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AimsThis study experimentally investigated the short-term (about 6 months) effects of roots of Cynodon dactylon on the gas permeability and gas diffusion coefficient of soils with different degrees of compaction.MethodsCompacted soil planted with Cynodon dactylon were left outdoors for about 6 months for plant growth. The measurements of rooted and bare soils included gas permeability, gas diffusion coefficient, root characteristics and soil microstructure. The relative effects of different root characteristic parameters on gas permeability and gas diffusion coefficient were compared through grey relational analysis.ResultsThe root volume ratio had a greater effect on gas permeability and gas diffusion coefficient, compared with root area index, root area ratio, root length density, and root biomass ratio. When the degree of compaction & GE; 85% (porosity & LE; 0.41, bulk density & GE; 1.56 g cm(-3)), the macro-pores at the root-soil interface increased gas permeability and gas diffusion coefficient, while negligible effects of roots on gas movement existed under degree of compaction of 80%. The increase in gas permeability by roots was more significant than that in gas diffusion coefficient. However, roots' increase of gas movement generally decreased at higher root volume ratio due to roots-occupied soil pores. Finally, gas permeability and gas diffusion coefficient of rooted soil were well predicted by newly-developed empirical models considering the effect of root volume ratio.ConclusionsMacro-pores at the root-soil interface tended to increase gas permeability and gas diffusion coefficient of soil with a degree of compaction & GE; 85%, while it is the opposite for root volume ratio.

It is well-recognised that stress can greatly affect the pore characteristics of unsaturated soil, such as pore size distribution (PSD) and pore shape (PS). So far, the effects of PSD and PS on the drying and wetting water retention curve at different stresses have not been well understood. To fill the knowledge gap, eight water retention tests were carried out to evaluate the effects of PSD and PS on hysteretic water retention behaviour at different stresses. Soil specimens purposely prepared at two compaction water contents and two initial void ratios were subjected to drying and wetting under two different net stresses (0 and 50 kPa). Moreover, mercury intrusion porosimetry (MIP) and micro-X-ray computed tomography (µ-XCT) were used to quantify the microstructure of specimens. The results showed an important role of pore structure in the hysteretic SDSWRCs. Compared to the specimens prepared on the dry side, specimens compacted on the wet side of the optimum are highly aggregated, possessing a lower water retention ability but a higher degree of hysteresis. The higher hysteresis observed in the highly aggregated specimens can be explained by the irregular-shaped pore characteristics observed in µ-XCT tests.

Biochar, a type of soil amendment, is widely used in heavy metal–polluted soil. It is also effective in enhancing plant performance. However, how biochar affects the soil heavy metal toxicity considering the plant–soil interactions is not well summarized. Therefore, this paper aims to review and summarize the effects of biochar on heavy metal availability in vegetated unsaturated soil. First, this review summarizes the effects of biochar on key parameters of unsaturated soil, including pore size distribution, water permeability, water retention curve, nutrient cycling and soil biota. Then, the mechanism of biochar for managing heavy metals in soil with vegetation is discussed, with a focus on both physical and biochemical aspects. Additionally, the impact of biochar-induced plant growth on the availability of heavy metals in the plant–soil system is explored. Finally, suggestions for future research and applications of biochar are provided. This paper supplies novel outlooks for further study and practice.

This book focuses on the engineering properties of unsaturated widely graded soils and their influence on slope stability. This book characterizes the natural widely graded colluvial soils from macroscale, mesoscale, and microscale viewpoints, introduces the techniques for measuring hydro-mechanical properties for unsaturated widely graded colluvial soils, and clarifies the hydro-mechanical behavior and the failure mechanism of widely graded colluvial soils subjected to environmental loads. This book improves the understanding of rainfall-induced landslides on natural slopes. Researchers and engineers in the field of civil engineering can benefit from the book. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023.

Vegetation has been found to improve the hydrological performance of landfill cover. However, existing studies ignore effects of hydrogen sulphide (H2S), which is a toxic odour generated from municipal solid wastes. H2S is a potential signalling molecule in regulating plant-stomata aperture, but how the plant-H2S interaction affects the water-gas transport in soil remains unknown. This study aimed to investigate effects of H2S on Bermuda grass growth and its influence on water-gas transport in unsaturated soils. Soil columns, with and without vegetation, were prepared and fumigated with different H2S concentrations. These columns were subjected to controlled drought and rainfall sequentially. The results showed that grass leaves almost wilted without H2S fumigation due to the Pythium diseases caused by antecedent waterlogged conditions. In contrast, H2S fumigation promoted grass growth because of the fungicidal effects of H2S. Upon drought, H2S fumigation caused stomata closure, which resulted in a lower matric suction in the vegetated soil than that without H2S fumigation. Accordingly, H2S concentration reduced because of oxidation by metal oxides in soil. Upon subsequent rainfall, more infiltration but lower matric suction was observed in vegetated soils than that in bare case, whereas H2S concentration decreased continuously due to gas dissolution in the pore water. Direct application of laboratory findings herein to field cases should be used with cautions, because the applied continuous lightening and low evapotranspiration rate of the soil column tests may not be representative to field conditions.

Material point method (MPM) offers an effective approach for analysing large-deformation problems, such as landslides that often involve unsaturated soil. Several MPM formulations for unsaturated soil have been reported in the literature, but they assumed that water retention ability and permeability function are independent of soil deformation. Furthermore, most studies used a single set of material points, so they cannot simulate some processes (e.g., the infiltration of free water into unsaturated soil). To address these problems, a two-point MPM formulation has been extended from saturated to unsaturated soils, considering the influence of porosity change on the water retention curve and permeability function. The information of solid and liquid phases is carried by two individual sets of material points, assuming zero air pressure. The elastoplastic mechanical behaviour is modelled by the Drucker-Prager model using Bishop's stress. The MPM simulations were compared with the results of physical model tests (for large-deformation problems) and finite element analysis (for small-deformation problems). It is evident that the proposed MPM formulation is able to capture various hydro-mechanical problems well. In particular, the computed failure processes of unsaturated soil slope under rainfall are consistent with results from physical model tests. © 2022 Elsevier Ltd

Air conductivity as a function of degree of saturation, ka(Sw), is a crucial air transport parameter of soil to evaluate soil aeration. This function controls the level of oxygen stresses introduced to plants. It has been found that plant roots affect ka(Sw) of soil where they grow in the soil pore space. However, there is no model available to predict ka(Sw) of root-permeated soils. Quantifying the effects of plant roots on ka(Sw) is vital to understand soil aeration. This study aims to derive a new model for the prediction of the ka(Sw) of unsaturated root -permeated soils. Then a verification experiment was conducted, aiming to validate the new model by comparing ka(Sw) predicted by the new model with ka(Sw) measured in the experiments. In the experiments, ka(Sw) of a compacted sandy loam vegetated with two grass species (Chrysopogon zizanioides and Cynodon dac-tylon) with different root densities was measured. The results showed that the proposed model predicts the ka(Sw) of root-permeated soils of various root densities well.

