Air Entry Value Research Articles

Exploring Soil–Water Characteristic Curves in Transitional Oil Sands Tailings

Soil–water characteristics curves (SWCC) have proved useful in estimating parameters used in modeling unsaturated geotechnical properties of soils including permeability and strength. Either saturation, gravimetric, and instantaneous and initial volumetric water content designation can be used to develop SWCCs. Studies have shown that any of the designations will give good estimates for soils that do not undergo volume change with suction change whereas, for soils that undergo substantial volume change, only saturation and instantaneous volumetric water content designation obtained by incorporating shrinkage curves can give correct estimates. Transition oil sands tailings have fines content that cannot be categorized as sandy or fine materials, and research on volume change with suction change in these materials is limited. In this study, HyProps, Tempe cells, and a chilled-mirror water potential meter were used to measure suction and corresponding water contents for samples that were prepared by mixing coarse sand and Fluid Tailing by ratios that mimic transition zone tailings. Shrinkage tests were also performed to observe the extent of volume change with suction increase. Air Entry Values (AEV) estimated from SWCCs based on gravimetric water content were found to be lower than those estimated from saturation-based SWCCs due to substantial volume changes in these materials with suction increase. The use of saturation water content designation is recommended in estimating AEV for transitional oil sands tailings. This is useful information in predicting the long term unsaturated geotechnical behavior of these materials for environmental management and safety purposes.

Hydromechanical behaviour of a slope reinforced by grass roots under rainfall conditions

Soil bioengineering using vegetation has been considered an environmentally friendly solution to improve slope stability. Although several studies have demonstrated the contribution of vegetation to slope stability, a gap in understanding the mechanisms of grass root–soil interactions under rainfall conditions remains. This study investigates the effects of the roots of vetiver grass (Chrysopogon zizanioides) on the hydromechanical behaviour of an unsaturated soil slope using the centrifuge modelling technique. The changes in pore water pressure and slope deformation were monitored during the test. The monitored data were subsequently back-analysed and interpreted using seepage–stability analyses. In addition, this study focused on evaluating the effect of roots on slope stability, considering safety and pore water pressure during rainfall. Results revealed that the vetiver roots remarkably affected the initial suction of the slope by increasing the soil's air-entry value. The increased suction and the additional cohesion provided by the roots enhanced slope stability under rainfall conditions.

Soil Water Characteristics Curves (SWCC) of Residual Soils Stabilized using Emerging and Novel Bioinspired Technique

The development of unsaturated soil mechanics stemmed from the inadequacies of classical soil mechanics in addressing the partial pore spaces found in real-world soil systems. However, assessing soil parameters under partial saturation is resource-intensive. To streamline this, the Soil-Water Characteristic Curve (SWCC) theory was introduced. Widely applied across geotechnical engineering, water resources, and agriculture, SWCC predicts parameters of unsaturated soil systems. In geotechnical engineering, SWCC is vital for designing systems near the Earth's surface, like slopes, clay liners and foundations. Furthermore, most of conventional soil improvement techniques pose a lot of environmental concerns. thus, emergence of green construction materials widely known as bio mediated and bioinspired soil improvement techniques like Enzymatic induced calcite precipitation (EICP). EICP, an emerging eco-friendly as it employs use of simply chemical compound that reduces carbon footprints, bioinspired technique as it mimics natural forming process, was applied to stabilize tropical soil for SWCC determination. SWCC parameters were derived by subjecting EICP-treated soils at varying concentration of cementation solutions (0 – 1.00 M) using pressure plate method. Results revealed that saturated water content θs, retention water content θr, and air entry value Ψa increased with higher cementation solution concentration. The values of θs, θr, and Ψa improved from of 0.663, 0.23405, and 3.4832 at 0 concentration of cementation solution to 0.701, 0.22864, and 7.7086, respectively upon treatment with 1.00 M cementation solution. Finally, it has been demonstrated EICP is capable of improving the SWCC parameters of residual soil, thus the EICP technique can be exploited in stabilization of residual for construction of compacted clay liner.

