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Gas breakthrough pressure is a significant parameter for the gas exploration and safety evaluation of engineering barrier systems in the carbon dioxide storage, remediation of contaminated sites, and deep geological repository for disposal of high-level nuclear waste, etc. Test for determining gas breakthrough pressure is very difficult and time-consuming, due to the low/ultra-low conductivity of the specimen. It is also difficult to get a comprehensive and high-precision model based on limited results obtained through individual experiments, as the measurements of gas breakthrough pressure were influenced by many factors. In this study, a collected database was built that covered a lot of former test data, and then, two models were developed by the random forest (RF) algorithm and multiexpression programming (MEP) method. The MEP model constructed with explicit expressions for the gas breakthrough pressure overcame the drawbacks of common "black box" models. Meanwhile, five significant indicators were selected from ten common features using the permutation importance algorithm. The RF model was interpreted by the Shapley value and the PDP/ICE plots, while the MEP model was analyzed through the proposed explicit expression, showing strong consistence with that in former studies. Finally, robustness analysis was conducted, and stability of the proposed two models was verified.

In recent years, more and more unique volumetric behaviors, including the nonlinear compression and unloading curves in the e-lnp plane, and different types of swelling and shrinkage curves obtained from suction-controlled swelling and shrinkage tests, etc., which are significantly different from those of nonexpansive clays, have been reported on compacted bentonite. More importantly, limitations were encountered when describing the unique behavior with existing constitutive models for unsaturated expansive soils. In this study, a new concept of critical saturation state (CSS) was proposed for compacted bentonite, in which, as the saturated state was reached, the matric suction still remained nonzero in the soil. Incorporated with this concept, a constitutive model was proposed. In this model, the CSS curve was defined in the suction-isotropic stress plane, delimiting the unsaturated zone. To validate the model, the volume change tests conducted on compacted GMZ bentonite under different stress paths were simulated. Good agreement confirmed that the proposed model could well describe the volume change behavior, including the swelling on hydration, shrinkage during drying, nonlinear compression, and unloading behavior.

In-situ remediation of the Cr(Ⅵ) contaminated clayey soil remains challenging due to the low hydraulic conductivity and high ion adsorption capacity of the clays. In this study, the injection tests of ferrous sulphate (FeSO4) for a clayey soil contaminated with hexavalent chromium (Cr(Ⅵ)) were performed to explore the feasibility of FeSO4 solution for remediation of Cr(Ⅵ) contaminated soil under different injection pressures. The results show that the injection of FeSO4 solution under the pressure of 30 kPa, 70 kPa, and 100 kPa caused the hydraulic conductivities of the specimens to decrease by 36.5%, 38.7% and 32.2%, respectively. This phenomenon could be attributed to the formation of the mixed iron(Ⅲ)/chromium(Ⅲ) hydroxide, a process for which scanning electron microscopy with energy-dispersive X-ray microanalysis provided direct evidence. Moreover, the mobility capacity of Fe(Ⅱ) was stronger under high pressure (70 kPa, 100 kPa) as indicated by Fe profiles in specimens after remediation under different injection pressures. The range of iron increment at 100 kPa was 3.2–11.9 (g/kg). However, the maximum iron increment at 30 kPa was only 5 (g/kg) and the migration of Fe(Ⅱ) was limited, which was less than 2.5 cm. In addition, the injection pressure of 70 kPa was the most efficient for remediation of Cr(Ⅵ) contaminated soil under the experimental condition in present work, which the remediation efficiency was 87.43%-95.12%. As injection pressure increased, Cr(Ⅵ) leaching increased. Therefore, these experimental results can be used as a reference for remediation of chromium contaminated soil by in-situ injection.

In this study, Lianyungang (LYG) soft marine clays were long-term leached by the distilled water and synthetic NaCl solutions, respectively, and then oedometer tests with cycle loading/unloading were conducted to investigate the desalination leaching effect on its compression/swelling behaviour. Test results reveal that the stress-induced creep generates a void ratio change (Δec), whilst the long-term desalination leaching accompanying colloidal erosion further results in the aging of clays’ structure, and hence, additional void ratio change (Δel) is identified. After long-term desalination leaching process, the compression/swelling behaviour of LYG clays in the elastic loading and unloading stage is changed slightly. However, in the elastoplastic loading stage, the desalination leaching leads to the significant compression at low stress level (less than 200 kPa), followed by the slight difference at high stress level (exceeding 200 kPa). Finally, the compression/swelling behaviour induced by clay’s aging for the long-term desalination leaching process was summarized.

