Coupled thermo-hydro-mechanical-chemical behaviour of MX80 bentonite in geotechnical applications

Due to their high swelling ability, high water retention capacity and low permeability, compacted bentonites have been considered as a key component of cover lining systems for storage of low and medium level toxic wastes and as barrier and backfilling materials for long-term safe storage of high level toxic waste in many countries. This thesis presents an experimental, theoretical and numerical study of thermo-hydromechanical- chemical behaviour of MX80 bentonite in geotechnical applications. The determination of swelling pressures of compacted bentonites is an important aspect of most bentonitebased barrier systems. Swelling pressures of bentonites are usually determined in the laboratory under constant volume conditions using oedometers. Powdered bentonites are usually compacted in stainless steel cylindrical specimen rings and tested immediately after the compaction process is completed. The swelling pressures thus measured are influenced by some post compaction residual stresses. However, bentonites in the form of pellets and bricks are prepared and used in repository conditions that, in turn, are free from any post compaction residual stresses. The influence of post compaction residual stress on swelling pressures of compacted bentonites for a range of dry density that are of interest has not been explored in the past. Such studies are of potential interest for the safe and efficient designs of toxic waste disposal repositories. Many of the waste repositories are commissioned in locations where the ground water either contains significant amount of salts or the repositories are anticipated to receive saline water from sea. Additionally, in some cases, the wastes that are disposed emit very high temperatures (e.g. spent fuel). Under these repository conditions, compacted bentonite barriers are subjected to both thermal and hydraulic loadings at opposite boundaries. A detailed study devoted to appreciate the combined influence of an elevated temperature, bulk fluid type and solute transport due to both hydraulic and thermal gradients are necessary to better the understanding of the mechanical behaviour of compacted bentonites in many practical engineering problems. Constant volume swelling pressure tests were carried out on compacted MX80 bentonite specimens in order to study the influence of post compaction residual stress, electrolyte concentration and temperature (for isothermal conditions) on the swelling pressure. The dry density of the bentonite was varied between 1.1 to 1.9 Mg/m3 to cover a wide range of compaction conditions. Theoretical assessments of swelling pressures were made using the Gouy-Chapman diffuse double layer theory and the Stern theory, as applicable to interacting clay platelet systems. Further, the experimental and the theoretical swelling pressures were compared in order to bring out the applicability of the electrical theories in assessing swelling pressures of bentonites for both compacted saturated and initially saturated slurried conditions. A series of thermal and thermo-hydraulic tests were carried out on bentonite specimens under laboratory scale settings. During the thermal tests, temperatures of 85 and 25 °C were applied at the bottom and top ends of the specimens, respectively. During the thermo-hydraulic tests, in addition to unequal temperature at opposite ends, distilled water was supplied from the top end of the specimens. The temperature and the relative humidity were monitored along predetermined depths of the specimens during both types of testing methods. The swelling pressure variations were monitored at the opposite end of the heat source. Changes in water content, dry density and concentrations of cations and anions along predetermined depths of the specimens were measured after termination of each of the tests. A thermo-hydro-mechanical finite element simulation was undertaken specifically for swelling pressures using the numerical code COMPASS (COde for Modelling PArtially Saturated Soils). Further, the experimental and the simulated results were compared both for thermal and thermo-hydraulic boundary conditions. Compacted bentonite specimens with post compaction residual stresses exhibited lesser swelling pressures as compared to their stress released counterparts. Agreements between the calculated swelling pressures from the Stern theory and the experimental swelling pressure results were found to be reasonable for compaction dry densities of less than 1.45 Mg/m3, whereas at higher dry densities, agreements between the measured swelling pressures and those calculated from the electrical theories were found to be poor. Conversely, compressibility behaviour of initially saturated slurried bentonites was found to be captured well by the electrical theories. On account of vapour flow under thermal gradients, compacted bentonite specimens exhibited swelling pressures at the opposite end of the heat source. The measured swelling pressure for the thermal gradient adopted varied between 0.5 to 1.2 MPa, whereas greater swelling pressures were noted due to an applied thermo-hydraulic gradient. Evaporation, condensation, diffusion and advection processes influenced the distribution of ions in compacted bentonite when subjected to both thermal and thermo-hydraulic gradients. The finite element code, COMPASS, enabled assessing changes in suction and swelling pressure of compacted bentonite satisfactorily under both thermal and thermo-hydraulic hydraulic gradients.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Coupled thermo-hydro-mechanical-chemical behaviour of MX80 bentonite in geotechnical applications

