For official use only

GOVERNMENT OF INDIA MINISTRY OF RAILWAYS

STUDY REPORT ON

USE OF COAL ASH IN

RAILWAY EMBAKMENT

REPORT NUMBER GE: 0 S005

February-2006

Geo-technical Engineering Directorate,

Research Designs and Standards Organisation Manak Nagar, Lucknow 11

PREFACE

This report is prepared on the basis of literature survey and field investigation. Views expressed in this report are subject to modification from time to time in the light of future developments on the subject and as such, do not represent the views of the Ministry of Railways (Railway Board), Government of India.

(Nand Kishore) Executive Director/Geotech. Engg,

INDEX

S. NO. TITLE

PAGE NO.

1 Introduction

1

2 Definition

1

3 Production of various types of ashes

2

4 Availability of coal ash

2-3

5 Engineering properties of coal ash

3

6 Discussion of test results

4

7 IRC Recommendation

4-6

8 Studies other than RDSO

6-22

9 Fly ash in railway embankment

23

10 Case Histories

24-28

11 Guidelines for road embankment by BIS

28-29

12 Conclusion

29

13 References

30

ANNEXURE

1 Test Results

31-36

1.0 INTRODUCTION

There is a shortage of topsoil in most urban areas for filling low-lying areas, as well as for constructing road/rail embankments. The other option is to use waste materials arising from different sectors such as domestic, industrial and mining etc. In this report, the focus is on the use of industrial solid waste like coal ash as a fill material for construction of railway embankment

2.0 DEFINITION

The term fill is used to describe ground that has been formed by material deposited by man. Thus fill or made ground, as it is some time called, results from human activity in contrast to natural soil, which has its origin in geological processes. The fill material can be classified as below:-

2.1 Non Engineered fill

Non engineered fill generally arise as the biproduct of human activities associated with the disposal of waste material. The fill is not placed with a subsequent engineering application. In view of little control that may have been exercised in placing the fill, there is extreme variability, which makes it very difficult to characterize the engineering properties of these fills and predict their behaviour.

2.2 Engineered fill

Engineered fill is a fill that has been selected, placed and compacted to an appropriate specification in order to achieve some required engineering performance. Thus the fill is designed and built with a specific use in mind

2.3 PFA

Pulverised fuel ash is formed of silt-sized particles, which are carried from the furnace of a coal-fired power station with the flue gases.

1

3.0 PRODUCTION OF VARIOUS TYPES OF ASH

In thermal power plants, coal ash is solid residue resulting from pulverized coal. The process of combustion produces two type of residues - one which settles at the bottom of the furnace and the other which is carried away by the flue gases to be collected by electro static precipitators. The former residue is called bottom ash and the latter fly ash.

3.1 Pond Ash

This refers to the ash stored in ash ponds by the hydraulic fill method. Usually, it is a mixture of bottom ash and fly ash at most thermal power plants in India.

3.2 Fly Ash

Fly ash is a finely divided residue resulting from the combustion of pulverised coal in boiler and collected from electrostatic precipitators. It is a pozzolanic material, which in the presence of water reacts with lime and forms cementitious materials.

3.3 Bottom Ash

This category of ash is collected at the bottom of boiler furnace as a resultant of coal burning activity. This is a comparatively coarse material characterized by better geotechnical properties. This is an excellent material for fill embankment and road construction but its availability are very less.

4.0 AVAILABILITY OF COAL ASH

Coal ash is available in large volume in coal based thermal power plants. Some coal based plants of NTPC are listed below:-

2

Sl. No.

Name of Power Station Address

1 Singrauli Super Thermal Power Station

P.O. Shaktinagar Distt. Sonebhadra U.P.

2 Korba Super Thermal Power Station

P.O. Pragati Nagar, Korba ( West) Distt.Korba

3 Ramagundam Super Thermal Power Station

P.O. Jyoti Nagar Distt. Karmnagar Andhra Pradesh.

4 Farakka Super Thermal Power Station

P.O. Nabarun Distt. Murshidabad West Bengal

5 Vindhyachal Super Thermal Power Station

P.O. Vindhyanagar Distt. Sidhi M.P.

6 Rihand Super Thermal Power Station

P.O. Rihand Nagar Distt.Sonebhadra U.P.

7 National Capital Power Station

P.O. Vidyut Nagar Dadri, Dhaulana Road, Distt. Gautam Budha Nagar U.P.

8 Feroz Gandhi Unchahar Super Thermal Power Station

P.O. Unchahar, Distt.Rae Barreilly U.P.

9 Badarpur Super Thermal Power Station

Badarpur, New Delhi

10 Kahalgaon Super Thermal Power Station

P.O. Deepti Nagar, Kahalgaon, Distt. Bhagalpur, Bihar

11 Talcher Kaniha Super Thermal Power Station

P.O. Kaniha, Distt. Angul Orissa

12 Talcher Thermal Power Station

P.O. Talcher Thermal, Distt. Angul, Orissa

13 Tandar Thermal Power Station

P.O. Tanda, Distt. Ambedkarnagar, U.P.

14 Simhadri Super Thermal Power Station

P.O. Simhadri, Distt. Vishakhapatnam A.P.

5.0 ENGINEERING PROPERTIES OF COAL ASH

Some coal ash samples from different thermal power plants were tested in GE Lab, RDSO. Test results of these samples are tabulated as Annexure-I.

3

6.0 DISCUSSION ON TEST RESULTS

Test results indicate that coal ash contain most of sand and silt particles and non plastic in nature. OMC & MDD test results indicate that fly ash having very high OMC and low MDD value. Shear parameter indicate that fly ash having very low cohesion value. Consolidation test results indicate that fly ash is having high void ratio as compare to ordinary soil. Uniformity coefficient of bottom ash is less than 7 indicate that the material is not well graded.

