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

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1 IGC 009, 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 5750, Andhra Pradesh, India. G.V. Rama Subbarao Assistant Professor, Dept. of Civil Engg., S.R.K. Institute of Technology, Vijayawada, Enikepadu 508, India. 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 45μ of these soils range from 50% to 0% and 5% to 70% respectively. The percent fly ash which replaced the soils is found to be in the range of 77% to 8%, 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 45μ 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.. INTRODUCTION Expansive soils also called swelling soils are prone to volume changes corresponding to change in moisture content. In India swelling soils are commonly known by the name Black Cotton soils. About one-fifth of the land area in India is covered by these soils. Several investigations were carried out in India and worldwide to stabilize expansive soils using different additives like cement, lime and industrial wastes like coal ashes. Use of coal ashes for stabilization of expansive soil resolve the clash between development and environment as it involves reuse and safe riddance of harmful coal ashes. In view of their good physical properties, they can be used beneficially in most of the geotechnical applications (Sridharan et al. 00). In recent years, the engineering community feels that bulk utilization of ash is possible through geotechnical applications (Pandian et al. 004). Compaction characteristics of soil-fly ash mixes were studied by several investigators (Basavanna & Ravi Kumar 990, Choudhary 994, Pandian 004, Prabakar 004, Phanikumar & Sharma 004, and Bhuvaneshwari et al. 005). Almost all these studies revealed that optimum moisture content increases due to addition of fly ash. However, several investigators reported decrease in maximum dry unit weight, while a few investigators reported increase in maximum dry unit weight due to addition of fly ash. From literature it is clear that no attempt has been made so far to quantify the change in dry unit weight of expansive soils due to addition of fly ash. The present investigation aims at quantifying the change in dry unit weight due to addition of fly ash at any given moisture content in terms of percent of fly ash which filled the voids in soil and percent of fly ash which replaced the soil. 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 45μ of these soils range from 50% to 0% and 5% to 70% respectively. The percent fly ash which replaced the soils is found to be in the range of 77% to 8% depending on liquid limit and coarse fraction present in the soil, irrespective of moisture content and percentage of fly ash added, 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 45μ 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. 338

2 . METHODS AND MATERIALS. Factorial Experimental Design In this strategy of experimentation, experiments are conducted by simultaneously varying the two factors over two levels (namely low level and high level). The two levels are so chosen that they cover the practical range of the parameters under consideration. Figure shows factorial experimental design consisting of two factors namely fraction coarser than 45μ denoted as Factor A and Liquid limit denoted as Factor B each factor having two different levels. The four treatment combinations arising out of the two factors at two different levels are represented by lower case letters namely (), a, b, and ab. Liquid Limit (Factor B) Fig. : Treatment of Combination in the Factorial Experimental Design Table summarises the details of numerical values of each factor adopted in this investigation for all the four treatment combinations along with experiment label. Soil Fraction Coarser than 45μ (Factor A) Table : Factorial Design Test Combinations Experiment label Fraction coarser than 45μ, factor A (%) Liquid limit, factor B (%) Soil- () Soil- a Soil-3 b 5.0 Soil-4 ab 70.0 Treatment combination Factor A low low Factor A high low Factor A low high Factor A high high. Materials Used.. Soils The soils used in the present investigation are obtained from two different places viz., Paritala and Yedurulanka having liquid limit of 5.0% and.0% respectively. Wet sieving is carried out on two collected soils using 45μ sieve, to determine the coarse faction in the natural soils. Now, two soils namely Soil-, Soil- having 5% and 70% of fraction coarser than 45μ respectively are artificially derived from Paritala Soil by mixing with a sand which is coarser than 45μ. Soil- and Soil- corresponds to treatment combinations () and a of Factorial Experimentation shown in Fig.. On the same lines two more soils namely Soil-3, Soil-4 having 5% and 70% of