The build up of pore air pressure associated with water in filtration into geomaterials under heavy rainfall condition

1 Master of Engineering dissertation TRAN TUAN ANH 16ME136 Graduate School of Science and Engineering Saitama University, Japan February, 2018 THE BUILD-UP OF PORE-AIR PRESSURE ASSOC

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Master of Engineering dissertation

TRAN TUAN ANH (16ME136)

Graduate School of Science and Engineering

Saitama University, Japan

February, 2018 THE BUILD-UP OF PORE-AIR PRESSURE ASSOCIATED

WITH WATER INFILTRATION INTO GEOMATERIALS

UNDER HEAVY RAINFALL CONDITION

A dissertation submitted to the Graduate School of Science and

Engineering in partial fulfillment of the requirement of the degree of

Professor Dr Masahiko Osada

Rock Mechanics Laboratory

Graduate School of Science and Engineering Department of Civil Engineering

Saitama University

Japan

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ABSTRACT

Water infiltration into unsaturated soils is an important geotechnical problem related to large deformation and failure of natural slopes and soil structures The failure of soils can be triggered by a wetting process from an unsaturated stage resulting from an increase in moisture content and a decrease in suction It is suggested that the pressure parameters play a significant role in investigation water infiltration phenomena Pore-air generally does not impede infiltration rates or wetting front movement when the water table is at depth However, pore-air entrapment is not often considered as a function of water infiltration process but affects Therefore, the study of water infiltration into unsaturated soils becomes an interesting topic due to the necessity of understanding the complex nonlinear interaction among the hydrological conditions, the hydraulic and pressure parameters of the unsaturated soils related to water infiltration

The objective of this study is to investigate the variation of pressure parameters associated with the water infiltration into geomaterials as a function to develop a complete influence rating procedure of heavy rainfall triggering landslide in further studies

To this end, a series of numerical simulation method associated with laboratory experiments based on the theory of multiphase-flow in porous media were carried out The laboratory experiments were conducted that there were two different column of sandy soil cases developed to evaluate the influence of pore-air entrapment on infiltration under different initial conditions The bottom of the soil column is bounded to make the air entrapment condition Neither the air nor the water can pass through the vertical column walls In the simulation method, a model was designed which fit with the laboratory experiments to investigate the behavior of pore air pressure during water infiltration in general Besides, a column of sandy soil was conducted that assumes water rising from the base as the effect of water table with soil

in open system The soil surface approaches the atmosphere and there is no air escape from the base The incoming water from the base force the water elevation toward the surface of the soil column as the effect of capillary pressure

The results showed that under closed conditions the wetting front migrates significantly slower following a rapid absorption at the early stage During closed infiltration, the only avenue for the movement of air phase is upwards through the advancing wetting zone to the soil surface and leak out as bubbles, which allows water absorbing to available pores space

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until the wetting front reaching the bottom of the soil column

The pore pressure behavior corresponds with the velocity of the movement of wetting front In the closed system, pore air pressure jumps up as fast as water infiltrating when it contacts to the soil surface During the interval, pore air pressure that is under the wetting front

is similar at any points within the soil system Pore air pressure at a specific position within soil will decrease only when it contacts to the wetting front whilst the remaining keep rising

At the moment, pore air pressure is approximately capillary pressure So that, the pore air pressure increases proportionally to the depth Pore air pressure also slows down the infiltration rate by the reduction of capillary pressure, the time lag between the pore air pressure at considered points indicates the velocity of advance of the wetting front Besides, the soil will not be fully saturated until pore air pressure is equal to zero

In open system, the air phase contacts to the atmosphere, and pore air pressure is approximatess zero in entire time The pore air pressure can still affect to the migration of wetting fluid, but negligible So that, the effect of pore air pressure can be ignored in the open system

Keywords:

Pore-air pressure, heavy rainfall, numerical simulation, infiltration

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ACKNOWLEDGEMENTS

It is my great pleasure to submit this thesis to the Graduate School of Science and Engineering, Department of Civil Engineering, Saitama University for the partial fulfilment of the degree in Master of Engineering This dissertation would not have being a real fulfillment without the backing and corporation from various individuals through various means It is a pleasure to convey my gratitude to all of them