Compacted bentonite/sand mixture have been considered as possible sealing/backfilling material in the deep geological disposal of high-level radioactive nuclear waste. The swelling pressure of the compacted bentonite/sand mixture is of significance in the design of the geological repository, which requires good consistency between data obtained by laboratory and field measurement. In this work, a series of swelling pressure tests were performed on compacted MX80 bentonite/sand specimens prepared by methods commonly adopted in laboratory (As-compacted, Transferred) and those mimicking the real block manufacturing process (Trimmed, Inserted). Results shown that with identical dry density (especially when the dry density was larger than 1.70 Mg/m3), largest swelling pressure was obtained in specimens prepared by method Inserted, followed by method As-compacted, Transferred and Trimmed. The distinct difference between the swelling pressure could be largely attributed to the effects of residual post-compaction lateral stress. More interestingly, specimens prepared by methods Trimmed and Transferred followed a similar swelling pressure-dry density relationship. From this point of view, method Transferred without causing mass loss, change of bentonite content and possible technological voids effect was recommended in lieu of method Trimmed for specimen preparation in laboratory.

Compacted Gaomiaozi bentonite–sand mixtures are regarded as attractive buffer/backfill materials for nuclear waste deep geological disposal. When the mixture blocks are emplaced, the bentonite dry density as well as the salinity of groundwater is of primary importance for their hydro-mechanical behavior. In this study, the influences of bentonite dry density and water salinity on the swelling pressure and saturated hydraulic conductivity of such mixtures are investigated using the constant volume method. Results indicated that for mixtures having a low sand content, the swelling pressure was only associated with the bentonite dry density, while the hydraulic conductivity was influenced by the presence of sand particles. Exponential relationships were noted among the final swelling pressure, the saturated hydraulic conductivity, and bentonite dry density on the case of distilled water and salt solutions infiltrating. As the salt concentration increased, the swelling pressure and hydraulic conductivity decreased and increased, respectively, and the effects became less significant as the bentonite dry density increased. The higher swelling pressure and lower hydraulic conductivity on specimens infiltrated with calcium chloride solutions were a result of cation exchange reactions and a competition between the swelling potential change caused by the calcium ions in the interlayer of smectite and electrolyte solutions. The present work can provide a reference for the design of dry density of bentonite–sand mixtures as buffer/backfill materials for deep geological disposal.

Bentonite pellets are recognized as good buffer/backfill materials for sealing technological voids in high-level radioactive waste (HLW) repository. Compared to that of a traditional compacted bentonite block, one of the most important particularities of this material is the initially discrete pellets and the inevitable heterogeneous porosity formed, leading to a distinctive water retention behaviour. In this paper, water retention and mercury intrusion porosimetry (MIP) tests were conducted on pellet mixture (constant volume), single pellet (free swelling) and compacted block (constant volume) of GMZ bentonite, water retention properties and pore structure evolutions of the specimens were comparatively investigated. Results show that the water retention properties of the three specimens are almost similar to each other in the high suction range (> 10 MPa), while the water retention capacity of pellet mixture is lower than those of the compacted block and single pellet in the low suction range (

Deep geological disposal has been adopted by many countries around the world as a method for disposal of high-level radioactive waste. During the construction and long-term operation of the repository, working as buffer/backfill materials, bentonite will hydrate and swell to fill up various voids in the repository including artificial gaps between canister and compacted bentonite blocks, blocks and blocks, blocks and natural barrier (host rock), etc. More importantly, this closure process will be inevitably affected by groundwater and its chemistry infiltrated. In this study, hydration tests with infiltration of solutions were conducted on GMZ bentonite specimens with artificial annular gaps. The hydration process was monitored by a camera, and the digital images taken were analyzed. Results show that the hydration process for highly compacted GMZ bentonite specimens with an annular gap can be divided into three phases, which can be identified by the development of cracks on the surface of the specimen. In the first phase, bentonite swells and gel generates upon hydration with gradually filling up of the annular gap. While in the second phase, cracks are observed in the center part on the specimen surface, and in the last phase, cracks gradually close up, and eventually disappear accompanied by macroscopic homogenization of the specimen. Different from that for the specimens infiltrated with deionized water, a more rapidly filling up of the artificial annular gap is observed for the specimens injected with salt solutions. Furthermore, with increasing concentration, both of the quantity and width of the cracks formed obviously decrease due to the reduction of both the DDL thickness and electrostatic repulsion forces. Meanwhile, with comparison to that infiltrated with NaCl solutions, a smaller scale of cracks is observed in the dry central part of the specimen infiltrated with CaCl2 solution. Finally, time consumed to reach the macroscopic homogeneous state for the specimen infiltrated with salt solutions is much shorter than that infiltrated with deionized water. Meanwhile, the time required for the specimen infiltrated with CaCl2 solution is shorter than that injected with NaCl solution. Results of the investigation on pore fluid chemistry effects on the swelling behavior of compacted GMZ bentonite with an artificial annular gap could be used for estimation of the sealing effects and time of the technological gaps in the disposal repository, as well as to provide reference for the design and even the final performance evaluation of the engineering barrier system.

