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Improvement of calcareous expansive soils in semi-arid environments
Journal of Arid Environments (2001) 47: 453–463 doi:10.1006/jare.2000.0726, available online at http://www.idealibrary.com on
Improvement of calcareous expansive soils in semi-arid environments
Zalihe Nalbantoglu & Emin Gucbilmez Department of Civil Engineering, Eastern Mediterranean University, Gazimagusa, Mersin 10, Turkey (Received 11 January 2000, accepted 14 September 2000, published electronically 19 February 2001) The semi-arid climate and geology of Cyprus have caused the formation of calcareous expansive soils on the island. In some areas, swelling has caused serious foundation problems. The industrial by-product Soma fly ash has been used to improve the engineering properties of the soil. Fly ash treatment has shown tremendous potential as an economical method for the stabilization of the soil. Significant reduction in the swell potential and an increase in the hydraulic conductivity values are obtained. Use of fly ash with a small percentage of lime produces even more dramatic results. Cation exchange capacity (CEC) values substantiate the formation of the new pozzolanic reaction minerals which result in more stable silt-sand like structures. 2001 Academic Press Keywords: calcareous; cementation; expansive; fly ash; hydraulic conductivity; semi-arid; stabilization; swell
Introduction Expansive soil is a term generally applied to any soil that has a potential for shrinking or swelling under changing moisture conditions. Expansive soils cause more damage to structures, particularly light buildings and pavements, than any other natural hazard, including earthquake and floods (Jones & Holtz, 1973). The amount of damage caused by expansive soils is alarming. In the United States, it has been estimated that losses from expansive soils exceed two billion dollars annually (Nelson & Miller, 1992). Unfortunately, expansive soil problems have not been recognized outside the area of their occurrence. In the underdeveloped nations, few people realise the magnitude of the damage caused by expansive soils. Expansive soils are abundant in arid or semi-arid regions where annual evapotranspiration exceeds precipitation. The alkaline environment and lack of leaching favour the formation of montmorillonite minerals which have a very high swell potential. The origin of expansive soils is related to the composition of the parent material and the degree of physical and chemical weathering to which the materials are subjected (Chen, 1988). Donaldson (1969) classified the parent materials that can be associated with expansive soils into two groups: the basic igneous rocks and the sedimentary rocks which contain montmorillonite as a constituent. The weak bonding force between the successive layers in montmorillonite allow water to enter between the individual sheets and cause swelling which lead to serious foundation problems. 0140}1963/01/040453#11 $35.00/0
2001 Academic Press
454
Z. NALBANTOGLU & E. GUCBILMEZ
Compacted clays subjected to low pressures may show significant expansion when inundated. Swell potential by definition is the percent swell of a soil compacted as in the standard compaction test and subjected to a 6)9 kPa surcharge before being immersed in water (Seed et al., 1962). A soil with a high swell potential usually exhibits a high swell pressure. The amount of swell is reduced by the overburden pressure. The higher the foundation pressure, the smaller the swell due to the inundation of the active supporting soil (Lee et al., 1983). The overburden pressure just sufficient to prevent swell upon inundation is termed the swell pressure. The semi-arid climate and geology of Cyprus, an island located at the eastern part of the Mediterranean Sea, have produced calcareous expansive soils in some areas. In Degirmenlik village located in the northern part of Cyprus, some serious cracks have been observed on many buildings and pavements. Especially light structures have been badly cracked. The evidence of structural damage in the area has motivated attempts to stabilize the soils prior to building construction. Soil stabilization with cement and lime and the mechanism of stabilization have been extensively studied by some researchers (Basma & Tuncer, 1991; Locat et al., 1996; Mathew & Narasimha, 1997). The improvements in the engineering properties of the soil as lime is added can be explained by two basic reactions: short-term reactions consisting cation exchange and flocculation and the long-term reaction, the pozzolanic activity. During the first stage of lime-clay reaction, excess of calcium ions in lime replace all other monovalent cations in the clay and change the electrical charge density around the clay particles. This results in an increase in the interparticle attraction, causing flocculation and aggregation and a consequent decrease in the plasticity of the soils. The pozzolanic reaction is time-bound and temperature dependent. During this process, the high pH causes silica and alumina to be dissolved out of the structure of the clay minerals and to combine with the calcium to produce the new cementitious compounds calcium silicate hydrates (CSH), calcium aluminate hydrates (CAH) and calcium alumino-silicate hydrates (CASH). These hydrates were observed by many researchers (Diamond et al., 1964; Sloane, 1965; Ormsby & Kinter, 1973; Choquette et al., 1987). Pozzolans are a source of silica or alumina with high surface area. They may be defined as a material which is capable of reacting with lime in the presence of water at ordinary temperature to produce cementitious compounds (Nicholson & Kashyap, 1993). The reaction between a pozzolan and lime in the presence of water is called a pozzolanic reaction (Chu & Kao, 1993). Possible sources of silica and alumina to form new hydrates are clay minerals, feldspar, micas, quartz, and other alumina-silicate (Choquette et al., 1987). In the present study, an industrial by-product Soma fly ash which has a well pozzolanic property has been used as a chemical stabilizer to improve the engineering properties of the soil. The study presents the effect of fly ash treatment on the volume change, hydraulic conductivity and mineralogy of the calcareous expansive soil. Currently, CEC values find limited use in the study of soil improvement. In the present study, an attempt has been made to use CEC values to substantiate the formation of the new mineral phases which are produced as a result of pozzolanic reaction. Background Thompson (1996) pointed out that calcareous soils are mineralogically quite similar to the parent materials and have not been subjected to extensive weathering and thus soil minerals with low weathering resistance still exist and provide excellent silica and/or alumina sources, contributing to a high level of lime reactivity.
