GENERAL INTRODUCTION
Landslides are significant natural hazards worldwide due to their frequent occurrence and severe impacts Defined as a mass wasting process, landslides involve the movement of rock, debris, or earth down slopes under gravity's influence They can manifest in various forms, including flowing, sliding, toppling, falling, or spreading, often exhibiting multiple movement types simultaneously Present on all continents, landslides significantly influence landscape evolution and pose serious risks to populations in affected areas.
Soil is a complex mixture of solid particles and interconnected voids that can hold air and water The pore water pressure significantly influences the balance of forces within the soil, often leading to slope failures associated with variations in pore pressure These variations typically follow a hydrological flow path influenced by precipitation or snowmelt, surface interception, evaporation, overland flow, infiltration, transpiration, percolation, groundwater recharge, and drainage into stream systems The coexistence of fluid and solid phases in soil is governed by the principle of effective stress, as established by Terzaghi in 1943.
The behavior of unsaturated soil during water infiltration is crucial in Geomechanics, as it often contributes to the failure of natural slopes, embankments, and artificial soil structures due to rainfall or melting snow Infiltration increases soil saturation and alters the movement of air within the soil, generating stress that can redirect water flow and deform the soil mass based on its inherent properties Deformation, defined as a change in shape or distortion from the original form, typically occurs in response to applied loads or stresses.
Thermal expansion and contraction, along with variations in water content, significantly impact soil behavior, particularly through the wetting process that leads to soil swelling The intensity of this swelling is largely influenced by the soil's infiltration capacity and the behavior of air within the soil mass During natural rainfall, water infiltration occurs at a rate that is dependent on the displacement of air, making the relationship between air and water infiltration crucial Capillary pressure within the soil can modify fluid flow, and if air is unable to escape, it becomes compressed, which in turn decreases the infiltration rate of water Consequently, the generation and evolution of pore pressure during the infiltration process are essential factors to consider.
In slope stability analysis, evaluating pore pressure distribution is crucial, especially during the early stages of a study This research focuses on the physical properties and behavior of pore pressure, particularly in relation to rainfall and water infiltration Understanding this relationship is essential for predicting erosion and landslide potential.
HEAVY RAINFALL AND WATER INFILTRATION
The infiltration theory of surface runoff is based on 2 fundamental concepts:
1 There is a maximum limiting rate at which the soil when in a given condition can absorb rain as it falls This is the infiltration-capacity (Horton, 1933)
2 When runoff takes place from any soil surface, large or small, there is a definite functional relation between the depth of surface detention or the quantity of water which accumulates on the soil surface, and the rate of surface runoff or channel inflow
The infiltration theory, established by Horton in 1945, is grounded in the concepts of continuity and storage equations During heavy rainfall, the process typically unfolds as illustrated in Fig 1.1 Initially, there is a time interval (t1) where the rainfall intensity is below the soil's infiltration capacity, allowing for complete absorption without any surface runoff or accumulation of surface detention However, this initial absorption reduces the infiltration capacity, and once the rainfall intensity exceeds this capacity at time t1, a subsequent interval (td) occurs where excess rainfall generates surface runoff.
When soil absorbs water, it fills surface depressions, preventing runoff Once these depressions are filled, any additional rainfall leads to surface detention, which eventually results in surface runoff.
Fig 1.1: Relation of rainfall to surface runoff, Ewing and Washington block,
In Figure 1.1, rainfall excess refers to the portion of rain that falls at intensities surpassing the soil's infiltration capacity, represented by the cross-sectioned area After the rainfall excess concludes, some surface detention remains, which is gradually released through infiltration or surface runoff During this period, it is common for rain to occur at intensities lower than the current infiltration capacity, contributing to surface runoff However, the overall surface runoff typically remains close to, or only slightly different from, the total rainfall excess (Horton, 1940).
