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

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 Master of Engineering by TRAN TUAN ANH 16ME136 Supervised by Professor Dr Masahiko Osada Rock Mechanics Laboratory Graduate School of Science and Engineering Department of Civil Engineering Saitama University Japan 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, poreair 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 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 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 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 TABLE OF CONTENT ABSTRACT ACKNOWLEDGEMENTS TABLE OF CONTENT LIST OF FIGURES LIST OF TABLES 11 CHAPTER 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 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 26 LABORATORY EXPERIMENTS OF WATER INFILTRATION 26 3.1 GENERAL 26 3.2 MATERIAL PROPERTIES 27 3.3 EXPERIMENTAL PREPARATION AND PROCEDURE 28 3.4 RESULT AND DISCUSSION 31 CHAPTER 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 53 CONCLUSIONS 53 5.1 GENERAL 53 5.2 FUTURE RECCOMANDATIONS 54 REFERENCES 55 LIST OF FIGURES Fig 1.1: Relation of rainfall to surface runoff, Ewing and Washington block, St Louis, Sept 7, 1916 14 Fig 1.2: Schematic cross section of a slope under a heavy rainfall condition 15 Figure 2.1 Conceptualization of water content profiles during infiltration, redistribution, and drainage (deep percolation) (Ravi et al 1998) 20 Figure 2.2: Air pressure with time: from top to bottom, capillary tube with different internal diameter (Culligan et al 2000) 22 Figure 2.2 Capillary pressure-water saturation relationship for various air and water flow regimes (Adam S, 2013) 24 Fig 2.3: Typical capillary functions for sand and clay 24 Fig 3.1: Schematic of the experiment of water infiltration in a closed system a Initial state, b After infiltration 26 Figure 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 29 Figure 3.5: Sensors arrangement (4 sensors) 30 Figure 3.6: Sensors arrangement (2 sensors) 31 Figure 3.6: Pore air pressure variation at different points during water infiltration process at the build-up stage (with sensors) Figure 3.7: Pore air pressure variation at different points during water infiltration process at the buil-up stage (with sensors) 32 Figure 3.8: Escape air from the soil system 33 Figure 3.9: Experiment with sensors after water fully infiltrated 34 Figure 4.1: Schematic diagram of water infiltration in closed system a Initial state, b After infiltration 40 Figure 4.2: Geometry of the model and boundary condition 41 Figure 4.3: Considered points and Meshing a – Considered points, b – Meshing 41 Fig 4.4: Water saturation profiles at time t=0 min(a), and t = (b) 42 Figure 4.5: Pore air and pore water pressure contribution at different positions of soil column 42 Figure 4.6: Pore air pressure profiles at different positions of soil column from times t=0 to t=60 43 Figure 4.7: Pore air pressure profiles at different positions of soil column in buil-up stage 43 Figure 4.8: Pore water pressure profiles at different positions of soil column from times t=0 to t=60 44 Figure 4.9: Capillary pressure profiles at different positions of soil column in entire process (a), and build-up stage (b) 45 Figure 4.10: Pore air pressure profiles at different positions of soil column a cm hydraulic head, b 10cm hydraulic head 46 Figure 4.11: Schematics of capillary raise model in opened system 48 Figure 4.12: Geometry of the model and boundary condition a – wetting phase; b – nonwetting phase; blue line: no flow 48 Figure 4.13: Considered points and Meshing a – Considered points, b – Meshing 49 Figure 4.14: Water saturation profiles from time t=0 min(a), and t = 60 (b) 49 Figure 4.15: Pore air and pore water pressure contribution at different positions of soil column 50 Figure 4.16: Pore air pressure profiles at different positions of soil column a in entire process, b at the early stage 51 Figure 4.17: Pore water pressure profiles at different positions of soil column a in entire process, b at the early stage 51 Figure 4.18: Capillary pressure profiles at different positions of soil column a in entire process, b at the early stage 52 10 Figure 4.6: Pore air pressure profiles at different positions of soil column from times t=0 to t=60 Figure 4.7: Pore air pressure profiles at different positions of soil column in buil-up stage 43 Figure 4.6 shows the behavior of pore air pressure in 60 minutes of water infiltration while Figure 4.7 illustrates its behavior at the early stage Overall, pore air pressure increases correlatively at the interval, and proportionally to the depth of the soil column It can be easily seen that the value of pore air pressure pointed at a lower position has a greater value compare to the higher positions in general (figure 4.6) It seems that pore air pressure increases rapidly at the beginning of the process, the entrapment air at any point raising steadily in about minutes before it jumps to the highest value at about minutes It also represents that the pore air pressure slows down the infiltration rate by the reduction of capillary pressure (Figure 4.9), the time lag between the sensors indicates the velocity of advance of the wetting front After that, the pore air pressure is decreased due to the wetting front approaching the bottom of the soil column, Figure 4.8 shows that the system is equilibrium from about minutes as the pore water pressure turning to positive and remaining stable The system is not saturation at the end of the considered process (60 minutes), but air would escape from the surface to the atmosphere and the system will be saturated at the point pore air pressure reaches the atmosphere pressure Figure 4.8: Pore water pressure profiles at different positions of soil column from times t=0 to t=60 44 Figure 4.6 and 4.9 show that 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 a b Figure 4.9: Capillary pressure