Soil chemical washing has the disadvantages of long reaction time, slow reaction rate and unstable effect. Thus, there is an urgent need to find a cost-effective and widely applicable alternative power to facilitate the migration of washing solutions in the soil, so as to achieve efficient removal of heavy metals, reduce the risk of soil compaction, and mitigate the damage of soil structure. Therefore, the study used a combination of freeze-thaw cycle (FTC) and chemical washing to obtain three-dimensional images of soil pore structure using micro-X-ray microtomography, and applied image analysis techniques to study the effects of freeze-thaw washing on the characteristics of different pore structures of the soil, and then revealed the effects of pore structure on the removal of heavy metals. The results showed that the soil pore structure of the freeze-thaw washing treatment (FT) became more porous and complex, which increased the soil imaged porosity (TIP), pore number (TNP), porosity of macropores and irregular pores, permeability, and heavy metal removal rate. Macroporosity, fractal dimension, and TNP were the main factors contributing to the increase in TIP between treatments. The porous structure resulted in larger effective pore diameters, which contain a greater number of branching pathways and pore networks, allowing the chemical washing solutions to fully contact the soil, increasing the roughness of the soil particle surface, mitigating the risk of soil compaction, and decreasing the contamination of heavy metals. The results of this study contribute to provide new insights into the management of heavy metal pollution in agricultural soils.

Biopolymer-bound soil composites (BSC) area novel class of cement-free building materials using biopolymer binders, many of which are sourced from the waste streams of major industries. This study investigates the recyclability of one particular BSC that uses kraft lignin as the biopolymer. Re-manufacturing of BSC was accomplished by mechanical disruption of the virgin material, followed by re-introduction of solvent, remixing, and remolding. The compressive strength of recycled lignin-based BSC was higher than that of BSC made with virgin ingredients. To understand the microstructure of lignin-based BSC, a series of X-ray micro-CT images of the test articles were obtained. Images produced by the micro-CT method reveal differences in the microstructure of the re-manufactured specimens indicating an enhancement of the association between lignin and aggregate particles. This study demonstrates the feasibility of recycling BSC and provides insight into the importance of biopolymer-aggregate association in determining the mechanical properties of BSC.

In many soil processes, including solute and gas dynamics, the architecture of intra-aggregate pores is a crucial component. Soil management practices and wetting-drying (W-D) cycles, the latter having a significant impact on pore aggregation, are two key factors that shape pore structure. This study examines the effects of W-D cycles on the architecture of intra-aggregate pores under three different soil management systems: no-tillage (NT), minimum tillage (MT), and conventional tillage (CT). The soil samples were subjected to 0 and 12 W-D cycles, and the resulting pore structures were scanned using X-ray micro-computed tomography, generating reconstructed 3D volumetric data. The data analyses were conducted in terms of multifractal spectra, normalized Shannon entropy, lacunarity, porosity, anisotropy, connectivity, and tortuosity. The multifractal parameters of capacity, correlation, and information dimensions showed mean values of approximately 2.77, 2.75, and 2.75 when considering the different management practices and W-D cycles; 3D lacunarity decreased mainly for the smallest boxes between 0 and 12 W-D cycles for CT and NT, with the opposite behavior for MT. The normalized 3D Shannon entropy showed differences of less than 2% before and after the W-D cycles for MT and NT, with differences of 5% for CT. The imaged porosity showed reductions of approximately 50% after 12 W-D cycles for CT and NT. Generally, the largest pores (>0.1 mm3) contributed the most to porosity for all management practices before and after W-D cycles. Anisotropy increased by 9% and 2% for MT and CT after the cycles and decreased by 23% for NT. Pore connectivity showed a downward trend after 12 W-D cycles for CT and NT. Regarding the pore shape, the greatest contribution to porosity and number of pores was due to triaxial-shaped pores for both 0 and 12 W-D cycles for all management practices. The results demonstrate that, within the resolution limits of the microtomography analysis, pore architecture remained resilient to changes, despite some observable trends in specific parameters.

In artificial freezing engineering, the freezing temperature is an important factor affecting soil frost heave deformation, and studying its impact is of great significance. The frost heave ratio of soil is a crucial factor for designing and predicting soil frost heave. However, it only considers vertical deformation while neglecting radial deformation. This paper introduces a simple unidirectional freezing apparatus specifically designed for three-dimensional x-ray computed tomography (CT) scanning, which allows for the investigation of internal structural changes in clay during freezing at four different freezing temperatures (i.e., -3 degrees C, -5 degrees C, -7 degrees C, and -9 degrees C). Freeze-necking of the soil was observed during freezing. An image processing method was proposed to segment the soil samples, and parameters such as length, equivalent diameter, and volume were measured to assess changes during freezing. The observed variations in necking depth and equivalent diameter indicate that freeze-necking is uniform. As the freezing temperature decreased, the necking depth reduced from 72.4 mm to 38.1 mm, and within this necking depth, the equivalent diameter decreased progressively from the bottom to the top. Moisture content increased near the cold end of the soil and decreased near the warm end, suggesting that freeze-necking is due to moisture migration within the soil. Considering freeze-necking, the volumetric frost heave ratio was defined to characterize soil frost heave deformation. This ratio also decreases as the freezing temperature decreases, and the values are smaller than those of the traditional frost heave ratio. The discrepancies become more pronounced at higher freezing temperatures, reaching up to 1.8% at -3 degrees C. The results indicate that lower freezing temperatures can reduce frost heave deformation, and freeze-necking requires greater attention in engineering at higher freezing temperature.