AIR ENTRAPMENT FORMATION BY WATER INFILTRATION
During the infiltration process, water penetrates the soil, creating internal flow within its porous structure As the soil wets, free air is replaced by water, leading to compression of the remaining air, which either dissolves or escapes as bubbles This results in increased pore pressure, as demonstrated by Fredlund (1993), who noted that both pore-air and pore-water pressures rise with total stress during undrained compression, causing a decrease in matric suction Experimental findings indicate that as total stress increases, pore pressures approach a single value, suggesting that even a slight increase in total stress can trigger a chain reaction that minimizes free air volume while matric suction increases indefinitely Consequently, this phenomenon implies that water infiltration may decrease with rising total stress or pore pressure.
It is accordant that a high intensity rainfall would create a runoff surface by an exceeding rate of infiltration capacity of the ground as a rise of pore pressure
Fig 1.2: Schematic cross section of a slope under a heavy rainfall condition
During the infiltration process, water penetrates the soil from the surface, creating internal flow within the porous media As the soil becomes wet, free air is displaced by water, leading to compression, while any remaining air either dissolves in the water or escapes as bubbles This results in an increase in pore pressure due to compression, as demonstrated by Fredlund (1993).
During undrained compression, both air and pore-water pressures rise with increasing total stress, leading to a decrease in matric suction Experimental evidence indicates a continuous rise in pore-air and pore-water pressures, converging towards a single value as total stress increases A minor increase in total stress may trigger a chain reaction that minimizes free air volume to nearly zero, while matric suction approaches infinity Consequently, this implies that water infiltration could be significantly reduced with an increase in total stress or pore pressure.
It is accordant that a high intensity rainfall would create a runoff surface by an exceeding rate of infiltration capacity of the ground as a rise of pore pressure
After heavy rainfall, the water level rises, leading to surface runoff, as illustrated in Fig 1.2 Beneath the water surface, a thin layer becomes saturated due to decreased matric suction, which impedes water infiltration This results in an unsaturated zone between the saturated layer and the water table In this zone, air escapes from the soil as bubbles and dissolves in the water, while the remaining free air is compressed due to water infiltration from both the surface and subsurface in the porous media Consequently, the volume of free air decreases, leading to potential failures from air entrapment formation.
RESEARCH OBJECTIVES
Water infiltration in unsaturated soils poses significant geotechnical challenges, particularly concerning the large deformations and failures of natural slopes and soil structures Such failures can be initiated by the wetting process, which increases moisture content and decreases suction in the soil This study emphasizes the crucial role of pressure parameters in understanding water infiltration phenomena, with a specific focus on the behavior of pore-air pressure during the infiltration process The primary objectives of this research are to investigate these dynamics and their implications for soil stability.
1 To observe behaviors of water movement and investigate behaviors of pore-air pressure in a porous media during water infiltration process
2 To understand the difference of entrapment air and free air within water infiltration process
3 To create a model which can phenomenalize the water infiltration process associated with the variation of pressure parameters Besides, the model could be easily modified to reach the phenomena with every geomaterial and connect with further models which would observe a full influence of heavy rainfall triggering slope failures in the future
LIMITATIONS OF THE STUDY
This study investigates the variation of pore-air pressure during water infiltration through laboratory experiments and numerical simulations, considering the hydraulic gradients and overburden pressure typically encountered in underground openings To enhance focus, specific limitations have been implemented in the research design.
1 Geomaterial is set with dry-air at at initial condition to clearly investigate the variation of pore pressure during infiltration process The situation, then, will be considered as a typical two phase-flow system of water and air in porous media
2 The experiments and simulation will be designed that achieve one dimensional infiltration toward which not allow water leaking from the system in case of consideration air entrapment behavior
3 The experiments and simulation are performed in a controlled environment which has temperature is equal to 20 o C.
THESIS OUTLINE
The chapters of the thesis are organized to explain the following targets of the research
This chapter outlines the study's background, detailing the rationale for selecting the current research, the scope of its objectives, and the primary aims of the investigation.
Chapter 2 explains the related theory behind the present research and some of the research methods and results of the previous workers
Chapter 3 Laboratory experiments of water infiltration
This chapter outlines the methods and instruments used in laboratory experiments, detailing the samples and procedures involved It also discusses the findings and provides recommendations based on the experimental results.