In the first place I owe my everlasting gratefulness to my supervisor, Professor, Dr Masahiko Osada for his keen supervision I take this opportunity to convey my heartiest gratitude to Professor Dr Masahiko Osada, my academic supervisor for the guidance and supervision rendered during my research to make it successful His truly scientist perception has made him as a constant oasis of ideas and passions in science, which exceptionally inspire and enrich my growth as a student, a researcher and a scientist Your patient guidance and valuable comments and making me well experienced on academic writing and resource handling

It is my pleasure to convey my noble thanks to Associate Professor Dr Tadashi Yamabe, Professor, Dr Kawamoto Ken for valuable advices Special gratitude goes for Associate Professor, Dr Chiaki T Oguchi who gave us great occasions to travel many locations

in Japan Thanks for your guidance and their willingness to share experience with us It is my pleasure to convey my thanks to Senior Professor Jiro Kuwano for giving us opportunity to join geotechnical field visits and enjoyable ski tour These field visits helped to learn new approaches in geotechnical field and also, we could explore many places around Japan

I appreciate the help that I got from Rock mechanics lab members as well as the friends

of Geosphere Research Institute It is a pleasure to pay a special tribute to KESCO, Ltd company especially Mr Kuo Ozawa, Mr Yuto Takahashi, and Mr Dahai Mi who guide me in various aspects of numerical simulation with COMSOL Multiphysics in my research work Further it is my duty to remember Mr Kenjiro Okada who being my tutor and a kind person in all my academic and nonacademic work Special thanks go to Tsuchiya san, Hosokawa san and Araya san for the continuous support during my laboratory experiments and friendship that share with me all the time

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My special thanks go to Asian Development Bank to offer me a valuable scholarship

to study in a world first class country like Japan without any financial difficulties I am very grateful to the international students, staff members of GRIS including Nara san, Foreign Student Office with Yuko Mori san and Sachiko Shimodaira san, Saitama University International House, International affairs office and graduate school staff and Japanese language teacher, Jonishi sensei for guide me the life in Japan

I would like to dedicate this dissertation to my loving family who show me the clear path of my life and being with me all the time Your courage, support and love helped me a lot

to achieve all my targets throughout the life

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TABLE OF CONTENT

ABSTRACT 1

ACKNOWLEDGEMENTS 4

TABLE OF CONTENT 6

LIST OF FIGURES 8

LIST OF TABLES 11

CHAPTER 1 12

INTRODUCTION 12

1.1 GENERAL INTRODUCTION 12

1.2 HEAVY RAINFALL AND WATER INFILTRATION 13

1.3 AIR ENTRAPMENT FORMATION BY WATER INFILTRATION 15

1.4 RESEARCH OBJECTIVES 16

1.5 LIMITATIONS OF THE STUDY 17

1.6 THESIS OUTLINE 17

CHAPTER 2 19

LITERATURE REVIEW 19

2.1 GENERAL 19

2.1.1 Previous studies on water infiltration behavior 19

2.1.2 Effect of pore pressure distribution to water movement within soil 21

2.2 MOTIVATION OF THIS STUDY 25

CHAPTER 3 26

LABORATORY EXPERIMENTS OF WATER INFILTRATION 26

3.1 GENERAL 26

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3.2 MATERIAL PROPERTIES 27

3.3 EXPERIMENTAL PREPARATION AND PROCEDURE 28

3.4 RESULT AND DISCUSSION 31

CHAPTER 4 35

NUMERICAL SIMULATION OF 1-DIMENSIONAL INFILTRATION PROBLEMS IN GEOMATERIAL 35

4.1 GENERAL INTRODUCTION 35

4.2 GOVERNING EQUATION 36

4.3 RESULTS AND DISCUSSION 39

4.3.1 Water infiltration in a closed system 39

4.3.2 Capillary rise in open system 46

CHAPTER 5 53

CONCLUSIONS 53

5.1 GENERAL 53

5.2 FUTURE RECCOMANDATIONS 54

REFERENCES 55

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LIST OF FIGURES

Fig 1.1: Relation of rainfall to surface runoff, Ewing and Washington block,St Louis, Sept

