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Title: Coupled thermo-hydro-mechanical-chemical behaviour of MX80 bentonite in geotechnical applications
Author: Bag, Ramakrishna
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2011
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TD Environmental technology. Sanitary engineering