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Title: Influence Of Osmotic Suction On The Swell And Compression Behaviour Of Compacted Expansive Clays
Authors: Thyagaraj, T
Advisors: Rao, Sudhakar M
Keywords: Soil Mechanics
Clay Soils - Compactification
Osmosis
Soil Compression
Clays - Swelling Behavior
Saturated Clays
Compacted Clays
Black Cotton Soil
Clays - Osmotic Compression
Osmotic Suction
Expansive Clay
Submitted Date: Sep-2006
Abstract: 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.
URI: http://hdl.handle.net/2005/383
Appears in Collections:Civil Engineering (civil)

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