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Lime treatment in cohesive soils generally causes a decrease in plasticity, dispersion, volume change potential, and an increase in particle size, permeability and strength (Broms & Boman, 1979; El-Rawi & Awad, 1981; Locat et al., 1990; Tuncer & Basma, 1991). Studies on lime, fly ash and lime-fly ash stabilization of soils have been conducted on soils in the many regions around the world (Transportation Research Board, 1987; Kamon & Nontanandh, 1991; Basma & Tuncer, 1991; Locat et al., 1996). A review of the present state of the art reveals that limited information is available, concerning the effect of curing time on the hydraulic conductivity of lime and fly ash treated soils. Some investigators (Townsend & Kylm, 1966; Brandl, 1981) concluded that hydraulic conductivity of a treated soil should increase with the increase in curing time whereas others (Terashi et al., 1980; Locat et al., 1996) noted a reduction in hydraulic conductivity. Hydraulic conductivity studies of lime and fly ash treated soils are widely varied and require further investigation. Materials Characteristics of the study area The site selected for the investigation was based on the reported structural damage in the area due to expansive soil which is a deposit of marine clays of Degirmenlik flysch. The flysch is an autochthonous formation which outcrops over an area of 1600 km in the north of the island, extending from east to west. The main facies of this Middle Miocene formation consist of turbidities, arenites, lutites, calcilutites and conglomerates (Dreghorn, 1978). The soil deposits derived from this formation give a blue-grey colour in fresh exposures and on hydration produce the khaki colour. Physical properties and mineralogical composition of the soil are shown in Table 1. According to the classification proposed by Seed et al. (1962), the soil can be characterized as highly expansive in terms of the values of percent clay sizes and activity. Table 1. Physical properties and mineralogical composition of the calcareous expansive soil
Properties Calcite (%) Quartz (%) Chlorite (%) Illite (%) Plagioclase (%) Dolomite (%) Kaolinite (%) Montmorillonite (%) Liquid limit (%) Plastic limit (%) Plasticity index (%) % Clay fraction ((2 lm) Activity Optimum water content (%) Max. dry unit weight (kN/m3) CEC (meq/100 g) pH
Test values 23)0 20)0 5)0 3)0 4)0 7)0 21)0 17)0 67)8 22)2 45)6 33)0 1)38 20)5 16)2 18)8 9)4
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Z. NALBANTOGLU & E. GUCBILMEZ
The presence of montmorillonite in the soil is a good indication of the high swell potential. Soma fly ash In the study, the industrial by-product Soma fly ash, produced in an electrical power plant in Turkey, was used as a chemical stabilizer to improve the engineering properties of the soil. In the plant, approximately 4 million tons of fly ash are produced every year. The fly ash has a fineness of 3818 cm2 g!1 and the fly ash particles retained on a No. 325 sieve (45 lm) is 18%. Class C Soma fly ash has a high percentage of calcium oxide (16%) which provides an inexpensive source of high-quality soil stabilizing agent. Specimen preparation and methods An experimental program was performed on the soil specimens collected from a depth of 1)5 m below the ground surface. Soil-fly ash mixtures were prepared with 15% and 25% fly ash by dry weight of the soil and one mixture with 15% fly ash plus an additional 3% lime. Pre-curing of soil-stabilizer-water mixture was allowed for 24 h, after which the various tests were performed on specimens. The optimum water content and maximum dry unit weight of the specimens were determined using the standard Proctor compaction test. For each test point, fresh samples were used. All specimens were compacted before testing by using the standard Proctor compaction effort at optimum water content. The prepared specimens were sealed in waxed paper and then dipped in hot parrafin to ensure a completely air-tight seal and allowed to cure at 223C and 70% relative humidity for periods of 0, 7 and 30 days. Specimens were subjected to swell tests, according to the procedure suggested by Seed et al. (1962). In the swell and compressibility tests, 20-mm high and 76-mm diameter cylinders were used. The swell pressure is obtained by consolidating the preswollen specimens to its initial height. In the study, the rate of compression is defined in terms of t90 which was determined from the standard one dimensional consolidation test for a pressure increment of 800 kPa. In the square root time method (Taylor, 1948) t90 is defined as the time required to complete 90% consolidation in the clay. The hydraulic conductivity values of the natural and treated soils were determined from the standard one dimensional consolidation test. They were calculated at the pressure level of 800 kPa and the following expression was used in the calculations: av cv cw k" 1#e
[1]
where k is the hydraulic conductivity, cv is the coefficient of consolidation, av is the coefficient of compressibility, cw is the unit weight of water and e is the void ratio. Results Figure 1 shows the effect of fly ash treatment on the plasticity index values, together with clay size fraction. From the figure, it can be seen that the plasticity index of the treated soils decreases as clay size content decreases with the increase in fly ash content. The lowest plasticity index and clay size fraction are obtained at 3% lime plus 15% fly ash treatment.
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Figure 1. Variation of plasticity index and percent clay size fraction with fly ash and lime-fly ash treatments. Untreated soil ( ), 15% fly ash ( ), 25% fly ash ( ), 3% lime#15% fly ash ( ).
The swell potential of fly-ash-treated soils is given in Fig. 2, indicating the reduction in the swell potential as fly ash content is increased. The specimen treated with 3% lime plus 15% fly ash gives an initial swell potential of 0)9% and with curing time, 7 and 30 days, the value of this swell drops to zero. When the swell potential values of natural, fly ash and lime-fly ash treated soils measured at different curing periods are compared (Fig. 3), even more dramatic results are obtained by using fly ash containing a low lime percentage.
Figure 2. Variation of swell potential with percent fly ash and curing time. 0 day ( ), 7 days ( ), 30 days ( ).
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Figure 3. Comparison among the swell potential values of natural, fly ash, and lime-fly ash treated soils. 0 day ( ), 7 days ( ), 30 days ( ).
