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A series of injection tests with compacted Gaomiaozi bentonite of varying dry density were performed using an innovative testing apparatus for hydraulic fracturing visualization, with emphases placed on fracturing macro-morphological dynamics and micro-mechanical mechanisms. Results showed that fracturing dynamics mainly consisted of three stages, namely the hydrating stage, the cracking stage, and the fracturing stage. As dry density increased, the circular hydration zone expanded, the cracking network became more intricate, and the fracturing pattern transitioned from long and clear to short and fuzzy. Breakthrough time and breakthrough pressure increased exponentially with increasing dry density, while injection rate decreased exponentially and breakthrough water volume increased linearly. The fracturing mechanism in compacted bentonite was turning from tensile to shear mode with increasing dry density. By comparing the shear strength with the sum of swelling pressure and tensile strength, a simple criterion for determining the limit state between tensile and shear modes was established, which can be popularized in other soils without swelling potential but under different confining pressures. Finally, a novel calculation method for breakthrough pressure was proposed based on cavity expansion theory, taking into account swelling effects and fracturing failure modes. [ABSTRACT FROM AUTHOR]

The swelling pressure of compacted bentonite under complex thermo-chemical conditions is a critical safety parameter of the engineering barrier system for a deep geological repository. To investigate the cyclic thermal and saline effects on the swelling pressure of densely compacted Gaomiaozi (GMZ) bentonite, a series of constant-volume swelling pressure tests were performed using deionized water and NaCl solutions. At 20 °C and 60 °C, the multi-step salinization-desalinization process generated a hysteresis of the swelling pressure, and multiple saline cycles led to its diminishing attenuation. In the framework of the double structure theory, the observed hysteresis of the swelling pressure was ascribed to the irreversible collapse of inter-aggregate pores under osmotic pressure, while the gradual attenuation was attributed to the further homogenization of the clay structure. Soaking tests were also conducted at various thermal and saline conditions to provide evidence for the cyclic saline effects. No salt precipitation was identified due to the negligible changes in the total suctions of the specimens after saline cycles conducted at 20 °C and 60 °C. The membrane effect (resistance of compacted clay to salt migration) was considered, and its degree reflected the changes in the osmotic suctions of the specimens occurred after the soaking tests. The resistance of compacted GMZ bentonite to salt migration was relatively strong during wetting with low saline solutions (0–0.5 mol/L) at 20 °C, and became weak during wetting with high saline solutions (1.0–2.0 mol/L) or at 60 °C. The variation of the swelling pressure during thermal cycling depended on the maximum temperature and concentration of the presaturation solution. By assuming the validity of the membrane effect at the maximum temperatures (valid at 40 °C and invalid at 60 °C), the inconsistent changes in the swelling pressure during thermal cycling were explained by considering the incomplete initial saturation and role of thermal osmosis. However, further verification of the proposed hypothesis by X-ray diffraction and membrane testing at elevated temperatures is required. • Collapse of inter-aggregate pores attenuates swelling pressure during saline cycling. • No salt storage is observed after a saline cycle. • Maximum temperature and salt content affect swelling pressure during thermal cycling. • Degradation of membrane resistance produces inconsistent swelling pressure trends. [ABSTRACT FROM AUTHOR]

Compacted bentonite has a dual-porosity structure characterized by intra- and inter-aggregate pores. The density of the water preserved in these two types of pores differs, and the interactions between these two types of pores lead to a complicated water retention behavior. In this study, this issue was addressed by establishing a dual-porosity water retention curve (WRC) based on the porewater density and pore structure evolution. The water retention behavior of densely compacted Gaomiaozi bentonite was tested. WRCs at three dry densities were obtained and unified using the water ratio e w (void ratio times the degree of saturation, eS r) as a state variable. Subsequently, mercury intrusion porosimetry (MIP) tests of samples taken from the specimens after the water retention tests were conducted. The intra- and inter-aggregate void ratios were determined based on the results of the MIP tests, and the pore structure evolution was described in terms of the intra-aggregate void ratio (e m). The evolution of the microstructure state variable ξ m (e m / e) along the confined wetting path and its boundary state after water saturation were also determined. Moreover, water contents of the intra- and inter-aggregate pores were differentiated using the boundary water contents at which the intra-aggregate pores reached saturation, and the degrees of saturation of the intra- and inter-aggregate pores were calculated based on their porewater density. Finally, van Genuchten type WRCs were introduced for the intra- and inter-aggregate pores, and the overall WRC was derived by integrating ξ m. The WRCs based on the porewater density and pore structure evolution are in better agreement with the testing data than those directly fitted from the experimental calculations, especially in the transition zone where the intra-aggregate pores approach water saturation. This dual-porosity WRC would help in modeling the hydromechanical behaviors of compacted bentonite in deep geological repositories for the disposal of high-level radioactive waste. • Density-dependent WRC was established and unified by using water ratio. • Pore structure evolution was characterized by using microstructural state variable. • Dual-porosity WRC was established based on pore structure evolution. [ABSTRACT FROM AUTHOR]

