Modification of black cotton soils by chemical admixtures is a common method for stabilizing the swell-shrink tendency of expansive soils. Advantages of chemical stabilization are that they reduce the swell-shrink tendency of the expansive soils and also render the soils less plastic. Among the chemical stabilization methods for expansive soils, lime stabilization is most widely adopted method for improving the swell-shrink characteristics of expansive soils. Lime stabilization of clays in field is achieved by shallow mixing of lime and soil or by deep stabilization technique. Shallow stabilization involves scarifying the soil to the required depth and lime in powder or slurry form is spread and mixed with the soil using a rotovator. The use of lime as deep stabilizer has been mainly restricted to improve the engineering behaviour of soft clays Deep stabilization using lime can be divided in three main groups: lime columns, lime piles and lime slurry injection. Lime columns refer to creation of deep vertical columns of lime stabilized material. Lime piles are usually holes in the ground filled with lime. Lime slurry pressure injection, as the name suggests, involves the introduction of a lime slurry into the ground under pressure. Literature review brings out that lime stabilization of expansive clays in field is mainly performed by mixing of lime and soil up to shallow depths. The use of lime as deep stabilizer has been mainly restricted to improve the engineering behaviour of soft clays. Use of lime in deep stabilization of expansive soils however has not been given due attention. There exists a definite need to examine methods for deep stabilization of expansive soils to prevent the deeper soil layers from causing distress to the structures in response to the seasonal climatic variations. In addition, there exists a need for in-situ soil stabilization using lime in case of distressed structures founded on expansive soil deposits. The physical mixing of lime and soil in shallow stabilization method ensures efficient contact between lime and clay particles of the soil. It however has limitation in terms of application as it is only suited for stabilization of expansive soils to relatively shallow depths. Studies available have not compared the relative efficiency of the lime pile technique and lime-soil mixing method in altering the physico-chemical, index and engineering properties of expansive black cotton soils. To achieve the above objectives laboratory experiments are performed that study: 1. the efficacy of lime piles in stabilizing compacted black cotton soil specimens from Chitradurga District in Karnataka. The efficiency of lime piles in chemically stabilizing the compacted black cotton soil mass was investigated as a function of: a)amount of lime contained in the lime pile b)radial migration of lime from the central lime pile c)migration of lime as a function of soil depth 2. the relative impact of the lime pile technique and lime-soil mixing method in altering the physico-chemical, index and engineering properties of expansive black cotton soil. The organization of this thesis is as follows After the first introductory chapter, a detailed review of literature performed towards highlighting the need to examine stabilization of expansive soils using lime pile technique is brought out in Chapter 2. Chapter 3 presents a detailed experimental programme of the study. 25 mm and 75 mm diameter lime piles were installed in the compacted soil mass to study the influence of amount of lime contained in the lime pile on the soil properties. The amount of quick lime contained in the 25 mm and 75 mm lime piles corresponded to 1 % and 3 % by dry weight of the soil mass respectively. Radial and vertical migration of lime from the central lime pile was examined by sampling soil specimens at different radial distances from the central lime pile and at different depths of soil sample. At a given depth and radial distance, migration of lime was estimated by comparing the exchangeable cation composition, pH and pore salinity of the treated soil with that of the natural (untreated) black cotton soil specimen. Alterations in the soil engineering properties at a given depth and radial distance were evaluated by comparing the index properties, swell potential and unconfined compressive strength of the lime pile treated soil specimen with those of the untreated specimen. To compare the relative efficiency of lime mixing and lime pile technique in altering the swelling behaviour of black cotton soil, batches of black cotton soil specimens were treated with 1 % and 3 % quick lime on dry soil weight basis. The compacted soil-lime mixes were cured at moisture contents of 31-34 % for a period of 10 days. The physico-chemical, index and engineering properties of the 1 % lime mixed specimens are compared with those of the 25 mm lime pile treated specimens. The properties of the 3 % lime mixed soil specimens are compared with those of the 75 mm lime pile treated specimens. Chapter 4 examines the efficacy of lime piles in stabilizing compacted black cotton soil specimens from Chitradurga District in Karnataka. Experimental results showed that controlling the swell potential of deep expansive soil deposits is possible by the lime pile technique. Treatment with lime pile caused migration of dissociated calcium and hydroxyl ions into the surrounding soil mass. In case of 25 mm lime pile, the experimental setup allowed measurement of migration of lime up to three times the lime pile diameter. In case of 75 mm lime pile, the experimental setup allowed measurement of migration of lime up to 1.6 times pile diameter. In both experiments, migration of lime was also uniform through out the soil depth of 280 mm. Migration of calcium and hydroxyl ions increased the pore salinity and pH of the treated soil mass. The increase in pH caused clustering of additional exchangeable calcium ions at the negative clay particle edges. The increased pore salinity and exchangeable calcium ions reduced the diffuse ion layer thickness that in turn suppressed the plasticity index and the swell potential of the compacted expansive soil. The laboratory results hence bring out that lime pile treatment in the field can substantially reduce the swell potential of the soil at least to a radial extent of 2 to 3 times the lime pile diameter. The 75 mm lime pile contained lime content in excess of the initial consumption of lime (ICL) value of the black cotton soil - namely 2.6 %. Laboratory results showed that migration of hydroxyl ions even from the 75 mm pile could not elevate the soil pH to levels required for soil-lime pozzoIonic reactions (pH ≥12). The very low solubility of lime in water (< 1 g/litre) and the impervious nature of the black cotton soil are considered to have impeded efficient interactions between lime and soil in course of treatment of the expansive soil with lime piles. Absence of soil-lime pozzolonic reactions precluded the formation of cementation compounds in the lime pile treated soil specimens. Cementation compounds formed by the soil-lime pozzolonic reactions are responsible for the much higher strengths of lime stabilized soils. Consequently, treatment with 25 mm pile had no impact on the unconfined compressive strength of the black cotton soil. Comparatively, treatment with 75 mm lime pile slightly increased the strength of the treated soil due to increased inter-particle attraction and particle flocculation. Chapter 5 compares the relative efficiency of the lime pile technique and lime-soil mixing method in altering the physico-chemical, index and engineering properties of expansive black cotton soil. Experimental results showed that mixing of soil and lime promote stronger chemical interactions between lime released hydroxyl ions and clay particles than that achieved by diffusion of lime from a central lime pile. The more alkaline pH of the lime mixed soil specimens rendered the clay particle edges more negative. Consequently, more calcium ions were adsorbed at the clay particle edges of the lime mixed soil specimens imparting them higher exchangeable calcium contents than the lime pile treated soil specimens. Also, at 3 % lime addition, the pH of the lime-mixed soil was sufficiently high (in excess of 12) to cause dissolution of silica and alumina from the clay lattice necessary for the formation of cementation compounds. The stronger lime modification reactions plus the lime-soil pozzolonic reactions (applicable for soil treated with lime content greater than ICL value) achieved by the lime mixing technique rendered the expansive soil much less plastic, much less expansive and much stronger than the lime pile treated specimens. The results of the laboratory study hence suggest that if a choice exists in the field between conventional method of spreading-mixing-compacting of soil-lime mixes and treating the ground with lime piles, the former technique should be adopted because of its greater efficacy in stabilizing the expansive soil. Chapter 6 summarizes the findings of the study.

