Mechanical and Transport Phenomena in Advanced Pollutants Containment Systems. A theoretical and experimental study

Geotechnical engineers and hydrologists have dedicated most of their efforts devoted to problems concerning environmental protection and human health since the 1970’s, by studying the interaction between soils and polluting substances. Such remarkable impetus of studies emerged mainly from the need to manage the disposal of radioactive materials produced by nuclear stations with solutions which would be able to guarantee an adequate level of safety in the long term (Daniel, 1993). Scientific community was soon called on to study industrial waste landfills and pollution of the sub-soil deriving from materials produced by chemical and petro-chemical industries: in particular, the main problem to solve was that of designing adequate pollutant barrier systems in order to limit contaminant discharge to groundwater and related migration in the subsoil (Benson, 2000). Initially, such barriers were made up by mineral type materials (drainage layers in sand and gravel, compacted clay liners, soil-bentonite and cement-bentonite cutoff-walls), since their properties for long-term use were considered sufficiently durable. Clay liners and clay caps were used respectively in order to provide isolation of waste leachate from the subsoil (in the case of liners), or to guarantee long-term control of percolation into the waste and control leachate generation (in the case of covers). The design of pollutant barrier systems has been innovatively modified since the mid-1980’s with the introduction of the so-called Geosynthetic Clay Liners (GCLs), which are defined as factory-manufactured hydraulic or gas barriers consisting of a layer of bentonite or other very low permeability material supported by geotextiles and/or geomembranes, mechanically held together by needling, stitching or chemical adhesives (Koerner and Koerner, 2010). The hydraulic resistance of conventional GCLs (i.e. which are not comprised of a geomembrane component) is attributed to the bentonite component of the GCL, which swells in the presence of water to form a tight sealing layer (Shackelford, 2007). GCLs have been utilized greatly as the lower portion of geomembrane/GCL composites in both landfill liners and final covers (e.g. Bouazza, 2002; Koerner, 2005); they have also been used in other containment applications and by themselves as single barrier systems when modified with a geofilm or polymer coating within the cover geotextiles. In most applications they have served as replacement materials for the more traditional compacted clay liners (CCLs) in cover systems or in bottom lining of waste containment facilities, since they present very low hydraulic conductivity to water and relatively low cost (Bouazza, 2002). GCLs have undergone great change since the 1990’s. Engineers had to explore new theoretical problems, such as the study of solute diffusion phenomena in soils and chemical osmosis in bentonites, since hydraulic performances of GCLs are greatly affected by the chemical composition of the environment surrounding the barrier (e.g. they can be worsen by a simple variation of the chemical and physical boundary conditions), and depend on the swelling and osmotic behaviour which bentonite, characterized by a high content in montmorillonite, may exhibit. Moreover, new specific design methodologies have been developed, whose distinctive features derive from the specific nature of the objective to reach, of the materials employed, of the boundary conditions and of the reference regulations; an important example of such new methodologies is represented by risk analysis. The research project developed during the PhD has been focused on bentonite barriers which are designed both in urban waste landfill, hazardous or radioactive wastes final disposal. The theoretical and experimental study has had the aim of studying the mechanical and chemico-osmotic behaviour of bentonite in contact with standard (i.e. de-ionized water, DW) and non standard liquids (i.e. sodium chloride solutions), acting on its state parameters, chemical composition, and boundary conditions at installation. The whole laboratory activity, comprising oedometer and chemico-osmotic tests, was carried out at “Politecnico di Torino”, in the Disaster Planning Laboratory (DIPLAB) of the Department of Structural, Geotechnical and Building Engineering (DISEG). The contents of the 3-year PhD research are reported in this thesis and are resumed in the short summary reported below: Chapter 1 – Bentonite barriers This chapter is aimed at introducing the main topics concerning the mineralogical, chemical and physical description of sodium bentonite; swelling and osmotic phenomena are also presented. Particular attention is devoted to the engineering properties which can be evaluated in order to characterize bentonite, in particular of Geosynthetic Clay Liners, for geoenvironmental applications: in this regard the most important characteristics of sodium bentonite which was employed for the PhD research activity are resumed. A propaedeutic study on mechanical behaviour of bentonites, referring in particular to the swelling phenomenon, is also described. Finally the main aspects and issues concerning practical application and proper use of Geosynthetic Clay Liners and Clay membrane barriers in general are introduced. Chapter 2 – Theoretical models for mechanical and osmotic behaviour of sodium bentonites The electric interaction between montmorillonite particles, which represent the main mineralogical component of bentonite, and the ions in pore solution determines macroscopic phenomena which cannot be modelled on the basis of the classical theories used to describe the movement of water and solutes through porous media. This chapter is focused on the theoretical approaches which can be adopted in order to model transport properties of bentonites and the related aspects of their mechanical behaviour. The phenomenological and physical theoretical approaches are complementary and represent a very useful tool for experimental data interpretation, in order to reduce the number of tests to be performed and simulate GCLs behaviour in applications, even in the long-term, under boundary conditions different from those adopted in laboratory. Chapter 3 – Mechanical and swelling behaviour of bentonites In the Paper included in this chapter, titled “MECHANICAL AND SWELLING BEHAVIOUR OF SODIUM BENTONITES IN EQUILIBRIUM WITH LOW MOLARITY NaCl SOLUTIONS UNDER OEDOMETRIC CONDITIONS”, a theoretical and experimental study on mechanical and swelling behaviour of tested sodium bentonite is presented. Several oedometer tests (i.e. with a conventional apparatus and a new testing device) were performed on sodium bentonite specimens in equilibrium with NaCl 0.01 M concentrated solutions in order to measure a phenomenological parameter, i.e. the swelling pressure, at different void ratios. The obtained experimental results have been interpreted on the basis of the theoretical model, by assuming that the microscopic deviations of the pore solution state variables from their average values are negligible: in such a way, it was possible to interpret the macroscopic behaviour on the basis of the physical and chemical properties of the bentonite mineralogical components, and, thus, characterize the microstructure of the material. In the last part of the chapter other oedometer tests are presented, with the aim of analysing the effect of different salt concentrations of the equilibrium solution on the mechanical and swelling behaviour of sodium bentonite. Chapter 4 – Chemico-osmotic behaviour of bentonites Membrane behaviour represents a potential benefit in engineered clay-based barriers for geoenvironmental applications, especially if such barriers consist of sodium bentonite. A theoretical and experimental study on membrane behaviour of tested sodium bentonite is presented in the Paper included in this chapter, titled “INFLUENCE OF SPECIMEN POROSITY AND SODIUM CHLORIDE PORE SOLUTION CONCENTRATION ON CHEMICO-OSMOTIC BEHAVIOUR OF SODIUM BENTONITES”. The effects of porosity on two natural sodium bentonite specimens submitted to multiple-stage chemico-osmotic tests were investigated, by evaluating two phenomenological parameters that affect transport properties of bentonite, i.e. the chemico-osmotic reflection coefficient and the osmotic effective diffusion coefficient. The experimental results were compared to literature data and interpreted on the basis of the proposed theoretical approach, under the hypothesis that the microscopic deviations of the state variables from their average values are negligible. Even in this case, it was possible to interpret the macroscopic behaviour on the basis of the physical and chemical properties of the bentonite mineralogical components. In the last paragraph of the chapter, two further chemico-osmotic tests, by means of a new testing apparatus, are presented: experimental data are interpreted through the proposed theoretical framework and compared with previous results.