Biogrouting has been proposed for improving mechanical properties of soils and rocks, whose performance greatly depends on the location of biocement at pore-scale. To enhance the performance of biogrouting, many strategies were proposed, including the addition of assistants, controlling curing moisture degree, and flocculation of bacteria. Clay is one such assistant which has been proven to be effective, with an assumption of increasing active biocement, i.e. those located between soil particles. In this work, we employed microfluidics to directly observe whether clay minerals can certainly control the location of precipitates and how they function. First of all, the capacity of bentonite and kaolin for adsorbing bacteria were investigated. Then, the location of CaCO3 crystals with and without clay minerals were visually observed using microfluidics. Pore-filling ratios and CaCO3 ratios, which are closely related to permeability and strength of biocemented soils, were quantitatively analyzed from collected images. Finally, the effects of bentonite and kaolin and their dosages on the location of biocement were comprehensively discussed. The results demonstrated that the performance of bentonite and kaolin on adsorbing bacteria and regulating biocement location is distinct due to differences in the morphologies of clays. These findings can help us to improve biogrouting performance on soil stabilization and propose new strategies in various practical applications, such as CO2 sequestration, heavy metal remediation, and oil recovery enhancement, all with a foundational understanding of their mechanisms.

The shear strength of compacted bentonite is crucial for preventing tilting and damage of the waste canisters in deep geological repositories (DGRs). The shear strength evolution along the confined wetting path also needs to be investigated, given the long saturation time of the bentonite buffer. This study conducted direct shear tests on densely compacted Gaomiaozi bentonite after suction control under confined conditions to determine its peak shear strength and strength parameters. Furthermore, the shear strength evolution along the confined wetting path was modeled on the basis of the effective stress principle. The results show that, for a given dry density, the peak shear strength at a given vertical pressure and the strength parameters exhibit an overall decrease along the confined wetting path. Moreover, the peak shear strength of the specimen that underwent confined wetting was considerably lower than that of the as-compacted specimen with the same total suction, indicating that the suction value and microstructure codetermine the peak shear strength of compacted Gaomiaozi bentonite. For this reason, the peak shear strength in the as-compacted state and the dual-porosity water retention curves established along the confined wetting path were used to model the shear strength evolution along the confined wetting path. The substitution equation for the effective stress parameter chi was selected on the basis of the experimental evidence. Finally, the model parameters were calibrated from the shear strength evolution at a given vertical pressure, and they reasonably reproduced the shear strength evolution under other vertical pressures. These findings can be helpful for the design and safe operation of DGRs under extreme geological conditions.

The long-term disposal of high-level radioactive waste (HLW) in deep geological repositories requires the reliable performance of engineered barrier systems (EBS). Compacted bentonite, widely used for its high swelling capacity, low permeability, and self-sealing properties, plays a critical role in these barriers. However, understanding the complex coupled thermo-hydro-mechanical (THM) behavior governing water infiltration dynamics remains a significant challenge, especially when gap spaces (or technological voids) are present. This study investigates water infiltration dynamics in bentonite-based EBS using a novel laboratory-scale experimental setup. Time-lapse photography was employed to monitor the evolution of hydration and swelling under thermal gradients and varying gap sizes, simulating repository conditions. The experimental program was designed to compare the effects of two gap sizes on infiltration rates, swelling behavior, and desiccation cracking. Results demonstrated that larger void spaces accommodated greater swelling, leading to lower dry density and higher permeability, while smaller gaps restricted desiccation cracking due to mechanical constraints. The correlation between pixel intensity and water content allowed the derivation of a linear calibration model, enabling real-time, non-destructive estimation of moisture distribution in bentonite. Findings in this study highlight the interplay between gap size, water infiltration, and thermal effects, emphasizing the need for optimized EBS designs to balance mechanical integrity and hydraulic performance. It is anticipated that the insights provided by this study contribute to the refinement of predictive models and advancing the safe and effective containment of HLW over geological timescales.

