Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Pokharel, Bigul
Master of Applied Science - MASc
University of British Columbia
The high compressibility and low shear strength characteristics of organic soil are significant challenges for the infrastructure development in Canada and across the world as well. This research investigated the use of pulp mill fly ash (PFA), alkaline activator and calcium bentonite (Ca-bentonite) to improve the engineering behaviour of the organic soil consisting of 26% organic matter, obtained from the wetlands in Wabasca region, Northern Alberta, Canada. Cementitious materials such as lime, and Portland cement have been successfully used to improve the engineering properties of problematic mineral soils having low organic content. However, limited studies have been devoted to stabilization of organic soils commonly encountered in forested regions of Canada. Series of fall cone tests and one dimensional oedometer tests were carried out to evaluate the effect of PFA, alkaline activator and Ca-bentonite on the engineering behaviour of the stabilized organic soil. In addition to fall cone and oedometer tests, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) test were conducted on regular and stabilized organic soil to analysis the change in microstructure and chemical components of the soil after stabilization. The results showed that the strength and compressibility behavior of the organic soil can be improved when treated with PFA and Ca-bentonite. Moreover, a potential large settlement of the organic soil can be reduced by adding PFA and Ca-bentonite to the organic soil. The amount of secondary compression index (Cα) was decreased significantly when compared to untreated organic soil, indicating that the PFA and Ca-bentonite stabilizer can effectively improve the creep behavior of organic soil. However, within the scope of this work, use of alkaline activator did not show any improvement in engineering behavior of the stabilized organic soil. This study implies that beneficial utilization of PFA and Ca- bentonite for stabilization of soil organic matter in forests and lowlands could be a practical step towards environmentally sound management of this waste byproduct.
EFFECT OF PULP MILL FLY ASH AND CALCIUM BENTONITE ON THE MECHANICAL AND MICROSTRUCTURAL BEHAVIOR OF ORGANIC SOIL FROM ALBERTA, CANADA by Bigul Pokharel BE in Civil Engineering, Tribhuvan University, Nepal, 2014 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) June 2020 © Bigul Pokharel, 2020 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis/dissertation entitled: Effect of pulp mill fly ash and Calcium bentonite on the mechanical and microstructural behavior of organic soil from Alberta, Canada submitted by Bigul Pokharel in partial fulfillment of the requirements of the degree of Master of Applied Science in Civil Engineering. Dr. Sumi Siddiqua, School of Engineering Supervisor Dr. Mahmudur Fatmi, School of Engineering Supervisory Committee Member Dr. Ramy Saadeldin, Adjunct Faculty, School of Engineering Supervisory Committee Member Dr. Mohammad Hossein Zarifi, School of Engineering University Examiner iii Abstract The high compressibility and low shear strength characteristics of organic soil are significant challenges for the infrastructure development in Canada and across the world as well. This research investigated the use of pulp mill fly ash (PFA), alkaline activator and calcium bentonite (Ca-bentonite) to improve the engineering behaviour of the organic soil consisting of 26% organic matter, obtained from the wetlands in Wabasca region, Northern Alberta, Canada. Cementitious materials such as lime, and Portland cement have been successfully used to improve the engineering properties of problematic mineral soils having low organic content. However, limited studies have been devoted to stabilization of organic soils commonly encountered in forested regions of Canada. Series of fall cone tests and one dimensional oedometer tests were carried out to evaluate the effect of PFA, alkaline activator and Ca-bentonite on the engineering behaviour of the stabilized organic soil. In addition to fall cone and oedometer tests, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) test were conducted on regular and stabilized organic soil to analysis the change in microstructure and chemical components of the soil after stabilization. The results showed that the strength and compressibility behavior of the organic soil can be improved when treated with PFA and Ca-bentonite. Moreover, a potential large settlement of the organic soil can be reduced by adding PFA and Ca-bentonite to the organic soil. The amount of secondary compression index (Cα) was decreased significantly when compared to untreated organic soil, indicating that the PFA and Ca-bentonite stabilizer can effectively improve the creep behavior of organic soil. However, within the scope of this work, use of alkaline activator did not show any improvement in engineering behavior of the iv stabilized organic soil. This study implies that beneficial utilization of PFA and Ca- bentonite for stabilization of soil organic matter in forests and lowlands could be a practical step towards environmentally sound management of this waste byproduct. v Lay Summary In Canada, about 18% of the total land is covered by organic soil, and a significant proportion of the highways, and oil and gas pipelines pass through the organic soil. Construction of road and pipelines in organic soil is challenging as the excessive settlement can be occurred due to its inherent characteristics. It is crucial to advance our understanding of the compression behavior and stabilization of organic soil to prevent the infrastructures from the excessive settlement and bearing failure. There has been significant research on stabilization of mineral soil; however, understanding of organic soil stabilization is still sparse. In this research, an effort was made to understand the chemical stabilization of organic soil using pulp mill fly ash, alkaline activator and calcium bentonite. One of the major contributions of this research is to evaluate the strength and compressibility behaviour of organic soil treated with environment friendly and cost-effective binders. vi Preface This thesis aimed at evaluating the effect of pulp mill fly ash (PFA), alkaline activator, and calcium bentonite (Ca-bentonite) on strength and compressibility behaviour of organic soil obtained from the wetlands in Wabasca region, Northern Alberta, Canada. The author prepared overall research plan, completed extensive literature reviews, conducted laboratory tests and data analysis, presented results, and prepared manuscripts for journal under the supervision of Dr. Sumi Siddiqua. The laboratory experiments were performed in the Geomaterial Research Laboratory at the School of Engineering. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) conducted at FiLTER - Fipke Laboratory for Trace Element Research, the University of British, Columbia Okanagan Campus (UBCO). X-ray diffraction (XRD) test was conducted in Vancouver campus. Ethics approval from the UBC Research Ethics Board was not required for this study. While earning my master’s degree in applied science (MASc.), I was able to submit two papers to journal for possible publication. The list of submitted and preparing papers related to this thesis is presented below. 1. Pokharel, B., and Siddiqua, S. (2020). “Understanding the effect of pulp mill fly ash on strength, compressibility and microstructures of organic soil.” International Journal of Geomechanics, ASCE. 2. Pokharel, B., and Siddiqua, S. (2020). “Effect of calcium bentonite clay and fly ash on the stabilization of organic soil from Alberta, Canada.” Engineering Geology. 3. Pokharel, B., and Siddiqua, S. (2020). “Performance of alkaline activator in stabilizing organic soil” (in preparation). vii Table of Contents Abstract...................................................................................................................................iii Lay Summary .......................................................................................................................... v Preface................ ..................................................................................................................... vi List of Figures ......................................................................................................................... xi List of Tables ........................................................................................................................ xvi List of Abbreviations ......................................................................................................... xviii List of Symbols ...................................................................................................................... xx Acknowledgments ............................................................................................................... xxii Chapter 1: Introduction ....................................................................................................... 1 1.1 Background ................................................................................................................ 1 1.1.1 Organic soil ......................................................................................................... 2 1.1.2 Pulp mill fly ash (PFA) ....................................................................................... 4 1.1.3 Alkaline activator ................................................................................................ 5 1.1.4 Calcium bentonite (Ca-bentonite) ....................................................................... 6 1.2 Problem statement and motivation ............................................................................. 6 1.3 Objectives ................................................................................................................. 10 1.4 Scope of research ..................................................................................................... 11 1.5 Organization of the dissertation ............................................................................... 13 Chapter 2: Literature Review ........................................................................................... 16 2.1 Organic soil .............................................................................................................. 16 2.1.1 Index properties ................................................................................................ 18 2.1.2 Strength ............................................................................................................. 20 2.1.3 Compressibility ................................................................................................. 21 2.2 Soil stabilization ....................................................................................................... 24 2.2.1 Organic soil stabilization .................................................................................. 24 2.3 pH of organic soil ..................................................................................................... 33 2.4 Soil binders ............................................................................................................... 33 2.4.1 Fly ash ............................................................................................................... 34 2.4.2 Pulp mill fly ash (PFA) ..................................................................................... 37 2.4.3 Calcium bentonite ............................................................................................. 38 2.4.4 Alkaline activator .............................................................................................. 41 viii 2.5 Curing ....................................................................................................................... 44 2.5.1 Curing period .................................................................................................... 44 2.5.2 Curing technique ............................................................................................... 46 2.6 Microstructural and chemical properties .................................................................. 48 2.6.1 SEM and EDS ................................................................................................... 49 2.6.2 XRD .................................................................................................................. 51 2.7 Summary .................................................................................................................. 52 Chapter 3: Geotechnical Characterization of Materials ................................................ 53 3.1 Organic soil .............................................................................................................. 53 3.1.1 Visual description ............................................................................................. 53 3.1.2 Soil index properties ......................................................................................... 54 3.1.2.1 Moisture content ............................................................................................ 55 3.1.2.2 Organic content ............................................................................................. 55 3.1.2.3 Fiber content .................................................................................................. 56 3.1.2.4 Degree of decomposition .............................................................................. 56 3.1.2.5 Particle size distribution ................................................................................ 56 3.1.2.6 Specific gravity ............................................................................................. 56 3.1.2.7 Density .......................................................................................................... 57 3.1.2.8 Void ratios ..................................................................................................... 57 3.1.2.9 Atterberg limit ............................................................................................... 58 3.1.2.10 Shrinkage limit .......................................................................................... 59 3.1.3 Shear strength.................................................................................................... 59 3.1.4 Soil Compressibility.......................................................................................... 60 3.1.5 Microstructural and chemical properties .......................................................... 61 3.2 Pulp mill fly ash (PFA) ............................................................................................ 62 3.2.1 Index properties ................................................................................................ 64 3.2.2 Microstructural and chemical properties .......................................................... 64 3.3 Calcium bentonite (Ca-bentonite) ............................................................................ 65 3.3.1 Index properties ................................................................................................ 66 3.3.2 Microstructural and chemical properties .......................................................... 66 3.4 Alkaline activator ..................................................................................................... 67 3.5 Summary .................................................................................................................. 67 ix Chapter 4: Methodology .................................................................................................... 69 4.1 Mix design ................................................................................................................ 69 4.2 Sample preparation ................................................................................................... 74 4.2.1 Mixing ............................................................................................................... 74 4.2.2 Compaction ....................................................................................................... 75 4.2.3 Curing ............................................................................................................... 76 4.3 Experimental phases ................................................................................................. 78 4.3.1 Fall cone test ..................................................................................................... 