Highway Research Bulletin

YEAR 2004-2005
Bulletin No. 73

USE OF REINFORCED FLYASH IN HIGHWAY EMBANKMENTS
Dr Praveen Kumar*, Dr H.C.Mehndiratta** & Siddhartha Rokade***

SYNOPSIS

It is often required to stabilise and reinforce the structurally unsound soil to bear the traffic load. Different types of materials are being increasingly employed in highway engineering to facilitate construction, ensure better performance and reduce maintenance. Based on comprehensive experimental and research programmes undertaken in developed countries non-metallic materials may be suitable for reinforcement. Geotextiles are the development in the field of reinforcing materials. These materials are thin, bi-dimensional, flexible, anti-corrosive and non-biodegradable hence have a long life. In the present study, a series of plate load tests and field CBR tests were conducted on locally available and categorised as SP soil as per IS classification, flyash, flyash in combination with lime and flyash combined with above soil. These tests were conducted on the above materials with and without geotextile reinforcement to see the reinforcing effect imparted by geotextiles. The geotextile reinforcement was also kept at varying depth and the tests were conducted. It was observed that the CBR of the above materials improved when geotextile was placed in it. Maximum increment in CBR was achieved when geotextile was placed near the surface. The introduction of geotextile reduced the deformation and increased the bearing capacity as was revealed by plate load tests.
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
* Associate Professor | Transportation Engineering Section, Civil Engineering
** Professor | Depptt., Indian Institute of Technology, Roorkee-247 667
*** Post Graduate Student | (Uttaranchal)
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1. INTRODUCTION
There are about 95 million  tonnes of flyash per annum as a waste material  in coal based thermal  power stations in the country. Because of pozzolanic  property of flyash, it can be converted in to meaningful wealth as an alternate cementing  material in civil engineering works. Use of flyash in roads, airfields and embankments are  some of the areas that have attracted the attention  and use in large quantities. In context  of roads and embankments, it can be said that flyash is a good  material for geotechnical application and can replace soil in most of the applications.


The existing 130 thermal power plants alone need about 60,000 acres of precious land for disposal of flyash in their life span of 30 years. About Rs. 700 crores has been invested to get rid of flyash where as a similar investment would have been sufficient to convert all flyash in to useful products. In addition to the disposal problem, air pollution gets increased in the ambient air. Table 1 shows various flyash producing thermal power plants in India.

 Table - 1. Some Major Thermal Power Stations in India

Western Region Eastern Region Northern Region Southern Region North Eastern Region
Amarkantak Bandel Badarpur Basin Bridge Bonogoigoan
Ballershah Barauni Bhatinda Ennore Chanderpur
Bhusawal Bokaro Faridabad Kathaugdem Kathalguri
Chola C.E.S.C. Harduaganj Nellore Lakwa
Dhuvaran Chandrapura Indraprastha Neyveli Namrup
Ghadhinagar Durgapur Kalakote Ramagundam  
Khaperkheda D.P.L. Kota Raichur  
Koradi Gouripur Obra Tuticorin  
Korba Kolaghat Panki Vijayawada  
Nasik Mulajore Panipat    
Paras Muzzaffarpur Paricha    
Parli New Cossipore Renusagar    
Rajghat Patratu Ropar    
Sabarmati Santhaldih Singrauli    
Satpura Talcher Ghaziabad    
Trombay Titagarh      
Ukai        
Uram        
Utran        
Wanakbari        

The concept of reinforcing poor soil has continued for a long time. Different types of material are being increasingly employed in various civil engineering activities and especially in highway engineering to facilitate construction , ensure better performance of the structure and reduce maintenance. Over the last few decades, the use of geosynthetics has recorded a tremendous increase. They have found wide acceptance in the construction industry all over. They are now looked upon as a cost effective solutions to many foundations and stability problems. Coal based thermal power stations produce flyash, depending on the quality of coal used and modes of burning and collection, flyash have varied pozzolanic properties. Nearly 50 per cent saving in the cost of road construction can be achieved if locally available flyash is used instead of costly stone aggregates.