profiles at different positions of soil column in entire process (a), and build-up stage (b) Figure 4.10 is the comparison of pore air pressure of the present model to a similar model with 10 cm of hydraulic head It illustrates that with the higher compressibility of water forcing, the pore air pressure can reach the higher value However, the behaviors of pressure parameters are quite similar 45 a b Figure 4.10: Pore air pressure profiles at different positions of soil column a cm hydraulic head, b 10cm hydraulic head The obtained result from the simulation of the behavior of pore air pressure agree with the obtained behavior of pore air pressure from experiments that are expressed in Chapter However, the value of pore air pressure is quite different due to the lack of the sensor calculating in a long time This leads to the leak of air and water out of the system This should be listed with limitation of the study 4.3.2 Capillary rise in open system The reaction of pore air pressure within soil coupled with the movement of water open a question of how it behaves in an open system The following model is designed to understand the behavior of pore air pressure with water movement in an open system (the air is not trapped) The open infiltration model setup for air and water, water enters from the base of a column made of air-dry Toyoura sand, while the top surface of the soil column approaches the atmosphere (Figure 4.12) The incoming water (the wetting phase) forces wetting front toward the surface of the column The air escape from the surface by the effect of water rising in the soil column Neither the air nor the water can pass through the vertical column walls The water at the inlet, which changes in time, corresponds to the height of fluid rise The column has a total height of 100 cm, a 15 cm radius The simulation covers in hour The initial conditions of the system are set as follow: 46 Wetting phase: pw0   w gD Nonwetting phase: pnw0  nw gD Assuming the water table level is cm, and it is controlled to be stable during the considered time, the boundary conditions are: At the base of the soil column: pw  w g 0.01 At the top surface of the soil column: pnw  Where D is the coordinate of vertical elevation (m), p is pressure (Pa), ρ is the fluid density (kg/m3), g is acceleration of gravity 47 Figure 4.11: Schematics of capillary raise model in opened system a b Figure 4.12: Geometry of the model and boundary condition a – wetting phase; b – nonwetting phase; blue line: no flow 48 The considered points are at 1, 20, 40, 60 cm from the bottom of the soil column (Figure 4.13a) a b Figure 4.13: Considered points and Meshing a – Considered points, b – Meshing a b Figure 4.14: Water saturation profiles from time t=0 min(a), and t = 60 (b) 49 Figure 4.15: Pore air and pore water pressure contribution at different positions of soil column 50 a b Figure 4.16: Pore air pressure profiles at different positions of soil column a in entire process, b at the early stage a b Figure 4.17: Pore water pressure profiles at different positions of soil column a in entire process, b at the early stage 51 a b Figure 4.18: Capillary pressure profiles at different positions of soil column a in entire process, b at the early stage The simulation showed that in the open system, the air phase contacts to the atmosphere, wetting front is increased only by effect of capillary pressure In entire considered time of the simulation, pore air pressure is approximately zero due to no compression The pore air pressure, somehow, can still affect to the migration of wetting fluid, but negligible For this case, it could be resulted that the effect of pore air pressure to the water movement can be ignored 52 CHAPTER CONCLUSIONS 5.1 GENERAL In the present study, the behavior of pore air pressure within soils by water infiltration was investigated through a series of laboratory experiments and numerical simulations of the one-dimensional water infiltration problems were carried out on a typical geomaterial The distribution of pore pressure coupled the behavior of water movement especially water infiltration was studied The conclusions obtained are described below The optically matched pore fluid – transparent soil allowed direct observation of the wetting front throughout the laboratory experiments Transparent soil also allowed direct monitoring of air phase movement from within the soil profile and measurement of distribution of saturation along the length of the column The numerical simulations incorporated the unsaturated properties of the two transparent soil gradations and captured the wetting front migration as well as the transient moisture regime throughout the experiments 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 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 53 The simulation for understanding the behavior of pore air pressure with water table rising as an effect of capillary in open system resulted that the air phase contacts to the atmosphere, and pore air pressure is about 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 5.2 FUTURE RECCOMANDATIONS 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 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 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 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Rep EPA/600/R-97/128a van Gaalen JF, Kruse S, Lafrenz WB, Burroughs SM (2013) Predicting water table response to rainfall events, central Florida Ground Water 51(3):350–362 ... downward within soil As the ideal gas law, the pressure of air must be raised by the reduction of air volume And this is the pressure of the remaining air in the system This is why the pore air pressure. .. 4.12) The incoming water (the wetting phase) forces wetting front toward the surface of the column The air escape from the surface by the effect of water rising in the soil column Neither the air. .. 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