Due to their high swelling ability, high water retention capacity and low permeability, compacted bentonites have been considered as a key component of cover lining systems for storage of low and medium level toxic wastes and as barrier and backfilling materials for long-term safe storage of high level toxic waste in many countries. This thesis presents an experimental, theoretical and numerical study of thermo-hydromechanical- chemical behaviour of MX80 bentonite in geotechnical applications. The determination of swelling pressures of compacted bentonites is an important aspect of most bentonitebased barrier systems. Swelling pressures of bentonites are usually determined in the laboratory under constant volume conditions using oedometers. Powdered bentonites are usually compacted in stainless steel cylindrical specimen rings and tested immediately after the compaction process is completed. The swelling pressures thus measured are influenced by some post compaction residual stresses. However, bentonites in the form of pellets and bricks are prepared and used in repository conditions that, in turn, are free from any post compaction residual stresses. The influence of post compaction residual stress on swelling pressures of compacted bentonites for a range of dry density that are of interest has not been explored in the past. Such studies are of potential interest for the safe and efficient designs of toxic waste disposal repositories. Many of the waste repositories are commissioned in locations where the ground water either contains significant amount of salts or the repositories are anticipated to receive saline water from sea. Additionally, in some cases, the wastes that are disposed emit very high temperatures (e.g. spent fuel). Under these repository conditions, compacted bentonite barriers are subjected to both thermal and hydraulic loadings at opposite boundaries. A detailed study devoted to appreciate the combined influence of an elevated temperature, bulk fluid type and solute transport due to both hydraulic and thermal gradients are necessary to better the understanding of the mechanical behaviour of compacted bentonites in many practical engineering problems. Constant volume swelling pressure tests were carried out on compacted MX80 bentonite specimens in order to study the influence of post compaction residual stress, electrolyte concentration and temperature (for isothermal conditions) on the swelling pressure. The dry density of the bentonite was varied between 1.1 to 1.9 Mg/m3 to cover a wide range of compaction conditions. Theoretical assessments of swelling pressures were made using the Gouy-Chapman diffuse double layer theory and the Stern theory, as applicable to interacting clay platelet systems. Further, the experimental and the theoretical swelling pressures were compared in order to bring out the applicability of the electrical theories in assessing swelling pressures of bentonites for both compacted saturated and initially saturated slurried conditions. A series of thermal and thermo-hydraulic tests were carried out on bentonite specimens under laboratory scale settings. During the thermal tests, temperatures of 85 and 25 °C were applied at the bottom and top ends of the specimens, respectively. During the thermo-hydraulic tests, in addition to unequal temperature at opposite ends, distilled water was supplied from the top end of the specimens. The temperature and the relative humidity were monitored along predetermined depths of the specimens during both types of testing methods. The swelling pressure variations were monitored at the opposite end of the heat source. Changes in water content, dry density and concentrations of cations and anions along predetermined depths of the specimens were measured after termination of each of the tests. A thermo-hydro-mechanical finite element simulation was undertaken specifically for swelling pressures using the numerical code COMPASS (COde for Modelling PArtially Saturated Soils). Further, the experimental and the simulated results were compared both for thermal and thermo-hydraulic boundary conditions. Compacted bentonite specimens with post compaction residual stresses exhibited lesser swelling pressures as compared to their stress released counterparts. Agreements between the calculated swelling pressures from the Stern theory and the experimental swelling pressure results were found to be reasonable for compaction dry densities of less than 1.45 Mg/m3, whereas at higher dry densities, agreements between the measured swelling pressures and those calculated from the electrical theories were found to be poor. Conversely, compressibility behaviour of initially saturated slurried bentonites was found to be captured well by the electrical theories. On account of vapour flow under thermal gradients, compacted bentonite specimens exhibited swelling pressures at the opposite end of the heat source. The measured swelling pressure for the thermal gradient adopted varied between 0.5 to 1.2 MPa, whereas greater swelling pressures were noted due to an applied thermo-hydraulic gradient. Evaporation, condensation, diffusion and advection processes influenced the distribution of ions in compacted bentonite when subjected to both thermal and thermo-hydraulic gradients. The finite element code, COMPASS, enabled assessing changes in suction and swelling pressure of compacted bentonite satisfactorily under both thermal and thermo-hydraulic hydraulic gradients

A finite difference based tool for analysing ground source heat pump system

The ground heat exchangers (GHE) consist of pipes buried in the soil and are used for transferring heat between the soil and the heat exchanger pipes of the ground source heat pump (GSHP). This paper presents the development of a numerical tool for anlysing the behaviour of horizontal ground source heat pump system. The tool was developed in Visual C++ environment. Impicilit finite difference heat conduction method was employed. The numerical solution was obtained by LU factorisation. For certain heat demand in a house and for known horizontal ground loop length, the numerical tool analyses whether the available ground loop length would be suffient to supply heat energy for the life time of GSHP system without reaching subzero temperature at any time

A finite difference based tool for analysing ground source heat pump system

The ground heat exchangers (GHE) consist of pipes buried in the soil and are used for transferring heat between the soil and the heat exchanger pipes of the ground source heat pump (GSHP). This paper presents the development of a numerical tool for anlysing the behaviour of horizontal ground source heat pump system. The tool was developed in Visual C++ environment. Impicilit finite difference heat conduction method was employed. The numerical solution was obtained by LU factorisation. For certain heat demand in a house and for known horizontal ground loop length, the numerical tool analyses whether the available ground loop length would be suffient to supply heat energy for the life time of GSHP system without reaching subzero temperature at any time