7.0 IRC RECOMMENDATIONS

The design of fly ash embankment is basically similar to design of soil embankment. The design process for embankments involves the following steps:

Site investigations Characterisation of materials Detailed design

7.1 Site investigations

The following information concerning the site and surrounding areas must be collected:

Topography Hydrology Subsoil investigations

7.2 Characterisation of materials

The materials to be used in embankment construction should be characterised to determine their physical and engineering properties. If fly ash is to be used, the following information is required for approval before commencement of work -

Particle size analysis OMC & MDD value determined by heavy compaction Densities of fly ash - density lower than 0.9 gm/cc not suitable

for embankment construction

4

Shear strength parameters required for evaluation of the stability of proposed slopes and the bearing capacity of foundations located and the fill

Compressibility characteristics - required for predicting the magnitude and duration of the fill settlement

Permeability and capillarity - to assess seepage and to design drainage system

Specification for compaction of the fill material Position of water table - High water table should be lowered by

providing suitable drains Details of intermediate horizontal soil layers between which ash

is to be sandwitched.

7.3 Typical geotechnical properties of fly ash as recommended byIRC

PARAMETER RANGE

Sp. GRAVITY 1.90-2.55 PLASTICITY NP MDD 0.9-1.6 OMC 38.0-18.0% COHESION NEGLIGIBLE ANGLE OF INTERNAL FRICTION

30-40

COEFFICIENT OF CONSOLIDATION Cv (cm2 /sec)

1.75X10-5-2.01X10 -3

COMPRESSION INDEX Cc

0.05-0.4

PERMEABILITY 8X10-6-7X10-4 PARTICLE SIZE DISTRIBUTION CLAY 1-10% SILT 8-85% SAND 7-90% GRAVEL 0-10% COEFFICIENT OF UNIFORMITY 3.1-10.7

5

7.4 General recommendation by IRC Fly ash to be used as fill material should not have soluble

sulphate content exceeding 1.9 gm. per litre (expressed as SO3) when tested according to BS: 1377.

Coal used in Indian thermal power plants has high ash content. As a result, enrichment of heavy metal is lower as compared to fly ash produced by thermal power plants abroad.

7.5 Detailed design as recommended by IRC

The design of fly ash embankment is similar to earthen embankments.

Special emphasis is required with respect to provision of earth cover.

The thickness of side cover would be typically in the range of 1 m to 3 m.

For embankment upto 3 m height, in general, the earth cover thickness about 1 m is sufficient

The side cover should be regarded as a part of embankment for design analysis.

The FOS for embankments constructed using fly ash should not less than 1.25 under normal serviceability conditions.

Intermediate soil layers are often provided in the fly ash embankment for ease of construction to facilitate compaction of ash and to provide adequate confinement.

Properly benched and graded slopes prevent the erosion of fly ash particles.

8.0 STUDIES OTHER THAN RDSO 8.1 Dr. Vimal Kumar and other in their paper on fly ash in road &

embankment published in National Seminar cum Business Meet on use of fly ash in Roads & embankment advocated that the fly ash is better material than soil in construction of road embankment. The brief details of their paper are given below:

Application areas of fly ash in road embankment

The use of fly ash / pond ash for road and embankment applications can be classified as follows:

6

In embankment construction (including RE wall) In sub base and base course In semi rigid and rigid pavements (concrete roads)

Fly Ash in Embankment ( including RE wall )

For material to be used in embankments construction, the properties of concern are specific gravity, compaction characteristics, workability, internal angle friction, cohesion etc. Indian fly ashes generally have comfortable scores on these properties.

The most important parameter for selection of a material for roads & embankments is compaction behaviour. Ash has a favorable point than soils here. Compaction curves (moisture content v/s dry density curve) for soil & pond ash show good compaction characteristics on addition of moisture. But the curve for soil shows steep rise in dry density with increase in moisture content upto optimum moisture content (OMC) and fall in dry density subsequently. Satisfactory compaction (dry density above 95% of that at OMC) is achieved for a limited range of moisture content (about 2% - 5%). On the other hand similar curve for pond ash is relatively flat and the corresponding range for pond ash is quite large (about 7-8%), which means that it can be compacted over a wide range of moisture content without much variations in dry density. Hence, provides more flexibility for use in different seasons. It may be noted that though the maximum dry density of fly ash at OMC is less than that of soil, but it is not due to loose compaction or presence of the voids, rather it is due to lower specific gravity of ash particles. Further, fly ash is easy to compact and can be compacted by using either static or vibratory rollers. Fly ash has internal angle of friction in the range of 300 to 420, which is quite high as compared to that of soils (280 to 350). Fly ash when moist possesses apparent cohesion too. So, it can provide greater stability of slopes as compared to soil and side slopes steeper than soils can be provided in the embankments.

Specific gravity of coal ash particles ranges from 1.6 to 2.4 as compared to that of soil, which is in range 2.55 to 2.75. Due to lightweight, it imparts less load on sub-grades, hence can be used on weak sub-grades.

7

Fly ashes have permeability in the range of 10-6 to 10-4 cm/sec. Its high permeability ensures free & efficient drainage. After rainfall, water gets drained out freely, which means its workability is better than soil, especially, during the monsoon. Work on fly ash fills / embankments can be re-started within a few hours of rain while in case of soils, one is required to wait for much longer periods. Further, fly ash gets consolidated at a faster rate and primary consolidation gets over very quickly. So, it has low compressibility & shows negligible subsequent settlements. Thus, it can be used in bridge abutments also. Further, fly ash provides better bonding with geogrid material, as it has more friction angle as compared to soil. Hence, it provides a better & steeper RE wall as compared to soil.

