IGC. 50 th. 50 th INDIAN GEOTECHNICAL CONFERENCE

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1 INDIAN GEOTECHNICAL CONFERENCE SCANNING ELECTRON MICROSTRUCTURE INVESTIGATIONS OF LIME TREATED BLACK COTTON SOIL AND MINE TAILINGS MIXTURE Ramesh H.N, Krishnaiah A.J and Venkataraja Mohan S D Professor, Department of Civil Engineering, UVCE, Bangalore University, Bangalore E mail-rheddur@yahoo.com Associate Professor, Department of Civil Engineering, MCE, Hassan E mail-ajkmce@gmail.com Professor, Department of Civil Engineering, Dr.AIT, Bangalore E mail-sdvclassical@yahoo.com ABSTRACT: This study is aimed at investigation on microstructures of lime-treated Black Cotton Soil (BCS) and Mine Tailings (MT) mixture in order to assess the consequent improvement on engineering properties such as strength and permeability. The soil used in this study was expansive clay with moderate natural moisture content. The strength of the remolded soil was 242 kpa and coefficient of permeability was 2.582x10-5 cm/s. The soil was mixed with lime in presence of mine tailings, after mixing the stabilized soil specimens were prepared and then tested in order to obtain strengths for curing times of immediate, 7 and 30 days and coefficients of permeability at immediate testing. The test results have indicated that the strength increased to 670 kpa and permeability decreased to 4.454x10-6 cm/s. In addition, microstructures were investigated on the same specimens after the tests using Scanning Electron Microscope (SEM). It was found that strengths of lime stabilized soil in presence of mine tailings markedly increased and permeability decreased. Reaction products and subsequent changes on microstructures due to chemical reactions were clearly seen based on the Scanning Electron Microscope (SEM) and EDS (Energy Dispersive Spectroscope) observations. In relation to strength development the major pozzolanic reaction compounds such as calcium silicate hydrate (CSH) and calcium aluminium silicate hydrate (CASH) were abundantly formed on clay fabrics. The cementitious compounds which contribute for the development of bonding between soil particles which have reduced pore spaces as soil structures became gradually hardened and resulting in an increase in strength and decrease in permeability. KEYWORDS: Lime treated soil, unconfined compressive strength, Scanning Electron Microscope, Coefficient of permeability, pozzolanic reaction compounds. INTRODUCTION Expansive soils are problematic in nature and pose several challenges for civil engineers. Such soils heave in presence of water and shrink when they dry out. Problems of expansive soils have appeared as cracking and break-up of pavements, railways, highway embankments, roadways, building foundations, canal and reservoir linings, irrigation systems, water lines and sewer lines (Al-Raws et al 2002). When geotechnical engineers faced problems with expansive soils, the engineering properties of those soils may need to be improved to make them suitable for construction. Soil stabilization technique is well established and is used in a veriety of applications like improving strength and stability, reduces the compressibility and permeability. Chemical additives such as lime, cement, fly ash and other chemical compounds have been used in expansive soil stabilization from many years with various degree of success (Eades and Grim 1960). Mine tailings is the residue of mine waste generated from mining industries after extraction of minerals from underground mined ores which causes environmental pollution. Mine tailings can be effectively utilized for civil engineering constructions which will minimize the disposal problems and reduce the environmental hazards (Pebble Project, 2005). Attempts have

