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 conform
Trang 1Ministry of education Ministry of agriculture and and training rural development
water resources university
-
Nguyen Thi Ngoc Huong
Study the effect of the shear strength of unsaturated soil to the stability of earth
Trang 2The scientific work has been finished at:
water resources university
Advisors: 1 Assoc Prof Dr Trinh Minh Thu
2 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
Trang 3INTRODUCTION
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:
1 To make clear the nature of unsaturated soil model and its parameters when compare to the traditional understanding about saturated soil mechanics
2 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
3 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
Trang 4The 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: do 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
Trang 5VII 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 1
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
Trang 6cross 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 1
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
Trang 7for 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
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
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: =
m
n
a e
1
(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
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
Trang 8Terzaghi (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 0 to 1
Fredlund et al (1978) suggested a shear strength equation for unsaturated soil by using stress state variables (s-ua) and (ua-uw) as follow:
f w a f
a f
' tan ' tan (2.13) where: ff – the shear stress on the failure surface at failure state, c’ - effective cohesion, (sf-
ua)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'sn u atan'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:
p
s
w s
p s
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
Trang 9satisfies 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 2
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 3
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 1 Songsat 2 Songsat 3 Khecat Particle size
>10.000 mm % 0,00 0,21 2,59 0,00 5,000 - 10,000 mm % 0,00 2,08 2,41 0,00 Gravel 2,000 - 5,000 mm % 0,00 4,89 6,25 9,00 Sand 0,500 - 2,000 mm % 0,00 11,19 10,30 7,00 0,250 - 0,500 mm % 15,34 5,80 5,54 6,00 0,100 - 0,250 mm % 15,11 9,69 10,30 13,00 0,050 - 0,100 mm % 33,28 30,17 29,69 8,00 Silt 0,010 - 0,050 mm % 12,22 9,68 8,93 15,00 0,005 - 0,010 mm % 0,97 1,29 1,22 11,00 Clay <0,005 mm % 23,07 25,00 22,78 31,00
Soil properties Notation Unit Yenbai 1 Yenbai 2 Yenbai 3 Yenbai 4 Yenbai 5
Trang 10Air pressure supply
High-air entry
Grain size
>10.000 mm % 6,08 0,00 0,00 0,00 0,00 5,000 - 10,000 mm % 8,08 8,23 0,85 0,17 2,49 Gravel 2,000 - 5,000 mm % 24,62 21,64 1,60 3,29 5,46
0,250 - 0,500 mm % 3,81 4,71 4,70 3,68 4,23 0,100 - 0,250 mm % 4,97 5,11 6,20 5,52 5,33 0,050 - 0,100 mm % 8,67 13,02 20,34 17,48 17,21 Silt 0,010 - 0,050 mm % 8,17 9,34 29,43 28,42 25,57 0,005 - 0,010 mm % 2,12 2,99 6,47 5,46 6,94 Clay <0,005 mm % 27,25 27,44 26,79 26,23 26,67
3.2 Tests for obtaining Soil-water characteristic curve (SWCC)
3.2.1 Apparatus for the Soil-water Characteristic Curve Tests
The pressure plate was used for obtaining the SWCC Figure 3.1 shows the set up of the pressure plate with a 5 bar high air-entry ceramic disk and a rubber membrane beneath the disk
3.2.2 Soil specimen preparation The 12 statically compacted soil specimens used were 20 mm in thickness, 62 mm in diameter and volume of 60cm3 The dry density was compacted at 95% of the maximum dry density and optimum water content The soil sample at Yenbai was trimmed with 20 mm in thickness and volume of 60cm3 The specimens used for obtaining SWCC were prepared in the same manner as the soil specimens for triaxial tests The space between the disk and the rubber membrane serves as a water compartment The water compartment is connected to a burette line that is opened to atmospheric pressure The number of specimens that can be tested in a pressure plate depends on the available disk space The ceramic disk was saturated prior to test
3.2.3 Saturation soil specimen and pressure plate
The saturation was done by pouring the de-aired distilled water on top of the disk and applying a high air pressure of 500 kPa while opening the valve of the burette line for about 1 hour Due to the high pressure in the chamber, the distilled de-aired water infiltrated through the ceramic plate The soil properties are presented in Tables 3.2a and 3.2b
Table 3.2 Soil properties of the compacted specimen
Properties Notation Unit Songsat 1 Songsat 2 Songsat 3 Khecat
Figure 3.1 Pressure plate for obtaining SWCC
Trang 11Wet unit weight cb g/cm 3 2,000 2,136 2,105 1,830
Coefficient of permeability at
saturated condition ks m/s 5,0.10-8 1,6.10-7 2,0.10-7 1,9.10-8Table 3.2b Properties of undisturbed soil specimen of Yenbai
Properties Notation Unit Yenbai 1 Yenbai 2 Yenbai 3 Yenbai 4 Yenbai 5
Unit weight w g/cm 3 1,712 1,649 1,613 1,741 1,613 Dry unit weight d g/cm 3 1,365 1,355 1,293 1,269 1,182 Volumetric water content
at saturated condition qs 0,447 0,410 0,496 0,510 0,515 Coeficient of permeability
at saturated condition ks m/s 6,5.10-7 2,4.10-7 9,1.10-8 9,6.10-8 6,2.10-73.2.4 Tests for obtaining SWCC