Damage caused by frost heave leads to costly maintenance in cold regions, like Hokkaido, Japan. Therefore, the study of the frost mechanism with experimental and numerical methods has been of great interest. Numerous models have been developed to describe the freezing process of saturated soil, which differs from the partially saturated conditions in the field. In fact, most subsurface soils are unsaturated. The freezing process of partially saturated soils is more complex than saturated soils, as the governing equations show strongly nonlinear characteristics. This study proposes a thermo-hydro-mechanical coupled model considering the heat transfer, water infiltration, and deformation of partially saturated soil to reproduce the freezing process of partially saturated frost susceptible soils distributed in Hokkaido. This model better considers the water-ice phase change and soil freezing characteristic curve (SFCC) during freezing under field conditions. The results from the multiphysics simulations agree well with the frost heave and water migration data from frost heave tests of Touryo soil and Fujinomori soil. In addition, this study discussed the influence of the various factors on frost heave amount, including temperature gradients, overburden pressures, water supply conditions, cooling rates, and initial saturation. The simulation results indicate that the frost heave ratio is proportional to the initial degree of saturation and is inversely proportional to the cooling rate and overburden pressure. Moreover, simulation under the open system generates much more frost heave than under the closed system. Finally, the main features of the proposed model are revealed by simulating a closed-system frost heave test. The simulation results indicate that the proposed model adequately captures the coupling characteristics of water and ice redistribution, temperature development, hydraulic conductivity, and suction in the freezing process. Together with the decreased hy

This study examines the effects of freeze-thaw actions and the concurrent seasonal fluctuations in water content, named as climate effect in this study, on the resilient modulus of subgrade materials to evaluate their mechanical behavior in cold regions. A series of suction-controlled resilient modulus tests on subgrade materials with variant freeze-thaw, wheel loads, and water contents conditions were conducted using a newly developed test apparatus. Test results were used to construct a simple model to estimate the climate effect on the resilient modulus by considering the synergistic effects between water content and freeze-thaw. Besides, this study calculated the fatigue life of eight local flexible pavement projects with variant subgrade layer moduli considering climate effect by combining the newly proposed model and long-term in-situ measured data. Results proved that climate related degradation of the subgrade materials decreases the fatigue life of asphalt pavements in cold regions.

Damage caused by frost heave leads to costly maintenance in cold regions, like Hokkaido, Japan. Therefore, the study of the frost mechanism with experimental and numerical methods has been of great interest. Numerous models have been developed to describe the freezing process of saturated soil, which differs from the partially saturated conditions in the field. In fact, most subsurface soils are unsaturated. The freezing process of partially saturated soils is more complex than saturated soils, as the governing equations show strongly nonlinear characteristics. This study proposes a thermo-hydro-mechanical coupled model considering the heat transfer, water infiltration, and deformation of partially saturated soil to reproduce the freezing process of partially saturated frost susceptible soils distributed in Hokkaido. This model better considers the water-ice phase change and soil freezing characteristic curve (SFCC) during freezing under field conditions. The results from the multiphysics simulations agree well with the frost heave and water migration data from frost heave tests of Touryo soil and Fujinomori soil. In addition, this study discussed the influence of the various factors on frost heave amount, including temperature gradients, overburden pressures, water supply conditions, cooling rates, and initial saturation. The simulation results indicate that the frost heave ratio is proportional to the initial degree of saturation and is inversely proportional to the cooling rate and overburden pressure. Moreover, simulation under the open system generates much more frost heave than under the closed system. Finally, the main features of the proposed model are revealed by simulating a closed-system frost heave test. The simulation results indicate that the proposed model adequately captures the coupling characteristics of water and ice redistribution, temperature development, hydraulic conductivity, and suction in the freezing process. Together with the decreased hy

The latest monitoring and asset management technologies for large infrastruc- tures involve digital representations that integrate information and physical models, exist in parallel to the real-life structures, and are continuously up- dated based on assimilated sensor data, in order to accurately represent the actual conditions in the structures. This type of technology is often referred to as Digital Twin. The implementation of such cutting-edge technology in monitoring assets like tailings dams, or embankment dams in general, and other large structures, implies the development of highly efficient numerical tools that, combined with sensor data, may support rapid, informed decision making.For the particular case of embankment dams, enabling this type of tech- nology requires an efficient numerical model that describes the coupled hydro-mechanical phenomena, pertinent to a dam functioning and safety. This may for instance be a Finite Elements (FE) model, describing the groundwater flow through unsaturated porous geomaterials.The process of updating and calibrating a model, such as the above mentioned FE model, based on sensor data is typically referred to as data assimilation. Often, this is achieved via an optimization approach, where a specific problem is solved multiple times for various parametric values, in search for the values that best describe the sensor data. The bottleneck in this type of application is typically the cost of multiple evaluations of the model, that may become prohibitive when the underlying FE model is large. In order to enable such applications, the present work proposes Model Order Reduction (MOR) methods tailored to the hydro-mechanical nonlinear problem at hand.MOR aims at the creation of a surrogate model that seeks an approximation of the FE solution in a reduced-order space. This is achieved by applying an offline-online strategy. In the offline stage, the solution manifold of thefull-order problem is sampled, in order to identify, Doctorat en Sciences de l'ingénieur et technologie, info:eu-repo/semantics/nonPublished

Damage caused by frost heave leads to costly maintenance in cold regions, like Hokkaido, Japan. Therefore, the study of the frost mechanism with experimental and numerical methods has been of great interest. Numerous models have been developed to describe the freezing process of saturated soil, which differs from the partially saturated conditions in the field. In fact, most subsurface soils are unsaturated. The freezing process of partially saturated soils is more complex than saturated soils, as the governing equations show strongly nonlinear characteristics. This study proposes a thermo-hydro-mechanical coupled model considering the heat transfer, water infiltration, and deformation of partially saturated soil to reproduce the freezing process of partially saturated frost susceptible soils distributed in Hokkaido. This model better considers the water-ice phase change and soil freezing characteristic curve (SFCC) during freezing under field conditions. The results from the multiphysics simulations agree well with the frost heave and water migration data from frost heave tests of Touryo soil and Fujinomori soil. In addition, this study discussed the influence of the various factors on frost heave amount, including temperature gradients, overburden pressures, water supply conditions, cooling rates, and initial saturation. The simulation results indicate that the frost heave ratio is proportional to the initial degree of saturation and is inversely proportional to the cooling rate and overburden pressure. Moreover, simulation under the open system generates much more frost heave than under the closed system. Finally, the main features of the proposed model are revealed by simulating a closed-system frost heave test. The simulation results indicate that the proposed model adequately captures the coupling characteristics of water and ice redistribution, temperature development, hydraulic conductivity, and suction in the freezing process. Together with the decreased hy