Soil-water retention capacity of expansive soil improved through enzyme induced carbonate precipitation-eggshell powder

Enzyme Induced Carbonate Precipitation (EICP) has been extensively investigated as a promising approach to improve engineering properties of soil, while Eggshell Powder (ESP) is an agricultural waste that effectively fills soil pores. The ESP provides abundant nucleation at sites for the EICP process, further promoting the effective precipitation of calcium carbonate. The research presented in this paper investigated the Soil Water Characteristic Curves (SWCC), permeability coefficient, and microstructure of expansive soil before and after EICP and EICP+ESP modification. A series of laboratory experiments were conducted, including soil water characteristic tests, permeability tests and Scanning Electron Microscopy (SEM). The results proved that the addition of EICP and EICP+ESP into natural expansive soil resulted in a gradual decline in air entry value, residual water content, and permeability coefficient, indicating an increase in water retention capacity and a decrease in permeability. Furthermore, with the intrusion of EICP and EICP+ESP, the contact between particles becomes smoother, and the soil pores become more equally distributed. Ultimately, there was an enhancement in water retention capacity of the natural expansive soil. This study emphasizes the synergistic potential of combining EICP and EICP+ESP as stabilizing additives to enhance the water retention capacity of expansive soil. Moreover, the reuse of ESP provides a sustainable solution for the resource utilization of agricultural waste and the improvement of expansive soil using bio-inspired methods.

The influence of rainfall patterns on factor of safety for clayey soil slopes

The persistent trend of rising temperatures and shifting weather patterns caused by climate change has prompted significant concern around the world. This research aims to evaluate the instability of slopes in Almaty, Kazakhstan, under various rainfall patterns, groundwater tables, and slope geometries by incorporating the principles of unsaturated soil mechanics. However, there have been a limited number of studies incorporating the principle of unsaturated soil mechanics with constant rainfall patterns in Central Asia, particularly in Kazakhstan, on the impact of rainfall-causing landslides. Hence, in this research, GeoStudio software (SEEP/W and SLOPE/W) was used to simulate the factor of safety (FoS) and pore water pressure for the investigated slopes under different rainfall patterns. Results from Hyprop and statistical method show that the saturated volumetric water content is 0.502, whereas the residual one is 0.147 and for the permeability function the conductivity coefficient started to sharply decrease at the suction value of 2 kPa when the air-entry value was 24 kPa. Findings from numerical analysis show the change in FoS for the slope of 10 m height and 27-degree slope angle was 6%, 7%, 7%, and 8% for cyclic, delayed, advanced, and normal distributions, respectively. For the slope with 20 m height and the same 27-degree angle, the change in FoS was 8%, 10%, 8%, and 11% for the cyclic, delayed, advanced, and normal distributions, respectively. These same patterns were shown in slopes with 35-degree and 45-degree angles, having the same 10 m and 20 m heights. Comparatively, this shows that slopes under cyclic rainfall patterns (240 mm of rain within 12 days) are less prone to failure compared to slopes under continuous, delayed, or regularly distributed rainfall patterns. Moreover, an increase in slope height and angle also affect the FoS negatively. It should be noted that the results obtained are only applicable to clayey-loam soil.

Understanding the impact of flow mechanisms in unsaturated layer on heat transfer near a partially submerged geothermal heat exchanger

