Plant microbial fuel cell (PMFC) is a promising technology that could be applied in green infrastructures such as green roofs for bioelectricity generation. While previous research has explored the influence of soil water characteristics on PMFCs in laboratory settings, this investigation extends to understanding natural ambient environmental conditions. A series of PMFCs was deployed for three months to examine bioelectricity generation, soil water characteristics, and ambient environmental conditions, employing three vegetation types, providing an intricate understanding of the coupled bio-hydrological behaviors in the field. PMFC performance was further enhanced by heightened solar radiation and precipitation, amplifying bio-electrical output. However, a 72 %-89 % decline in electrical current and potential occurred at the air entry value due to disrupted ion transport which caused increased soil electrical resistance. This phenomenon underscores the intricate balance between ambient conditions and PMFC performance, laying a foundation for optimizing PMFCs for specific ecological and hydrological contexts. These findings emphasize the potential of PMFCs in real-time, in-situ reflecting of hydrological soil characteristics, offering an innovative approach to ecosystem management and hazard mitigation efforts.

Electromagnetic induction (EMI) technique has been used in the large-scale geological mapping investigation; however, few studies have associated small-scale EMI responses and soil pore characteristics. We developed a simple theoretical framework to interpret EMI responses of geomaterials and used a parametric study to show the dependency of high-frequency electromagnetic induction (HFEMI) signals on the electrical conductivity and the associated soil pore characteristics. A series of EMI tests were performed on geomaterials with varying water contents, void ratios, and porewater salinities. Results showed the salient dependence of HFEMI signals on the frequency, water content, void ratio, and porewater salinity. Additionally, it was found that the low-frequency noise might affect the data interpretation of frequency-dependent EMI signals, and the log-log plotting could identify the actual materials' responses in the high-frequency domain. The association between EMI responses and soil properties was explained by the underlying soil electrical conduction phenomenon. A semiempirical equation was derived to interpret soil electrical conductivity from the EMI spectrum, and a good correlation was found between frequency-dependent EMI quadrature signals and soil properties. This study highlights the considerable potential of adopting the HFEMI technique for nonintrusive geomaterials characterization and proposes a systematic methodology for performing EMI screening to obtain reliable EMI signals for geotechnical evaluation.

The volumetric and hydrological responses of clayey soils subjected to drying-wetting (D-W) cycles are of paramount importance for the integrity of geoenvironmental infrastructures. The study aimed to investigate the cracking behavior of clayey soils under D-W cycles by using advanced 2D imaging and 3D scanning techniques to capture the initiation and propagation of desiccation cracks within a soil specimen. The temporal variation in the soil water content and the corresponding 2D digital photography and 3D morphology of cracks were simultaneously monitored, and the cracking characteristics were interpreted. It was found that the time-dependent evaporation process was independent of the D-W cycles. Both 2D and 3D characterization showed the cracking hysteresis phenomenon in the unsaturated soil, which indicates the dependency of the crack opening and closure on the degree of saturation. D-W cycles led to the formation of subcracks and the increase in the total crack length, reflecting the soil degradation. Additionally, it was demonstrated that the 3D characterization exhibited the advantage of capturing the volumetric change and the subtle change in the macroporosity of the cracked soil over the 2D visualization. The current study provides a perspective of combining 2D and 3D characterization for interpreting the volumetric change of cracked soils and enhancing the understanding of the hydromechanical responses and the soil-atmosphere interactions.

