YEAR 2006

By Dr Praveen Kumar*, Dr H.C.Mehndiratta** and A.K.Durga Rao***


It is often required to stabilize and reinforce the structurally unsound soil to bear the traffic. Based on comprehensive experimental and research programmes undertaken in developed countries, non-metallic materials are bound to be suitable for reinforcement. Geosynthetics are being widely used in highway engineering, to solve a variety of problems related to drainage, separation and reinforcement. Geomembranes are the second largest group in geosynthetics. They are impervious thin sheets of rubber or plastic material used primarily for lining and covers of liquid or solid storage barriers. In cases, where drainage is not a major problem, plastic sheet (polyethylene) can be used as Geosynthetics. It is cheap and easy to use.

In the present study, a series of lab and field CBR tests alongwith plate load tests were conducted on fly ash in combination with Roorkee soil (locally available soil). These tests were conducted on the flyash/soil material with and without plastic sheet reinforcement to evaluate the reinforcing effect imparted by plastic sheet. The plastic sheet reinforcement is also kept at varying depths and tests were conducted. It was observed that the CBR of the above material improved when plastic sheet was placed in it. Maximum increment in CBR was achieved when plastic sheet is placed at a depth of 5 cm from surface. Introduction of plastic sheet reduced the deformation and increased the bearing capacity as was revealed by plate load tests. Direct shear test is conducted on fly ash to find out the shear parameters. The triaxial tests were also conducted on fly ash. These tests were conducted on the above material with and without plastic sheet, and to see the reinforcing effect on shear parameters (C and F)


1.1 General

Due to industrialization and rapid economic growth, demand for electricity has risen tremendously. To meet this demand, a number of coal based thermal power plants have been set up. At present, in India about 100 million tonnes1 of fly ash is produced in every year from thermal power plants and this is likely to blow up to a staggering 200 million tonnes by 2012. At the present level of ash utilization, this amount of fly ash is expected to lock 1.60 lakh acres of precious land in ash ponds.

Fly ash is causing environmental pollution, health hazards and requires large area of disposal1. Due to increasing concern for environmental protection and growing awareness of the ill effects of pollution, disposal of fly ash generated at thermal power plants has become an urgent and challenging task. Fly ash can be utilized in many ways as shown through extensive R& D efforts. However, Bulk utilization can be done in the field of civil engineering application especially for construction of roads and embankments because of its pozzolanic property. In the context of roads and embankments, it can be said that fly ash is a good material for geotechnical applications and can replace soil in most of the applications.


2.1 Properties of Flyash

The chemical, physical and engineering properties of ash depend 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 fly ash vary from plant to plant and even within the same plant2. Chemical composition of ashes governs pozzolanic and self-hardening characteristics.

2.1.1 Chemical composition of fly ash

The chemical composition varies from particle to particle and from one sample to another. The principal constituents of ash are silica, alumina and iron oxide with smaller amounts of calcium oxide, magnesium oxide and sulphur and unburnt carbon3.

Table 1 shows composition of fly ash from various power plants4 in India. Based on calcium oxide content, fly ash 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 properties4. Chemical composition of Indian, British and American ashes is presented in
Table 2 5,6,7.

It is seen from these tables that Indian fly ashes are more siliceous than the British and American ashes. Other constituents fall more or less in the same range. It is also noticed that Indian fly ashes contain higher content of unburnt carbon.

Chemical constituents most likely to affect the engineering properties of fly ash are free lime and unburnt carbon. Free lime contributes to the age hardening properties of fly ash when compacted. Unburnt carbon also affects the strength and compaction characteristics8.

2.1.2 Physical properties of fly ash Grain size distribution

Table 3 indicates the grain size distribution of pond ash, fly ash and bottom ash from different power plants. It is observed from table that fly ash is predominantly silt size (<75µ) and bottom ash is predominantly sand sized (0.15mm to 10mm). Pond ash near the inflow point is akin to bottom ash where as pond ash near the out flow point is closer to fly ash.

A general range4 found by sieve and hydrometer analysis are given below

Silt size (75 µ-2 µ)                 : 80 to 90 %
Sand size (4.75mm-75 µ)      : 10 to 15 %
Clay size (< 2 µ)                    : 0 to 10 %
Fly ash is generally silt size material Specific gravity

Specific gravity of ashes is found to be varying over a wide range. Literature indicates values of specific gravity as low as 1.53 to as high as 2.81, as against 2.65 to 2.8 for soils. Fly ash is characterized by low specific gravity due to presence of unburnt carbon and the presence of large proportion of cenospheres or hollow particles in fly ash8. Table 4 presents the range of specific

gravity values for various ashes. Due to its lightweight, it imparts less load on subgrade, hence can be used on weak subgrade. The large variation in the values of specific gravity is because of the combination of many other factors such as composition and segregation. The specific gravity of ash is higher when it has high iron content (Fe2O3). If the iron content is more than 20%, the fly ash exhibits a specific gravity more than 2.59. Most of the Indian fly ashes show low specific gravity ranging between 1.84 -2.67. It is also indicated that pond ash has smaller value (1.53 –2.39) of specific gravity than fly ash/ bottom ash10.