Heat transfer near geothermal heat exchangers in the presence of groundwater flow has been extensively studied over the past years. However, previous studies often overlooked the presence of an unsaturated layer in the shallow subsurface soil, or disregarded phase change and fluid flow within this layer. In this study, a comprehensive hydro-thermal model is developed to evaluate heat transfer near a partially submerged geothermal borehole in groundwater flow while incorporating phase change and liquid and vapor flow in the unsaturated layer. The effect of unsaturated soil hydraulic analysis on heat transfer and energy efficiency of the geothermal piles is explored under varying soil permeabilities, air-entry suctions, solid thermal conductivities, and soil types. Consideration of water content variation and unsaturated fluid flow in the model enhances heat injection rate by up to 11.23 % in highly permeable soils. However, in scenarios with higher solid thermal conductivity, or lower air-entry suction values, the heat injection rate reduces by as much as 16.13 % due to unsaturated soil hydraulic analysis. Furthermore, it is found that consideration of unsaturated soil hydraulic analysis is of greater importance in Bonny silt than sandy soil, lowering the heat injection rate by 8.53 % and 2.53 %, respectively.

Soil-water characteristic curve of expansive soils considering cumulative damage effects of wetting and drying cycles

A novel experimental program was undertaken to explore crack development inside an expansive soil column under cyclic Wetting and Drying (W-D) and its influences on the Soil-Water Characteristic Curve (SWCC). Experimental investigations revealed that in the first drying cycle, fine and evenly distributed cracks developed from initial defects in the upper surface. In the subsequent drying cycles, the unhealed cracks during the wetting cycle further grow in width, length and depth while the initiation zone of new cracks was limited. The crack development characteristics within the soil column were described using fractal dimensions. The cracking degree of soil cross-sections gradually reduced with increasing depth. Also, each W-D cycle process contributed to a similar increment in the crack fractal dimension of soil at the same depth. In addition, the saturated water content, air-entry value and residual suction of cracked expansive soils gradually reduced with an increase in the number of W-D cycles. An obvious bimodal phenomenon was only observed in the SWCC of soil samples after the third W-D cycle. In order to quantify the damage induced by crack development to the soils, the damage variable was proposed based on the crack fractal dimensions. Furthermore, the Damage Impact Factor (DIF), as a function of the volume damage variable and soil plasticity index, was introduced into the traditional Fredlund and Xing (F-X) model to estimate the SWCC of cracked expansive soil. The form of modified F-X model is relatively simple and can be conveniently extended into geotechnical engineering practice applications.

Preliminary Experiences in Determining the Soil–Water Characteristic Curve of a Sandy Soil Using Physical Slope Modeling

Relating soil moisture content to soil suction, the soil–water characteristic curve (SWCC) represents an essential feature in unsaturated soil mechanics that enables estimation of different unsaturated soil property functions and modeling of the macro-scale soil behavior. However, depending on the soil and processes under consideration, proper hydraulic characterization of a soil through direct laboratory measurements can be difficult, time-consuming, and involve many uncertainties. In the case of uniformly graded sands, there is a highly nonlinear and steep shape of the SWCC, with only a few kPa of soil suction separating saturated and residual soil moisture conditions, which makes measurements for determinations of SWCC especially challenging. This study encompasses an investigation of the sandy type of soil’s behavior and presents some preliminary results and experiences on the determination of SWCC through the use of physical slope model tests. The 30 cm deep slope, inclined at 35 degrees and instrumented with soil moisture and pore water pressure sensors, was exposed to series of rainfall intensities, ranging from 37 up to 300 mm/h. The results indicated that the data on hydraulic response in monitored points are not only useful for the determination of SWCC, but that the approach is useful for investigation of hydraulic hysteresis phenomena, as well as its effects on soil moisture and pore water pressure conditions, which also affects the stability conditions of a slope. In particular, the best-fit parameters of the van Genuchten model suggested air entry values of 1.6 and 1.1 kPa for the drying and the wetting curves of the SWCC, respectively, with the two branches shifted by about 1 kPa of soil suction.