Total suction of unsaturated soils is contributed by matric and osmotic suctions. Matric suction arises from capillary actions in the soil structure and varies with changes in moisture content of the soil. Pore fluid osmotic suction is related to the dissolved salt content in soil water (soil water salinity) and increases with pore water salinity. Exposure of clay soils to chemical solutions (example landfill leachate, brine pond solutions) induces osmotic suction difference between soil water and the chemical reservoir. Soil water refers to the aqueous solution residing in soil pores that is chemically composed of H2O molecules and dissolved salt molecules. Osmotic suction difference between soil water and the chemical reservoir is dissipated through the following modes. Salt molecules diffuse from the chemical reservoir to the soil water and H2O molecules from soil water flows to chemical reservoir to equalize salt concentrations in the two chambers. This flow of H2O molecules is called an osmotic flow. During osmotic flow, if the clay particles behave as perfect semi-permeable membranes, only water exchanges between clay voids and the external solution in response to chemical concentration gradients. Clay particles however function as imperfect semi-permeable membranes and transfer dissolved salts in addition to water. The outward flow of H2O molecules from soil water (dilute solution chamber) to chemical reservoir (concentrated solution chamber) causes negative pore fluid pressures to develop within the compacted clay, which then leads to increase in effective stress and the consequent volume decrease is termed as osmotic induced consolidation. Conversely, diffusion of salt molecules from chemical reservoir to soil water in response to chemical concentration gradient reduces the thickness of the diffuse ion layers around the clay particles causing a decrease in the electrical repulsion forces between them. This in turn reduces the separation of the clay particles and, consequently, compresses the clay sample to a lower void ratio; the process being termed as osmotic consolidation. Tests described by researchers show that osmotic consolidation usually has a larger effect than the osmotically induced one. Review of the literature shows that most of the available theoretical and experimental analysis in literature only focuses on the behaviour of clay samples reconstituted from slurries and not on the one of compacted clays. Compacted clays are exposed to osmotic suction gradients under field situations such as landfills and brine ponds where compacted clay liners are in contact with leachate/brine solutions. Examining the impact of osmotic suction dissipation on the swell/compression behaviour of compacted clays forms the focus of the present thesis. Statement of Problem Compacted clays differ from clay samples reconstituted from slurries as they are characterized by both matric suction and osmotic suction. As a result, besides dissipating osmotic suction gradients by diffusion of salt molecules and flow of H2O molecules, compacted clays absorb salt solution in their partly saturated void spaces to dissipate matric suction and in the process may develop swelling strains. However, absorption of salt solution to dissipate matric suction and salt diffusion in response to osmotic suction difference will alter the diffuse double layer (DDL) thickness as the latter is affected by the dissolved salts concentration of soil water; alterations in DDL thickness will in turn affect the swelling behaviour of the compacted clays. The influence of alterations in DDL thickness from dissipation of matric suction and osmotic suction difference on the swelling magnitudes of compacted expansive clays exposed to salt solutions needs to be examined. The direction of salt diffusion in response to dissipation of osmotic suction difference will also impact the swelling behaviour of compacted clays exposed to osmotic suction gradients. Diffusion of salts from external reservoir to soil water (salinization path) in response to osmotic suction gradients will reduce the swell potential of the compacted expansive clay from increased dissolved salts concentration in soil water. Conversely, diffusion of salts from soil water to external reservoir (desalinization path) should facilitate the compacted clay to swell more from reduction in its dissolved salts concentration. The influence of direction of salt diffusion during dissipation of osmotic suction gradient on the swell behaviour of compacted expansive clays needs to be examined. The volumetric response of compacted clays exposed to salt solutions may be different compared with identically compacted specimens wetted with distilled water at same total vertical pressure value. As previously mentioned, exposure of compacted clays to salt solutions, besides destroying capillary bonds will alter the soil water chemistry of the compacted clay specimens from absorption of salt solution to dissipate matric suction and salt diffusion in response to osmotic suction gradients. Alterations in soil water chemistry in turn alter the swell pressures of compacted clay specimens from concomitant changes in electrical repulsion forces. If the modified swell pressure of the compacted specimen exceeds the total vertical pressure, diminished swelling strains result at the macroscopic level. Conversely, the compacted clay will experience compressive strains at the macroscopic level if the total vertical pressure exceeds the modified swell pressure of the compacted specimen. Alterations in the wetting induced volumetric response of compacted clays from modifications in swell pressure upon exposure to salt solutions needs to be examined. Earlier researchers had re-plotted the compressibility data for sodium- montmorillonite clays remolded with sodium chloride solutions using the osmotic suction of the remolding fluids as a stress state variable in a three-dimensional space. Along a plane in which osmotic pressure (π) is constant, the coefficient of volume compressibility (mv) was obtained. Along a plane in which the effective stress [(σ - uw)] is constant, the slope defined the osmotic coefficient of volume compressibility (mπ). The above concept is useful to predict the osmotic consolidation strains of clay specimens upon exposure to salinization paths at constant effective stress. Salt diffusion into soil water in response to osmotic suction gradients may alter the exchangeable cation composition of saturated clay specimens. Alterations in exchangeable cation composition alters the diffuse ion layer thickness of clay particles which in turn may impact the osmotic swelling strains developed by saturated saline clay specimens upon exposure to desalinization path and osmotic consolidation strains developed by saturated desalinated clay specimens upon exposure to salinization path. Saturated saline specimens refer to saturated clay specimens that are exposed to salinization (saturated specimens are inundated with salt solution) path. Saturated desalinated specimens are obtained by exposing saturated saline specimens to desalinization (inundated with distilled water) path. Osmotic swelling refers to the swelling strains developed by saturated saline specimens on exposure to desalinization path. These strains result from outward migration of salts in response to osmotic suction gradients. The influence of cation exchange reactions on the osmotic swelling strains developed by saturated saline clay specimens upon exposure to desalinization path and osmotic consolidation strains developed by saturated desalinated clay specimens upon exposure to salinization path needs examination. The swelling magnitudes of compacted specimens are influenced by variations in dry density, water content and consolidation pressure. However, the effect of variation in compaction dry density and water content on the osmotic swell behaviour of saturated saline specimen exposed to desalinization path and osmotic consolidation behaviour of saturated desalinated specimen exposed to salinization path is not known and needs examination. Based on the statement of the problem, the following objectives emerge: • To examine the influence of dissipation of matric suction and osmotic suction difference on the swelling behaviour of compacted expansive clays exposed to osmotic suction gradients (salinization path). • To examine the influence of direction of salt diffusion during dissipation of osmotic suction gradients on the swell behaviour of compacted expansive clays. • To examine alterations in the wetting induced volumetric strain response of compacted clays from modifications in swell pressure upon exposure to salt solutions at range of total vertical pressures. • To predict the osmotic consolidation strains of saturated clay specimens upon exposure to salinization paths at constant effective stress. • To examine the influence of cation exchange reactions on the osmotic swelling strains developed by saturated saline clay specimens upon exposure to desalinization path and osmotic consolidation strains developed by saturated desalinated clay specimens upon exposure to salinization path. • To examine, effect of variation in compaction dry density and water content on the osmotic swell behaviour of saturated saline specimen exposed to desalinization path and osmotic consolidation behaviour of saturated desalinated specimen exposed to salinization path. The organization of the thesis is as follows: After the first introductory chapter, a detailed review of literature is performed towards highlighting the need to examine the influence of dissipation of osmotic suction gradients on the swell-compression behaviour of compacted expansive clays in Chapter 2. Chapter 3 presents a detailed experimental program of the study. Chapter 4 examines the influence of dissipation of matric suction and osmotic suction difference on the swelling behaviour of compacted expansive clays exposed to salinization path. The chapter also examines the influence of direction of salt diffusion durin dissipation of osmotic suction gradients on the swell behaviour of compacted expansive clays. Black cotton soil from Karnataka State was used as the expansive clay specimen to examine these objectives. Inundating compacted expansive clay specimens with (0.1 M to 4 M) sodium chloride solutions at a total vertical pressure