Effect of leaching on geotechnical properties of busan marine clay and stabilized dredged clay

Thesis for the Degree of Doctor of Philosophy EFFECT OF LEACHING ON GEOTECHNICAL PROPERTIES OF BUSAN MARINE CLAY AND STABILIZED DREDGED CLAY by Thanh-Hai Do Department of Ocean Engine

Thesis for the Degree of Doctor of Philosophy

EFFECT OF LEACHING ON GEOTECHNICAL PROPERTIES OF BUSAN MARINE CLAY AND

STABILIZED DREDGED CLAY

by Thanh-Hai Do

Department of Ocean Engineering

The Graduate School Pukyong National University

Effect of leaching on geotechnical properties of Busan marine clay and

stabilized dredged clay

Advisor: Prof Yun-Tae Kim

by Thanh-Hai Do

A thesis submitted in partial fulfillment of the requirements

for the degree of

Doctor of Philosophy

in Department of Ocean Engineering,

The Graduate School, Pukyong National University

February, 2011

ACKNOWLEDGEMENTS

It is great pleasure to thank everyone who helped me write my dissertation successfully Looking back upon the past four years, my deepest gratitude is to my advisor, Professor Yun-Tae Kim His patience and support helped me overcome many crisis situations and finish this dissertation I hope that one day I would become as good a supervisor as him

I am pleased to thank the useful comments and suggestions to complete

my dissertation by committee members Professors: Jin-Ho Jeong, Du-Hwoe Jung, Gi-Cheol Kwoen and Tae-Hyung Kim

I own sincere and thankfulness to all Professors of Ocean Engineering in Pukyong National University and Civil Engineering University in Ho Chi Minh University of Technology Specially, I have been indebted to Professor Phan Vo for his help and instruction of my teaching and research work

I am also grateful to the former or current staff at Ocean Engineering Department, Pukyong National University and Geosystems Laboratory Particularly, a note of gratitude goes to Mr Dong-Joo Lim for their special care to me Also, thanks are sent to my girlfriend (Ngoc Anh) for her love and companionship to me in all difficult times during studying in Korea Most importantly, I am heartily thankful to the love and patience of my family The advice of my father and confidence of my mother support me strength to overcome all troubles Also, the talk and discussion with my brother (Xuan Hoa) encourage me better in studying Without all of you, this dissertation would not have possible

EFFECT OF LEACHING ON GEOTECHNICAL PROPERTIES

OF BUSAN MARINE CLAY AND STABILIZED DREDGED CLAY

Do Thanh Hai

Department of Ocean Engineering, The Graduate School,

Pukyong National University

of the upper and lower clay deposits, the soil salinity of lower clay layer is significantly lower than that of upper clay layer, which results in higher settlement in this clay Moreover, salt has some negative effect on strength development and compressibility for stabilized dredged clay as dredged clay

is mixed with cement

A leaching process can change the natural marine clay and salt-rich dredged clay into leached clay and non-salt dredged clay, respectively This process causes the change in geotechnical properties not only in natural marine clay but also in stabilized dredged clay Moreover, few researches have been carried out on the leaching effect on the geotechnical properties of Busan marine clay, neither in undisturbed state nor in stabilized dredged clay Therefore, the objective of this study is to evaluate how leaching affects the geotechnical properties of Busan leached marine clay and non-salt dredged clay in comparison with unleached marine clay and salt-rich dredged clay, respectively

In this study, soil salinity is checked using the CPC-401 salinity meter with the soil-extract method The in-situ salinity of unleached marine clay and salt-rich dredged clay is about 16-18 g/l and 28-30 g/l, respectively The leaching process is a time-consuming procedure to obtain unleached clay and non-salt dredged clay For natural marine clay, leaching process to obtain a leached specimen required permeating distilled water from the top to the bottom of the specimen In order to get leached undisturbed specimens from unleached undisturbed specimens after leaching, the constant rate of strain (CRS) equipment is modified to perform the leaching process in the cell chamber Leached specimen is assumed to reach the soil salinity of 3 g/l obtained after about 25 days For salt-rich dredged clay, the leaching process

is soil-washing using fresh water several times until the soil salinity reaches 0g/l The non-salt dredged clay is obtained after 6 times of soil-washing, then

dried in natural condition to have same initial water content of salt-rich dredged clay