7, 1916 14Fig 1.2: Schematic cross section of a slope under a heavy rainfall condition 15Figure 2.1 Conceptualization of water content profiles during infiltration, redistribution, and drainage (deep percolation) (Ravi et al 1998) 20Figure 2.2: Air pressure with time: from top to bottom, capillary tubewith different internal diameter (Culligan et al 2000) 22Figure 2.2 Capillary pressure-water saturation relationship for various air and water flow regimes (Adam S, 2013) 24Fig 2.3: Typical capillary functions for sand and clay 24Fig 3.1: Schematic of the experiment of water infiltration in a closed system

a Initial state, b After infiltration 26Figure 3.2: Water-retention characteristic curve for Toyoura sand 27

Fig 3.3: Sensor measurement equipment (Source: http://www.keyence.com/) 28

Figure 3.4: Schematic diagram of water infiltration experiment

and pore air pressure measurement 29Figure 3.5: Sensors arrangement (4 sensors) 30Figure 3.6: Sensors arrangement (2 sensors) 31Figure 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) 32Figure 3.8: Escape air from the soil system 33Figure 3.9: Experiment with 2 sensors after water fully infiltrated 34Figure 4.1: Schematic diagram of water infiltration in closed system

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a Initial state, b After infiltration 40Figure 4.2: Geometry of the model and boundary condition 41Figure 4.3: Considered points and Meshing

a – Considered points, b – Meshing 41Fig 4.4: Water saturation profiles at time t=0 min(a), and t = 5 min (b) 42Figure 4.5: Pore air and pore water pressure contribution at different positions

of soil column 42Figure 4.6: Pore air pressure profiles at different positions of soil column

from times t=0 min to t=60 min 43Figure 4.7: Pore air pressure profiles at different positions of soil column

in buil-up stage 43Figure 4.8: Pore water pressure profiles at different positions of soil column

from times t=0 min to t=60 min 44Figure 4.9: Capillary pressure profiles at different positions of soil column

in entire process (a), and build-up stage (b) 45Figure 4.10: Pore air pressure profiles at different positions of soil column

a 7 cm hydraulic head, b 10cm hydraulic head 46Figure 4.11: Schematics of capillary raise model in opened system 48Figure 4.12: Geometry of the model and boundary condition

a – wetting phase; b – nonwetting phase; blue line: no flow 48Figure 4.13: Considered points and Meshing

a – Considered points, b – Meshing 49Figure 4.14: Water saturation profiles from time t=0 min(a), and t = 60 min (b) 49Figure 4.15: Pore air and pore water pressure contribution

at different positions of soil column 50Figure 4.16: Pore air pressure profiles at different positions of soil column

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LIST OF TABLES

Table 3.1: Material properties of Toyoura sand _ 27Table 3.2: Experiments parameters _ 30Table 4.1: Material parameters 38

of rock, debris, or earth down a slope, under the influence of gravity (Cruden and Varnes, 1996 and Hung et al., 2013) Landslides involve flowing, sliding, toppling, falling, or spreading, and many landslides exhibit a combination of different types of movements, at the same time or during the lifetime of the landslide Landslides are present in all continents and play an important role in the evolution of landscapes In many areas they also pose a serious threat to the population (Petley, 2012)

Soil is a complex package of solid particles of various sizes and shapes The network

of interconnected voids between the solid particles may contain air and water The pressure of water in the pores strongly affects the equilibrium of forces in a soil, and indeed many slope failures are related to pore pressure variations basically following a standard hydrological flow path of precipitation or snowmelt, above surface interception and detention, and consequent evaporation or overland flow, infiltration, transpiration, percolation and (perched) groundwater recharge, and drainage toward stream/river systems The main consequence of the coexistence

of fluid and solid phases in a soil is the principle of effective stress (Terzaghi, 1943; Abramson

et al., 1996)

It has been well recognized that the behavior of unsaturated soil subjected to water infiltration plays an important role in Geomechanics This is because the failure of natural slopes, embankments, and artificial soil structures is most often due to both short and long infiltrations caused by rainfall or melting snow Water infiltrating into unsaturated soils results

in an increase of saturation As the movement of water during infiltration, behaviors of free air

in soil may lead to some stress generation This stress leads to the diversion of infiltration and deformed the soil mass to some extent depending on its inherent susceptibility Deformation of

an object is defined as change in shape or other forms of distortion from its original shape Deformation occurs usually in response to an applied load or stress, but it may result from