Fly Ash in Sub Base and Base Course

Fly ash can be usefully employed for construction of sub base/base course. Mixing of soil and fly ash in suitable proportions improves the gradation as well as plasticity characteristics in the mix, thereby improving the compacted strength.

Fly ash (preferable) / Pond ash can be used for sub base and base course construction and stabilization. The fly ash is usually used in combination with lime to form the matrix that cements the aggregate particles together. Generally clay soils are stabilized with fly ash alone whereas silty soils respond well to stabilization with fly ash and lime or cement.

Physical, Chemical and Geo-Technical Properties of Fly Ash in India

Physical Properties of Fly ash vs. Natural Soil .

Properties Fly Ash Natural Soil Bulk Density (gm/cc) 0.9 1.6 1.3 1.8 Specific Gravity 1.6 2.4 2.55 2.75 Plasticity Very low

or non plastic Low to high

Shrinkage Limit Very low Low to high Grain size Silty/Sandy Clay size present Clay content - Could be much higher

8

Free Swell Index Very low Variable Classification Sandy silt to silty sand Variable

Water Holding Capacity (WHC) per cent

30-60 20-60

Typical Geo-Technical Properties of Fly Ash India Parameter Range

Specific Gravity 1.6 - 2.4

Plasticity Non-Plastic

Maximum Dry Density (gm/cc) 0.9 - 1.6

Optimum Moisture Content ( per

cent)

18.0 38.0

Cohesion (kN/m2 ) Negligible

Angle of Internal Friction () 30 - 42

Coefficient of Consolidation Cv

(cm/sec)

1.75 X 10-5- 2.01 X 10-3

Compression index Cc 0.05 0.4

Permeability (cm/sec) 8 X 10-6 7 X 10-4

Particle size Distribution ( percent of materials)

Clay size fraction 1 10

Silt size fraction 8 85

Sand size fraction 7 90

Gravel size fraction 0-10

Coefficient of Uniformity 3.1-10.7

9

10

11

12

8.2 Dr. Sudhir Mathur and others in their paper on Construction of

Road embankment and reinforced earth wall using fly ash published in National Seminar cum Business Meet on use of fly ash in Roads & embankment studied Engineering properties of fly ash and some case history. The brief detail of his paper is given below:

Fly ash obtained from coal fired electric power plants can be used as alternative material for construction of road embankments. The engineering behavior of fly ash would be similar to silt or fine sand. Usage of fly ash for embankment construction leads to its bulk utilization, replacing good earth and is especially attractive in urban areas where borrow material has to be brought from long distances.

Engineering properties of fly ash

The properties of ash depend primarily on type of coal and its pulverization, burning rate, temperature, method of collection, etc. The significant properties of fly ash that must be considered when it is used for construction of road embankments are gradation, compaction characteristics, shear strength, compressibility and permeability properties. Individual fly ash particles are spherical in shape, generally solid, though some times hollow. Fly ash possesses a silty texture and its specific gravity would be in the range of 2.2 to 2.4, which is less than natural soils. Fly ash is a non-plastic material. Fly ash displays a variation of dry density with moisture content that is smaller than the variation exhibited a well-graded soil. The tendency of fly ash to be less sensitive to variations in moisture content than natural soils can be explained by the higher void content of fly ash. Normal soils have 1 to 5 per cent air voids when compacted at maximum dry density. Fly ash contains 5 to 15 per cent air voids at maximum dry density. The higher air voids tend to limit the build up of pore-water pressures during compaction, thus allowing the fly ash to be compacted over a large range of moisture content. For the same reason, fly ash does not experience density increases from the changes in the compactive efforts of the same magnitude as experienced in case of fine-grained soils.

Fly ash exhibits shear strength characteristics similar to those of a cohesionless soil. It has a significant value of undrained angle of

13

internal friction and a minimal cohesion intercept in partially saturated condition. The friction angle for fly ash usually varies from 300 to 350 and some time especially for coarse ash, friction angle can be as high as 400. Any apparent cohesive behaviour displayed will be lost upon complete saturation. Majority of Indian power plants use bituminous coal and hence ash produced does not have significant free lime content. As a result such a fly ash is not hydraulic. Any latent strength development due to self-hardening would be very insignificant and cannot be counted on for design purposes. The compressibility of fly ash can be estimated in the laboratory using the oedometer. The typical values of compression index, Cc, for virgin compression ranges from 0.05 to 0.4 with a majority of values usually from 0.1 to 0.15. Recompression index, Cr ranges from 0.006 to 0.04. These values show that compaction can significantly reduce the compressibility of fly ash fills. The permeability of fly ash ranges from 8 x 10-6 cm/sec to 7 x 10-4 cm/sec. Generally medium to coarse type of ash have permeability values of about 10-4 cm/sec and hence can be considered to have good permeability.

Embankment construction using fly ash

Successful field trials have shown the suitability of fly ash as a fill material for construction of road embankments. Both reinforced as well as un-reinforced type of embankments have been constructed using fly ash. Reinforced embankments, popularly known as Reinforced Earth walls (RE walls) are used in urban areas for approaches to flyovers and bridges. RE walls have several advantages like faster rate of construction, economy, aesthetic look and saving the land required for construction of an unreinforced embankment. Fly ash is an ideal backfill material for RE wall construction because of its higher angle of internal friction and better drainage property. Geosynthetic materials like geogrids or geotextiles can be used as reinforcement for construction of reinforced fly ash embankments.