In this investigation, the effect of leaching on geotechnical properties of both Busan marine clay and stabilized dredged clay were experimentally evaluated The experimental method of investigation consists of determining the physical properties, unconfined compressive strength, stress-strain and compressibility characteristics of both the unleached and leached specimens The behavior characteristics of the leached and unleached marine clay specimens were evaluated using several series of constant rate of strain (CRS) tests with various strain rates While the non-salt and salt-rich dredged clays are mixed with cement to form non-salt rich stabilized dredged material and salt-rich stabilized dredged material, which are composite geomaterials, noted as CGM-N and CGM-S, respectively The microstructures, strength and compressibility characteristics of CGM-N and CGM-S were assessed using scanning electronic microscope (SEM), unconfined compression test, and oedometer test at 7 and 28 days of curing time

The experimental results reveal that the compressibility of leached clay increases as its salinity decreases However, the void ratio, Atterberg limits, and preconsolidation pressure in the leached samples are lower than those of

in unleached clay after leaching and CRS test The increased compressibility and decreased preconsolidation pressure may be induced from a weakening

of the interparticle bonds in the leached soil skeleton The CRS test results with various strain rates reveal that higher strain rates correspond with higher

levels of effective stress and higher apparent preconsolidation pressure in both leached and unleached clays

Also, experimental results exhibit that salt concentration of clayey soil affect not only to change microstructure of CGM but also to reduce the significantly the strength and yield stress of mixture However, the compressibility of CGM-S is larger than that of CGM-N The yield stress has linear relationship with unconfined compressive strength for all CGM mixtures with the magnitude of 0.33, regardless of non-salt or salt-rich dredged clay

Keywords: leaching, salinity, Busan marine clay, stabilized dredged clay, strain rate, compressibility characteristics, and strength

