Indian Geotechnical Conference (December 18-20, 2003) s8

Quantification of Change in Dry Unit Weight of Mechanically Stabilised Expansive Soils Using Fly Ash IGC 2009, Guntur, INDIA

QUANTIFICATION OF CHANGE IN DRY UNIT WEIGHT OF MECHANICALLY STABILISED EXPANSIVE SOILS USING FLY ASH

K. Mallikarjuna Rao Professor, Dept. of Civil Engineering, S.V. University, Tirupati–517502, Andhra Pradesh, India. E-mail: [email protected] G.V. Rama Subbarao Assistant Professor, Dept. of Civil Engg., S.R.K. Institute of Technology, Vijayawada, Enikepadu–521108, India. E-mail: [email protected]

ABSTRACT: Mechanical stabilization of expansive soils using fly ash is one viable option for fly ash utilization in huge quantity. Addition of fly ash results in change of both the compaction characteristics namely maximum dry unit weight and optimum moisture content. The change in dry unit weight is attributed to replacement of soil by fly ash and possible filling of void spaces by fly ash. The present investigation aims at quantifying the change in dry unit weight in terms of percent of fly ash which replaced the soil and percent of fly ash which filled the voids in soil at constant moisture content. Laboratory experiments were carried out to evaluate compaction characteristics of four different expansive soils on addition of fly ash ranging from 5% to 80% by weight. The Liquid Limit and Fraction Coarser than 425μ of these soils range from 50% to 120% and 25% to 70% respectively. The percent fly ash which replaced the soils is found to be in the range of 77% to 82%, balance being the percent of fly ash filling the voids. An experimental strategy called Two Factor Factorial Design is adopted in conducting experiments which enables relative quantification of effect of two factors namely fraction coarser than 425μ and liquid limit of the actual soil as well as their interaction effect on percent fly ash which replaced the soil and percent fly ash which filling the voids of the soil.

1. INTRODUCTION unit weight due to addition of fly ash. From literature it is clear that no attempt has been made so far to quantify the Expansive soils also called swelling soils are prone to change in dry unit weight of expansive soils due to addition volume changes corresponding to change in moisture of fly ash. content. In India swelling soils are commonly known by the name Black Cotton soils. About one-fifth of the land area in The present investigation aims at quantifying the change in India is covered by these soils. Several investigations were dry unit weight due to addition of fly ash at any given carried out in India and worldwide to stabilize expansive moisture content in terms of percent of fly ash which filled soils using different additives like cement, lime and the voids in soil and percent of fly ash which replaced the industrial wastes like coal ashes. Use of coal ashes for soil. Laboratory experiments were carried out to evaluate stabilization of expansive soil resolve the clash between compaction characteristics of four different expansive soils development and environment as it involves reuse and safe on addition of fly ash ranging from 5% to 80% by weight. riddance of harmful coal ashes. In view of their good The Liquid Limit and Fraction Coarser than 425μ of these physical properties, they can be used beneficially in most of soils range from 50% to 120% and 25% to 70% respectively. the geotechnical applications (Sridharan et al. 2001). In The percent fly ash which replaced the soils is found to be in recent years, the engineering community feels that bulk the range of 77% to 82% depending on liquid limit and utilization of ash is possible through geotechnical coarse fraction present in the soil, irrespective of moisture applications (Pandian et al. 2004). content and percentage of fly ash added, balance being the percent of fly ash filling the voids. An experimental strategy Compaction characteristics of soil-fly ash mixes were studied called Two Factor Factorial Design is adopted in conducting by several investigators (Basavanna & Ravi Kumar 1990, experiments which enables relative quantification of effect of Choudhary 1994, Pandian 2004, Prabakar 2004, Phanikumar two factors namely fraction coarser than 425μ and liquid & Sharma 2004, and Bhuvaneshwari et al. 2005). Almost all limit of the actual soil as well as their interaction effect on these studies revealed that optimum moisture content percent fly ash which replaced the soil and percent fly ash increases due to addition of fly ash. However, several which filling the voids of the soil. investigators reported decrease in maximum dry unit weight, while a few investigators reported increase in maximum dry 2. METHODS AND MATERIALS

