

DCP Test for fly ash mixed gravelly sand

TABLE 5 COMPARISON OF CBR VALUES

% Fly

ash DCPI60 %CBR From

DCPI60

%CBR from CBR Test

0 5 55.1 85

5 6 45.5 80

10 7.1 38 31

15 8.8 30.4 17

20 11.2 23.5 46

25 14.4 18 43

4.0 SUMMARY AND CONCLUSIONS

Use of locally available waste materials such as fly ash can find a viable option in controlling the costs of gravelly sand transport from far places to the construction site. Use of locally available fly ash along with the gravelly sand will solve three major issues such as (i) environmental issue and (ii) gravelly sand transport cost will be min-

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imized and (iii) wastage of agricultural land can be avoided. The CBR values obtained from DCPI60 (DCP test) and laboratory CBR test are decreasing as the percentage of fly ash increases from 0% to 25%. The CBR values obtained from DCP test are decreasing linear-ly with percentage of fly ash and whereas the CBR values obtained from CBR test are decreasing in a curve linear fashion. The CBR values obtained from DCP test are so low as compared to the values obtained from laboratory CBR test. Even 25% addition of fly ash to the gravelly sand, the CBR value showed more than 20%. In general in majority flexible pavement design, though the CBR value is more than 20%, its value is limited to 20%. Hence, from this it can be pro-posed that even up to 25% addition of fly ash can make the econom-ic construction of pavement without compromising any strength as-pects.

REFERENCES

[1] S. Bhuvaneshwari, R.G. Robinson and S.R. Gandhi, “ Stabilisation of ex-pansive soils using fly ash”, Fly ash Utilization Programme (FAUP), TIFAC, DST, New Delhi, Fly ash India 2005, VIII 5.1.

[2] Erdal Cokca, “Use Of Class C Fly Ashes for the Stabilization – of an Expan-sive Soil”. Journal of Geotechnical and Geoenvironmental Engineering, Vol. 127, July’ 2001, pp. 568-573.

[3] G. Ferguson and S.M. Leverson, “Soil and Pavement Base Stabilization with Self-Cementing Coal Fly Ash”, American Coal Ash Association, 1999, Al-exandria, VA.

[4] IRC: SP: 72-2007, “Flexible Pavement Design for Rural Roads”. [5] IS: 2720 (Part 7)-1980, “Methods of test for soils”: Determination of water

content-dry density relation using compaction. [6] IS: 2720 (Part l6)-1979, “Methods of test for soils”, Laboratory determina-

tion of CBR. [7] Lin Li, B. Tuncer Edil and H. Craig Benson, “Properties of pavement geo-

materials stabilized with fly ash”, World coasl ash (WOCA) Conference, May 4-7, 2009, Lexington, KY, USA, pp.1-11.

[8] D.B. Mahrt, “Reclaimed class C Iowa fly ash as a select fill material: hy-draulic conductivity and field testing of strength parameters.” MSc thesis, 2000, Iowa State University, Ames, IA.

[9] A. Misra, “Stabilization characteristics of clays using class c fly ash.”

Transportation Research Record 1611, Transportation Research Board, 1998, pp.46-54.

[10] P. Paige-Green, “Recent Developments in Soil Stabilization”, Proceedings of 19th ARRB Conference, Sydney, Australia, Dec 1998, pp.121-135.

[11] N.S. Pandian, K.C. Krishna and B. Leelavathamma, “Effect of Fly Ash on the CBR Behaviour of Soils”, Indian Geotechnical Conference 2002, Allah-abad, Vol.1, pp.183-186.

[12] “Utility bonanza from dust”, State Environment Related Issues, Department of Forests, Ecology & Environment, Government of Karnataka, Parisara ENVIS Newsletter, 2007, Vol.2, No.6,.

[13] S.M. Prasanna Kumar, “Cementitious compounds formation using pozzola-nas and their effect on stabilization of soils of varying engineering proper-ties”, International conference on environment science and engineering, IP-CBEE, 2011, Vol.8, pp.212-215, IACSIT Press, Singapore.

[14] M. Robert Brooks, “Soil stabilization with fly ash and rice husk ash”, Inter-national Journal of Research and Reviews in Applied Sciences, 2009, Vol-ume 1, Issue 3, pp.209-217.

[15] M.P. Rollings, and R.S. Rollings Jr, “Geotechnical Materials in Construc-tion”, 1996, McGraw-Hill, New York.

[16] R. Srinivas Kumar, “Highway Engineering”, 2010, University Press India Pvt., Ltd., Hyderabad.

[17] TRL Transport Research Laboratory, “A user manual for a program to analyze DCP data”, Overseas Road Note 8, Dept. of Transport, UK.

[18] R.L. Terrel, J.A. Epps, E.J. Barenberg, J.K. Mitchell, Thompson and M.R. Thompson, “Soil stabilization in pavement structures: A user’s manual” FHWA-IP-80-2, 1979a, Vol. 1, Department of Transportation, Federal Highway Administration, Washington, D.C.

[19] D.J. White, “Hydrated fly ash fines as soil stabilizers.” ISU-ERI-Ames Re-port 01177, 2000, Engineering Research Institute, Iowa State University, Ames, IA.

[20] D.J. White, “Reclaimed hydrated fly ash as select fill under pcc pavement Wapello County demonstration project – Field Performance.” ISU-ERI-

Ames Report 01176, 2002a, Engineering Research Institute, Iowa State Uni-versity, Ames, IA.

[21] J. Zheng, R. Zhang and H. Yang, “Highway Subgrade Construction in Expansive Soil Areas”, Journal of Materials in Civil Engineering, 2009, vol. 21, No. 4, pp. 154-162.

ACKNOWLEDGEMENTS

The authors are happy to express their sincre thanks are due to the staff of geotechnical laboratory at JNTU Hyderabad in India for continued support extended to complete this project. Also, sincere thanks are due to the officials of ASTU, Adama, Ethiopia for their moral support and facility provided to pre-pare the manuscript.

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