3.1 Materials Used

3.1.1 Soil : The soil used in the study11 was the locally available Roorkee soil. It is a cohesionless soil

which is classified as A-3 as per PRA classification and (SP) as per IS classification. Various properties of Roorkee soil are given in Table 5.

3.1.2 Fly ash : The flyash used in the study11 was brought from National Thermal Power Station situated at Dadri (Ghaziabad) in Uttar Pradesh, India, which was available free of cost. Flyash is classified as silts of low compressibility (ML). The physical and chemical properties of flyash are given in Table 6.

3.1.3 Plastic sheet : The plastic sheet (polyethylene) used in the study11 was obtained from Roorkee market. The properties of this material are given in
Table 7.


A test pit of size 1.5m x 1.5m x 0.5m was dug in the pavement testing hall of Transportation Engineering Section of IIT, Roorkee. It was planned to conduct the field CBR and plate load tests for following subgrade conditions.
(i) Two layers of Roorkee soil (top &bottom) and four layers of fly ash (middle) without plastic sheet.
(ii) Two layers of Roorkee soil (top &bottom) and four layers of fly ash (middle) with plastic sheet at 2.5 cm from top.
(iii) Two layers of Roorkee soil (top &bottom) and four layers of fly ash (middle) with plastic sheet at 5.0 cm from top.

Details of thickness of various layers in unreinforced and reinforced sections are given in Figs 1(a) and 1 (b).

Measured quantities of Roorkee soil and fly ash were mixed with water so as to prepare the mix at OMC and spread uniformly to make respective layers. Hand compaction was carried out for all layers due to site location constraint. For each layer, densities were measured using sand replacement method and compaction was carried out till at least 95% of laboratory maximum density (each of fly ash and soil separately) was achieved. After sample was prepared, it was levelled through a levelling edge.


4.1 Compaction Test Results

IS heavy compaction tests were conducted to determine the optimum moisture content (OMC) and maximum dry density (MDD) of the following materials.
(a) Roorkee Soil (SP)
(b) Flyash
(c) 75% Flyash +25% Roorkee Soil
(d) 70% Flyash +30% Roorkee Soil
(e) 65% Flyash +35% Roorkee Soil
(f) 60% Flyash +40% Roorkee Soil.

The results are given in Table 8. It is seen that the compacted dry density of fly ash 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. It is also observed that the variation in OMC is very less if the Roorkee soil is more than 40%. Hence its percentage was restricted up to 40.

However, considering the problem of mixing the fly ash with soil in actual construction, CBR and other tests were conducted in layered system consisting of separate layers of soil and fly ash.

3.2 Lab CBR Test Results

Results obtained from conventional laboratory CBR test on fly ash combination with Roorkee soil are given in Table 9. These tests were conducted without plastic sheet reinforcement and with reinforcement placed in the material at 2.5 cm, 5.0 cm, 6.3 cm and 7.5 cm from the surface in order to study the effect of position of plastic sheet on CBR. These tests were also conducted with plastic sheet provided in multilayer to find the reinforcement effect and to find the most effective position of plastic sheet in the embankment.

From the above table it is observed that introduction of plastic sheet in the above mentioned combination has an effect on its CBR value. CBR value generally depends on the type of soil, amount of compaction and position of plastic sheet in the material. It is observed that the CBR value increases with the introduction of plastic sheet placed in the material. The CBR value is nearly 2.5 times when plastic sheet was kept in the material at 5 cm from surface to that without plastic sheet condition. From Table 9 it is seen that there is a gradual decrease in CBR value when plastic sheet used at 5, 6.3cm and 7.5cm from surface. The maximum value of CBR was obtained when plastic sheet was kept 5.0cm from top. The percentage increase in lab CBR values without plastic sheet condition to with plastic sheet at different depths from surface are shown in Fig 2. It is also observed that the CBR value of single layer reinforcement section is more than the multilayer reinforcement.

4.3 Results of Field CBR Tests

The field CBR tests were conducted on fly ash in combination with Roorkee soil for the conditions having best results in the lab i.e. plastic sheet at 2.5 cm & 5.0 cm from top. For comparison, field CBR was also found for same combination without with out plastic sheet. The results are tabulated in Table 10. The sections are the same as mentioned earlier in Fig 1. The percentage increase in Field CBR values without plastic sheet condition to with plastic sheet at different depths from surface are shown in Fig 3.