The most distinguishing feature of un-reinforced fly ash embankment would be use of fly ash as core material with earth cover. In case of un-reinforced embankments, side slope of 1:2 (Vertical: Horizontal) is generally recommended. Providing good earth cover using loamy soil should protect the slopes of the embankments. The thickness of side cover would be typically in the range of 1 to 3 m. The thickness of cover depends on the height

14

of the embankment, site conditions, flooding if expected, etc. This cover material can be excavated from the alignment itself and reused as shown in the Fig below. Stone pitching or turfing on this cover is necessary to prevent erosion due to running water. Intermediate soil layers of thickness 200 to 400 mm are usually provided when height of embankment exceeds 3 m. These intermediate soil layers facilitate compaction of ash and provide adequate confinement. Such intermediate soil layers also minimise liquefaction potential. Liquefaction in a fly ash fill generally occurs when fly ash is deposited under loose saturated condition during construction. To avoid the possibility of any liquefaction, fly ash should be properly compacted to at least 95 per cent of modified proctor density and in case water table is high, it should be lowered by providing suitable drains or capillary cut-off. Fly ash can be compacted using either vibratory or static rollers. However vibratory rollers are recommended for achieving better compaction. Compaction is usually carried out at optimum moisture content or slightly higher. The construction of fly ash core and earth cover should proceed simultaneously. High rate of consolidation of fly ash results in primary consolidation of fly ash before the construction work of the embankment is completed. The top 0.5 m of embankment should be constructed preferably using selected earth to form the subgrade for the road pavement.

Excavation of earth from alignment of embankment for providing side cover

15

Earlier fly ash embankment projects

Delhi PWD in association with CRRI pioneered in the construction of first reinforced flyash approach embankment on one side of the slip roads adjoining NH-2 in the Okhla fly over project. The length of the approach embankment is 59 m while the height varied from 7.3 m to 5.3 m. Geogrids were used for reinforcement of fly ash and a total quantity of about 2700 cum of ash from Badarpur thermal power plant was used for filling. The flyover was opened to traffic in Jan 1996.

First un-reinforced fly ash embankment in the country was constructed for eastern side approach of second Nizamuddin Bridge Project. A typical cross-section of the embankment shown in fig below. Pond ash produced at nearby Indraprastha Power Station was used for construction. The project is unique of its kind, since pond ash has been used for construction of high embankment in flood zone. A total quantity of about 1.5 lakh cubic metres of pond ash was used in this project. CRRI were the consultants for this project and provided design of the embankment and were associated for quality control supervision during construction. The project was completed and the road section opened to traffic in September 1998. The experiences gained during this project led to formulation of Guidelines on use of fly ash for embankment construction.

Cross-Section of Fly ash Embankment

16

Recent experiences of using fly ash for road embankment

Brief details of some of the projects executed in the recent past are given below:

Use of Pond Ash for Road Embankment on NH-6

As part of ongoing National Highway Development Programme, four-laning work of NH-6 from Dankuni to Kolaghat near Kolkata in West Bengal was taken up. The height of the embankment on this road section varied from 1.5 m to 4 m and requirement of earth fill was approximately 20 million cubic metres. However, good earth was not available near the proposed embankment site and the lead would be more than 100 km. This was leading to enormous increase in the project cost and also resulting in delays in the completion of the project. Hence the task on evaluating pond ash as alternative construction material was taken up. Required quantity of pond ash was available near the site at Kolaghat Power Plant. The pond ash samples collected from the Kolaghat power plant and bottom ash from Budge-Budge power plant (Kolkata) were tested in the laboratory to determine their engineering properties. The properties of pond ash, local soil and local sand are given in table below:-

Property Local

Sand Bottom ash from Budge-Budge Thermal Power Plant

Pond ash from Kolaghat Thermal Power Plant

Local Soil

Percentage material passing 75 Sieve

02 20 65 29

Modified Proctor test Maximum Dry Density (gm/cc) Optimum moisture content (%)

1.71 12.2

1.17 31.0

1.33 25.0

2.15 9.4

Permeability ( cm./ sec)

3.11x 10-3 6.26x10-3 7.2x10-4 -

Liquid Limit (%) - - - 35.4 Plasticity Index NP NP NP 15.7 Direct Shear Test Cohesion C kg/ cm2

0

0

0

0.23

17

Angle of internal friction ()

32 30 34 25

The proposed road alignment passes through waterlogged area. The water table in the area is very shallow and rises up to or above the ground during the rainy season. The subsoil at site generally consisted of silty clay or clayey soil up to a considerable depth. Such soils settle even under smaller loads imposed due to embankment of low height. However, if a lightweight material like pond ash is used in place of soil, the amount of settlement would certainly reduce.

The results of the stability analysis indicated improvement in factor of safety when fly ash was adopted as fill material. Results of stability analysis are given in table below. Keeping in view the site conditions, availability of materials near the construction site, it was suggested that after dewatering, geotextile wrapped sand or bottom ash layer of 0.5 m thickness be laid as base of the embankment. Pond ash embankment protected with 1.5 m thick soil cover was designed as given in figure below. However due to contractual constraints, the embankment was constructed by mixing pond ash and sand in ratio of 85:15 and subgrade was constructed using pond ash and soil in the ratio of 75:25

Condition Fill material Minimum Factor Of Safety Soil 1.62 Unsaturated Condition Pond Ash 1.92 Soil 1.36 Saturated Condition Pond Ash 1.50

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Slope failure of fly ash embankment

The Noida-Greater Noida Express-highway near Delhi, which is 23 km long, was constructed in a total span of approximately 3 years. It is a six lane express highway with divided carriageway. The height of the embankment on the total stretch is generally 1 to 2 m. However, at certain locations where the alignment crosses an under pass, the height of approach embankment varies from 6 to 8 m. The entire embankment was constructed using fly ash in its core and soil cover was provided along the slope and top portion of the embankment to prevent erosion. Intermediate soil layers were also provided with in the fly ash core. The highway was opened to traffic in the year 2003. In August 2004, after the heavy rainfall in quick succession, it was observed that the side slopes in high embankment portion had severely eroded and gullies were formed through out the high embankment slope. It was also observed at few spots that due to the piping action, the water had undermined the entire soil cover provided on the side slopes resulting in the exposure of fly ash layers. Detailed investigations were undertaken and causes of failure were identified as follows:

Severe erosion on the superlevated portion had taken place due

to heavy run-off from six-lane carriageway, which was discharged on one side of the embankment.