78 4.3.2 Oedometer test .................................................................................................. 79 4.3.3 pH tests.............................................................................................................. 80 4.3.4 SEM/EDS and XRD ......................................................................................... 81 4.4 Summary .................................................................................................................. 82 Chapter 5: Performance of Pulp Mill Fly Ash, and Alkaline Activator........................ 83 5.1 Effect of pulp mill fly ash ........................................................................................ 83 5.1.1 Undrained shear strength .................................................................................. 83 5.1.2 Compressibility behaviour ................................................................................ 85 5.1.2.1 Stress-strain behaviour .................................................................................. 85 5.1.2.2 Change in void ratio ...................................................................................... 87 5.1.2.3 Coefficient of consolidation and compression index .................................... 88 5.1.2.4 Hydraulic conductivity .................................................................................. 91 5.1.2.5 Secondary compression index ....................................................................... 93 5.1.2.6 Evaluation of Cα/Cc ratio ............................................................................... 94 5.1.3 Microstructural analysis and chemical analysis ................................................ 95 5.1.3.1 SEM analysis ................................................................................................. 95 5.1.3.2 Energy Dispersive Spectroscopy (EDS) analysis.......................................... 97 5.1.3.3 Variation of soil pH with PFA treatment ...................................................... 99 5.2 Effect of Alkaline Activator (AA) ......................................................................... 100 5.2.1 Undrained shear strength ................................................................................ 100 5.2.2 Compressibility behaviour .............................................................................. 102 5.3 Summary ................................................................................................................ 104 Chapter 6: Performance of Calcium Bentonite and Pulp Mill Fly Ash ...................... 105 6.1 Undrained shear strength ........................................................................................ 105 6.2 Effect of curing method.......................................................................................... 107 x 6.3 Compressibility behavior ....................................................................................... 108 6.3.1 Consolidation settlement ................................................................................. 108 6.3.2 Stress-strain behavior ...................................................................................... 109 6.3.3 Change in void ratio ........................................................................................ 110 6.3.4 Primary compression index, coefficient of consolidation and constrained modulus.... ..................................................................................................................... 111 6.3.5 Hydraulic conductivity.................................................................................... 114 6.3.6 Creep Deformation.......................................................................................... 116 6.4 Morphological and chemical analysis .................................................................... 118 6.4.1 SEM/EDS analysis .......................................................................................... 118 6.4.2 X-ray diffraction (XRD) analysis ................................................................... 122 6.4.3 Variation of soil pH with Ca-bentonite ........................................................... 124 6.5 Summary ................................................................................................................ 124 Chapter 7: Conclusions and Recommendations ............................................................ 126 7.1 Summary ................................................................................................................ 126 7.2 Scientific contributions .......................................................................................... 127 7.3 Major findings ........................................................................................................ 127 7.3.1 PFA stabilization ............................................................................................. 128 7.3.2 Alkaline activator stabilization ....................................................................... 129 7.3.3 Ca-bentonite and PFA stabilization ................................................................ 130 7.4 Limitations and future works ................................................................................. 132 References...... ...................................................................................................................... 134 xi List of Figures Figure 1.1. Flow chart of the study ................................................................................................ 12 Figure 2.1. Typical correlation between organic content and specific gravity (modified from Huat et al. 2009). ............................................................................................................................. 19 Figure 2.2. Creep deformation of coarse peat with time (modified from O’Kelly 2005). ............. 21 Figure 2.3. The value of Cα/Cc for organic soil and variation of Cα/Cc with cement content. ....... 23 Figure 2.4. Effect of organic material on creep deformation (modified from Venda Oliveira 2012).................................................................................................................................................. 24 Figure 2.5. Fundamental procedures of soil reinforcement (modified from Hezaji et al. 2012). .. 25 Figure 2.6. Unconfined compressive strength for various mixes for Raheenmore peat (modified from Hebib and Farrell 2003). ................................................................................................ 26 Figure 2.7. Cα versus Cc for cement mixed Ballydermot peat (modified from Hebib and Farrell 2003). ...................................................................................................................................... 27 Figure 2.8. Stress-strain curves of untreated and lime-treated clays (modified from Sakr et al. 2009).................................................................................................................................................. 28 Figure 2.9. Effect of cement on compressibility and hydraulic conductivity of the treated organic soil (modified from Bobet et al. 2011).................................................................................... 29 Figure 2.10 Effect of cement and fly ash dosage and curing period on UCS (modified from Kolay et al. 2011). ............................................................................................................................. 30 Figure 2.11. Effect of fly ash on stress and resilient modulus (inverse of volume compressibility, mv) of organic soil (modified from Tastan et al. 2011)........................................................... 31 xii Figure 2.12. Effect of concentration of NaOH and activator to binder ratio on compressive strength of concrete (modified from Cristelo et al. 2013). ................................................................... 43 Figure 2.13. Effect of curing period on UCS of stabilized soil (modified from Siddiqua and Barreto 2018). ...................................................................................................................................... 45 Figure 2.14. Effect of curing period on stress-strain behavior of the stabilized soil (modified from Hassan et al. 2019). ................................................................................................................. 46 Figure 2.15. Effect of curing technique on UCS of the stabilized soil with cement (modified from Kalantari and Prasad 2014). .................................................................................................... 47 Figure 2.16. Effect of curing period and technique on compressibility of organic soil treated with various material (modified from Dehghanbanadaki et al. 2016). ........................................... 48 Figure 2.17. SEM image of (a) peat soil and (b) stabilized peat soil with fly ash (Kolay et al. 2011)................................................................................................................................................. 50 Figure 3.1. (a) Organic soil (b) SEM image, and (c) EDS spectrum. ............................................ 53 Figure 3.2. Plasticity chart to characterise the soil based on Atterberg limits. .............................. 59 Figure 3.3. X-Ray diffraction results with a mineral indication for soil, fly ash (PFA), and Bentonite clay (Ca-bentonite).................................................................................................................. 63 Figure 3.4. (a) Dumped PFA in the local pulp mill industry in Kamloops, BC, and (b) close-up view of PFA. .................................................................................................................................... 63 Figure 3.5. (a) SEM image, and (b) EDS spectrum of PFA........................................................... 65 Figure 3.6. (a) Calcium bentonite (b) SEM image, and (b) EDS spectrum of Ca-bentonite. ........ 66 Figure 4.1. Compacted (a) oedometer sample and (b) samples for fall cone test. ......................... 76 Figure 4.2. (a) Water curing for oedometer test (b) water curing for fall cone test, and (c) air curing.................................................................................................................................................. 77 xiii Figure 4.3. (a) Set up for fall cone test and (b) penetration of cone into soil after release from its initial position. ........................................................................................................................ 79 Figure 4.4. (a) Setup for oedometer test, (b) consolidated sample in oedometer ring, and (c) sample after consolidation test. ........................................................................................................... 80 Figure 5.1. Effect of PFA content on the shear strength of organic soil. ....................................... 84 Figure 5.2. Influence of PFA on compression behavior of organic soil. ....................................... 86 Figure 5.3. Effect of surcharge load and PFA content on the change in void ratio of organic soil.................................................................................................................................................. 87 Figure 5.4. (a) Variation of Cv with respect to normal stress and PFA content, and (b) Variation of Cc with respect to normal stress and PFA content. ................................................................. 89 Figure 5.5. Variation of mv with respect to PFA and normal stress, ’v. ...................................... 90 Figure 5.6. Effect of PFA on (a) compression index (Cc) and (b) coefficient of volume compressibility (mv). ............................................................................................................... 90 Figure 5.7. Variation of k value with applied pressure and PFA content. ..................................... 92 Figure 5.8. (a) Change in void ratio during creep with PFA content, and (b) variation of Cα with PFA content. ........................................................................................................................... 93 Figure 5.9. Variation of C /Cc with PFA or cement content. ........................................................ 95 Figure 5.10. SEM image of the organic soil after consolidation test (a) untreated soil, (b) with 5% PFA, (c)10% PFA, (d) 20% PFA, (e) 30% PFA, (f) 40% PFA, and (g) 50% PFA. ............... 96 Figure 5.11. EDS spectrum of untreated and treated organic soil (a) 0% PFA-untreated soil, (b) 5% PFA, (c) 10 PFA, (d) 20% PFA, (e) 30% PFA, (f) 40% PFA, and (g) 50% PFA. ................. 98 Figure 5.12. Normalized undrained shear strength for various AA/PFA and NaOH concentration................................................................................................................................................ 101 xiv Figure 5.13. Effect of curing period on undrained shear strength. .............................................. 102 Figure 5.14. Consolidation settlement of the samples treated different concentration and dosage of alkalinity. .............................................................................................................................. 103 Figure 6.1. Variation of undrained shear strength (su) with % of Ca-bentonite........................... 106 Figure 6.2. Comparison of consolidation settlement of the specimens cured by water and air. .. 107 Figure 6.3. Effect of Ca-bentonite dosage on the primary consolidation behavior of treated and untreated organic soil. ........................................................................................................... 109 Figure 6.4. (a) Compression curve from 1D consolidation test and (b) void ratio after each loading increment............................................................................................................................... 110 Figure 6.5. Variation of Cc with normal stress and Ca-bentonite dosage. ................................... 111 Figure 6.6. Variation of (a) coefficient of consolidation and (b) constrained modulus with normal stress and Ca-bentonite dosage. ............................................................................................ 112 Figure 6.7. Variation of (a) constrained modulus and (b) primary compression index with Ca-bentonite dosage.................................................................................................................... 114 Figure 6.8. Hydraulic conductivity of the samples as a function of (a) normal stress and (b) void ratio at the end of each loading stage. ................................................................................... 115 Figure 6.9. Influence of Ca-bentonite content on one-dimensional creep. .................................. 116 Figure 6.10. Variation of Cα/Cc with Ca-bentonite dosage and compassion of Cα/Cc with previous research. ................................................................................................................................ 117 Figure 6.11. Micrograph and chemical composition of untreated organic soil. .......................... 118 Figure 6.12. Micrograph and chemical composition of new materials of 30% PFA treated soil. 119 Figure 6.13. Micrograph and chemical composition CASH and ET materials from soil treated with 10% Ca-bentonite with 30% PFA. ........................................................................................ 120 xv Figure 6.14. Micrograph and chemical composition soil treated with 15% Ca-bentonite with 30% PFA. ...................................................................................................................................... 120 Figure 6.15. X-ray diffraction spectrum for untreated soil, soil treated with 30% PFA and soil treated with 10% Ca-bentonite and 30% PFA. ..................................................................... 122 xvi List of Tables Table 2.1. Classification of peat based on degree of decomposition (Karlson and Hansbo 1981; Jarret 1995) ...................................................................................................................... 17 Table 2.2. Classification of organic soil based on their organic content (Jarret 1995; ASTM D4427) ............................................................................................................................. 18 Table 2.3. USDA classification of peat (Huat 2004) .............................................................. 18 Table 2.4. Index properties of the fibrous peat (Mesri and Ajlouni 2007). ........................... 