Figure 4 shows a significant decrease in the swell pressure values of fly ash and lime-fly ash treated soils. It can be seen that the swell pressure values of the treated soils decrease further with the increase in curing time. The effect of fly ash, lime-fly ash treatment and curing time on the rate of compression is shown in Fig. 5. In the figure, the rate of compression is defined in terms of t90. The figure indicates that the time required to complete 90% consolidation (t90) decreases with the increase in percent fly ash and curing time. Figure 6 shows the increase in the hydraulic conductivity values with the increase in fly ash content and lime-fly ash treatment. Further increases in the hydraulic
Figure 4. Effect of fly ash, lime-fly ash and curing time on the swell pressure values. Fly ash ( ), fly ash#3% lime ( ). 0 day ( ), 7 days ( ), 30 days ( ).
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Figure 5. Effect of fly ash, lime-fly ash and curing time on the rate of compression for pressure p"800 kPa. Fly ash ( ), fly ash#3% lime ( ). 0 day ( ), 7 days ( ), 30 days ( ).
conductivity values were obtained with curing time. The most dramatic increase in hydraulic conductivity was obtained for the soil treated with 3% lime plus 15% fly ash. Cation exchange capacity (CEC) values of fly ash and lime-fly ash treated soils measured at 30 days are given in Fig. 7. The fly ash and lime-fly ash treated soils give
Figure 6. Variation of hydraulic conductivity with percent fly ash, lime-fly ash and curing time for pressure p"800 kPa. Fly ash ( ), fly ash#3% lime ( ). 0 day ( ), 30 days ( ).
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Figure 7. Effect of fly ash, lime-fly ash treatment on cation exchange capacity values measured at 30 days. Fly ash ( ), fly ash#3% lime ( ).
a decrease in CEC values, indicating the change in the mineralogy of the treated soils which are no longer representative of the untreated soil. Discussion Figure 1 indicates the reduction in plasticity index values as clay size fraction decreases with the increase in fly ash treatment. This is explained by the reduced thickness of the double layer of the clay due to cation exchange reaction which causes an increase in the attraction forces and leads to a better flocculation of the particles. Unlike the pozzolanic reaction, this process tends to modify the soil without producing the new secondary minerals (Marks & Haliburton, 1972). Reduction in swell potential values of fly ash treated soils is shown in Fig. 2. The figure indicates that at zero curing time fly ash treatment does not seem to be very effective in reducing the swell potential. With 25% fly ash, the swell potential of the soil reduces only to 3)7%. However, with a curing time of 30 days, the swell potential of the same soil gives zero value. The increasingly granular nature of the stabilized soils with time results in further reduction in swell potential, indicating the reduced water absorption tendency of the treated soils. This suggests that pozzolanic reactions cause changes in the mineralogy of the treated soils and result in more stable silt-sand like structures. Similarly, reduction in the swell pressure values shown in Fig. 4 is ascribed to the cementing ability of fly ash and lime-fly ash treated soils which reduces the water absorption tendency of the calcium-saturated clays. Cementation between particles is a major factor in limiting volume increase of clays on swelling (Yong & Warkentin, 1966). During this process, the high pH causes silica and alumina to be dissolved out of the structure of the clay minerals and to combine with the calcium to produce the new cementitious compounds. As a result of this reaction, a highly open fabric arrangement and a consequent decrease in the water absorption tendency of the treated soils are obtained.
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The rate of compression is directly related to the rate at which the pore water can escape from the soil. The difference in the rate of dissipation of excess pore water pressure depends on the difference in soil’s hydraulic conductivity. The dissipation of excess pore water pressure is instantaneous in soils with high hydraulic conductivity whereas the dissipation of excess pore water pressure in soils, with relatively low hydraulic conductivity, is very slow and time dependent. In the study, the rate of compression is defined in terms of t90. The decrease in t90 values in Fig. 5 indicates that the rate of compression of the treated soils increases with the increase in percent fly ash and curing time. This means that with fly ash and lime-fly ash treatment the soils approach granular soil behavior, suggesting an increase in the hydraulic conductivity of the treated soils. To further substantiate these findings, hydraulic conductivity of the natural and treated soils was determined from the standard one dimensional consolidation test (Fig. 6). Further increase in hydraulic conductivity values was obtained with curing time, substantiating the findings that, with fly ash treatment and curing time, soils become more granular in nature and result in less water absorption potential. Cation exchange capacity values of the treated soils also explain the increase in hydraulic conductivity with the increase in curing time. Cation exchange capacity is the quantity of exchangable cations required to balance the charge deficiency on the surface of the clay particles (Mitchell, 1993). It is a permanent feature of the crystal and does not depend upon the composition of the ambient solution (Mathew & Narasimha, 1997). Clays with larger specific surface area usually have higher CEC, higher surface activity and consequently higher water absorption potential. The decrease in CEC values in Fig. 7 verifies the formation of the new pozzolanic reaction minerals which result in more stable silt-sand like structures and give less water absorption potential and higher hydraulic conductivity values. In contrast to some of the previously published research, further increase in hydraulic conductivity values of the treated soils is obtained with time. This is in agreement with the reduced CEC values indicating that fly ash, lime-fly ash treatment and curing time cause changes in the mineralogy of the treated soils and produce the new secondary minerals with more stable silt-sand like structures. In the field, fly ash should be mixed with soils so as to produce similar results obtained from the soil-fly ash mixtures prepared in the laboratory. For heavy clay soils two-stage pulverization and mixing is required. Disc harrows and grader scarifiers are suitable for preliminary mixing and high-speed rotary mixers or single-pass travel plant mixers are required for final mixing. Compaction should immediately follow the mixing operation and prior to compaction the moisture content of the soil-fly ash mixture should be brought to two percent above the optimum moisture content. Several illustrated publications (National Lime Association, 1985; Sherwood, 1993) contain valuable information on the field construction methods. Conclusion In arid and semi-arid environments, the modification of engineering properties of expansive soils becomes very important to geotechnical engineers as the foundation soils ideal for the intended purpose become less and less available. The industrial waste product Soma fly ash shows tremendous potential for the stabilization of the calcareous expansive soil. Significant reduction in the plasticity index and swell potential and an increase in hydraulic conductivity values are obtained. Treatment of expansive soils using fly ash which requires costly disposal provides an inexpensive source of high quality stabilizing agent. The use of this industrial byproduct for soil stabilization would be beneficial for improving soils for construction
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purposes, while at the same time would be one of many viable answers for handling the fly ash waste problem.