The reuse of by-products has become increasingly important as a means of minimising the consumption of natural resources and reducing waste disposal. This study examines the potential reuse of steel slag for soil stabilisation, with benefits such as conserving natural resources and mitigating the greenhouse gas emissions associated with the production of conventional stabilising agents. It focuses on evaluating the effect of pozzolanic reactions on the strength and stiffness of both loess silt and silt-bentonite mixtures. The experimental tests included the physical characterisation of granular materials, reactivity tests of the pozzolanicity of soil mixtures, compaction tests, unconfined compression tests, and hydraulic conductivity tests. The impact of the curing period was also analysed to quantify the effects of natural cementation and the development of hydrogels within soil pores on the compacted soil properties. The findings suggest that adding steel slag can significantly increase the strength and the stiffness of compacted loess silts by over 300% and 500%, respectively, after 56 days of curing, substantially reducing the hydraulic conductivity of granular materials, such as the tested silt, as hydrogels partially occupy the pores available for liquid flow. It should be noted that the chemical reactions during hydrogel formation may hinder the free expansion of clay mixtures and release Ca2+ ions, thereby counteracting the expected reduction in hydraulic conductivity when bentonite is added to compacted earthen barriers. [Graphics]

The study examines the effects of pressmud (PM), an innovative, sustainable, and beneficial alternative for soil treatment, on the hydromechanical behaviors of sand-bentonite (SB) through a series of laboratory tests, including unconfined compressive strength (UCS), free swelling, and 1D consolidation. SB mixtures with 10 and 15% bentonite (by sand weight) and PM at 4, 8, and 12% (by SB weight) were evaluated. Additionally, microstructural analyses using X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed to elucidate the mechanisms of soil improvement. The results indicate that PM incorporation reduces the maximum dry unit weight while significantly enhancing the UCS of SB mixtures, with maximum strength increases of 111.5 and 65.5% observed in mixtures containing 10 and 15% bentonite, respectively, after 28 days of curing. These changes help not only enhance the strength properties but also decrease the self-loading on the landfill liner. Moreover, higher PM contents exhibit a dual impact of reducing swelling strain while slightly increasing both compressibility and permeability. Despite the marginal increase in permeability, it still meets the necessary criteria for serving as a landfill liner (< 10(-7) cm s(-1)). The observed effects of PM on SB behavior can be ascribed to the synergistic effects of natural fibers and cementitious gels, as confirmed by microstructural analyses. The natural fibers mainly contribute to the shear resistance of the interface, while the cementitious gels bridge and interlock the solid particles, thereby enhancing the hydromechanical performance of SB. It is concluded that the reuse of PM in soil stabilization not only enhances the hydromechanical performance of SB but also results in better PM management.

This study investigates the influence of primary variables selection on modeling non-isothermal two-phase flow, using numerical simulation based on the full-scale engineered barrier system (EBS) experiment conducted at the Horonobe Underground Research Laboratory (URL) as part of the DECOVALEX-2023 project. A thermalhydraulic coupled model was validated against analytical solution and experimental data before being applied to simulate the heterogeneous porous media within the EBS. Two different primary variable schemes were compared for discretizing the governing equations, revealing substantial differences in results. Notably, using capillary pressure as a primary variable instead of saturation resulted in closer alignment with analytical solutions and real-world observations. While the modeling work at the Horonobe URL generally exhibited trends consistent with experimental data, discrepancies were attributed to the operational conditions of the heater and the influence of the Excavation Damaged Zone (EDZ) near the borehole.