20 Table 2.5. Mixed design for Irish peat stabilization (Hebib and Farrell 2003)....................... 26 Table 2.6. summary of compression and creep index (Bobet et al. 2011). ............................. 29 Table 2.7. Comparison of chemical properties of OPC and fly ash (Feng and Clark 2011) .. 35 Table 2.8. Comparison of chemical and physical properties of OPC and fly ash (Zeng et al. 2012). ............................................................................................................................... 36 Table 2.9 Typical oxide compositions of PFA (Cherian and Siddiqua 2019). ....................... 38 Table 2.10 Typical physical properties of Calcium bentonite (Dananaj et al. 2005). ............ 40 Table 2.11. Typical chemical components of Calcium bentonite (Dananaj et al. 2005). ....... 40 Table 3.1. Geotechnical properties of the soil, PFA and Ca-bentonite................................... 54 Table 3.2. Chemical properties of soil, PFA, and Ca-bentonite. ............................................ 62 Table 4.1. Mix design for sample preparation for the Phase 1 test program. ......................... 70 Table 4.2. Mix design for Fall cone tests. ............................................................................... 71 Table 4.3. Mix design for one-dimensional oedometer tests. ................................................. 72 Table 4.4. Mix design for fall cone and oedometer tests ........................................................ 73 Table 4.5. Loading scheme for consolidation test. ................................................................. 80 xvii Table 5.1. pH value and EDS analysis of soil treated with different % of PFA at 7 days curing periods.............................................................................................................................. 99 Table 6.1. EDS analysis of soil treated with different % of Ca-bentonite + 30% PFA. ....... 121 xviii List of Abbreviations AA Alkaline activator ASTM American Society for Testing and Materials BC British Columbia C Cement Ca-bentonite Calcium bentonite CAH Calcium aluminate hydrate CASH Calcium aluminate silicate hydrate CBR California bearing ratio CEC Cation exchange capacity CPL Cement, fly ash, lime CSH Calcium silicate hydrate EDS Energy dispersive spectroscopy EOP End of primary consolidation ET Ettringite ICP-AES Inductive coupled plasma-atomic emission spectroscopy LOI Loss on ignition OC Organic content OM Organic matter OPC Ordinary Portland cement PC Percentage cement PFA Pulp mill fly ash S Blast furnace slag SC Blast furnace slag, cement SCBA Sugarcane bagasse ash SEM Scanning Electron microscopy xix SG Blast furnace slag, gypsum TCLP Toxicity characteristic leaching procedure UCS Unconfined compressive strength USA United states of America USCS Unified Soil Classification System XRD X- ray diffraction xx List of Symbols H Consolidation settlement 'p Pre-consolidation pressure s Density of soil w Density of water ∆e Change in void ratio °C Degree Celsius A Area Al Aluminum Al2O3 Aluminum Oxide C Carbon Ca Calcium Cao Calcium oxide Cc Primary compression index CO2 Carbon dioxide Cs Swelling index Cv Coefficient of consolidation Cα Secondary compression index e Void ratio ei Initial void ratio Gs Specific gravity H cone penetration depth Hi Initial height Hs Height of solid mass HT Total height Hv Height of void PI Plasticity index K Cone factor k Hydraulic conductivity kPa kilopascal xxi M Molal Ms Mass of soil solid mv Coefficient of volume compressibility N loss on ignition Na2SiO3 Sodium Silicate NaOH Sodium Hydroxide Pt Highly organic soil Q Total cone weight quc Shear strength Si Silica su Undrained shear strength Vs Volume of solid mass wL Liquid limit wP Plasticity limit σ′v Effective vertical stress  Total porosity d Dry bulk density I Initial density xxii Acknowledgments I express my sincerest gratitude to all the great individuals who supported me throughout this journey. I would like to thank my supervisor, Dr. Sumi Siddiqua, for providing me with a huge opportunity to shape my knowledge and skills at the University of British Columbia. I am so grateful to you for introducing me to the world of research and constantly supporting me through every step of my journey. It truly would not have been possible to complete this thesis without your continuous guidance and dedication. I appreciate the time and thoughtful consideration that you gave me to improve my work. I am extremely grateful to all my supervisory committee members. My deepest appreciation to The Natural Sciences and Engineering Research Council of Canada (NSERC), TransCanada, and ConeTec for the generous funding, samples, and in-kind support needed for this research. I am indebted to UBC writing research center and the technicians Dr. Sudeep Shrestha, Dr. Mark Button, and Michelle Tofteland. I am thankful to the biology research group who extremely helped me with the freeze-drying of samples in their lab. I would like to thank all my colleagues from my research group for their support and for providing me with a great work and study environment during my experiment in the laboratory. I cannot forget the Nepalese students at UBCO who have always been there to support one another in the difficult times of COVID-19. I am thankful to my roommate, Ashmita Rai, whose love and support during my hard times are worth more than I could express on paper. I truly appreciate the generous support from Bhuwan Paudel and Sarin Raj Pokhrel during my studies. I can not end without thanking my beloved parents and in-laws for showering me with love and encouragement. My parents whose restless effort in supporting my education inspired xxiii me to accomplish this goal and gave me the strength I needed throughout my studies. I have always dreamed of making my mom proud and I believe the accomplishments in this thesis are going to make her happy. Thanks to my dearest sister Kshitij Pokharel, brother Biliv Pokharel for their encouragement and love. Most importantly, I would like to thank my dearest husband who has always been my strength throughout this journey. Your emotional and academic support and your sacrifices have always given me hope to move forward in life. Without you, I would not be the person I am today. xxiv Dedicated to my beloved parents Sita Pokharel, Govind Raj Pokharel and my dear husband Keshab Sharma Chapter 1: Introduction 1.1 Background Organic soil covers about 436 million hectares of earth’s total landmass (Andriesse 1988). Canada and the USA have about 110 million and 30 million hectares of peatland respectively (Harper and Kershaw 1997). A significant proportion of the world’s highways and oil and gas pipelines pass through organic soil (Harper and Kershaw 1997; Stark et al. 2019; Gharedaghloo and Price 2019). Existing practices for the construction of infrastructure over organic soil mostly involves preloading or the replacement of organic soil with granular materials to a sufficient depth. These methods are reported to be time-consuming and not particularly cost-effective. The chemical stabilization technique is considered to be a more economical and time-saving option for organic soil (Kazemian 2017). Contrary to mineral soil, the engineering properties of organic soil are poorly understood, and research on the stabilization of organic soil is sparse (Tremblay et al. 2002; Bobet et al. 2011). It is crucial to understand the engineering properties and stabilization of organic soil to prevent the infrastructure from experiencing bearing capacity failure and excessive settlement. This research focuses on the utilization of industrial by-product pulp mill fly ash (PFA), alkaline activator (AA), and locally available raw calcium bentonite (Ca-bentonite) for organic soil stabilization. PFA, alkaline activator, and Ca-bentonite are considered as green binders, and can be an energy-efficient and cost-effective alternative for ordinary Portland cement (OPC) in geotechnical engineering applications. 2 The effect of PFA, alkaline activator, and Ca-bentonite on the strength and compressibility behavior of organic soil were investigated in this research through a series of laboratory tests, and microstructural and chemical analysis. The laboratory tests were comprised of fall cone tests, one-dimensional consolidation tests, and SEM/EDS and XRD tests for microstructural and chemical composition determination. The results obtained were analyzed in terms of undrained shear strength, primary and secondary (creep) compression index, coefficient of consolidation, the ratio of secondary compression index to primary compression index, and the change in microstructural and chemical composition of the stabilized soil. This chapter discusses the background of the thesis, the problem statement of this study, and the objectives and scope of the stabilization of the organic soil along with an outline of the dissertation. 1.1.1 Organic soil Organic soil is derived from plant residues and can cover a wide range of soil with high organic content like fibrous peat to soil with a significant decomposition of organic material and mineral contamination (Radforth and Brawner 1977; Riedy 2006; Sakr et al. 2009; Tastan et al. 2011). Organic soil is well known as a problematic soil for engineering construction because of its intrinsic characteristics such as low shear strength and density, high water content and compressibility, and significant creep behavior (Huat et al. 2004; Farrell 2012; Venda Oliveira 2013). Organic soils undergo significant primary consolidation in a very short period and continues in creep stages (Mesri et al. 1997). As a result, organic soils pose significant engineering problems for infrastructure development in the form of bearing capacity failure, shear failure or excessive settlement (Radforth and Brawner 1977; Acharya 2015). 3 In Canada, about 18% (1.5M km2) of the total land is covered by organic soil, and the spatial variation of the physical and chemical properties of the soil is primarily based on the origin of organic materials and mineral contaminations (Farrell 2012; Acharya 2015). A significant proportion of highways, and oil and gas pipelines in Canada pass through peat and muskeg (bog wetland), which are major sources of organic soil. Strong soils are necessary to ensure safe transportation of oil via pipelines. The high compressibility and low strength of organic soil present challenges for the design of buried pipelines against potential relative ground movements. Due to significant creep deformations that may take place under the mass of pipe and fuel, creep of bed soil leading to possible buckling could take place in buried pipelines, creating challenging for the design and construction of the pipeline. Infrastructure development is necessary on organic soils in spite of its problematic nature. For example, most oil sands mining sites in Northern Alberta are underlain by thick deposits of peat or highly organic soils (Bedair 2013). Pipelines provide one of the safest modes of transportation of liquids and gas over large distances, and buried pipelines are important for transporting oil and gas in Canada. Canadian pipelines have experienced significant failures caused by organic soils in the past. For example, Terasen’s mainline pipeline failed near Abbotsford, British Columbia, in 2005. The field investigations showed that the failed site was located in a wet peat bog. The failure of the pipeline may have been attributed to excessive settlement of peat beneath the pipeline (Pipeline Investigation Report 2005). Similarly, the energy pipeline managed by Husky Energy failed near the North Saskatchewan River in July 2016. Investigations showed that heavy rainfall and weak soil led to excessive ground movement, causing the pipeline failure (Saskatoon Phoenix 2016). All development projects, especially road and pipeline 4 constructions, require a specific construction method when dealing with organic soils (Huat et al. 2005; Talib et al. 2015; Nath et al. 2017). It is crucial to advance our understanding of the compression behavior and stabilization of organic soil to prevent highways and pipelines from experiencing excessive settlement which could lead to failure of the structures. 1.1.2 Pulp mill fly ash (PFA) Pulp mill fly ash (PFA), a solid waste by-product of wood combustion in power boilers of pulp and paper mills, is regarded as a green binder, and can be an energy-efficient and cost-effective substitute for OPC in geotechnical engineering applications (Nalbantoglu 2004; Novais et al. 2018). In general, PFA is produced by burning bark, wood residue, re-circulated ash, and sludge with microorganisms and fibers in an incinerator at a high temperature of 730 °C (Arm et al. 2014). The pulp and paper industries are expanding significantly in recent years as the market demand for paper, tissue paper, pulp for personal hygiene products, cardboard and packaging paper, and paper textile applications has substantially increased (Cherian and Siddiqua 2019). About one million tons of PFA is produced every year in Canada, occupying valuable landfill space and causing concerns for its management for the paper and pulp industries (Hannam 2017; Bouzoubaâ and Fournier 2018). Hannam (2017) reported that about 235,000 tons of wood ash were produced in BC in 2014 alone. More than 2.9 million tons of PFA are produced in the United States of America every year (Carabin et al. 2000). However, PFA remains a highly underused by-product due to low marketable value and demands, primarily because of a lack of technical standards and stringent government policies. The utilization of non-commonly used PFA can significantly contribute to reducing energy demand and greenhouse gases as it replaces the use of cement for soil stabilization. PFA is a green stabilizer, and also reduces the requirement of landfills which often threaten natural resources 5 (Cherian and Siddiqua 2019; Nath et al. 2017). Moreover, PFA reduces the heat of hydration when mixing with soil; hence, it requires less water than OPC and is easier to handle and use in cold weather conditions which prevail in Canada. The use of OPC and lime for soil stabilization is costly and less environmentally friendly than fly ash and other industrial by-products, which are inherently more sustainable and energy-efficient, as PFA can be reused beneficially to stabilize soil as an alternative to landfilling (Chang and Cho 2012). PFA is considered to be a potential candidate for supplementary cementitious material, due to its fineness and high surface area, as well as its high calcium and silica contents (Novais et al. 2014; Cherian and Siddiqua 2019). Previous studies have demonstrated its potential as a raw material for construction and geotechnical applications, soil amendment in agriculture and forestry, as well as for adsorbing inorganic and organic compounds from contaminated land and water bodies. (Laohaprapanon et al. 2010; Kogbara and Al-Tabbaa 2011; Camberato et al. 2011; Simão et al. 2019). 1.1.3 Alkaline activator A combination of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) was used as an alkaline activator to activate the PFA. The chemical process that activates fly ash is known as geopolymerization. The main objective of the alkaline activator is to activate the silica and alumina components of PFA and to produce the geopolymer which binds to the soil particles. Previous studies showed that an alkaline activator combination of NaOH and Na2SiO3 can break and dissolve silica and alumina bonds from an aluminosilicate mineral and accelerate the pozzolanic reaction and setting time of the fly ash. Duchesne et al. (2010) reported that NaOH as an alkaline activator enhances the pozzolanic reaction and produce a less smooth gel. Similarly, Palomo et al. (1999) highlighted that Na2SiO3 accelerates the reaction between the 6 source material and the solution and produces cementitious materials. The concentration of NaOH and the mass ratio of the alkaline activator to PFA were varied systematically to understand the effect of alkaline activator on stabilizing the organic soil using activated PFA. 1.1.4 Calcium bentonite (Ca-bentonite) Calcium bentonite consists of a smectite mineral group and has unique physical and chemical properties such as a high absorption capacity, low swelling capacity, high surface area, and high viscosity. Ca-bentonite has a low swelling capacity compared to sodium bentonite. Ca-bentonite contains enough silica which contribute to pozzolanic reactions during stabilization (Fan et al. 2014; Wayal et al. 2012). Ca-bentonite is highly reactive to CaOH generated from cement hydrolysis for the development of pozzolanic cementation bond (Sabrin et al. 2019). Because of these physical and chemical properties, calcium bentonite can be used as a natural pozzolan to partially replace OPC in the stabilization of the soil. Formation of such pozzolanic cementation bonds is known to ensure continued strength gain and improve the microstructure of cemented materials. As Ca-bentonite consists of a large portion of fine materials, it also works as a filler to reduce the void space of stabilized soil. Ca-bentonite has been used to produce environmentally-friendly concrete, mortar, and brick, as well as to stabilize inorganic soil (Isaia et al. 2003; Habert et al. 2008). Calcium bentonite has been widely used for reducing the permeability of backfill material. 