Effects of dynamic capillarity on the shear strength of sandy soils during transient two‐phase flow: Insights from non‐equilibrium triaxial simulations

AbstractModeling two‐phase flow in unsaturated porous media is not only important to vadose zone hydrology but also of great value in diverse disciplines. Common approaches use a simplified relationship between fluid pressure difference and saturation, neglecting the influence of saturation change rates. However, many studies have suggested that the applicability of this approach is limited to situations where the rate of change in saturation is insignificant. Despite several studies highlighting the importance of non‐equilibrium capillarity effects in unsaturated flow modeling, its significance in the mechanical response of the porous medium remains unclear. This study thus aims to address this gap by comparing the simulation results of the traditional static approach and an advanced approach that incorporates dynamic capillarity effects. The comparison is conducted under various flow boundary conditions to assess the magnitude of the differences between the two approaches. The results indicate that as the hydraulic boundary conditions’ absolute values increase, the contrast between the mechanical response of the two simulation scenarios (dynamic and static) becomes more significant. For instance, the dynamic model can predict shear strengths up to 50% higher than the static model. This highlights the importance of considering non‐equilibrium effects while modeling the mechanical behavior of an unsaturated porous medium. Finally, the parametric study of the effect of dynamic coefficient, air entry value, and saturated conductivity reveals the more pronounced effect of the dynamic coefficient on the mechanical response.

Temperature effects on adsorption and capillarity water retention mechanisms in constrained unsaturated soils

AbstractThis paper focuses on the impact of elevated temperatures on the adsorptive and capillarity water retention mechanisms of unsaturated soils under constrained (constant volume) conditions. This topic is critical for simulating the thermo-hydraulic behavior of soils in hydrogeological or geotechnical applications, including climate change effects on near surface soils, energy piles or soil borehole thermal energy storage systems in unsaturated soil layers, and buffers for geological nuclear waste repositories. A nonisothermal soil water retention curve (SWRC) that separately considers the temperature-dependency of the key parameters governing adsorptive and capillarity water retention mechanisms and soil physical parameters (e.g., surface tension, contact angle, adsorption capacity, cation exchange capacity, mean cavitation suction, air entry value and equilibrium film thickness) was developed to provide insights into the impact of temperature on water retention over the full suction range. The nonisothermal SWRC was validated using experimental data on high plasticity clays, with a good prediction of temperature effects on adsorption and capillarity water retention mechanisms in constrained unsaturated soils.

Enhanced resistance to desiccation cracking of polymer–bentonite mixtures: an experimental investigation of underlying mechanisms

Polymers have been shown to enhance the resistance of swelling clay soils to desiccation cracking, a critical property in engineering applications, particularly in waste containment facilities. However, the microscopic and macroscopic mechanisms driving this improvement remain poorly understood. Additionally, the influence of different mixing methods on these mechanisms is not well-established. While dry mixing (DM) is more convenient for onsite implementation, wet mixing (WM) offers intercalation between clay and polymer, resulting in potentially more durable stabilization outcomes. In this paper, key properties related to desiccation cracking of a polymer–clay mixture were measured. The mixture was synthesized by amending Na-bentonite with sodium carboxymethyl cellulose (Na-CMC) using DM and WM. Soil water-retention characteristics curves (SWCC), swelling and shrinkage potential, tensile strength, and pore-size distribution by mercury intrusion porosimetry (MIP) were measured for both mixtures and untreated bentonite. Compared to pure bentonite, mixtures were found to have slightly reduced air-entry values, significantly lower swelling and shrinkage potentials, and higher tensile strengths. In all experiments, DM exhibited superior performance compared to WM. MIP analysis of the amended mixtures revealed a more porous structure compared to untreated bentonite.

Assessment of Two SWCC models for predicting the curve fitting parameters of lateritic soil: bentonite mixtures