TABLE OF CONTENTS

ACKNOWLEDMENTS i

ABSTRACT ii

TABLE OF CONTENTS vi

LIST OF TABLES xi

LIST OF FIGURES xii

CHAPTER 1: INTRODUCTION 1.1 Overview and scientific background 1

1.1.1 Overview 1

1.1.2 Scientific background 4

1.1.2.1 Geotechnical properties of Busan clay 4

1.1.2.2 Field leaching in Busan clay 6

1.1.2.3 Dredged clay in Busan and stabilizing technique 12

1.1.2.4 Soluble salt 13

1.2 Scope and objectives 16

1.3 Organization 17

CHAPTER 2: LITERATURE REVIEW 2.1 Introduction 19

2.2 Soil structure 20

2.2.1 Early concepts 20

2.2.2 Microfabric observation methods 24

2.3 Laboratory leaching test 26

2.3.1 The salt leaching theory 26

2.3.1.1 Before leaching 27

2.3.1.2 After leaching 28

2.3.2 Leaching apparatuses 29

2.3.2.1 Rowe hydraulic cell 31

2.3.2.2 Modified oedometer equipment 32

2.3.3 Some important factors of leaching test 35

2.3.3.1 Target salinity value 36

2.3.3.2 Duration of leaching 36

2.3.3.3 Sequence of leaching test 38

2.4 Effect of leaching on geotechnical properties of marine clay 39

2.5 Consolidation by constant rate of strain (CRS) test 43

2.6 Strain rate effect on the compressibility of soft clay 46

2.7 Effect of leaching on stabilized dredged clay 50

2.8 Summary 52

CHAPTER 3: EXPERIMENTAL METHOD 3.1 Introduction 53

3.2 Definition of salinity 54

3.2.1 Water salinity 55

3.2.2 Soil salinity 55

3.3 Laboratory method for measuring salinity 56

3.3.1 CPC-401 salinity meter 56

3.3.2 Soil-extract method 57

3.3.3 Calibration step 58

3.3.4 Measuring procedure 58

3.4 Salinity measurement value 60

3.4.1 Natural marine clay 60

3.4.2 Salt-rich dredged clay 64

3.5 Laboratory leaching test 65

3.5.1 Natural marine clay 65

3.5.1.1 Diagram of leaching test 65

3.5.1.2 Modified constant rate of strain (CRS) equipment 67

3.5.2 Salt-rich dredged clay 71

3.5.2.1 Diagram of leaching test 71

3.5.2.2 Scanning electronic microscope (SEM) test 73

3.5.2.3 Unconfined compression test 73

3.5.2.4 Oedometer test 74

3.6 Summary 76

CHAPTER 4: EFFECT OF LEACHING ON GEOTECHNICAL PROPERTIES OF MARINE CLAY AND ITS RATE DEPENDENCY 4.1 Introduction 78

4.2 Materials for testing 79

4.3 Leaching procedure and CRS testing 81

4.4 Effect of leaching on physical properties 83

4.4.2 Effect of leaching on Atterberg limits 85

4.5 Effect of leaching on compressibility characteristics and its strain rate dependency 87

4.5.1 Effect of leaching on e-logσ′v curves 87

4.5.2 Effect of leaching on stress-strain curves 91

4.5.3 Effect of leaching and strain rate on preconsolidation pressure 93

4.5.4 Effect of leaching on compression index and swelling index 98

4.5.5 Effect of leaching on hydraulic conductivity 100

4.6 Summary 103

CHAPTER 5: EFFECT OF LEACHING ON STABILIZED DREDGED CLAY 5.1 Introduction 104

5.2 Materials and leaching process 105

5.3 Experimental program 106

5.4 Effect of leaching on physical properties 108

5.4.1 Specific gravity 108

5.4.2 Unit weight 108

5.4.3 Post curing water content 110

5.4.4 Post curing initial void ratio 110

5.5 Effect of leaching on stress-strain behavior 113

5.6 Effect of leaching on unconfined compressive strength 113

5.7 Effect of leaching on compressibility characteristics 115

5.7.1 Stress-strain curves and void ratio-vertical pressure curves 115

5.7.2 Compression index and swelling index 117

5.7.3 Yield stress 119

5.8 Summary 121

CHAPTER 6: CONCLUDING REMARKS 6.1 General conclusions 122

6.1.1 Leaching process on Busan marine clay and dredged clay 122

6.1.2 Effect of leaching on physical properties 123

6.1.3 Effect of leaching on mechanical characteristics 123

6.1.3.1 Compressibility and its rate dependency of marine clay 123

6.1.3.2 Strength and compressibility of stabilized dredged clay 124

6.2 Recommendations 125

REFERENCES 126

LIST OF TABLES

Table 1.1 Survey of artesian pressure head in the Nakdong River plain 18

Table 2.1 Target salinity value and duration of leaching test 37

Table 2.2 Literature review of properties of natural and leached clay 40

Table 3.1 Initial salinity of Hwajoen samples 62

Table 3.2 Salinity value of salt-rich dredged soil 66

Table 4.1 Initial soil properties of three samples in Hwajeon site 80

Table 5.1 Properties of salt-rich dredged clay 107

Table 5.2 Mixing and testing conditions 107

LIST OF FIGURES

Fig 1.1 Nakdong river and its vicinity (Kim, 2008) 3

Fig 1.2 Longitudinal section of deltaic deposits of Nakdong River mouth (Kim, 2008) 5

Fig 1.3 Artesian pressure measured at BH-26 of Busan New Port site (Huh, 2003) 8

Fig 1.4 Salinity and OCR value for clay at BH-4, Hwajoen site

(Rao, 2004) 10

Fig 1.5 Annual generation of dredged soil in Busan, Korea (re-produced after Kim et al., 2006) 14

Fig 1.6 Solubility of common soil minerals (re-produced after Lide, 1994) 15

Fig 1.7 Sketch of evaporation process in landfill 15

Fig 2.1 Honeycomb arrangement (Terzaghi, 1925) 23

Fig 2.2 Structure of undisturbed marine clay (Casagrande, 1932) 23

Fig 2.3 Proposed model in fresh water and seawater (Lambe, 1953) 23

Fig 2.4 Effect of leaching on the structure of undisturbed marine clay (After Skepton and Northey, 1952) 29

Fig 2.5 Proposed flow chat for collapse potential evaluation set up (After Mansour et al., 2008) 30

Fig 2.6 Schematic diagram of Rowe hydraulic cell (After Rowe and

Barden, 1969) 31

Fig 2.7 Modified oedometer set up (After Al-Amoudi and Abduljauwad, 1995) 34

Fig 2.8 Schematic diagram of leaching experiment on modified oedometer (After Tolerance, 1976) 34

Fig 2.9 One-dimensional compression of Berthierville clay (a) preconsolidation pressure as function of strain rate; (b) normalized effective stress-strain curve 47