of 6.25 kPa in oedometer cells exposed the clay specimens to salinization paths. Measurements of changes in swelling strains, matric suction (measured by filter paper method) and pore water chemistry with time provided insight into the relative influence of matric suction and salt diffusion on the kinetics of swell. Examining the time-axial deformation behaviour of compacted specimens exposed to salinization paths in the post-primary swell region delineated the influence of osmotic suction dissipation on the volume change behaviour of compacted expansive clays. The influence of direction of salt diffusion in response to osmotic suction gradients on the swelling behaviour of compacted expansive clay was examined in the following manner. Salt diffusion from external reservoir to soil water (salinization path) was accomplished by inundating compacted clay specimens with 0.4 M and 4 M sodium chloride solutions in oedometer cells at 6.25 kPa. Salt diffusion from soil water to external reservoir (desalinization path) was accomplished by inundating salt-amended specimens with distilled water in oedometer cells at 6.25 kPa. Salt-amended specimens refer to expansive clay specimens remolded with 0.4 M/4 M sodium chloride solution at desired moisture content and compacted to the design density. Experimental results illustrated that compacted specimens dissipated matric suction by absorption of distilled water and sodium chloride solutions. The initial osmotic suction difference was dissipated by inward diffusion of salts; salt solutions absorbed to dissipate matric suction also contributed to dissipation of osmotic suction difference. The compacted clay specimens swelled on inundation with sodium chloride solutions as dissipation of matric suction and the attendant growth of diffuse ion layer repulsion dominated compacted clay behaviour exposed to salinization paths. However exposure to salinization path reduced swell magnitudes of compacted clay specimens from reductions in diffuse ion layer thickness. The time-swell plots of the compacted clay specimens exposed to salinization path categorized into initial, primary and secondary swell regions. Rates of primary swell were 5 to 21 times larger than rates of secondary swell. Experimental data suggested that primary swell develops relatively rapidly as it is linked to rate of matric suction dissipation. Secondary swell developed more slowly as it is controlled by diffusion of salts and adsorption-desorption reactions. Increase in dissolved salts concentration in soil water during primary swell occurs from salt solution absorbed in response to matric suction and salt diffused in response to osmotic suction difference. Comparatively, increase in dissolved salts concentration in soil water during secondary swell occurs from diffusion of salts in response to osmotic suction gradients. Exposure of salt-amended clays to desalinization path caused outward diffusion of salts to dissipate osmotic suction difference and absorption of distilled water to quench the matric suction of the salt-amended specimens. The salt-amended specimens developed greater swell potentials than compacted specimens inundated with distilled water owing to reduction in dissolved salt concentration of soil water and replacement of native exchangeable calcium and magnesium ions by sodium ions. The time-swell behaviour of salt-amended specimens exposed to desalinization path categorize into four regions: small initial swell region followed by large primary swell and small secondary swell regions and lastly a large tertiary swell region. Complete dissipation of matric suction coincides with end of primary swell and both processes terminate in 120-240 minutes after inundation for salt-amended specimens exposed to desalinization paths. Further, only small fraction (16 to 18 %) of possible salt extrusion occurs at the end of primary swell and bulk of salt extrusion occurs during secondary and tertiary swell. Secondary swell developed at a slower rate than primary swell, as the rate of osmotic suction dissipation during secondary swell was smaller than rate of matric suction dissipation during primary swell. Likewise, tertiary swell developed at similar or faster rate than primary swell, as rate of osmotic suction dissipation during tertiary swell is similar or quicker than rate of matric suction dissipation during primary swell for the salt-amended clays. Analysis of the laboratory results showed that greater magnitude of outward salt diffusion mobilizes larger magnitudes of secondary + tertiary swell in response to dissipation of osmotic suction difference in case of the salt-amended clay specimens. Comparison of swelling behaviour of specimens exposed to salinization and desalinization paths revealed that the direction of salt diffusion impacts their swelling behaviour. Inward salt diffusion during salinization path reduces the swell magnitude of the compacted specimens. Bulk of the swell occurs during primary swell. Outward salt diffusion during desalinization path imparts a larger swell magnitude to the salt-amended specimens in comparison to the compacted specimen inundated with distilled water. Bulk of the swell occurs during secondary + tertiary swell. Dissipation of matric suction was rapid and coincided with the end of primary swell during salinization and desalinization paths. Bulk diffusion of salts during secondary and tertiary swell was a relatively slow process. Chapter 5 examines alterations in the wetting induced volumetric response of compacted clays from modifications in swell pressure upon exposure to salt solutions at range of total vertical pressures (6.25 kPa to 200 kPa). The chapter delineates the manner in which dissipation of matric suction (arising due to unsaturated status of compacted clay) and osmotic suction difference (arising due to chemical concentration gradients between soil water and chemical reservoir) impacts the DDL repulsion pressure/swell pressure and wetting-induced volume change behaviour of compacted expansive clays as a function of total vertical pressures (6.25 kPa to 200 kPa). Alterations in the diffuse double layer repulsion pressure of compacted clays from salt diffusion are calculated based on Gouy- Chapman diffuse double theory. The diffuse double layer repulsion pressures of compacted clays exposed to salinization paths are compared with the oedometer swell pressures. The impact of modifications in swell pressure from salt diffusion on the nature of wetting-induced volumetric strains (swell/compression) experienced by the compacted expansive clay specimens exposed to salinization paths is also examined. The nature of wetting-induced volume change behaviour is analyzed in context of the total vertical pressure to swell pressure ratio of specimens exposed to salinization paths. Salinization experiments are performed in conventional oedometers with the chemical boundary conditions imposed in an “open air” fashion. In the salinization experiments, salt solutions in the oedometer reservoir were in contact with the soil water through wet porous stones. Experimental results revealed that dissipation of initial osmotic suction difference between soil water and oedometer reservoir via salt migration impacted the diffuse double layer repulsion pressure and the wetting-induced volume change behaviour of compacted clays. Osmotic suction varies directly; while, the diffuse double layer thickness inversely varies with dissolved salt concentration of soil water. Consequently, inundation with sodium chloride solutions increase the initial osmotic suction difference at the expense of the diffuse double layer repulsion pressures developed by the compacted clay specimens. Salt diffusion in response to dissipation of osmotic suction difference reduced the theoretical (DDL repulsion pressure) and experimental swell pressures of compacted clays inundated with sodium chloride solutions. The theoretical swell pressures however greatly differed from the experimental swell pressures. The total vertical pressure to modified experimental swell pressure ratio determined the nature of axial strains (swell or compression) experienced by compacted clays on exposure to osmotic suction gradients. When the total vertical pressure to modified swell pressure ratio less than unity, the compacted clay specimens experienced net swelling on inundation with sodium chloride solutions. Conversely, when the total vertical pressure to modified swell pressure ratio exceeded unity, the compacted clay experienced net compression on inundation with sodium chloride solutions. When the total vertical pressure to modified swell pressure ratio was unity, the compacted clay did experience any net axial strains on inundating with sodium chloride solution. The ingress of sodium chloride solutions in response to matric suction saturated the void spaces of the compacted specimens prior to commencement of compression. As a result, compression strains experienced by the compacted specimens on exposure to salt solutions were mainly contributed by osmotic consolidation strains. The amount of salt diffused into soil water had direct bearing on the magnitude of osmotic consolidation strains experienced by the compacted specimens at given total vertical pressure value. The time-rates of primary consolidation are approximately 20 to 100 times quicker than rates of osmotic consolidation. The much slower rates of osmotic consolidation arise, as this process is mainly diffusion controlled in comparison to primary consolidation that is mainly dependent on the soil’s permeability to water flow under load-imposed hydraulic gradients. Primary consolidation strains exceed the osmotic consolidation strains at total vertical pressures of 100 kPa and 200 kPa on exposing the compacted specimen to 1 M sodium chloride solution. The osmotic consolidation strain exceeds the primary consolidation strain on exposing the compacted specimen to 4 M sodium chloride solution at total vertical pressure of 200 kPa. Chapter 6 develops a method to predict the osmotic consolidation strains of saturated clay specimens upon exposure to salinization paths at constant effective stress, examines the influence of cation exchange reactions on the osmotic swelling strains developed by saturated saline clay specimens upon exposure to desalinization path and osmotic consolidation strains developed by