2. PROPERTIES OF FLYASH
The chemical, physical and engineering properties of ash depends on the type and source of coal used, method and degree of coal preparation, cleaning and pulverization, type and operation of power generation unit, ash collection, handling and storage methods etc. So the properties of flyash vary from plant to plant and even within the same plant.

The mineral group present in coal such as hydrated silicate group (kaolinite and montmorillonite etc.), sulphate group (quartz, feldspar, apatite etc.) and their varying composition play a major role in determining the chemical composition of ash. During combustion the above minerals gets transformed. So the overall composition varies from particle to particle and from one sample to another. The principle constituents of ash are silica, alumina and iron-oxide with smaller amounts of calcium oxide, magnesium oxide, sulphur and unburnt carbon.

Table 2 shows average composition of flyash from various power plant in India. It is observed that Indian fly ash generally tends to be more siliceous and also contains higher concentration of unburnt coal than the flyash from foreign countries and therefore is less reactive. Unburnt carbon acts as diluent of the pozzolanic matter in flyash. Based on the amount of calcium oxide present, flyash is divided in to two categories as per ASTM. Type F (Cao less than 5%) and type C (Cao more than 5 %). The type F fly ash is derived from burning of anthracite or bituminous coal and type C fly ash is obtained from burning lignite coal. Type C fly ash may contain lime content higher as well as pozzolanic properties.

 Table - 2. Composition of Flyash from Various Thermal Power Plants

Location SiO2 Al2O3 Fe2O3 CaO MgO SO3 Loss on Ignition
Delhi 59.00 28.10
4.30
2.50
0.30
0.10 4.40
Singrauli 56.80 28.80 7.80 2.70 0.60 0.10 0.40
Obra 64.40 23.00 6.50 0.70 0.20 0.21 3.40
Panki 58.00 25.10 10.00 1.10 0.40 0.30 4.30
Harduaganj 61.00 16.10 5.60 3.10 0.40 0.40 11.80
Korba 66.53 18.90 8.90 3.60 2.60 0.20 0.53
MPEB 58.30 24.60 4.40 5.40 3.90 -- 3.30
Durgapur 49.30 20.05 19.60 2.28 1.53 Trace 6.13
Barauni 60.28 22.68 8.45 1.10 0.31 1.40 6.0
Bokaro 51.60 22.00 4.70 0.86 0.85 2.10 19.40
Chandrapur 60.30 26.80 5.30 1.18 0.51 1.60 5.20
Phulpur 59.77 23.92 9.56 2.51 1.28 -- --
Indraprastha 60.10 18.60 6.40 6.30 3.60 -- 4.90
Neyveli 45.59 23.33 0.64 5.16 1.50 Trace 1.20


3. CASE STUDIES OF FLYASH UTILISATION IN EMBANKMENTS
3.1 Highway 402 Interchange Embankment , Ontario, Canada
This site was underlain by 30 m of compressible silty clay and layers of sand totaling 7.0m [6]. Substantial differential settlements between the approach embankments supported on this clay and the pile-supported piers were expected if conventional construction materials were used. To avoid the costly and inconvenient alternative of staged construction, flyash was selected as light weight fill for the embankment.

Coal ash from Ontario Hydro’s Lambton generating station was used in the over- pass construction involving 82640 tonnes of bottom ash and 64210 tonnes of flyash. Different embankment, 8m high at the overpass were constructed for the east bound and west bound lanes. The embankment design consisted of a lower layer of bottom ash, a flyash core, and an upper topping of bottom ash. Owing to possible post action in the silt-sized flyash, about 1.2m of bottom ash was used to top the embankment. Compaction was done with smooth drum, self propelled, vibratory compactors. Proctor dry densities were 1587 kg/m3 at 20.6% water content for the flyash.

A complete set of instruments was placed at two stations 50m a part. During the monitoring period, frost depths ranging from 0.6 m to 1.03m were recorded using instruments. The frost heave sensor exhibited responses ranging from 5.0 mm settlement to 8.0 mm heave. On the basis of sub soil investigation, an embankment settlement of 460 mm was calculated for conventional earth fill material, where as the light weight flyash fill used at these site showed settlements of the order of 230 mm or less. Up to 2.45 mm of settlement was measured within the 8.0 m fill height by settlement rods, which is a negligible amount.