Biochar amendment have been explored recently for improving hydro-mechanical properties of landfill cover system and enhancing the vegetation for the ecological restoration of post closure landfills. Existing studies have predominantly focused on the hydro-mechanical properties of biochar-amended rooted soils and yet few studies have comprehensively investigated their gas permeability functions in response to unsaturated conditions. The objective of this study is to experimentally and theoretically investigate the coupled effects of biochar and root intensity on CO2 gas permeability in unsaturated soil. Biochar-amended rooted soil was prepared by growing Chrysopogon zizanioides on granite residual soils with biochar at a mas ratio of 5 %. Gas permeability and unsaturated soil properties (i.e., suction and degree of saturation) were monitored simultaneously. Due to the presence of roots, the maximum gas permeability, hydraulic conductivity, and suction increased by 200 %, 600 %, and 50 %, respectively, compared with bare soil. This was attributed to preferential flow, micropore increasement and negative pressure induced by root growth. However, biochar addition decreased the maximum gas permeability and hydraulic conductivity by 21 % and 33 %, respectively. This was attributed to the pore filling function of biochar, increased capillarity due to intrapore of biochar, and presence of surface functional groups in biochar promoting CO2 adsorption. A newly developed model was proposed to predict gas permeability with respected to unsaturated soil properties. It was revealed that the gas permeability function of rooted soils showed a strong dependence on the measured root length density. This study highlights the significance of adopting biochar in mitigation of gas emissions in vegetated landfill cover system.

The thermo-hydro-mechanical (THM) behaviors of frozen soils are often modeled based on the thermodynamic fluxes of moisture and heat. However, existing models disregard the pore-scale granular interaction between the soil grain and ice crystal for saturated partially frozen soils. The pore-scale mechanism of pore-filling and load-bearing for the ice phase in the loaded soil skeleton has not been explored. An alternative constitutive model is therefore proposed by considering the microscopic temperature-dependent distribution of the ice phase for pore-filling and load-bearing in the soil interpore. This reflects the influence of the ice phase on the soil stress states and the associated THM behaviors as interpreted based on the critical state framework. The model was validated by published experimental results and considerably captured undrained shearing behaviors of frozen soils at various temperatures and confining pressures. A numerical parametric study was conducted to investigate the dependency of the phase relationship, stress state, undrained shear strength, and soil stiffness on temperature. The modeling suggests that the ice crystals filling in the pore are partially load-bearing to affect the soil stiffness and partially unloaded to alter the stress state. It shows that freezing turns the soil into a heavily consolidated state by increasing the specific volume and decreasing the effective granular void ratio. The undrained shear strength of frozen soils increases with a decrease in temperature because the dilatancy is enhanced due to the ice invasion in the interpore. It also demonstrates that soil stiffness is influenced by not only the stress state but the freezing history. This study highlights the temperature-dependency of mechanical behaviors and the validity of using the concept of stress states to interpret the pore-scale mechanistic soil-water-ice interactions for frozen soils.

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.

The present study focuses on the effect of root density on gas permeability and water retention in biochar-amended soil used as a cover material in landfills. Biochar amendment of soil is important measurement in landfill cover, while vegetation is an important for ecological restoration. Despite its importance, little attention has been given to the adaptation of vegetated biochar-amended soil in landfill cover. The interactions between the biochar, vegetation, and soil concerning the gas permeability remain unknown, which may lead to unexpectedly increased waste gas emissions in landfill covers. To enhance the utilization efficiency of biochar amendment and vegetation techniques in landfill covers, further investigation of their coupled effects on gas permeability and water retention is necessary. Four different treatments were applied to manufacture series of soil columns: bare soil (BS), biochar-soil composite (BSC), vegetated biochar-soil composite with low planting density (VBSCL) and high planting density (VBSCH). The soil water characteristic curve and gas permeability were observed under natural wetting and drying cycles. The results showed that VBSCL increased gas permeability by 142% as compared to BSC. VBSCH enhanced gas permeability by 168% as compared to BSC. This was due to the spreading and decaying root systems forming preferential pathways for gas transfer. Additionally, VBSCL and VBSCH made around a 10% increase in volume water content at the whole suction range, while BSC just enhanced around 20% water content at the low suction range (less than 10 kPa). The combination of roots and biochar have significantly enhanced water retention during entire suction range due to capillarity.