Absence of longitudinal kerb channel and chutes allowed water to drain off along the slope.

19

Deep pits were made in the embankment slopes to fix utilities like electric poles and crash barriers, which were backfilled with loose soil.

Run off water entered into the embankment side cover and caused deep cavities exposing fly ash at many locations.

The remedial measures suggested included filling of the cavities with granular material and compaction of side slopes, provision of toe wall, provision of kerb channel and chutes at regular intervals to take away the rain water safely and provision of stone pitching along with filter medium on the side slopes. The repair and restoration of the embankment is under progress.

8.3 Sri A.Trivedi & V.K.Sud in their paper Collapse behaviour of

coal ash published in Journal Of Geotechnical And Geo-environmental studied Engineering ASCE/April-2004 described an investigation carried out to examine the factors influencing the collapse settlement of the compacted coal ash due to wetting. The brief detail of their paper are given below:

The soils that exhibit collapse have an open type of structure with a high void ratio as expected in the case of ashes. According to Barden et al. (1969) the collapse mechanism is controlled by three factors; (1) a potentially unstable structure, such as flocculent type associated with soils; (2) a high applied pressure which further increases the instability; and (3) a high suction which provides the structure with only temporary strength which dissipates on wetting. As per an empirical study by Meckechnie (1989), the dry unit weight and water content are generally considered as important parameters that control the collapse of metastable structure of soils, if the dry unit weight is less than 16 kN m-3. The tentative dry unit weight of the coal ashes in Ropar ash pond was often found to be less than 10 kN m-3 suggesting possibility of collapse.

Conclusions given by author

The collapsibility of coal ash is one of the most important parameters for using ash as a fill material. Based upon the test results, various outcomes of this study are summarized as:

20

1. The collapse potential obtained by the oedometer test is a dependent parameter of several factors such as grain size characteristics, stress level, testing technique, degree of compaction, a finite consolidation ratio, moisture content, soluble substance, etc.

2. At prewetting critical moisture content and in the critical stress range (50-125 kpa), the ashes tend to collapse more than those in the dry condition. The observed collapse potential was proportional to the collapsibility factor identified from the maximum and minimum void state of the ashes. The ashes with more than 50% of the particles in silt size range were found to be collapsible.

3. The dry disposed ashes were more collapsible due to the presence of soluble substances as compared to that obtained by the wet disposal. Therefore, a correction was applied in the observed collapse potential of the dry disposed ashes to obtain a common correlation with the mean size as of the wet disposed ashes

4. The generally recognized lower limit of collapse potential for the collapsible soils in the oedometer is 0.01. It was observed that the coal ash with a collapse potential of 0.0075 at 80% degree of compaction (Dc) collapsed in model tests at 87% and 94% Dc. Increasing the density of this ash arrested the collapse in the model test. The coal ash with a lower collapse potential (0.0037 at 80% Dc) did not collapse at all while an ash with a higher collapse potential (0.021 at 80% Dc) collapsed at all the densities examined in the model test. Therefore, the lower limit of collapse potential of the collapsible ashes was recommended as 0.0075 at 80% degree of compaction in the oedometer.

5. In field, the collapse may occur due to the accidental wetting or a rise of water table. In such cases, the magnitude of measured collapse is a function of the depth of wetting front from the ground level. If the wetting front ratio is more than 1.8, a threat of collapse is bare minimum. The field collapse test is recommended under an actual condition of wetting, if ashes are to be used as a structural fill.

8.4 Sri Manoj Dutta in his paper Use of Coal Ash in Embankment

Stability Analysis and Design Consideration published in Journal of Civil Engineering & Construction Review, April 1999 and Engineering properties of Coal Ash published in Journal of Indian Geotechnical Conference 1998 studied behavior of coal ash in embankment. The brief details of his paper are given below:

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The use of bottom ash in embankment construction results in

slope 2:1 being stable. These slopes are much steeper than slopes, which are stable when pond ash or fly ash is used.

The use of pond ash yields stable embankment slope of 2.5:1 ( for ru =0 case)and 3:1( for ru =0.2 case)

Fly ash, which is a poorly draining material and has low strength, yields the least embankment slopes (4:1 for ru =0.2) for a factor of safety of 1.5.

Special design features of embankment constructed with ash:

Fugitive dust emission occurs at construction sites during hot

dry month due to wind erosion from ash, which is spread, as well as stock piled at site. This has to be avoided by

i) Suspending construction during hot dry months ii) Providing continuous water sprinklers iii) Providing intermediate earth cover

The possibility of surface water runoff pollution due to erosion

by rainwater during construction in the monsoon month has to be recognized and appropriate design measures adopted. Contained- cells construction technique can also reduce surface water pollution.

The stability of side slopes of thick fills has also to be ensured

against long-term wind erosion and water erosion (in the absence of slope maintenance after construction) by providing self sustaining erosion control measures such as thick soil covers with side lopes with turfing.

For embankment which are likely to experience ponding of

water during monsoon months, well designed slope protection measures (such as stone pitching/rip rap) are required with proper toe protection

Compactibility of wet ash during monsoon months under

condition of excess moisture has to be established.