2 Ramesh H N, Krishnaiah A J and Venkataraja Mohan S D been made elsewhere to use industrial wastes like fly ash, rice husk ash, ground granulated blast furnace slag for ground improvement ( Basha et al 2005). A similar approach has been made and studies have been carried out to assess the suitability of mine tailings in presence of lime for improving the engineering properties of expansive black cotton soils. To study of water flow inside the soil mass, the coefficient of permeability k should be known. In soils, k is one of the key parameters to study seepage or the steady state flow of water through soil. The older type of dam, the buildup of excess pore pressures within the embankment and seepage can be a problem, especially for a reservoir having high or rapidly fluctuating water levels for long periods for a dam having impervious foundations. If seepage is excessive can lead to instability and eventual failure of all or part of the downstream face. Mine tailings can be used as construction material for improving the properties of the soil (Pebble project 2005). In view of the above, in the present investigation an attempt is made to utilize the mine tailings with black cotton soil in presence of lime for construction of embankment and to study the hydraulic conductivity. The Scanning Electron Microscope (SEM) is one of the most versatile instrument used to study the microstructure of soil particles in a state of more advanced hydration. Secondary electrons emitted from a sample surface and appeared to be threedimensional images. The SEM has X20 to X150, 000 magnification range and a depth of field which is 300 times greater than that of the light microscope. These characteristic coupled with the fact that clay particles themselves and fracture surfaces through soil masses may be viewed directly, have led to extensive use of SEM for the study of clay particles. The Scanning Electronic Microscope (SEM) study along with EDS (Energy Dispersive Spectroscope) or EDX (Energy Dispersive X-ray detector system) which is more popular as this detector can display all the elements present in the specimen as they are being collected and enables rapid identification to be performed. MATERIALS Black Cotton Soil: In the present investigation the black cotton soil was obtained from Davanagere, Karnataka state, India. This is a residual soil and was collected from an open excavation, at a depth of 1.5 meter below the natural ground surface. The black cotton soil was air dried, pulverized and passed through 425micron BIS sieve was used for the present investigation. Mine tailings: Mine tailings was collected from an open dump from Kolar Gold Fields (KGF), Kolar, Karnataka, India. After removing the vegetations from the mine tailings, it was air dried, pulverized and passing through 425 micron BIS sieve was used in the present investigation. Lime: Lime [Ca (OH) 2] used in the present investigation was obtained from Fisher Scientific Chemicals private limited, Mumbai, India. The physical properties of Black cotton soil and Mine tailings are presented in Table.1 Table 1.Physical properties of Black Cotton Soil and Mine tailings (Ramesh et al 2013) Properties Values BCS MT Colour Black Pale gray Specific Gravity Liquid limit (%) Plastic limit (%) 47.2 Non plastic Plasticity Index (%) 34.8 Non plastic Shrinkage limit (%) Fine sand fraction (%) Silt fraction (%) Clay fraction (%) MDD (kn/m 3 ) OMC (%) UCS (kpa)

3 INDIAN GEOTECHNICAL CONFERENCE Table 2. Chemical properties of Black Cotton Soil and Mine tailings (Krishnaiah 2014) Properties Values BCS MT Silica (SiO 2 ) Alumina (Al 2 O 3 ) Calcium (CaO) Cynide Absent - METHODS The compaction tests were conducted using mini compaction test apparatus as per the procedures of Sridharan and Sivapullaiah (2005). Unconfined compressive strength and Permeability tests were carried out as per BIS: 2720 (part X) [1973] for various combinations of lime treated black cotton soil with mine tailings. All samples were prepared at their respective maximum dry density and optimum moisture content. The ZEISS EVO (R) MA and LS Series Scanning Electron Microscope (SEM) operated at 20KV was used to study the morphology and elemental composition of the soil samples at a voltage range of 20KV. The oven dried (105±5 C) soil samples (10 mm * 10mm) were placed on a aluminium stub covered with double sided carbon tape and the tested specimens were glued on aluminum holders for scanning. RESULTS AND DISCUSSIONS The compaction tests were carried out on black cotton soil and mine tailings mixtures treated with lime to know the maximum dry density and optimum moisture content. Further, with respect to maximum dry density and optimum moisture content the unconfined compressive strength tests were conducted for optimizing the percent of mine tailings in black cotton soil as well as lime content for black cotton soil and mine tailings mixture. It was observed that 30% of mine tailings for black cotton soil was found to be optimum and 3% lime for black cotton soil and mine tailings mixture was found to be optimum (Ramesh et al 2013). These optimum combinations are used to study the morphology of the stabilized soil and the rate of percolation or seepage of water through soil media are presented in the following section. Compaction characteristics of Lime treated Black cotton soil and Mine tailings mixture The maximum dry density and optimum moisture content of black cotton soil and mine tailings were found to be 14.3 kn/m 3, 15.7 kn/m 3 and 29.9%, 21.5% respectively. Addition of optimum 3% lime to black cotton soil and mine tailings mixture, the maximum dry density decreases and optimum moisture content increases. The reduction in maximum dry density may be attributed to replacement of lime soil mixture which has relatively lower specific gravity than that of soil and mine tailings and also due to the flocculation contracting the effect of compactive effort to attain its maximum dry density (Udayashankar and Puranik 2012).The increase in optimum moisture content is probably a consequence of the additional water held within the flocculent soil structure resulting from lime interaction and exceeding water absorption by lime as a result of its lower specific gravity. Unconfined compressive strength of Lime treated Black cotton soil and Mine tailings mixture The unconfined compressive strength of black cotton soil and mine tailings were found to be 242 kpa and 122 kpa respectively. Addition of optimum 3% lime to black cotton soil and mine tailings mixtures the strength increased to 491 kpa and 670 kpa for 7 days and 30 days of curing period respectively. This is attributed to the strength gain in lime treated mine tailings with black cotton soil involves two reactions such as hydration and pozzolanic reaction to form insoluble compounds of calcium aluminium silicate hydroxide (CASH) (Mohammed and Anita 1998). The microstructure analysis shows that due to formation of different cementitious compounds found from XRD and SEM studies attributed to strength gain in black cotton soil. The variation of

