tóm tắt luân án tiếng anh nghiên cứu ảnh hưởng của cường độ chống cắt đất không bão hòa đến sự ổn định đập đất

Ministry of education and training Ministry of agriculture and rural development water resources university - Nguyen Thi Ngoc Huong Study the effect of the shear strength of unsaturated soil to the stability of earth fill dams Specialization: Geotechnical Engineering Code number: 62 - 58 - 60 - 01 THESIS OF DOCTOR OF ENGINEERING IN BRIEF HANOI, 2013 The scientific work has been finished at: water resources university Advisors: Assoc Prof Dr Trinh Minh Thu Prof Nguyen Cong Man Critic person 1: Critic person 2: Critic person 3: The PhD thesis will be defended at the thesis assessment committee at the Water Resources University - 175 - Tayson street – Dongda - Hanoi At……oclock, date/month/year…………………………… The PhD thesis can be obtained: The national library or The Water Resources University 175 – Tayson – Dongda - Hanoi -1INTRODUCTION I THE NECESSITY OF THIS STUDY The properties of unsaturated soil on stress - strain relationship, pore pressure variation, soil shear strength, and coefficient of seepage are not conformed to the theories of saturated soil mechanics In reality the slope in nature (residual) or artificial embankment (local material dams), are generally a saturated/unsaturated soil system, so all theories for saturated soil mechanics are not adequately applied for saturated/unsaturated soil environment In Vietnam, earth dams are generally used with in-situ soils having low clay content (especially earth dams at the central part of Vietnam) The knowledge, experience, theory for calculation, apparatus … for unsaturated soils are still very limited In our country some of earth dams good operate until present time, but they were unstable by calculated checking, so it should be related to have no taking account of influencing the parameters of the unsaturated soil In a research project about vertical slopes in Hong Kong, some studies also gave the same result So, except from normal calculation method, the consideration of the influence of unsaturated soil parameters when calculating earthen structures is very important and necessary, it shows a completed calculation method for unsaturated soil mechanics Until now, Vietnam has not had much research on the characteristics of unsaturated soil mechanics when calculation the stability of earthen structures, especially the studies of the influences of the shear strength of unsaturated soils to the stability of earthen structures In other words, at the present Vietnam almost does not have laboratory equipments for obtaining unsaturated soil properties To reach advanced countries in the world, the construction and establishment of equipments for defining characteristics of unsaturated soils is an important problem in our country Therefore, this thesis “Study the effect of the shear strength of unsaturated on the stability of earth fill dams” is an urgent issue and it has big scientific and piratical meaning II rESEARCH objectives The main objectives of this thesis are: To make clear the nature of unsaturated soil model and its parameters when compare to the traditional understanding about saturated soil mechanics To establish relationships between the characteristics of unsaturated soils and the relationships between the characteristics of unsaturated soils and saturated soils as well as between different methodologies in laboratory tests Find the relationship for calculating the functions of unsaturated soil characteristics used in Vietnam conditions To find the possibility and conditions to apply the achieved research results, the aim is to reduce the construction cost when using the unsaturated soil parameters in assessing the stability of earthen slopes III SUBJECT AND SCOPE OF THE STUDY This thesis would try to study some clayey soil and clay loam samples Except from normal physical and mechanical properties of soil, this study would mainly concentrate on important properties of unsaturated soils that related to the stability of earthen slopes, such as: The relationship between the volumetric water content, coefficient of permeability and shear strength, then applying for the calculation of dams that are filled by local materials and natural slopes: representing for the fill soil in North – East area is the Khecat reservoir (Quangninh province), fill soils that are used in for the dam in Songsat reservoir (Ninhthuan province) representing for the fill soils in the Central and Northwest area (Yenbai province) in Vietnam IV CONTENT OF THE THESIS -2The main content of this study aims to solve the following problems: (1) Study an overview about earthen dams in general and instability problems of earthen slopes, the saturated and unsaturated soil environment, the present situation and application of the physical and mechanical parameters of unsaturated