3.2 Visvesvarya Setu Project
Delhi PWD in association with CRRI and Flyash Mission constructed the first reinforced flyash retaining wall on one side of the slip roads adjoining NH-2 in the above mentioned project [3]. The length of approach embankment is 59.0 m while the height varied from 7.3 m to 5.3 m. The substructure was reinforced with bi-oriented geogrids using bottom ash as a filler material. For constructing the super structure cast in situ facing panels were placed over previously laid RCC foundation block. Mono-oriented geogrids were laid and anchored to facing panels. Construction of conventional retaining wall would have resulted in acquirement of private property as its foundation would have encroached in to adjoining lands. A total quantity of about 2700 cubic metre (compacted) of pond ash was used for filling. The flyover was opened to traffic in Jan 96 and has been performing well.

3.3 Second Nizamuddin Bridge Project at Delhi
A new four lane bridge is constructed across the river Yamuna, a little down stream of the existing Nizamuddin Bridge along NH-24 [1]. For construction of embankment for approach road flyash (pondash) was used. Flyash from the ponds of Indraprastha thermal power plant Delhi was used as fill material. It is the major flyash utilisation project in India till date. Approximately 0.15 million tonnes of flyash was utilised in the construction along with soil and gravel. The total length of the eastern approach is 1.826 km and the height of embankment ranges from 6 m to 9 m and it consists of two or three flyash cells surrounded by soil cover made up of low permeability soil. The reason for providing 0.40 m thick intermediate barrier is to separate the flyash in to several individual cells, this has the advantage in controlling any built up of hydrostatic pressure within the confined flyash cell over a period of time. Each individual cell is constructed with a clear height of 2.0 to 2.3 m approximately.

Specifications adopted for the materials are pond ash compacted to 95% of MDD and it should be 1.2 gm/c.c. The moisture variation for compaction was OMC 2%. Soil was compacted to a dry density not less than 1.65 gm/cc and PI value in the range of 8 to 10%. Design of embankment was carried out on the basis of slope stability analysis. The analysis of flyash embankment was evaluated by using the simplified Bishop’s method of slices and arrived the side slopes 1V : 2 H. Figure 1 shows the typical cross section of fly ash embankment.



3.4 Embankment with Geogrid and Anchor Reinforcement
The newly developed combined system technique using anchor and geogrids is found to be suitable for a wide range of fill material [7]. Varuna bridge approach

Structure proposed to be constructed consists of 10.5 m height. The construction work is to be done in three stages of 3.5 m of flyash each. The retaining structure is reinforced with anchors and geogrids in alternate layers. Construction of embankment is proposed with the obvious advantage that it would allow access for inspection maintenance and construction purpose.

3.5 Lime Treated Flyash as Embankment Material
Lime treated flyash was used in the construction of embankment over soft clay at a site near north Madras thermal power plant to carry ash slurry pipe line near Attipattu at Madras. An experimental work has been carried out to know the stability and settlement characteristics. The flyash samples were collected from Ennore thermal power station, Madras. The lime used in this embankment was of reagent calcium oxide (CaO) of 95% purity, optimum lime content was found to be 1.6 per cent.

4. DETAILS OF EXPERIMENTAL PROGRAMME
4.1 Soil :
The soil used in the study was locally available Roorkee Soil. It is a cohesionless soil which is classified as A-3 as per revised PRA (Public Roads Administration) classification and (SP) as per IS classification. The various properties of Roorkee soil are given in Table.3.

 Table - 3  Properties of  Soil

Property Value
Classification as per Revised PRA A-3
OMC (%) 13.5
Maximum Dry Density (gm/cc) 1.8
Specific Gravity 2.65
Uniformity Coefficient, Cu 2.19
Coefficient of Curvature, Cc 0.90
Liquid Limit (%) Non Plastic
Plastic Limit (%) Non Plastic
Plasticity Index (%) Non Plastic

4.2 Flyash :
The flyash used in the study was brought from National Thermal Power Station situated at Ghaziabad which was available free of cost. Flyash is classified as silts of low compressiblity (ML). The physical and chemical properties of flyash are given in Table 4 .

