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

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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

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

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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.