References Basma, A.A. & Tuncer, E.R. (1991). Effect of lime on volume change and compressibility of expansive clays. Transportation Research Record, 1295: 52}61. Brandl, H. (1981). Alteration of soil parameters by stabilization with lime. Proceedings, 10th International Conference on Soil Mechanics and Foundation Engineering. Stockholm, 587}594 pp. Broms, B. & Boman, P. (1979). Lime columns—a new foundation method. Journal of Geotechnical Engineering Division, ASCE, 105: 539}556. Chen, F.H. (1988). Foundation on Expansive Soils. Amsterdam: Elsevier Science Publisher. 463 pp. Choquette, M., Berube, M.-A. & Locat, J. (1987). Mineralogical and microtextural changes associated with lime stabilization of marine clays from Eastern Canada. Applied Clay Science, 2: 215}232. Chu, S.C. & Kao, H.S. (1993). A study of engineering properties of a clay modified by fly ash and slag. In: Fly Ash for Soil Improvement. Geotechnical Special Publication, ASCE, 36: 89}99. Diamond, S., White, J.L. & Dolch, W.L. (1964). Transformation of clay minerals by calcium hydroxide attack. Proceedings 12th National Conference on Clays and Clay minerals, New York: Pergamon Press, pp. 359}379. Donaldson, G.W. (1969). The occurrence of problems of heave and the factors affecting its nature. Second International Research and Engineering Conference on Expansive Clay Soils. Texas: A & M Press. Dreghorn, W. (1978). Landforms in the Girne Range-Northern Cyprus. Ankara: The Mineral Research and Exploration Institute of Turkey. 222 pp. El-Rawi, M.N. & Awad, A.A.A. (1981). Permeability of lime stabilized soils. Journal of Transportation Engineering Division, ASCE, 107: 25}35. Jones, D.E. & Holtz, W.G. (1973). Expansive soils—the hidden disaster. Civil Engineering, ASCE, 43: 49}51. Kamon, M. & Nontanandh, S. (1991). Combining industrial wastes with lime for soil stabilization. Journal of Geotechnical Engineering, ASCE, 117: 1}17. Lee, I.K., Ingles, O.G. & White, W. (1983). Geotechnical Engineering. Massachusetts: Pitman Publishing Incorporation. 508 pp. Locat, J., Berube, M.-A. & Choquette, M. (1990). Laboratory investigations on the lime stabilization of sensitive clays: shear strength development. Canadian Geotechnical Journal, 27: 294}304. Locat, J., Tremblay, H. & Leroueil, S. (1996). Mechanical and hydraulic behaviour of a soft inorganic clay treated with lime. Canadian Geotechnical Journal, 33: 654}669. Marks, B.D. & Haliburton, T.A. (1972). Acceleration of lime-clay reactions with salt. Journal of Soil Mechanics and Foundations Division, ASCE, 98: 327}339. Mathew, P.K. & Narasimha, R.S. (1997). Effect of lime on cation exchange capacity of marine clay. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 123: 183}185. Mitchell, J.K. (1993). Fundamentals of Soil Behavior. New York: John Wiley and Sons Incorporation. 437 pp. National Lime Association. (1985). Lime Stabilization Construction Manual. (8th Edn). Arlington, VA: National Lime Association of America. 48 pp. Nelson, J.D. & Miller, D.J. (1992). Expansive Soils—Problems and Practice in Foundation and Pavement Engineering. New York: John Wiley and Sons Incorporation. 259 pp. Nicholson, P.G. & Kashyap, V. (1993). Flyash stabilization of tropical Hawaiian soils. Fly Ash for Soil Improvement. Geotechnical Special Publication, ASCE, 36, 15}29 pp. Ormsby, W.C. & Kinter, E.B. (1973). Effects of dolomitic and calcitic limes on strength development in mixtures with two clay minerals. Public Roads, 37: 149}160. Seed, H.B., Woodward, R.J. & Lundgren, R. (1962). Prediction of swelling potential for compacted clays. Journal of Soil Mechanics and Foundations Division, ASCE, 88: 53}87. Sherwood, P.T. (1993). Soil stabilization with cement and lime. State-of-The-Art Review, HMSO Publications Centre.
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Sloane, R.L. (1965). Early reaction determination in two hydroxide-kaolinite systems by electron microscopy and diffraction. Proceedings, 13th National Conference on Clays and Clay Minerals, New York: Pergamon Press, 331}339 pp. Taylor, D.W. (1948). Research on consolidation of clays. Cambridge: Massachusetts Institute of Technology, Report Serial No. 82, Department of Civil Engineering. Terashi, M., Tanaka, H., Niidome, Y. & Sakanoi, H. (1980). Permeability of treated soils. Proceedings, 15th Japan Conference on Soil Mechanics and Foundation Engineering, 773}776 pp. Thompson, M.R. (1996). Lime reactivity of Illinois soils. Journal of Soil Mechanics and Foundations Division, ASCE, 92: 67}92. Townsend, D.C. & Kylm, T.W. (1966). Durability of lime-stabilized soils. Highway Research Board Bulletin, 139: 25}41. Transportation Research Board (1987). Lime stabilization: reaction, properties, design and construction. Committee on Lime and Lime-fly ash Stabilization. State-of-the-Art-Report, 5, Washington, D.C., 1}59 pp. Tuncer, E.R. & Basma, A.A. (1991). Strength and stress-strain characteristics of a lime treated cohesive soil. Transportation Research Record, 1295: 70}79. Yong, R.N. & Warkentin, B.P. (1966). Introduction to Soil Behavior. New York: The Macmillan Book Company. 451 pp.