This study focuses on the coupled thermo-hydro-mechanical processes in unsaturated soils surrounding energy piles. Energy piles are deep foundations that are used to provide support to buildings and for exchanging heat between the ground and an overlying structure. While their performance in saturated and dry soil layers have been well investigated, their performance in unsaturated soils is an emerging topic that poses special challenges. Changes in temperature may induce drying of the soil, which will affect not only the heat transfer process but also the mechanical response at the soil-pile interface. In particular, this study evaluates the coupled processes in unsaturated soils surrounding energy piles using numerical simulations considering coupled heat transfer and water flow with nonequilibrium phase change and enhanced vapor diffusion processes. Further, this study evaluates the changes in yield stress of unsaturated soils during changes in suction which may be induced by thermally induced drying using a novel experimental technique. The numerical simulations and experiments in this study are focused on a low-plasticity soil referred to as Bonny silt, which was selected for this study because the solid particles are chemically inert, the soil retains water over a wide range of suction magnitudes, and many experiments and numerical studies have been performed on the soil that provide a point of comparison for the thermo-hydro-mechanical processes investigated.In the numerical simulations, a three-dimensional coupled heat transfer and water flow model was calibrated using tank-scale tests on Bonny silt then applied to investigate the long-term behavior of solitary energy piles and energy piles in groups having different boundary conditions and thermo-hydraulic initial conditions. The model was first applied to understand the impact of heating and cooling cycles for conditions representative of both heat exchange and heat storage for energy piles in geometric co

The use of recycled construction demolition (C&D) waste in geo-environmental infrastructure as a carbon neutralization strategy has gained traction recently. This review aims to provide a state-of-the-art on the unsaturated hydro-mechanical properties of C&D waste and its unexplored application in possible geo-environmental infrastructures. More than 100 studies comprising theoretical and numerical modelling, laboratory investigation, and field tests on the use of C&D waste have been discussed in this review systematically. The effects of particle size distribution, water absorption capacity, abrasion index, flakiness index, pH, reactivity index and microstructural cracks of C&D waste on its unsaturated hydro-mechanical properties were evaluated. Due to its porous nature and the presence of adhesive ettringite and needle-like minerals on the C&D waste's surface, its water absorption capacity is usually 2.3–4.6 times higher than natural aggregates. The applications of soil-recycled aggregates composite in geo-environmental infrastructures, such as slopes, landfill cover system, subgrade in pavements, geosynthetic reinforced backfill soil, green roof, are discussed in the context of unsaturated soil behaviour. From this review, the effects of freeze-thaw cycling, high precipitation events, and extreme desiccation on the performance of C&D waste based geo-environmental infrastructure were seen to be lacking in literature. Even though the C&D waste is deemed to be a suitable adsorbent due to its chemical composition, very few studies have explored C&D waste as a filter material for heavy metal retention in geo-environmental infrastructure. The thermodynamic properties of C&D waste as an adsorbent indicated that the sorption process is exothermic, and the entropy and Gibbs free energy being positive and negative, respectively. The review identifies new research themes for developing C&D waste as an adsorbent by optimizing the C&D waste particle size, adsorbent concentrat

The spatial variability is an inherent characteristic of soils. In addition to isotropic and transversely anisotropic soils, rotated anisotropic soils can also be observed in nature owing to deposition and weathering. Recent studies revealed that the spatially variable soil strength parameters can significantly affect rotated anisotropically deposited soil slope stability. However, the influence of spatial variability of saturated water permeability (ks) on this type of slope stability has been not fully understood, especially for the three-dimensional (3D) slope analysis. This paper aims to investigate unsaturated soil slope reliability with 3D ks rotated anisotropy random fields under rainfall infiltration. Due to the low efficiency of the covariance matric decomposition method (CMD) in simulating large-scale 3D rotated anisotropy random fields, a modified CMD method is proposed to save the computational cost. Moreover, comparisons between 2D and 3D random finite element analyses are also discussed. The results indicate the modified CMD method can markedly improve computational efficiency by more than 3.3 times compared with the CMD method. Only applying variations of pore water pressure (PWP) of a section or groundwater table (GWT) cannot sufficiently account for the uncertainty of slope stability. The cross-dip slope, where the dip direction of the slope is perpendicular to the dip direction of strata, has a larger reliability index (1.3−3.0 times) than the dip slope and reverse slope at the same soil bedding plane orientation. The most unfavorable soil bedding plane orientation for the cross-dip slope and the dip/reverse slope is 0° (horizontally deposited) and 30° (slightly larger than slope angle), respectively. It is attributed to the relatively low ks along these two orientations being more likely to form the aquiclude, affecting overall variations of PWP, leading to a larger standard deviation of the factor of safety and a lower reliability index. Furtherm

Loess slopes along the Yellow River in China are prone to failure in response to intensive irrigation. Existing studies found that the wetting front depth subjected to intensive farming water could barely reach 6 m in loess terrace. Long-term field monitoring revealed that irrigation water takes a short time to recharge the groundwater. Yet the dynamic process and mechanism of irrigation water percolation through thick unsaturated loess remains unrevealed. To address this problem, the Heifangtai terrace was selected as the study area and a series of unique full-scale controlled field irrigation experiments were carried out combined with numerical modelling. The variations in water content, pore-water pressure, and electrical resistivity over time were monitored. From the field testing, the unsaturated stratified loess layer could be divided into two vadose zones based on the water percolation characteristics, namely, transient flow zone (0–6.5 m) and steady flow zone (>6.5 m). Variation in water content presented a hysteresis effect in the transient flow zone compared with the steady flow zone. In the whole seepage process, the vertical percolation rate of 0.5 m/day was double that of the horizontal percolation rate. Furthermore, the irrigation water percolating process was modelled using a finite element code, PANDAS, which was validated by the field testing. Numerical simulation results demonstrated that the movement of wetting front was hampered by the air outflow within transient flow zone and that irrigation water reached the deep groundwater table through the thick unsaturated stratified loess within 100 days, which was consistent with the field monitoring data. This research aids in a better understanding of the irrigation water percolation process in unsaturated stratified loess in the loess region. © 2021 Elsevier B.V.

