The soil-water characteristic curve (SWCC) is a fundamental relationship in unsaturated soil mechanics that relates the amount of water in the soil to the corresponding matric suction. Hysteresis, which refers to the dependence of the SWCC on the wetting and drying path, has been observed in experimental studies. Several factors have been proposed as contributors to SWCC hysteresis, including air entrapment, contact angle hysteresis, ink-bottle effect, and changes in soil fabric due to swelling and shrinkage. However, the significance of each factor's contribution is still debated. To investigate the underlying source of SWCC hysteresis, pore-scale numerical simulations were conducted using the multiphase lattice Boltzmann method (LBM). The LBM is a numerical method that allows for simulating fluid with multiple phases (liquid and gas) at the pore-scale. While previous studies have explored various mechanisms using LBM, none have specifically examined hysteresis effects. In this study, both single-phase and multiphase LBM simulations were performed. The single-phase LBM discretizes a fluid domain into a grid of equal spacings called lattice nodes. Each lattice node represents a collection of fluid molecules or particles with discrete velocities. Macroscopic quantities such as mass density and fluid velocity can be calculated from particle distribution functions at each lattice node. The multiphase LBM extends this approach to simulate multiple phases or components within a fluid domain. In this case, liquid and gas phases were considered. The simulation involved wetting (pore filling) and drying (pore emptying) experiments on 2D and 3D granular packings. The results showed that even when controlling for factors such as air entrapment, contact angle hysteresis, ink-bottle effect, and changes in soil fabric due to swelling and shrinkage, SWCC hysteresis still occurred. This indicated the presence of an underlying source that was not accounted for by these factors. By comparing the liquid/gas phase distributions during wetting and drying experiments, it was observed that during wetting, many liquid bridges expanded simultaneously and joined together to fill the pores from smallest to largest. This allowed for menisci with larger radii of curvature (lower matric suction). On the other hand, during drying, only existing gas clusters could expand, but they became constrained by the size of the pore openings surrounding them. As a result, menisci with smaller radii of curvature (higher matric suction) were formed. These findings suggest that the simultaneous expansion and joining of liquid bridges during wetting play a crucial role in SWCC hysteresis. This mechanism is not captured by current factors proposed as contributors to hysteresis. The study highlights the importance of considering pore-scale phenomena in understanding SWCC hysteresis and provides valuable insights for future research in unsaturated soil mechanics.
The soil-water characteristic curve (SWCC) is a way to show how much water is in the soil based on how tightly the soil holds onto it. Hysteresis is when the SWCC changes depending on if the soil is getting wet or drying out, and there are different reasons for this. Scientists used computer simulations to study hysteresis effects in the SWCC, focusing on how water moves through small spaces in the soil. They did experiments with wetting and drying on packed soil to see what happens. Even when they tried to control other factors, they still saw hysteresis happening, which means there must be something else causing it. The joining of liquid bridges during wetting is an important part of hysteresis that we don't fully understand yet. Studying small-scale things happening in the soil can help us learn more about hysteresis and do more research in the future."
Definitions
- Soil-water characteristic curve (SWCC): A way to show how much water is in the soil based on how tightly the soil holds onto it.
- Hysteresis: When something changes depending on if it's getting wet or drying out.
- Simulations: Using computers to create models or experiments.
- Wetting: When something gets wet with water.
- Drying: When something loses moisture and becomes dry again.
- Packed soil: Soil that has been pressed down tightly into a solid form.
- Factors: Different things that can affect or
Title: Investigating the Source of Hysteresis in Soil-Water Characteristic Curves Using Pore-Scale Numerical Simulations
Introduction:
The soil-water characteristic curve (SWCC) is a fundamental relationship in unsaturated soil mechanics that describes the amount of water in the soil as a function of matric suction. This relationship plays a crucial role in understanding and predicting the behavior of unsaturated soils, which are prevalent in many engineering applications such as slope stability, foundation design, and landfills. However, experimental studies have shown that SWCCs exhibit hysteresis, meaning that they differ depending on whether the soil is being wetted or dried. This phenomenon has been observed for decades but its underlying source remains debated.