The application of energy geotechnology and artificial ground freezing technique always encounters the frozen soil layer in the subsurface. In cold regions, accelerated climate change induces freeze-thaw cycles on earthen structures. It is essential to develop a constitutive model for thermo-hydro-mechanical (THM) behaviors of frozen soils to assess the stability and serviceability of geotechnical infrastructure in the frozen ground. This study developed a constitutive model to investigate the pore-scale granular interaction between soil grains and ice crystals in saturated partially frozen soils. The microscopic temperature-dependent distribution of the ice phase in the soil pore spaces was anticipated as pore-filling and load-bearing particles. It was found that not all the ice crystals were being loaded in the soil skeleton. This microscopic morphological distribution of the ice phase was found to be associated with the soil stress states, indicating the dependency of THM behaviors of frozen soils on stress state and temperature. The model was validated by existing experimental results and showed considerable accuracy in the prediction of undrained shearing behaviors of frozen soils under various temperature and confining conditions. It also revealed the process of the ice invasion in the pore spaces, affecting the stress state and yield surface. Furthermore, the model might also explain the soil degradation due to freeze-thaw cycles by considering the variation in the effective granular void ratio, as reflected by the soil freezing characteristic curves. This study provides insight into the pore-scale mechanistic soil-water-ice interactions for frozen soils. The proposed model is compatible with the critical state framework and could be applied to geomaterials' characterization in cold regions.

The risk of geohazards associated with frozen subgrades is well recognized, but a comprehensive framework to evaluate frost susceptibility from microstructural characteristics to macroscopic thermo-hydro-mechanical (THM) behaviors has not been established. This study aims to propose a simple framework for quantitatively assessing frost susceptibility and compressibility in frozen soils. A systematic THM model was devised to predict heat transfer, soil freezing characteristics, and stress states in frozen soils. Constant freezing experiments and oedometer compression tests were performed on bentonite clays under varying temperatures (−5°C, −10°C, and −20°C) and stress levels to validate the proposed model. Additionally, soil electrical conductivity measurements were employed to assess the temperature- and stress-dependent volumetric and mechanical properties of frozen soils. The model used Fourier’s law to compute the transient soil temperature profile and estimated the volume change and stress states based on the soil freezing characteristic curve. Experimental results showed that frost heave of bentonite reached between 9.0% and 26.6% of axial strain, which was largely predicted by the proposed model. It also demonstrated that the frost heave was mainly attributed to the fusion of the porewater. Additionally, the preconsolidation pressure of frozen soils exhibited a rapid increasing trend with decreasing temperature, which was explained by the temperature-dependent ice morphology in the soil interpore. Furthermore, the findings also demonstrated a remarkable sensitivity in the electrical conductivity in response to the soil temperature during the frost heave process and the stress state under the loading or unloading path.

Landfill cover systems should exhibit low gas permeability to minimize the overflow of greenhouse gases and subsequent air pollution. Microbially induced carbonate precipitation (MICP), a biocementation technique, has been applied for subsurface soil stabilization by improving the shear strength of the soil. However, the impact of MICP on the gas permeability of unsaturated soils remains unknown. Considering the role of biocementation in the modification of soil interpores, this study investigated the feasibility of using the MICP technique to reduce the gas permeability of granite residual soils in response to unsaturated conditions. The biocemented soil samples were prepared by mixing soils with MICP chemical solutions at different chemical concentrations. Water retention tests and measurements of gas permeability were performed, in which suction, volumetric water content and gas permeability were continuously monitored. Additionally, energy-dispersive X-ray spectroscopy and X-ray diffraction analyses were performed to investigate the formation of CaCO3 precipitates; scanning electron microscopy was used to study the impact of MICP on the soil interpore structure, and the acid washing method was used to determine the CaCO3 content. The results showed that soils treated with higher concentrations of MICP chemical solutions had higher air entry pressures and residual water contents. This indicates the improvement of water retention due to the presence of MICP, which increases the microstructural porosity and enhances the capillarity, as observed via microscopy. Furthermore, the results revealed that biocementation significantly reduced the gas permeability of the soil and that the change in the maximum gas permeability strongly correlated with the MICP chemical solution concentration and the CaCO3 content. This study highlights the role of MICP in soil–water–air interface studies and the potential application of this biocementation technique to minimizing gas emission issues in landfill cover systems.