In the analysis of stability of slopes in terms of effective stresses, the pore pressure distribution is of fundamental importance and its evaluation is one of the prime objectives in the early stages of any stability study Therefore, the present study with particular emphasis placed on the physical properties and behavior of pore pressure related to phenomenon that are occurred within rainfall especially water infiltration stage Understanding the relation and how

it is behaving as the initial step and to establish the prediction of the capabilities of erosion or

landslide

1.2 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

These two concepts, in connection with the equation of continuity or storage equation, from the basis of the infiltration theory (Horton, 1945) The march of events during a heavy rainfall which produces surface runoff is usually as shown by Fig 1.1 At the start, there is an interval t1 of initial rain at intensity less than infiltration-capacity During this interval, the rain

is all absorbed by the soil, no surface runoff occurs, and no surface detention accumulates The infiltration-capacity is, however, reduced by this rain until, at the time t1, it becomes less than the rain intensity Then during a second interval td the excess rain above, the amount absorbed

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by the soil goes to fill the surface depressions and no runoff occurs When the surface depressions are filled, rainfall excess continuing produces, first, surface detention, and, from this, surface runoff

Fig 1.1: Relation of rainfall to surface runoff, Ewing and Washington block,

St Louis, Sept 7, 1916

On Fig 1.1, the part of the rain which falls at intensities exceeding infiltration-capacity

is designated rainfall excess and this is indicated by the cross-sectioned area At the end of rainfall excess, tn, previously accumulated surface detention still remains and is gradually disposed of by infiltration or by surface runoff During the interval while surface detention is disappearing, there may be and usually is rain at an intensity less than the then infiltration-capacity of the soil, and this residual rain does in part into surface runoff, but the total surface runoff in most cases is sensibly equal to or at least not greatly different from the total rainfall excess (Horton, 1940)

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1.3 AIR ENTRAPMENT FORMATION BY WATER INFILTRATION

At the beginning of the infiltration process, water absorbs into soil body from surface and produces internal flow within the porous media During the wetting process, the free air is replaced

by water and compressed, the remaining air is dissolved into water or escaped as bubbles Consequently, the pore pressure increases by compression Fredlund (1993) showed that the pore-air and pore-water pressures increase as the total stress increases during undrained compression This cause to a decrease of the matric suction The experimental evidence supports a continual increase of the pore-air and pore-water pressure, approaching a single value as the total stress is increased It would appear that a slight increase in total stress could cause a chain reaction process which would reduce the free air volume to an infinitesimal size while the matric suction goes to infinity This means the water infiltration could be reduced as a rise of 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

At the beginning of the infiltration process, water absorbs into soil body from surface and produces internal flow within the porous media During the wetting process, the free air is replaced

by water and compressed, the remaining air is dissolved into water or escaped as bubbles Consequently, the pore pressure increases by compression Fredlund (1993) showed that the pore-

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air and pore-water pressures increase as the total stress increases during undrained compression This cause to a decrease of the matric suction The experimental evidence supports a continual increase of the pore-air and pore-water pressure, approaching a single value as the total stress is increased It would appear that a slight increase in total stress could cause a chain reaction process which would reduce the free air volume to an infinitesimal size while the matric suction goes to infinity This means the water infiltration could be reduced as a rise of 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 shows a concept that the water level increases and runoff surface occur after heavy rainfall A thin layer under the water surface is saturated (In reality, there is often not a sharp wetting front and/or the soil above the wetting front may not saturated) which is created by a decrease of matric suction reducing water infiltration It traps an unsaturated zone between itself and the water table In this zone, there is number of porous air escapes from soil as bubbles and dissolves into water, the remaining free air is compressed by replacing of water infiltration from water surface and subsurface in porous media As the result, the volume of free air further decreases This creates failures as the formation of air entrapment

1.4 RESEARCH OBJECTIVES

Water infiltration into unsaturated soils is an important geotechnical problem related to large deformation and failure of natural slopes and soil structures The failure of soils can be triggered by a wetting process from an unsaturated stage resulting from an increase in moisture content and a decrease in suction It is suggested that the pressure parameters play a significant role in investigation water infiltration phenomena Understanding the behavior of pore-air pressure during water infiltration process is mainly concerned in this study The objectives of this study are:

