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Investigation of the swelling pressure of compacted bentonite is of great importance in evaluating the performance of the engineered barrier system in deep geological repositories for disposal of high-level radioactive waste. In this study, three methods including Cui method ( Cui et al., 2013 ), swell-consolidation method and constant-volume method were employed for determining the swelling pressure of compacted GMZ01 bentonite with infiltration of different concentrations of salt solutions. The swell-consolidation method gave higher value of swelling pressure, while the other two methods produced almost the same values. A critical void ratio was determined for separating the swelling induced by combination effects of double-layer swelling and crystalline swelling from that caused by the effect of crystalline swelling. Based on this, the salt solution effect on swelling pressure can be well explained. For specimens with a void ratio higher than this critical value, swelling pressure decreases with the increase of concentration of NaCl solution, due to the inhibition of the double-layer swelling and crystalline swelling. On the contrary, in case of void ratio near this critical value, the salt solution effects become negligibly. Conclusions reached on GMZ01 bentonite in this work are useful for determination of geological engineering parameters for construction of the Chinese deep geological repository.

Gas breakthrough is one of the key characteristics for saturated geological materials with low permeability encountered in deep geological disposal of high-level radioactive waste, shale gas extraction and CO2 capture-geological storage, etc. Although gas breakthrough phenomenon is widely investigated in previous researches, the essential mechanisms are still not clearly explained. In the present work, a new approach for determination of gas breakthrough mechanism was proposed and a series of gas injection tests were conducted for verification. According to the theoretical analysis, the capillary effect, mechanical effect, as well as the interfacial leakage can induce gas breakthrough. The capillary breakthrough indicates the displacement of liquid phase driven by the capillary effect to form continuous flow pathway across the specimen. The mechanical breakthrough indicates the mechanically induced dilatancy of flow pathways followed by a significant decrease of the capillary flow resistance in the specimen. The interfacial breakthrough indicates the failure of the sealing capacity of the lateral boundary during the gas injection test. Based on the inherent characteristics, deformation properties of the specimen, as well as the boundary conditions, relationships between the capillary/mechanical/interfacial breakthrough pressure and the confining pressure were established, which can be used to determine the triggering mechanism of the gas breakthrough. Constant-volume and constant-isotropic stress gas injection tests were subsequently conducted for verification of the proposed approach. Results showed that the mechanical breakthrough is the most likely mechanism for the low permeability specimens tested in the present work.

The design of sealing/backfill elements in high level radioactive waste disposal repository often depends on the laboratory results in terms of swelling pressure and hydraulic conductivity. In this study, a series of swelling pressure test were performed on compacted specimens of MX80 bentonite and Callovo-Oxfordian (COx) claystone mixture with different initial stress state (with and without residual stress relaxation), aiming at investigating the effects of compaction induced residual lateral stress on the swelling pressure. Two methods named “Trimming” and “Transfer” were applied for the residual stress relaxation; whereas specimens were directly compacted in the hydration ring to simulate the case without residual stress relaxation. A higher swelling pressure was obtained on the specimens directly compacted in the testing ring compared to their counterparts with residual stress relaxation. Regarding to the relaxation methods, “Transfer” is more practical for specimen preparation in the laboratory, as it allows the relaxation of the stress without changes in specimens' diameter and mass.

The technological voids, which are inevitably formed during the construction of the deep geological repository for high-level radioactive nuclear waste, have negative influence on the hydro-mechanical properties of the engineered barrier. In this study, hydration tests along with image acquisition were carried out on compacted GMZ bentonite samples with simulated technological voids to investigate the self-sealing behavior and research emphasis was placed on the cracks appearing during the hydration. Results showed that with the presence of technological voids, the self-sealing process of the hydrating sample can be visually divided into cracking and closing stages, both of which were affected by the sample preparation process, the void width, and the sample height. The hydration cracks occurred at the early phase was considered as tensile cracks induced by the failure of tensile strength against the tensile stress induced by the difference of swelling strain. At the end of hydration, though the cracks were closed with the sealing of technologicalvoids, the distributions of water content and dry density along the sample diameter were found in a decreasing and an increasing trend with the increasing distance from the technological voids, respectively. By applying the image processing method, the cracking and sealing behaviors were further studied with the reciprocal verification of digital images and grey scale profiles. A good agreement between the grey scale profiles and digital images was found and the quantifiable grey values could accurately reflect the color difference on the samples which was difficult to be distinguished by the naked eye, showing the applicability and superiority of image processing technology.