Chapter 4 Numerical simulation of 1-dimensional infiltration problems in geomaterial
Numerical simulations of the 1-dimentional water infiltration problems are presented in this
18 chapter to discuss the variation of pore-air pressure during water infiltration process
The conclusions of this dissertation and recommendations for future works are given
GENERAL
Numerous studies have investigated the infiltration problem in unsaturated soils through column tests (Juan David M D et al 2017, K Kamiya et al 2014, Hong Yang et al 2004) and analytical solutions (Touma, J et al 1986, Shao et al 2015, Paulina, S et al 2016) However, due to the complexities of initial and boundary conditions, multi-layered soils, varying rainfall intensities, and intricate engineering geometries, numerical simulations have become essential for analyzing unsaturated soil infiltration While many numerical studies have addressed these complexities, the behavior of pore pressure, particularly pore air pressure coupled with water movement, remains underexplored This study focuses on the transient vertical infiltration problems, specifically examining the behavior of pore air pressure related to water movement and the concentration of pressure parameters during heavy rainfall conditions.
2.1.1 Previous studies on water infiltration behavior
The vadose zone plays a crucial role in the hydrological cycle, significantly affecting processes such as infiltration, storm runoff, evapotranspiration, interflow, and aquifer recharge Water movement within this zone is typically understood through three key stages: infiltration, redistribution, and drainage or deep percolation Infiltration marks the initial phase where water enters the soil from the surface, often driven by precipitation or surface activities Research by Romano et al (1998) indicates that this process is influenced by capillary and gravity forces, and is generally modeled as one-dimensional vertical flow Following infiltration, the redistribution stage occurs, where water is redistributed within the soil profile.
After water application stops, infiltrated water is redistributed throughout the soil profile, influenced by both capillary and gravitational forces During this redistribution phase, evapotranspiration occurs simultaneously, affecting the quantity of water that can penetrate deeper into the soil The process culminates in deep percolation or recharge, which happens when the wetting front reaches the water table (Ravi et al 1998).
Figure 2.1 Conceptualization of water content profiles during infiltration, redistribution, and drainage (deep percolation) (Ravi et al 1998)
Rainfall-induced infiltration in unsaturated porous media is a widespread phenomenon, particularly in low-lying areas like valleys and slopes Extreme weather conditions, driven by climate change, are expected to increase the variability of infiltration characteristics and the water table levels During rainfall, infiltration alters water uptake, leading to observable skeletal deformations and changes in the water table The rise in the water table following rainfall is a complex process influenced by factors such as soil permeability, initial soil-water conditions, water table position, evapo-transpiration, land cover, and rainfall intensity.
In reality, rainfall infiltration causes soil structure variations Wu et al 2016 explained that during water infiltration into unsaturated porous medium, the porosity in the porous
Changes in the saturation level of a medium impact its properties, with stress modifications in unsaturated porous media leading to alterations in porosity This deformation affects water flow within these media According to Boogaard et al (2016), the hydrology surrounding a landslide area is crucial for pore pressure buildup in the soil, which diminishes shear strength due to the buoyancy force of water in saturated soils and the soil suction in unsaturated conditions.
2.1.2 Effect of pore pressure distribution to water movement within soil
Pore water pressure and capabilities of landslide occurring due to rainfall have been examined by many researchers on different characterizing behavior and mechanical properties
Research has identified various factors contributing to landslides, focusing on the interplay between effective stresses, shear strength, and water flow Steep unsaturated soil deposits can remain stable due to soil suction enhancing shear strength; however, when these soils become wet, reduced suction can trigger shallow landslides (Bogaad et al., 2016) Additionally, soil hydraulic conductivity may be influenced by deformation under saturated conditions, with significant volumetric changes from soil suction potentially leading to the formation of shrinkage cracks (Fredlund et al., 1993).