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

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1.5 LIMITATIONS OF THE STUDY

This study focuses on studying the variation of pore-air pressure during water infiltration with laboratory experiments and numerical simulation, the actual situation in underground openings experiences hydraulic gradients and overburden pressure Limitations are made to simplify and

make this study more focus as follow:

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 20oC

Chapter 2 Literature review

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

In this chapter methods and instrumentation for laboratory experiments are explained with the details of samples and experimental procedures Besides, elaborates the findings and recommending reasons for the results of the experiments

Chapter 4 Numerical simulation of 1-dimensional infiltration problems in geomaterial

Numerical simulations of the 1-dimentional water infiltration problems are presented in this

al 1986, Shao, et al.2015, Paulina, S et al 2016,) Recently, numerical solutions have been used to analyze the problem of unsaturated soil Numerical simulations are necessary because

of the complicated initial and boundary conditions, the multi-layered soils, the different rainfall intensities, and the geometry of many engineering problems, whereas the analytical solutions cannot be obtained Many numerical studies that can account for the inherent complexities of the infiltration problem into unsaturated soils have been presented () In spite of all the valuable works mentioned, the behaviors of pore pressure especially pore air pressure coupled water movement within soils is an interesting topic that has not been fully studied yet In this study, the behavior of pore air pressure associated with water movement on the transient vertical infiltration problems are studied This chapter review the important studies in the literature within the scope of present study, concentration of pressure parameter associated with water infiltration under heavy rainfall condition

2.1.1 Previous studies on water infiltration behavior

The vadose zone is an integral component of the hydrological cycle, directly influencing infiltration, storm runoff, evapotranspiration, interflow, and aquifer recharge Water movement in the vadose zone is generally conceptualized as occurring in the three stages

of infiltration, redistribution, and drainage or deep percolation as illustrated in Figure 2.1 For this conceptualization, infiltration is defined as the initial process of water entering the soil resulting from application at the soil surface Romano et al (1998) showed that Infiltration through an unsaturated soil is generally assumed to be the result of precipitation or surface processes that involve the use of water The dynamic of such processes is mainly controlled by capillary and gravity forces, and for most practical problems is formulated as a one-dimensional flow in the vertical direction Redistribution occurs as the next stage where the

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infiltrated water is redistributed within the soil profile after the cessation of water application

to the soil surface During redistribution, both capillary and gravitational effects are important Evapotranspiration takes place concurrently during the redistribution stage and will impact the amount of water available for deeper penetration within the soil profile The final stage of water movement is termed deep percolation or recharge, which occurs 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 pervasive in nature Heavy rainfall under extreme weather conditions, largely attributed to the effects of climate change,

is expected to produce increased variations to the infiltration characteristics and the level of the water table in low lying areas such as valley and slopes (Schnellmann et al 2010; Tsai and Wang, 2011) Infiltration during rainfall alters the water uptake giving rise to skeletal deformations; i.e., there are deformations that can be observed during water infiltration with a resulting water table change A rise in the water table in response to a rainfall event is a complex process influenced by several factors including permeability, the initial soil–water conditions, the position of the water table, evapo-transpiration, land cover and use, rainfall intensity, etc (Mansuco et al 2012; van Gaalen et al 2013)

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

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medium changes with the level of saturation At the same time, stress modification in the unsaturated porous medium leads to porosity changes Deformation in the unsaturated porous medium leads to variations in the porosity, which influences the water flow in the unsaturated porous medium Boogaard et al (2016) showed that, the hydrology in around a landslide area

is key to pore pressure build up in the soil skeleton which reduces shear strength due to the buoyancy force exerted by water in a saturated soil and to soil suction in an unsaturated soil

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

In fact, researches have been carried out to assess about reasons causing to landslide and its influence by various ways Based on the deep interplay between effective stresses, shear strength and water flow, some steep unsaturated deposits rest at equilibrium thanks to the contribution of soil suction to shear strength When the soil gets wet, the reduction of suction may lead to shallow landslide triggering (Bogaad et al., 2016) Soil hydraulic conductivity could be affected by soil deformation in saturated conditions It also means volumetric deformations induced by soil suction may be so large to lead to the development of shrinkage cracks (Fredlund et al., 1993)