This study developed a new method to simultaneously measure gas dispersion coefficient and gas coefficient of permeability (GCP) of an unsaturated soils with due theoretical consideration of steady-state gas transport mechanisms. Correspondingly, a soil column apparatus was developed. Based on the proposed method, gas mixture containing air (with a volumetric concentration >90%) and tracer gas were applied to the column base. The gas dispersion coefficient was estimated based on a measured steady-state tracer gas concentration gradient and a dispersive flux of the tracer gas, the latter of which was determined based on mass conservation of the tracer gas. On the other hand, the GCP was determined based on the total flux of gas mixture and gas pressure gradient following Darcy's law. The new method has the following advantages over existing methods: (1) has an ability to determine gas dispersion coefficient and GCP at any soil depth rather than an average value of a test region; (2) measures the two transport properties at varying soil moisture conditions rather than being limited by uniform moisture distribution; and (3) requires only a simple hand-calculation method to estimate the gas dispersion coefficient without the need to apply nonlinear regression on a gas breakthrough curve. The proposed method to determine GCP and gas dispersion coefficient of a silty sand was verified by independent element tests. © 2022 American Society of Civil Engineers.

This study developed a new method to simultaneously measure gas dispersion coefficient and gas coefficient of permeability (GCP) of an unsaturated soils with due theoretical consideration of steady-state gas transport mechanisms. Correspondingly, a soil column apparatus was developed. Based on the proposed method, gas mixture containing air (with a volumetric concentration >90%) and tracer gas were applied to the column base. The gas dispersion coefficient was estimated based on a measured steady-state tracer gas concentration gradient and a dispersive flux of the tracer gas, the latter of which was determined based on mass conservation of the tracer gas. On the other hand, the GCP was determined based on the total flux of gas mixture and gas pressure gradient following Darcy's law. The new method has the following advantages over existing methods: (1) has an ability to determine gas dispersion coefficient and GCP at any soil depth rather than an average value of a test region; (2) measures the two transport properties at varying soil moisture conditions rather than being limited by uniform moisture distribution; and (3) requires only a simple hand-calculation method to estimate the gas dispersion coefficient without the need to apply nonlinear regression on a gas breakthrough curve. The proposed method to determine GCP and gas dispersion coefficient of a silty sand was verified by independent element tests. © 2022 American Society of Civil Engineers.

Loess slopes along the Yellow River in China are prone to failure in response to intensive irrigation. Existing studies found that the wetting front depth subjected to intensive farming water could barely reach 6 m in loess terrace. Long-term field monitoring revealed that irrigation water takes a short time to recharge the groundwater. Yet the dynamic process and mechanism of irrigation water percolation through thick unsaturated loess remains unrevealed. To address this problem, the Heifangtai terrace was selected as the study area and a series of unique full-scale controlled field irrigation experiments were carried out combined with numerical modelling. The variations in water content, pore-water pressure, and electrical resistivity over time were monitored. From the field testing, the unsaturated stratified loess layer could be divided into two vadose zones based on the water percolation characteristics, namely, transient flow zone (0–6.5 m) and steady flow zone (>6.5 m). Variation in water content presented a hysteresis effect in the transient flow zone compared with the steady flow zone. In the whole seepage process, the vertical percolation rate of 0.5 m/day was double that of the horizontal percolation rate. Furthermore, the irrigation water percolating process was modelled using a finite element code, PANDAS, which was validated by the field testing. Numerical simulation results demonstrated that the movement of wetting front was hampered by the air outflow within transient flow zone and that irrigation water reached the deep groundwater table through the thick unsaturated stratified loess within 100 days, which was consistent with the field monitoring data. This research aids in a better understanding of the irrigation water percolation process in unsaturated stratified loess in the loess region. © 2021 Elsevier B.V.

This study focuses on the coupled thermo-hydro-mechanical processes in unsaturated soils surrounding energy piles. Energy piles are deep foundations that are used to provide support to buildings and for exchanging heat between the ground and an overlying structure. While their performance in saturated and dry soil layers have been well investigated, their performance in unsaturated soils is an emerging topic that poses special challenges. Changes in temperature may induce drying of the soil, which will affect not only the heat transfer process but also the mechanical response at the soil-pile interface. In particular, this study evaluates the coupled processes in unsaturated soils surrounding energy piles using numerical simulations considering coupled heat transfer and water flow with nonequilibrium phase change and enhanced vapor diffusion processes. Further, this study evaluates the changes in yield stress of unsaturated soils during changes in suction which may be induced by thermally induced drying using a novel experimental technique. The numerical simulations and experiments in this study are focused on a low-plasticity soil referred to as Bonny silt, which was selected for this study because the solid particles are chemically inert, the soil retains water over a wide range of suction magnitudes, and many experiments and numerical studies have been performed on the soil that provide a point of comparison for the thermo-hydro-mechanical processes investigated.In the numerical simulations, a three-dimensional coupled heat transfer and water flow model was calibrated using tank-scale tests on Bonny silt then applied to investigate the long-term behavior of solitary energy piles and energy piles in groups having different boundary conditions and thermo-hydraulic initial conditions. The model was first applied to understand the impact of heating and cooling cycles for conditions representative of both heat exchange and heat storage for energy piles in geometric co

This study developed a new method to simultaneously measure gas dispersion coefficient and gas coefficient of permeability (GCP) of an unsaturated soils with due theoretical consideration of steady-state gas transport mechanisms. Correspondingly, a soil column apparatus was developed. Based on the proposed method, gas mixture containing air (with a volumetric concentration >90%) and tracer gas were applied to the column base. The gas dispersion coefficient was estimated based on a measured steady-state tracer gas concentration gradient and a dispersive flux of the tracer gas, the latter of which was determined based on mass conservation of the tracer gas. On the other hand, the GCP was determined based on the total flux of gas mixture and gas pressure gradient following Darcy's law. The new method has the following advantages over existing methods: (1) has an ability to determine gas dispersion coefficient and GCP at any soil depth rather than an average value of a test region; (2) measures the two transport properties at varying soil moisture conditions rather than being limited by uniform moisture distribution; and (3) requires only a simple hand-calculation method to estimate the gas dispersion coefficient without the need to apply nonlinear regression on a gas breakthrough curve. The proposed method to determine GCP and gas dispersion coefficient of a silty sand was verified by independent element tests. © 2022 American Society of Civil Engineers.