Research on air pressure behavior in response to water infiltration remains inconclusive, with many mechanical scenarios treating pore air pressure as atmospheric pressure Culligan et al (2000) demonstrated that measuring air pressure can help predict water flux into a column and cumulative infiltration By accurately measuring air pressure in various capillary tubes, it is possible to evaluate how sensitive hydraulic conductivity and sorptivity are to slight increases in air pressure Additionally, the minimal air pressure before the wetting front, approximately 1/4 cm, has a negligible impact on infiltration.
Figure 2.2: Air pressure with time: from top to bottom, capillary tube with different internal diameter (Culligan et al 2000)
K Kamiya and S Yamada (2014) showed that pore air pressure in the soil is generated by the water infiltration This generation is related to the reduction of air permeability at near the soil surface by rainfall And, the soil structure could be affected by the larger pore air pressure This phenomena that is defined as a collapse is also described by Fredlund in 1993, but pore air pressure had not been clearly considered as a main variable
A 2014 study by G.A Siemens et al examined the variations in pore air pressure during water infiltration, highlighting that Richard's equation typically assumes no impedance from the pore air phase However, the findings revealed that even a minor heterogeneity can significantly affect the pore pressure response during the infiltration process.
The D10 gradation difference is minimal, yet the fine layer significantly alters the transient pore pressure regime and the migration of the wetting front This layer leads to ponding during the descent of the wetting front and behaves similarly to a leaky bedrock layer at depth, effectively limiting flow and reducing the speed of the wetting front Under closed conditions, the presence of the fine layer results in prolonged hydrostatic conditions, causing entrapment of the air phase and small water pockets.
23 heterogeneities cannot be ignored to predict the transient pore pressure response during infiltration
Capillary pressure refers to the pressure difference between two immiscible fluid phases within the same pores, resulting from the interfacial tension that must be surpassed for flow to begin This pressure is typically dependent on the level of water saturation in the pores.
In hydrophilic porous media, capillary pressure remains nonnegative, with an increase in capillary pressure (pc) occurring as water saturation decreases, while pore water pressure simultaneously declines.
The relationship between capillary pressure and water saturation is commonly referred to as the suction function, retention function, or soil water characteristic function This concept has been explored in various studies (Pinder et al 2008; Or D and Wraith J, 2002; Kutílek M et al., 1994; Lu N and Likos W).
In a porous medium, as illustrated in Figure 2.2, the interaction between air and water phases is crucial Typically, when the medium is completely saturated with water, the air can only penetrate if its pressure surpasses the water pressure by a certain threshold.
The capillary pressure can be related to the air relative humidity by the Kevin equation: ln( ) gas w c w
Where R gas is the universal gas constant (8.31 J mol -1 K -1 ), T is the Kelvin temperature, Mw is the mole mass of water (0.018 kg mol -1 ), H is the relative air humidity
Figure 2.2 Capillary pressure-water saturation relationship for various air and water flow regimes (Adam S, 2013)
Fig 2.3: Typical capillary functions for sand and clay
MOTIVATION OF THIS STUDY
The literature review highlights the critical role of pore pressure in water movement as a significant factor affecting slope and embankment failures during heavy rainfall This process not only alters soil strength properties but also compromises slope stability Current studies primarily focus on infiltration rates and pore water pressure behavior related to deformation or failures, while the concept of pore air pressure remains largely overlooked The limited attention given to pore air pressure in previous research underscores the need for a comprehensive understanding of its behavior during water infiltration This study aims to address this gap by investigating pore air pressure dynamics through laboratory experiments and numerical simulations, ultimately contributing to the development of a comprehensive influence rating procedure for landslide triggers due to heavy rainfall in future research.
GENERAL
During heavy rainfall, surface runoff can lead to flooding as water accumulates in specific areas This water quickly infiltrates the soil, but if the inflow rate exceeds the soil's infiltration capacity, ponding occurs The infiltration process can be visualized as water moving vertically through a cylindrical tube, reaching the saturation zone above the water table In this scenario, the system is treated as a closed system, preventing air from escaping at the soil column's base Laboratory experiments were conducted to explore this phenomenon, as illustrated in the schematic design of the experiment before and after infiltration.