The safety assessment for the long-term operation of a deep repository is an essential work for the engineering of geological disposal for high-level radioactive waste (HLW). Bentonite selected as a barrier/backfill material in repositories, can be exposed to the chemical effects of alkaline site water owing to cement degradation, which poses a great threat to the long-term safety of the repository. This work is focusing on the swelling property of bentonite with alkaline pore water in order to investigate the evolution of vertical strain of compacted Gaomiaozi bentonite under the simulated long-term environments of Chinese HLW repository. Fourier transform infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy tests were utilized to analyze the bentonite after it reacted with the pore water solutions. Results reveal that the swelling strain is significantly influenced by the composition and concentration of the solutions, and that swelling time decreases as temperature increases. In the secondary swelling stage of the bentonite hydrated by solutions containing K+, the swelling strain decays because K ions enter the montmorillonite crystal layer and generate low-swelling or non-swelling minerals. The solutions (particularly high alkaline solutions) containing K ions strongly decrease the swelling performance of bentonite, suggesting that alkaline cement water can degrade the swelling property of the buffer material in a high-level radioactive waste (HLW) disposal system. • The chemical of solutions has obvious influence on the swelling strain of bentonite. • In the secondary swelling stage, the swelling strain of bentonite with YCW decays. • Montomorillonite dissolutes and generates low-swelling minerals in K- solutions. • The swelling property of bentonite was irreversibly reduced by the YCW solution. [ABSTRACT FROM AUTHOR]

Knowledge of the mechanical properties of compacted bentonite is essential to assessing the performance of an engineered barrier system in a high-level radioactive waste repository. In this study, direct shear tests and uniaxial compression tests were conducted on compacted Gaomiaozi bentonite with a dry density of 1.7 g/cm3 to investigate its stiffness and strength characteristics at various initial suction levels. Specimens were found to exhibit brittle failure within an initial suction range of 11–223 MPa. The peak shear strength and shear modulus were found to increase with increasing initial suction and to begin to decrease at an initial suction of 126 MPa. The residual shear strength was found to increase with increasing initial suction within 11–223 MPa. The uniaxial compressive strength and elastic modulus were found to increase with increasing initial suction and to begin to decrease at an initial suction of 62 MPa. Models were developed to predict the stiffness and strength of the specimens as a function of the initial suction and effective stress parameter (χ). For the peak shear strength, shear modulus, uniaxial compressive strength, and elastic modulus, using the degree of saturation (S r) and effective degree of saturation (S e) in place of χ produced the same trends which diverged from the test results at initial suction levels lower than 12 MPa and higher than 126 MPa. However, using S r and S e in place of χ reasonably reproduced the residual shear strength within an initial suction range of 11–223 MPa. In contrast, using (S r)α (S r raised to an exponent α) in place of χ reproduced the test results well. For the peak shear strength, shear modulus, uniaxial compressive strength, and elastic modulus, the simulations required a value of 2.4 for the α. For the residual shear strength, the best value of α was 1.0. Based on the testing method and model approach developed in this study, the evolution of the stiffness and strength of compacted bentonite along wetting and drying paths can be further explored. • Stiffness and strength were determined via direct shear and uniaxial compression tests. • Stiffness and strength were modeled using the effective stress parameter χ. • Peak strength and stiffness modeling requires α = 2.4 when using (S r)α as the χ. • Residual shear strength modeling requires α = 1.0 when using (S r)α as the χ. [ABSTRACT FROM AUTHOR]