Bentonite has been considered as a potential buffer/backfill material for construction of engineering barriers in deep geological repository for disposal of high-level nuclear waste. Anisotropy will be inevitably generated during static compaction during preparation of bentonite blocks, leading to induce possible influences on buffering function formation of the engineering barrier system and even the safety of the whole repository. In the present work, swelling deformation tests along the directions both perpendicular and parallel to the compaction plane were conducted on unsaturated compacted GMZ bentonite specimens using a self-developed lateral confined swelling test setup. Influences of initial dry density and vertical pressure on the final swelling strain and its anisotropy, as well as the mechanism of evolution of anisotropy coefficient on hydration, were analysed. Results demonstrate that the swelling time, average strain rate and final swelling strain increase with increasing initial dry density and decrease with increasing vertical pressure. Meanwhile, the swelling strain along the perpendicular direction is much higher than that along the parallel direction, exhibiting an obvious anisotropic swelling characteristic. The anisotropy coefficient is relatively large at the initial swelling stage, and then gradually decreases to be stabilised. The final anisotropy coefficient increases with initial dry density and decreases with vertical pressure.

Compacted laterite and bentonite, as well as a mixture of the two, are strongly impermeable and can effectively prevent the migration and diffusion of contaminants. This study explored the feasibility of employing a laterite/bentonite mixture as a vertical barrier in landfills, tailings ponds, or sites contaminated by heavy metals. Permeability tests using chemical solutions of different concentrations and compression tests were performed on mixed bentonite/laterite samples with different dry densities and mixing ratios. The permeability and consolidation characteristics were verified using scanning electron microscopy. The hydraulic conductivity of the laterite samples decreased with increased dry density, and the hydraulic conductivity of the mixed-soil samples decreased with increased bentonite content. Adding bentonite significantly improved seepage control performance. The hydraulic conductivity of the pure laterite was 4.7 × 10−7 cm/s, whereas that of the laterite sample mixed with 20% bentonite was 1.6 × 10−8 cm/s. The compacted laterite with a dry density of 1.5 g/cm3 was demonstrated to satisfy the permeability specifications to serve as a landfill seepage control system. When the concentration of the heavy metal or salt solutions increased, the laterite flocculation increased, and the hydraulic conductivity decreased. The laterite in the mixed soil samples substantially affected these characteristics, particularly decreasing the hydraulic conductivity. As the concentration of the chemical solutions increased, the yield stress of the compacted samples and the compression index decreased, and the compression performance improved. These findings indicate that chemical solutions significantly affect the hydromechanical behavior of a laterite/bentonite mixture used as an engineered barrier.

A series of temperature-controlled swelling pressure and swelling strain tests were performed on Gaomiaozi (GMZ) bentonite to study the temperature effects on the swelling behavior. The influence of sample dry densities (for swelling pressure tests) and applied various vertical stresses (for swelling strain tests) were taken into account. Emphasis was put on the stress-dependent properties of temperature effects. Results show that at a certain temperature, there is a unique relationship between the stress level and the void ratio of saturated bentonite specimens for either the swelling pressure or swelling strain tests. This relationship could be defined as “swelling stable line” (SSL), which was bilinear shaped and reflects a stress-dependent swelling behavior of compacted bentonite. This stress-dependent swelling behavior is attributed to the different hydration mechanisms under lower and higher stresses. A comparison of SSLs at different temperatures shows an obvious stress-dependent behavior of temperature effects on swelling properties. These stress-dependent temperature effects on swelling behavior of GMZ bentonite are further confirmed by data collected from literatures on various types (i.e., Ca- or Na-bentonite) of bentonites: regardless of the predominant cations, the elevation of temperature increased the swelling potential in the lower stresses part of SSL, whereas it reduced the swelling potential in the higher stresses part. This phenomenon is related to the influences of temperature on the swelling processes (i.e., hydration, diffuse double-layer repulsion, and separation of elementary layers) under different stress levels.