Critical soil suctions (threshold, tipping point, and permanent wilting) corresponding to initial drought response, near-death stage, and complete mortality, respectively; is essential for formulating irrigation schemes of vegetation grown in compacted soil under drought conditions. The effect of soil types on these critical soil suctions are unexplored and is crucial in understanding the soil-specific plant water functions. This study aims to establish the drought response of Axonopus compressus (grass), based on stomatal conductance (g(s)) and chlorophyll fluorescence parameters (CI) grown in different soil types. A. compressus were grown in six soil types (2 coarse-grained and 4 fine-grained soils) for 8 weeks, followed by continued drought condition. The g(s) and CI were monitored along with soil suction and moisture content. Both leaf and root growth were observed to be higher in coarse-grained soils than fine-grained soils, even though the water retention of the coarse-grained soils were comparatively less. Drought stress initiation in plants was captured by psi(threshold) from the CI (especially in fine-grained soils) before the g(s) response. The three critical soil suctions estimated from the correlation between CI and psi were found to be increasing with higher soil clay fraction. Corresponding plant available water contents (based on v/v volumetric water content) with each of three critical soil suctions were found to be dependent on the relative growth of canopy to root growth that occurred in different soil medias. Especially, plant available water in 'tipping suction' was dependent on the soil clay fraction (i.e., higher fraction could restrict root water uptake) and is presented with a simple empirical correlation for A. compressus.

Plant microbial fuel cell (PMFC) newly emerges as a sustainable technology in green infrastructures such as green roofs, reactive barriers, and environmental biosensors. PMFCs generate bioelectricity driven by microbial redox reactions in the rhizosphere. However, the interactions between the bioelectricity and soil–water characteristics remain unknown, and PMFCs are vulnerable to drought stress. Therefore, this study aims to reveal the coupled bio-hydrological behaviors of PMFCs and propose biochar amendment on PMFCs for improving bioelectricity generation under drought conditions. Biochar-aided PMFCs were prepared by growing green-roof vegetation, Hydrocotyle vulgaris, on sandy lean clay with biochar at a mass ratio of 0, 5, and 10

Pavements constructed on frost-susceptible subgrade layers are prone to deterioration from freezing in cold regions. A thermal insulation layer above the frost-susceptible subgrade layer in a pavement structure could effectively mitigate the deterioration due to frost action. The objective of study was to evaluate the thermal insulation performance of different insulating materials experimentally and numerically. Therefore, five largescale pavement boxes, including one uninsulated pavement and four insulated pavements using different materials (i.e., XPS board, tire chips, foam glass aggregates (FGA), and foamed concrete), were constructed. The thermal insulation performance was studied under constant freezing condition and freeze-thaw cycles where the temperature profile of pavements was monitored. A finite element model was developed by considering the transient heat flow and the associated latent heat of fusion in the unbound granular layers. Correspondingly, the numerical simulation was conducted to extrapolate the temperature profile for pavement structures made by different insulating materials. The experimental results show that the XPS and FGA have the best overall insulation performance, followed by foamed concrete and tire chips. Results of the numerical simulation suggest that to achieve an equivalent level of insulation performance as a 5 cm thick XPS board under the same freezing conditions, insulating materials such as tire chips layer, FGA layer, foamed concrete layer, or recycled concrete aggregate should have insulation layer thicknesses of 40 cm, 25 cm, 30 cm, or 68 cm, respectively. In terms of cost-benefit analysis, the foamed concrete insulated pavement is the most expensive option, followed by uninsulated pavement, insulated pavements using FGA, tire chips, and XPS. This study highlights the critical role of the insulating material and the insulation layer thickness in cost-benefit thermal insulation for pavements in cold regions.