From the Table 10 it is observed that the CBR value of fly ash with Roorkee soil increases with plastic sheet. The CBR value, when this plastic sheet is at 5.0 cm depth is nearly 2 times the value obtained without plastic sheet condition.

4.4 Results of Plate Load Tests

Plate load tests were conducted to study the effect of plastic sheet reinforcement on bearing capacity and settlement characteristics. The values of modulus of subgrade reaction and modulus of elasticity were calculated for the material fly ash in combination with Roorkee soil for both without plastic sheet and with plastic sheet at varying depths of 2.5 cm and 5.0 cm from surface. Results obtained from plate load tests are given in Table 11. Figs. 4 to 6 show pressure-settlement curves for various conditions. The value of modulus of subgrade reaction was calculated as follows.

It is observed that modulus of subgrade reaction, k increases by reinforcing the material with plastic sheet and maximum advantage can be drawn by placing the plastic sheet at a depth of 5.0 cm from surface. From the above table it can be observed that the increase in strength of subgrade by placing plastic sheet at 5.0 cm from top in the material is 1.5 times than without plastic sheet. The percentage increase in lab modulus of subgrade reaction values without plastic sheet condition to with plastic sheet at different depths from surface are shown in Fig 3.

4.4 Modulus of Elasticity

From plate load tests conducted on fly ash in combination with Roorkee soil as already mentioned earlier, E values for different conditions of reinforcement were obtained. Mean settlement bearing pressure curve was drawn for each condition and value of modulus of elasticity was calculated using burmister's equation. Elastic moduli calculated for different conditions are tabulated in Table 12. The percentage increase in modulus of elasticity values without plastic sheet condition to with plastic sheet at different depths from surface are shown in Fig 3 . The values of modulus of elasticity are calculated as follows.

From the above table it can be observed that the increase in elastic moduli of without plastic sheet condition and with plastic sheet at 5 cm from top is 62%. Hence, it can be concluded that the strength of subgrade in terms of modulus of elasticity is increased by providing plastic sheet reinforcement.

4.5 Triaxial Test Results

Shear strength parameters are angle of shearing resistance and cohesion which are found from triaxial test, for the condition unconsolidated undrained (without saturation) accordance with IS 2720 (Part-xi). The test results are shown in Table 13. Figs 7-9 show the Mohr’s circles for fly ash with and without plastic sheet


The following conclusion can be drawn from the present study:

(1) The plastic sheet reinforcement improves the CBR value considerably. The laboratory CBR values at OMC showed an increasing trend with plastic sheet at 7.5, 6.3 and 5.0 cm from surface as compared to without plastic sheet reinforcement.

(2) The position of plastic sheet reinforcement affects the CBR value significantly and maximum CBR value achieved when plastic sheet was kept at 5.0 cm from surface in both lab & field CBR tests.

(3) In the lab CBR tests it is observed that the CBR value is more in the case of single layer reinforcement than multilayer reinforcement.

(4) The values of modulus of subgrade reaction as well as modulus of elasticity increase with inclusion of plastic sheet reinforcement to reinforcement at 2.5 and 5.0 cm respectively.

(5) The shear parameters, C and F are increased by introduction of plastic sheet in the test specimen.

(6) The plastic sheet reinforcement is useful in the embankments, where there is no problem of drainage. The Reinforcement of plastic sheet may be provided at 5.0 cm from surface of embankment.


1. Rural Roads Manual, SP-20, Indian Roads Congress,
New Delhi, 2002

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3. Trivedi A. and Sood V.K., "Coal Ash as Embankment and Structural Fill Material" Proc of Workshop on Coal Ashes, TIET, Patiala, 1999, pp 130-137.

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8. Grey, D.H. and Lin, Y.K., " Engineering Properties of Compacted Fly Ash", Journal of Soil Mechanics and Foundation Engineering Division, ASCE, Vol. 98 No. 4, 1972, pp. 361-380.

9. Singh, D.N., "Influence of Chemical Constituents of Fly Ash Characteristics", Proceedings of the Indian Geotechnical Conference-1996, Madras, 1996, pp. 227-230.

10. Dayal, U., Chandra, S and Bohra N.C., "Geotechnical Investigation of Ash Properties for Dike Construction at Ramagundam Super Thermal Power Project", Report of National Thermal Power Corporation Ltd., New Delhi, 1989.

11. Durga Rao, A.K., A Study on Plastic Sheet Reinforced Flyash Embankments, M.Tech. Thesis, Department of Civil Engineering, Indian Institute of Technology, Roorkee,
April, 2003.