Effect of pore medium chemistry ( e.g. bioethanol fluids) on permeability and on basic geotechnical properties of clay materials.

The permeability of clay materials hosted in aquifers, in natural barriers and in soils is affected by the amount and type of clay and its physical properties like liquid limit, plastic limit, shrinkage limit, the particle size distribution, soil structure, and the physico-chemical properties of pore fluids. Most previous researches have studied the effects produced by surfactants, electrolyte solutions and hydrocarbon-based contaminants such as gasoline, diesel, combusted oils, etc. In several places around the world, (e.g., Brazil, USA, China, European Community), increasing use is being made of high-ethanol-content fuel blends. In this framework, prolonged spillages from storage tanks containing conventional and biofuels may trigger changes of hydrogeological and geotechnical properties, facilitating the migration of pollutants into the aquifer and environment. The research examines the effect of bioethanol-based fluids on the coefficient of permeability of kaolinitic/montmorillonitic clays and on other selected geotechnical parameters (e.g., plasticity characteristics, activity, specific surface, void ratio, etc.). Test results indicate that the adding of high fractions of ethanol in the gasoline deeply increases the permeability of kaolinite up to two order of magnitude with respect to water: changes are produced by the formation of clusters (flocculation) and the developing of cracks and pores identifiable at both macro and micro scale by high-resolution Scanning Electron Microscope. It is very difficult to assess the relative importance of various factors affecting the permeability and other engineering properties like strength and compressibility, since many of them are interdependent. Replacement of exchangeable sodium by calcium ions resulted in almost significant increase in the coefficient of permeability, increase in strength and decrease in compressibility at a given void ratio. Earlier investigations have shown that the increase in the thickness of diffuse double layer associated with the particle surfaces is the cause of the decrease in the permeability of highly plastic clays by constricting the flow channels, thus reducing the amount of effective pore space of water flow by mobilising a dispersed clay fabric, which increases the tortuocity factor and bringing a decrease in the permeability. The undrained strength behavior as affected by pore medium strongly dependent on either the clay percent in the soil is primarily kaolinitic or montmorillonitic. As the dielectric constant decreases both strength and permeability increases and compressibility decreases.