Fig 2.10 Ranges of strain rates usually occurred in laboratory tests and in the field (After Leroueil et al., 1988, CGT: constant gradient test; CRS: constant rate of strain; MSL: multiple stage loading test) 49 Fig 3.1 CPC-401 equipment 57

Fig 3.2 Calibration procedure before salinity testing 59

Fig 3.3 Soil solutions before measuring salinity 63

Fig 3.4 Checking soil salinity of samples in Hwajoen site 63

Fig 3.5 Dredged clay taken in Busan New Port site 64

Fig 3.6 Diagram of leaching procedure for natural marine clay 66

Fig 3.7 Standard automatic CRS equipment 69

Fig 3.8 Schematic diagram of assembly leaching test in modified CRS apparatus 69

Fig 3.9 Modification of cell chamber to obtain leachate 70

Fig 3.10 Checking salinity of leachate 70

Fig 3.11 Diagram of leaching procedure for salt-rich dredged clay 72

Fig 3.12 Specimens and unconfined compression machine 75

Fig 3.13 Oedometer test diagram 75

Fig 4.1 Variation of soil salinity with time of leaching process 82

Fig 4.2 Void ratio changes with salinity 84

Fig 4.3 Variation of Atterberg limits with salinity (a) Liquid limit,

(b) Plastic limit 86

Fig 4.4 CRS tests with the leached and the unleached specimens of HJ-1 specimens with ε v =10−3%/sec 88

Fig 4.5 CRS tests with the leached and the unleached specimens of HJ-2 specimens with ε v =10−4%/sec 89

Fig 4.6 CRS tests with the leached and the unleached specimens of HJ-3 specimens with ε v =10−5%/sec 90

Fig 4.7 Strain rate effect on unleached and leached specimens 92

Fig 4.8 Variation of preconsolidation pressure with salinity 94

Fig 4.9 Linear relationship between preconsolidation pressure and strain rate 97

Fig 4.10 Variation of compression index with salinity 99

Fig 4.11 Variation of swelling index with salinity 99

Fig 4.12 Comparison of hydraulic conductivity between leached and unleached specimens 102

Fig 5.1 Variation of salinity with soil-washing times 105

Fig 5.2 Specific gravity changes with mixtures of CGM-S and

CGM-N 109

Fig 5.3 Unit weight changes with mixtures of CGM-S and CGM-N 109

Fig 5.4 Post-curing water content changes with mixtures of CGM-S and

CGM-N 111

Fig 5.5 Post-curing initial void ratio changes with mixtures of CGM-S and CGM-N 111

Fig 5.6 SEM analysis of CGM-N and CGM-S at 28 days 112

Fig 5.7 Stress-strain curves of CGM-N and CGM-S at 7 days 114

Fig 5.8 Unconfined compressive strength of CGM-S and CGM-N at 7 and 28 days 114

Fig 5.9 Vertical pressure-vertical strain curves for CGM mixtures 116

Fig 5.10 Vertical pressure-void ratio curves for CGM mixtures 116

Fig 5.11 Compression index of CGM-S and CGM-N at 28 days 118

Fig 5.12 Swelling index of CGM-S and CGM-N at 28 days 118

Fig 5.13 Yield stress of CGM-S and CGM-N at 28 days 120

Fig 5.14 Relationship between unconfined compressive strength and yield stress 120

south And now a new large port is under construction along the coastline (see Fig 1.1) In the future the area will be developed as a key industrial belt

in Korea

At the initial stage of development, a heavy industrial complex was planned on the river mouth but the project was soon abandoned because of a poor soil condition Local engineers are still facing a lot of difficult problems because of an unusual thick clay deposit and its softness During carrying out

a lot of projects here, engineers have experienced several failures such as collapse of a breakwater, tilting of bridge piers, unusual large settlements and

so on These failures were mainly caused by insufficient knowledge of the deposit and characteristics of Busan clay

In the last few decades, the demands of reclamation for construction of large industry zone along coastal area are more and more necessary due to a rapid economic growth It is important to investigate the geotechnical properties of Busan clay, which have an influence on the compressibility of natural Busan clay, and also to find a method for stabilizing the dredged clay after excavating under sea

Fig 1.1 Nakdong River and its vicinity (Kim, 2008)

1.1.2 Scientific background

1.1.2.1 Geotechnical properties of Busan clay

The soil deposits of the Nakdong River plain, called Busan clay, are very thick in which its thickness varies from 30m to 60m depending on site locations, especially more than 80m at the mouth of the Nakdong River