Compacted bentonite as an artificial barrier/backfill material is susceptible to heat when combined with the hyperalkaline conditions of deep geological repositories. In order to investigate the st...

Laboratory tests, including compressibility, permeability, and microstructure tests, were conducted on tailings samples using custom-designed test apparatus to investigate the effect of metal contamination (Cu2+) on the hydromechanical behavior of compacted tailings. Infiltrating samples with various dry densities with distilled water or CuSO4 solution at various concentrations showed that the void ratio of compacted tailings decreased with increased dry density. An increase in the metal contaminant concentrations from 0 to 0.1 mol/L increased the compression coefficient of the tailings from 0.14 to 0.84 MPa−1 under a vertical load of 0.01 to 2.0 MPa, while the yield stress of the tailings decreased from 204.3 to 98.7 kPa, respectively. The linear relationship between permeability coefficient (k) and void ratio (e) is described by k = − 6.48 + 17.17e. Microstructure test results showed that the diffusion double layer thinned, and the surface potential decreased, indicating that the contaminant of Cu2+ enhanced the compressibility and permeability of the tailings. The microstructure test results also showed that the amount of fine-grained soil in the copper tailings was significantly less after the hydromechanical test. Therefore, the permeability and compressibility of copper tailings increased. The experimental results are in good agreement with the estimated results.

Gaomiaozi (GMZ) bentonite is the most extensively used buffer/backfill material in deep geological repositories used for the disposal of high-level nuclear waste in China. Highly alkaline ground water solutions derived from the cement dissolution may cause changes in the mineralogical composition and in the adsorption capacity of bentonite, and may affect the long-term performance of the engineering barrier system. In the present study, the alteration of montmorillonite and its influence on the adsorptive properties of GMZ bentonite were investigated. The X-ray diffraction patterns of the mineral composition showed that the main compound of the GMZ bentonite is montmorillonite. By contrast, its hydration with a solution of KOH at 80 °C produced cementitious materials, and transformation could likely lead to kaolinisation. Thermodynamic results show that the adsorption of Sr (II) on GMZ bentonite is exothermic and follows quasi-secondary adsorption kinetics. For an initial Sr (II) concentration C0 = 100 mg/L, the maximum amount of Sr (II) adsorbed (qe) on GMZ bentonite was 9.72 mg/g. The adsorption capacity of GMZ bentonite on Sr (II) decreased gradually and as a function of the mineral transformation time in a mixture of GMZ bentonite and KOH. The Freundlich isotherm adsorption model can describe in a satisfactory manner the adsorption of Sr (II) on GMZ bentonite or the transformation of bentonite at different degrees of mineral transformations.

Compacted Gaomiaozi (GMZ) bentonite has been considered as the first choice of buffer materials in the deep geological disposal of high-level radioactive wastes (HLW) in China. During the l...

Compacted bentonite is often considered as a possible buffer/backfill material for deep geological disposal of high-level radioactive waste. During the long-term lifespan, the bentonite barriers will undergo coupled thermo-hydro-mechanical (THM) progresses, which induce complex volume changes. In this work, the volume change behavior of compacted bentonites undergoing THM interactions was analyzed based on the framework of Barcelona Expansive Model, in which the thermal volume changes were taken into account by adopting the approach of Tang and Cui (Geotechnique 59:185–195, 2009). After analyzing the published experimental results of wetting/drying and heating/cooling cycles, new fI and fD functions were put forward for describing the interaction between micro- and macro-structures, while a thermal loading yield function curve was established for describing the TM behavior under unsaturated conditions. Some cyclic wetting/drying and thermal loading tests reported and modelled in literature were simulated to verify the relevance of the improvements undertaken. Comparisons among the original model, the modified model as well as the test results clearly showed a satisfactory performance of the model in describing the volume changes of compacted swelling clays upon THM processes.