Loess slopes along the Yellow River in China are prone to failure in response to intensive irrigation. Existing studies found that the wetting front depth subjected to intensive farming water could barely reach 6 m in loess terrace. Long-term field monitoring revealed that irrigation water takes a short time to recharge the groundwater. Yet the dynamic process and mechanism of irrigation water percolation through thick unsaturated loess remains unrevealed. To address this problem, the Heifangtai terrace was selected as the study area and a series of unique full-scale controlled field irrigation experiments were carried out combined with numerical modelling. The variations in water content, pore-water pressure, and electrical resistivity over time were monitored. From the field testing, the unsaturated stratified loess layer could be divided into two vadose zones based on the water percolation characteristics, namely, transient flow zone (0–6.5 m) and steady flow zone (>6.5 m). Variation in water content presented a hysteresis effect in the transient flow zone compared with the steady flow zone. In the whole seepage process, the vertical percolation rate of 0.5 m/day was double that of the horizontal percolation rate. Furthermore, the irrigation water percolating process was modelled using a finite element code, PANDAS, which was validated by the field testing. Numerical simulation results demonstrated that the movement of wetting front was hampered by the air outflow within transient flow zone and that irrigation water reached the deep groundwater table through the thick unsaturated stratified loess within 100 days, which was consistent with the field monitoring data. This research aids in a better understanding of the irrigation water percolation process in unsaturated stratified loess in the loess region. © 2021 Elsevier B.V.

With the ballast aging, the changes in the size and shape of the ballast particle reduce the drainage capacity, as well as cause a greater permanent deformation of the railroad ballast. Therefore, it is meaningful to investigate the effect of aging on the mechanical behavior of unsaturated ballast, and to estimate the cyclic plastic deformation by considering the aging effects. Here, "aging effect" means the increase in fine fraction content and the particle shape becomes rounded and smooth as compared with fresh ballast. In this study, the influence of aging on the cyclic plastic deformation of unsaturated ballast was evaluated through a series of cyclic loading triaxial compression tests. Test results indicate that the cyclic plastic deformation of ballast is seriously affected by water content, fine fraction content and drainage condition, and the increasing trend becomes more remarkable at the water-rich aged ballast under the fully undrained condition since the effective confining pressure decreases due to the generation of excess pore water pressure. Furthermore, the applicability of two types of estimation models (i.e., a semi-empirical model named University of Illinois at Urbana-Champaign (UIUC) model, and an elastoplastic model named subloading surface extension (SSE) model) to the prediction for cyclic plastic deformation of unsaturated ballast is also verified in this study by comparing with results of cyclic loading triaxial compression tests. As the result, it is revealed that the UIUC model is suitable for predicting the cyclic plastic deformation of fresh ballast (or slightly aged ballast) with different water contents under the fully drained condition, and the SSE model shows good potential to estimate the cyclic plastic deformation of fresh and aged ballasts by considering the effects of water content and drainage condition. The findings of this study indicate that drainage conditions have a significant effect on predicting the cyclic plastic defor

With the ballast aging, the changes in the size and shape of the ballast particle reduce the drainage capacity, as well as cause a greater permanent deformation of the railroad ballast. Therefore, it is meaningful to investigate the effect of aging on the mechanical behavior of unsaturated ballast, and to estimate the cyclic plastic deformation by considering the aging effects. Here, "aging effect" means the increase in fine fraction content and the particle shape becomes rounded and smooth as compared with fresh ballast. In this study, the influence of aging on the cyclic plastic deformation of unsaturated ballast was evaluated through a series of cyclic loading triaxial compression tests. Test results indicate that the cyclic plastic deformation of ballast is seriously affected by water content, fine fraction content and drainage condition, and the increasing trend becomes more remarkable at the water-rich aged ballast under the fully undrained condition since the effective confining pressure decreases due to the generation of excess pore water pressure. Furthermore, the applicability of two types of estimation models (i.e., a semi-empirical model named University of Illinois at Urbana-Champaign (UIUC) model, and an elastoplastic model named subloading surface extension (SSE) model) to the prediction for cyclic plastic deformation of unsaturated ballast is also verified in this study by comparing with results of cyclic loading triaxial compression tests. As the result, it is revealed that the UIUC model is suitable for predicting the cyclic plastic deformation of fresh ballast (or slightly aged ballast) with different water contents under the fully drained condition, and the SSE model shows good potential to estimate the cyclic plastic deformation of fresh and aged ballasts by considering the effects of water content and drainage condition. The findings of this study indicate that drainage conditions have a significant effect on predicting the cyclic plastic defor

With the ballast aging, the changes in the size and shape of the ballast particle reduce the drainage capacity, as well as cause a greater permanent deformation of the railroad ballast. Therefore, it is meaningful to investigate the effect of aging on the mechanical behavior of unsaturated ballast, and to estimate the cyclic plastic deformation by considering the aging effects. Here, "aging effect" means the increase in fine fraction content and the particle shape becomes rounded and smooth as compared with fresh ballast. In this study, the influence of aging on the cyclic plastic deformation of unsaturated ballast was evaluated through a series of cyclic loading triaxial compression tests. Test results indicate that the cyclic plastic deformation of ballast is seriously affected by water content, fine fraction content and drainage condition, and the increasing trend becomes more remarkable at the water-rich aged ballast under the fully undrained condition since the effective confining pressure decreases due to the generation of excess pore water pressure. Furthermore, the applicability of two types of estimation models (i.e., a semi-empirical model named University of Illinois at Urbana-Champaign (UIUC) model, and an elastoplastic model named subloading surface extension (SSE) model) to the prediction for cyclic plastic deformation of unsaturated ballast is also verified in this study by comparing with results of cyclic loading triaxial compression tests. As the result, it is revealed that the UIUC model is suitable for predicting the cyclic plastic deformation of fresh ballast (or slightly aged ballast) with different water contents under the fully drained condition, and the SSE model shows good potential to estimate the cyclic plastic deformation of fresh and aged ballasts by considering the effects of water content and drainage condition. The findings of this study indicate that drainage conditions have a significant effect on predicting the cyclic plastic defor

To investigate unsaturated soil-water interaction at micro-scale, this study extends the numerical manifold method (NMM) by incorporating a soil-water coupling model considering specific capillary water distribution and capillary force calculation. The soil skeleton is constructed by a soil skeleton generation algorithm with random polygons. To more realistically capture the interaction between soil grains and capillary water, a capillary mechanics-based geometric algorithm is proposed to iteratively calculate the capillary water distribution. The capillary forces corresponding to the capillary water distribution are calculated based on the Young-Laplace equation. The proposed capillary water solving framework is first verified by reproducing the soil-water characteristic curve and the capillary water distribution of an ideal contact-disk model against analytical solutions. To further validate the ability of the capillary water solving framework to predict hydraulic behavior of the real soil, a laboratory test on the Toyoura sand is reproduced numerically. Then an ideal direct shear test is performed to further validate the two-way soil-water coupling procedure, in which a comparison between the numerical and analytical results regarding the shear strength and matric suction is presented. Finally, microscopic hydraulic and compression tests are conducted on two soil specimens with the same porosity and mean grain diameter but different uniformity coefficients. The results elucidate that the extended method is a potential tool to explore unsaturated soil behaviors at micro-scale.