Investigation on compression behavior of bentonite is an important issue for designing the artificial barriers in a geological repository for nuclear waste disposal. However, significantly different from that of non-expansive clays, normal consolidation line of bentonite may be nonlinear in e-lnp plane. Obviously, the linear equation of normal consolidation line in conventional constitutive models could not describe nonlinear compression behavior of bentonite. In this work, compression tests were carried out on GMZ01 bentonite slurry and compacted specimens with suction-control (0, 1, 4.2, 9, 38 and 113 MPa). The mechanisms of nonlinear normal consolidation line for saturated and unsaturated bentonite were analyzed, respectively. According to the test results, an elastoplastic constitutive model framework with nonlinear normal consolidation line for describing compression behavior of bentonite was proposed. In which, a unified equation of normal consolidation line for saturated bentonite specimen related to water content was proposed. Meanwhile, a critical saturation curve was defined in a s-p space to describe the transformation from unsaturated to saturated states during the compression process of unsaturated bentonite specimens. Based on the critical saturation curve, a nonlinear equation of normal consolidation line for unsaturated bentonite was proposed. Finally, the compression test results for bentonite slurry and compacted specimens were simulated by the model newly proposed. Validation shows that good agreements are reached between the simulated results and experimental ones. Obviously, the nonlinear equation of normal consolidation line proposed in this paper could be used in development of constitutive models for describing nonlinear compression behavior of bentonite.

Investigation on influences of cyclic salinization-desalinization processes on clay is of great importance for evaluation of the behavior of engineering barrier in deep geological repository for disposal of high-level radioactive waste. In this study, one-dimensional free swelling tests were conducted on densely compacted GMZ01 bentonite specimens, which has an initial dry density of 1.7 Mg/m 3 , with cyclically infiltration of NaCl, CaCl 2 or KCl solution at different concentrations and de-ionized water. Results show that the volume change of the compacted clay could be influenced by concentrations and cation types of solutions, as well as number of salinization-desalinization cycles during the salinization-desalinization processes. The total swelling strain of GMZ01 bentonite specimens measured on the first salinization with salt solutions at a same concentration follows an order: NaCl > CaCl 2 > KCl. This observation could be explained by the influences of basal space, DDL theory and K-linkage. A cation with higher replacing power can be replaced by another cation with lower replacing capacity when it has a higher concentration in the pore solution. The concentration effect on the swelling behavior of GMZ01 bentonite was explained using the concept of osmotic suction for specimens initially infiltrated with NaCl solutions. For specimens initially infiltrated with CaCl 2 solutions, in addition to the osmotic suction effect, the distance between the unit layers of montmorillonite also played a role in explaining the swelling behavior. The swelling behavior of specimens initially infiltrated with KCl solutions could be explained by the strong K-linkage.

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.

In this work, cyclic free swelling/shrinkage tests were conducted on uniaxial compacted GMZ bentonite specimens with consideration of influences of initial dry density and suction. For conducting the cyclic swelling/shrinkage test, vapour equilibrium technique was employed for suction control and a laser displacement sensor was adopted to measure the specimen dimension. Results reveal that influence of initial dry density on water retention property depends on suction. For suctions lower than 14.2 MPa, specimen with higher initial dry density exhibits stronger water retention capacity, while for suctions higher than 38 MPa, initial dry density hardly influences water retention property. Unrecoverable swelling or shrinkage volumetric deformation is accumulated during the cyclic suction-variation processes and influenced by initial suction of the specimen. Meanwhile, GMZ bentonite specimen tested exhibits a significant anisotropy on volume change, which is more distinct for specimens with higher initial dry density, with swelling anisotropy stronger than shrinkage one. For specimen with high suctions (≥3.3 MPa), suction variation along a single wetting or drying path hardly influence this volumetric anisotropy. However, swelling anisotropy decreases with increasing suction cycles experienced and eventually equals to the shrinkage one after several suction cycles.

Metakaolin (MK), widely used for high-performance concrete admixtures, is introduced into improving the percolation behaviors of cement-stabilized soft clays, which is a key performance when analyzing the permeability and consolidation in the grounds modified by deep mixing methods etc., the underground water seepage and the migration of pollutants in waterproof curtains by grouting methods etc. To investigate MK effect on the hydraulic conductivity of cement stabilized soils, the flexible wall permeameter was developed and mercury intrusion porosimetry (MIP) tests were performed. The results show that the addition can reduce the hydraulic conductivity 10–100 times when MK content arrives at 3% to 5%, which attributes to less total porosity and pore diameter. As the cement stabilized soils not only belong to a kind of special soils in geotechnical engineering, but also belong to cement-based reinforced materials, it is further discussed whether the unique expression of the hydraulic conductivity of these porous materials (clay, cement stabilized soils, cement pastes and concretes) existed. The results present that there is a good correlation between the hydraulic conductivity, the void ratio and median throat pore diameter for each kind of materials from clays to concretes respectively, and the hydraulic conductivity qualitatively increases with the void ratio and median throat pore diameter whatever clays, cemented soils and pastes or concretes. After several attempts, the combined variable nD502 was adopted to normalize the macro and micro characterization of porosity, and the relatively unique relation depicting the hydraulic conductivity of these materials was proposed.