Improvement of calcareous expansive soils in semi-arid environments
Zalihe Nalbantoglu & Emin Gucbilmez Department of Civil Engineering, Eastern Mediterranean University, Gazimagusa, Mersin 10, Turkey (Received 11 January 2000, accepted 14 September 2000, published electronically 19 February 2001) The semi-arid climate and geology of Cyprus have caused the formation of calcareous expansive soils on the island. In some areas, swelling has caused serious foundation problems. The industrial by-product Soma fly ash has been used to improve the engineering properties of the soil. Fly ash treatment has shown tremendous potential as an economical method for the stabilization of the soil. Significant reduction in the swell potential and an increase in the hydraulic conductivity values are obtained. Use of fly ash with a small percentage of lime produces even more dramatic results. Cation exchange capacity (CEC) values substantiate the formation of the new pozzolanic reaction minerals which result in more stable silt-sand like structures. 2001 Academic Press Keywords: calcareous; cementation; expansive; fly ash; hydraulic conductivity; semi-arid; stabilization; swell
Introduction Expansive soil is a term generally applied to any soil that has a potential for shrinking or swelling under changing moisture conditions. Expansive soils cause more damage to structures, particularly light buildings and pavements, than any other natural hazard, including earthquake and floods (Jones & Holtz, 1973). The amount of damage caused by expansive soils is alarming. In the United States, it has been estimated that losses from expansive soils exceed two billion dollars annually (Nelson & Miller, 1992). Unfortunately, expansive soil problems have not been recognized outside the area of their occurrence. In the underdeveloped nations, few people realise the magnitude of the damage caused by expansive soils. Expansive soils are abundant in arid or semi-arid regions where annual evapotranspiration exceeds precipitation. The alkaline environment and lack of leaching favour the formation of montmorillonite minerals which have a very high swell potential. The origin of expansive soils is related to the composition of the parent material and the degree of physical and chemical weathering to which the materials are subjected (Chen, 1988). Donaldson (1969) classified the parent materials that can be associated with expansive soils into two groups: the basic igneous rocks and the sedimentary rocks which contain montmorillonite as a constituent. The weak bonding force between the successive layers in montmorillonite allow water to enter between the individual sheets and cause swelling which lead to serious foundation problems. 0140}1963/01/040453#11 $35.00/0
2001 Academic Press
454
Z. NALBANTOGLU & E. GUCBILMEZ
Compacted clays subjected to low pressures may show significant expansion when inundated. Swell potential by definition is the percent swell of a soil compacted as in the standard compaction test and subjected to a 6)9 kPa surcharge before being immersed in water (Seed et al., 1962). A soil with a high swell potential usually exhibits a high swell pressure. The amount of swell is reduced by the overburden pressure. The higher the foundation pressure, the smaller the swell due to the inundation of the active supporting soil (Lee et al., 1983). The overburden pressure just sufficient to prevent swell upon inundation is termed the swell pressure. The semi-arid climate and geology of Cyprus, an island located at the eastern part of the Mediterranean Sea, have produced calcareous expansive soils in some areas. In Degirmenlik village located in the northern part of Cyprus, some serious cracks have been observed on many buildings and pavements. Especially light structures have been badly cracked. The evidence of structural damage in the area has motivated attempts to stabilize the soils prior to building construction. Soil stabilization with cement and lime and the mechanism of stabilization have been extensively studied by some researchers (Basma & Tuncer, 1991; Locat et al., 1996; Mathew & Narasimha, 1997). The improvements in the engineering properties of the soil as lime is added can be explained by two basic reactions: short-term reactions consisting cation exchange and flocculation and the long-term reaction, the pozzolanic activity. During the first stage of lime-clay reaction, excess of calcium ions in lime replace all other monovalent cations in the clay and change the electrical charge density around the clay particles. This results in an increase in the interparticle attraction, causing flocculation and aggregation and a consequent decrease in the plasticity of the soils. The pozzolanic reaction is time-bound and temperature dependent. During this process, the high pH causes silica and alumina to be dissolved out of the structure of the clay minerals and to combine with the calcium to produce the new cementitious compounds calcium silicate hydrates (CSH), calcium aluminate hydrates (CAH) and calcium alumino-silicate hydrates (CASH). These hydrates were observed by many researchers (Diamond et al., 1964; Sloane, 1965; Ormsby & Kinter, 1973; Choquette et al., 1987). Pozzolans are a source of silica or alumina with high surface area. They may be defined as a material which is capable of reacting with lime in the presence of water at ordinary temperature to produce cementitious compounds (Nicholson & Kashyap, 1993). The reaction between a pozzolan and lime in the presence of water is called a pozzolanic reaction (Chu & Kao, 1993). Possible sources of silica and alumina to form new hydrates are clay minerals, feldspar, micas, quartz, and other alumina-silicate (Choquette et al., 1987). In the present study, an industrial by-product Soma fly ash which has a well pozzolanic property has been used as a chemical stabilizer to improve the engineering properties of the soil. The study presents the effect of fly ash treatment on the volume change, hydraulic conductivity and mineralogy of the calcareous expansive soil. Currently, CEC values find limited use in the study of soil improvement. In the present study, an attempt has been made to use CEC values to substantiate the formation of the new mineral phases which are produced as a result of pozzolanic reaction. Background Thompson (1996) pointed out that calcareous soils are mineralogically quite similar to the parent materials and have not been subjected to extensive weathering and thus soil minerals with low weathering resistance still exist and provide excellent silica and/or alumina sources, contributing to a high level of lime reactivity.