To preclude the possibility of piping of ash in condition of seepage through ash, properly designed filters and drains (both internal & external) have to be provided

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9.0 FLY ASH IN RAILWAY EMBANKMENT

Railway used fly ash from Kolaghat thermal Power plant in Tamluk-Digha new rail link project from ch.10.92 to ch. 12.28 in about 1.30 km in length. This location is situated near Haldi bridge where embankment had failed twice during construction. Fly ash was also used in a near-by location on Panskura-Haldia section between ch.(-600) to ch.775. Section adopted for embankment construction is given below:-

RDSO officials visited TamlukDigha section of South Eastern Railway along with railway officials for performance appraisal of fly ash embankment in July 05. This section opened for traffic on 20.11.03. Main observations of visit are as under -

The axle load of the section is 17 tons and GMT 0.22. The sectional speed is 80 kmph and speed restriction of 30

kmph is imposed between km 12/0 to km 13/3 where fly ash was used due to erosion.

The track attention near Haldi bridge location was reported as 2 per month.

The variation in cross level is reported as 4.0 mm to 8.0 mm. Erosion of side slope occurred due to removal of side cover of

fly ash embankment on account of movement of cattle or any other reason. The large amount of fly ash had eroded and settlement occurred between hume pipe bridge no. 24 and 25 in Dec.04.

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10.0 CASE HISTORY OF FLY ASH EMBANKMENT IN DMRC

For the rail corridor of mass rapid transport system for Delhi, the base car depot was planned at Shastri Park on the east side of river Yamuna near ISBT. This area of 64 hectare is located at the reduced level of 203 to 204 meters above sea level where as 100 years HFL is 208.9 m hence it was decided to fill the base depot area with soil.

On review it was decided that if a potion of earth work can be done by using pond ash being generated at IP and Rajghat power stations of Delhi Vidyut Board, then a lot of saving in terms of time and money can be achieved due to reduced lead of carting of flyash as compared to the soil which is generally not available near Delhi. Mass filling at Shastri Park was to be completed in a short span of 12 months so as to give adequate time for consolidation of newly built embankment before construction of structures or starting the track laying over the filled up soil. A total quantity of about 17 lakhs cum of earth was required for constructing the main line track embankment at 6 m to 9 m height and about 30 ha of depot area at 6 m height.

Advantages considered

Due to following advantages, it was decided to use flyash to the extent of two- thirds requirement of the earthwork: Utilization of soil in place of flyash would have resulted in

erosion of topsoil from a large area of agricultural land and resultant degradation of land.

Disposal of flyash is a big problem for thermal power stations, hence scientific disposal of flyash by DMRC will pave the way for large scale utilization of flyash in the construction of earthen embankment for roads and rails.

Due to quick draining characteristics, work can be continued in monsoon.

Being a friction material, (no cohesion) proper compacted flyash shows very small long-term settlement.

Construction speed is faster as compaction vs moisture content curve is more even resulting in wider range of moisture content for compaction.

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Lower density than soil, hence low overburden pressures for the same height of embankment, hence less chances of toe failure.

Assured availability free of cost. The cost of transportation of flyash was less as compared to

soil. Flyash being lighter material, required less haulage and hence was economical.

Availability of good quality soil in such huge quantities was difficult.

Embankment Design

Embankment was designed on the same line as for soil embankment. A base layer of soil varying from 1.2 m to 1.6 m was provided over which 2 m thick layer of flyash was provided with intermediate soil layer of 0.4 m. On both sides of embankment, soil shoulder of 4.47 m width was provided for ensuring minimum soil cover of 2 m at all the locations. On the top, a minimum soil cover of 400 mm with 300 mm cover of blanket material has been provided for ensuring no erosion of flyash. A typical curve of embankment is shown in figure below -

The side slopes of the embankment have been kept to two horizontal to one vertical. The average height of embankment varies from 6 m to 9 m. Wherever height is more than 6 m, a 3 m berm has been provided at a depth of 6 m below the top. Soil and

25

flyash are fixed as 98% and 95% of modified proctor test ( IS-2720) respectively. Figure below shows typical compaction vs moisture content curve for soil and flyash.

Typical compaction vs moisture content curve for Soil

Typical compaction vs moisture content curve for Fly ash

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Description of work

The project site is situated in national capital of Delhi near Kashmere Gate, ISBT in the flood planes of river Yamuna and within the eastern marginal bund or Shahdara marginal bund. The work was started in Oct.1998 and full site was under standing water of a few cm to 1.0 m. Large scale dewatering by high discharging pumps and a network of drains was planned to make the ground dry. The top layer of soil having vegetation or poor soil was removed and ground was compacted by sheep foot roller of 10 to 12 tons with 8 to 10 passes so as to achieve a minimum dry density 95% of modified dry density. After the base layer of soil was laid and compacted in layer thickness of 15 cm, 98% of modified dry density was achieved. Base soil layer of 1.2 m to 1.6 m was laid before starting laying of flyash layer of 15 cm thickness each. The embankment designed as shown in figure above was completed by laying layers of soil and flyash as per the requirement of design.

Problems faced and solutions found:

During the winter season, the moisture content in the pond ash was very high and due to non availability of sun-shine consecutively for many weeks, moisture content could not be kept near OMC. Hence, stacking and rehandling of material was done to achieve the desired compaction level of 95% of modified dry density.

During transportation, there were chances of spillage of fly ash, if it is dry even after keeping the dumpers fully coverd by tarpulin and in case of higher moisture content, fly ash used to flow from the opening in the dumper body as it liquefies very quickly. Hence extensive dewatering was done in ash ponds at loading point itself.