Pore air pressure

Several studies have been conducted on the behavior of air pressure response to water infiltration but not so clear, and even for most of mechanical situations of water infiltration, the pore air pressure is considered as atmosphere pressure and neglected Culligan et al 2000 presented that the air pressure measurement could be used in predicting the water flux into the column and hence the cumulative infiltration By using the precise air pressure measurements for the various capillary tubes, it is possible to assess the sensitivity of hydraulic conductivity and sorptivity to minor increases in air pressure Besides, the small air pressure ahead of the

wetting front, i.e h a ~ 1/4cm, has near-negligible effect on infiltration

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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

Another study concerned about pore air pressure variation whilst water infiltration from G.A Siemens et al in 2014 pointed that an infiltration process, if it is modelled by Richard’s equation, will typically assume there is no impedance from pore air phase However, the study resulted that the impact of a relatively minor heterogeneity on the pore pressure response The D10 of the two gradations is less than an order of magnitude different; however, the fine layer changed the transient pore pressure regime and the wetting front migration The fine layer caused ponding during wetting front descent and acted similar to a leaky bedrock layer at depth The fine layer acted to limit flow, thus reducing the wetting front speed At equilibrium under closed conditions, hydrostatic conditions were observed over a significantly longer period of time with the addition of the fine layer Thus, entrapment of the air phase and small

The relationship between capillary pressure and water saturation is known by a number

of names, such as the suction function, retention function, or soil water characteristic function, etc (Pinder et al 2008; Or D and Wraith J, 2002; Kutílek M et al., 1994; Lu N and Likos W, 2004) Figure 2.2 shows its general features in relation to various configuration of air and water

in a porous medium Usually, if the medium is fully water-saturated, it can be invaded by the air phase only if the air pressure exceeds the water pressure by a specific value

The capillary pressure can be related to the air relative humidity by the Kevin equation:

Where R gas is the universal gas constant (8.31 J mol-1K-1), T is the Kelvin

temperature, Mw is the mole mass of water (0.018 kg mol-1), H is the relative air humidity

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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

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2.2 MOTIVATION OF THIS STUDY

The review of literatures showed that the reactions of pore pressure with water movement was considered as one of the significant factor influencing failures of a slope or embankment under heavy rainfall condition This process also alters the strength properties of soil and damages the slope stability The current literatures in this context are also devoid clearly solving for water movement in a multiphase system Most studies concern with infiltration rate or behavior of pore water pressure with deformation or failures However, the concept of pore air pressure is almost neglected or not usually used to describe the behavior of water infiltration Moreover, the effect of pore air pressure was not much pronounced in previous literature Therefore, it is important to look for the behavior of pore air pressure whilst water infiltration to aim a fully understanding of the process This has been a great motivation

to perform the present study with particular emphasis placed on pore air pressure behavior associated with water infiltration using a series of laboratory experiments and numerical simulations to be a fundamental basement of developing a complete influence rating procedure

of heavy rainfall triggering landslide in further studies

Fig 3.1: Schematic of the experiment of water infiltration in a closed system

a Initial state, b After infiltration

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3.2 MATERIAL PROPERTIES

Toyoura sand (standard Japanese sand) is used as a represent geomaterial through the present research for both laboratory experiments and numerical simulations The unique values of the material parameters of Toyoura sand, listed in Table 3.1 are the same

as those in the works by Sato et al (2003)

Table 3.1: Material properties of Toyoura sand Variable Unit Description Value

Figure 3.2: Water-retention characteristic curve for Toyoura sand

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3.3 EXPERIMENTAL PREPARATION AND PROCEDURE

The laboratory experiments are being conducted using a pressure sensor measurement system to investigate the behaviour of pore-air pressure during water infiltration process The equipment used in the experiments are provided by KEYENCE including sensors AP-C35, control unit CU-21A, Multi-input data loggers NR-TH08, NR-500 and Keyence Wave Logger software to record and analyse pore air pressure value of the experiments Fig 3.3 Besides, a system of tube, scale, calculators, rulers, and cameras are used as tools to serve experiments

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 shows a schematic diagram of the sand column and the location of the sensors used to measure the pore air pressure during infiltration process The major components

of the system are: acrylic cylinder, pore air pressure sensors, and the measurement system The acrylic columns were instrumented with the pore air pressure sensors located at 1, 5, 10 and 15