IMPROVEMENT OF CALCAREOUS EXPANSIVE SOILS
455
Lime treatment in cohesive soils generally causes a decrease in plasticity, dispersion, volume change potential, and an increase in particle size, permeability and strength (Broms & Boman, 1979; El-Rawi & Awad, 1981; Locat et al., 1990; Tuncer & Basma, 1991). Studies on lime, fly ash and lime-fly ash stabilization of soils have been conducted on soils in the many regions around the world (Transportation Research Board, 1987; Kamon & Nontanandh, 1991; Basma & Tuncer, 1991; Locat et al., 1996). A review of the present state of the art reveals that limited information is available, concerning the effect of curing time on the hydraulic conductivity of lime and fly ash treated soils. Some investigators (Townsend & Kylm, 1966; Brandl, 1981) concluded that hydraulic conductivity of a treated soil should increase with the increase in curing time whereas others (Terashi et al., 1980; Locat et al., 1996) noted a reduction in hydraulic conductivity. Hydraulic conductivity studies of lime and fly ash treated soils are widely varied and require further investigation. Materials Characteristics of the study area The site selected for the investigation was based on the reported structural damage in the area due to expansive soil which is a deposit of marine clays of Degirmenlik flysch. The flysch is an autochthonous formation which outcrops over an area of 1600 km in the north of the island, extending from east to west. The main facies of this Middle Miocene formation consist of turbidities, arenites, lutites, calcilutites and conglomerates (Dreghorn, 1978). The soil deposits derived from this formation give a blue-grey colour in fresh exposures and on hydration produce the khaki colour. Physical properties and mineralogical composition of the soil are shown in Table 1. According to the classification proposed by Seed et al. (1962), the soil can be characterized as highly expansive in terms of the values of percent clay sizes and activity. Table 1. Physical properties and mineralogical composition of the calcareous expansive soil
Properties Calcite (%) Quartz (%) Chlorite (%) Illite (%) Plagioclase (%) Dolomite (%) Kaolinite (%) Montmorillonite (%) Liquid limit (%) Plastic limit (%) Plasticity index (%) % Clay fraction ((2 lm) Activity Optimum water content (%) Max. dry unit weight (kN/m3) CEC (meq/100 g) pH
Test values 23)0 20)0 5)0 3)0 4)0 7)0 21)0 17)0 67)8 22)2 45)6 33)0 1)38 20)5 16)2 18)8 9)4
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The presence of montmorillonite in the soil is a good indication of the high swell potential. Soma fly ash In the study, the industrial by-product Soma fly ash, produced in an electrical power plant in Turkey, was used as a chemical stabilizer to improve the engineering properties of the soil. In the plant, approximately 4 million tons of fly ash are produced every year. The fly ash has a fineness of 3818 cm2 g!1 and the fly ash particles retained on a No. 325 sieve (45 lm) is 18%. Class C Soma fly ash has a high percentage of calcium oxide (16%) which provides an inexpensive source of high-quality soil stabilizing agent. Specimen preparation and methods An experimental program was performed on the soil specimens collected from a depth of 1)5 m below the ground surface. Soil-fly ash mixtures were prepared with 15% and 25% fly ash by dry weight of the soil and one mixture with 15% fly ash plus an additional 3% lime. Pre-curing of soil-stabilizer-water mixture was allowed for 24 h, after which the various tests were performed on specimens. The optimum water content and maximum dry unit weight of the specimens were determined using the standard Proctor compaction test. For each test point, fresh samples were used. All specimens were compacted before testing by using the standard Proctor compaction effort at optimum water content. The prepared specimens were sealed in waxed paper and then dipped in hot parrafin to ensure a completely air-tight seal and allowed to cure at 223C and 70% relative humidity for periods of 0, 7 and 30 days. Specimens were subjected to swell tests, according to the procedure suggested by Seed et al. (1962). In the swell and compressibility tests, 20-mm high and 76-mm diameter cylinders were used. The swell pressure is obtained by consolidating the preswollen specimens to its initial height. In the study, the rate of compression is defined in terms of t90 which was determined from the standard one dimensional consolidation test for a pressure increment of 800 kPa. In the square root time method (Taylor, 1948) t90 is defined as the time required to complete 90% consolidation in the clay. The hydraulic conductivity values of the natural and treated soils were determined from the standard one dimensional consolidation test. They were calculated at the pressure level of 800 kPa and the following expression was used in the calculations: av cv cw k" 1#e
[1]
where k is the hydraulic conductivity, cv is the coefficient of consolidation, av is the coefficient of compressibility, cw is the unit weight of water and e is the void ratio. Results Figure 1 shows the effect of fly ash treatment on the plasticity index values, together with clay size fraction. From the figure, it can be seen that the plasticity index of the treated soils decreases as clay size content decreases with the increase in fly ash content. The lowest plasticity index and clay size fraction are obtained at 3% lime plus 15% fly ash treatment.
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Figure 1. Variation of plasticity index and percent clay size fraction with fly ash and lime-fly ash treatments. Untreated soil ( ), 15% fly ash ( ), 25% fly ash ( ), 3% lime#15% fly ash ( ).
The swell potential of fly-ash-treated soils is given in Fig. 2, indicating the reduction in the swell potential as fly ash content is increased. The specimen treated with 3% lime plus 15% fly ash gives an initial swell potential of 0)9% and with curing time, 7 and 30 days, the value of this swell drops to zero. When the swell potential values of natural, fly ash and lime-fly ash treated soils measured at different curing periods are compared (Fig. 3), even more dramatic results are obtained by using fly ash containing a low lime percentage.
Figure 2. Variation of swell potential with percent fly ash and curing time. 0 day ( ), 7 days ( ), 30 days ( ).
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Figure 3. Comparison among the swell potential values of natural, fly ash, and lime-fly ash treated soils. 0 day ( ), 7 days ( ), 30 days ( ).