This work-in-progress paper discusses the development of an educational game to provide integrated geotechnical engineering education modules that connect theoretical concepts, laboratory testing, field investigation, and engineering design. The game, Earth Trek, is developed based on the design of geothermal piles, which are an innovative and sustainable geotechnical engineering approach to combat climate change. Virtual reality is applied to visualize the field environments, laboratory conditions, and design components for structural simulation. The game uses a combination of storytelling and tasks to engage students with geotechnical concepts in an enjoyable way. With the newly developed game, geotechnical engineering instructors can provide students with exposure to laboratory testing and field environments, improving the quality of geotechnical engineering education. The use of multiphysics enriched mixed reality gaming allows for a visual representation of the connections between theoretical concepts, laboratory testing, field investigation, and engineering design. Additionally, this study discusses the challenges that geotechnical students face when dealing with worldwide concerns such as energy demand, environmental protection, infrastructure sustainability, and hazard reduction. Earth Trek allows students to apply geotechnical engineering knowledge to explore the underground space and the associated geothermal energy to tackle the engineering problems using only their smartphones. Through exploring the virtual environment and completing game tasks, students can obtain different testing tools used for geotechnical experiments, including thermal conductivity measurement and direct shear test. They are also trained to conduct parametric study to explore the influence of boundary conditions on thermal transfer efficiency of the geothermal pile. The key contribution of this work is to illustrate an educational paradigm based on mixed reality, moving towards creative engineering education in geotechnical engineering. The newly developed educational game and Earth Trek are expected to enhance geotechnical engineering education and provide students with an interdisciplinary knowledge to tackle worldwide concerns.

This study aims to develop a multiphysics-enriched mixed reality game system that provides an integrated geotechnical learning experience with visualization, collaboration, and simulation tools. This paper summarizes the game development works. Essential game components, such as player characters, geotechnical testing devices and tools, and laboratory environment, were created for the virtual game. Multiple mini-games incorporating geotechnical concepts are designed to further encourage the players to play the game. The thermal conductivity and direct shear tests will be performed in a virtual walk-in environmental chamber with controllable temperature. Using these experimental results as modeling inputs, finite element simulations are integrated to support the geothermal pile analysis under field conditions. Players will be able to visualize the simulation results through the augmented reality technique. The entire game is created as a mobile game platform, which enables players to use personal smartphone or tablet devices. The work is expected to improve undergraduate students' interests and success in geotechnical engineering coursework, as well as provide developmental insights and educational background to inform researchers who seek to develop similar games.

The slope stability of saline soil always poses a hidden danger. Excessive changes caused by pore saline in suction and volumetric can damage the stability of green infrastructure with slopes, such as landfill liners. There is a scarcity of studies that have utilized sedimentation tests to examine the influence of different pore fluid salinities on the hydromechanical behavior of clay. Furthermore, research investigating the coupling effects of biochar and salinity on clay’s hydromechanical behavior by calculating effective stress is limited. Based on the research background, water retention, volumetric shrinkage, and sedimentation tests were conducted in a temperature-controlled chamber in this study, with continuous monitoring of the change in soil volume (including height). Soil-water characteristic curves were determined under different relative humidity conditions by using Kelvin’s equation. An increase in porewater salinity led to a lower void ratio at a given gravimetric water content, while biochar addition decreased the volumetric expansion of clays by 0.4–14

Erodibility assessment of soil by hole erosion test (HET) is popular due to its advantages of being economical, rapid, simple procedure and better control over soil properties. Modelling the erosion process in HET is vital for interpreting the measurements and unambiguous quantification of the erodibility of compacted soil in geotechnical infrastructures. This study developed a new theoretical model by considering power (instead of shear stress) to describe the concentrated leak erosion processes of compacted soil in HET using Bernoulli’s Energy equation and work-energy principle. The model accounts for soil self-weight, fluid flow rate, fluid energy transformation, hole enlargement, and compaction state of soil for obtaining the soil erodibility. For this purpose, a relationship was developed between erosion rate and fluid energy rate. A new analytical equation was proposed for determining the temporal hole enlargement in HET as well as the erodibility of soil without the knowledge of the piezometric head. The theoretical and analytical model was validated by (a) performing HETs with repetitions and (b) comparing with the existing interpretation method. The proposed theoretical and analytical methods showcased similar results in terms of the erodibility of soils under varying hydraulic conditions. The use of the new models considerably simplifies the interpretation of HET and addresses the problem of unambiguous estimation of soil erodibility.

