Clays are used for long for the control of soil and water pollution as they are inexpensive natural materials with a high adsorption capacity for a wide range of pollutants. However their use as components in engineered waste containment systems is often limited when it comes to the control of organic contaminants as the clays are organophobic in nature. Organic modification of the natural clays, by replacing the exchangeable inorganic cations of clay with organic cations, can facilitate to overcome this limitation. On modification the clays become organophilic which can enhance their sorption capacities for organic contaminants. There are several ways by which natural clays can be modified with organic cations. The type of clay, the type of modifier, and the extent of modification play an important role in enhancing the organic sorption capacity. Sorption of organics by the organo clays depends on a large extent on the specific interactions that occur between modified clay and the organic contaminants. The interaction between the clay and the contaminants depend on the physico-chemical properties of modified clay and nature of organic contaminants. Since the properties of natural clays are likely to be altered by the modification a detailed study has been taken up to understand the physico chemical characteristics of organo clays which essentially control their organic sorption efficiency. Apart from bentonite which is widely used as a component of barrier systems, the characteristics of other types of clays on organic modification also needs to be assessed as they can also form part of the containment system frequently. Further the modification of clays is bound to bring in significant changes on their geotechnical properties which may affect their performance when used as barrier material. Only limited research has been conducted in the past on the geotechnical characteristics of organo clay. Therefore extensive studies have been carried out on the evaluation of the geotechnical characteristics of organo clays and the effect of organic modification on important geotechnical properties. Since very often inorganic and organic contaminants can occur simultaneously, admixtures of bentonite and organically modified clays needs to be employed as a part of clay barrier system. Moreover clay alone is very rarely used as component of barrier systems and significant portion of barrier material usually include non clay fraction. Hence studies have been carried out on mixtures containing different proportions of organo clay and bentonite and sand – organo clay / bentonite to evaluate their geotechnical behavior. Important geotechnical properties considered for detailed studies are swelling, compressibility and permeability. Detailed studies on the organic sorption capacities of different organically modified and unmodified clays, mixtures of bentonite and organo clays have also been conducted. The results of studies conducted are presented in 9 chapters. The organization of the thesis is as follows: Chapter 1 gives detailed background information on the sources and hazards of organic contaminants, inadequacy of conventional barriers to contain organic contaminants, the need for modification of natural clays, and the methods for organic modification of clays. Extensive review of literature has highlighted the need to study the effect of organic modification on the physico chemical and geotechnical properties of clay in different pore fluids. Organo clays were prepared using a wide range of clays viz., two types of bentonites of different regions, black cotton clay and commercially available kaolinite with a long chain organic cation. The extent of organic modification was varied by varying the amount of organic cation exchanged as function of total cation exchange capacity of the clays. Detailed physico chemical characterization of these modified and unmodified clays has been carried out with the help of different state of art techniques. The Chapter 2 brings out the effect of modification, role of type of clay and type of modifiers on the characteristics of organo clays by comparing the physico chemical characteristics of different modified and unmodified clays. The organic modification of montmorillonitc clays with long chain organic cation is found to increase their lattice spacing with the amount of modification whereas no such increase was observed on modification of kaolinitic clays even when all the exchangeable inorganic cations were replaced with the organic cations. The XRD studies revealed that the intercalated organic cations of the modified montmorillonite clays assumed mono, bi, or pseudo tri layer depending on the extent of organic modification. Irrespective of the type of clay modified or the modifier used all the organo clays tend to become e hydrophobic, and the surface area of the clays was found to decrease. A comparison of the characteristics of clays modified in laboratory with organo clay obtained commercially revealed that the organic modification was more effective for the organo clay prepared in the laboratory. As the index properties of all clays are generally correlated with their geotechnical characteristics, the effect of organic modification on the index