82 Quantification of Change in Dry Unit Weight of Mechanically Stabilised Expansive Soils Using Fly Ash 2.1 22 Factorial Experimental Design 2.2.1 Soils In this strategy of experimentation, experiments are The soils used in the present investigation are obtained from conducted by simultaneously varying the two factors over two different places viz., Paritala and Yedurulanka having two levels (namely low level and high level). The two levels liquid limit of 52.0% and 112.0% respectively. Wet sieving are so chosen that they cover the practical range of the is carried out on two collected soils using 425m sieve, to parameters under consideration. Figure 1 shows 22 factorial determine the coarse faction in the natural soils. Now, two experimental design consisting of two factors namely soils namely Soil-1, Soil-2 having 25% and 70% of fraction fraction coarser than 425μ denoted as Factor A and Liquid coarser than 425μ respectively are artificially derived from limit denoted as Factor B each factor having two different Paritala Soil by mixing with a sand which is coarser than levels. The four treatment combinations arising out of the 425m. Soil-1 and Soil-2 corresponds to treatment combinations 2 two factors at two different levels are represented by lower (1) and a of 2 Factorial Experimentation shown in Fig. 1. On case letters namely (1), a, b, and ab. the same lines two more soils namely Soil-3, Soil-4 having 25% and 70% of fraction coarser than 425μ respectively are artificially derived from Yedurulanka soil by mixing + 425m sand. Soil-3 and Soil-4 corresponds to treatment combinations b and ab of 22 Factorial Experimentation shown in Figure 1.

) The addition of sand coarser than 425μ size to clayey soil B

r increases its fraction coarser than 425μ value without altering o t

c its liquid limit value. a F (

i 2.2.2 Fly Ash L

u Fly ash has been obtained from Vijayawada Thermal Power q i

L Station, Ibrahimpatnam. The chemical composition of fly ash as supplied by VTPS authorities are given in Table 2. Based on the chemical composition, the fly ash used in this Fraction Coarser than 425 (Factor A) investigation comes under category of Class F (ASTM Fig. 1: Treatment of Combination in the 22 Factorial C618). The geotechnical properties of the fly ash used in this Experimental Design study are presented in Table 3.

Table 1 summarises the details of numerical values of each Table 2: Chemical Composition of Fly Ash factor adopted in this investigation for all the four treatment Name of the chemical % by weight combinations along with experiment label. Silica (SiO2) 64.08 Alumina (Al2O3) 20.21 Table 1: 22 Factorial Design Test Combinations Ferric Oxide (Fe2O3 + Fe3O4) 4.17 Titanium Dioxide (TiO2) 0.42 Soil Experiment Fraction Liquid Treatment Calcium Oxide (CaO) 6.20 label coarser than limit, combination Magnesium Oxide (MgO) 0.91 425μ, factor B Sulfate (SO4) 1.24 factor A (%) (%) Loss on Ignition (LOI) 1.07 Factor A low Soil-1 (1) 25 52.0 and Factor B Table 3: Geotechnical Properties of Fly Ash low Property Value Factor A high Specific Gravity 2.10 Soil-2 a 70 52.0 and Factor B Fine Sand 23.74% low Fines 74.93% Factor A low Maximum Dry Unit weight 13.63 kN/m3 Soil-3 b 25 112.0 and Factor B Optimum Moisture Content 21.4% high Factor A high 2.3 Tests Conducted 112.0 Soil-4 ab 70 and Factor B I.S. light compaction tests were conducted on all the above high four soils with and without adding fly ash in different proportions. The amount of fly ash added is 0%, 5%, 10%, 2.2 Materials Used 15%, 20%, 25%, 40% and 80% by dry weight of the soils.