Factors Contributing to Hysteresis:
Several factors have been proposed as contributors to SWCC hysteresis, including air entrapment, contact angle hysteresis, ink-bottle effect, and changes in soil fabric due to swelling and shrinkage. Air entrapment occurs when air pockets get trapped within the pores during wetting and can affect the flow of water through the soil. Contact angle hysteresis refers to differences in contact angles between advancing (wetting) and receding (drying) menisci at solid-liquid interfaces. The ink-bottle effect describes how pore size distribution affects capillary pressure-saturation relationships. Changes in soil fabric due to swelling and shrinkage occur when clay minerals absorb or release water causing changes in their volume.
Pore-Scale Numerical Simulations:
To investigate the underlying source of SWCC hysteresis, researchers turned to pore-scale numerical simulations using the multiphase lattice Boltzmann method (LBM). LBM is a numerical method that allows for simulating fluid with multiple phases at a microscopic level by discretizing fluid domains into grids called lattice nodes.
Methodology:
In this study, both single-phase and multiphase LBM simulations were performed on 2D and 3D granular packings. The single-phase LBM simulated a fluid domain with only one phase (liquid or gas) while the multiphase LBM extended this approach to simulate multiple phases within a fluid domain. Wetting and drying experiments were conducted by filling and emptying pores with liquid and gas phases.
Results:
The results of the simulations showed that even when controlling for previously proposed factors contributing to hysteresis, SWCC hysteresis still occurred. This indicated the presence of an underlying source that was not accounted for by these factors. By comparing the liquid/gas phase distributions during wetting and drying experiments, researchers observed that during wetting, many liquid bridges expanded simultaneously and joined together to fill the pores from smallest to largest. On the other hand, during drying, only existing gas clusters could expand but were constrained by pore openings surrounding them. As a result, menisci with smaller radii of curvature (higher matric suction) were formed.
Conclusion:
The findings from this study suggest that simultaneous expansion and joining of liquid bridges play a crucial role in SWCC hysteresis at the pore-scale level. This mechanism is not captured by current factors proposed as contributors to hysteresis. The study highlights the importance of considering pore-scale phenomena in understanding SWCC hysteresis and provides valuable insights for future research in unsaturated soil mechanics.
Implications:
Understanding the source of SWCC hysteresis can lead to improved models for predicting soil behavior under different moisture conditions. This knowledge can also aid in developing more accurate methods for measuring soil properties such as hydraulic conductivity and shear strength.
Future Research:
This study opens up new avenues for future research on SWCC hysteresis using numerical simulations at a microscopic level. Further investigations could explore how different soil types or particle sizes affect hysteresis and how it varies under different boundary conditions. Additionally, the findings from this study could be validated through experimental studies.
Conclusion:
In conclusion, the pore-scale numerical simulations using LBM have provided valuable insights into the source of SWCC hysteresis. The study highlights the importance of considering pore-scale phenomena in understanding soil behavior and provides a new perspective on the underlying mechanism behind SWCC hysteresis. This research has implications for improving our understanding of unsaturated soils and can lead to more accurate predictions in engineering applications.
The soil-water characteristic curve (SWCC) is the most fundamental relationship in unsaturated soil mechanics, relating the amount of water in the soil to the corresponding matric suction. From experimental evidence, it is known that SWCC exhibits hysteresis (i.e. wetting/drying path dependence). Various factors have been proposed as contributors to SWCC hysteresis, including air entrapment, contact angle hysteresis, ink-bottle effect, and change of soil fabric due to swelling and shrinkage, however, the significance of their contribution is debated. From our pore-scale numerical simulations, using the multiphase lattice Boltzmann method, we see that even when controlling for all these factors SWCC hysteresis still occurs, indicating that there is some underlying source that is not accounted for in these factors. We find this underlying source by comparing the liquid/gas phase distributions for simulated wetting and drying experiments of 2D and 3D granular packings. We see that during wetting (i.e. pore filling) many liquid bridges expand simultaneously and join together to fill the pores from the smallest to the largest, allowing menisci with larger radii of curvature (lower matric suction). Whereas, during drying (i.e. pore emptying), only the limited existing gas clusters can expand, which become constrained by the size of the pore openings surrounding them and result in menisci with smaller radii of curvature (higher matric suction).