Fig 3.1: Schematic of the experiment of water infiltration in a closed system a Initial state, b After infiltration
MATERIAL PROPERTIES
Toyoura sand, recognized as standard Japanese sand, serves as the representative geomaterial in this research for laboratory experiments and numerical simulations The material parameters of Toyoura sand, detailed in Table 3.1, align with those established in the studies conducted by Sato et al (2003).
Table 3.1: Material properties of Toyoura sand
Variable Unit Description Value θr Residual volumetric fraction 0.045 θs Saturation volumetric fraction 0.43 α 1/m Van Genuchten alpha parameter 3.6 n Van Genuchten N parameter 4.2 m Van Genuchten M parameter 1-1/N l Van Genuchten L parameter 0.5
K m/s Material hydraulic conductivity 1.7e-4 ρw kg/m 3 Water density 1000 ε Material porosity 0.44
Figure 3.2: Water-retention characteristic curve for Toyoura sand
EXPERIMENTAL PREPARATION AND PROCEDURE
Laboratory experiments are being conducted to study pore-air pressure behavior during water infiltration using a pressure sensor measurement system The equipment, supplied by KEYENCE, includes AP-C35 sensors, a CU-21A control unit, NR-TH08 and NR-500 multi-input data loggers, and Keyence Wave Logger software for recording and analyzing pore air pressure values Additionally, a variety of tools such as tubes, scales, calculators, rulers, and cameras are utilized to facilitate the experiments.
Sensors AP-C35 Control unit CU-21A
Multi-input data loggers NR-TH08, NR-500 Keyence Wave logger
Fig 3.3: Sensor measurement equipment (Source: http://www.keyence.com/)
Figure 3.4 illustrates a schematic diagram of a sand column, highlighting the placement of sensors designed to measure pore air pressure throughout the infiltration process The system comprises key components, including an acrylic cylinder, pore air pressure sensors, and a measurement system Notably, the acrylic columns are equipped with sensors positioned at depths of 1, 5, 10, and 15 centimeters.
At a depth of 29 cm from the bottom of the sand column, measurements of CH4, CH3, CH2, and CH1 were taken (refer to Figures 3.5 and 3.6) The sensors are linked to a data logger, which transfers the data to a computer for recording via specialized software The pore air pressure data is measured in kilopascals (kPa).
Figure 3.4: Schematic diagram of water infiltration experiment and pore air pressure measurement
Experiments involving varying masses of sand and corresponding volumes of water are conducted to analyze surface runoff and flooding formation Water is applied to the soil surface to effectively simulate these phenomena.
Density of Sand used (g/cm3) 1.46 1.38
Density of water used (g/cm3) 1 0.99
Height of water remaining (cm) 0 2.50
RESULT AND DISCUSSION
Figures 3.6 and 3.7 depict the pore air pressure behavior during the build-up stage for both scenarios In each case, there is a noticeable increase in pore air pressure as water begins to absorb into the soil system.
To describe the phenomena, the ideal gas law can be used nw nw
The ideal gas law is expressed by the formula P nw = (nRT) / V nw, where P nw represents the pressure of the gas in Pascals, V nw denotes its volume in cubic meters, n indicates the amount of substance measured in moles, T is the absolute temperature in Kelvin, and R is the ideal gas constant, valued at 8.314 J/K mol.
Figure 3.6: Pore air pressure variation at different points during water infiltration process at the build-up stage (with 4 sensors)
Figure 3.7: Pore air pressure variation at different points during water infiltration process at the buil-up stage (with 2 sensors)
The designed experiment controls airflow to move exclusively through the top surface of a soil column, assuming a constant initial volume of air in the soil During water infiltration, the water fills the porous spaces previously occupied by air, causing the displaced air to escape as bubbles through the soil surface, which remains submerged in water This process results in a decrease in air volume as water moves downward, leading to an increase in pore air pressure according to the ideal gas law, as the pressure of the remaining air rises with the reduction in volume.