The sedimentation of sand and gravel in the valley was followed by the deposition of different compositions of materials, affected by marine transgression and regression through a long geological history The deposits can be divided into five distinguished layers as shown in Fig 1.2 The bottom gravel and sand layer are thick and an aquifer holding fresh water flowing from the upper stream The middle sand layer is sandwiched by the upper and lower clay layers but it is missing or very thin in the coastal and offshore areas The silty clay layer is consisted of upper and lower layers, based on the age of sedimentation and engineering properties The upper clay layer becomes a top deposit in seaside but the layer in the landside is covered by a deposit of silty sand, which has been transported by the floodwater after the sea water level becomes stable The clay has been normally consolidated and

is soft to medium in consistency The lower clay is stiff to very stiff as the layer has been subjected to a heavier overburden pressure The dominant clay mineral is illite with some chrolite and kaolinite (Kim, 2008)

Shinho Hwajeon Myungji

Kimhae airport Jangyu Yangsan

Fill

Sand

Fig 1.2 Longitudinal section of the deltaic deposits of the Nakdong river mouth (Kim, 2008)

1.1.2.2 Field leaching in Busan clay

Presence of artesian pressure in Busan clay is well known from several site investigations Table 1.1 presents the results of artesian pressure measurement carried out at different locations (Kim, 2008) Higher artesian pressure appears at locations in Yangsan and Busan New Port Other locations have lower artesian pressure Figure 1.3 shows a distribution of artesian pressure measured at a borehole at Busan New Port site In this figure, a geological term IS (inner shelf) denotes the marine environment, while NS (near shore) denotes the brackish environment An aquifer is found

to locate 57m below ground level, at which the measured pressure head is 2.96m (Huh, 2003) It is also noted that the artesian pressure increases almost linearly with an increase in depth, as shown in Fig 1.3 It is obvious from Table 1.1 and Fig 1.3 that the Busan clay in Nakdong River plain is subjected to artesian pressure induced from an aquifer The aquifer providing artesian pressure is the sand and gravel sand layer at the bottom of the deposition The artesian pressure is caused by higher water tables of the upper stream and surrounding mountains, which are associated with the aquifer in the Nakdong River plain (Kim, 2008)

Table 1.1 Survey of artesian pressure head in the Nakdong River plain

Location

El Measured (-m)

Artesian pressure head (m)

References

Fig 1.3 Artesian pressure measured at BH-26 of Busan New Port site (Huh,

CoastNS(U)IS(U)NS(U)

2.576

Geotechnical profile of Busan clay is shown in Fig 1.4 Under about 10m silty sand is sedimentary clay deposit of approximately 40m in thickness, followed by sandy layer which usually has gravel in its lower part Sedimentary clay deposit consists of upper clay and lower clay layers Upper clay layer with thickness of about 20m has N values from 0 to 7 and soil salinity is ranged between 13g/l to 17g/l with an average value of 15g/l However, lower clay layer has N value greater than 8 The soil salinity decreases from about 12g/l to 5g/l with depth In Busan clay, it is noted that the salinity of the lower clay layer is significantly smaller than that of the upper clay layer This clearly indicates the leaching of salt has been taking place for a long time This natural leaching process was caused by upward flow of fresh water due to artesian pressure acting on the sand layer (Kim, 2008)

The value of OCR in Fig 1.4 is evaluated from the results obtained by the oedometer test Since the soil disturbance may be induced by the sampling of undisturbed soil from deeper depth, Schmertmann’s correction was applied for the determination of the preconsolidation pressure The results show that the values of OCR in upper clay deposit are about 1 while those in lower clay deposit become less than 1 This may be due to the artesian pressure applied to the lower portion of the soil layer and one of the specific characteristics of Busan clay (Kim, 2008)

It has been well known that leaching affects plasticity and compressibility of soft clay Kim (2008) reported a valuable discussion regarding to leaching effects on geotechnical properties based on both laboratory and in-situ test results The liquidity index (IL) at Shinho and Yangsan showed almost constant through depth with a value of unity Main concern is that liquidity index in the lower portion of the clay layer is as high

as unity even in the depth of 30m Of course, this is due to the leaching, which causes the decrease of liquid limit while maintaining almost constant natural water content

1.1.2.3 Dredged clay in Busan and stabilizing technique

In order to keep harbors and waterways navigable, prevent rivers from flooding, and restore the ecosystem of marine environment, dredging technique is commonly used (Forstner and Calmano, 1998; Kim et al., 2008)