A two-layer hydro-mechanically coupled material point method has been formulated in the framework of unsaturated soil mechanics. The method employs two layers of material points to represent coupling effects between the solid and fluid phases. Several numerical examples demonstrate unique features of the method, including free water infiltration into the unsaturated soil, dynamic soil–water interaction in an example of underwater soil collapse and initially saturated soil column collapse. Furthermore, rainfall-induced failure in unsaturated soil slopes is analyzed. The proposed method can well capture change in matric suction, development of shallow and deep-seated shear bands, and finally, large-deformation post-failure behavior. Parametric studies also demonstrate soil cohesion, dilatancy and friction angle play significant roles on the slope failure mechanisms. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

The hydraulic conductivity of unsaturated soils can span several orders of magnitude. The measurement of hydraulic conductivity over a wide suction range is a challenging task for decades. In this study, numerical tests are carried out to check the assumption, error sources, and applicable conditions of the wetting front advancing method and instantaneous profile method. Based on the numerical test results, a digital wetting front advancing method is proposed to measure hydraulic conductivity of unsaturated soils in the relative high suction range and the traditional instantaneous profile method is found to be robust in measuring hydraulic conductivity in the low suction range, e.g. for soil with a degree of saturation above 0.9. Further, a combination of the digital wetting front advancing method and the traditional instantaneous profile method is implemented in the same soil column test and leads to an accurate soil hydraulic conductivity function over a wide suction range. The proposed method is applicable to various soil types and the test duration is only 1 ~ 5 days, thus makes the measurement of hydraulic conductivity a routine experiment for unsaturated soil mechanics.

Artificially-hydrophobized soil has been used in geotechnical applications, such as slope stabilization. This application requires a thorough understanding of the soil's hydromechanical properties to inform stability assessment. The shearing behavior of dry and fully saturated hydrophobized sand has been extensively investigated in the literature. Yet, knowledge on the effects of unsaturation on the soil's hydromechanical properties is scarce. Thus, how hydrophobic coating affects geotechnical properties such as compressibility and stress–dilatancy relation is unclear. This study conducted a comprehensive and systematic test program to, for the first time, quantify the hydromechanical properties of unsaturated hydrophobized Toyoura sand under wide ranges of confining pressure (25–300 kPa) and degrees of saturation (S; 0%–100%). The sand was hydrophobized by dichlorodimethylsilane (DMDCS). Constant-water-content shearing tests and compression tests were performed using the direct-shear box and oedometer apparatus, respectively. The test results revealed that the hydrophobic coating (i) made the compressibility index, swelling index, and peak friction angle independent of S; (ii) switched the sand's stress–strain behavior from strain-softening to strain-hardening at any degree of saturation; (iii) made the low-stress nonlinearity associated with sand dilatancy almost vanish; and (iv) made the peak friction angle to be practically independent of stress. © 2021 Elsevier B.V.

Expansive soils exhibit significant changes in their swell and shrink behaviour upon variation of moisture content. The majority of soils in Queensland are expansive; consequently, failures in natural slopes with expansive characteristics are commonly reported under climate-induced soil moisture variations. In order to understand the failure mechanism of such soil slopes, it is vital to analyse slope stability under unsaturated conditions. The current study employed undisturbed soil samples from natural slopes in Queensland and determined their basic soil properties, including the soil-water characteristic curve (SWCC), and unsaturated shear strength to facilitate the analysis of rainfall-induced slope failure. A series of conventional direct shear tests were conducted for different moisture contents under various vertical stresses. The test results were analysed to obtain the variation of the apparent cohesion and the effective friction angle of the soil with moisture content and suction. The study presents an empirical equation to predict the cohesive component in the shear strength of unsaturated expansive soils as an exponential function of suction. The formulation originated from multiple linear regression analyses for data sets obtained from shear tests using undisturbed soils with varying moisture contents. The empirical equations can realistically predict the reduction in soil cohesion due to wetting (R2 = 0.8666). The study provides an alternative to the quantitative estimation of unsaturated shear strength of expansive soils.

Artificially-hydrophobized soil has been used in geotechnical applications, such as slope stabilization. This application requires a thorough understanding of the soil's hydromechanical properties to inform stability assessment. The shearing behavior of dry and fully saturated hydrophobized sand has been extensively investigated in the literature. Yet, knowledge on the effects of unsaturation on the soil's hydromechanical properties is scarce. Thus, how hydrophobic coating affects geotechnical properties such as compressibility and stress–dilatancy relation is unclear. This study conducted a comprehensive and systematic test program to, for the first time, quantify the hydromechanical properties of unsaturated hydrophobized Toyoura sand under wide ranges of confining pressure (25–300 kPa) and degrees of saturation (S; 0%–100%). The sand was hydrophobized by dichlorodimethylsilane (DMDCS). Constant-water-content shearing tests and compression tests were performed using the direct-shear box and oedometer apparatus, respectively. The test results revealed that the hydrophobic coating (i) made the compressibility index, swelling index, and peak friction angle independent of S; (ii) switched the sand's stress–strain behavior from strain-softening to strain-hardening at any degree of saturation; (iii) made the low-stress nonlinearity associated with sand dilatancy almost vanish; and (iv) made the peak friction angle to be practically independent of stress. © 2021 Elsevier B.V.

A two-layer hydro-mechanically coupled material point method has been formulated in the framework of unsaturated soil mechanics. The method employs two layers of material points to represent coupling effects between the solid and fluid phases. Several numerical examples demonstrate unique features of the method, including free water infiltration into the unsaturated soil, dynamic soil–water interaction in an example of underwater soil collapse and initially saturated soil column collapse. Furthermore, rainfall-induced failure in unsaturated soil slopes is analyzed. The proposed method can well capture change in matric suction, development of shallow and deep-seated shear bands, and finally, large-deformation post-failure behavior. Parametric studies also demonstrate soil cohesion, dilatancy and friction angle play significant roles on the slope failure mechanisms. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

The hydraulic conductivity of unsaturated soils can span several orders of magnitude. The measurement of hydraulic conductivity over a wide suction range is a challenging task for decades. In this study, numerical tests are carried out to check the assumption, error sources, and applicable conditions of the wetting front advancing method and instantaneous profile method. Based on the numerical test results, a digital wetting front advancing method is proposed to measure hydraulic conductivity of unsaturated soils in the relative high suction range and the traditional instantaneous profile method is found to be robust in measuring hydraulic conductivity in the low suction range, e.g. for soil with a degree of saturation above 0.9. Further, a combination of the digital wetting front advancing method and the traditional instantaneous profile method is implemented in the same soil column test and leads to an accurate soil hydraulic conductivity function over a wide suction range. The proposed method is applicable to various soil types and the test duration is only 1 ~ 5 days, thus makes the measurement of hydraulic conductivity a routine experiment for unsaturated soil mechanics.