1.2 Problem statement and motivation Mixing of chemical stabilizer as an additive with soft soil is a common practice to improve the engineering properties of the soft soil. Chemical stabilizers are categorized as traditional and non-traditional stabilizers (Marto et al. 2014). Traditional stabilizers consist of ordinary 7 Portland cement (OPC), lime, gypsum, bituminous materials, and fly ash which are widely used to improve the engineering properties of soil. On the other hand, non-traditional stabilizers consist of various polymers, silicates, bio-cement enzymes, resins, clay, and acids. Traditional stabilizers such as OPC, lime, gypsum, and coal fly ash have been commonly used for stabilizing weak mineral soils with low organic content (< 3% organic matter) (Tremblay et al. 2001; Bobet et al. 2011; Horpibulsuk et al. 2011; Cong et al. 2014; Bahmani et al. 2016; Kang et al. 2017; Siddiqua and Barreto 2018; Sabrin et al. 2019; Muhammad and Siddiqua 2019). However, stabilization using these materials may have an adverse impact on the environment due to leaching of hazardous compounds into surface water and groundwater. Further, the production of OPC requires a huge amount of energy and releases a significant amount of greenhouse gases (Worrell et al. 2001). It is estimated that around 0.7–1.1 ton of carbon dioxide is generated per ton of OPC production and contribute 5% – 8% of total carbon dioxide emissions (WBCSD 2009; Mehta 2010). Carbon dioxide emission has been increasing by 1% per year and expected to reach 4.4 billion tones by 2050. Cement treatment of organic soil may not be a viable option for Canada as the organic soil covers a large area and the price of OPC is rising significantly in recent years (Wong et al. 2009). Even though the fly ash is broadly used to stabilize with mineral soil (Kang et al. 2016; Rios et al. 2017; Siddiqua and Barreto 2018), the performance of fly ash on stabilizing organic soil is still poorly understood. Hence, there is an imperative need for developing an environmentally-friendly and sustainable technology for the successful stabilization of the problematic organic soil. Contrary to OPC as a binding agent, PFA is recognized as an environmentally-friendly material because it is a by-product and consumes low energy during production unlike OPC production, (Huberman and Pearlmutter 2008). The environmental impact assessment of PFA 8 in terms of the major and trace element concentrations in the leachate showed that it has no serious adverse effects on the ecosystem. (Pitman 2006; Toller et al. 2009; Nordmark et al. 2014). Although fly ash has been used to improve the engineering properties of weak mineral soils (Cristelo et al. 2012; Zentar et al. 2012; Latifi et al. 2016; Alsafi et al. 2017; Zhang et al. 2017; Furlan et al. 2018; Siddiqua and Barreto 2018; Coudert et al. 2019; Gu and Chen 2020; Lang et al. 2020), the effect of fly ash on the engineering properties of organic soils is still poorly understood. Chemical stabilization of organic soils is more difficult to attain than mineral soils, chiefly due to the ill effects of organic matter on the cementation process (Hebib and Farrell 2003; Bobet et al. 2011). The presence of high organic content obstructs the pozzolanic reactions by suppressing soil silica and alumina dissolution, and lead to detriment of the strength of stabilized soils (Develioglu and Pulat 2019). About 6 million tons of fly ash is produced every year in Canada, occupying valuable landfill space (Nordin et al. 2017). Fly ash appears to be the only material that is underused and that represents a potential for soil stabilization on a large scale. This can significantly contribute to reducing carbon dioxide (CO2) emissions as the fly ash replaces the use of cement for soil stabilization. Non-traditional additives, including Ca-bentonite, are mostly naturally occurring materials or are produced through a natural process without any adverse effect on the environment (Shazim et al. 2012; Junaid et al. 2013; Vummaneni and Reddy 2017). Extensive research has been conducted on the effects of traditional stabilizers on the engineering properties of the soil. However, the research on non-traditional additives are very limited and are primarily focused on mineral soil. Numerous studies have been carried out to evaluate the effects of natural pozzolan on cemented materials, and stabilized mineral soil (Sabir et al. 2001, Isaia et al. 2003, Tangpagasit et al. 2005; Habert et al. 2008; Velosa and Cachim 2009; Cristelo 9 et al. 2012). It is believed that a low clay content can be enough to drastically improve strength and compressibility behavior of soil with fly ash and lime treatment. Wayal et al. (2012), Fan et al. (2014), and Du et al. (2015) reported the improved engineering behavior of the weak mineral soil mixed with Ca-bentonite. Hence, adding a low quantity of clayey materials in addition to fly ash can make sand stabilization possible. However, investigation on performance of Ca-bentonite as a binder to stabilize organic soil are relatively sparse. Alkaline activators (NaOH and Na2SiO3) have been used to produce green concrete and mortar using fly ash to replace OPC (Hadi et al. 2019; Shoaei et al. 2019). However, the performance of alkaline activator when mixed with PFA treated organic soil is poorly understood. The engineering behavior of clay and sand are well known and various stabilization methodologies for clay and sand are well established; however, investigation on organic soil is limited. It is very crucial to advance our understanding of the engineering behavior of organic and stabilizing the organic soil to prevent the excessive settlement or bearing failure which could lead to failure of the infrastructures built on or in it. In this study, Ca-bentonite and PFA were extensively used in the stabilization of the organic soil sample, which is called the chemical stabilization process. Several researchers presented PFA and Ca-bentonite as an energy-efficient and cost-effective substitute for OPC in geotechnical engineering applications (Chang and Cho 2012, Pokharel and Siddiqua 2020). Existing research primarily focuses on improving the strength parameter of the weak soil. The compressibility behavior of the soil is equally important as a strength parameter. Moreover, the compressibility behavior of soil is highly critical for developing infrastructure in organic soil (Mesri et al. 1997; McVay and Nugyen 2004; Riedy 2006). Numerous research studies have been carried out on the stabilization of mineral soil such as clay and silty clay by 10 using cement and lime (Du et al. 2014; Eisazdeh et al. 2011; Kassim et al. 2005); however, stabilization of organic soil is still poorly understood (Huat et al. 2005; Talib et al. 2015). Moreover, peat also demonstrates a high degree of spatial variability as well as between different formations within a single peatland (Hobbs 1986; Elsayed 2003; Acharya 2015). Acharya (2015) highlighted the need for site-specific research on stabilizing the peat soil for any infrastructure intended to be developed on that site. Moreover, the effect of PFA and Ca-bentonite on the engineering properties of organic soil is poorly understood. Despite PFA and Ca-bentonite having numerous advantages as a green stabilizer, there is currently no widespread commercial application of PFA and Ca-bentonite in stabilizing organic soil in the construction industry. This lack of widespread acceptance might be attributed to a lack of research to verify the improved engineering behavior of the PFA and Ca-bentonite stabilized soil, standard design guidelines and methodologies, and laboratory and field-testing procedures for the evaluation of the performance of PFA and Ca-bentonite stabilized organic soil (Tingle and Santoni 2008). This research will help to establish an innovative practice, standards, and technology to stabilize the organic soil for future construction, with the potential for improving human and environmental health, environmental performance, and economic returns of construction. 1.3 Objectives The main purpose of this research was to investigate the effects of PFA and Ca-bentonite on the strength and compressibility behavior of organic soils from Wabasca, Northern Alberta, Canada. Also, this reseach sought to develop mix design criteria for optimum PFA and Ca-bentonite contents necessary to achieve the desired engineering behavior. The specific objectives of this research were: 11 ➢ Understand the effect of pulp mill fly ash on strength, compressibility, and microstructures of organic soil. ➢ Investigate the performance of alkaline activator on strength and compressibility of the stabilized organic soil. ➢ Evaluate the performance of combination of Ca-bentonite and PFA as binder to stabilize the organic soil. ➢ Characterize the creep behaviour of the stabilized organic soil using the ratio between secondary compression (Cα) and primary compression index (Cc). 1.4 Scope of research The objectives of this experimental program were achieved by conducting a series of laboratory experiments that were carefully planned to investigate the engineering behavior of the organic soil mixed with different proportions of PFA and Ca-bentonite. The flow chart of this research is shown in Figure 1.1. The scopes of this study are summarized as follows: ➢ Perform various laboratory tests such as specific gravity, water content, and atterberg limits, to characterize the organic soil and PFA and Ca-bentonite from local industries. ➢ The research is performed in three phases. In the first phase of the research, the organic soil is treated with different dosages of PFA to evaluate the effect of PFA on the strength and compressibility behavior of the organic soil. The second phase consists of a series of tests on the organic soil treated with alkaline activated PFA. PFA is activated using alkaline activator to accelerate the pozzolanic reactions. The third phase of the research is to evaluate the effect of different dosages of Ca-bentonite on the strength and compressibility behavior of the organic soil mixed with a fixed percentage of PFA. 12 Figure 1.1. Flow chart of the study 13 ➢ Perform the fall cone test to investigate the change in undrained shear strength of the organic soil mixed with a different proportions of PFA, activated PFA, and Ca-bentonite. ➢ Perform incremental loading tests on the unstabilized organic soil and stabilized organic soil with a varying dosage of PFA, activated PFA, and Ca-bentonite to investigate the primary consolidation behavior. ➢ Perform constant loading consolidation (creep) tests under the highest incremental load with varying PFA and Ca-bentonite to investigate the creep behavior. ➢ Perform SEM/EDS and XRD tests on stabilized organic soil using PFA and Ca-bentonite to investigate the change in the internal structures and chemical components of the stabilized organic soil. ➢ Perform a pH test on stabilized soil mixed with PFA and Ca-bentonite to evaluate the environment for the pozzolanic reaction after adding PFA or Ca-bentonite as a binding material. 1.5 Organization of the dissertation This dissertation consists of seven chapters, as follows: Chapter one presents the characteristics and problems associated with organic soil. Moreover, the engineering behavior of organic soil is highlighted. Chapter one presents a problem statement and the motivation explaining why this research was undertaken. This chapter emphasizes the objectives of this study that can contribute to advancing the knowledge on stabilizing the organic soil using PFA, alkaline activator, and Ca-bentonite. Finally, the scope of the research objectives is highlighted. 14 Chapter Two reviews the strength and compressibility behavior of organic soil. This chapter highlights the primary and secondary (creep) consolidation of organic soil. Status-quo of the organic soil stabilization using various binding materials such as cement, lime, fly ash, and bentonite clay is briefly reviewed. Chapter Three presents geotechnical characteristics of organic soils and binding materials used in the research, i.e. pulp mill fly ash (PFA) and Ca-bentonite. Preliminary tests were performed to characterize the organic soil, PFA, and Ca-bentonite. Chapter Four highlights the methodologies used to evaluate the effect of various binding materials on the strength and compressibility of the stabilized organic soil. Moreover, this chapter describes a mix design for preparing samples, and the fall cone, oedometer, EDS/SEM, and XRD tests. Chapter Five presents the results of the performed tests on untreated and PFA-treated organic soil. Effect of PFA on strength and compressibility behavior of the organic soil through an in-depth analysis of test results and microstructural investigations are presented as well. This section analyses primary and creep deformation of the stabilized organic soil by calculating various coefficients. Chapter Six presents the results of the laboratory test on untreated and Ca-bentonite and PFA treated organic soil. Effect of Ca-bentonite with constant 30% PFA on strength and compressibility behavior of the organic soil through an in-depth analysis of test results and microstructural investigations are presented as well. Effect of curing techniques on strength and compressibility are presented. This section analyses primary and creep deformation of the stabilized organic soil by calculating various coefficients. 15 Chapter Seven summarizes the dissertation and presents the research findings obtained throughout the study. Moreover, this chapter presents the limitations of this study and the recommendations for future research to advance the understanding of the characterization and stabilization of organic soil. 16 Chapter 2: Literature Review This literature review covers the characterization of organic soil and associated problems, the compressibility behavior of organic soil, and organic soil stabilization with various additives. This chapter also focuses on the utilization of pulp mill fly ash (PFA) and calcium bentonite (Ca-bentonite) as the additive for organic soil stabilization. Activation of the PFA using sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) to accelerate and enhance the pozzolanic reaction are reviewed and briefly explained in this chapter. Moreover, primary and secondary consolidation behavior of the stabilized soil are reviewed. This chapter demonstrates that additives significantly improve the soil strength, soil compressibility, bearing capacity, and subsequently, decrease the settlement of the organic soil. 2.1 Organic soil Organic soil forms when partially decomposed plant materials accumulate in a saturated wetland. Generally, decomposition of the plant materials is a slow process due to a shortage of oxygen (Hua 2016). The process of chemical reactions, fossilization, and vegetation form the organic soil in the wetlands (Hashim 2008). The organic soil is known for its poor engineering properties such as having low density, high compressibility and water content, and low shear strength, and significant creep behavior (Huat et al. 2004; Farrell 2012; Venda Oliveira et al. 2013). Macfarlane and Williams (1974) reported that Canadian railway embankments built over peat suffered from large continuous settlement, requiring extensive track maintenance to continue railway operations. Macfarlane and Williams (1974) carried out extensive research on engineering aspects of peat soil in Canada and highlighted the general problems for 17 infrastructure such as road and railways, buildings, transmissions tower foundations, and pipelines on peat soil. The classification of organic soils is difficult, whereas classification of mineral soil is well defined and developed based on grain size distribution, plasticity, texture, and color (Huat 2004). The Unified Soil Classification System (USCS) separates the organic soil from mineral soil and divides the organic soil into broader categories. Though there are several minute divisions for mineral soil, highly organic soil (Pt) is a single major division for organic soil. Several researchers have classified organic soil based on organic content, fiber content, and degree of decomposition, as presented in Tables 2.1-2.3. The geotechnical characteristics of the organic soil depend on the source of organic matter, degree of decomposition, and spatial variation (Acharya 2015). Some typical values of index properties, strength, and compressibility from previous studies are presented in the following section. Table 2.1. Classification of peat based on degree of decomposition (Karlson and Hansbo 1981; Jarret 1995) Designation Group Description Fibrous peat H1-H4 Low degree of decomposition fibrous structure. Easily recognizable plant. Pseudo-fibrous peat H5-H7 Intermediate degree of decomposition. Recognizable plant structure. Amorphous peat H8-H10 High degree of decomposition. No visible plant structures. Mushy consistency. 