83 Quantification of Change in Dry Unit Weight of Mechanically Stabilised Expansive Soils Using Fly Ash The compaction curves are plotted for all the tests conducted The second action causes decrease in dry unit weight since with and without addition of fly ash. the specific gravity of fly ash is very less. The sum total of these two effects determines the observed change in dry unit 3. RESULTS AND DISCUSSIONS weight due addition of fly ash. Identification and classification properties of the four soils The following procedure is adopted to evaluate the percent of namely Soil-1, Soil-2, Soil-3, and Soil-4 along with the Fly Ash filling the voids of the Soil and the percent of Fly compaction characteristics obtained from the I.S. Light Ash which replaced the Soil: compaction tests and classification of soils according to The phase diagram of fly ash treated and untreated soil at any Indian Standard Classification System (IS: 1498; 1970) are given moisture content are shown in Figure 3. The volume of presented in Table 4. Typical compaction curves for fly ash solid phase in untreated soils is owing to soil solids only treated soil are shown in Figure 2 along with the compaction where as in fly ash treated soils the volume of solid phase curve of actual soil without addition of any fly ash. comprises of volume of soil solids and volume of fly ash added. The weight of fly ash is further divided into two Table 4: Properties of the Soils Used components namely ‘p’ and ‘q’; where p = Weight of Fly Property Paritala soil Yedurulanka soil Ash filling the voids of the Soil and q = Weight of Fly Ash Soil-1 Soil-2 Soil-3 Soil-4 which replaced the Soil. Specific 2.61 2.57 2.71 2.68 Gravity Gravel 0% 0% 0% 0% p Sand 47.33% 78.94% 25.25% 70.10% Vr q Silt and Clay 52.67% 21.06% 74.75% 29.90% Vs  Liquid limit 52% 52% 112% 112% Ws =  d soil  d Soil  FlyAsh Plastic limit 27% 27% 43% 43% Plasticity Index 25% 25% 69% 69% I.S.S.C. System CH SC CH SC (a) Untreated Soil (b) Fly Ash treated Soil Free Swell 50% 50% 130% 130% Fig. 3: Phase Diagrams of Untreated and Fly Ash Index Treated Soil Maximum Dry 17.66 18.84 14.52 17.85 3 3 3 3 Unit weight kN/m kN/m kN/m kN/m (p + q) = Total Weight of Fly Ash added Optimum Moisture 17.0% 12.9% 25.0% 18.7% Dry weight of Soil and Fly Ash (Fig. 3(b)) = [Dry wt of Soil Content (Fig. 3(a))] + {Dry Wt of Fly Ash filling the voids of the Soil (p) + Dry wt of Fly ash which replaced the Soil (q)} – Dry DryDry Unit Unit Weight Weight Vs Vs moisture Moisture Content Content 18 weight of soil which was replaced by Fly ash (r)] 17.8 ) 17.6 Soil-1+0% Fly Ash m

u 17.4  c Soil-1+5% Fly Ash  d  = [   + (p+q) –r] (1) / 17.2 SoilFlyAsh d soil N k

17 Soil-1+10% Fly Ash n i

( 16.8

t Soil-1+15% Fly Ash Let Vr = Volume of fly ash whose weight is equal to q

h 16.6 g i 16.4 Soil-1+20% Fly Ash e 16.2 W

t 16 Soil-1+25% Fly Ash   i q n 15.8  

U  Vr = 15.6 Soil-1+40% Fly Ash   y G  r 15.4  FA w 

D Soil-1+80% Fly Ash 15.2 15 r = Dry weight of soil which was replaced by Fly ash 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 Moisture Content (%) = Weight of soil solids whose volume is equal to

Fig. 2: Typical Compaction Curves of Soil With and Without Vr = VrGs  w Fly Ash addition where Gs = Specific gravity of solids

From Figure 2 it is clear that the addition of fly ash results in  q   q      G  change in dry unit weight at any given moisture content r =   Gs  w =  s  irrespective of % of fly ash added. The change in dry unit  GFA w   GFA  weight may be attributed to two possible mechanisms: (i) Fly From eqn. (1): ash occupying the void spaces in solid phase (ii) replacement of soil by fly ash. The first one leads to increase in volume of  q    G    d SoilFlyAsh = [  d soil + (p + q) –  s  ] solid phase and hence causes an increase in dry unit weight.  GFA 

