The Paper reports the mechanisms controlling the Atterberg limits and compressibility characteristics of bentonite, homoionized with a series of monovalent, divalent and trivalent cations. An increase in cationic valency reduces the liquid limit and the compressibility of the clay material, while an increase in the hydrated ionic radius (for a constant valency) increases the liquid limit and compressibility. The size effect is more pronounced for the monovalent bentonites than for the divalent and trivalent clays. The influence of ionic size is explained qualitatively by the Stern model. In the potassium and ammonium bentonites, besides the relatively higher adsorption of cations in the Stern layer, the partial fixation of the cations in the hexagonal oxygen holes of the surface silicate layer also contributes to the marked changes in the properties studied. The consolidation rate and permeability are also significantly affected by the valency and size of the adsorbed cations. An increase in the valency of the adsorbed cations leads to quicker rates of consolidation and higher permeabilities, while, for a constant valency, an increase in the hydrated radius of the adsorbed cation results in a slower rate of consolidation and a lower permeability. Once again the size effect is more pronounced for the monovalent bentonites and is relatively small in the divalent and trivalent bentonites. The variations in the coefficient of secondary compression with cationic valency and size are similar to that noted for the compression index. L'article décrit les mécanismes qui contrôlent les limites d'Atterberg et les limites de compressibilité de la bentonite homoionisie avec une série de cations monovalents, divalents et trivalents. Une augmentation de la valence cationique réduit la limite de liquidité et la compressibilité de l'argile, tandis qu'une augmentation du rayon ionique hydraté (à valence constante) provoque un accroissement de la hmite de liquidité et de la compressibilité. L'effet dimensionnel est plus prononcé pour les bentonites monovalentes que pour les argiles divalentes et trivalentes. L'influence de la dimension ionique s'explique quahtativement par le modèle de Stern. Dans le cas des bentonites à potasse et à ammonium, outre l'adsorption relativement plus élevée des cations dans la couche de Stern, la fixation partielle des cations dans les trous hexagonaux d'oxygène de la couche de silicate superficielle contribue aussi aux changements prononcés dans les propriétés étudiées. La vitesse de consolidation et la permeabihté sont aussi affectées de façon importante par la valence et les dimensions des cations adsorbés. Une augmentation de la valence des cations adsorbés produit des vitesses plus élevées de consolidation et des permeabilités supérieures, tandis qu'une augmentation du rayon hydraté du cation adsorbé (à valence constante) entraine une moindre vitesse de consolidation et une moindre permeabilité. Encore une fois le facteur dimensionnel est plus prononcé pour les bentonites monovalentes et il reste relativement peu élevédans le cas des bentonites divalentes et trivalentes. Les variations dans le coefficient de compression avec la valence cationique et les dimensions sont semblables à celle qui a été enregistrte pour l'indice de compression.
The impact of the variation in compaction condition on the swelling and shrinkage behavior of three soils has been examined. Two natural soils, namely red soil and black cotton soil, and one artificially mixed soil sample of commercial bentonite with well-graded sand, were studied. Compaction curve for Standard Proctor conditions were plotted and four compaction conditions were selected. Experimental results showed that clay mineralogy dominates over compaction conditions in influencing the swelling and shrinkage behavior of the tested soils. Monitoring of void ratio (e)-water content (w) relations during shrinkage showed that soil specimens generally shrunk in three distinct linear stages. A small reduction in void ratio occurred on reduction in water content during the first shrinkage stage and was termed as initial shrinkage. In second stage, void ratio decreased rapidly with reduction in water content and was termed as primary shrinkage. In third and final stage, reduction in water content is accompanied by a marginal change in void ratio and it's called residual shrinkage. Irrespective of initial compaction conditions studied, the transition from primary to residual shrinkage for all the specimens occurred within a narrow range of water content (10-15%).
Groundwater is a significant water resource in India for domestic, irrigation, and industrial needs. By far the most serious natural groundwater-quality problem in India, in terms of public health, derives from high fluoride, arsenic, and iron concentrations. Hydrogeochemical investigation of fluoride contaminated groundwater samples from Kolar and Tumkur Districts in Karnataka are undertaken to understand the quality and potability of groundwater from the study area, the level of fluoride contamination, the origin and geochemical mechanisms driving the fluoride enrichment. Majority of the groundwater samples did not meet the potable water criteria as they contained excess ([1.5 mg/L) fluoride, dissolved salts ([500 mg/L) and total hardness (75-924 mg/L). Hydrogeochemical facies of the groundwater samples suggest that rock weathering and evaporation-crystallization control the groundwater composition in the study area with 50-67% of samples belonging to the Ca-HCO 3 type and the remaining falling into the mixed Ca-Na-HCO 3 or Ca-Mg-Cl type. The saturation index values indicated that the groundwater in the study area is oversaturated with respect to calcite and under-saturated with respect to fluorite. The deficiency of calcium ion concentration in the groundwater from calcite precipitation favors fluorite dissolution leading to excess fluoride concentration.
The present study examines the influence of extraneous salt addition on pore-fluid osmotic suction of a clay soil. The dependence of swell potentials of the salt-amended clay specimens on initial pore-fluid osmotic suction is also examined. The osmotic suctions predicted by Van't Hoff's equation are in excess or smaller than the values calculated from the pore water electrical conductivity, depending on whether the Van't Hoff factor in the Van't Hoff equation is included or not. Experimental results suggest that the salt-amended specimens absorbed water and swelled in response to matric suction and chemical concentration gradients on inundation with water in oedometer cells. Salt also diffused from the soil pores of salt-amended specimens to the reservoir water in oedometer cells in response to chemical concentration gradients. Reduction in effective stress from osmotic flow into the soil specimen and increase in interparticle repulsion from reduction in pore-water salt concentration rendered the total swell potentials of salt-amended specimens independent of initial pore-fluid osmotic suction. The initial pre-fluid osmotic suction does, however, significantly affect the rate of swelling.Key words: clays, chemical properties, swell potential, suction.
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