In this test, the air pressure was applied at different level The pore-air pressure, ua was
applied by air pressure, while water pressure opened to atmospheric pressure (i.e., u a =
20 kPa and u w = 0 kPa), therefore the matric suction changing due to the change in applying air pressure The tests for obtaining SWCC were done with the matric suction of 10kPa, 20 kPa, 50 kPa, 100 kPa, 200 kPa and 400 kPa
3.2.5 Test results
Figures 3.3a and 3.3b show the SWCC of 9 compacted soil specimens As indicated in Figure 3.3a, there was a significant decrease in volumetric water content when the matric suction in the specimen exceeded the air-entry value The soil-water characteristic curve of the specimen indicated that the air-entry value (AEV) of the Khecat compacted soil specimen was 40 kPa The air-entry values of the Songsat soil specimens 1, 2 and 3, were 20,04kPa, 20,08kPa and 11,8kPa respectively The air-entry values of the Yenbai soil specimens 1, 2, 3, 4 and 5, were 31kPa, 32kPa, 28kPa and 29kPa as show in figure 3.3b The results show that the increase Ip with increasing the air-entry value
0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55
Figure 3.3a SWCC of Khecat compacted soil
specimens
Figure 3.3b SWCC of the Yenbai compacted soil specimens
3.2.6 Calculation coefficient of permeability from SWCC
3.2.6.1 Calculation SWCC by using Fredlund vµ Xing (1994) method
Fredlund and Xing (1994) method have been widely used However, Fredlund and Xing (1994) method was developed based on the test results in oversea countries mainly from North America, that have some different soil properties in Vietnam Therefore, the study
Air-entry value = 29 kPa Air-entry value = 32 kPa
Air-entry value = 31 kPa
Air-entry value = 40 kPa Air-entry value = 11,8 kPa
Air-entry value
= 20,04 kPa
Air-entry value = 20,08 kPa
Trang 12proposed the equation to determine values of m, n based on the Fredlund and Xing (1994) equation as follow:
(3.2) n =
2 *
31,
Comparison the results with Fredlund vµ Xing (1994) method
The detailed of the calculation SWCC based on this study are presented in tables III.2 and III.6 Appendix III of the full thesis The comparison calculation of SWCC by using fredlund and Xing (1994) and the results from this study are presented in Figures from 3.4 to 3.7
Predicted (Fredlund and Xing, 1994)
Predicted (Proposed Equation)
0,0 0,1 0,2 0,3 0,4 0,5 0,6
Predicted (Fredlund and Xing, 1994)
Predicted (Proposed Equation)
0,0 0,1 0,2 0,3 0,4 0,5 0,6
Figure 3.6 SWCC of the Songsat
compacted soil specimen 2
Figure 3.7 SWCC of the Songsat compacted soil specimen 3
The comparison results of the Yenbai soil specimen are presented in Appendix III of full thesis The figures show that the prediction results from this study are good agreement to the experimental results than those results from Fredlund and Xing (1994) method
3.2.6.2 Prediction coeficient of permeability function from SWCC
This study was calculated coeficient of permeability at different volumetric water content by using equation 2.10 with m and n from equation 3.2 and 3.3 The prediction of coefficient
of permeability from SWCC by this study and Fredlund and Xing 1994 for compacted soil specimens are presented in Figures 3.8, 3.9, 3.10 and 3.11
Figure 3.8 Coefficient of permeability
versus matric suction of Khecat specimen
Figure 3.9 Coefficient of permeability versus matric suction of Songsat specimen 1
Trang 13Figure 3.10 Coefficient of permeability
versus matric suction of Songsat specimen 2
Figure 3.11 Coefficient of permeability versus matric suction of Songsat specimen 3 The results from figures show that the coefficient of permeability with respect to matric suction based on this study is good agreement to the experimental data
3.3 Obtaining SHEAR STRENGTH OF UNSATURATED SOILS USING DIRECT SHEAR APPARATUS
3.3.1 Direct shear apparatus
Layout of the direct shear apparatus is presented in Figure 3.12
3.3.2 Direct shear test procedure Right after suction equilibrium at each matric suction value, three soil samples have been tested using direct shear apparatus at normal stress of 100kPa, 200kPa and 300kPa The soil specimens have been tested immediately after taken out of pressure plate
in order to keep suction unchanged The shearing rate was slow enough in order to prevent pore-water pressure increase The shearing rate in the direct shear test of 0,02mm/minute has been chosen Shearing was terminated when the shear stress reach a peak value have been observed
3.3.3 Testing program
The study has been tested for spacemen at Khecat compacted sample represented for the North and represented for the Central part was Songsat compacted specimens 2 and 3 Each soil type was tested with 12 specimens
3.3.4 Presentation of the result
compacted soil
Figure 3.14 show the experimental results on extended Morh-Coulomb failure envelope The results from figure 3.14 show that the effective friction angle, ’ = 230 and effective cohesion, c’ = 34 kPa The effective cohesion increase with increasing in matric suction, but effective friction angle was remained nearly unchanged and b decrease The b = ’ when the matric suction was smaller than air-entry value
The intersection lines between extended Coulomb failure envelope and ~ (s - ua) plane are show in Figure 3.15 The results from figure 3.15 show that the shear strength the increase
Figure 3.12 Lay out of the direct shear
apparatus
Figure 3.14 Extended Mohr-Coulomb
failure envelope of Khecat specimen