 Table - 4 Physical and Chemical Properties of Flyash

Properties Test Value

1. Physical Properties

(a) Specific Gravity (G) 2.16
(b) Fineness by Sieving
Sand particles (%) 3.5
Silt particles (%) 95.5
Clay particles (%) 1.0
(c) Proctor OMC (%) 22.0
(d) Maximum dry density (gm/cc) 1.4
2. Chemical Properties
(a) Silica (SiO2) (%) 58.78
(b) Iron oxide (Fe2O3) (%) 9.31
(c) Alumina (Al2O3) (%) 26.92
(d) Calcium Oxide (CaO) (%) 1.77
(e) Magnesium Oxide (MgO) (%) 0.68
(f) Total sulphur (%) 0.1
(g) Sodium Oxide (N2O) (%) 0.28
(h) Potassium Oxide (K2O) (%) 1.44
(i) Loss on Ignition (%) by weight 0.72

4.3 Lime :
The Lime used in the study was commercial lime available in Roorkee market. Intentionally, scientific lime was not used because in actual only commercial lime will be used.

4.4 Geotextile :
Only one Type of the Geotextile was used which was of non-woven type in the present study. The properties of this material is given in Table 5 .

 Table -5 Properties of Geotextile

Fabric Construction Needle punched non-woven
Material composition 100% Polypropylene
Weight per sq. metre 309 gms
Thickness 2.97 mm
Resistance to Chemicals Excellent
Resistance to bio-degradation Excellent
Roll available 4.5 m wide and
  25-30 m long

4.5 Test Pit Preparation :
Test pits of size 1.5m X 1.5m X 1.0m were dug in the pavement test hall of Transportation Engineering Section of IIT, Roorkee. The pits were prepared with following materials:
a. Soil
b. Flyash
c. Flyash + 4 percent Lime
d. 75 percent Flyash + 25 percent Soil
e.Two such pits were prepared at a time, one unreinforced and the other reinforced with geotextile at varying depth of 50cm    (H/2), 30cm (H/3) and 10cm (H/10) from top. Where H is the total depth of the pit for each of the above materials. Compaction    was done at OMC in five layers of 20 cm each with the help of tamping rod. Densities were measured using aluminum cups and    compaction was carried out till at least 95% of laboratory maximum dry density was achieved. Vibratory roller was also used on    the surface to ensure uniformity in compaction. When lime was used in combinations with flyash the compacted surface was left    for 3 days of air curing. This was done to ensure the reactivity of lime with other materials.

5. ANALYSIS OF LABORATORY TEST RESULTS
5.1 Compaction Tests
Standard proctor tests were conducted to determine the optimum moisture content (OMC) and maximum dry density (MDD) of various materials. The results of proctor test carried on soil , flyash, flyash + 4% lime, 75% flyash + 25% soil are given in Table 6. It is seen that the compacted dry density of flyash is well below that of most conventional fill materials. This is advantageous if the fill or embankment has to be placed on ground of low bearing capacity or where long term settlement is possible. Moreover it is seen in case of flyash that for greater variation in water content there is little variation in maximum dry density.

Table – 6 Maximum Dry Density and OMC of Various Materials

S. No. Type of Material Optimum Moisture Content (%) Meximum Dry Density (gm/cc)
1 Soil

14.0

1.85
2 Flyash 22.0 1.43
3 Flyash + 4% Lime 23.5 1.64
4 75% Flyash + 25% Soil 18.5 1.59

5.2 Field CBR Test
Results obtained from field CBR test on soil (SP) , flyash, flyash + 4% lime and 75% flyash + 25% soil (SP) are given in Table 7 . These tests were conducted without geotextile reinforcement and with reinforcement placed in the materials at 50 cm , 30cm and 10cm from the surface in order to study the effect of position of geotextile on CBR and eventually to find the position which gives maximum benefit .