In Korea, a large amount of dredged clay has been deposited from navigation channels and construction sites of large-scale port and harbour projects such

as Busan New Port Dredged clay is very soft soils which have natural water contents higher than their liquid limit, and low shear strength that can not be reused as backfill without treatment Figure 1.5 illustrates the annual generation of dredged soil from 1990 to 2004 in Busan, Korea, where the dredging is continuously increased due to large construction projects associated with new industrial complexes Dredged clay is usually dumped in waste disposal sites at sea This, however, is becoming increasingly difficult due to environmental regulations and pressure has been increasing to reuse the dredged clay in port and harbor construction projects

The soil-cement stabilization for recycling dredged clay or soft clay is well-known method and widely used in the world (e.g., Uddin et al., 1997; Watabe et al., 2000; Petchgate et al., 2001; Yin, 2001; Bergado et al., 1999; Lee et al., 1999) However, for case where small to medium loads (e.g from

1 to 3m high road embankment or backfill) were applied, the use of 10-30% cement content to improve soft mud ground would be too conservative (Feng

et al., 2001) Thus, the appropriate use of recyclable waste by-products in

cement-admixed clay should encompass both environmentally and friendly practice as well as cost-saving element (Kim et al., 2010)

1.1.2.4 Soluble salt

Soluble salts or minerals that are commonly found in soils can be classified in water as readily soluble, moderately soluble and weakly soluble based on the degree of solubility Figure 1.6 shows the solubility for the commonly occurring soluble soil minerals (Lide, 1994) In general, the graph shows that minerals with the highest solubility are Chlorides

Marine clays deposited along a coastal area usually contain an amount of soluble salt in the pore of soil skeleton The soil salinity depends on the salt concentration of water or seawater environmental deposition The salinity of seawater from which the marine clays are sedimented is of the order 35g/l (Sverdrup et al., 1942)

Most of the marine dredged clay is taken under sea water level of Busan New Port area Because the clay-dominated soil is infiltrated by salt, there is

an occurrence of high salinity The dredged clay is then stored in landfill and for a long time, its inside sea water is evaporated As a result, salt remains in soil as shown in Fig 1.7

Fig 1.5 Annual generation of dredged soil in Busan, Korea (re-produced

after Kim et al, 2006)

Fig 1.6 Solubility of common soil minerals (re-produced after Lide, 1994)

Dredged Soil

Evaporated

Fig 1.7 Sketch of evaporation process in landfill

1.2 Scope and objective

Main reason that the values of OCR in the lower clay are less than 1 may

be a soil disturbance induced by the soil sampling, particularly in samples from deeper location, even though the advanced sampling techniques are used (Chung et al., 2003a) This soil disturbance took place in the leached clay in the deep location (a leached clay is very sensitive to the soil disturbance) The leaching process in the lower clay resulted from artesian pressure acting on the sandy layer (Kim, 2008) Therefore, it is necessary to investigate the leaching effects on the preconsolidation pressure as well as compressibility characteristics of Busan leached clay

Salt leaching can change the natural marine clay and salt-rich dredged clay into leached clay and non-salt dredged clay, respectively This process causes the change in geotechnical properties not only in natural marine clay but also in stabilized dredged clay Moreover, few researches have been carried out on the leaching effect on the geotechnical properties of Busan marine clay, neither in undisturbed state nor in stabilized dredged clay

Therefore, the objective of this study is to evaluate how leaching affects the geotechnical properties of Busan leached marine clay and non-salt dredged clay in comparison with unleached marine clay and salt-rich dredged clay, respectively

1.3 Organization

This dissertation contains six chapters, which provide two main sections The fist part describes the salt concentration in Busan marine clay and dredged clay and modified equipment on leaching procedure to get undisturbed leached specimen and non-salt dredged clay The second part focuses on the effect of leaching on the geotechnical properties of Busan leached marine clay and non-salt dredged clay in comparison with unleached marine clay and salt-rich dredged clay, respectively

In chapter 2, literature review of soil structure, leaching effect, constant rate of strain, strain rate effect on consolidation of soft soil and stabilized dredged clay are described