Figure 4 shows a significant decrease in the swell pressure values of fly ash and lime-fly ash treated soils. It can be seen that the swell pressure values of the treated soils decrease further with the increase in curing time. The effect of fly ash, lime-fly ash treatment and curing time on the rate of compression is shown in Fig. 5. In the figure, the rate of compression is defined in terms of t90. The figure indicates that the time required to complete 90% consolidation (t90) decreases with the increase in percent fly ash and curing time. Figure 6 shows the increase in the hydraulic conductivity values with the increase in fly ash content and lime-fly ash treatment. Further increases in the hydraulic
Figure 4. Effect of fly ash, lime-fly ash and curing time on the swell pressure values. Fly ash ( ), fly ash#3% lime ( ). 0 day ( ), 7 days ( ), 30 days ( ).
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Figure 5. Effect of fly ash, lime-fly ash and curing time on the rate of compression for pressure p"800 kPa. Fly ash ( ), fly ash#3% lime ( ). 0 day ( ), 7 days ( ), 30 days ( ).
conductivity values were obtained with curing time. The most dramatic increase in hydraulic conductivity was obtained for the soil treated with 3% lime plus 15% fly ash. Cation exchange capacity (CEC) values of fly ash and lime-fly ash treated soils measured at 30 days are given in Fig. 7. The fly ash and lime-fly ash treated soils give
Figure 6. Variation of hydraulic conductivity with percent fly ash, lime-fly ash and curing time for pressure p"800 kPa. Fly ash ( ), fly ash#3% lime ( ). 0 day ( ), 30 days ( ).
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Figure 7. Effect of fly ash, lime-fly ash treatment on cation exchange capacity values measured at 30 days. Fly ash ( ), fly ash#3% lime ( ).
a decrease in CEC values, indicating the change in the mineralogy of the treated soils which are no longer representative of the untreated soil. Discussion Figure 1 indicates the reduction in plasticity index values as clay size fraction decreases with the increase in fly ash treatment. This is explained by the reduced thickness of the double layer of the clay due to cation exchange reaction which causes an increase in the attraction forces and leads to a better flocculation of the particles. Unlike the pozzolanic reaction, this process tends to modify the soil without producing the new secondary minerals (Marks & Haliburton, 1972). Reduction in swell potential values of fly ash treated soils is shown in Fig. 2. The figure indicates that at zero curing time fly ash treatment does not seem to be very effective in reducing the swell potential. With 25% fly ash, the swell potential of the soil reduces only to 3)7%. However, with a curing time of 30 days, the swell potential of the same soil gives zero value. The increasingly granular nature of the stabilized soils with time results in further reduction in swell potential, indicating the reduced water absorption tendency of the treated soils. This suggests that pozzolanic reactions cause changes in the mineralogy of the treated soils and result in more stable silt-sand like structures. Similarly, reduction in the swell pressure values shown in Fig. 4 is ascribed to the cementing ability of fly ash and lime-fly ash treated soils which reduces the water absorption tendency of the calcium-saturated clays. Cementation between particles is a major factor in limiting volume increase of clays on swelling (Yong & Warkentin, 1966). During this process, the high pH causes silica and alumina to be dissolved out of the structure of the clay minerals and to combine with the calcium to produce the new cementitious compounds. As a result of this reaction, a highly open fabric arrangement and a consequent decrease in the water absorption tendency of the treated soils are obtained.
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The rate of compression is directly related to the rate at which the pore water can escape from the soil. The difference in the rate of dissipation of excess pore water pressure depends on the difference in soil’s hydraulic conductivity. The dissipation of excess pore water pressure is instantaneous in soils with high hydraulic conductivity whereas the dissipation of excess pore water pressure in soils, with relatively low hydraulic conductivity, is very slow and time dependent. In the study, the rate of compression is defined in terms of t90. The decrease in t90 values in Fig. 5 indicates that the rate of compression of the treated soils increases with the increase in percent fly ash and curing time. This means that with fly ash and lime-fly ash treatment the soils approach granular soil behavior, suggesting an increase in the hydraulic conductivity of the treated soils. To further substantiate these findings, hydraulic conductivity of the natural and treated soils was determined from the standard one dimensional consolidation test (Fig. 6). Further increase in hydraulic conductivity values was obtained with curing time, substantiating the findings that, with fly ash treatment and curing time, soils become more granular in nature and result in less water absorption potential. Cation exchange capacity values of the treated soils also explain the increase in hydraulic conductivity with the increase in curing time. Cation exchange capacity is the quantity of exchangable cations required to balance the charge deficiency on the surface of the clay particles (Mitchell, 1993). It is a permanent feature of the crystal and does not depend upon the composition of the ambient solution (Mathew & Narasimha, 1997). Clays with larger specific surface area usually have higher CEC, higher surface activity and consequently higher water absorption potential. The decrease in CEC values in Fig. 7 verifies the formation of the new pozzolanic reaction minerals which result in more stable silt-sand like structures and give less water absorption potential and higher hydraulic conductivity values. In contrast to some of the previously published research, further increase in hydraulic conductivity values of the treated soils is obtained with time. This is in agreement with the reduced CEC values indicating that fly ash, lime-fly ash treatment and curing time cause changes in the mineralogy of the treated soils and produce the new secondary minerals with more stable silt-sand like structures. In the field, fly ash should be mixed with soils so as to produce similar results obtained from the soil-fly ash mixtures prepared in the laboratory. For heavy clay soils two-stage pulverization and mixing is required. Disc harrows and grader scarifiers are suitable for preliminary mixing and high-speed rotary mixers or single-pass travel plant mixers are required for final mixing. Compaction should immediately follow the mixing operation and prior to compaction the moisture content of the soil-fly ash mixture should be brought to two percent above the optimum moisture content. Several illustrated publications (National Lime Association, 1985; Sherwood, 1993) contain valuable information on the field construction methods. Conclusion In arid and semi-arid environments, the modification of engineering properties of expansive soils becomes very important to geotechnical engineers as the foundation soils ideal for the intended purpose become less and less available. The industrial waste product Soma fly ash shows tremendous potential for the stabilization of the calcareous expansive soil. Significant reduction in the plasticity index and swell potential and an increase in hydraulic conductivity values are obtained. Treatment of expansive soils using fly ash which requires costly disposal provides an inexpensive source of high quality stabilizing agent. The use of this industrial byproduct for soil stabilization would be beneficial for improving soils for construction
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purposes, while at the same time would be one of many viable answers for handling the fly ash waste problem.