84 Quantification of Change in Dry Unit Weight of Mechanically Stabilised Expansive Soils Using Fly Ash

 q   3.1 Influence of Fraction Coarser than 425μ on ‘Percent        G   ( p  q) d soil d SoilFlyAsh  s   Fly Ash Filling the Voids’  GFA   Hence, at any given moisture content and for a given soil, the Factorial experimentation permits the relative quantification weight of fly ash which replaced the soil (q) is obtained by of factors studied (namely Liquid limit and Fraction coarser equating the observed difference in dry unit weight to the than 425 microns) and their interaction on response of difference between weight of fly ash added (p + q) and the interest (Percent Fly Ash filling the voids). The Percent Fly weight of soil replaced by fly ash. Ash filling the voids are found to have values when fly ash added is equal to 19.5%, 18.3%, 22.5%, and 21.6% for the GFA q    d    d   ( p  q) four soils tested in this investigation. For treatment soil SoilFlyAsh G Soil combinations (1) and a liquid limit is constant at low value The weight of fly ash filling the void spaces (P) is obtained which is equal to 52.0%. For treatment combination b and ab by subtracting ‘q’ from total fly ash added. the liquid limit is constant and is at high value which is equal to 112.0%. The Percent Fly Ash filling the voids is observed Percent of fly Ash filling the voids of the Soil = P = [p/(p + q)] to decrease from 19.5% to 18.3% at low level of liquid limit Percent of Fly Ash which replaced the Soil= Q = [q/(p + q)] and from 22.5% to 21.6% at high level of liquid limit as the fraction coarser than 425μ is varying from 25% to 70%. The The values of P and Q obtained from the above procedure relative quantification of the effect of fraction coarser than are presented in Tables 5 and 6. 425μ on Percent Fly Ash filling the voids can be determined by taking average of the effect of fraction coarser than 425μ Table 6: % Fly Ash replaced the Soil and % Fly Ash Filled at high level of liquid limit and the effect of fraction coarser the Soil for Soils Treated with Fly Ash than 425μ at low level of liquid limit which may be given by at any Moisture Content the following equation. Type of % Fly ash % Fly ash filling Effect of main factor A = Difference between average soil replaced the soil the voids = response at low level of factor A and response at high level = Q = q/(p + q) P = p/(p + q) of factor A Soil-1 80.5 19.5 Soil-2 81.7 18.3  a  ab   (1)  b        (2) Soil-3 77.5 22.5  2   2  Soil-4 78.4 21.6 1  a  ab  b  (1)` The percent fly ash which replaced the soils is found to be in 2 the range of 77% to 82% depending on liquid limit and 1 coarse fraction present in the soil, irrespective of moisture  18.3 21.6  22.5 19.5  1.0 units 2 content and percentage of fly ash added, balance being the percent of fly ash filling the voids.

Table 5: Values of ‘p’ and ‘q’ at Different Moisture Contents (For soil-1 added with 5% Fly Ash) Water Dry Dry unit % Fly Specific Specific Weight of Weight Specific % Fly ash % Fly ash content of unit weight of ash gravity gravity fly ash of fly gravity replaced filled the soil + weight soil + fly added of soil-1 of fly which ash of soil-1 the soil = Q soil = P = fly ash of ash ash replaced filling +fly ash = q/(p + q) p/(p + q) (%) soil (in the soil, q the (in kN/m3) (in N) voids of kN/m3) the soil, p (in N) 10.40 15.30 15.7 5 2.61 2.1 0.868 0.211 2.586 80.5 19.5 11.54 15.68 16.09 5 2.61 2.1 0.868 0.211 2.586 80.5 19.5 14.01 16.64 16.97 5 2.61 2.1 0.868 0.211 2.586 80.5 19.5 15.56 17.27 17.76 5 2.61 2.1 0.868 0.211 2.586 80.5 19.5 17.37 17.70 17.56 5 2.61 2.1 0.869 0.211 2.586 80.5 19.5 19.84 17.32 16.19 5 2.61 2.1 0.869 0.211 2.586 80.5 19.5 3.2 Influence of Liquid Limit on ‘Percent Fly Ash For treatment combinations (1) and b the fraction coarser Filling the Voids’ than 425μ is constant at low value which is equal to 25%. For treatment combination a and ab the fraction coarser than 425μ is constant at high value which is equal to 70%. The