Effect of pile and heat exchanger properties on total heat extraction of an energy pile - A numerical study

Geothermal energy is one of the potential energy resources to meet future energy demand keeping environmental pollution under control. This paper presents the use of geothermal energy for space heating from energy pile. An energy pile with a single U tube heat exchanger of polyethylene (PE) pipe was modeled in this study. The effect of pile and heat exchanger properties on the total heat extraction was studied by the finite element analysis using COMSOL Multiphysics. The 3D model was developed and validated based on the literature reported results of an experimental thermal performance of a borehole equipped with a single and double U tube heat exchanger. Tetrahedral elements were considered for simulation of a 3D model. The model of a single energy pile of certain dimensions with different soil layers was considered, each soil layers were associated with different temperature. The effect of various parameters such as the length of concrete pile, the diameter of concrete pile, the thickness of U pipe, the inner diameter of U pipe and velocity of fluid inside the U pipe on amount of heat extraction was studied for an energy pile equipped with a single U tube heat exchanger. It was observed that the most influential parameters in increasing the outlet temperature of the heat exchanger loop are the diameter of the concrete pile, the inner diameter of U pipe and the velocity of fluid inside the U pipe

Effect of Stern-layer on the compressibility behaviour of bentonites

The Stern theory as applicable to interacting parallel clay platelet systems was used to study the compressibility behaviour of bentonites. For a constant surface electrical potential, the distribution of the total electrical charge among the Stern-layer and the Gouy-layer was found to have significant influence on the electrical potential at the midplane between clay platelets. Consideration of the Stern-layer was found to reduce the repulsive pressure or the swelling pressure between clay platelets at large platelet spacing, whereas the repulsive pressure increased significantly when the interacting Gouy-layers were pushed aside. A far greater repulsive pressure was noted for Ca-bentonite than that occurred for Na-bentonite at a platelet distance close to 1.0 nm. Similarly, strong interaction between clay platelets was noted due to suppressed Gouy-layers when the bulk fluid concentration was increased. The repulsive pressure generated due to the overlapping of the Stern-layers was found to be sensitive to changes in the specific adsorption potential, the dielectric constant of the pore fluid in the Stern-layer, and the surface electrical potential. Comparisons of the calculated pressure–void ratio relationships from the Stern theory and the Gouy-Chapman diffuse double layer theory with the experimental consolidation test results of Na- and divalent-rich bentonites showed that, in general, the Stern theory improved the predictions of pressure–void ratio relationships, particularly for pressures greater than 100 kPa; however, strong agreements were lacking in all the cases studied

Geoenvironmental application of bentonites in underground disposal of nuclear waste: characterization and laboratory tests

In the context of underground storage of nuclear waste, this paper presents the characterization and laboratory tests on compacted MX80 bentonite. Various laboratory testing techniques were used for establishing the water retention behavior of the bentonite. Compacted bentonite specimens were hydrated with fluids of low and high salt concentrations both at ambient and an elevated temperature of 70°C. The swelling behavior of the bentonite was also studied by stepwise wetting compacted specimens at various suctions under isochoric condition. The influence of thermal and thermohydraulic gradients on the mechanical behavior of the bentonite was studied in a thermohydraulic column cell. The investigation showed that the development of swelling pressure in compacted bentonites upon a decrease in the applied suction is accompanied by a decrease in the magnitude suction stress, which gets manifested in the development of the interparticle repulsive pressure and a decrease in the effective stress. The influence of an elevated temperature and an increase in concentration of salt in the hydrating fluid was found to reduce the swelling pressure of the bentonite. Both thermal and thermohydraulic gradients were found to be manifested on the development of axial pressures. Applications of thermal and thermohydraulic gradients were found to cause redistributions of water content, dry density, degree of saturation, and suction within compacted bentonites

Effects of post-compaction residual lateral stress and electrolyte concentration on swelling pressures of a compacted bentonite

The effects of post-compaction residual lateral stress and salt concentration in the hydrating fluid on swelling pressures of compacted MX80 bentonite is brought out in this paper. In order to release the residual lateral stresses, following the static compaction process during preparation of specimens, compacted bentonite specimens were extruded from the specimen rings and then inserted back prior to testing them for swelling pressures in isochoric condition. The swelling pressure tests were carried out at several dry densities of the bentonite with distilled water and solutions of NaCl (0.1 and 1.0 M) as the hydrating fluids. With water, the test results showed that specimens that underwent extrusion and insertion processes exhibited about 10–15 % greater swelling pressures as compared to the specimens those that were compacted and tested. The influence of saline solutions was found to reduce the swelling pressure of the bentonite, but their impact was less significant at high compaction dry densities