In deep geological repositories for spent fuel and high-level radioactive waste (SF/HLW), a great deal of gas will be produced by anaerobic corrosion of metals, radiolysis of water and microbial degradation of organic materials. Due to the extremely low permeability of saturated compacted bentonite, gas will accumulate in the space in the bentonite around the waste canister, resulting in generation of a high gas pressure nearby the waste canister. Migration of gas in the engineered barrier system can be divided into four phases according to the gas pressure level: advection and diffusion of dissolved gas, visco-capillary two-phase flow, dilatancy-controlled gas flow and gas flow along macroscopic tensile fractures. Based on the characteristics of gas flow (i.e., gas migrates along discretely distributed pathways and is influenced by the stress state) and the modified gas pressure dependent permeability method from literatures, a modified approach with consideration of stress state was established. With this modified approach, the calculated results were improved, while it cannot describe the discrete distribution of gas flow pathways. Therefore, further modification with consideration of heterogeneity was proposed. On the basis of the existing experimental results under different boundary conditions, verification and comparison of the evolutions of the gas fluxes and the degree of saturation were conducted. Results show that the proposed approach in this article can effectively simulate the performance of gas permeability and the evolution of the degree of saturation during the two-phase flow and the dilatancy-controlled flow phases.

Studies on the hydraulic behavior of GMZ bentonite have been performed since 1980s. Based on a review of the former studies, achievements on experimental and theoretic results obtained on the hydraulic aspects of compacted GMZ bentonite are presented in this paper. Results show that, for high suctions (> 4 MPa) the water retention capacity of compacted GMZ bentonite is almost independent of the constraint conditions; for low suctions ( 70 MPa).

During the long-term operation of a deep geological repository, infiltration of groundwater with different chemical compositions can affect the buffer/backfill properties of compacted bentonite. Using a newly developed apparatus, swelling pressure and permeability tests were carried out on densely compacted GMZ01 bentonite samples, which has an initial dry density of 1.70 Mg/m3, with de-ionized water as well as NaCl and CaCl2 solutions at different concentrations. Salinity effects of infiltrating solutions on swelling pressure and hydraulic conductivity of tested samples were investigated. Results obtained show that the swelling pressure of GMZ01 bentonite decreases with increasing concentration of infiltrating solutions, while the degree of the impact decreases with the increase of concentrations. Moreover, swelling pressure reaches stability more rapidly in case of high concentrations. The hydraulic conductivity of GMZ01 bentonite increases with the increase of solution concentrations. Comparison shows that the impact of NaCl solutions on the swelling pressure and hydraulic conductivity is higher than that of CaCl2 solutions at same concentrations. This may be explained by the impact of cation types on the microstructure of bentonite.

GMZ bentonite has been selected as a potential material for the construction of engineered barrier in the Chinese program of geological nuclear waste disposal, for its high montmorillonite content, high cation exchange capacity (CEC) and large specific surface etc. Studies on mineralogy and chemical composition, mechanical properties, hydraulic behavior, swelling behavior, thermal conductivity, microstructure and volume change behavior of GMZ bentonite were performed from 1980s. Based on a review of the former studies, achievements on experimental and theoretic results obtained on compacted GMZ bentonite specimens including basic properties, thermal, hydraulic and mechanical behaviors are presented in this paper. Results show the thermal conductivity of GMZ bentonite and the bentonite-based mixtures influenced by its dry density, water content, mixture of other materials and degree of saturation etc. Water retention capacity of highly-compacted GMZ bentonite decreases as the temperature increases under confined and unconfined conditions. The hysteretic behavior in the water retention curves of the compacted GMZ bentonite is not so significant at 20 or 40 °C. The unsaturated hydraulic conductivity of compacted GMZ bentonite under unconfined conditions is higher than that of under confined conditions. This is possibly induced by the difference in the mechanism of microstructural changes during hydration under different confining conditions. The compaction curves for GMZ bentonite with different dry densities are clearly step-phased. And the optimum water content for GMZ bentonite is about 15%. An exponential relationship between swelling pressure and dry density of highly-compacted GMZ bentonite was determined for the prediction of swelling pressure. Furthermore, the void ratio after swelling for unconfined sample also can be predicted using diffuse double layer (DDL) theory.