On the onset of summer, the peculiar problem of flying of flyash was encountered at ash pond as well as site. A very elaborate arrangement of net work of nine bore wells was developed for sprinkling water continuously over the exposed flyash slopes and top and in addition to that,12 tankers of 6000 to 10000 litres capacity were deployed during peak requirement to sprinkle water at all the roads on which machinery was moving. But as the site is surrounded by

27

During summer, it was observed that rate of loss of water from the compacted layer was very high resulting into loss of compaction, which used to get aggregated by movement of dumpers which came for dumping of flyash. This problem was solved by increasing thickness of compacted layer of fly ash from 150 mm to 300 mm. This helped in speeding the construction, reducing the open area of flyash and reduced loss of moisture and compaction.

Spillage of flyash on roads during transportation is an area of great concern and carelessness can create a very serious pollution problem within the city. Hence preventive and corrective measures were taken to control this menace. On daily basis, it was ensured that all dumpers carrying flyash were fully covered by tarpulin overloading of dumpers was strictly not allowed.

Quality control

A self contained fully equipped soil-testing laboratory was established before starting the work. Some of the tests were Sieve analysis, Moisture content determination, Modified proctor test as per IS 2720, Liquid Limit and Plastic Limit. Following were the main considerations as part of the quality assurance programme - Each source of soil flyash was decided and approved before

bringing the material at site. Field control on compaction was achieved by ensuring moisture

contents near OMC, adequate plain and vibro passes of compactor and compaction level by core cutter method were ensured.

. 11.0 GUIDELINE FOR EMBANKMENT (As per BIS 10153-1982) Studies have shown the suitability of fly ash as a fill material for

the construction of embankments. The properties to be kept in view are grain size, density; shear strength, compaction characteristics & permeability. The fly ash has to be compacted at OMC, which is normally in range of 15-30 percent. Because of low density the

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material is suitable for location where clayey soils get consolidated under overburden material. The permeability of compacted fly ash is low so in cases where the water table is very high or surface water likely to percolate down the embankment, it is advisable to provide for drainage a layer of coarse material 300-450 mm thick below the fly ash

12.0 CONCLUSION Railway embankment is quite different from road embankment due

to the fact that they are designed for higher axle loads and very tight safety tolerance therefore, coal ash can not be used directly in railway embankment. The study reveals that fly ash is cohesionless and highly erodible in nature, has low density and high void ratio, as such, it may not behave as ideal material for construction of railway embankments. To over come these inherent geotechnical short comings, construction of embankment with fly ash requires specialised method wherein fly ash has to be used in combination with naturally occurring soil. Extensive monitoring of field performance of embankment constructed with fly ash on an experimental basis is required before usage of fly ash could be propagated on wider scale.

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REFERENCES

1 Use of Coal Ash in Embankment Stability Analysis and Design

Consideration by Sri Manoj Dutta

2. Engineering properties of Coal Ash by Sri Manoj Dutta

3 Collapse behaviour of coal ash by Sri A.Trivedi & V.K.Sud

4. Construction of Road embankment and reinforced earth wall using fly

ash by Dr. Sudhir Mathur and others

5. Fly ash in road & embankment byDr. Vimal Kumar and other

6. Engineered Fills by Thomas Telford

Annexure-I

Ash Samples Properties

Coal Ash samples

Properties S.No.

NTPC Vidhya Nagar, M.P. (Fly Ash)

NTPC Vidhya Nagar, M.P. (Pond Ash)

Singrauli Super Thermal Power Station (Fly Ash)

Singrauli Super Thermal Power Station ( Bottom Ash)

Kolag- hat Thermal Power (Fly Ash)

Barauni Ther- mal Power (Fly Ash)

Uncha har Thermal Power (Fly Ash)

Badar pur Thermal Power ( Bottom Ash)

1. Classific -ation

ML SM SM SP-SM, SM SP-SM ML SP-SM, SP,SM

Grain size distribution Gravel (%) 00 01 06 09 00 00 00 00-6 Sand (%) 20 72 56 84 61 94-95 16-30 80-92 Silt (%) 80 27 38 07 39 05-06 70-84 03-17 Clay (%) 00 00 00 00 00 00 00 00

2.

Fines passing 75 sieve (%)

80 27 38 07 39 05-06 70-84 03-17

Consistency Limits (%) 3. Liquid Limit NP NP NP NP NP NP NP

31

Plastic Limit NP NP NP NP NP NP NP NP Plasticity Index

NP NP NP NP NP NP NP NP

OMC (%) 13.5 21.0 22.5 25.0 19.2 17.60-19.45

18-24.5 - 4.

MDD (g /cm3)

1.51 1.31 1.28 1.27 1.26 1.641-1.660

1.22-1.42

-

Shear parameters: C(kg/cm2) 0.02 0.00 0.024 0.012 .0143 - (degree) 33.83 35.33 33.65 33.5-

33.65 32.98 -

5.

C (kg/cm2) 0.10 0.0-0.01 - - (degree) 11.2 24 - - 6 Specific gravity

2.19 2.06 1.82 2.13 2.2 2.34-2.41

- -

7. Consolidation test parameters:

Compression Index (Cc) 0.2086 0.2428 0.1107 0.0674-

0.0719 - -

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Coefficient of Consolidation (Cv) (cm2/min) at: 2 Kg/cm2 4 Kg/cm2 8 Kg/cm2

0.1671 0.2169 0.1702

0.1394 0.1466 0.2291

0.1196

0.1849

0.2552

0.1302-0.3629 0.2627-0.3581 0.2515-0.3509

-

-

Pre- Consolidation Pressure (Pc) (kg/cm2)

1.00

0.90

0.75

0.95-1.02

-

-

Initial Void Ratio (e0)

0.5795

0.4894

1.1275 1.0870-1.3154

-

-

8 Uniformity Coefficient ( Cu)

10.47

4.52

66.67

3.66

-

3.05-3.38

8.54-11.96

3.54-5.0

9 Coefficient of curvature ( Cc)

0.76 1.47 9.80 1.47 - 1.23-1.47

0.63-1.37

0.74-1.31

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S.No.