fraction coarser than 45μ respectively are artificially derived from Yedurulanka soil by mixing + 45μ sand. Soil-3 and Soil-4 corresponds to treatment combinations b and ab of Factorial Experimentation shown in Figure. The addition of sand coarser than 45μ size to clayey soil increases its fraction coarser than 45μ value without altering its liquid limit value... Fly Ash Fly ash has been obtained from Vijayawada Thermal Power Station, Ibrahimpatnam. The chemical composition of fly ash as supplied by VTPS authorities are given in Table. Based on the chemical composition, the fly ash used in this investigation comes under category of Class F (ASTM C68). The geotechnical properties of the fly ash used in this study are presented in Table 3. Table : Chemical Composition of Fly Ash Name of the chemical % by weight Silica (SiO ) Alumina (Al O 3 ) 0. Ferric Oxide (Fe O 3 + Fe 3 O 4 ) 4.7 Titanium Dioxide (TiO ) 0.4 Calcium Oxide (CaO) 6.0 Magnesium Oxide (MgO) 0.9 Sulfate (SO 4 ).4 Loss on Ignition (LOI).07 Table 3: Geotechnical Properties of Fly Ash Property Value Gravity.0 Fine Sand 3.74% Fines 74.93% Maximum Dry Unit weight 3.63 kn/m 3 Optimum Moisture Content.4%.3 Tests Conducted I.S. light compaction tests were conducted on all the above four soils with and without adding fly ash in different 339

3 proportions. The amount of fly ash added is 0%, 5%, 0%, 5%, 0%, 5%, 40% and 80% by dry weight of the soils. The compaction curves are plotted for all the tests conducted with and without addition of fly ash. 3. RESULTS AND DISCUSSIONS Identification and classification properties of the four soils namely Soil-, Soil-, Soil-3, and Soil-4 along with the compaction characteristics obtained from the I.S. Light compaction tests and classification of soils according to Indian Standard Classification System (IS: 498; 970) are presented in Table 4. Typical compaction curves for fly ash treated soil are shown in Figure along with the compaction curve of actual soil without addition of any fly ash. Table 4: Properties of the Soils Used Property Paritala soil Yedurulanka soil Soil- Soil- Soil-3 Soil-4 Gravity Gravel 0% 0% 0% 0% Sand 47.33% 78.94% 5.5% 70.0% Silt and Clay 5.67%.06% 74.75% 9.90% Liquid limit 5% 5% % % Plastic limit 7% 7% 43% 43% Plasticity Index 5% 5% 69% 69% I.S.S.C. System CH SC CH SC Free Swell Index 50% 50% 30% 30% Maximum Dry Unit weight kn/m 3 kn/m 3 kn/m 3 kn/m 3 Optimum Moisture Content 7.0%.9% 5.0% 8.7% Dry Unit Weight (in kn/cum) Dry Dry Unit Unit Weight Vs Vs moisture Moisture Content Moisture Content (% ) Soil-+0% Fly Ash Soil-+5% Fly Ash Soil-+0% Fly Ash Soil-+5% Fly Ash Soil-+0% Fly Ash Soil-+5% Fly Ash Soil-+40% Fly Ash Soil-+80% Fly Ash Fig. : Typical Compaction Curves of Soil With and Without Fly Ash addition From Figure it is clear that the addition of fly ash results in change in dry unit weight at any given moisture content irrespective of % of fly ash added. The change in dry unit weight may be attributed to two possible mechanisms: (i) Fly ash occupying the void spaces in solid phase (ii) replacement of soil by fly ash. The first one leads to increase in volume of solid phase and hence causes an increase in dry unit weight. The second action causes decrease in dry unit weight since the specific gravity of fly ash is very less. The sum total of these two effects determines the observed change in dry unit weight due addition of fly ash. The following procedure is adopted to evaluate the percent of Fly Ash filling the voids of the Soil and the percent of Fly Ash which replaced the Soil: The phase diagram of fly ash treated and untreated soil at any given moisture content are shown in Figure 3. The volume of solid phase in untreated soils is owing to soil solids only where as in fly ash treated soils the volume of solid phase comprises of volume of soil solids and volume of fly ash added. The weight of fly ash is further divided into two components namely p and q ; where p = Weight of Fly Ash filling the voids of the Soil and q = Weight of Fly Ash which replaced the Soil. V s (a) Untreated Soil Fig. 3: Phase Diagrams of Untreated and Fly Ash Treated Soil (p + q) = Total Weight of Fly Ash added Dry weight of Soil and Fly Ash (Fig. 3(b)) = [Dry wt of Soil (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 weight of soil which was replaced by Fly ash (r)] ( γ d ) = [ ( ) Soil+ FlyAsh d soil γ + (p+q) r] () Let V r = Volume of fly ash whose weight is equal to q V r = G FA q γ w r = Dry weight of soil which was replaced by Fly ash = Weight of soil solids whose volume is equal to V r = V r G s γ w where G s = gravity of solids r = G FA q γ w G s W s = ( ) soil d γ ( ) Soil FlyAsh q γ w = G FA V r Gs p q (b) Fly Ash treated Soil γ d + 340