saturated desalinated clay specimens upon exposure to salinization path and effect of variation in compaction dry density and water content on the osmotic swell behaviour of saturated saline specimen exposed to desalinization path and osmotic consolidation behaviour of saturated desalinated specimen exposed to salinization path Experimental results illustrated that for a given osmotic suction difference (∆π), larger osmotic consolidation strains are predicted at the lower range of consolidation pressures (25-100 kPa), than at the higher range of consolidation pressures (200-400 kPa) as physico-chemical effects dominated the deformation behaviour at the lower stresses, while; mechanical effects (frictional effects, particle interference) became important at higher range of stresses due to proximity of particles and particle groups. Comparatively, at constant consolidation pressure, the magnitudes of osmotic consolidation strains developed by the saturated clay specimens depend on the magnitude of osmotic suction difference (∆π) imposed on the specimens. The slope of the axial strain versus osmotic suction curve defined the coefficient of osmotic compressibility (mπ). Likewise, slope of the axial strain versus effective stresses plot defined the mv values for the specimens. The mπ values are 10 to 20 times smaller than the mv values indicating that the saturated clay specimens experience smaller osmotic consolidation strains from unit increase in osmotic pressure than consolidation strains from unit increase in consolidation pressure. The predicted osmotic consolidation strains were 1.9 to 2.9 times larger than the experimentally determined values. The experimental values were lower as the saturated clay specimens did not compress sufficiently enough on exposure to salinization at concerned effective stress as the well developed diffuse ion layer of the saturated clay specimen inhibited (osmotic) consolidation of the clay specimen. Ion-exchange reaction has a profound influence on the osmotic swelling developed by the saturated saline specimens and osmotic consolidation strains developed by saturated desalinated specimens upon exposure to osmotic suction gradients. Saturated saline specimens are obtained by salinization of the distilled water aturated specimen with sodium chloride solution at desired vertical stress. During salinization ion exchange occurs between sodium ions of inundating fluid and native divalent exchangeable cations of the clay surface. Upon desalinization in distilled water environment, the saturated saline specimen developed 9.2 % osmotic swelling strain at consolidation pressure of 200 kPa over period of 2560 hours. Comparatively, the unsaturated compacted specimen developed much smaller swelling strain of 0.32 % over period of 26 hours upon inundation with distilled water at consolidation pressure of 200 kPa. The 100-fold larger duration needed by saturated saline specimen to develop larger osmotic swelling strain arose from diffusion controlled outward migration of salts from soil water to distilled water reservoir. The saturated saline specimen exhibited 29-fold larger swell magnitude than the compacted clay specimen at same consolidation pressure as the combined effects of reduction in dissolved salt concentration (from outward diffusion of salts) and enhanced exchangeable sodium concentration increased the diffuse ion layer thickness around clay particles to an extent that the saline specimens swelled by 9 % at 200 kPa. Experimental results also indicated that after ion-exchange equilibrium was established, subjecting saturated saline specimens to cycles of desalinization yielded similar magnitudes of osmotic swelling strains. Likewise saturated desalinated specimen subjected to cycles of salinization yielded similar magnitudes of osmotic consolidation strains. Also the magnitudes of osmotic swelling and osmotic consolidation strains exhibited by the saturated saline and saturated desalinated specimens were of similar magnitudes. Variations in compaction density of the compacted clay specimens had bearing on the osmotic swelling developed by the saturated saline specimens and osmotic consolidation strains developed by the saturated desalinated specimens in response to dissipation of osmotic suction gradients. Desalinization caused the 1.42 Mg/m3series saturated saline specimen to experience 2 fold larger swelling strain than the 1.28 Mg/m3 series saline specimen from outward salt diffusion in response to dissipation of osmotic suction gradient. Similarly, salinization caused the 1.42 Mg/m3 series saturated desalinated specimen to experience 1.46 fold larger osmotic consolidation strain from inward salt diffusion than the 1.28 Mg/m3 desalinated specimen. The much larger swell potential exhibited by the 1.42 Mg/m3saline specimen than the 1.28 Mg/m3 series saline specimen indicates that the influence of compaction dry density persists even after saturation and alterations in exchangeable cation composition of the compacted clay specimens. Experimental results demonstrated that variations in compaction water do not have a bearing on the osmotic swelling and osmotic consolidation strains subsequently developed by the saturated saline and desalinated specimens. Chapter 7 summarizes the main findings of this study.

The enormous rate of increase in waste generation across the world is a serious threat to the future generation, if not handled properly, due to the creation of health hazards and global warming. This was awakened many engineers and researchers to find an appropriate solution for efficient management of waste. The land filling of the waste is the most widely adopted method for its disposal, whose efficiency mainly depends on the engineered barrier system in place. Though possessing many limitations, clay liner solely or along with Geo-membrane is often used to avoid ground and surface water contamination. The thickness of the liner of a given breakthrough time depends on the transport rates of the selected contaminants. To estimate the transport rate of any given contaminant, it is necessary to understand the different migration processes of contaminants through the liner material. It was observed from the literature that, the transport rate of contaminants mainly depends on Dispersion coefficient (D) and Distribution coefficient (K) which are the main contaminant transport parameters. The amount of contaminant transport through the liner system for a desired time period is thus estimated from these contaminant transport parameters using the Advection-Dispersion Equation (ADE). The unregulated open dumps are another cause of serious environmental problem, where the contaminants are free to migrate in any direction through the underground soil. The percolation rate and the accumulation of leachate increase during the rainy season, which picks up more contaminants from the waste and thus the threat of the leachate increases. The leachate normally migrates in vertical and lateral directions, causing contamination of ground and surface water resources, and hence, there is a need to estimate the transport rates of contaminants in the porous media. These transport rates are not only useful for designing barrier systems, but also useful to find a suitable remediation technique for the removal of contaminants from a contaminated site. Thus, determination of transport rate is very important in effective waste management systems. Most of the researchers have obtained the contaminant transport parameters through the column tests to simulate one dimensional flow. Often, it is a lengthy process and there is a need to find an easy and effective method of determining these parameters which can reduce the time and effort. Generally, the metallic contaminants such as Cadmium (Cd), Copper (Cu), Lead (Pb), Mercury (Hg), Nickel (Ni) and Zinc (Zn) which are most hazardous are considered for the contaminant migration studies. In the present study, the transport rates of two heavy metals Copper and Zinc through locally available Black Cotton soil and Red soil were studied. Column experiments were conducted to simulate the field conditions under two types of test conditions i.e., Constant and Decreasing source concentrations. For Black Cotton soil as the hydraulic conductivity was very less and was taking a long time for achieving complete breakthrough, the soil sectioning method was used to get the depth versus concentration. The soil sectioning method involves the determination of pore water concentration of any given contaminant in different sections of the soil column. The depth versus concentration profile can serve as the same purpose as that of complete column test after breakthrough. The column experiments can be done only up to a relative concentration (C/C0) of about 0.2 instead of 0.8 or more. The soil samples were compacted to different densities to know the effect of density on transport parameters. The Black Cotton Soil samples were compacted to 0.76-0.97 times of maximum dry density and Red Soil samples were compacted to 0.81-0.98 times of maximum dry density. The samples were compacted to lesser densities to reduce the experimentation time. The transport parameters for field densities can be determined by setting „Forecast Trend Lines‟ to the density versus dispersion coefficient and density versus distribution coefficient plots. The contaminant transport was modeled by various methods i.e., Analytical, Semi-analytical, Explicit Finite Difference and Implicit Finite Difference methods. These models can be extended to predict the contaminant migration through soil liners constructed with similar soils. During the lifetime of a landfill, it may be subjected to both constant and decreasing source concentration conditions and thus the contaminant transport parameters determined by both constant and decreasing tests will be useful to estimate the optimum thickness of soil liner. The disposal of waste solutions and sludges by industries has led to problems with the contamination of both soil and groundwater. Much research work has not been carried out in the past for the remediation of contaminated soils in India. Thus an attempt has been made to study in detail the different remediation techniques on various contaminated soils. Three heavy metal contaminated soils were studied with two remediation techniques i.e., Soil washing and