properties of clays was studied. Chapter 3 presents the effect of organic modification on the plasticity and free swell behavior of clays. The index properties of commercially available organo clay and the unmodified clay used for its preparation were evaluated with pore fluids of different dielectric constants. Fluids of varied dielectric constants were chosen as it is one of the important characteristics to understand the behavior of clays. It was observed that the organic modification of clays reduced the plasticity of the clays in water and increased the plasticity in less polar liquids like ethanol. As the organo clays are more hydrophobic, the water holding capacity and plasticity in water is decreased to a large extent. The free swell behavior of clays in different pore fluids were assessed in terms of the modified free swell index. It was found that trend of variation of free swell index with dielectric constant for modified and unmodified clays, as in the case of plasticity is quite opposite. The swell volume of the modified clays was observed to be controlled more by surface solvation than by the development of the inter particle repulsive forces and diffused double layer. The effect of incorporating unmodified bentonite with organically modified clay on the index properties of bentonite has also been studied. The results suggested that the effect of organo clay addition to bentonite was always to reduce its plasticity and free swell in water. However in pore fluids of lower dielectric an increase in the plasticity and free swell was observed with increasing organo clay content in the mixture. This owes to the fact that organo clays can interact strongly with organic fluids, changing its fabric arrangement. As reported from literature it is well established that the swell of clays has conflicting role on the stability and permeability of clay barriers. Swelling of clays is liable to cause a reduction in hydraulic conductivity, enhance the retention times of contaminants and attribute self healing capacity to the liners. Even though extensive studies have been carried out on the swell behaviour and mechanism of swell of unmodified clays, no systematic research is reported on the effect of organic modification on swell behavior of clays especially in the presence of different pore fluids. Chapter 4 describes the results of oedometer swell tests carried out on compacted samples of modified, unmodified clays and organo clay –bentonite mixture in the presence of different pore fluids such as water, ethanol, and their mixture and carbon tetra chloride. Swelling ability of the unmodified clays was not completely suppressed even in the presence of low polar miscible organic liquids as they were molded at water contents corresponding to the optimum moisture content (OMC). The order of the swelling for the unmodified bentonites was in the order of the polarity of the pore fluids used, while the order is reversed upon organic modification of clays. The mechanism of swell in the case of organo clays in organic liquids was related to the solvation of the organic liquid by the intercalated organic cations. And unlike in the case of unmodified clays, the organo clays showed “solvent induced swelling”. Both organic modification and addition of organo clay to bentonite resulted in the suppression of the swelling of clays in water irrespective of the type of modifier or the extent of organic modification. The Chapter 5 gives a detailed account of the compressibility behavior of organically modified clays and its mixtures with bentonite when the samples were molded with water at their respective OMC and later inundated with different fluids. Significant differences were observed on the compressibility of modified and unmodified clay in different fluids. Organic modification of clays reduced their affinity to water and resulted in lowering the compressibility. However there was an increased compressibility for the organo clays when the samples were inundated with non polar liquids and the compression of the organo clay in non polar fluid was not influenced by the nature of clay nor by the type of modifier. The compressibility of the mixtures of organo clay and bentonite in non polar liquids was generally controlled by the organo clay component of the mixture. Organo clays can be recommended as additives in bentonite slurries for construction of slurry walls in order to improve the containment of organics. But the amendment should not compromise the stability and integrity of the slurry walls. Moreover the influence of addition of sorptive material like organo clay on the compressibility behavior of bentonite slurry has received little attention and needs serious consideration as the studies in the previous chapter has brought out that the compressibility of compacted bentonite reduced significantly on organic modification as well as on addition of orgno clay. The Chapter 6 deals with the compressibility behavior of slurries of unmodified bentonite, organo clay, and their mixtures molded with respective liquid limits with water and later inundated with fluids of different dielectric constants as the slurries frequently get in contact with fluids other than water during their operational