4 From eqn. (): ( γ d ) = [ ( γ ) Soil+ FlyAsh d soil + (p + q) q G ] s G FA q [( γ ) ( ) ] d γ = G ( p + q ) soil d Soil + FlyAsh s G FA Hence, at any given moisture content and for a given soil, the weight of fly ash which replaced the soil (q) is obtained by equating the observed difference in dry unit weight to the difference between weight of fly ash added (p + q) and the weight of soil replaced by fly ash. G FA q = {[( γ ) ( γ ) ] + ( p + q )} d soil d Soil + FlyAsh The weight of fly ash filling the void spaces (P) is obtained by subtracting q from total fly ash added. Percent of fly Ash filling the voids of the Soil = P = [p/(p + q)] Percent of Fly Ash which replaced the Soil= Q = [q/(p + q)] The values of P and Q obtained from the above procedure are presented in Tables 5 and 6. Table 6: % Fly Ash replaced the Soil and % Fly Ash Filled the Soil for Soils Treated with Fly Ash at any Moisture Content Type of soil % Fly ash replaced the soil = Q = q/(p + q) G Soil % Fly ash filling the voids = P = p/(p + q) Soil Soil Soil Soil The percent fly ash which replaced the soils is found to be in the range of 77% to 8% depending on liquid limit and coarse fraction present in the soil, irrespective of moisture content and percentage of fly ash added, balance being the percent of fly ash filling the voids. 3. Influence of Fraction Coarser than 45μ on Percent Fly Ash Filling the Voids Factorial experimentation permits the relative quantification of factors studied (namely Liquid limit and Fraction coarser than 45 microns) and their interaction on response of interest (Percent Fly Ash filling the voids). The Percent Fly Ash filling the voids are found to have values when fly ash added is equal to 9.5%, 8.3%,.5%, and.6% for the four soils tested in this investigation. For treatment combinations () and a liquid limit is constant at low value which is equal to 5.0%. For treatment combination b and ab the liquid limit is constant and is at high value which is equal to.0%. The Percent Fly Ash filling the voids is observed to decrease from 9.5% to 8.3% at low level of liquid limit and from.5% to.6% at high level of liquid limit as the fraction coarser than 45μ is varying from 5% to 70%. The relative quantification of the effect of fraction coarser than 45μ on Percent Fly Ash filling the voids can be determined by taking average of the effect of fraction coarser than 45μ at high level of liquid limit and the effect of fraction coarser than 45μ at low level of liquid limit which may be given by the following equation. Effect of main factor A = Difference between average response at low level of factor A and response at high level of factor A a + ab = () + b = [ a + ab b () ]` = [ ] =.0 units () Water content of soil + fly ash (%) Table 5: Values of p and q at Different Moisture Contents (For soil- added with 5% Fly Ash) Dry unit weight of soil (in kn/m 3 ) Dry unit weight of soil + fly ash (in kn/m 3 ) % Fly ash added gravity of soil- gravity of fly ash Weight of fly ash which replaced the soil, q (in N) Weight of fly ash filling the voids of the soil, p (in N) gravity of soil- +fly ash % Fly ash replaced the soil = Q = q/(p + q) % Fly ash filled the soil = P = p/(p + q)

5 3. Influence of Liquid Limit on Percent Fly Ash Filling the Voids For treatment combinations () and b the fraction coarser than 45μ is constant at low value which is equal to 5%. For treatment combination a and ab the fraction coarser than 45μ is constant at high value which is equal to 70%. The Percent Fly Ash filling the voids is observed to increase from 9.5% to.5% at low level of Factor A (fraction coarser than 45μ) and from 8.3% to.6% at high level of Factor A as the liquid limit is varying from 5% to %. The relative quantification of the effect of liquid limit on Percent Fly Ash filling the voids can be determined by taking average of the effect of liquid limit at high level of Factor B and the effect of liquid limit at low level of Factor B which may be given by the following equation. Effect of main factor B = Difference between average response at low level of factor B and response at high level of factor B b + ab () + a = (3) = [ ab + b a () ] = [ ] = 3. units 3.3 Influence of Interaction of Fraction Coarser than 45μ and Liquid Limit on Percent Fly Ash Filling the Voids For treatment combinations along the diagonal () and ab both the factors namely fraction coarser than 45μ and liquid limit are either at low level or at high level. Both the factors at high level for ab and at low level for (). On the other hand, for treatment combinations along the other diagonal b and a, one factor is at high level and the other factor is at low level. The Percent Fly Ash filling the voids is observed to increased from 9.5% to.6% along the diagonal () and ab whereas it is increased from 8.3% to.5% along the diagonal a and b. The relative