Coal Ash samples

Properties

NTPC Rihand (Fly Ash)

NTPC Rihand (Bottom Ash)

NTPC Ramagun-dam A.P (Fly Ash)

NTPC Ramagun-dam, A.P. (Bottom Ash)

NTPC Ramagun-dam, A.P. (Bottom Ash))

NTPC Kahalgaon (Fly Ash)

NTPC

Kahalgaon (Bottom

Ash)

1. Classifica-tion

ML SM ML SP-SM SP-SM ML SP-SM

Gravel (%) 00 00 00 03 01 00 00 Sand (%) 01 55 07 92 91 15 94 Silt (%) 79 45 93 05 08 85 06 Clay (%) 20 00 00 00 00 00 00

2.

Fines passing 75 sieve (%)

99 45 93 05 08 85 06

Liquid Limit NP NP NP NP NP NP NP Plastic Limit NP NP NP NP NP NP NP

3.

Plasticity Index

NP NP NP NP NP NP NP

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OMC (%) 15.0 20.0 16.0 27.0 27.0 15.50 27.0 4. MDD (g /cm3)

1.58 1.26 1.51 1.17 1.17 1.45 1.2

C(kg/cm2) . (degree)

5.

C (kg/cm2) (degree) 6 Specific gravity

2.15 1.864

7. Compression Index (Cc)

Coefficient of Consolidation (Cv) (cm2/min) at: 2 Kg/cm2 4 Kg/cm2 8 Kg/cm2

35

36

Pre- Consolidation Pressure (Pc) (kg/cm2)

Initial Void Ratio (e0)

-

8 Uniformity Coefficient ( Cu)

7.69

14.12

3.27

3.56

3.50

9 Coefficient of curvature ( Cc)

0,69 3.53 0.80 1.13 0.75

For official use onlyGOVERNMENT OF INDIAMINISTRY OF RAILWAYSSTUDY REPORT ONUSE OF COAL ASHINRAILWAY EMBAKMENT February-2006Geo-technical Engineering Directorate,Research Designs and Standards OrganisationManak Nagar, Lucknow 11PREFACE3.0 PRODUCTION OF VARIOUS TYPES OF ASH In thermal power plants, coal ash is solid residue resulting from pulverized coal. The process of combustion produces two type of residues - one which settles at the bottom of the furnace and the other which is carried away by the flue gases to be collected by electro static precipitators. The former residue is called bottom ash and the latter fly ash. AddressFly Ash in Sub Base and Base CourseTypical Geo-Technical Properties of Fly Ash IndiaParameterRangeSpecific Gravity1.6 - 2.4PlasticityNon-PlasticMaximum Dry Density (gm/cc) Optimum Moisture Content ( per cent)18.0 38.0Cohesion (kN/m2 )NegligibleAngle of Internal Friction ()30 - 42Coefficient of Consolidation Cv (cm/sec)Compression index Cc0.05 0.4Permeability (cm/sec)8 X 10-6 7 X 10-4Particle size Distribution ( percent of materials)Clay size fraction1 10Silt size fraction8 85Sand size fraction7 90Gravel size fraction0-10Coefficient of Uniformity3.1-10.7ConditionFill material

Conclusions given by author Annexure-IS.No.Coal Ash samples Properties 1.Classific -ationMLSMSMSP-SM, SMSP-SMMLSP-SM, SP,SM2.Grain size distribution Gravel (%)000109000000-6Sand (%)207256846194-9516-3080-92Silt (%)802738073905-0670-8403-17Clay (%)00 00000000 000000Fines passing 75( sieve (%)802738073905-0670-8403-173.Consistency Limits (%)Liquid LimitNPNPNPNPNPNPNPPlastic LimitNPNPNPNPNPNPNPNPPlasticity IndexNPNPNPNPNPNPNPNP4.OMC (%)13.521.022.525.019.217.60-19.4518-24.5-MDD (g /cm3)1.511.311.281.271.261.641-1.6601.22-1.42-5.Shear parameters:C((kg/cm2)0.02 0.00 0.0240.012.0143-(((degree)33.83 35.33 33.6533.5-33.6532.98 -C (kg/cm2)0.100.0-0.01--( (degree)11.224--Specific gravity2.192.061.822.132.22.34-2.41--Consolidation test parameters:CompressionIndex (Cc)0.2086 0.2428 0.11070.0674-0.0719--Coefficient of Consolidation (Cv) (cm2/min) at: 2 Kg/cm24 Kg/cm2Pre- Consolidation Pressure (Pc) (kg/cm2)0.750.95-1.02Initial Void Ratio (e0) 1.12751.0870-1.3154S.No.Coal Ash samples Properties NTPCNTPC(Bottom Ash) 1.Classifica-tionMLSMMLSP-SMSP-SMMLSP-SM2.Gravel (%)0000000300Sand (%)01550792911594Silt (%)79459305088506Clay (%)20 0000 00000000Fines passing 75( sieve (%)994593050885063.Liquid LimitNPNPNPNPNPNPNPPlastic LimitNPNPNPNPNPNPNPPlasticity IndexNPNPNPNPNPNPNP4.OMC (%)15.020.016.027.027.015.5027.0MDD (g /cm3)1.581.261.511.171.171.451.25.C((kg/cm2).(((degree)C (kg/cm2)( (degree)Specific gravity2.15 1.864 CompressionIndex (Cc)Coefficient of Consolidation (Cv) (cm2/min) at: 2 Kg/cm24 Kg/cm2Pre- Consolidation Pressure (Pc) (kg/cm2)Initial Void Ratio (e0)