life. However it was observed that the effect of polarity of the inundating liquid is masked in all the cases by the presence of large amount of initial molding water as the possible specific chemical interactions between organo clay and non polar fluids were restricted in the presence of large amount of molding water. But the slurry samples molded and inundated with non polar carbon tetra chloride showed that the organo clay samples are more compressible when molded with carbon tetrachloride. The chapter also gives a brief discussion on the effect of initial molding water content on the compressibility of organo clays and its mixtures. The compression was found to increase with increase in initial water content irrespective of the type of inundating fluid in agreement with the behavior observed in the case of unmodified clays. However the effect was less pronounced at higher applied pressures. The Chapter 7 brings out the volume change behavior of organo clay amended sand bentonite mixtures (SOB) which form potential barrier to prevent and /or remove contaminants. The compaction behavior of mixtures showed that the degree of compaction achieved was controlled mainly by the sand content and proportion of organo clay in the total fine fraction. The volume change behavior of the SOB mixtures were assessed with the help of oedometer tests conducted on mixtures compacted at OMC conditions and inundated with different fluids same as those used for the swell tests. The samples with higher sand content showed no observable swell when inundated with liquids viz., water, ethanol and their mixture as all the swollen finer particles were accommodated in the voids created by sand particles. However a high swell percentage was measured when samples with high organo clay content were inundated with carbon tetrachloride. Moreover with increased amounts of organo clay in the mix the swelling of bentonite was suppressed and the same trend continued even when the pore fluids were changed to liquids of medium polarity. The organo clays are capable of interacting strongly with non polar liquids like carbon tetra chloride, and hence an appreciable swell was noted when inundated with them especially in the case of mixtures with high organo clay content. The swell behavior of SOB mixtures with lower sand contents were controlled mostly by the interaction of the pore fluid with bentonite and organo clay, interactions between organo clay and bentonite and the polarity of the pore fluid. As the pore fluid polarity was decreased the influence of organo clay component of the mixture was more pronounced. The Chapter 8 explains the hydraulic performance of modified and unmodified clays along with that of the mixtures of organo clay with bentonite and SOB. The coefficient of permeability was calculated from the consolidation data obtained on sample molded at OMC. The permeability variations observed on changing the pore fluids were studied at each applied pressure. The hydraulic conductivity showed a decreasing trend with the increase in applied pressure for all the clays. The specific interactions of the organo clay with the pore fluids and the clay content were found to play a role in controlling the permeability. Limited tests were carried out to simulate a condition where a SOB liner is proposed as a secondary liner below a punctured geo membrane and its hydraulic performance was evaluated with diesel and water as pore fluids. The permeability coefficients with diesel as permeant were observed to decrease with increase in organo clay content of the mixture irrespective of the applied pressure where as the reverse was true when permeated with water. Thus the use of SOB as secondary liner below storage tanks so as to control the transport of contaminants leaking containments systems is established. The organic sorption efficiency of the modified and unmodified clays and the mixture were evaluated in terms of removal of total organic carbon (TOC) and reduction in chemical oxygen demand (COD) of the different leachates including municipal solid waste (MSW) leachate when treated with different types of modified and unmodified clays. All the modified clays irrespective of the type of clay or the type of modifier used showed improved organic sorption capacity. The sorption of TOC was found to follow a linear sorption mechanism in the case of organo clays and the organic contaminants were partitioned on to the organic phase attached to the organo clays. The composition, age and type of leacahte played a major role in controlling the organic sorption efficiency of organo clays in the case of MSW leachates. The studies done with different mixtures of organo clay and bentonite and SOB mixtures clearly proved that the addition of organo clay always enhanced the organic sorption efficiency of the mixtures. The results are discussed in Chapter 9. The Chapter 10 highlights the major conclusions drawn from the study. The study, apart from satisfying the research zeal on understanding the behavior of organo clays, has generated important information useful for the geo environmental engineer to arrive at appropriate design of barrier systems incorporating organically modified clay, based on the characteristics of pore fluid.