immobilization. As a case study, Zinc contaminated soil was collected from Hindustan Zinc Limited located near Udaipur in Rajasthan State, India and column leach tests were conducted on this soil with different leaching solutions to study the efficiency of the soil washing technique. The leaching solutions used for removing zinc from this soil were 0.1N HCl, 0.1N EDTA, 0.1N HCl+0.1N EDTA and 0.1N FeCl3. It was found that 0.1N FeCl3 was more efficient to remove zinc from this soil. The removal efficiency was also high with 0.1N HCl+0.1N EDTA solution. The transport rates were determined by matching the theoretical elution curves with experimental elution curves. The contaminant transport for column leach tests was modeled using analytical solution based on the Leaching Mass Ratio approach. These transport rates are useful to estimate the rate of treatment as well as the amount of flushing solution required to remove Zinc knowing the area of contamination and in-situ soil conditions. One of the potential sources of soil and ground water contamination with toxic metal ions is Effluent Treatment Plant sludge (ETP Sludge). The efficiency of soil washing technique was also studied on ETP Sludge using five leaching solutions i.e., distilled water, 0.1N HCl, 0.1N EDTA, 0.1N HCl+0.1N EDTA and 0.1N FeCl3. ETP sludge was collected at a filter press, KIADB industrial area, Doddaballapur, Bangalore. The removal efficiencies of these leaching solutions for removal of different metal ions (Copper, Zinc, Iron, Nickel, Cadmium, Lead and Chromium) were studied. The highest removal efficiencies were observed with 0.1N FeCl3 and 0.1N HCl+0.1N EDTA. The transport rates of different metals were determined which will be useful to estimate the quantity of leaching solution required in the field to remediate this sludge using soil washing technique. Even though soil washing technique is more effective than immobilization, for less permeable soil with more clay content, it is not a cost effective method. In such cases immobilization technique can be used to remediate the contaminated soil. The immobilized metals will not migrate through soil to groundwater and will not give adverse environmental hazards in their treated state. In the present study, immobilization technique was studied on two materials, (i) contaminated soil from open dump and (ii) ETP Sludge. The contaminated soil was collected from an open dump located at the Bingipura dumping yard, Bangalore and was tested for the presence of heavy metal ions. The efficiency of treatment to immobilize the metals was studied with different additives. The chemical agents with which can decrease the solubility product will be effective to immobilize the metal ions. The stabilizing agents used for treating these materials were lime water, NaOH and cement. These stabilizing agents were selected after preliminary batch tests. Since most of the heavy metals in soils become less mobile with increase in pH, the lime water / NaOH was added to the soil/sludge to adjust the pH of the mixture to 7.0, 8.5 and 10.0. The cement: soil ratios used were, 1:100 (pH=6. 8), 1:50 (pH=8. 1) and 1:25 (pH=9. 8) by weight. Leaching tests were conducted on the amended soils to know the long term efficiencies of the chemical agents for immobilizing the metal ions. The work carried out in this thesis is presented in different chapters as given below: For the design of the liner system, it is necessary to know the different contaminant transport processes, the determination of their rates and modeling. For remediation of contaminated soil, it is required to find the suitable remediation technique based on the amount and type of pollutants, the type of soil and other geological conditions. The detailed information about sources of heavy metals, effects of heavy metal contamination on health and the environment, contaminant transport processes, methods of determining transport rates, and different modeling techniques for contaminant transport are explained in Chapter 1. The Background information along with the scope and objectives of this study are presented in this chapter. The extensive review of literature related to column experiments, various solutions to Advection-Dispersion Equation, and different remediation techniques to treat the contaminated soil, is also presented in this chapter. Chapter 2 gives detailed information about various materials and methods used in this study. The characteristics of soils used in the present study and preparation of different chemical solutions were explained. The experimental procedures of batch tests, column tests and soil sectioning to determine the contaminant transport parameters were given in detail. The experimental procedures that are required for assessing the efficiency of soil washing technique i.e., Batch leach tests and column leach tests were also explained. The laboratory assessment of immobilization efficiency through leaching test was explained briefly. The analytical and numerical solutions used for this study were discussed in detail. This chapter also includes a method of prediction of breakthrough curves from the incomplete column test data. The contaminant transport parameters of metal ion Copper in two locally available soils i.e., Black cotton soil and Red soil were determined by various techniques i.e., Analytical (using MATLAB v7 software), semi-analytical (using POLLUTE v7 software), Explicit Finite Difference Method with two software tools (MATLAB v7 and M.S.EXCEL 2010), Implicit Finite Difference method with three schemes (BTCS, UPWIND & CRANK NICOLSON) using two software tools (MATLAB v7 and M.S.EXCEL 2010). Modifications were done in the spreadsheet solution of non-reactive solute available from the literature to incorporate the retardation factor as the solutes used in this study are reactive in nature. These results are presented in Chapter 3. The contaminant transport parameters determined for different test conditions (constant and variable source concentrations) and for different densities of soil are reported in this chapter. Determination of transport rates corresponding to maximum dry density using trend lines and preparation of design charts to estimate the thickness of the liner are also discussed in this chapter. The contaminant transport parameters were also determined for metal ion Zinc in the same soils with the same techniques as that of Copper and the migration rates were compared for both the ions. These models and comparative results are presented Chapter 4. It was observed that with increase in density, the dispersion coefficient decreases and Distribution coefficient increases. It was also found that the dispersion coefficient of Black Cotton Soil was lower than that of Red Soil whereas the distribution coefficient of Black Cotton soil is much higher than that of Red Soil. Further, it was observed that the dispersion coefficient of Copper was less than that of Zinc whereas the distribution coefficient of Copper was higher than Zinc. The design of liner thickness, based on transport rates of Zinc is briefly discussed in this chapter. A case study has been explained for the remediation of Zinc contaminated sandy soil using soil washing technique. The undisturbed soil samples collected from four locations of waste disposal site of Hindustan Zinc Limited located near Udaipur in Rajasthan State of Western India were assessed to find the suitable leaching solution and number of pore volumes for the effective removal of Zinc from this soil. The chelates/ solvents used for this soil were 0.1N HCl, 0.1N EDTA, 0.1N HCl+0.1N EDTA and 0.1N FeCl3. The contaminant transport parameters were also determined from the column leach tests based on the Leaching Mass Ratio approach and the results are presented in Chapter 5. From the experimental study it was observed that 0.1N FeCl3 and 0.1N HCl+0.1N EDTA are the most suitable leaching solutions to treat this soil. The Chapter 6 contains the sludge analysis of an industrial ETP sludge, column leach test results of this sludge with different leaching solutions, removal efficiencies of different solutions used and the transport rates of different contaminants. The leaching solutions used for this sludge were distilled water, 0.1N HCl, 0.1N EDTA, 0.1N HCl+0.1N EDTA and 0.1N FeCl3. It was observed that 0.1N FeCl3 and 0.1N HCl+0.1N EDTA are the most suitable leaching solutions to treat this sludge. Other solutions have also removed the contaminants by more than 50%, but the number of pore volumes required to leach out the contaminants was high. The order of removal efficiencies of different solutions is presented below: 0.1N FeCl3 > 0.1N HCl + 0.1N EDTA > 0.1N EDTA > 0.1N HCl > distilled water. The transport rates of different contaminants (Cu, Zn, Cd, Fe, Ni, Pb and Cr) were determined using analytical solution and are presented in this chapter. These transport rates are useful to estimate the quantity of leaching solution required in the field to remediate the sludge using soil washing technique. A contaminated soil collected from an open dump site within Bangalore city and ETP Sludge were analyzed to know the efficiency of immobilization/ solidification technique of remediation using three chemical agents lime, NaOH and cement. The soil samples were mixed with different proportions of these chemicals to adjust the pH of the mixtures to 7.0, 8.5 and 10.0. Leaching tests were conducted on the modified soils to know the long term efficiency of these chemical agents to immobilize the contaminants and these results are discussed in Chapter7. The results showed that highest immobilization efficiencies can be achieved with lime for this contaminated soil and cement is the most suitable chemical agent to treat this sludge. The immobilization efficiencies of different stabilizing agents for various metals were studied and the results analyzed. The Chapter 8 includes the major observations and conclusions of the present research work which will be useful for Geotechnical and Geo-environmental engineers to estimate the transport rates of contaminants, to design the soil liners, to assess the efficiency of soil washing technique to remediate the contaminated soil, to estimate the quantity of leaching solution required in the field for soil washing and to find the suitable chemical agent for remediating the contaminated soil by immobilization technique.