Pavement Subgrade provides the basic support to the structural layers of a pavement system and hence, it is important to perform a reliable assessment of the condition of the subgrade prior to designing and constructing the road. Because, the failures in the subgrade level, during the design life of the pavement, will have the consequence of occurring pavement distresses before the end of expected pavement life. The condition of the subgrade can be assessed in a laboratory environment by collecting samples from the selected site. However, it is a time-consuming process and would be difficult to perform in more frequent intervals on a selected pavement section. Accordingly, in-situ subgrade assessment methods have gained the popularity owing to the expedite results and the ability to conduct at more frequent intervals in less time. This paper reports a comparison of subgrade assessment conducted on a selected experimental road stretch. In that road stretch, test sections were constructed to assess the field performance of geogrid reinforced roads. The bearing capacity of the subgrade (CBR value) was assessed by conducting borehole tests prior construction stage and Dynamic Cone Penetration Test (DCP), Variable Dynamic Cone Penetration Test (PANDA®), Light Weight Deflectometer Test (LWD) and Falling Weight Deflectometer (FWD) test were performed during and at the end of construction. The comparison of the subgrade CBR from different test methods reveals that DCP provides closer assessment to laboratory investigated CBR value while other test methods show an overestimation.

Type 2.3 granular material is widely used as an unbound pavement material in Queensland, Australia. The mechanical properties such as the permanent and resilient response of this unbound granular material (UGM) under cyclic loading are required in pavement design. Thus, accurate evaluation of these properties through laboratory testing is a prime concern of pavement engineers. These properties are greatly affected by the relative compaction of the material. Therefore, this study was conducted to investigate the effect of relative compaction on the resilient and permanent deformation behaviour of Type 2.3 UGM. A series of Reported Load Triaxial (RLT) tests were performed on the selected UGM by varying the relative density while maintaining the moisture content and stresses constant for all tested specimens. The results of this study indicate that the maximum permanent strain is reduced when the relative density is increased. However, the resilient modulus is approximately constant up to 95% relative compaction but increases when the density is further increased.

A two-layer hydro-mechanically coupled material point method has been formulated in the framework of unsaturated soil mechanics. The method employs two layers of material points to represent coupling effects between the solid and fluid phases. Several numerical examples demonstrate unique features of the method, including free water infiltration into the unsaturated soil, dynamic soil–water interaction in an example of underwater soil collapse and initially saturated soil column collapse. Furthermore, rainfall-induced failure in unsaturated soil slopes is analyzed. The proposed method can well capture change in matric suction, development of shallow and deep-seated shear bands, and finally, large-deformation post-failure behavior. Parametric studies also demonstrate soil cohesion, dilatancy and friction angle play significant roles on the slope failure mechanisms. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.

Artificially-hydrophobized soil has been used in geotechnical applications, such as slope stabilization. This application requires a thorough understanding of the soil's hydromechanical properties to inform stability assessment. The shearing behavior of dry and fully saturated hydrophobized sand has been extensively investigated in the literature. Yet, knowledge on the effects of unsaturation on the soil's hydromechanical properties is scarce. Thus, how hydrophobic coating affects geotechnical properties such as compressibility and stress–dilatancy relation is unclear. This study conducted a comprehensive and systematic test program to, for the first time, quantify the hydromechanical properties of unsaturated hydrophobized Toyoura sand under wide ranges of confining pressure (25–300 kPa) and degrees of saturation (S; 0%–100%). The sand was hydrophobized by dichlorodimethylsilane (DMDCS). Constant-water-content shearing tests and compression tests were performed using the direct-shear box and oedometer apparatus, respectively. The test results revealed that the hydrophobic coating (i) made the compressibility index, swelling index, and peak friction angle independent of S; (ii) switched the sand's stress–strain behavior from strain-softening to strain-hardening at any degree of saturation; (iii) made the low-stress nonlinearity associated with sand dilatancy almost vanish; and (iv) made the peak friction angle to be practically independent of stress. © 2021 Elsevier B.V.

The hydraulic conductivity of unsaturated soils can span several orders of magnitude. The measurement of hydraulic conductivity over a wide suction range is a challenging task for decades. In this study, numerical tests are carried out to check the assumption, error sources, and applicable conditions of the wetting front advancing method and instantaneous profile method. Based on the numerical test results, a digital wetting front advancing method is proposed to measure hydraulic conductivity of unsaturated soils in the relative high suction range and the traditional instantaneous profile method is found to be robust in measuring hydraulic conductivity in the low suction range, e.g. for soil with a degree of saturation above 0.9. Further, a combination of the digital wetting front advancing method and the traditional instantaneous profile method is implemented in the same soil column test and leads to an accurate soil hydraulic conductivity function over a wide suction range. The proposed method is applicable to various soil types and the test duration is only 1 ~ 5 days, thus makes the measurement of hydraulic conductivity a routine experiment for unsaturated soil mechanics.

This is the peer reviewed version of the following article: [Cao, Z, Chen, J, Alonso, EE, et al. A constitutive model for the accumulated strain of unsaturated soil under high‐cycle traffic loading. Int J Numer Anal Methods Geomech. 2021; 45: 990– 1004.], which has been published in final form at https://doi.org/10.1002/nag.3188. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving., The road base is normally situated above the water table and thus in unsaturated state. Experimental results show that the accumulated strains of the unsaturated road base aggregate under high-cycle traffic loads are significantly influenced by the matric suction. To predict the accumulated strain of unsaturated road base aggregate under high-cycle traffic loads, a constitutive model was developed based on the Barcelona Basic Model (BBM) and the shakedown concept. In this model, the shakedown and plastic creep boundaries of the aggregate under cyclic loads were supposed to exist and to have the same shape as the “static” yield surface in BBM. The strain accumulation rates were described as an exponential function of the distance between the peak cyclic stress point and the conjugated point at the current cyclic yield surface. An explicit calculation methodology was adopted to avoid large calculation errors and to improve the calculation efficiency of the model. Comparison between model predictions and testing results proved the accuracy of the proposed model, which can be used as a basic model to predict the long-term deformation of unsaturated road base aggregate under high-cycle traffic loads., The National Natural Science Foundation of China (Grant Nos. 51978611, 51778571)., Peer Reviewed, Postprint (author's final draft)

