Study On Physical Property, Water Retention And Deformation Behaviour Of Bentonite Based On The Interlamellar Hydration Model

With the development of nuclear industry, a large amount of high level radioactive waste has been produced. The deep geological disposal is regarded as the most reasonable way to save high level radioactive waste in our county. Bentonite is selected as the buffer/backfill material in the deep disposal repository for its excellently physical and mechanical property. It is necessary to study the hydro-mechanical behavior of unsaturated soils at different temperatures since non-isothermal conditions are inevitable in the practice of geotechnical engineering, e.g., unsaturated buffer/backfill materials for barriering nuclear waste will experience increasing temperature due to the dissipation of the radioactive energy.In this dissertation, based on the theory of the interlamellar hydration microscopic structure of montmorillonite proposed by Forslind, through a variety of methods and means,(i.e., filter paper method, X-ray diffraction, thermogravimetric Analysis and the swelling deformation test), the physical property and thermo-hydro-mechanical behavior of compacted Gaomiaozi(GMZ) calcium bentonite was studied, and the water retentaion and swelling deformation behavior of the bentonite are revealed at different temperatures. The main study content and conclusions are as follows:1. Suction is a key variable in predicting unsaturated soil’s hydro-mechanical responses and the filter paper method is a simple but effective method to measure the suction of unsaturated soils. This study presents the results of an experiment examining the effects of temperature on the calibration curves of Whatman No. 42 and Double Circle No. 203 filter papers and the application of these curves to the measurement of the total suction of compacted GMZ bentonite at different temperatures. The calibration curves between the suction and water content for the two filter papers were determined at temperatures of 20, 40, 60 and 80 ℃ by the vapour equilibrium technique using fifteen types of saturated salt solutions. The test results show that calibration curves of the two filter papers shift down with increasing temperature. The calibration equations for the two filter papers are proposed at temperatures of 20 ℃, 40 ℃, 60 ℃ and 80 ℃. The obtained calibration curves were used to determine the soil-water characteristic curve of GMZ calcium bentonite, a possible buffer material for the deep geological disposal of high level radioactive waste(HLRW) in Chinese repositories. It is found that the soil-water characteristic curves of the bentonite are similar using the two filter papers and shift down clearly with increasing temperature, especially above 60 ℃. However, the effect was also not significant on the soil-water characteristic of the bentonite by temperature during higher suction.2. According to theory of the interlamellar hydration microscopic structure of montmorillonite proposed by Forslind, an equation is established for predicting water content of montmorillonite at end of the hydration process. When montmorillonite absorbs only monolayer of water molecules, the water content is considered as the residual water content. The influences of the initial dry density and temperature on the residual water content of bentonites are investigated by using measured soil-water characteristic curves of four different kinds of bentonites. The results show that the residual water content of bentonite is proportional to the specific surface area of bentonite, the thickness and the density of hydrate layer of water molecules, regardless of the initial density and temperature. The comparison of measured and predicted water contents at residual shows that the formula is reasonable.3. According to theory of the interlamellar hydration microscopic structure of montmorillonite proposed by Forslind, an equation is established for predicting specific gravity of montmorillonite at end of the hydration process. The larger the specific surface area and thickness of interlamellar hydration layers of bentonite is, the greater the specific gravity value of bentonite is. The interlayer thicknesses of the Gaomiaozi calcium bentonite with different initial water contents saturated using ethylene glycol, which was determined by X-ray diffraction, are approximately the same. By comparing measured and predicted values of specific gravity of sand and bentonite using pure water and kerosene, respectively, it is found that the reagent has influences on specific gravity of bentonite. The measured specific gravity value using pure water is larger than that using kerosene, which verifies the rationality of the formula.4. To investigate the effect of temperature on highly compacted GMZ calcium bentonite speciments, the swelling deformation and swelling pressure tests and X-ray diffraction test were conducted at different temperatures. The test results show that the relationship between the montmorilonte void ratio and vertical pressure at full swelling is a straight line in semilogarithmic scale. The montmorilonte void ratio versus vertical pressure curve moves down with increasing temperature. Moreover, the influence of temperature on the final void ratio of bentonites with highly initial dry density after saturation is not significantly. At the same time, the interlayer thicknesses of the GMZ calcium bentonite at full swelling were determined by the X-ray diffraction. The water content of montmorillonite is close to the gravimetric water content of GMZ calcium bentonite measured using drying method.