The aim of the thesis has been to evaluate and understand the effect of caustic alkali solution of varying composition on the behaviour of expansive soil containing mixed layered minerals. Mixed layered minerals are formed of two or more kinds of inter grown layers, not physical mixtures. Illite - smectite is the most abundant and wide spread of the mixed layered clay minerals in sedimentary rocks and soils and also more common than either discrete illite or smectite. In geotechnical engineering much attention has not been paid to the behaviour of soils containing mixed layered minerals. Much less is known about the behaviour of these soils in polluted environment. Mixed layered minerals are more susceptible to environmental changes as the structural linkages between the layer minerals are weak compared to normal layered phyllosilicates. One important pollutant that can have considerable effect on the behaviour of soils is the caustic alkali contamination released from various industries. Recent studies have shown that the behaviour of even stable minerals is affected by alkali contamination. However, the effect of caustic alkali contamination on the behaviour of soils containing mixed layered minerals is not known and has been chosen for detailed study. Also to understand the mechanism of their interaction with alkali, it is necessary to study the effect of alkali solutions on the constituent clay minerals viz., montmorillonite and illite under similar conditions. To elucidate the mechanism of soil alkali interaction limited tests were conducted with simple electrolyte solution, as the alkali solution also acts as electrolyte apart from being alkaline. To confirm the mechanism of interaction, tests are also conducted on these soils with industrial spent liquor containing high caustic alkali and suspended alumina obtained from an alumina extraction plant treating bauxite with high alkali solutions at high temperatures. The results obtained in the laboratory are compared with the soil samples contaminated with leaking industrial Bayer's liquid in the field. Studies are also conducted to suggest remedial measures to control the adverse effects of alkali solutions on soil containing mixed layer minerals. The content of the thesis is broadly divide into 8 Chapters - viz., Introduction, Background and overview, Experimental program and procedures, Behaviour of soils containing mixed layer mineral illite - smectite (BCSI), Behaviour of montmorillonite and illite, Influence of Bayer's liquor and study on the field contaminated soils, Measures to control the influence of alkali contamination on BCSI and Summary and conclusions. The broad outline of these chapters is given in Chapter 1. A review of literature on the behaviour of soils containing different types of clay minerals with emphasis on mixed layer minerals has been presented in Chapter 2. The influence of different inorganic contaminants on the properties of soils in terms of their physical and chemical characteristics as well as their concentration has been summarized. The importance of changes in surface characteristics of soil particles and the changes in the thickness of diffuse double layer in altering the property of soils at low concentration of contaminants and changes in the mineralogy with high concentrated contaminants such as acids and alkalis has been highlighted. This forms the background information necessary to bring out the scope of the study. Four soils having different mineralogy have been used in this study. These soils are, black cotton soil containing predominantly mixed layer mineral illite - smectite mineral called rectorite, illite, montmorillonite (common smectite) and black cotton soil containing predominantly montmorillonite. The properties of the soils used are described in Chapter 3. Caustic alkali solutions of 1N, 4N concentration prepared in the laboratory and industrial alkali-spent liquor are used as contaminants. The spent Bayer's liquor had about 4N alkali concentration and 10% alumina in suspension. To simulate the effect of suspended alumina, two more caustic alkali solutions of 1N and 4N solutions containing 10% alumina by weight of solutions are also prepared. To isolate the effect of electrolyte solutions from that of alkali solution, two electrolyte solutions of 1N and 4N sodium chloride solutions are also used. Test procedures for conducting various tests such as pH, water adsorption characteristics, X-ray diffraction studies, SEM studies, thermal characteristics and geotechnical properties such as Atterberg limits, Oedometer tests and Shear Strength are given in this chapter. The test procedures are modified, wherever necessary, to bring out the effect of contaminants, particularly the effect of duration of interaction on the properties of soils. The source and properties of black cotton soil are presented in Chapter 4. Detailed x-diffraction studies have confirmed the presence of inter layered illite-smectite mineral viz., rectorite, which is uncommon in Indian expansive soils, and is classified as CH (Clay of high compressibility) as per ASTM soil classification. Effect of alkali and salt solutions of 1N and 4N concentration on all physico chemical and geotechnical properties are studied in this chapter. As it is known that presence of certain elements such as aluminium influence the soil alkali interaction, the effect of suspended alumina along with alkali solution has also been investigated. The effect of contaminating fluids such as 1N NaOH, 4N NaOH with and without alumina, 1N NaCl and 4N NaCl on the geotechnical properties of the soil has been studied. Mineralogical changes were observed by XRD and thermal studies in the soil treated with 4N NaOH solution and 4N NaOH + 10% alumina. The interlayer potassium of illite is released and potassium hydroxide is formed in soil treated with 4N NaOH. Swelling compounds such as sodium aluminium silicate hydroxide hydrate (SASH) has formed due to attack of 4N NaOH + 10% alumina on silica rather than on rectorite. Thus the studies clearly bring out that the rectorite present in the soil is dissociated only in the presence of strong alkali solutions of concentration of about 4N. The liquid limit of soil decreased with increase in the electrolyte concentration in the case of NaCl solutions. With 1N NaOH, the liquid limit of soil increased due to increase in the thickness of diffuse double layer due to increased pH. However, Proctor's maximum dry density increased and optimum moisture content decreased with 1N NaOH. With increase in the concentration of alkali solution to 4N, the rectorite dissociates into constituent minerals with the formation potassium hydroxide. The liquid limit of soil decreased probably due to the dominating influence of electrolyte nature of hydroxide solution over the effect of increased negative charge on clay particles due increase in the pH on the constituent minerals. Proctor's maximum dry density decreased and optimum moisture content increased with 4N NaOH. Sediment volume and oedometer free swell at seating/nominal