To investigate unsaturated soil-water interaction at micro-scale, this study extends the numerical manifold method (NMM) by incorporating a soil-water coupling model considering specific capillary water distribution and capillary force calculation. The soil skeleton is constructed by a soil skeleton generation algorithm with random polygons. To more realistically capture the interaction between soil grains and capillary water, a capillary mechanics-based geometric algorithm is proposed to iteratively calculate the capillary water distribution. The capillary forces corresponding to the capillary water distribution are calculated based on the Young-Laplace equation. The proposed capillary water solving framework is first verified by reproducing the soil-water characteristic curve and the capillary water distribution of an ideal contact-disk model against analytical solutions. To further validate the ability of the capillary water solving framework to predict hydraulic behavior of the real soil, a laboratory test on the Toyoura sand is reproduced numerically. Then an ideal direct shear test is performed to further validate the two-way soil-water coupling procedure, in which a comparison between the numerical and analytical results regarding the shear strength and matric suction is presented. Finally, microscopic hydraulic and compression tests are conducted on two soil specimens with the same porosity and mean grain diameter but different uniformity coefficients. The results elucidate that the extended method is a potential tool to explore unsaturated soil behaviors at micro-scale.

Expansive soils are predominant in arid and semi-arid regions, and they are highly moisture sensitive. The investigation of the climate-ground interaction of expansive soils is extremely important to understand the hydro-mechanical responses of such soils. During the past decade, the low cost and simple methods to reliably monitor the soil volumetric water content have been popular among geotechnical practitioners. As a result, EC-5 moisture sensors have been widely used in agricultural and scientific projects due to the minimized salinity and textural effects on the final volumetric water content readings. The changes in soil volumetric water content profile can be effectively monitored using soil-specifically calibrated EC-5 sensors. However, due to the temperature sensitivity of EC-5 sensors, a suitable correction factor is essential to determine the actual volumetric water content of the expansive soil. In this study, the authors present a practical and straightforward approach to calibrate EC-5 sensors incorporating the temperature effect. This simple calibration approach enhances the accuracy of the EC-5 moisture profiling and eventually leads to safe decision-making of geotechnical practitioners.

This study investigates the thermal soil-geosynthetic interaction mechanisms of reinforcing geotextiles confined in compacted silt that may be encountered when using mechanically-stabilized earth (MSE) walls as geothermal heat sinks. A thermo-mechanical geosynthetic pullout device was used that incorporates standard components for geosynthetic pullout or creep testing but also heating elements at the top and bottom of the soil box to apply boundary temperatures and dielectric sensors embedded in the soil layer to monitor distributions in temperature and volumetric water content. Two test series were performed: the first involves monotonic pullout of woven polypropylene geotextiles after reaching steady-state conditions under different boundary temperatures without a seating load, and the second involves monotonic pullout of woven polyethylene-terephthalate geotextiles after reaching steady-state conditions under different boundary temperatures with a seating pullout load. The results indicate that the pullout resistance of both geotextiles decreased with increasing temperature. Although heating led to drying of the unsaturated silt layers as expected, measurements from the second test series indicate accumulation of water at the silt-geotextile interface. An effective stress analysis considering thermal softening of soils indicates that the increase in effective saturation at the silt-geotextile interface was the cause of the decrease in pullout resistance with heating.

Understanding colloid transport in porous media under transient-flow conditions is crucial in understanding contaminant transport in soil or the vadose zone where flow conditions vary constantly. In this article, we provide a review of experimental studies, numerical approaches, and new technologies available to determine the transport of colloids in transient flow. Experiments indicate that soil structure and preferential flow are primary factors. In undisturbed soils with preferential flow pathways, macropores serve as main conduits for colloid transport. In homogeneously packed soil, the soil matrix often serves as filter. At the macroscale, transient flow facilitates colloid transport by frequently disturbing the force balance that retains colloids in the soil as indicated by the offset between colloid breakthrough peaks and discharge peaks. At the pore-scale and under saturated condition, straining, and attachment at solid–water interfaces are the main mechanisms for colloid retention. Variably saturated conditions add more complexity, such as immobile water zones, film straining, attachment to air–water interfaces, and air–water–solid contact lines. Filter ripening, size exclusion, ionic strength, and hydrophobicity are identified as the most influential factors. Our review indicates that microscale and continuum-scale models for colloid transport under transient-flow conditions are rare, compared to the numerous steady-state models. The few transient flow models that do exist are highly parameterized and suffer from a lack of a priori information of required pore-scale parameters. However, new techniques are becoming available to measure colloid transport in real-time and in a nondestructive way that might help to better understand transient flow colloid transport. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality.

The importance of soil water retention characteristics in modelling the hydro-mechanical response of unsaturated soils has been well recognised by many investigators in recent years. Determination of strain-dependent soil water retention curve (SWRC) is likely to be extraordinarily difficult. The first two authors have recently shown that SWRC can be computed from the gradation curve and the calculation result is consistent with the experimental results obtained from pressure plate tests. In this paper, based on a hypothesis related to change in the pore size distribution (POSD) due to volumetric strain of soil skeleton, a method to compute strain-dependent SWRC is presented. It is found that at initial degrees of saturation higher than 0.8, the influence of volumetric strain may be marginal whilst at initial degrees of saturation lower than 0.8, its influence is likely to be substantial. In all cases, the gradation curve of the soil affects the SWRC. © 2020 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences

Thermally driven moisture and vapor transport in unsaturated soil under a thermal loading applied by underground structures affects the magnitude and distribution of matric suction. There is a lack of data about the temperature dependency on the hydraulic conductivity function (HCF). This study (1) developed and used a temperature-controllable soil column for directly and simultaneously measuring the temperature dependency on both the soil-water retention curve (SWRC) and HCF using the instantaneous profile method, and (2) conducted water-vapor heat transport analyses to illustrate the effects of temperature dependency of SWRC and HCF on matric suction. The measurements showed that for sand there was a suction threshold of 2 kPa, below which the hydraulic conductivity of sand heated to 50 degrees C became higher than that at 20 degrees C due to decreased water viscosity. Above this threshold, the hydraulic conductivity at 50 degrees C became lower, by as much as an order of magnitude at 10 kPa suction, because of temperature-induced reduction of water-retention ability. A parametric study revealed that using SWRC or/and HCF obtained at 20 degrees C overestimated the magnitude of suction for soil heated to a temperature higher than 20 degrees C. The overestimations were greater when heating took place in drier soil. The effects of vapor transport were negligible during the early stage of heating, and became prominent only after 7-8 days of continuous constant heating of the soil, regardless of the amount of initial soil suction considered.