85 Quantification of Change in Dry Unit Weight of Mechanically Stabilised Expansive Soils Using Fly Ash Percent Fly Ash filling the voids is observed to increase from 3.4 Influence of Fraction Coarser than 425μ, Liquid 19.5% to 22.5% at low level of Factor A (fraction coarser Limit, and Interaction of Fraction Coarser than 425μ than 425μ) and from 18.3% to 21.6% at high level of Factor and Liquid Limit on ‘Percent Fly Ash which A as the liquid limit is varying from 52% to 112%. The Replaced the Soil’ relative quantification of the effect of liquid limit on Percent The Percent Fly Ash which replaced the Soil are found to Fly Ash filling the voids can be determined by taking have values equal to 80.5%, 81.7%, 77.5%, and 78.4% for average of the effect of liquid limit at high level of Factor B the four soils tested in this investigation. and the effect of liquid limit at low level of Factor B which may be given by the following equation. Table 7: Effect of Factors on Percent Fly Ash which Effect of main factor B = Difference between average Replaced the Soil response at low level of factor B and response at high level Factor Effect of factor on percent fly of factor B ash which replaced the soil  b  ab   (1)  a        (3) Fraction coarser 425μ 1.05 units  2   2    Liquid Limit 3.15 units 1  ab  b  a  (1) Interaction of Fraction 0.15 units 2 coarser than 425μ and 1 Liquid Limit  21.6  22.5 18.319.5  3.2 units 2 From Table 7, it is clear that liquid limit of the soil has a dominating influence on Percent Fly Ash which replaced the 3.3 Influence of Interaction of Fraction Coarser than Soil. The effect of fraction coarser than 425μ is fairly good. 425μ and Liquid Limit on ‘Percent Fly Ash Filling The interaction effect of both liquid limit and fraction coarser the Voids’ than 425μ in the soil is marginal. For treatment combinations along the diagonal (1) and ab both the factors namely fraction coarser than 425μ and liquid 4. CONCLUSIONS limit are either at low level or at high level. Both the factors at high level for ab and at low level for (1). On the other Based on the test results presented in this investigation, the hand, for treatment combinations along the other diagonal b following conclusions are drawn: and a, one factor is at high level and the other factor is at low  Addition of fly ash is observed to result in change in the level. The Percent Fly Ash filling the voids is observed to dry unit weight of mechanically stabilized expansive increased from 19.5% to 21.6% along the diagonal (1) and soils at any given moisture content. This may be ab whereas it is increased from 18.3% to 22.5% along the attributed to two factors namely filling of void spaces diagonal a and b. The relative quantification of the interaction between soil solids by fly ash and replacement of soil by effect of fraction coarser than 425μ can be determined by fly ash. taking average difference between the effect of fraction  The percent fly ash which replaced the soils is found to coarser than 425μ at high level of liquid limit and effect of be in the range of 77% to 82% depending on liquid limit fraction coarser than 425μ at low level of liquid limit. and coarse fraction present in the soil, irrespective of Interaction effect of AB= Average difference between Effect moisture content and percentage of fly ash added, of A at high level of B and Effect of A at low level of B balance being the percent of fly ash filling the voids.  It is found that liquid limit of the soil is a dominating ab  b[a  (1)]    (4) influence on Percent Fly Ash filling the voids due to  2  addition of fly ash. The effect of fraction coarser than 1 425μ is fairly good. The interaction effect of both liquid  ab  (1)  a  b limit and fraction coarser than 425μ in the soil is 2 marginal. 1  21.6 19.5 18.3  22.5  0.2 units  It is also found that liquid limit of the soil is a 2 dominating influence on Percent Fly Ash which replaced From this, it is found that liquid limit is the factor having a the Soil. The effect of fraction coarser than 425μ is fairly dominating influence on Percent Fly Ash filling the voids good. The interaction effect of both liquid limit and due to addition of optimum fly ash. The effect of fraction fraction coarser than 425μ in the soil is marginal. coarser than 425μ is fairly good. The interaction effect of both liquid limit and fraction coarser than 425μ in the soil is REFERENCES marginal. ASTM C618 – 08, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolana for Use as a

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