surcharge load of 6.25 kPa of soil increased in 1N and 4N caustic alkali solutions, though by different mechanisms. The increase with 1N solution is essentially due to increased negative charges on clay mineral surface. However, the increase in swelling with 4N solution is associated with the dissociation of rectorite mineral and occurs in two distinct phases unlike in the case of 1N solution. While the first phase can be attributed to the effect of alkaline nature of the solution after reduction in its concentration due to reaction with rectorite and the consequent reduction in its electrolyte nature. The second phase is due to the swelling of the separated constituent minerals in the presence of excess of alkali and occurs after much delay. Consolidation behaviour of rectorite in 1N and 4N alkali solutions has been studied in two ways: 1). Loading without waiting for the second stage of swelling to occur, as in standard consolidation procedure and 2). Loading after completion of second stage of swelling which is occurring after considerable delay as explained earlier. Normally one would initiate loading after equilibrium is reached at the end of first stage of swelling and second stage of swelling is not suspected. As there is no second stage of swelling with 1N solutions, these two types of consolidation tests produced the same results. Abnormal rebound is observed during unloading with 4N solution in which loading cycle is initiated without waiting for second stage of swelling to complete. It is interesting to note that while the liquid limit of soil decreased with increase in the concentration of alkali solution, the swelling increased. The testing procedure and period of interaction as well as the concentration of alkali solution during the test in these two tests are different. The effects of alkali solution are more severe in case of liquid limit because of thorough mixing and consequent effective reaction during testing. Similarly, the volume changes in soil that has already reacted with 4N alkali solution when exposed to further to alkali contamination are considerably less compared to uncontaminated soil exposed to fresh contamination. The shear strength of soil treated with 4N-alkali solution has increased particularly after long period of interaction. This indicates that the soil after mineralogical changes posses good strength. Chapter 5 presents the effect of alkali and salt solutions on the physico chemical and geotechnical properties of component minerals of mixed layered illite/smectite. For this study, commercially obtained montmorillonite (bentonite), naturally occurring black soil containing montmorillonite and commercially pure illite are used. It was observed that montmorillonite alkali reactions would not produce significant mineralogical changes where as illite is dissociated into smectite with the formation of potassium silicate by the interaction of released potassium with soluble silica. This confirms that the ultimate products of rectorite with alkali solutions would be smectite and compounds of potassium. In the absence of mineralogical alterations the liquid limit of montmorillonite decreases due to suppression of diffuse double layer thickness due to dominating influence of alkali solutions on this highly active clay. However a small increase in liquid limit is observed in illite with alkali solutions. Thus the net effect of alkali on rectorite is to decrease the liquid limit with increase in alkali concentration. While the free swell and oedometer swelling of montmorillonite generally decreases with increase in the alkali concentration, they increase in illite. However, in both the minerals the swelling occurs only in one phase. Thus the second phase of swelling that has been observed in rectorite can be attributed to delayed swelling of montmorillonite that has been released by the attack of alkali on rectorite. The behaviour of black soil containing mixed layer mineral contaminated in the field and laboratory by leaking Bayer's spent liquor in an alumina extraction plant has been studied in Chapter 6. The Atterberg limits of the samples treated with liquor are reduced and sediment volume increased. Similarly the swelling at seating load in consolidation test is higher in sample compacted with water and inundated with liquor. X-ray diffraction studies showed that the mineralogical changes are similar to those occurred with 4N caustic alkali solution. The mineralogical and micro structural changes in the soil samples that are contaminated by leaked spent liquor in the field are relatively more marked. Also the behavior of highly montmorillonite clay, bentonite, has been studied contaminated with liquor in the laboratory. The study on the effect of high concentrated alkali solutions on montmorillonite can be useful to study the effect of interaction on the dissociated montmorillonite. These studies are helpful to suggest some possible remedial measures to control the adverse effect of alkali on soils. Possible Remedial schemes that can be adopted before and after contamination of the soil to control the adverse effect of alkali solutions on the black cotton soil containing mixed layered mineral are listed and their effectiveness examined in Chapter 7. The suggested remedial measures include flushing with water to dilute the effect of alkali, neutralisation with dilute hydrochloric acid, stabilisation of soil with lime and calcium chloride and use of impervious membrane to separate the foundation soil from alkali solution. The effectiveness of different measures as well as the method of their application has been described. Efforts are made to understand the mechanism of remedial action. Consolidation tests conducted on soil contaminated with 4N alkali solution and inundated with water showed increased swelling due to dilution of the alkali concentration. Though the swelling of contaminated soil can be controlled by passing dilute hydrochloric acid (1N), the method is not advocated as it can lead to ground water contamination. Mixing the soil with solutions containing up to 5% by weight of calcium compound in water could not prevent the alkali induced heave in the long run when inundated with 4N alkali solution. This was due to dissolution of silica by the strong alkali solutions and formation of swelling compounds such as sodium aluminium silicate hydroxide hydrate (SASH). The formation of sodium aluminates occurred only when the alkali solution contained alumina or soil contained calcium compounds. There are no significant variations in the effects of calcium chloride or calcium hydroxide on contaminated soil. Replacing the foundation soil with soil thoroughly contaminated with 4N alkali solutions and controlling the migration of contaminants into the foundation soil using high-density polyethylene (HDPE) geosynthetic membranes can be an effective measure to control the heaving in alkali contaminated foundation soil containing interstratified illite – smectite. Summary and the major conclusions of the thesis are presented in Chapter 8.

