The Effects of Swelling and Porosity Change on Capillarity: DEM Coupled with a Pore-Unit Assembly Method

In this study, a grain-scale modelling technique has been developed to generate the capillary pressure–saturation curves for swelling granular materials. This model employs only basic granular properties such as particles size distribution, porosity, and the amount of absorbed water for swelling materials. Using this model, both drainage and imbibition curves are directly obtained by pore-scale simulations of fluid invasion. This allows us to produce capillary pressure–saturation curves for a large number of different packings of granular materials with varying porosity and/or amount of absorbed water. The algorithm is based on combining the Discrete Element Method for generating different particle packings with a pore-unit assembly approach. The pore space is extracted using a regular triangulation, with the centres of four neighbouring particles forming a tetrahedron. The pore space within each tetrahedron is referred to as a pore unit. Thus, the pore space of a particle packing is represented by an assembly of pore units for which we construct drainage and imbibition capillary pressure–saturation curves. A case study on Hostun sand is conducted to test the model against experimental data from literature and to investigate the required minimum number of particles to have a Representative Elementary Volume. Then, the capillary pressure–saturation curves are constructed for Absorbent Gelling Material particles, for different combinations of porosity values and amounts of absorbed water. Each combination yields a different configuration of pore units, and thus distinctly different capillary pressure–saturation curves. All these curves are shown to collapse into one curve for drainage and one curve for imbibition when we normalize capillary pressure and saturation values. We have developed a formula for the Van Genuchten parameter \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha $$\end{document}α (which is related to the inverse of the entry pressure) as a function of porosity and the amount of absorbed water.

Bridging Effective Stress and Soil Water Retention Equations in Deforming Unsaturated Porous Media: A Thermodynamic Approach

The finite deformation of an unsaturated porous medium is analysed from first principles of mixture theory. An expression for Bishop’s effective stress is derived from (1) the deformation-dependent Brooks and Corey’s water retention curve and (2) the restrictions on the constitutive relationships of an unsaturated medium subject to finite deformation. The resulting expression for the effective stress parameter \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\chi $$\end{document}χ is reasonably consistent with experimental data from the literature. Hence, it is shown that Bishop’s equation is constitutively linked to water retention curves in deforming media.

New non-ureolytic heterotrophic microbial induced carbonate precipitation for suppression of sand dune wind erosion

The detrimental effects of sand storms on agriculture, human health, transportation network, and infrastructures pose serious threats in many countries worldwide. Hence, wind erosion is considered a global challenge. An environmental-friendly method to suppress wind erosion is to employ microbially induced carbonate precipitation (MICP). However, the by-products of ureolysis-based MICP, such as ammonia, are not favorable when produced in large volumes. This study introduces two calcium formate-bacteria compositions for non-ureolytic MICP and comprehensively compares their performance with two calcium acetate-bacteria compositions, all of which do not produce ammonia. The considered bacteria are Bacillus subtilis and Bacillus amyloliquefaciens. First, the optimized values of factors controlling CaCO₃ production were determined. Then, wind tunnel tests were performed on sand dune samples treated with the optimized compositions, where wind erosion resistance, threshold detachment velocity, and sand bombardment resistance were measured. An optical microscope, scanning electron microscope (SEM), and X-ray diffraction analysis were employed to evaluate the CaCO₃ polymorph. Calcium formate-based compositions performed much better than the acetate-based compositions in producing CaCO₃. Moreover, B. subtilis produced more CaCO₃ than B. amyloliquefaciens. SEM micrographs clearly illustrated precipitation-induced active and inactive bounds and imprints of bacteria on CaCO₃. All compositions considerably reduced wind erosion.

Inulin biopolymer as a novel material for sustainable soil stabilization

The development of new urban areas necessitates building on increasingly scarce land, often overlaid on weak soil layers. Furthermore, climate change has exacerbated the extent of global arid lands, making it imperative to find sustainable soil stabilization and erosion mitigation methods. Thus, scientists have strived to find a plant-based biopolymer that favors several agricultural waste sources and provides high strength and durability for sustainable soil stabilization. This contribution is one of the first studies assessing the feasibility of using inulin to stabilize soil and mitigate erosion. Inulin has several agricultural waste sources, making it a sustainable alternative to traditional additives. Soil samples susceptible to wind erosion were collected from a dust-prone area in southwest Iran and treated with inulin at 0%, 0.5%, 1%, and 2% by weight. Their mechanical strength was evaluated using unconfined compressive strength tests and a penetrometer. In addition, wind tunnel tests (at 16 m/s) were performed to investigate inulin's wind erosion mitigation potential. The durability of treated samples was evaluated after ten wetting-drying cycles to assess the effect of environmental stressors. The results indicated a 40-fold increase in the unconfined compressive strength (up to 8 MPa) of the samples treated with 2% inulin and only 0.22% weight loss after ten wetting-drying cycles. SEM images revealed the formation of biopolymer-induced particle-to-particle bonds. Moreover, Raman spectroscopy indicated molecular (hydrogen) bonding of the biopolymer hydrogel-soil particles facilitated by the hydroxyl groups of inulin. The deterioration in stiffness and strength of treated samples was less noticeable after 3rd dry-wet cycle, indicating the durability of the samples. The durability of samples against wet-dry cycles was attributed to molecular bonding of soil-biopolymer hydrogel, as revealed by FTIR analysis.

A pore-scale study of fracture sealing through enzymatically-induced carbonate precipitation (EICP) method demonstrates its potential for CO2 storage management

Geological fractures are mechanical breaks in subsurface rock volumes that provide important subsurface flow pathways. However, the presence of fractures can cause unwanted challenges, such as gas leakage through fractured caprocks, which must be addressed. In this study, the dynamics of enzymatically induced carbonate precipitation in rock fractures and their subsequent influence on CO2 leakage were investigated from a pore-scale perspective for the first time. This was achieved through real-time monitoring of the injection of the solution into a rock-microfluidic flow cell using optical and scanning electron microscopy. It was revealed that the main growth dynamics occur during the first three injection cycles, with growth continuing until the fracture aperture is fully closed in the 6th cycle. Based on the flow simulation, a significant reduction of up to 25% in the CO2 conductivity of the original fracture is expected even after the first treatment cycle. Future studies are suggested to explore different resolutions, testing conditions, and to conduct 3-dimensional investigations of the growth dynamics.