Theoretical And Experimental Study On Unsaturated Soils And Bentonite-sand Mixtures

As the essential part of soil mechanics, saturated soil mechanics is applied in geotechnical engineering practice. However, unsaturated soils are widely existed in nature, with more complex behaviour. Nowadays, unsaturated soil mechanics is developing, in which shear strength theory and constitutive equations are far from perfect. Because unsaturated soil is composed of solid phase, fluid phase and gas phase, mechanical property is more complicated than that of saturated soil. The control and measurement of pore air pressure and pore water pressure in tests are very difficult. Testing data of unsaturated soils are also very shortage. Furthermore, with the use of nuclear energy and development of nuclear engineering, high-level radioactive nuclear waste disposal issues have gained increasing attention all over the world and become universal difficult problems. Research on the deep geological disposal of high-level radioactive nuclear waste is underway in all nuclear energy generation countries. In comparison, the research is at the initial stage in China, especially on bentonite-sand mixtures, which are used as the back-filling materials for high-level radioactive nuclear waste repositories. Testing data of bentonite-sand mixtures are very valuable. Thus, in this dissertation, studies are emphasized on constitutive equations, nonlinearity of shear strength and crack propagation depth of unsaturated expansive soils in unsaturated soil mechanics. Then, utilizing GDS unsaturated triaxial apparatus and pressure plate extractor, the shear strength, swelling properties and soil-water characteristic of bentonite-sand mixtures are studied carefully. The main investigations are as follows:1. The coupling parameter between micro- and macro-structural deformation of the elasto-plastic model for unsaturated expansive soils, and yield suction of unsaturated soils are studied. Research results indicate that the amount of micro-structural strain accumulates and tends to a stationary value during wetting-drying cycles. So the micro-structural volumetric strain can be divided into n stages with different micro-structural parameters. The times of wetting-drying cycles can be used to difine the value of n. Equation of the coupling parameter between micro- and macro-structural deformation is put forward, which uses test results of wetting-drying cycles well and enhances the relationship of micro- and macro-parameters. Considerting different states of pore water and different discharge ways during isotropic compression test and triaxial shrinkage test of unsaturated soils, the definition and equation of boundary suction is established. The zone, which is enclosed by the yield curves (SI and LC) and the coordinate axes, is divided into two zones: elastic and elasto-plastic, by boundary suction. And there are three stages in the suction increasing process of unsaturated soils, that is, elastic, elasto-plastic and plastic stages. An estimating formula of yield suction of unsaturated soils is suggested.2. The tensile crack propagation depth of unsaturated expansive soils is studid. A linear elastic solution of the tensile crack propagation depth is presented, which considers the contributions of effective cohesion and effective internal friction angle at the same time. Reduced coefficient of effective cohesion is definited, which considers the reduction of effective cohesion when cracks exit. Expressions for critical matrix suction at the ground surface, and the crack propagation depth with no influence of the depth of the groundwater level are derived from the equation of crack propagation depth. Relationships with influencing factors are compared, such as effective cohesion, reduced coefficient of effective cohesion, effective internal friction angle, Poisson's ratio, matrix suction at the ground surface and the depth of the groundwater level. Based on above, the influences of tensile cracks on tension zone depth are analyzed, considering three typical matrix suction profiles along the depth, that is, constant, linear and parabolic distributions.3. Shear test results on unsaturated soils indicate that there exists nonlinearity in the shear strength versus matrix suction failure envelope. The angleφ~b is variable and a function of the applied matrix suction. The nonlinear relationship of total cohesion versus matrix suction is analyzed, and an equation with nonlinear coefficient is presented. Then a new shear strength equation for unsaturated soils is derived from it. The reliability and practicability of the shear strength equations with nonlinear coefficient are verified, using four kinds of shear test data on different unsaturated soils. Results show that nonlinear coefficient is a significant parameter for unsaturated soils, just like effective cohesion and effective internal friction angle, and do not vary with matrix suction. Different values can be taken to indicate nonlinear characteristics of different unsaturated soils. When the value of nonlinear coefficient is equal to 1, the new shear strength equation has a smooth transition towards the shear strength equation presented by Fredlund et al., one special form of the new shear strength equation in this study. Utilizing GDS unsaturated triaxial apparatus, the shear strength of bentonite-sand mixtures is studied, under the condition of controlling matrix suction. Mixtures of Heishan calcium-type bentonite and Fujian standard sand in a proportion of 50:50 are used. The relationship between the shear strength and matrix suction of unsaturated bentonite-sand mixtures is analyzed. Results indicate that the shear strength equation with nonlinear coefficient can express the shear strength of bentonite-sand mixtures well.4. The soil-water characteristic curve (SWCC) represents the ability of a soil to retain water. SWCC reflects the relationship between water content or volumetric water content or degree of saturation versus suction. Utilizing the pressure plate apparatus with high air entry ceramic plate, the SWCCs of bentonite-sand mixtures between degree of saturation versus matrix suction are determined and analyzed. Three states of bentonite-sand mixtures are investigated, which are different dry density, different times of wetting-drying cycle and different proportions of mixture. Results show that for the bentonite-sand mixtures with different dry density, the logarithm of air-entry value increases linearly as dry density increases. It suggested that greater values of matrix suction are needed to reduce the values of degree of saturation, for the sake of greater values of dry density corresponding to smaller pores. For the bentonite-sand mixtures with different times of wetting-drying cycle, the SWCCs have the same shape. The air-entry values increase linearly as the times of wetting-drying cycle increase. The slope coefficients of straight-line section of the SWCCs are similar. For the bentonite-sand mixtures in different proportions of mixture, the air-entry value is influenced by relative packing ratio of pore. When bentonite absorbs water and swells, pores are packed. Greater packed density result in the greater air-entry values.5. Utilizing GDS unsaturated triaxial apparatus and consolidation apparatus, a series of one dimension and triaxial swelling tests are conducted on bentonite-sand mixtures with different dry density and different initial water content in order to investigate the swelling properties. Based on the experimental data obtained, relationships between swelling strain and time, swelling strain and axial stress, swelling strain and water-absorbing volume, axial stress and water-absorbing volume are analyzed. The results show that the curves of swelling strain versus time can be expressed by the hyperbola well. Swelling strain increases linearly as water-absorbing volume increases. Swelling strain versus logarithm of stress has a relationship of linearity, the same as the relationship of water-absorbing volume versus logarithm of stress. Swelling strain increases as water-absorbing volume increases, but decreases as stress increases. Water-absorbing volume decreases as stress increases. For a given value of stress, water-absorbing volume has a maximum value at the end of swelling process. Then, a swelling constitutive equation of the bentonite-sand mixtures is established by the mothed of multiple linear regression analysis, which reflects the influences of axial stress and water-absorbing volume on swelling strain at the same time. It can reflect the swelling properties of bentonite-sand mixtures actually.