18 Table 2.2. Classification of organic soil based on their organic content (Jarret 1995; ASTM D4427) Basic soil type Description Organic content (%) Clay or silt or sand Slightly organic 3-20 Organic soil Organic 20-75 Peat Highly organic >75 Table 2.3. USDA classification of peat (Huat 2004) Type of peat Fibre content Von Post Scale Fibric peat Over 66% H4 or less Hemic peat 33-66% H5 or H6 Sapric peat Less than 33% H7 2.1.1 Index properties Index properties of organic soil largely depend on the organic content, fiber content, and degree of decomposition. Void ratio, moisture content, and liquid limit increase with an increase in organic content, while specific gravity (Figure 2.1), and bulk density decrease with an increase in organic content (Huat et al. 2009). Riedy (2006) characterized the organic soil as a high void ratio up to 14, high water holding capacity up to 652%, and high organic content up to 92%. Hashim (2008) conducted a series of tests to characterize the peat soil and reported moisture content, organic content, and fibre content of about 555.55%, 96.45%, and 90.39% respectively. A high percentage of moisture content is expected as the organic soil is generally formed in a wetland. Specific gravity of 1.24, bulk density of 1037.73 kg/m3, and pH of 3.51 were reported by Hashim (2008). Huat et al. (2014) reported the natural water content of the organic soil up to 1500% and the bulk density in the range of 400-1200 kg/m3. 19 Dehghanbanadaki et al. (2019) reported fibrous peat having the organic content and natural water content about 91% and 495%, respectively. Mesri and Ajlouni (2007) presented the index properties of the fibrous peats, namely Middleton peat, Wisconsin, and James Bay peat, Quebec (Table 2.4). The organic content of the Middleton peat and James Bay peat was 90-95% and 96% respectively. Åhnberg (2006) reported the index properties of Swedish Dömle peat; water content, specific gravity, and organic content were 2000%, 1.44, and 97% respectively. Figure 2.1. Typical correlation between organic content and specific gravity (modified from Huat et al. 2009). 20 Table 2.4. Index properties of the fibrous peat (Mesri and Ajlouni 2007). Property Middleton peat, Wisconsin James Bay peat, Quebec Natural water content (%) 510-850 1,000-1,340 In situ void ratio 8.3-14.2 18.0-23.5 Initial degree of saturation (%) 89-100 93-100 Organic content (%) 90-95 96 Specific gravity of solids 1.53-1.65 1.50-1.64 Effective overburden pressure (kPa) 29 3 Preconsolidation pressure (kPa) 30-43 5-9 2.1.2 Strength The soil strength is a crucial parameter when designing any structures to be constructed on or in soil. Organic soil is characterized as a low strength material. Huat (2005) reported that the organic content has a adverse impact on the shear strength parameters of the soil; higher organic content lowers the shear strength of the soil in the long term. Riedy (2006) carried out a comprehensive laboratory investigation to evaluate unconfined compressive strength (UCS) of undisturbed organic soil from Florida, USA, and reported the UCS ranging from 17 to 23 kPa for the soil at a ground level and 29 to 40 kPa for the soil at a depth of about 4.5 m. Hebib and Farrell (2003) reported that the UCS of natural peat was equal to 5-10 kPa. Nath et al. (2017) investigated the UCS of the organic soil having 36.9% organic content. The unit weight and water content of the soil was 11.17 kN/m3 and 87.12% respectively. Nath et al. reported that the UCS of the soil was about 38 kPa. Kamaruidzaman et al. (2019) reported that the UCS of peat having about 75% of organic matter can be as low as 8 kPa. 21 2.1.3 Compressibility Organic soil is known for its high compressibility and significant creep behavior. Fox et al. (1992) carried out long-duration consolidation tests on Middleton peat, which showed the significant contribution of creep to total settlement. Mesri et al. (1997) examined the creep behavior of the peat under various vertical stresses, concluding that the creep due to secondary consolidation can contribute to half of the total consolidation settlement in organic soil. McVay and Nugyen (2004) conducted a series of oedometer tests on organic soil from Florida, USA, and observed that the long-term settlements were mostly from creep, which can contribute up to 50% of the total consolidation settlement. O’Kelly (2005) evaluated the compressibility behavior of organic soil from Ireland with different organic content and fabrics. It was observed that peaty silt and bog peats undergo considerable creep deformation even under minimal loading conditions (Figure 2.2), which contribute to a large portion of the consolidation (O’Kelly 2005). The value of Cα was reported as 0.003–0.011. Figure 2.2. Creep deformation of coarse peat with time (modified from O’Kelly 2005). 22 Organic soil with higher organic content shows the primary compression index (Cc) up to 15, where medium stiff clay often has Cc ranges from 0.5 to 9.2 (Huat 2004). Riedy (2006) carried out a comprehensive laboratory investigation to evaluate the compressibility behavior of undisturbed organic soil from Florida, USA. Riedy reported that the coefficient of consolidation (Cv) of organic silt and peat was 0.024 cm2/s and 0.023 cm2/s respectively and concluded that the secondary compression contributes more than 50% of the total consolidation. The value of Cc and C for organic silt and peat were reported as 1.78 and 0.029, and 5.32 and 0.074 respectively (Riedy 2006). Riedy (2006) observed the C/Cc ratio (ratio of secondary compression index, Cα, to primary compression index, Cc) for Florida organic soil as 0.06. The concept of using the Cα/Cc ratio to evaluate the compressibility behavior of soil was proposed by Mesri et al. (1997) and has since been applied by many researchers (Hwang et al. 2005; Sobhan et al. 2007 and 2012; Sarsour 2014; Talib et al. 2015). The value of Cα/Cc can be used to characterize or define geomaterials. Peat material has the highest value of Cα/Cc, while the lowest value of Cα/Cc is related to granular material (Santagata et al. 2008). As the value of Cα/Cc decreases, engineering behavior of soil is considered to change from that of weak organic soil to mineral soil, and ultimately to a stiff granular material (Hwang et al. 2005; Sobhan et al. 2007; Talib et al. 2015). Hwang et al. (2005) and Sarsour (2014) investigated the primary and secondary consolidation behaviour of organic soil. The organic conent of the soil was 58.0%. Hwang et al. (2005) and Sarsour (2014) reported that the value of Cα /Cc was about 0.06 as shown in Figure 2.3. Hashim (2008) investigated the organic soil in Malaysia and reported Cc and swelling index (Cs) ranged from 2.43 to 2.84 and 0.014 to 0.016 respectively. 23 Sobhan et al. (2012) conducted a series of oedometer tests on organic soils collected from Palm Beach County, Florida, USA. The organic content, moisture content, and void ratio ranged from 25% to 92%, 160% to 650%, 3.2 to 13.9, respectively. Soban et al. (2012) reported the value of Cα /Cc as 0.1 for organic soil (Figure 2.3), indicating that the creep due to secondary consolidation can contribute to significant settlement in organic soil. Figure 2.3. The value of Cα/Cc for organic soil and variation of Cα/Cc with cement content. Venda Oliveira (2012) investigated the effect of organic matter (OM) on the creep behavior of the organic soil as presented in Figure 2.4. Figure 2.4 demonstrates that the creep deformation increases with an increase in organic matter, which was an expected behavior of the organic soil. 24 Figure 2.4. Effect of organic material on creep deformation (modified from Venda Oliveira 2012). 2.2 Soil stabilization Figure 2.5 presents various methods which have been used in reinforcing and stabilizing soft soil-like organic soil. Three fundamental approaches that are widely used in soft soil reinforcement are physical, mechanical, and chemical techniques. The effectiveness of various methods in terms of cost and performance is based on the soil types, desired engineering properties, and quantity of soils (Edil 2003; Kazemian and Huat 2009; Hejazi et al. 2011). Of the three fundamental procedures, the chemical stabilization technique is considered to be a more economical and time-saving option for organic soil (Kazemian 2017). 2.2.1 Organic soil stabilization Lime, gypsum, ordinary Portland cement (OPC), coal fly ash, and other chemicals have been used for stabilizing organic soil. Nevertheless, limited research has been conducted on organic soil stabilization. Sstabilization of organic soils is more difficult than mineral soils, chiefly due to the ill effects of organic matter on the cementation process (Hebib and Farrell 2003; Bobet 25 et al. 2011). The presence of high organic content obstructs the pozzolanic reactions by suppressing soil silica and alumina dissolution, compromising the strength of stabilized soils (Develioglu and Pulat 2019). Figure 2.5. Fundamental procedures of soil reinforcement (modified from Hezaji et al. 2012). Hebib and Farrell (2003) investigated the effects of various binders on engineering properties of the Irish peat using unconfined compression, consolidation, and triaxial tests. The compressive strength of peat mixed with different binders such as cement, fly ash, lime, blast furnace slag, and gypsum (Table 2-5) increased significantly as shown in Figure 2-6. The researchers also observed that the voids of the mixed soil were largely filled with the binding agents. The compressive strength increased significantly with an increase in the dosage of cementing materials and an increase in the curing period as shown in Figure 2.6. Also, the compressibility index of the stabilized soil decreased significantly. The ratio of Cα to Cc was 0.054 for the sample cured for 28 days, and the ratio decreased with an increase in curing days, Soil reinforcement methods1. Physical: vibration, freeze and thaw, thermo-electrical2. Mechanicala) Using fibrous materialsb) Compaction: e.g. surcharge load3. Chemical a) Conventional materials: cement, lime and bitumenc) Polymeric resins: e.g. Polyvinyl alcohol, polyvinyl acetate, Poly methyl mehta acrylate (PMMA)b) Enzymes: e.g. PZ-22xa) Geosynthetic family:Geogrid, Geotextile, Geocell b) Randomly distribution fibers:Natural, manmade, and mineral26 as shown in Figure 2.7. Cement was found to be superior compared to other binding agents such as fly ash, lime, and gypsum (Figure 2.6). The additive prepared by mixing 85% of blast furnace slag and 15% gypsum exhibited higher strength, as shown in Figure 2.6. Table 2.5. Mixed design for Irish peat stabilization (Hebib and Farrell 2003). C Cement CPL (80, 20) Cement 80%; PFA 20%; lime 3% CPL (60, 40) Cement 60%; PFA 40%; lime 3% CPL (40, 60) Cement 40%; PFA 60%; lime 3% CPL (20, 80) Cement 20%; PFA 80%; lime 3% S Blast furnace slag SC (60, 40) Blast furnace slag 60%; cement 40% SG (85, 15) Blast furnace slag 85%; gypsum15% PFA: Pulverized fuel ash Figure 2.6. Unconfined compressive strength for various mixes for Raheenmore peat (modified from Hebib and Farrell 2003). 27 Figure 2.7. Cα versus Cc for cement mixed Ballydermot peat (modified from Hebib and Farrell 2003). Chen and Wang (2006) stabilized organic soil with organic content which ranged from 12.12% to 17.42% using cement. It was concluded that the stabilization of organic soil is difficult to achieve exclusively using cement. Chen and Wang recommended using additional additives with cement to obtain a higher strength of organic soil. Sakr et al. (2009) conducted comprehensive tests on organic clay soil mixed with lime in the order of 1%, 3%, 5%, and 7% by dry mass of soil. The samples were cured for 7, 15, 30, and 60 days. Stress-strain behavior of the soil mixed with a higher amount of lime showed a brittle behavior as a highly dense sample, while untreated soil exhibited a ductile behavior as a less dense sample (Figure 2.8). 28 Figure 2.8. Stress-strain curves of untreated and lime-treated clays (modified from Sakr et al. 2009). Bobet et al. (2011) studied the compressibility behavior of cement-treated organic soil. The liquid limit of the soil studied was about 400%. It was observed that the compressibility of the soil decreased with an increase in cement dosage, but the hydraulic conductivity of the soil increased with an increase in cement dosage (Figure 2.9). Bobet et al. (2011) claimed that a change in the fabric and a higher void ratio of the stabilized soil resulted in higher hydraulic conductivity of the stabilized soil. This result is similar to previous studies by Hebib and Farrell (2003), Kang (2011), and Venda Oliveira et al. (2012). The change in Cc and Cα with increase in cement content is presented in Table 2-6. 29 Figure 2.9. Effect of cement on compressibility and hydraulic conductivity of the treated organic soil (modified from Bobet et al. 2011). Table 2.6. summary of compression and creep index (Bobet et al. 2011). PC (%) Cc Cα Cα/Cc 0 1.548 ± 0.207 0.123 0.095 8 1.444 0.106 0.085 20 1.326 0.080 0.063 50 1.699 0.070 0.042 100 1.382 0.029 0.021 Kolay et al. (2011) conducted a series of UCS test on fibrous peat from Malaysia. The peat consisted of high organic content (greater than 75%) with high moisture content ranging from 360% to 623%. OPC cement, quick lime, and fly ash were used to stabilize the peat soil. Results showed that there was a significant improvement in UCS of treated peat soil. The optimum UCS was obtained for the peat sample mixed with 15% fly ash and cured for 28 days. However, no optimum dosage of quick lime and OPC was noticed, as shown in Figure 2.10. The authors also performed SEM tests to investigate the microstructure of the stabilized and unstabilized soil. 30 Figure 2.10 Effect of cement and fly ash dosage and curing period on UCS (modified from Kolay et al. 2011). Tastan et al. (2011) evaluated the effect of fly ash on UCS and resilient modulus of various organic soils with organic content ranging from 1% to 27%. This study was conducted with three different organic soils and six different fly ashes mixed with varying proportions. It was observed that both the UCS and the resilient modulus increased with an increase in fly ash; however, the increment of UCS and resilient modulus depends on soil types and organic content (Figure 2.11). The UCS of organic soil increased from 40 kPa (control sample) to 400 kPa when about 30% of fly ash was mixed with organic soil. The effect of organic content in soil is detrimental; an increase in organic content leads to an exponential decrease in UCS and resilient modulus of the soil (Tastan et al. 2011). Hwang et al. (2012) investigated the stabilization of organic soil from West Lafayette, Indiana, USA, using cement as a binding agent. It was observed that the value of Cα/Cc dropped noticeably when organic soil was mixed with cement, as shown in Figure 2.3. The profound effect of cement on organic soil stabilization was observed when the cement exceeded 50% of the dry soil mass. Shoban et al. (2012) presented that the value Cα /Cc for peat decreased from 31 0.12 to 0.024 when about 86% of cement was added to peat (Figure 2.3). The value of Cα /Cc decreased significantly with addition of cement, indicating that the creep behavior of organic soil can be improved using cementitious material. Sarsour (2014) reported that additives such as cement, gypsum, and slag can improve the creep behavior of South Florida organic soil, as shown in Figure 2.3. It was observed that the combination of cement (20%), gypsum (20%), and slag (20%) can provide promising results in reducing the creep behavior of the soil. Figure 2.11. Effect of fly ash on stress and resilient modulus (inverse of volume compressibility, mv) of organic soil (modified from Tastan et al. 2011). Oliveria et al. (2013 and 2014) investigated the creep behavior of Portuguese soft soil stabilized with binders. Natural water content and organic content of the Portuguese soft soil was reported as 80.9% and 19.4% respectively. Creep behavior of both stabilized and remolded 32 soil was investigated under six different stress levels ranging from 100 to 1648 kPa. It was observed that the secondary compression index increased as the stress level increased. The results depict that the mixing of additives with organic soil results in a significant improvement in the engineering properties of soil such as strength, density, compressibility, void ratio, and coefficient of consolidation. Moreover, the secondary compression index of the soil mixed with various additives was reduced by up to 2600%. The optimum binder composition used to reduce the secondary compression for the Portuguese soil was 75% OPC mixed with 25% of slag or fly ash (Oliveria et al. 2013 and 2014). Fly ash treated specimen exhibited less compressible behaviour than the blast furnace granulated slag treated specimen since fly ash contained a higher amount of silica and alumina, accelerating pozzolanic reactions. Guzman and Alfaro (2017) studied the engineering properties of peat from Northern Manitoba, Canada. The water content of the soil ranged from 200% to 900% at different depths. A one-dimensional consolidation test and UCS test were conducted to determine the engineering properties of peat soil from a newly constructed road. The test results showed that hydraulic conductivity significantly decreased with an increase in effective stress. The hydraulic conductivity of the amorphous peat was found to be lower than the fibrous peat. Jais et al. (2019) performed a series of UCS tests on modified peat to evaluate the effect of fly ash dosage and curing time on strength. The dosage of fly ash used was 5%, 10%, 15%, and 20% of the dry mass of soil while a constant dosage of flexible geopolymer activator was used (i.e. 40% of the dry mass of soil). The test was conducted after 0, 7, 14, 28, and 56 days of curing. The value of UCS of the modified peat increased significantly as the fly ash content and curing time increased. 