soil in Vietnam and foreign countries (2) Concentrate deeply on the theories and methods for defining the unsaturated soil parameters such as: the soil water characteristic curve; the coefficient of permeability and shear strength (3) Based on the achieved results, propose the procedure for the triaxial test for unsaturated soil, especially with the modified triaxial equipment, suitably used under Vietnamese conditions (4) Experimental study to find out the soil – water characteristic curve for different soils used in practical structures and the shear strength of soil corresponding to different matric suctions, defining a curve that show the relationship between the shear strength  and the matric suction (ua-uw) (5) Study the relationship between the soil water characteristic curve and the soil shear strength and the permeability coefficient of unsaturated soils, calculating to define the permeability coefficient of soil in saturated/ unsaturated condition (6) Propose an experimental equation that shows the soil – water characteristic and the relationship between the soil shear strength and the matric suction of the soil samples that were suitably used in Vietnam (7) Comparing, confronting the achieved results from the suggested equations with the experimental results From the achieved result, petitioning about the possibility of applying these proposed equations in calculating the coefficient of permeability and the shear strength of unsaturated soils in Vietnam (8) Applying the achieved results to analyze, assess about the stable state of the real structures (the earthen dam at Khecat reservoir and Songsat reservoir) and natural slopes in Yenbai province V Research METHODOLOGY The following methods are applied in this thesis: +) Theoretical method: study the theory of the soil – water characteristic curve, the shear strength of unsaturated soil and the permeability coefficient of saturated – unsaturated soil +) The finite element method: modeling and analyze the seepage problem for earthen dams under the saturated/ unsaturated environment +) The slope stability analysis method: calculate the stability of the slope of earthen dam when considering the soil parameters under saturated, unsaturated conditions +) The experimental method: the laboratory tests to find out the properties and parameters of unsaturated soils, for instance: tests to define the soil – water characteristic curve, tests to find the relationship between the unsaturated soil shear strength versus different matric suctions VI THE SCIENTIFIC AND practical MEANING OF THE THESIS This thesis is done would contribute knowledge that is more suitable in the scientific base about the unsaturated soil parameters and their influence to the stability state of earthen slopes Study the experimental results on the Vietnamese soils about the soil-water characteristic curve, shear strength and coefficient of permeability versus different matric suction, and suggest experimental equations that allow calculating relation curves that suit the results of laboratory tests on Vietnamese soil From the results that withdraw from experimental studies, this thesis would apply to study the stability state of earthen dams and from that point out the amount of influence of the unsaturated soil shear strength to the slope stability factor of safety This thesis also has large contribution on applying the experimental equations in modeling unsaturated soil parameters in Vietnam, examine the influence of unsaturated soil parameters in calculating and designing earthen dams to define the suitable dam cross section that satisfy both the scientific and economic conditions, so contributing to the application of an advance in the development of the hydraulic construction field in Vietnam -3VII SUMmARY OF NEW CONTRIBUTIONs OF THIS THESIS The thesis has had some contribution in scientific and realistic meaning as follow: (1) The triaxial compression apparatus for unsaturated soil that have been modified from the triaxial compression apparatus for saturated soil at the Geotechnical Laboratory - Water Resources University based on the principles given by Fredlund and Rahardjo (1993) (2) The soil – water characteristic curves was obtained for some typical soil in Vietnam and also for defining the permeability coefficient and shear strength function for these soils Develop a graph for obtaining the correction coefficient  versus Ip for different soil (from slight clay, loam, heavy loam to clay) in Vietnam The results from this study of the shear strengths show that shear strength parameters (’, c’ and b) from the same soil but with different apparatus (direct shear test, triaxial consolidation drain test and constant moisture content triaxial test) gave relatively close to each other Therefore it would be suggest that when lacking of the triaxial compression apparatus for unsaturated