This study compares the capabilities of soil water characteristic curve (SWCC) models by Brooks-Corey (BC) and van Genuchten (vG) in estimating the curve fitting parameters for lateritic soil—bentonite mixtures. The SWCCs of soil treated with 0–10% bentonite and compacted with British standard light (BSL) energy at compaction states representing dry of optimum, optimum, and wet of optimum conditions were measured by sequential desorption using pressure plate extractor. The fitting parameters of the two equations were determined by a non-linear fitting program. The fitting capabilities of the models on the measured data were compared by statistical indices namely the root mean square error (RMSE), linearity (R2) and index of agreement (d). Results revealed that volumetric water content increased as bentonite content increased with specimens containing 10% bentonite recording the highest and therefore has greater capacity to retain water/contaminants, while the air entry value (AEV) for the various soil mixtures also increased with higher bentonite content. The study also found that the estimated volumetric water contents approximated the measured values at all suctions with a high degree of accuracy with RMSE values that ranged from 0.0035 to 0.0150 for vG model which are somewhat lower than the values for BC equation. Similarly, R2 for the vG equation (≥ 0.99) are, on average, slightly higher than those of the BC fits. However, the d values associated with the BC model which varied between 0.788 and 0.971 are higher than those of the vG (0.784–0.968). Overall, the study established that the vG model proved marginally superior in respect of goodness of fit.

Analysis of Soil–Water Characteristic Curve and Microstructure of Undisturbed Loess

Long-term irrigation promotes the infiltration of water in the thick, stratified loess layer, significantly raising the groundwater table and triggering a series of landslides in loess platform areas. The soil–water characteristic curve (SWCC) of loess buried at different depths affects the unsaturated infiltration process and is intricately connected to the soil’s microstructure. The SWCCs, scanning electron microscope (SEMs), and pore size distributions (PSDs) for five sets of undisturbed loess samples at depths ranging from 3.4 to 51.9 m are shown in this paper. The results indicate that the fitting parameter air entry value (AEV) of the SWCC rises from 13.67 kPa to 40.19 kPa as the depth increases from 3.4 to 51.9 m. And the saturated volumetric water content drops by 10.9%, with a notable SWCC shape difference between the transition and residual zones observed. Additionally, the total porosity of undisturbed loess falls by 12% when the depth increases from 3.4 to 51.9 m, while the macropores and mesopores reduce by 3.6% and 12.1%, respectively. These findings highlight the control of the pore structure on the SWCC and emphasize the correspondence between the SWCC and PSD. The conclusions also illustrate that the compaction effect changes the microstructure characteristics of loess, thereby affecting the soil’s water retention behavior.

Investigation of Unsaturated Soil Hydraulic Properties for Subgrade Improvement using Marble Dust Waste

Introduction The distribution of suction, which has implications for seepage and shear strength, plays a crucial role in determining the stability of unsaturated soil. The Air Entry Value (AEV) and Residual Water Contents (RWC) change over time to create the Soil-Water Characteristic Curve (SWCC), which is very important for figuring out how the suction is distributed. During a flood occurrence, the subgrade becomes inundated with water, leading to adverse effects on the AEV and RWC of the subgrade. Consequently, the performance of the subgrade is diminished. This study focuses on the use of Marble Dust Waste (MDW) as an addition to the subgrade to improve the strength of the subgrade. Aims This study aims to investigate the variations in air AEV in relation to the optimal MDW in unsaturated soil found in Malaysia. Methods The study uses control samples and soil that has been mixed with different amounts of MDW: 5%, 10%, 15%, 20%, and 25%. The SWCC is generated using the pressure plate extractor device. Results The results indicate a significant increase in the AEV for the mixed soil sample when compared to the control sample. While the soil-MDW mixture containing 5% of this material has an AEV value of 23 kPa, the control sample's value is 10 kPa. Conclusion The findings of the study suggest that the utilization of MDW yields a beneficial influence on the AEV, hence potentially improving the performance of the subgrade. The researchers aim to evaluate the potentially dangerous waste and convert it into a substance that is appropriate for engineering applications. This study provides empirical evidence that aligns with the objectives outlined in Chapter 8 of the 12th Malaysia Plan for the period 2021–2025. The topic under discussion pertains to environmental sustainability, specifically focusing on enhancing the ability to withstand the adverse effects of climate change and disasters, as well as the Green Technology Master Plan Malaysia 2017–2030, with a specific emphasis on Chapter 6, which addresses the issue of waste management.