Figure 3.8: Escape air from the soil system
Figure 3.7 illustrates the changes in pore air pressure during two water infiltration experiments at the build-up stage Notably, the pore air pressure at CH 1 (5 cm) dropped to zero around second 250, while CH 2 continued to record an increase in pore air pressure This phenomenon occurs as water completely infiltrates the soil column, leaving no external water on the soil surface (refer to Figure 3.9) As water continues to move downward, the surface transitions to the drainage stage, allowing atmospheric air to enter the system and causing the pore air pressure at the 6 cm point to decrease to atmospheric levels Meanwhile, the lower section of the wetting front remains a separate closed system, resulting in a continued increase in pore air pressure as water moves towards the bottom of the soil column.
Figure 3.9: Experiment with 2 sensors after water fully infiltrated
GENERAL INTRODUCTION
Recent focus on numerical simulation for unsaturated soils is rising due to their significance in geotechnical issues, including slope failures and expansive soils These failures often result from both short-term and long-term water infiltration caused by rainfall or melting snow As water infiltrates saturated soils, it increases saturation levels, leading to changes in pore pressure, reduced suction, and decreased shear strength Consequently, one-dimensional infiltration into saturated soils has emerged as a critical area of study, essential for understanding pore pressure behavior and the response of saturated soils.
This study investigates the infiltration process and its impact on pore pressure parameters during one-dimensional water infiltration, particularly during wetting events caused by heavy rainfall, such as flooding or ponding Utilizing multiphase flow theory, various formulas are employed to characterize water infiltration The geomaterial is modeled as a combination of solid and gas phases at the initial state, effectively illustrating the soil's response to water infiltration.
There are two models are performed in the present study to analysis the response of water movement in geomaterial to behavior of pore-air pressure
The experimental program on water infiltration into geomaterials was expanded by utilizing the multiphase flow module from COMSOL Multiphysics This modeling aimed to deliver a comprehensive pore pressure profile during the infiltration process, enabling a better understanding of how air drainage influences the observed behavior.
GOVERNING EQUATION
The governing equations for two-phase flow in porous media are derived from the mass conservation principle, specifically applied to a Darcy-scale representative elementary volume (REV) The total mass change of fluid phase α within the REV must be balanced by the mass flux across its boundary The velocity of each fluid phase relative to the solid phase is described by the extended Darcy formula (Adam, 2013) By substituting the Darcy equation into the mass balance for each phase, we obtain a system of two coupled partial differential equations.
nw nw nw s rnw nw nw 0 nw
In the context of fluid dynamics, subscripts denote the non-wetting (air) and wetting (water) phases, represented as nw and w, respectively Key parameters include fluid density (ρ in kg/m³), phase saturation (S), matrix porosity (𝜙), dynamic viscosity (μ in Pa s), absolute permeability (κs in m²), relative permeability (kr), fluid pressure (p in Pa), and gravitational acceleration (g in m/s²).
Immiscible fluids are distinguished by a clear interface, known as the surface tension (σaw) at the air-water boundary At 20°C, the surface tension is measured at 0.0726 N/m, which decreases as temperature rises.
The permeability s and hydraulic conductivity K (m/s) are related to the viscosity μ and density ρ of the fluid, and the acceleration of gravity g by
Capillary pressure (p c) is defined as the difference in pressure between the non-wetting phase and the wetting phase, expressed as p c = p nw - p w This pressure can be converted into an equivalent height of water, known as the capillary pressure head, measured in meters (m).
The van Genuchten model (1980) relates the volumetric water content (θ) to the hydraulic head (h)
Where θ r and θ s are the residual moisture content and the saturation moisture content; α and n are fitting parameters; m = 1 – 1/n
Assuming that the porous media is nondeformable implies that w nw 1
The capillary pressure-saturation function is explained by the van Genuchten equation (VG)
Sew represents the effective saturation of wetting fluids, calculated using the formula Sew = (θw - θwr)/(θws - θwr) In this equation, θws refers to the saturated wetting fluid saturation, while θwr indicates the residual wetting fluid saturation Additionally, α vg and n are fitting parameters, with m defined as m = 1 - 1/n.