Elucidating the effects of long-term cultivation on pore structure and infiltration characteristics is essential for understanding soil degradation mechanism and improving cropland sustainability. In this study, we evaluated a number of indicators regarding soil properties, pore structure, and water infiltration during long-term cultivation and the relationship of basic properties and pore structure with soil infiltration in northeast Mollisol region of China. The studied cropland involved four cultivation durations (20, 40, 60, and 100 years) and one forest (FR) as the control. X-ray computed tomography (CT) and mini disk infiltrometer were applied to assess the effects of long-term cultivation on soil pore structure and infiltration characteristics at soil depths of 0-100 cm. The pore properties including soil porosity (TP), pore number (PN), pore size distribution, mean shape factor (MSF), fractal dimension (FA), anisotropy (DA), and Euler number (EN), and infiltration properties including mean (IR), initial (IIR), stable infiltration rate (SIR), and saturated hydraulic conductivity (Ks) were planned to be measured in this study. The results showed that soil organic carbon (SOC) in cropland was significantly lower than that in FR, while bulk density (BD) and mean weight diameter (MWD) exhibited an opposite trend. Long-term cultivation altered the soil pore size and shape distribution, and decreased the macroporosity (>500 mu m) and elongated porosity, which suggested that long-term cultivation results in a simpler pore network. FR took a longer time to reach a stable state in infiltration curve over time, while cropland had lower IR, IIR, SIR and Ks, indicating negative effects of cultivation on water infiltration. > 500 mu m porosity and elongated porosity were positively correlated with infiltration characteristics (P<0.01). The infiltration parameters were mainly affected by soil pore properties (MSF, PN, and DA) and basic properties (SOC, MWD, and BD), highlighting the importance of pore morphology in the infiltration process. Our results provide new insights into the evolution of soil structure and properties and explanations for water infiltration dynamics under long-term cultivation from the microscale.

Different sedimentary zones in coral reefs lead to significant anisotropy in the pore structure of coral reef limestone (CRL), making it difficult to study mechanical behaviors. With X-ray computed tomography (CT), 112 CRL samples were utilized for training the support vector machine (SVM)-, random forest (RF)-, and back propagation neural network (BPNN)-based models, respectively. Simultaneously, the machine learning model was embedded into genetic algorithm (GA) for parameter optimization to effectively predict uniaxial compressive strength (UCS) of CRL. Results indicate that the BPNN model with five hidden layers presents the best training effect in the data set of CRL. The SVM-based model shows a tendency to overfitting in the training set and poor generalization ability in the testing set.The RF-based model is suitable for training CRL samples with large data. Analysis of Pearson correlation coefficient matrix and the percentage increment method of performance metrics shows that the dry density, pore structure, and porosity of CRL are strongly correlated to UCS. However, the P-wave velocity is almost uncorrelated to the UCS, which is significantly distinct from the law for homogenous geomaterials. In addition, the pore tensor proposed in this paper can effectively reflect the pore structure of coral framework limestone (CFL) and coral boulder limestone (CBL), realizing the quantitative characterization of the heterogeneity and anisotropy of pore. The pore tensor provides a feasible idea to establish the relationship between pore structure and mechanical behavior of CRL. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Microbial-induced calcium carbonate precipitation (MICP) is a new biotechnology that can be used to improve the strength of soils. Unsaturated soils are common in nature and saturation is a significant factor affecting the efficiency of bio-cementation. This study investigated the properties of MICP under different grouting saturation conditions. Unconfined compressive strength (UCS) tests confirmed that biocemented sand could get higher strength under unsaturated grouting conditions with the same calcium carbonate content which helps reduce the material cost. Scanning electron microscopy (SEM) test results show that at lower saturation, the size and amount of calcium carbonate crystals were insufficient but calcium carbonate mainly gathered between the particles. At higher saturation, larger calcium carbonate crystals were produced and exited in pores and on the particle surface, increasing the filling effect. Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) test results show that the dominant calcium carbonate morphology detected in samples was calcite, which was the most stable one. X-ray computed tomography (CT) test results show that after cementation, the measured contact surface area became uniform and the coordination number was higher. The flow direction of bacteria and the cementing solution did not induce significant anisotropy in the cementation process. The effective cementation and content of calcium carbonate jointly influenced the improvement of soil mechanical properties.

Cracking behavior can reduce soil hydraulic and mechanical properties and is a preferential pathway for water flow and pollutant transportation, resulting in polluted environment, such as application to landfill liners and capping. Recently, researchers have advocated the use of waste materials for clay mixtures using various measurement and analysis methods. Therefore, this study aims to conduct a bibliometric analysis of the scientific literature published between 2002 and 2021 obtained from Scopus to quantitatively identify research trends, key research areas, and future research paths in this field on desiccation and crack behavior using waste materials as landfill liners. The VOS viewer software was used to analyze 41 articles in which the paper selection process was filtered. The results showed that the fly ash mixture's application as a landfill liner could reduce cracking significantly. Furthermore, fractal analysis and X-ray computed tomography measurements have proven to be good candidates for measuring cracks because they are the most accurate for calculating the crack value. Waste materials such as fly ash can be applied as landfill liners with other materials, such as bentonite and coconut coir fibers. This study is beneficial for improving the design and selecting the appropriate materials for landfill liners.