Hysteretic water-retention behaviour of unsaturated hydrophobised soils

Hydrophobised soils found in the superficial region of earthen infrastructure can affect the hydrological processes of these structures. The hysteretic water-retention curve (WRC), which governs these processes, for hydrophobised soils has rarely been reported. Existing apparatus is unable to measure the WRC of unsaturated soils as it could not control the condition of pore water pressure (uw) in excess of pore air pressure (ua) when the contact angle was larger than 90°. In this study, a new apparatus was created, which sandwiches a soil sample between a high water-entry value (WEV) membrane and a high air-entry value (AEV) ceramic disc, to control the uw < ua and uw > ua conditions. Contact angle hysteresis and menisci evolution in the test materials during wet–dry cycles were measured to interpret the WRC. The WEV of soil increased with decreasing chemical heterogeneity and particle size. When drying the hydrophobised soils, they could retain water before reaching a certain negative uw. The WEV identified from the second wetting cycle was lower than that from the first cycle, whereas the AEV remained largely unchanged. The WEV and AEV decreased when the particle size of the hydrophobised materials was increased, resulting in a smaller hysteresis size.

Study on Effect of Particle Size Distribution on Water-Retention Capacity of Coral Sand from Macro and Micro Perspective

The reclamation coral sand (CS) layer is the survival environment for island reef vegetation in the South China Sea. The root system within the CS bed draws water necessary for vegetation growth, implying that the water-retention capacity of CS plays a pivotal role in determining vegetation viability. Particle size distribution (PSD) significantly influences the water-retention capacity of geomaterials. This study examines the impact of PSD on the water-retention capacity of CS from both macro (soil–water characteristic curve, SWCC) and micro (pore water distribution) perspectives using the pressure plate test and nuclear magnetic resonance technique, and an F&X model was used to analyze the SWCC of CS. The findings indicated that the F&X model aptly describes the SWCC of CS with different PSDs. Both the air entry value and residual water content rise with an increased content of fine grains (d < 0.25 mm), suggesting that the presence of fine grains augments the water-retention capacity of CS. It is considered that a size range of d = 0.075–0.25 mm predominantly impacts the water-retention capacity of CS. The PSD primarily influences the water-retention capacity by affecting the pore size distribution of CS. The volume of small pores swells with the surge of fine-grain content, while the maximum pore size contracts with increasing fine-grain content. Limited pore connectivity in CS means macropores can retain water even under high suction, bolstering the water-retention capacity of CS. These findings offer theoretical guidance for selecting gradation parameters for the planting layer on island reefs.

Microstructural investigation of the unsaturated hydraulic properties of hydrochar-amended soils

Hydrochar is an urban soil amender that supports plant growth on slopes. It is a biomass-derived carbon-rich material produced by the hydrothermal carbonation process; thus, it has a different pore structure from biochar produced by pyrolysis. The effects of hydrochar on the hydraulic properties of unsaturated compacted soils and the underlying pore-level soil-hydrochar interaction mechanisms are unclear. In this study, silty-clay sand was amended by grass-derived hydrochars produced at two hydrothermal carbonisation temperatures (180 and 240 °C, denoted as H-180 and H-240, respectively) with different mass proportions (fH\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{{\ ext{H}}}$$\\end{document}). The pore and throat size distributions, water retention curves (WRCs) and hydraulic conductivity functions (HCFs) of the soils with and without amendment were measured. The results showed that hydrochar improved the soil water retention capability as the smaller hydrochar particles filled the soil pores with diameters larger than 290 μm. Compared with the increase in air-entry value made by H-180, the one made by H-240 was more substantial as this type of hydrochar altered the soil pores to be smaller in size. Hydrochar also increased the hydraulic conductivity of the soil by half to one order of magnitude due to the increase in the throat frequency and the presence of hydrochar intra-pores. One exception was the amendment by H-180 at fH\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{{\ ext{H}}}$$\\end{document} of 2.5%, wherein the HCF was reduced due to pore clogging at throat diameters less than 90 μm and more than 250 μm. Finally, amending the soil with either H-180 or H-240 at fH\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${f}_{{\ ext{H}}}$$\\end{document} of 5% always improved the WRC and HCF, thereby benefiting plant water uptake.