The pore space in a medium can only be occupied by one fluid at a time, which directly influences the effective saturation of each phase It is essential to note that the total saturation of air and water combined must equal one.
The specific capacity of the wetting phase is influenced by variations in effective saturation related to capillary pressure In contrast, the specific capacity of the nonwetting phase is determined through the relationships of effective saturations between the two phases.
The hydraulic properties relative to the wetting fluid in van Genuchten retention are described as following:
Where α, n, m, and L denote soil characteristics Note that with two-phase flow, the van Genuchten-Mualem formulas hinge on the value of Hc
For the nonwetting fluid, the properties are
The simulation utilizes Toyoura sand as the primary material, with its properties derived from previous studies and calculations (Mori et al., 1986; Feike J Leji et al., 1996; Sato et al., 2003) The accompanying table presents essential data for the fluid-flow model.
Variable Unit Description Value θr Residual volumetric fraction 0.045 θs Saturation volumetric fraction 0.43 α 1/m Van Genuchten alpha parameter 3.6 n Van Genuchten N parameter 4.2 m Van Genuchten M parameter 1-1/N
K m/s Material hydraulic conductivity 1.7e-4 ρw kg/m 3 Water density 1000 ρa kg/m 3 Air density 1.28 μw Pa.s Water dynamic viscosity 0.001 μa Pa.s Air dynamic viscosity 1.81e-5 σaw N/m Interfacial tension air-water 0.0726 ϕ Material porosity 0.44
GENERAL
This study examines pore air pressure behavior in soils during water infiltration, utilizing laboratory experiments and numerical simulations of one-dimensional water infiltration in typical geomaterials The research focuses on the relationship between pore pressure distribution and water movement, particularly during the infiltration process Key findings are summarized below.
The use of optically matched pore fluid in transparent soil enabled direct observation of the wetting front and air phase movement within the soil profile during laboratory experiments This setup facilitated the measurement of saturation distribution along the column length Numerical simulations that accounted for the unsaturated properties of two transparent soil gradations effectively captured the migration of the wetting front and the transient moisture regime Results indicated that under closed conditions, the wetting front migrates significantly slower after an initial rapid absorption phase During closed infiltration, air movement is restricted to upward movement through the advancing wetting zone, leading to bubble formation at the soil surface, while water continues to absorb into available pore spaces until the wetting front reaches the bottom of the soil column.
Pore pressure behavior is closely linked to the velocity of the wetting front's movement In a closed system, pore air pressure increases rapidly as water infiltrates upon reaching the soil surface During this process, the pore air pressure beneath the wetting front remains consistent across various points within the soil A decrease in pore air pressure at a specific location occurs only when it interacts with the wetting front, while pressure levels in other areas continue to rise.
Currently, pore air pressure is roughly equal to capillary pressure and increases with depth This pore air pressure reduces the infiltration rate by lowering capillary pressure, while the time lag in pore air pressure at specific points reflects the speed of the wetting front's advancement Furthermore, soil saturation is only achieved when pore air pressure reaches zero.
The simulation demonstrated that in an open system, the rising water table influences pore air pressure, which remains close to zero throughout the process due to its contact with the atmosphere While pore air pressure can influence the movement of wetting fluids, its impact is minimal and can generally be disregarded in this context.
FUTURE RECCOMANDATIONS
1 The effects of initial water content and external water flow are not considered in the present experiments and simulations Therefore, it is meaningful to apply above conditions simultaneously with pore pressure measurements
2 Finite element method (FEM) can be used to evaluate the distribution of residual air within soil system with aim of better understanding pore pressure behavior with the effect of water infiltration
3 The water infiltration coupled capillary rise from water table should be applied in a general model to fully understand the phenomena of the behavior of pore pressure associated with water movement within soil
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