Clay-engineered barriers might be subjected to soil salinization issues under climate change. A recently emerged desalinization method is achieved by modifying clays using biochar. However, unsaturated soil responses of biochar-engineered clays in saline environments under drought conditions remain unknown. This study aims to investigate soil shrinkage and water retention characteristics of biochar-amended kaolin and bentonite under saline conditions. Soil shrinkage and water retention tests were conducted on clays (with and without biochar addition) with various porewater salinity (i.e., 0%-10%). Physiochemical properties (including zeta potential and porewater pH) were measured to interpret particle-fluid interactions. Shrinkage characteristics of kaolin and bentonite exhibited sensitivity and insensitivity to the porewater salinity, respectively. This phenomenon was explained by hydrogen-sodium ion exchange and deprotonation phenomenon occurring on kaolin and bentonite, respectively. Biochar significantly alleviated the salinity-induced shrinkage of clays by increasing the shrinkage limit of kaolin and bentonite by 6%-14% and 50%-107%, respectively (p < 0.05). This was attributed to the porous structure and hydrophilic functionality of biochar that immobilized sodium ions through ion exchange and protonation reactions. The air entry value of clays significantly increased with porewater salinity and biochar addition due to the reduction of void ratio and enhanced capillarity, respectively. An empirical equation was proposed to predict the shrinkage limit of clay in various saline conditions. It highlighted that the application of biochar-engineered clays could contribute to the desalination and the improvement of resistance to shrinkage damage in hydro-chemical barriers.

Biochar amended clay layer has emerged as a sustainable hydraulic barrier for hazardous municipal waste containment system. The effects of pore fluid salinity on soil shrinkage and water retention characteristics of biochar amended clay are unknown. This study aims to investigate the behavior of soil shrinkage and water retention of biochar amended kaolin under different pore fluid salinity. A series of volumetric shrinkage and water retention tests were conducted on biochar amended kaolin in sodium chloride solution at initial concentrations of 1 %, 5 %, and 10 %. Biochar addition increased the shrinkage limit and minimum void ratio of kaolin by up to 17 % and 11 %, respectively. Air entry value of kaolin increased by 6-88 times with an increase in pore fluid salinity, caused by interparticle aggregation. Micrographs showed that biochar intrapore was filled by kaolin particles, partially hindering the interparticle aggregation of clay in the salt solution. Biochar addition lowered zeta potential on the surface of kaolin particles by 50-75 %, indicating that the immobilisation of excess sodium ions was achieved by biochar. Correspondingly, osmotic suction of pore fluid decreased by 21-64 % due to biochar's ion absorption. The findings highlighted that biochar addition to kaolin specimens minimises NaCl-induced soil shrinkage and reduces the pore fluid salinity. This study indicates that biochar could be potentially helpful for desalinisation and mitigating volumetric change issues for geo-environmental infrastructures.

Determination of erosion characteristics is of great significance to assess the erodibility of geomaterials that are subjected to seepage force. The erosion characteristics indicate soil particle removal in term of internal erosion that might occur in earthen structures. Hole erosion test (HET) is a simple and effective approach to determine erosion characteristics. It is noted that there are not many studies that focus on the development of a theoretical model describing the erosion characteristics and the associated process of soil particle detachment in HETs. The aim of this study is to propose a simple model based on Bernoulli's principle to interpret erosion characteristics of geomaterials in HETs. An analytical equation was deduced from a physically based model incorporating Bernoulli's principle and erosion constitutive law for internal erosion within a soil pipe driven by pressure gradient. The analytical equation could be applied to determine soil particle removal, radial erosion propagation, erosion coefficient, and critical shear stress. A series of HETs were performed under different flow rate to verify the proposed model. The obtained results demonstrated that the proposed model allowed for reasonably predicting the amount of soil particle removal and understanding erosion characteristics of soils through the HET.