soil, the direct shear test can be used for preliminary determining the shear strength parameters of unsaturated soils (3) When the matric suction in the soil changes, the effective cohesion c’ would change, however the internal friction angle, ’, is almost unchanged for some type of soil in Vietnam (4) Obtaining a set of unsaturated soils parameters that representing for some types of soil in Vietnam as well as successfully proves the effect of unsaturated soil parameters to the stability of earth fill slope in Vietnam Propose a method to apply the unsaturated soil parameters in calculating the stability of slope that satisfy the safety and economic conditions for soils in Vietnam VIII THE STRUCTURE OF THE THESIS Introduction Chapter 1: Overview about earthen dam and unsaturated soil Chapter 2: The theoretical basis Chapter 3: Experimental research Chapter 4: Apply to calculate the stability of slope for real structures (earthen dam at Songsat reservoir and another one at Khecat reservoir) and natural slopes in Yenbai Conclusions and future works CHAPTER OVERVIEW ABOUT EARTHEN DAM AND UNSATURATED SOIL 1.1 OVERVIEW ABOUT STUDIES OF EARTHEN DAM AND UNSATURATED SOIL 1.1.1 General overview about earthen dam Earthen dam is a popular type of structure that is used to raise the water level Earthen dam can be classified based on the type of the dam body as follow: 1) Homogeneous dam: The dam body is filled by one type of soil 2) Heterogeneous dam: The dam is filled with different soil types 3) The dam with an inclined rigid wall or flexible wall 4) The dam with a flexible or rigid core-wall 5) Combination type dam: The body part at upstream site is filled with different soil types, the body part at downstream site is rock masses 1.1.2 Problems related to the instability of earthen slopes Large scale slope incidents in the world and in Vietnam were mostly related to the unsaturated state of soil Instability problems often happen in earthen slopes that are formed by residual soils with deep ground water table Nowadays, most of the present methods that are used to analyze the stability of slope assume the slip surface is a circular sliding surface due to the fact that the using of this type with the -4cross section is a segment of a circle would give a reasonable result with high degree of accuracy without complicated calculation In general, effective shear strength parameters (c’ and ’) are used when the saturated earthen slopes are analyzed It is possible to assume that the negative pore water pressure can be neglected for cases that larger part of the slip surface is located under the water level However, in the situation that the ground water level is deep or when the shallow failure is predicted to happen, it would be not reasonable to ignore the negative pore water pressure 1.2 OVERVIEW ABOUT THE SATURATED AND UNSATURATED SOIL CONDITIONS Saturated soil is the biphase one (solid and liquid phases) and existing positive pore water pressure Unsaturated soil is the multiphase one and existing negative pore water pressure Lambe and Whitman (1979) defined that unsaturated soil is a three - phase soil system including: solid, liquid and air According to Fredlund and Morgensten (1977), when analyzing the stresses in a multiphase continuous environment, it is important to note that the intermediate air - water phase behaves as an independent phase, so that the unsaturated soil would be a system of four phases: solid, air, water and the surface tension phase The matric suction, soil water characteristic curve, permeability coefficient and shear strength are basic parameters of unsaturated soil The shear strength of unsaturated soil is different from saturated soil by the cohesion due to matric suction This additional cohesion depends on (ua - uw), and the value of b 1.3 THE SITUATION OF STUDYING THE UNSATURATED SOIL PARAMETERS IN THE WORLD AND IN VIETNAM 1.3.1 The situation of studying the unsaturated soil parameters in the world The theory about unsaturated soil mechanics was established from many decades ago Before 1950, scientists started to study about basic properties of unsaturated soil; however, most of their care only concentrated on the flow of capillary In the end of 1950s, new developments have started by the studying of the volumetric strain and shear strength of unsaturated soil The above study leads to suggestions about some stress equations that were called effective stress for unsaturated soil Then there was a slow progression with the direction to accept two independent stress state variables (Fredlund and Mongensten, 1977) Until now, we have had a quite stable background about the theory of unsaturated soil mechanics 1.3.2 Overview about researches in soil shear strength Terzaghi (1936) used the Morh – Coulomb criteria and the definition about effective stress to describe the shear strength of saturated soil