It may be observed that introduction of geotextile in soil has an effect on its CBR value. CBR value depends on type of soil, amount of compaction and position of geotextile in soil. It is observed that the CBR values show an increasing trend with the introduction of geotextile placed in the materials at 50cm, 30 cm and 10 cm from surface.

The CBR values in case of soil (SP) is nearly doubled when geotextile was kept in it at 10 cm from surface to that when geotextile was not used as reinforcement. From Table 4.2 it is seen that there is a gradual increase in CBR values when geotextile used at 50cm, 30cm and 10cm from surface to that when geotextile was not used. Maximum values of CBR were obtained when geotextile was kept at 10cm from top, which shows that for maximum benefit the geotextile should be kept near the surface. Lime showed very good affinity with flyash and CBR values were greatly enhanced for both with and without geotextile.. Similarly the mixture of flyash with soil showed improved results both with and without geotextile.

Table-7 CBR Values for Various Types of Materials Including Soil

S. No. CBR without geotextile (%) CBR with geotextile at 50 cm from surface (%) CBR with geotextile at 30 cm from surface (%) CBR with geotextile at 10 cm from surface (%)
  at 2.5 mm at 5.0 mm at 2.5 mm at 5.0 mm at 2.5 mm at 5.0 mm at 2.5 mm at 5.0 mm
Roorke Soil (SP) 11.42

10.95

12.85 12.38

17.14

16.66

22.14

21.42
Flyash 15.71 14.28 17.85 17.14 23.57 21.90 28.57 26.66
Flyash + 4% Lime 22.85 20.00 27.85 26.19 35.0 30.95 40.71 34.76

75% Flyash + 25% Soil (SP)

18.57 14.76 22.14 18.57 27.85 24.28
34.28 30.00

5.3 Plate Load Tests
From the values of the modulus of subgrade reaction and modulus of elasticity of various materials with and without reinforcement it shows that the inclusion of geotextile adds to the strength of the subgrade. Table 8 reveals that the modulus of subgrade reaction increases as the geotextile layer moves towards the surface of material. Table 9 gives the values of modulus of elasticity for various types of materials. Modulus of elasticity increases as the geotextile is nearer to the surface.

Table-8 Modulus of Subgrade Reaction (K) for Various Materials

  K value without geotextile (kg / cm3) K value with Geotextile at 50 cm from surface (kg /cm3) K value with geotextile at 30cm from surface (kg/cm3) K value with geotextile
at 10 cm from surface (kg /cm3)
Roorke Soil (SP) 2.464

3.040

4.352

4.736

Flyash 4.160 4.608 5.056 5.880
Flyash + 4% Lime 5.568 5.952 7.072 8.032
75% Flyash + 25% Soil (SP) 4.416 5.120 5.792 6.720


Table-9 Modulus of Elasticity (E) for Various Materials

  E value without geotextile (kg / cm2) E value with Geotextile at 50 cm from surface (kg /cm2) E value with geotextile at 30cm from surface (kg/cm2) E value with geotextile
at 10 cm from surface (kg /cm2)
Roorke Soil (SP) 120.6
130.7

155.7

160.3
Flyash 140.4 147.6 194.4 221.4
Flyash + 4% Lime 196.2 223.2 248.4 264.6
75% Flyash + 25% Soil (SP) 151.2 180.0 199.8 216.0


Table10-12 shows the percentage increase in field CBR, Modulus of Subgrade Reaction and Modulus of Elasticity values after use of Geotextile.