33 Kamaruidzaman et al. (2019) evaluated the effect of cement and sugarcane bagasse ash (SCBA) on the strength and compressibility behavior of peat having about 75% of organic matter from Malaysia. Series of UCS and one-dimensional consolidation tests were conducted on untreated and SCBA treated peat. The UCS and compression index of the peat was 8 kPa and 3.76 respectively. The value of UCS increased to 190 kPa when the peat soil was mixed with 5% SCBA and 95% cement of dry mass of peat. The treated sample was cured for 7 days. The ratio of Cα/Cc decreased significantly from 0.056 to 0.027 when the peat soil was mixed with 5% SCBA and 95% cement of dry mass of soil. The significant decrement of Cα/Cc indicates that the stabilized mixture can efficiently decrease the creep deformation. 2.3 pH of organic soil The humic acid component of organic soil causes the soil to be acidic in nature, with pH values ranging between 3.0-4.5 (Åhnberg 2006; Kalantari 2017). Mutalib et al. (1991) reported that the pH of the organic soil was in the range of 3.0 to 4.0. Normally, the pozzolanic reaction is poor in acidic conditions and more active in alkaline environments (Tremblay et al. 2002; Wong et al. 2013). Organic soil contains mainly low pH which adversely effects bonding between the particles. Tremblay et al. (2002) also suggested that hydration reactions are likely to be retarded if the pH of the soil and additives mixture is less than 9. Mixing the organic soil with fly ash can increase the pH, which is sufficient to produce a cementitious bond by neutralizing the acid (Tremblay et al. 2002; Nath et al. 2017). 2.4 Soil binders Soil binders are categorized as traditional binders and non-traditional binders (Marto et al. 2014). Traditional binders consist of OPC, lime, gypsum, bituminous materials, and fly ash. 34 Stabilization of organic soil using OPC and lime may adversely affect the environment, as hazardous compounds can leach into the surface and groundwater (Chang and Cho 2012). The product of each metric ton of OPC and lime releases approximately 1.0 metric tons and 0.86 metric tons of carbon dioxide to the atmosphere respectively (Ivanov and Chu 2008; Thangaraj and Thenmozhi 2013). Several researchers presented fly ash as an energy-efficient and cost-effective substitute for OPC in geotechnical engineering applications (Chang and Cho 2012; Pokharel and Siddiqua 2020). The non-traditional binders consist of various polymers, silicates, bio-cement enzymes, resins, clay, and acids. The non-traditional binders are mostly naturally occurring materials or are produced through a natural process without any adverse effect on the environment (Shazim et al. 2012; Junaid et al. 2013; Vummaneni and Reddy 2017). Extensive research has been conducted on the effects of traditional binders on the engineering properties of the soil. However, the research on non-traditional binders is limited and the available studies are primarily focused on mineral soil. 2.4.1 Fly ash Fly ash is a fine mineral residue obtained as a by-product from various incinerator or combustion processes. The main source of fly ash in North America has been derived from the combustion process of the coal industry in USA and Canada. Moreover, there are many different types of fly ash produced from different sources such as pulp mill fly ash, sugarcane bagasse ash, solid waste ash, and rice husk ash. (Basha et al. 2005; Olsson et al. 2006; Cherian and Siddiqua 2019; Kamaruidzaman et al. 2019). Fly ash is generally grey-colored powder and consists of finely divided spherical glassy particles. Similar to the OPC, fly ash is rich with pozzolanic substances that form cementing materials in the presence of water through the 35 hydration process (Feng and Clark 2011; Chang and Cho 2012). Since major chemical and physical components of the fly ash are similar to the OPC (Tables 2-7 and 2-8), fly ash has been used as an alternative to the OPC in the construction industry. Table 2.7. Comparison of chemical properties of OPC and fly ash (Feng and Clark 2011) OPC, % by mass Fly ash, % by mass Cao SiO2 Al2O3 Fe2O3 MgO SO3 Na2O K2O TiO2 LOl 64.01 205.31 5.78 2.35 1.19 3.53 0.11 0.77 0.37 1.63 0.37 – 27.68 27.88 – 59.40 5.23 – 33.99 1.21 – 29.63 0.42 – 8.79 0.04 – 4.71 0.20 – 6.90 0.64 – 6.68 0.24 – 1.73 0.21 – 28.37 The grain size distribution of the fly ash is similar to the OPC and ranges from 10 to 100 m. The chemical and physical properties of the fly ash depend on the material burnt in the incinerator and the combustion process. ASTM (ASTM C 618-19) divides the fly ash from coal into two groups, i.e. Class C and Class F based on their chemical components. Since Class C fly ash consists of cementitious materials similar to OPC, it has been widely used in concrete, mortar, and brick. Class C fly ash has also been widely used for soil stabilization (Mir and Sridharan 2013). Though millions of tons of fly ash are produced as a by-product every year, only about 18% of the total fly ash is used for construction purposes (Baykal and Doven 2000). 36 Table 2.8. Comparison of chemical and physical properties of OPC and fly ash (Zeng et al. 2012). Chemical composition /physical properties Cement Fly ash Silica (SiO2, %) Alumina (Al2O3, %) Iron oxide (Fe2O3, %) Calcium oxide (CaO, %) Magnesium Oxide (MgO, %) Sulfur trioxide (SO3, %) Sodium Oxide (Na2O (eq), %) Free calcium oxide (CaO (f), %) Chloride (Cl, %) Loss on ignition (LOI, %) Density (g/ml) Specific area (m2/kg) 22.93 4.29 2.89 66.23 1.92 0.35 0.70 0.64 0.006 1.70 3.12 343 57.60 21.90 2.70 7.80 1.68 0.41 1.05 _ _ 7.05 2.06 355 Feng and Clark (2011) investigated the fly ash from forty-nine different sources in USA and Canada. It was reported that most of the fly ash met the requirement for fines content and loss on ignition (LOI) for use in concrete and mortar (Feng and Clark 2011). Several studies have also been conducted using fly ash as a binding material for soft soil stabilization (Laohaprapanon et al. 2010; Kolay et al. 2011; Kogbara and Al-Tabbaa 2011; Camberato et al. 2011; Cristelo et al. 2012; Zentar et al. 2012; Latifi et al. 2016; Alsafi et al. 2017; Zhang et al. 2017; Furlan et al. 2018; Siddiqua and Barreto 2018; Simão et al. 2019; Coudert et al. 2019; Gu and Chen 2020; Lang et al. 2020). Most of these studies were focused on mineral soil stabilization; however, research on stabilization of organic soil is sparse. 37 2.4.2 Pulp mill fly ash (PFA) Pulp mill fly ash (PFA), a solid waste by-product of wood combustion in power boilers of pulp and paper mills, is considered a green binder, and can be an energy-efficient and cost-effective alternative for OPC in geotechnical engineering applications (Huberman and Pearlmutter 2008; Novais et al. 2018; Cherian and Siddiqua 2019). In general, PFA is produced by burning bark, wood residue, re-circulated ash, and sludge with microorganisms and fibers in an incinerator at a high temperature of 730 °C (Arm et al. 2014). The reactive alumina, silica, and calcium are the primary elements in the PFA that provide the cementing properties and can be used for partial replacement with cement (Cherian and Siddiqua 2019). The typical oxide compositions of PFA reported by various researchers are presented in Table 2-9. The environmental impact assessment of PFA for the major and trace element concentrations in the leachate showed that it has no serious adverse effects on the environment (Pitman 2006; Toller et al. 2009; Nordmark et al. 2014). Moreover, PFA reduces the heat of hydration when mixing with soil; hence, PFA requires less water than OPC and is easier to handle in cold weather conditions which prevail in Canada (Cherian and Siddiqua 2019; Nath et al. 2017). Arm et al. (2014) investigated the stabilization of an unpaved road in Sweden using 30% PFA by dry mass of soil. Water infiltration and bearing capacity tests were conducted after a two-year period of road stabilization. The authors observed that the infiltration rate and bearing capacity of the road was significantly improved. Arm et al. observed that the top surface of the paved road was hard and stiff after rainfall. It was summarized that PFA is a cost-effective green stabilizer for soil stabilization, reduces landfill waste, and protects the environment. 38 Table 2.9 Typical oxide compositions of PFA (Cherian and Siddiqua 2019). References Composition of Major and minor Oxides (%) SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 Muse and Mitchell (1995) N/P 1.25 0.63 12.0 0.77 0.14 1.33 N/P Pitman (2006) N/P 9.1 5.1 1.66 0.11 0.01 0.26 0.51 Naik et al. (2003) 26.5 9 5.4 16.0 3.0 1.7 5.0 N/P Vassilev et al. (2013) 31.6 13.2 5.12 28.9 5.4 1.42 13.2 N/P Sarafimova et al. (2011) N/P N/P N/P 52 1.32 N/P 2.39 N/P Ohno and Eric (1993) N/P 8.21 1.43 9.49 0.65 0.67 1.03 N/P Abdullahi (2006) 31.8 28 2.34 10.5 9.3 6.5 10.4 N/P Chowdhury et al. (2015) 50.7 8.2 2.1 19.6 6.5 2.1 2.8 N/P Rani and Jenifer (2016) conducted an index properties test, a proctor test, and a California bearing ratio (CBR) test on the PFA stabilized clay soil intended for use for earthen road pavement. The dosage of PFA was 5% and 10%. The authors found that the plasticity index and free swelling index decreased with an increase in PFA, and also reported that the value of CBR increased three-fold when 10% of PFA was mixed with clay soil. Several other researchers (Magnusson 2005; Karthikeyan and Vinod Kumar 2017; Jhariya and Parte 2018) have used PFA to stabilize the weak mineral soil. All these studies were focused on improving the engineering behaviour of the mineral soil using PFA; however, the effect of PFA on engineering behaviour of the organic soil is still poorly understood. 2.4.3 Calcium bentonite Calcium bentonite (Ca-bentonite) consists of a smectite mineral group and has unique physical and chemical properties such as a high surface area, high absorption capacity, low swelling 39 capacity, and high viscosity. The physical and chemical properties of typical Ca-bentonite are presented in Tables 2-10 and 2-11. The Ca-bentonite has a relatively low swelling capacity compared to sodium bentonite. Major chemical components of Ca-bentonite are similar to OPC or fly ash, though the amount of chemical elements may differ. Oxide of Si, Al, Fe, Ca, Mg, Na, K, and S are common chemical components observed in Ca-bentonite, which also has pozzolanic properties (Dananaj et al. 2005; Du et al. 2015). Ca-bentonite has been widely used for reducing the permeability of backfill material for retaining walls. Fan et al. (2014) conducted a series of oedometer tests with different percentages of Ca-bentonite on a dry mass basis and reported that bentonite percentage in the range of 5% to 15% is enough to reduce the hydraulic conductivity of the soil. Wayal et al. (2012) studied the effect of bentonite on dune sand stabilization using lime. An optimum UCS was obtained at 15% Ca-bentonite of dry mass of soil. It was concluded that the bentonite helps to develop the cohesive bond, resulting in higher strength. From the EDS test, the authors demonstrated that bentonite containing adequate silica helps in pozzolanic reaction formation during stabilization (Fan et al. 2014; Wayal et al. 2012). The formation of pozzolanic cementation bonds ensures continuous strength gain and improves the microstructure of stabilized soil. Du et al. (2015) conducted series of slump and consolidation tests to evaluate the effect of Ca-bentonite on workability, compressibility, and permeability of the kaolin clay, and reported improved workability and compressibility behavior of kaolin soil mixed with Ca-bentonite. 40 Table 2.10 Typical physical properties of Calcium bentonite (Dananaj et al. 2005). Property Value Clay fraction <0.002 mm (%) 40 – 7 Dry bulk density d (gcm-3) 1.73 Total porosity () 0.29 Liquid limit wL (%) 101 Plasticity limit wP (%) 28.2 Plasticity index IP (%) 72.8 Total content of expanding clay minerals (%) 63 – 67 Smectite content of the clay fraction <0.002 mm (%) 90 – 95 Illite content of the clay fraction <0.002 mm (%) 0 – 5 Kaolinite content of the clay fraction <0.002 mm (%) 0 – 5 Chlorite content of the clay fraction <0.002 mm (%) 0 Cation exchange capacity CEC (mmol/ 100g) 80 Table 2.11. Typical chemical components of Calcium bentonite (Dananaj et al. 2005). Composition Smectite content (%) 68% 92% SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) Na2O (%) K2O (%) SO3 (%) Ignition loss: to 1000 °C (%) 64.38 18.29 2.6 1.91 3.05 0.53 0.94 0.01 7.77 60.45 20.67 2.41 2.09 3.66 0.18 0.34 0.01 9.74 41 2.4.4 Alkaline activator Hydration reactions transform the hydrated paste (cement or fly ash paste) into a hardened material and contribute to the overall strength development. The hydration rate influences the properties of the stabilized soil such as strength and compressibility (Mangat and Lambert 2018). The hydration rate of fly ash such as PFA is very slow when mixed with water, resulting in slow strength development. An alkaline activator is used to accelerate the hydration rate when PFA is mixed with organic soil, and also develops a favorable alkaline environment for the pozzolanic reaction (Radhakrishna 2015; Mangat and Lambert 2018). Sodium hydroxide (NaOH), potassium hydroxide (KOH), and sodium silicate (Na2SiO3) have been widely used as alkaline activators (Vargas et al. 2011; Radhakrishna 2015; Atis et al. 2015; Mangat and Lambert 2018) to develop geopolymer and alkaline activated material. Provis (2014) defined geopolymer and alkali-activated material based on the chemistry of the activated product. Geopolymer is an activated material with a low calcium and high aluminum content, while alkali-activated material is activated material with a high calcium content. Geopolymer and alkali-activated materials are used in concrete, mortar, and brick for engineering construction (Power Pile 2013; Sarker 2008; Shrest 2013; Uretek 2014; Radhakrishna 2015; Mangat and Lambert 2018). Moreover, geopolymer and alkali-activated materials have been used in stabilizing weak soil (Cristelo et al. 2012 and 2013; Zhang et al. 2013). An alkaline activator has been considered as a replacement for OPC in various applications. Previous studies showed that alkaline activator as a combination of NaOH and Na2SiO3 can break and dissolve silica and alumina bonds from an aluminosilicate mineral and accelerate the hydration rate and setting time of fly ash. Duchesne et al. (2010) reported that NaOH as an alkaline activator