Effects of Compaction Water Content on Water Retention and Deformation Behavior of Compacted Loess

Compacted loess is widely used as a construction material in engineering practices. The compaction dry density and compaction water content have significant effects on the hydromechanical behavior of compacted loess, thereby influencing the serviceability and safety of engineering structures. The former was widely studied in previous studies, while the latter is rarely known. In this study, the water retention, compression, and collapse behavior of compacted loess at different compaction water contents are investigated through pressure plate and oedometer tests. Microstructure analysis was carried out for insight analysis of the test results. The air entry value and yield stress of compacted loess decreased by 64% and 50%, respectively, with the compaction water content increasing from 12.4% to 19.0%. This is due to the fact that the number of clods increases with increasing the compaction water content, leading to many large-sized pores (i.e., diameter > 1,000 μm) and weaker soil skeletons. The influence of compaction water content on the water retention and compression behavior of loess is more pronounced at lower compaction dry densities and lower testing water contents, respectively. In addition, the specimen shows smaller collapse indexes at higher compaction water contents, mainly because of the larger deformation induced by the initial compression.

Bioelectricity generation in plant microbial fuel cells: Influence of vegetation density and unsaturated soil properties

Plant microbial fuel cells (PMFCs) have promising potential in various geoenvironmental engineering applications such as green roofs, biosensors, water purification and soil remediation. However, the vulnerability of PMFCs to drought stress and the unknown interplay between bioelectricity, planting density, and unsaturated soil properties pose significant challenges for their applications. The research objective is to investigate the effects of planting density on bioelectricity generation under drought conditions. The research was conducted using vegetation Hydrocotyle vulgaris at four different planting densities in silty soil. Bioelectricity and unsaturated soil properties, including water content and suction, were monitored. The results indicated that moderate increase in planting density could enhance the power density of PMFCs up to 4.5 times. This is because of the increased availability of root exudates for microbial activity with increasing root biomass and decreased internal resistance by extending the soil pore space. It was also found that plant roots could affect bioelectricity generation by altering unsaturated soil properties, such as the air-entry value of soils and soil pore structure. However, high planting densities reduced bioelectricity due to increased internal resistance from roots occupancy and evapotranspiration-induced crack initiation. This research provides valuable insight into optimizing PMFC performance and sustainability.

Comparative study on suction obtained using the membrane filter method from triaxial apparatus and pressure plate apparatus

To investigate the stability of the earth embankments and slope failures during natural disasters, it is necessary to consider that soil is naturally unsaturated. Hence, it is essential to understand the variations of pore air pressure and pore water pressure, in terms of suction. The relationship between soil suction and soil moisture content is generally attained by soil water characteristic curve (SWCC), which is critically important in characterizing the mechanical behaviour of unsaturated soils. The SWCC is obtained from pressure plate apparatus by imposing the suction continuously by draining water from the saturated specimen and/or supplying water to unsaturated soil, which differs from the actual soil compaction process at the site. Therefore, the current study employs a methodology of suction measurement directly at a certain degree of compaction and degree of saturation using the membrane filter method in the triaxial apparatus, to meet the realistic conditions in the field. A comparative study has been done using SWCC with the imposed suction and the measured suction (Natural suction) at each compacted state. The results from both the methods exhibited an increase in the suction and air entry value with the increase in the degree of compaction, proving that an increasingly positive effect of suction contributed while increasing the density. Whereas the suction measured immediately after the compaction (Natural suction) in triaxial apparatus is more consistent with the suction imposed in the wetting process of SWCC. Further deep understanding of the microstructure behaviour of each compacted state and the SWCC is necessary.