References Basma, A.A. & Tuncer, E.R. (1991). Effect of lime on volume change and compressibility of expansive clays. Transportation Research Record, 1295: 52}61. Brandl, H. (1981). Alteration of soil parameters by stabilization with lime. Proceedings, 10th International Conference on Soil Mechanics and Foundation Engineering. Stockholm, 587}594 pp. Broms, B. & Boman, P. (1979). Lime columns—a new foundation method. Journal of Geotechnical Engineering Division, ASCE, 105: 539}556. Chen, F.H. (1988). Foundation on Expansive Soils. Amsterdam: Elsevier Science Publisher. 463 pp. Choquette, M., Berube, M.-A. & Locat, J. (1987). Mineralogical and microtextural changes associated with lime stabilization of marine clays from Eastern Canada. Applied Clay Science, 2: 215}232. Chu, S.C. & Kao, H.S. (1993). A study of engineering properties of a clay modified by fly ash and slag. In: Fly Ash for Soil Improvement. Geotechnical Special Publication, ASCE, 36: 89}99. Diamond, S., White, J.L. & Dolch, W.L. (1964). Transformation of clay minerals by calcium hydroxide attack. Proceedings 12th National Conference on Clays and Clay minerals, New York: Pergamon Press, pp. 359}379. Donaldson, G.W. (1969). The occurrence of problems of heave and the factors affecting its nature. Second International Research and Engineering Conference on Expansive Clay Soils. Texas: A & M Press. Dreghorn, W. (1978). Landforms in the Girne Range-Northern Cyprus. Ankara: The Mineral Research and Exploration Institute of Turkey. 222 pp. El-Rawi, M.N. & Awad, A.A.A. (1981). Permeability of lime stabilized soils. Journal of Transportation Engineering Division, ASCE, 107: 25}35. Jones, D.E. & Holtz, W.G. (1973). Expansive soils—the hidden disaster. Civil Engineering, ASCE, 43: 49}51. Kamon, M. & Nontanandh, S. (1991). Combining industrial wastes with lime for soil stabilization. Journal of Geotechnical Engineering, ASCE, 117: 1}17. Lee, I.K., Ingles, O.G. & White, W. (1983). Geotechnical Engineering. Massachusetts: Pitman Publishing Incorporation. 508 pp. Locat, J., Berube, M.-A. & Choquette, M. (1990). Laboratory investigations on the lime stabilization of sensitive clays: shear strength development. Canadian Geotechnical Journal, 27: 294}304. Locat, J., Tremblay, H. & Leroueil, S. (1996). Mechanical and hydraulic behaviour of a soft inorganic clay treated with lime. Canadian Geotechnical Journal, 33: 654}669. Marks, B.D. & Haliburton, T.A. (1972). Acceleration of lime-clay reactions with salt. Journal of Soil Mechanics and Foundations Division, ASCE, 98: 327}339. Mathew, P.K. & Narasimha, R.S. (1997). Effect of lime on cation exchange capacity of marine clay. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 123: 183}185. Mitchell, J.K. (1993). Fundamentals of Soil Behavior. New York: John Wiley and Sons Incorporation. 437 pp. National Lime Association. (1985). Lime Stabilization Construction Manual. (8th Edn). Arlington, VA: National Lime Association of America. 48 pp. Nelson, J.D. & Miller, D.J. (1992). Expansive Soils—Problems and Practice in Foundation and Pavement Engineering. New York: John Wiley and Sons Incorporation. 259 pp. Nicholson, P.G. & Kashyap, V. (1993). Flyash stabilization of tropical Hawaiian soils. Fly Ash for Soil Improvement. Geotechnical Special Publication, ASCE, 36, 15}29 pp. Ormsby, W.C. & Kinter, E.B. (1973). Effects of dolomitic and calcitic limes on strength development in mixtures with two clay minerals. Public Roads, 37: 149}160. Seed, H.B., Woodward, R.J. & Lundgren, R. (1962). Prediction of swelling potential for compacted clays. Journal of Soil Mechanics and Foundations Division, ASCE, 88: 53}87. Sherwood, P.T. (1993). Soil stabilization with cement and lime. State-of-The-Art Review, HMSO Publications Centre.
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Sloane, R.L. (1965). Early reaction determination in two hydroxide-kaolinite systems by electron microscopy and diffraction. Proceedings, 13th National Conference on Clays and Clay Minerals, New York: Pergamon Press, 331}339 pp. Taylor, D.W. (1948). Research on consolidation of clays. Cambridge: Massachusetts Institute of Technology, Report Serial No. 82, Department of Civil Engineering. Terashi, M., Tanaka, H., Niidome, Y. & Sakanoi, H. (1980). Permeability of treated soils. Proceedings, 15th Japan Conference on Soil Mechanics and Foundation Engineering, 773}776 pp. Thompson, M.R. (1996). Lime reactivity of Illinois soils. Journal of Soil Mechanics and Foundations Division, ASCE, 92: 67}92. Townsend, D.C. & Kylm, T.W. (1966). Durability of lime-stabilized soils. Highway Research Board Bulletin, 139: 25}41. Transportation Research Board (1987). Lime stabilization: reaction, properties, design and construction. Committee on Lime and Lime-fly ash Stabilization. State-of-the-Art-Report, 5, Washington, D.C., 1}59 pp. Tuncer, E.R. & Basma, A.A. (1991). Strength and stress-strain characteristics of a lime treated cohesive soil. Transportation Research Record, 1295: 70}79. Yong, R.N. & Warkentin, B.P. (1966). Introduction to Soil Behavior. New York: The Macmillan Book Company. 451 pp.