To determine the stress state for unsaturated soil, more and more researches have accepted the using of two independent stress state variables (Fredlund and Morgenstern, 1977) 1.3.3 The research situation about the properties of unsaturated soil in general and soil shear strength in specific in our country In our country, problems related to unsaturated soil mechanics was just started to study in recent years A few publications and researches about unsaturated soil has been announced, the theory of unsaturated soil mechanics related to permeability, stability, stress - strain has been applied to calculate the stability of structures Especially, the geotechnical engineering laboratory - Water Resources University has had an equipment to define the soil water characteristic (SWCC) and the shear strength of unsaturated soil, contributing to experimental research for defining unsaturated soil parameters in Vietnam 1.4 CONCLUSION OF CHAPTER The problem to study and apply laboratory equipment, laboratory procedures to determine unsaturated soil parameters and apply these parameters in calculating the stability of earth structures in Vietnam have a great meaning and necessary, it starts a new research direction -5for Vietnamese scientists Together with the world’s scientists we also have a great contribution on the development and fulfillment the theory in unsaturated soil mechanics In this thesis, the author suggests a study to define unsaturated soil parameters for some soil types in Vietnam and apply these parameters in earth dam stability calculation Chapter THEORETICAL BASIS OF UNSATURATED SOIL 2.1 STRESS STATE VARIABLES IN SOIL ENVIRONMENT Bishop (1959) suggested an experimental equation to define the effective stress and it has been applied popularly (for example the lecture in Oslo, Norway, 1955): s’ = (s - ua) + (ua - uw) (2.2) where: ua – pore air pressure;  - the parameter related to the degree of situation In 1977, Fredlund and Morgenstern has studied and concluded that any two of three normal stress variables (total stress s, pore water pressure uw and pore air pressure ua) can be used to describe the stress state of unsaturated soil In other words, three combinations can be used to describe stress state variables, compatible with soil structure and the surface tension in unsaturated soil: (s-ua) and (ua-uw); (s-uw) and (ua-uw); (s-ua) and (s-uw), where: s - total stress; ua – pore air pressure; uw – pore water pressure 2.2 THE SOIL WATER CHARACTERISTICS In unsaturated soil mechanics, the relation curve between the soil moisture versus matric suction is defined as the soil water characteristic It has a great meaning in solving seepage problems in unsaturated soil mechanics, controlling parameters of unsaturated soil such as the permeability coefficient, shear strength and volumetric strain of soil  Equations of the soil water characteristic curve Many types of experimental equations have been proposed to perform the soil water characteristic curve These equations were suggested to model SWCC based on the assumption that the shape of SWCC depends on the distribution of the pore size in soil The equation form that is used to illustrate the relationship between the matric suction and moisture content is the equation of Fredlund & Xing (1994)       Fredlund and Xing:  = C   n         ln e   a           m (2.8) where: , a, n, m – constants (different parameters of soil),  - matric suction,  volumetric water content,  = (q - qr) / (qs – qr) (qs is the volumetric water content at saturation, qr is the residual volumetric water content, and q is the volumetric water content at a specific matric suction), e – the log base number, and C() – the adjusted coefficient  Defining the soil water characteristic curve by experiment The soil water characteristic curve can be defined by the pressure plate method in the laboratory In the laboratory, the matric suction can be acted on the sample by keeping the pore water pressure equal to zero and putting into the sample a positive air pore pressure Therefore the matric suction in the soil sample can be changed [(ua - uw) where uw is kept equal to zero by acting different air pressure into the sample This method is in the group of “axial transitivity” technique 2.3 THE SHEAR STRENGTH OF UNSATURATED SOIL 2.3.1 The saturated soil shear strength equation -6Terzaghi (1936) used the Mohr – Coulomb criteria and the effective stress definition to describe saturated soil shear strength: ff = c’ + (sf - uw)f tan’ (2.11) where: ff – shear stress on the failure plane at failure; c’ – the effective cohesion; (sf - uw)f – the effective normal stress on the failure plane at failure; ’ – the effective internal friction angle 2.3.2 The shear strength equation of unsaturated soil Bishop (1959) proposed a shear strength equation as follow:  = c’ + [(s - ua) + (ua - uw)] tan’ (2.12) where: c’ - the effective cohesion; ’ - the effective internal friction angle of saturated soil, s - the total normal stress, ua - pore air pressure, and  - a parameter related to the soil degree of saturation, vary from to Fredlund et al (1978) suggested a shear strength equation for unsaturated soil by using stress state variables (s-ua) and (ua-uw) as follow: (2.13)  ff  c ' s f  u a  f tan  ' u a  u w  f tan  b where: ff – the shear stress on the failure surface at failure state, c’ - effective cohesion, (sfua)f – net normal stress on the failure surface at failure state, ’ – effective internal friction angle corresponding to the net normal stress (sf-ua)f, (ua-uw)f – matric suction at failure state, and b – the angle that shows the velocity of the increase in shear strength corresponding to the increase in the matric suction (ua-uw)f at failure state The shear strength of unsaturated soil is usually defined from the consolidated drained triaxial test (the CD test) or the triaxial test with constant moisture content (the CW test) Vanapalli et al (1996) and Fredlund et al (1996) suggested a function to predict the shear strength of unsaturated soil from the SWCC and effective shear strength parameters (c’ and ’) as follow:   c's n  u a  tan  'u a  u w   tan  ' (2.15) where:  - a adjusted argument used to find the calculated values that fit the measured values;  - the volumetric water content that has been normalized ( = qw/qs); qw – volumetric water content; qs – volumetric water content at saturation 2.4 THE METHOD TO ANALYZE THE PERMEABILITY IN THE SATURATED AND UNSATURATED ENVIRONMENT The soil permeability coefficient can be determined by indirect method from SWCC or direct method (the permeability test) Leong and Rahardjo (1977) suggested an equation to predict the permeability coefficient based on the saturated permeability coefficient and the soil water characteristic curve, as follow: q  k w  k s   ks  w  q   s p p (2.19) where: p is a constant Fredlund et al (2011) has determined constant p for many pair of data and has found out the variation of p from 2,4 to 5,6 for different soils The average value of p for any type of soil is 3,29 2.5 The earthen stability analysis method The limit equilibrium method using effective stress and pore water pressure for determining the slope stability factor of safety is applied popularly in reality to analyze and assess the stability of earth dams When calculating the stability of slope concerning the negative pore water pressure, it is possible to use the “total cohesion” method - add the matric suction to the soil cohesive (Ching and et, al, 1984) to find out the factor of safety equation that -7satisfies both positive and negative pore pressure (Fredlund, 1989, 1995; Rahardjo and Fredlund, 1993), or using nonlinear relationship between the shear strength and matric suction (Rahardjo, Fredlund and Vanapali, 1992) In the “total cohesion” method, unsaturated soil is considered to have total cohesive including the effective cohesion and matric suction 2.6 CONCLUSION of CHAPTER The characterised parameters for unsaturated soils are the SWCC, permeability coefficient and shear strength SWCC is defined using experimental method and calculated equations The permeability coefficient and shear strength of unsaturated soil can be defined indirectly through the SWCC or directly through the laboratory tests When analyzing the slope stability, it is possible to apply the “completed cohesive” method to consider the influence of unsaturated soil parameters to the factor of safety CHAPTER EXPRERIMENTAL Research FOR OBTAINING UNSATERATED SOIL PROPERTIES 3.1 BASIS SOIL PROPERTIES Thesis is concentrated to study on unsaturated soil at three areas in Vietnam: on the North – West, North - East and in the Central part The compacted soils used for testing are at the Ninhthuan dam of Phuocthang village, Bacai Distric and Ninhthuan province The second compacted soils for testing is at the Khecat earth fill dam of Hailang village, Tienyen district and Quangninh province The third soils are undisturbed samples at the natural slope of Yenbai city, Yenbai province The procedure of the soil testing was following TCVN 1995 standards and soil properties are presented in Tables 3.1a and 3.1b Table 3.1a Soil properties of the compacted specimens Soil properties Notation Unit Songsat Songsat Songsat Particle size >10.000 mm % 0,00 0,21 2,59 5,000 - 10,000 mm % 0,00 2,08 2,41 Gravel 2,000 - 5,000 mm % 0,00 4,89 6,25 Sand 0,500 - 2,000 mm % 0,00 11,19 10,30 0,250 - 0,500 mm % 15,34 5,80 5,54 0,100 - 0,250 mm % 15,11 9,69 10,30 0,050 - 0,100 mm % 33,28 30,17 29,69 Silt 0,010 - 0,050 mm % 12,22 9,68 8,93 0,005 - 0,010 mm % 0,97 1,29 1,22 Clay 10.000 mm 5,000 - 10,000 mm Gravel 2,000 - 5,000 mm Sand 0,500 - 2,000 mm 0,250 - 0,500 mm 0,100 - 0,250 mm 0,050 - 0,100 mm Silt 0,010 - 0,050 mm 0,005 - 0,010 mm Clay