quantification of the interaction effect of fraction coarser than 45μ can be determined by taking average difference between the effect of fraction coarser than 45μ at high level of liquid limit and effect of fraction coarser than 45μ at low level of liquid limit. Interaction effect of AB= Average difference between Effect of A at high level of B and Effect of A at low level of B [ ab b] [a ()] = = [ ab + () a b] = [ ] = 0. units (4) From this, it is found that liquid limit is the factor having a dominating influence on Percent Fly Ash filling the voids due to addition of optimum fly ash. The effect of fraction coarser than 45μ is fairly good. The interaction effect of both liquid limit and fraction coarser than 45μ in the soil is marginal. 3.4 Influence of Fraction Coarser than 45μ, Liquid Limit, and Interaction of Fraction Coarser than 45μ and Liquid Limit on Percent Fly Ash which Replaced the Soil The Percent Fly Ash which replaced the Soil are found to have values equal to 80.5%, 8.7%, 77.5%, and 78.4% for the four soils tested in this investigation. Table 7: Effect of Factors on Percent Fly Ash which Replaced the Soil Factor Effect of factor on percent fly ash which replaced the soil Fraction coarser 45μ Liquid Limit Interaction of Fraction coarser than 45μ and Liquid Limit.05 units 3.5 units 0.5 units From Table 7, it is clear that liquid limit of the soil has a dominating influence on Percent Fly Ash which replaced the Soil. The effect of fraction coarser than 45μ is fairly good. The interaction effect of both liquid limit and fraction coarser than 45μ in the soil is marginal. 4. CONCLUSIONS Based on the test results presented in this investigation, the following conclusions are drawn: Addition of fly ash is observed to result in change in the dry unit weight of mechanically stabilized expansive soils at any given moisture content. This may be attributed to two factors namely filling of void spaces between soil solids by fly ash and replacement of soil by fly ash. The percent fly ash which replaced the soils is found to be in the range of 77% to 8% depending on liquid limit and coarse fraction present in the soil, irrespective of moisture content and percentage of fly ash added, balance being the percent of fly ash filling the voids. It is found that liquid limit of the soil is a dominating influence on Percent Fly Ash filling the voids due to addition of fly ash. The effect of fraction coarser than 45μ is fairly good. The interaction effect of both liquid limit and fraction coarser than 45μ in the soil is marginal. 34

6 It is also found that liquid limit of the soil is a dominating influence on Percent Fly Ash which replaced the Soil. The effect of fraction coarser than 45μ is fairly good. The interaction effect of both liquid limit and fraction coarser than 45μ in the soil is marginal. REFERENCES ASTM C68 08, Standard ation for Coal Fly Ash and Raw or Calcined Natural Pozzolana for Use as a Mineral Admixture in Portland cement Concrete, American Society for Testing of Materials, Pennsylvania, USA. Basavanna, B.M. and Itagi Ravi Kumar (990). Use of Coal Ash to Improve Some Properties of Black Cotton Soil, Indian Geotechnical Confrence 990 on Advances in Geotechnical Engineering, Indian Geotechnical Society, Bombay, India, pp Bhuvaneshwari, S., Robinson R.G. and Gandhi, S.R (005). Use of Coal Ash to Improve Some Properties of Black Cotton Soil, Fly Ash Utilization Programme (FAUP), TIFAC, Department of Science and Technology, New Delhi, India, pp. VIII Choudhary, A.K. and Jha, J.N (006). Stabilization of Expansive Soils using Fly Ash, Proceedings of CIVIL ENGINEERING: Meeting the Challenges of Tomorrow, Gurunanak Dev Engineering College, Ludhiana, Panjab, India, pp Montgomery, D.C. (00). Design and Analysis of Experiments, John Wiley and Sons, Inc., New York. Pandian, N.S. (004). Fly ash Characterization with reference to Geotechnical Applications, Journal Indian Institute of Science, 84, pp Pandian, N.S. (004). Stabilization of Expansive Soil with Fly Ash, Proceedings of National Symposium on Advances in Geotechnical Engineering, Karnataka Geotechnical Center of Indian Geotechnical Society, India, pp Phanikumar, B.R. and Sharma, R.S. (004). Effect of Fly Ash on Engineering Properties of Expansive Soils, Journal of Geotechnical and Geoenvironmental Engineering, Vol. 30, No. 7, pp Prabakar, J., Nitin Dendorkar and Morchhale, R.K. (004). Influence of Fly Ash on Strength Behaviour of Typical Soils, Journal of Construction and Building Materials, Elsevier Publishers, Vol. 8, pp Sridharan, A. and Prakash, K (007). Geotechnical Engineering Characterization of Coal ashes, CBS Publishers and Distributors, New Delhi, India. SP 36 (Part-I): 987 Compendium of Indian Standard on soil Engineering: Laboratory testing of soils. 343