In chapter 3, the scope of testing and equipment are considered The CRS test is modified for leaching procedure Also, the leaching procedure is described to obtain the natural leached clay and non-salt dredged clay The testing procedure including CRS test with different strain rates and other tests

on the mixtures are carried out

In chapter 4, experimental results focus on evaluating how leaching and the strain rate affect the compressibility behavior of leached and unleached marine clays Firstly, the effect of leaching on physical properties is investigated Secondly, the effect of leaching and strain rate on stress-strain curves leached and unleached marine clays are compared Thirdly, the

hydraulic conductivity obtained from CRS test after leaching is discussed Finally, the slope of logσ′pc against logε v for Busan marine clay is given

In chapter 5, the effect of leaching on stabilized dredged clay is continuously investigated by mixture with non-salt and salt-rich dredged clay with cement The unconfined compressive strength, compression characteristics of composite geomaterial is carried out by unconfined compression test and oedometer test The results of these tests are analyzed and compared to obtain the effect on strength development of non-salt and salt-rich stabilized dredged clay

In chapter 6, conclusions and recommendations are given

of soil salinity depends on the salt concentration in seawater and changes with geological environment over long periods

Natural marine clay deposited in seawater condition tends to have higher void ratio and low compressibility due to flocculated structure Moreover, salt has some negative effect on strength development and compressibility for stabilized dredged clay as dredged clay is mixed with cement However, a leaching process can change the natural marine clay and salt-rich dredged clay into leached clay and non-salt dredged clay, respectively Then, the geotechnical properties are changed due to leaching Many researchers have investigated how leaching affects geotechnical properties of marine clay This chapter describes these researches focused on soil structure, cause of leaching in the field, leaching method in laboratory, and effect of leaching on geotechnical properties of marine clay and stabilized dredged clay

2.2 Soil structure

Many models have been proposed by many researchers to describe the soil structure from the early concepts to microfabric observation methods However, this study focuses on the difference in soil structure of sedimentation in seawater (marine clay) and sedimentation in fresh water It has been well known that the marine clay has a flocculated structure while sedimentation in fresh water has a dispersed structure In order to know the previous model of these structures, a literature review from the early concepts

to microfabric observation methods is given

2.2.1 Early concepts

The early concepts of soil structure have been proposed primarily on the basic understanding of internal electrical forces by many researchers such as Terzaghi (1925), Casagrande (1932), Lambe (1953), Schofield and Samson (1954) The first model is assumed by Terzaghi (1925) that the individual clay mineral grains stick to each other at the points of contact for building honeycomb arrangement as shown in Fig 2.1 It has been suggested that the flocculent structures are formed upon coagulation of the suspension by an electrolyte and therefore the sediment obtains the whole structure at the time

of its formation However, this model does not describe the difference of sedimentation in seawater and fresh water

Later, Casagrande (1932) proposed that “honeycomb” arrangement existed in sensitive marine clays as shown in Fig 2.2 In seawater, the

coagulating effect of the salts is so strong that it causes all the particles to form flocs Actually, this model is basically similar to Terzaghi’s concept Because of the molecular attraction, these flocs and individual grains settle into a very loose honeycomb structure The clay flocs located in the smallest gaps between adjacent silt particles were highly compressed whereas those contained in the larger spaces between silt particles had little compression, which is called “bond clays” and “matrix clay”, respectively Based on this model, it can be postulated that the undisturbed marine clay have high rigidity due to “bond clays”, but once bonding is destroyed, such as remolding process, the soil becomes very soft Therefore, it has high sensitivity due to flocculent structure

In later years, the consideration is involved with the double layer theory Lambe (1953) considered the structure of inorganic soils and presented clay mineral arrangements associated with deposition in a range of electrochemical environments from freshwater to marine In freshwater, he stated that sedimentation in a very low electrolytic concentration leads to flocculation due to net electrical force between adjacent particles being attractive Under such conditions the individual plates form an open arrangement of edge-edge or edged-face contacts, which referred to as “non-salt flocculated” (Fig 2.3a) In marine, he also stated that sedimentation in a high electrolytic concentration leads to flocculation with a higher degree of particle parallelism than in the low electrolytic concentration (freshwater) Under such conditions the individual plates form an open stepped

arrangement of edged-edge, face-face and edge-face contacts, which referred

to as “salt-flocculated” (Fig 2.3b) Lambe (1958) stated that sedimentation in

a high electrolytic concentration leads to flocculation, while in intermediate electrolytic concentrations, such as brackish water lead to dispersed arrangement (Fig 2.3c)