Sub-surface soil pollution by various processes with high concentration of contaminants can significantly alter geotechnical properties of soils causing unexpected failures of structures founded on them. The changes can occur due to alteration in soil water interaction processes and/or by intense chemical interactions leading to mineralogical and microstructural changes. Behaviour of soil upon contamination with alkali pollutant is one of the major concerns faced by the geotechnical researchers in recent years. In the present study an attempt has been made to understand the role of mineralogical and morphological changes on the volume change (swelling and compressibility) behaviour of soils by prolonged interaction with caustic alkali pollutant. Based on the results it has been proposed to develop remedial measures to nullify and/or control the detrimental effects. A comprehensive experimental program has been planned to achieve these objectives. The experimental investigations carried out and results obtained are presented in eight chapters as follows. The broad outline of thesis is given in Chapter 1. A detailed review of literature on the type of phyllosilicate minerals present in various soils is presented in Chapter 2 with a view to select most common soils for the study. Various sources of contaminants and their effect on the properties of soils have been summarised. Present understanding on the mechanisms leading to changes in the soil properties has been elucidated. The occurrence of alkali contamination has been reviewed in this chapter which enabled to select the ranges of alkali concentration for the study. Based on the review of various methods employed to improve the soil behaviour, the use of salt solutions such as potassium chloride (KCl) and magnesium chloride (MgClB2B) and pozzolanic fly ash has been considered to counteract the alkali effects. Based on this detailed survey, the scope of the present investigation has been elaborated at the end of the chapter. Chapter 3 presents different materials used and various methods adapted in the current study. Three soils having different mineralogy have been used in this study to bring out the effect of alkali solutions on their volume change behaviour. While two soils were classified as CH, the third one was of CL. The CH soils used in this study are called Black Cotton Soils in India. One soil contained predominantly mixed layer illite-smectite mineral (BCS I) and the other contained predominantly montmorillonite mineral (BCS M). The locally available CL soil used is referred as red earth (RE) whose predominant mineral is kaolinite. Alkali solutions of concentration ranging from 1N to 4N are prepared using sodium hydroxide pellets (NaOH). Slat solutions viz. potassium chloride and magnesium chloride and pozzolanic fly ash obtained from Neyveli thermal power plant (NFA) are used as additives. Procedures to determine the geotechnical properties of the soils such as Atterberg limits, specific gravity, grain size distribution and compaction characteristics are given in this chapter. Procedures for identifying the mineral and microstructure of the soils such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) are also presented in this chapter. Standard oedometer tests with fixed ring apparatus were performed to study the volume change behaviour of soils under various conditions. Volume change behaviour of soils in the presence of alkali solutions is presented in Chapter 4. In order to assess the effect of alkali solution on the volume change behaviour of soils it is necessary to study their behaviour in water. Relatively very high swell was observed in BCS M, whereas the swell in RE and BCS I soil specimens was very low and moderate respectively. Adsorption of water to form diffuse double layer near the negative surface of clay mineral particles leads to swelling in soils. The thickness of the double layer depends on the cation exchange capacity of soil. Higher cation exchange capacity leads to development of higher thickness of double layer thereby inducing swell. The higher is the swell the higher would be the compression. The effect of different concentrations (1N, 2N and 4N) of alkali solutions on volume change behaviour of three types of soil is presented in this chapter. All the three soils studied, irrespective of their mineralogical composition, exhibited high swell when contaminated with alkali solution compared to water. However, the extent and nature of swell varied both with the type of mineral present in the soil and concentration of sodium hydroxide solution. The swell in BCS I increases with increase in the concentration of the alkali solution. In 1N alkali solution the high swell occurred is due to the breaking up of interstratified mineral into constituent minerals initiated by the leaching of potassium from soil due to high pH. In 2N and 4N alkali solutions, the observed high swell occurs in two stages: the first stage of swelling is due to breaking up of interstratified mineral into constituent minerals initiated by the leaching of potassium from soil due to high pH, and the second stage of swelling is due to the formation of new minerals (Zeolite P in case of 2N NaOH and Sodalite in case of 4N NaOH). The nature of swell is influenced by the formation of minerals depending on the concentration of alkali solution. Thus the studies clearly indicate that the swelling is due to the release of potassium from soil at higher pH and due to mineralogical changes depending upon the concentration of alkali solution. Confirmative tests were conducted to support the release of potassium during first stage of swelling and mineralogical alteration after second stage of swelling. The high swell in BCS M becomes higher in 1N alkali solution. The increased swell in the soil with 1N alkali solution is due to increase in the ion exchange capacity of soil at higher pH. The swell which is very high with 1N alkali solution decreases with 2N alkali solution. With increase in concentration of alkali solution to 2N, the increase in the negative charges due to alkalinity becomes less and the swell decreases due to dominant influence of electrolyte effect. With increase in the concentration of alkali solution to 4N, both these influences become less and the amount of swell remains the same. Significant increase in the amount of swell is observed with alkali solution even in non-swelling red earth. The nature of swell as well as the formation of minerals is not altered by the change in the concentration of alkali solution. At any concentrations of alkali solution the observed swell is noticed in two stages – very small first stage of swell due to lower ion exchange capacity and considerable second stage of swell due to the formation of new mineral (Sodalite) with any concentration of alkali solution. It has been observed that the normal hyperbolic swell – compression relationship does not apply for the alkali contaminated soils. The higher swell does not result in higher compression, as the swollen soil remains fairly incompressible. Analysis of the results and detailed studies on micro-structure and mineralogy of soils bring out mechanism of alkali effects. Comparing the swell behaviour of soils with alkali solutions brings out the relative importance of various mechanisms proposed for induced heave. The effect of salt solutions used viz., potassium chloride and magnesium chloride to restrict the influence of alkali solution on the volume change behaviour of BCS I is presented in Chapter 5. These salts react with alkali solution to form partly soluble potassium hydroxide (KOH) and sparingly soluble magnesium hydroxide (Mg(OH)B2B) respectively. Presence of ionic potassium can bring out potassium linkages, by bridging potassium ion between the unit layers of expansive minerals reducing the swell. Magnesium ions can restrict swell, by replacing the monovalent exchangeable ions present in soil and/or by formation of magnesium hydroxide which is a weak cementing agent. The effect of potassium hydroxide on the volume change behaviour of soil has been studied and the results clearly indicate that fixation of potassium is facilitated by high pH of KOH solution. Addition of potassium chloride has partially controlled the alkali induced heave in soil. Of the two stages of swelling observed in soil in the presence of 4N alkali solution, only the first phase of swelling is reduced which may be due to electrolyte effect and/or due to fixation of potassium. The second phase of swelling that occurs in soil due to mineralogical changes can not be controlled with the use of potassium chloride. Addition of magnesium chloride salt solution also reduced the effect of alkali solution mostly due to suppression of thickness of diffuse double layer that develops near clay surface. The nature of reduction in the swell of alkali solution during the two stages by magnesium chloride is similar to that of potassium chloride. The partial reduction in swell of soil in the presence of salt solutions leads to reduction in the compressibility of soil. Detailed data and analysis, presented in this chapter, bring out the role of microstructure and mineralogy on soil behaviour. The abnormal volume changes due to mineralogical changes affected by high concentration of sodium hydroxide could not be controlled with salt solutions, attempts are made to utilize fly ash to control the alkali induced heave. The pozzolanic compounds produced by hydration of compounds presented and/or produced by lime silica reactions can bind the soil particles controlling the swelling. The results on the effectiveness of fly ash on BCS I soil are presented in Chapter 6. The physical and chemical properties of fly ash along with the mineralogical composition and the microstructure of the fly ash are also presented in this chapter. Before studying the effect of fly ash to control the volume change behaviour of soils in presence of alkali solutions, the effect of alkali solutions on the volume change behaviour of fly ash itself has been studied. The results showed no noticeable changes in swell and compressibility of fly ash, encouraging its use for controlling the alkali induced swell. The ability of different percentages (10%, 20% and 50%) of fly ash to control alkali induced volume changes in soil with varying concentrations of alkali solutions, viz., 1N, 2N and 4N has been studied. The results indicate that the addition of fly ash effectively reduces alkali induced swell in BCS I. The effectiveness of fly ash increases with increase in its content. The reduction in swelling of soil is partially due to replacement of soil with fly ash and mainly due to cementation of soil particles by pozzolanic compounds produced. More than 25% of fly ash is generally required to significantly reduce the swell in alkali solutions. The reduction in swell with addition of fly ash also leads to lower compressibility of soil. The role of microstructure and mineralogy in controlling the volume change behaviour are also presented in this chapter. The effectiveness of fly ash in controlling the volume changes in RE and BCS M due to alkali solutions are studied in Chapter 7. The addition of fly ash completely eliminates the swelling in both the soils. The reduction in swelling up on addition of fly ash is essentially due to efficient binding of particles by pozzolanic reaction compounds. Addition of even 10% of fly ash is sufficient in completely arresting the swelling of RE and BCS M by alkali solution. Detailed data and analysis of the results to bring out the role of microstructure and mineralogy on the behaviour of soils are presented. It is clear that relatively higher amounts of fly ash is required to control the alkali induced heave in BCS I than in other soils at higher concentrations of alkali solution. The major conclusions from the study are presented in Chapter 8. The thesis demonstrates that alkali contamination alters mineralogy and morphology of soils affecting the volume change behaviour significantly. The study also brings out that fly ash can control the undesirable swell that occurs in most types of soils by cementing the soil particles to resist swelling. Though the amount of fly ash required to control the alkali induced heave varies, 25% of fly ash is often sufficient.