enhances the pozzolanic reaction and produces a less smooth 42 gel. Similarly, Palomo et al. (1999) highlighted that Na2SiO3 accelerates the reaction between the source material and the solution and produces cementitious materials. Ravikumar (2010) investigated the effect of concentration of NaOH and activator to binder ratio on compressive strength of stabilized soil and reported that the compressive strength decreased with increased activator to binder ratio. The higher compressive strength can be achieved with an increase in the concentration of NaOH. Several researchers (Palomo 1999; Chindaprasirt 2007; Bakri et al. 2011 and 2012; Mustafa et al. 2011; Cristelo et al. 2013; Radhakrishna 2015) reported that the ratio of alkaline activator to the fly ash and ratio of Na2SiO3 to NaOH significantly influences strength and compressibility behavior of the geopolymer or alkali-activated material. Mustafa et al. (2011) suggested using the ratio of Na2SiO3 to NaOH between 2 to 2.5, at which higher compressive strength can be achieved, whereas Chindaprasirt et al. (2007) reported that maximum compressive strength can be obtained when the ratio of Na2SiO3 to NaOH was 0.67 to 1.00. The optimum amount of alkaline activator to fly ash and Na2SiO3 to NaOH depends on the chemical composition of the fly ash and the soil to be stabilized (Mustafa et al. 2011; Radhakrishna 2015). Cristelo et al. ( 2013) stabilized the weak soil using alkaline-activated fly ash where the fly ash was activated using different dosage and molal concentration of NaOH (in order of 10M, 12.5M, and 15M). Cristelo et al. reported a strong correlation between the strength of the stabilized soil and alkaline activator dosage. The strength of the stabilized soil increased with a decrease in alkaline activator to fly ash ratio, as shown in Figure 2.12. Yusuf et al. (2014) conducted a series of UCS tests on alkali-activated ground slag mortar/concrete with different molal concentrations of NaOH (in order of 4M, 8M, 10M, 12M, and 16M ). The strength of concrete that can be achieved in 28 days without using any activator 43 was achieved in 3 days using NaOH as an activator. Yusuf et al. (2014) reported that the strength of the concrete/mortar largely depends on the proportion of Na2SiO3 to NaOH. Figure 2.12. Effect of concentration of NaOH and activator to binder ratio on compressive strength of concrete (modified from Cristelo et al. 2013). Rios et al. (2016) used alkaline activated fly ash to improve the engineering behavior of the silty sand. It was observed that both the strength and stiffness of the silty sand evolved significantly after activating the fly ash. Previous studies suggested that alkaline activator can be used to accelerate the rate of hydration. Alkaline activator has been suceessfully applied to produce green concrete, mortar, and brick (Cristelo et al. 2013; Radhakrishna 2015), and has also been used in stabilizing mineral soil. After reviewing several literature, it can be concluded that alkaline activator has a high potential for chemical stabilization of organic soil. 44 2.5 Curing Curing is an important step of the soil stabilization process, and without proper curing, the chemical process of hydration is not complete. Cementitious materials such as cement, lime, gypsum, and fly ash hydrate when in contact with water. The stabilized soil gains strength over the curing period due to the hydration process which begins when the binder comes in contact with water. Previous studies showed that the engineering behavior of the stabilized soil depends on the curing period and method (Hebib and farrell 2003; Edil and Staab 2005; Wong 2010; Correia 2011; Oliviera et al. 2012; Dehghanbanadaki et al. 2016). This section is intended to review the previous studies on the effect of curing period and method on soil stabilization. 2.5.1 Curing period Hebib and Farrell (2003) studied the effect of curing period on the strength of cement-stabilized peat. Considerable increase in strength was observed with time over a period of one year. The 28 days and 90 days UCS was about 50% and 70% of the one-year strength respectively (Figure 2.6). Hebib and Farrell (2003) reported that the value of Cα/Cc decreased from 0.054 to 0.036 when the curing period was extended from 28 days to 90 days (Figure 2.7). Sakr et al. (2009) conducted UCS tests on organic clay mixed with lime which cured for 7, 15, 30, and 60 days. The UCS of the sample increased significantly with time over a period of 30 days (Figure 2.8); however, no significant increase in strength occurred after 30 days of curing. Kolay et al. (2011) carried out UCS tests on peat samples stabilized with cement and fly ash. The stabilized samples were cured for 7 days, 14 days, and 28 days. The UCS of 45 cement-treated sample increased with time. The 7 days UCS was about 60% of the 28 days UCS. The effect of curing period on UCS was observed to be less significant at higher cement dosage of cement and fly ash, as shown in Figure 2-8. The increase in UCS of fly ash-stabilized soil with time was observed to be less significant, as shown in Figure 2-8. Oliveira et al. (2012) reported that C reduced by 60% (from 0.00102 to 0.00063) when the curing period was extended from 28 to 60 days. Siddiqua and Barreto (2018) carried out a series of UCS tests on chemically stabilized (calcium carbide residue and fly ash) rammed earth. Siddiqua and Barreto reported that the UCS increased with time over a period of 60 days. The authors observed that the effect of the curing period on UCS was more significant as the dosage of cement increased (Figure 2-13). Figure 2.13. Effect of curing period on UCS of stabilized soil (modified from Siddiqua and Barreto 2018). Hassan et al. (2019) carried out a series of UCS tests on organic soil treated with biomass silica (SH-85) cured for three different periods, i.e. 3 days, 7 days, and 28 days. Figure 46 2-14 indicates that most of the strength developed within 3 days and increased slightly with time. Figure 2.14. Effect of curing period on stress-strain behavior of the stabilized soil (modified from Hassan et al. 2019). 2.5.2 Curing technique Two different curing methods such as water curing (moist), and air curing are widely used in the stabilization of weak soil (Kalantari and Prasad 2014; Dehghanbanadaki et al. 2016). Soil and additives are mixed to prepare samples in water curing and are then kept submerged in water for a given curing period. Water curing is applied to the soil with low water holding capacity such as sand. Water curing also resembles the normal field condition where the groundwater table is generally high. Air curing can be achieved by placing mixed soil in a mold or plastic bag without the addition of any water in the mixture during the curing period. The air curing method can be adopted for organic soil as organic soil can hold enough water to complete pozzolanic reactions (Kalantari and Huat 2008; Oliviera et al. 2012; Kalantari and Prasad 2014; Dehghanbanadaki et al. 2016). Kalantari et al. 201). Venda Oliviera et al. (2012) conducted a series of oedometer tests on water-cured (moist) and buoyant (air) cured cement and slag stabilized samples. Organic content of the 47 tested soil ranged from 0.33% to 9.29%. It was observed that the water-cured stabilized sample exhibited less creep than the air-cured sample. Kalantari and Prasad (2014) evaluated the influence of curing technique on compressive strength of cement stabilized peat. The moisture content of the peat ranged from 198% - 417% and the organic content was 80.23%. Both air curing and moist curing were adopted to cure the samples for 28 days, 90 days, and 180 days. Air curing exhibited higher compressive strength than moist curing, as shown in Figure 2.15. The compressive strength of the air-cured sample was almost two times that of the water-cured sample. Figure 2.15. Effect of curing technique on UCS of the stabilized soil with cement (modified from Kalantari and Prasad 2014). Dehghanbanadaki et al. (2017) evaluated the compressibility of the peat treated with cement and fly ash. The samples were cured in both water and air for 14 days, 28 days, and 90 days. The organic content and moisture content of the peat was 495% and 91% respectively. It was observed that Cc decreased with curing time over a period of 90 days, as shown in Figure 48 2.16. Air-cured samples exhibited lower compressibility than water-cured samples (Figure 2.16). However, no significant difference was observed between air and water curing when the soil was mixed with a higher dosage of cement or fly ash. Figure 2.16. Effect of curing period and technique on compressibility of organic soil treated with various material (modified from Dehghanbanadaki et al. 2016). 2.6 Microstructural and chemical properties Microstructural evolution of the soil as a result of chemical interactions between additives such as cement, lime, fly ash, and gypsum, and soil components has been investigated using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS) analysis (Tang et al. 2007; Wong et al. 2012; Quang and Chai 2015). Similarly, X-ray diffraction analysis (XRD) has been used to determine the mineral composition of the soil and 49 additives (Yi et al. 2013). This section briefly explains the application of SEM/EDS and XRD for characterizing elemental compositions of the stabilized soil. 2.6.1 SEM and EDS Scanning electron microscopy (SEM) of the test specimens are used to analyze its microstructure and to provide substantial evidence on the cementation mechanism (Jha and Sivapullaiah 2015). Energy Dispersive Spectroscopy (EDS) tests were conducted to chemically characterize elemental compositions of the untreated and treated organic soil (Jha and Sivapullaiah 2015; Hassan et al. 2019). The EDS test primarily focused on investigating the change in the proportion of major chemical elements (i.e. Ca, Si, and Al) which play a crucial role in developing cementitious bond between soil particles through the pozzolanic reaction (Morsy 2014; Siddiqua and Barreto 2018). Kolay et al. (2011) demonstrated the experiment on fibrous peat from Malaysia. The peat consisted of high organic content (>75%) and the moisture content ranged from 360% to 623%. OPC, quick lime, and fly ash were used to stabilize the peat. The SEM test was conducted to visualize the microstructure of stabilized and unstabilized soil. Figure 2.17 shows that the peat was composed of a more discontinuous structure, woody and porous in nature. In contrary to untreated peat, a thin skeleton-like structure was observed in treated peat, which was a sign of cementitious bonds forming between the soil particles. The SEM image clearly showed that the soil particles were tightly packed. White lumps, a product from fly ash induced pozzolanic reactions (Jha and Sivapullaiah 2015; Latifi et al. 2016), are clearly seen in the pores of organic soil in Figure 2.17. 50 Figure 2.17. SEM image of (a) peat soil and (b) stabilized peat soil with fly ash (Kolay et al. 2011) Wong et al. (2012) stabilized the peat soil using kaolin clay and cement and studied the microstructural properties of untreated peat and treated peat soil by conducting SEM and EDS tests. The SEM test revealed that untreated peat soil consists of loose spongy and hollow materials. The EDS test revealed that untreated peat consisted of a high percentage of oxide and carbon, while Ca, Si, and O were the dominant chemical elements in stabilized peat. The EDS test showed that increasing percentage of Ca resulting in formation of Ca ion, which aids to produce new cementing materials through the hydration process. Quang and Chai (2015) evaluated the hydraulic conductivity of the Ariake clay and riverbed deposit treated with lime. The microstructure of the lime-treated Ariake clay and riverbed soil was studied using SEM imaging. SEM analysis revealed that the cementatious materials formed by hydration fill the intra-aggregate pores of soil particles, resulting in decreasing the permeability and significantly increasing the shear strength of stabilized soil. 51 2.6.2 XRD The XRD test is conducted to determine the crystalline components present in the soil. Major crystalline minerals and their proportion in the soil are analyzed to evaluate the development of cementitious compounds such as calcium aluminate hydrate (CAH) and calcium silicate hydrate (CSH) in the stabilized soil (Jha and Sivapullaiah 2018). Chrysochoou et al. (2009), Yi et al. (2013), Ahmed (2015), Jha and Sivapullaiah (2018), and many others conducted, XRD tests to evaluate the change in major crystalline minerals of the stabilized soil. Jha and Sivapullaiah (2018) conducted SEM/EDS and XRD tests to determine the change in the internal structure and chemical composition of stabilized gypseous soil using fly ash and lime. The XRD results showed the multiple peaks of cementitious compounds CAH and CSH gel at 20% of fly ash. SEM images were explained based on the XRD test as well, which showed that newly formed cementing crystals were silicate and aluminate-hydrated cementitious compounds. The SEM analysis showed that the formation of new cementing compounds increased as the dosage of the fly ash increased, resulting in improving the strength of the soil. It was observed that the amount of Si, Al, and Ca increased with an increase in dosages of fly ash to the soil. Sekhar and Nayak (2017) studied the stabilization of organic clay using granulated blast furnace slag. Both SEM and XRD analyses were conducted to evaluate the change in the microstructure of the stabilized soil. The XRD analysis of the untreated soil showed kaolinite, illite, halloysite, and quartz as major crystalline minerals. The XRD analysis of the stabilized soil revealed the replacement of major crystalline minerals of the soil by minerals of granulated blast furnace slag. 52 2.7 Summary Organic soil is characterized by low density and low shear strength, large void ratios, high compressibility and high-water content, and significant creep behavior. As a result, organic soil poses significant engineering problems for infrastructure development in the form of shear failure or excessive settlement. Ordinary Portland cement (OPC), lime, gypsum, coal fly ash, and other chemicals, have been commonly used for stabilizing the weak mineral soil with low organic content. However, stabilization using these materials may have an adverse impact on the environment, due to leaching of hazardous compounds into the surface and groundwater, consumption of large amount of energy, and significant emissions of greenhouse gases. Although fly ash has been used to stabilize the weak mineral soil, the performance of fly ash on stabilizing organic soil is still poorly understood. Ca-bentonite has cementitious properties and been widely used for reducing the permeability of backfill material. Ca-bentonite can be used as a natural pozzolan in stabilizing the organic soil. The hydration rate influences the engineering properties of the stabilized soil. The hydration rate of fly ash, such as PFA, is very slow compared to OPC, resulting in slowly developing the strength. An alkaline activator is used to accelerate the hydration rate when PFA is mixed with organic soil. Previous studies showed a beneficial effect of alkaline activator when mixed with fly ash and soil. Despite having numerous advantages, PFA, Ca-bentonite, and alkaline activator as a green stabilizer, there is currently no widespread commercial application of these binders in the construction industry. SEM/EDS tests have been used to analyze microstructural evolution of the soil as a result of chemical interactions between soil and additives, and XRD tests have been used to determine the mineral composition of the soil and additives. 