This study delves into the mechanical properties and mechanisms of bentonite-modified cement soil, a reinforced material formed through the physicochemical reactions of cement, soil, and water. Recognizing the material's widespread application in foundation treatment, slope reinforcement, and seepage control, alongside the environmental pressures of cement production, this research explores the potential of bentonite as a partial cement substitute. Through indoor unconfined compressive strength and permeability tests, varied by curing age, bentonite type, and mix ratio, the study assesses the impact of these factors on the material's performance. Microscopic analyses further elucidate the intrinsic mechanisms at play. Key findings include: a non-linear relationship between bentonite content and modified cement soil strength, with sodium-based bentonite enhancing strength more effectively than calcium-based; a significant reduction in permeability coefficient with increased bentonite content, particularly with sodium-based bentonite; and a detailed examination of the material's microstructure, revealing the critical role of cement and bentonite content in pore reduction and strength enhancement. The study underscores the paramount influence of cement content on both strength and permeability, proposing a prioritized framework for optimizing modified cement soil's performance. (c) 2025 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Heavy metal contamination in soil poses significant environmental and geotechnical challenges, requiring effective stabilization to limit contaminant mobility, enhance soil stability, and reduce deformation. This study investigates the dynamic response and microstructural changes in heavy metal-contaminated clayey sand, emphasizing the effects of clay type (kaolin and bentonite) and zeolite stabilization at varying contents (5%, 10%, and 15%). Laboratory tests, including cyclic triaxial, bender element, adsorption, sedimentation, pH measurements, Atterberg limits, and SEM analyses, were performed. Results reveal that contamination significantly reduces liquefaction resistance, with kaolin-based mixtures more susceptible than bentonite-based ones due to differences in plasticity, specific surface area, and swelling capacity. Zeolite stabilization, especially at 10% content, improves resistance by strengthening the soil structure and mitigating pore pressure under cyclic loading. Contamination affects shear modulus and damping ratio differently for kaolin and bentonite mixtures, with zeolite amplifying these impacts at higher contents through enhanced particle dispersion. Heavy metal adsorption increases with bentonite and zeolite addition, with bentonite exhibiting 180% greater lead adsorption than kaolin. Optimal adsorption performance is achieved with 10% zeolite. Microstructural analysis indicates contamination disrupts hydrogen bonding of kaolin, induces flocculation in bentonite, and has minimal effect on the stable structure of zeolite. These findings highlight the importance of clay type, zeolite content, and soil composition in mitigating contamination effects, providing insights into effective soil stabilization strategies.

Thermal backfill is an integrated part of underground electrical cable infrastructures systems, ground heat source pumps and radioactive waste repositories, as it minimizes resistance to heat transfer away from these systems. The heat transfer capacity and current carrying capability of underground electrical cables are significantly affected by thermal conductivity of backfill material and the surrounding soil media. Therefore, this research paper compares the thermal conductivity and shrinkage results of compacted (low to high densities) fly ash- and sand-bentonite mixtures with bentonite contents of 30%, 50%, 60%, 80% and 100%. The thermal conductivity of mixtures increased from 1.05 Wm-1K-1 to 1.20 Wm-1K-1 with the addition of fly ash content from 20 to 70% by weight in bentonite. The thermal conductivity bentonite-sand mixture was also found to be increased from 1.21 Wm-1K-1 to 1.83 Wm-1K-1 with increasing sand content. Additional to this, the bentonite-sand and bentonite-fly ash-based backfill materials surrounding heat-sensitive structures experience shrinkage and desiccation cracking due to thermal drying. Therefore, the desiccation volumetric shrinkage tests of bentonite-sand and bentonite-fly ash mixtures were conducted and found that the presence of sand or fly ash reduces shrinkage strain. Based on the experimental results, this study suggests a sustainable utilization of fly ash up to 50%-70% as an effective thermal backfill material in electrical cable infrastructure systems. Thus, the application of fly ash as a construction material reduces environmental impact and cost, aligning with the goals of sustainable development.

Bentonite grouting is utilized widely in geotechnical engineering to stabilize the excavation and prevent seepage in sandy soils. However, the infiltration behavior of bentonite slurry in sandy soil is not well understood, primarily due to rheological blocking and the formation of a filter cake. This study performed infiltration column tests to investigate the infiltration behavior under various conditions, including slurry concentration, sand properties, grouting pressure, and infiltration duration. Monitoring included infiltrated distances (calculated from drainage volume), pore pressure at different depths, and bentonite distribution using methylene blue titration. Results indicate that rheological blocking occurs during the infiltration process as bentonite slurry, which is a shear-thinning fluid, increases in viscosity with a decreased shear rate. This phenomenon is more pronounced with higher slurry concentrations, leading to reduced infiltration distances. Additionally, in soils with pore throats smaller than bentonite particles, a filter cake forms above the surface of the grouted soil, decreasing the pore pressure and further reducing infiltration distance. The distribution of bentonite content remains consistent across the infiltrated zone, resulting in a linear pressure drop. Based on these findings, the study proposes a novel model that combines the generalized Darcy's law, the Herschel-Bulkley rheological model, and mass conservation of slurry to predict the spatiotemporal progression of the infiltration front. This model, which was validated using experimental data, accurately predicts the effects of rheological properties and filter cake formation on infiltration. The results of this study provide valuable insights into infiltration processes and enhance the application of bentonite slurry in grouting.