4 Ramesh H N, Krishnaiah A J and Venkataraja Mohan S D strength of black cotton soil treated with optimum mine tailings with lime is as shown in Fig.1 UCS (kpa) Curing period in days BCS alone MT alone BCS + 30% MT BCS + 30% MT + 3 % Lime Fig. 1 Variation of UCS of BCS and MT treated with lime Permeability characteristics of Lime treated Black cotton soil and Mine tailings mixture Permeability is the measure of the rate at which the fluid passes through soil media. The permeability values were obtained from consolidation test results using the following expression K=C v m v γ w where C v = coefficient of consolidation, m v = coefficient of volume change and γ w = unit weight of water. The permeability of black cotton soil alone for 200 kpa is 2.582x10-5 cm/sec, addition of 3% lime to black cotton soil treated with optimum mine tailings permeability decreased to 4.454x10-6 cm/sec. It indicate that permeability reduced by 82% compare to black cotton soil alone, the permeability of black cotton soil treated with mine tailings in presence of lime depends on the grain size, degree of compaction and pozzolanic activity. The increase in consolidation pressure decreases the void ratio and addition of lime decrease the permeability, this is due to formation of gelatinous compounds which block the pores (Brandl, 1981). The variation of permeability of black cotton soil treated with mine tailings in presence of lime as shown in Fig.2 Permeability (m/sec) 6.1E E E E E E-05 1E Pressure in kpa BCS Alone BCS + 30% MT BCS+30%MT+3 %Lime Fig. 2 Variation of permeability of BCS and MT treated with lime Scanning Electronic Microscope (SEM) Studies of Lime treated Black cotton soil and Mine tailings mixture To investigate the mechanisms underlying the increase in compressive strength with curing time for mine tailings-lime treated black cotton soil, microanalysis using scanning electron microscope (SEM) combined with energy dispersive X-ray (EDX) spectroscope was carried out on cured soil specimens of optimum combinations. SEM images observations indicate that the abundant presence calcium aluminium silicate hydroxide (CASH) formation which increased the strength with curing time (Xu, A and Sarkar, S L 1994, Lav A.H. and Lav M.A. 2000). The microstructural changes in cured samples as shown in Fig. 3,5,7 and 9 and EDS micrographs conform that the elements (Ca, S and Al) peaks which are supporting for increase in strength as shown in Fig.4,6,8, and 10. Fig. 3 SEM image of mine tailings alone

5 INDIAN GEOTECHNICAL CONFERENCE Fig. 4 EDS micrograph of mine tailings alone Fig. 7 SEM image of BCS and MT mixture for immediate testing Fig. 5 SEM image of black cotton soil alone Fig. 8 EDS micrograph of BCS and MT for immediate testing Fig. 6 EDS micrograph of black cotton soil alone Fig. 9 SEM image of BCS and MT mixture for 30 days of curing period