Table-10 Percentage Increase in Field CBR Values with Geotextile

  % increase in field CBR values when geotextile at 50cm from surface % increase in field CBR values when geotextile at 30cm from surface % increase in field CBR values when geotextile at 10cm from surface
Roorke Soil (SP) 12.52
50.08

93.87

Flyash 13.62 50.03 81.85
Flyash + 4% Lime 21.88 53.17 78.16
75% Flyash + 25% Soil (SP) 19.22 49.97 84.59


Table-11 Percentage Increase in Modulus of Subgrade Reaction (K) with Geotextile

  % increase in  K values when geotextile at 50cm from surface % increase in K values when geotextile at 30cm from surface % increase in K values when geotextile at 10cm from surface
Roorke Soil (SP) 23.37 76.62

92.20

Flyash 10.76 21.53 41.34
Flyash + 4% Lime 6.89 27.01 44.25
75% Flyash + 25% Soil (SP) 15.94 31.15 52.17


Table –12 Percentage increase in Modulus of Elasticity (E) with Geotextile

  % increase in  E values when geotextile at 50cm from surface % increase in E values when geotextile at 30cm from surface % increase in E values when geotextile at 10cm from surface
Roorke Soil (SP) 8.37 29.10 32.91
Flyash 5.12 38.46 57.69
Flyash + 4% Lime 13.76 26.60 34.86
75% Flyash + 25% Soil (SP) 19.04 32.14 42.85


6. CONCLUSIONS
(1) The geotextile reinforcement improves the CBR value considerably.
(2 The field CBR value at optimum moisture content of Roorkee soil (SP), flyash, flyash + 4% lime and 75% flyash + 25% soil (SP)      increased by 93.87%, 81.85%, 78.16%,and 84.59% respectively when geotextile was kept at 10 cm from the surface.
(3) The field CBR values at optimum moisture content showed an increasing trend when calculated without geotextile       reinforcement ,with geotextile at 50cm, 30cm and 10cm respectively.
(4) The position of geotextile reinforcement affects the field CBR value tremendously and maximum field CBR values were achieved       when geotextile was kept at 10cm from the surface. Thus for maximum benefit the geotextile should be placed near the       surface.
(5) Lime mixed with soil (SP) showed very good affinity with flyash and their mixture of flyash + 4% lime and 75% flyash +25% soil      (SP) gave higher field CBR values.
(6) The values of modulus of subgrade reaction as well as modulus of elasticity for all the four materials of soil (SP), flyash, flyash +      4% lime and 75% flyash + 25% soil (SP) increase when position of geotextile shifts from 50 cm to 10 cm. Further, the inclusion      of geotextile adds to the strength of the subgrade.
(7) Out of the four different materials taken in the present study, flyash with 4% lime gave the highest strength values in both      plate load test and field CBR test. The values of modulus of elasticity also showed increasing trends with the addition of      geotextile layer.

REFERENCES
(1) Report on Use of Flyash in Eastern Approach Embankment Second Nizamuddin Bridge CRRI, New Delhi, February, 1999.
(2) A. Hilmi Lav and M. Aysen Lav “Microstructural Development of Stabilised Fly ash as Pavement Base Material”, Journal of Materials      in Civil engineering ASCE Vol. 13-1, May 2000, pp 157-162.
(3) Murty AVSR “Utilisation of Flyash For Embankment Construction” Experience Sharing Meet on Use of Flyash in Roads and      Embankments, CRRI, New Delhi, 1988, pp 15-23.
(4) Trivedi. A and Sood V.K. “Coal Ash as Embankment and Structural Fill Material” Dept. of Civil Engineering TIET, Patiala, pp       130-137.
(5) Swami R.K., Dhawan P.K., AVSR Murty “Stabilisation of Flyash for Road Construction” SSRR division, CRRI, New Delhi, pp       161- 169.
(6) Karri Srinivas Rao “Design of Highway Embankments Using Flyash in Alternate Layers” M.E., Thesis, Civil Engineering Dept., UOR,      Roorkee, 2001.
(7) Srinivas S. “Studies on Shear Strength Behaviour of Coal Ashes”, Proc. of the National Seminar on Flyash Characterization and its       Geotechnical Application, IISC Bangalore, 1999.
(8) Rokade Siddhartha, A Parametric Study on Reinforced Flyash in Highway Embankments, M.E.Thesis, Indian Institute of      Technology, Roorkee, 2002.
(9) Indian Roads Congress, Rural Roads Manual, SP:20-2002, New Delhi.