6 Ramesh H N, Krishnaiah A J and Venkataraja Mohan S D stages of pozzolanic activity and shows the pronounced peaks (S, Al and Ca) which will contribute for increases the strength with curing period. 5. The microstructure investigation of black cotton soil treated with lime and mine tailings is found to be effective in increasing unconfined compressive strength with increase in curing period. It is also effective in decreasing the permeability of the soil mass. ACKNOWLEDGEMENT Fig. 10 EDS micrograph of BCS and MT for immediate testing CONCLUSIONS Based on the analysis and results some of the following conclusions have been drawn: 1. The unconfined compressive strength of black cotton soil and mine tailings mixture treated with lime increases with increase in curing period due to hydration and pozzolanic reaction. 2. The permeability of the black cotton soil treated with mine tailings in presence of lime is decreases due to formation gelatinous compounds by pozzolanic reactions which block the pores of the soil media. 3. The SEM analysis of lime treated back cotton soil and optimum mine tailings observed that, the strength increased due to formation of calcium silicate hydrate (CSH) and calcium aluminium silicate hydroxide (CASH) cement hydration compounds with curing period. 4. The SEM images with EDS micrographs have been observed that the hardened paste microstructure has developed with soilmine tailings-lime mixtures in different The authors would like to acknowledge Late Mr. Venugopala S.R and Msr. Supriya M.D, former P. G. Students, Faculty of Engineering (Civil), UVCE, Bangalore University, Jnanabharathi, Bangalore , India, for his help during experimental investigation. REFERENCE 1. Al-Raws AA, Taha R, Nelson JD, Al-Shab TB and Al-Siyabi H (2002), A Comparative evolution of various additives in the stabilization of expansive soils. Geotech Test J Vol. 25(2), Bhasha E.A, Hashmi R, Mahmud H.B And Muntohar A.S (2004), Stabilization of Residual Soil with Rice Husk Ash and Cement, Construction and building Materials, Elsevier, Vol.19, BIS 2720 (Part X) (1973). Determination of Unconfined Compressive Strength. Bureau of Indian Standards, New Delhi 4. BIS 2720 (Part X) (1973). Determination of Unconfined Compressive Strength. Bureau of Indian Standards, New Delhi 5. BIS 2720 (Part X) (1973). Determination of Permeability. Bureau of Indian Standards, New Delhi 6. Brandl H (1981), Alteration of soil parameters by Stabilization with Lime,

7 INDIAN GEOTECHNICAL CONFERENCE Procedings X International Conference on Soil Mechanics and Foundation Engineering, Stockholm, 4, Eades J L, Grim R E (1960), The reaction of hydrated lime with pure clay minerals in soil stabilization, U.S Highway Research Board Meeting 8. Krishnaiah A J, Strength and Compressibility Behaviour of Compacted Fine grained Soils in Presence of Mine tailings and other Additives, Ph. D Thesis submitted to Bangalore University, Lav A.H. and Lav M.A. (2000), Micro structural Development of Stabilized Fly ash as pavement basic material, Jour. of Materials in Civil Engg. ASCE, Vol 12(2), Mohamed A.M.O and AnitaH.E Geoenvironmental Engineering. Elsevier, Amsterdam, Pebble Project, Tailings and Tailings Managements, Northern Dynasty Mines Inc., 2005, Poon C.S, Lio K.W, and Tang C.I (1997), A systematic study of cement/ PFA chemical stabilization/solidification process for the treatment of heavy metal waste. Waste management and research. Vol.19, No.4, Sridhan A and Sivapullaiah P.V (2005), Minicompaction test apparatus for fine grained soils, Geotechnical Testing Journal, 28 (3), Udayashankar.D.Hakari and S.C.Puranik (2012), Stabilization of Black Cotton Soils using Fly Ash, Hubballi-Dharwad Municipal Corporation Area, Karnataka, India Global Journal of Researches in Engineering Civil & Structural Engineering, Vol.12, Xu, A and Sarkar, S L (1994) Micro structural developments in high-volume fly ash cement system. Journal of materials in Civil Engg., ASCE, Vol(1),