53 Chapter 3: Geotechnical Characterization of Materials Series of geotechnical laboratory tests were carried out to characterize the materials used in this study. The organic soil materials were collected from Wabasca, Alberta, the pulp mill fly ash (PFA) was collected from a local industry in Kamloops, BC, and the calcium bentonite (Ca-bentonite) was received from a local company, Pacific Bentonite Ltd., Kamloops, BC. 3.1 Organic soil 3.1.1 Visual description The color of the organic soil was black to dark brown, as shown in Figure 3-1. The soil contained small wood chips, grassroots, and insect cells. Bunches of grass fibers were easily visible in the soil. The texture of the soil was spongy, and water was easily expelled when squeezed by fingers due to its high-water content. Figure 3.1. (a) Organic soil (b) SEM image, and (c) EDS spectrum. (a) (b) (c)54 3.1.2 Soil index properties A series of laboratory tests were conducted to determine the index properties of the tested soil after visual inspection. The soil index properties studied included: moisture content, organic content, fiber content, particle size distribution, specific gravity, void ratios, and Atterberg limits. The index properties of the soil are presented in Table 3.1. Table 3.1. Geotechnical properties of the soil, PFA and Ca-bentonite. Parameter Average value Soil PFA Ca-bentonite Field density (kg/m3) 1213 - - Natural moisture content (%) 260±26 - 4.91 Organic content (%) 26 17.42 - Swelling index(mL/2g) - - 12 Loss on ignition (LOI %) 28.8 20.6 - Specific gravity 2.17 2.7 2.49-2.72 Ash content (%) 74 - - Fiber content (%) 23 - - Shrinkage limit (%) 35 - - pH 7.19 13 8.26 Liquid limit (%) 104 106.8 209 Plastic limit (%) 84 66 21 Plasticity index 20 40.8 188 Particle size distribution (%) 97.2* > 0.075 mm 18 20 - 0.002 mm – 0.075 mm 60 63 - < 0.002 mm 22 17 - *Fines fraction 55 3.1.2.1 Moisture content The moisture content of the organic soil was determined as per ASTM standard (ASTM D2974-07A) and was found to vary significantly for each test. Three samples were taken in tares and kept in the oven for 24 hours at 105-1100 C. The average moisture content was calculated after 24 hours by measuring the dry weight of three tares with soil. The moisture content of the soil was 260±26%, indicating that there was mineral contamination, and that the soil was at an intermediate stage of decomposition. As organic soils are able to absorb and retain large amounts of water, pure peat can have moisture contents as high as 3000% (MacFarlane 1969). 3.1.2.2 Organic content The soil organic content was measured according to ASTM (ASTM D2974-07A). Three samples were obtained to measure the organic content of the soil. The weighted moist soil was kept in the oven at 105-1100 C for 24 hours to measure the dry mass of the samples. Porcelain basins were heated in 5500 C for 3 hours and then placed in a desiccator to cool down. After cooling down, the dry soil was placed in the oven at 5500 C for 4 hours. The tare of soil was placed in the desiccator after heating the soil at high temperature. Next, the weight was measured and a loss on ignition test was conducted to determine the organic content of the soil, using the following equation: OC (%) = [1 – 1.04 (1 – N)] ×100 [3-1] where OC= organic content % and N= loss of ignition %. The average value of organic content was measured as 26%. This value also indicates mineral soil contamination and a large degree of decomposition. Based on the organic content, the soil 56 used in this research is known as muskeg soil as shown in Table 2.2 (Jarret 1995; McVay 2004). 3.1.2.3 Fiber content Fiber content is an important parameter when characterizing organic soil. Fiber content was obtained from the dry mass of fibers retained on sieve No. 100 and expressed in terms of percentage of the oven-dried mass of the organic soil (ASTM D1997-13). 3.1.2.4 Degree of decomposition Based on the humification, the soil could be categorized as H7, as shown in Table 2.3 (Heat 2004). A soil that falls between H5-H7 is known as pseudo fibrous peat and has an intermediated degree of decomposition with recognizable plant structures (Karlson and Hansbo 1981; Jarret 1995). 3.1.2.5 Particle size distribution Both the sieve analysis and hydrometer tests were conducted in accordance with ASTM D422 to determine the particle size distribution of the organic soil. Sieve analysis was conducted for the soil particles larger than 0.075 mm. The hydrometer test was used for soil particles passing through sieve #200 (i.e. 0.075 mm sieve). The organic soil consisted of a high fines content, with 82% passing through the 75 m sieve. The proportions of the sand, silt, and clay are 18%, 60%, and 22%, respectively (Table 3-1). 3.1.2.6 Specific gravity The specific gravity of the soil was measured according to ASTM D854-14 using a pycnometer. Specific gravity is required to calculate the void ratio of the soil sample and found 57 to be 2.17. A specific gravity greater than 2.0 for organic soils indicates a significant amount of mineral soil contamination. The correlation between organic content and a specific gravity of soil, recommended by Radforth and Brawner (1973), is applicable for this organic soil as well. 3.1.2.7 Density Jennifer (2018) carried out a series of field investigation to characterize the in-situ properties of organic soil deposits in Northern Alberta, Canada. Jennifer (2018) collected undisturbed soil samples using Shelby tube soil samples to measure the field density of the organic soil. The bulk density measured in the field was 1213 kg/m3. Both fall cone and 1D consolidation tests were performed on samples prepared at the field density of the organic soil. 3.1.2.8 Void ratios The organic soil has a high water-holding capacity with a high void ratio, meaning that the soil was highly compressible in nature. The initial void ratio of the organic soil was calculated by the following equation. The specific gravity of the soil solid can be defined by Equation 3-2: 𝐺𝑠 =𝜌𝑠𝜌𝑤 [3-2] where, s, and w are the density of soil and water respectively. The density of the soil is the ratio of the mass of soil to the volume of solid. Then Equation 3-2 can be rewritten as: 𝐺𝑠 =𝜌𝑠𝜌𝑤= 𝑀𝑠𝑉𝑠 𝑤=𝑀𝑠𝐴𝐻𝑠𝑤 [3-3] 58 where Ms is the mass of solid soil (=dry mass of soil), Vs is volume of solid mass, A is area of solid mass, and Hs is height of solid mass. Equation 3-2 can be rearranged to obtain height of solid soil as follows (Equation 3-4): 𝐻𝑠 = 𝑀𝑠𝐴𝐺𝑠𝑤 [3-4] Then, the height of void (Hv) can be obtained by subtracting the height of solid (Hs) from the total height of the soil sample which can be measured directly. 𝐻𝑣 = 𝐻𝑇 − 𝐻𝑠 [3-5] Finally, void ratio of the soil sample can be calculated using Equation 3-6. 𝑒0 =𝑉𝑣𝑉𝑠=𝐻𝑣 𝐴𝐻𝑠 𝐴=𝐻𝑣 𝐻𝑠 [3-6] Void ratio of the sample at any time during the consolidation test can be calculated by subtracting the consolidation settlement at that time from initial height of samples (Equation 3-7): 𝐻𝑇 = 𝐻𝑖 − 𝐻 [3-7] where Hi is the initial height of the sample and H is the consolidation settlement. 3.1.2.9 Atterberg limit Atterberg limits (liquid limit and plastic limit) are a crucial parameter to characterize to understand the behavior of the soil. Atterberg limits of the tested soil were determined by both the Casagrande method (ASTM D4318) and the fall cone method (Wasti 1987). The liquid limit of the soil was 104% with an associated plastic limit of 84% and a plasticity index of 20% 59 (Table 3-1). The soil falls below the A-line in the MH/OH region of the plasticity chart m (Figure 3-2). Figure 3.2. Plasticity chart to characterise the soil based on Atterberg limits. 3.1.2.10 Shrinkage limit The shrinkage limit is the water content of a soil at which a further decrease in water content does not cause a decrease in volume (ASTM D4943-18). Shrinkage limit of the organic soil was determined as 35% according to ASTM D4943-18, as presented in Table 3-1. 3.1.3 Shear strength The fall cone test (BS 1377 1990) is widely used to estimate the undrained shear strength (su) of a fine-grained soil based on the cone factor, K (Zentar et al. 2009; Llano-Serna et al. 2018). The fall cone test is a quick method used to estimate the undrained shear strength (su) of soil (Zentar et al. 2009). The fall cone tests were conducted on soil samples to estimate undrained shear strength according to BS 1377 (BS 1377 1990). The detailed methodology and 60 calculation procedures for the fall cone tests are presented in Chapter 4. It should be noted that the organic soil showed minimal shear strength of about 5 kPa at in-situ density as the samples were prepared at a natural moisture content higher than the liquid limit of that soil. The samples were fully saturated at the time of testing in order to represent the field conditions of natural deposits. 3.1.4 Soil Compressibility The compressibility behavior of the organic soil was evaluated by a one-dimensional consolidation test using an oedometer apparatus (Bowles 1992). Incremental loading was applied with a stress range of 6.25-100 kPa for primary consolidation, and constant creep loading of 100 kPa for secondary consolidation. For each loading stage, the loading was maintained for 24hrs and the data was recorded as suggested in ASTM D2435 standards. The details of the one-dimensional consolidation test are presented in Chapter 4. The compressibility behavior of the organic soil was characterized using results from the oedometer test on the organic soil. The stress history (pre-consolidation pressure, 'p), primary compression index (Cc), coefficient of volume compressibility (mv), coefficient of consolidation (Cv), hydraulic conductivity, and creep coefficient (Cα) were calculated. As expected with remolded untreated organic soils, no preconsolidation stress (stress history) was apparent in the results. The value of Cc, mv, and Cv were about 2.0, 0.011 kPa-1, 60 m/min respectively for untreated soil. The value of k of untreated soil decreased by thousand times from the initial value of 6.1×10-6 m/min at σ′v = 6.25 kPa (& e = 5.2) to 2.3×10-9 m/min at σ′v = 100.0 kPa (& e = 2.5). The value of C of untreated organic soil at 100 kPa creep stress was about 0.14. 61 3.1.5 Microstructural and chemical properties The microstructural and chemical composition of the organic soil was characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) testing. Figure 3-1b shows the corresponding SEM image of organic soil. A dispersed, irregular, and discontinuous microstructure with elongated fibrous structures and large voids was observed in the organic soil. The granular matrix of the soil, in addition to the irregularly shaped sheet-like particles, can be seen in Figure 3-1b. The organic components are more difficult to differentiate from the mineral components in Figure 3-1b; however, the mineral soil is angular while the organic soil has irregular shapes. The sheet-like particles comprise the organic component of the soil. Figure 3-1b indicates that organic particles cover a significant portion of the soil volume, though organic particles only account for about 26% of the mass, as determined by the loss on ignition test. The chemical composition of the organic soil was determined with EDS analysis. The list of major elements and their proportions are presented in Table 3-2. Figures 3-1b and 3-1c show the corresponding SEM image and EDS spectrum of the organic soil. The EDS analysis revealed that the major elements present in the organic soil were Silica (Si), Aluminum (Al), Iron (Fe), Potassium (K), and Carbon (C) as shown in EDS spectrum in Figure 3-1c. Similar chemical compositions of organic soil were found by EDS analysis by Leong et al. (2012) and Hassan et al. (2019). The results from the EDS and SEM image analysis revealed that the organic soil was more granular and amorphous, indicating a large degree of mineral contamination, as explained in the previous section. 62 Table 3.2. Chemical properties of soil, PFA, and Ca-bentonite. Elements Soil (wt%) PFA (wt%) Ca-bentonite (wt%) O 41.01 34.5 54.54 Si 10.42 9.67 27.38 Al 1.9 1.83 8.71 Ca 2.99 17.72 1.67 K 0.57 4.72 1 Fe 2.73 1.83 3.79 C 34.94 25.76 - S 1.33 1.75 - P - 0.78 - Na 0.2 - 1.58 Others 3.91 1.43 1.33 The XRD test was carried out to determine the crystalline components present in the soil. Figure 3-3 shows numbers of visible peaks for different chemical elements along with quantitative results of the mineral composition of the organic soil. The organic soil exhibited sharps peaks of oxygen, quartz and calcite. The organic soil consisted of calcite (14.6%), illite/muscovite (11%), clinochlore (3.5%), K-feldspar (5.4%), plagioclase (8.4%), and quartz (52.6%). The sharp peak of clinochlore on the graph was observed at 2 of 7.23°, 14.23°. The sharp peak of quartz displayed at 2 of 24.30°, 31.19°, 42.75°, 58.94°, and 70.96°. The other peaks showed broad corresponding to 2 of different angles. 3.2 Pulp mill fly ash (PFA) The PFA was collected from an electrostatic precipitator in a pulp mill located in Kamloops, BC, Canada (Figure 3-4), which was used for stabilizing the organic soil. The PFA is produced by burning bark, wood residue, re-circulated ash, and sludge with microorganisms and fibers in an incinerator at a high temperature of 730 °C (Arm et al. 2014). 63 Figure 3.3. X-Ray diffraction results with a mineral indication for soil, fly ash (PFA), and Bentonite clay (Ca-bentonite). Figure 3.4. (a) Dumped PFA in the local pulp mill industry in Kamloops, BC, and (b) close-up view of PFA. (b)(a)64 3.2.1 Index properties The properties of PFA used in this research are reported in Table 3-1. The loss on ignition of PFA was 20.6%. The liquid and plastic limits of the PFA were 106.78% and 66%, respectively. The maximum particle size of PFA was 0.15 mm, with 80% less than 0.075mm and 17% less than 0.002 mm. 3.2.2 Microstructural and chemical properties The SEM and EDS tests were carried out to characterize the morphological and chemical properties of PFA. The SEM image of the PFA shown in Figure 3-5a reveals that PFA particles were irregular in shape. The porous cellular forms observed in Figure 3-5a might be attributed to partially burned wood. The EDS test did not show concerning heavy metals in the PFA. Figure 3-5 shows the corresponding EDS spectra of the PFA; from these results, it can be observed that the dominant elements present were Si, Al, Fe, K, and C. Some chemical components such as Si, Al, and Ca revealed that PFA has pozzolanic properties which can act as a good stabilizer. The proportion of various elements in the PFA is presented in Table 3.2. A toxicity characteristic leaching procedure (TCLP) was performed according to Environment Canada (1990). The leachate collected from acid-digested samples was analyzed for trace element concentrations using an inductive coupled plasma-atomic emission spectroscopy (ICP-AES) technique. The results showed heavy metal concentrations in PFA such as Zn and Ba in high concentrations, compared to minor levels of As, Cd, Co, Cr, Cu, Mo, Ni, Pb, Se, and V. It is important to note that all heavy metals and other toxic elements were well below the permissible limits recommended by CSA 14001-96 (1996) standard guidelines. 65 Figure 3.5. (a) SEM image, and (b) EDS spectrum of PFA. The sharp peak of PFA displayed on the XRD graph (Figure 3-3) was calcium hydroxide, quartz, syngenite, and calcite. Some chemical elements observed in PFA during XRD were calcium hydroxide (9.5%), calcite (62.1%), hydrocalumite (4.4%), syngenite (13.6%), and quartz (5.2%). The multiple numbers of syngenite were displayed at 2 of 10.87°- 60.12° and quartz was displayed at 2 of 24.28° - 76.80°. Calcium hydrate and calcite were displayed at 2 of 12.80° - 65.34° and 22.35°- 75.11° respectively. The presence of syngentite, calcite, and calcium hydrate prove that the fly ash has cementitious materials. Syngentite has self-hardening properties that reflect its binding to soil particles. 3.3 Calcium bentonite (Ca-bentonite) Ca-bentonite was received from Pacific Bentonite Ltd, in Kamloops. The calcium bentonite was in fine powder form and cream in color (Figure 3-1a) and was used as an additive for the stabilization of organic soil. This bentonite was classified as low-swelling calcium bentonite compared to sodium bentonite, containing 41.1% montmorillonite. Ca-bentonite contains considerable amount of cementing elements such as Si, Al, and Ca. Ca-bentonite has high 66 specific area. These characteristics satisfy the suitability of calcium bentonite for soil stabilization. Figure 3.6. (a) Calcium bentonite (b) SEM image, and (b) EDS spectrum of Ca-bentonite. 3.3.1 Index properties Various relevant laboratory tests were conducted to characterize Ca-bentonite, and the physical properties are presented in Table 3-1. The moisture content of Ca-bentonite was 4.91%. The liquid limit, plastic limit, and plasticity index of Ca-bentonite are 209%, 21%, and188% respectively. Fines passing from 0.75µm were 97.2% and specific gravity was 2.49-2.72. 3.3.2 Microstructural and chemical properties Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) was conducted to evaluate the morphology and chemical properties of Ca-bentonite (Figures 3-1b and 3-1c). The chemical properties of calcium bentonite are presented in Table 3.2. The EDS test showed that calcium bentonite contains a rich weight percentage of silica components at 27.38%. High percentage of silica in Ca-bentonite can be a supplement for low silica (a) (b) (c)67 percentage in PFA, and that Ca-bentonite is a good additive for soil stabilization when used along with PFA. The XRD graph on Ca-bentonite displayed with 41.1% of montmorillonite. The montmorillonite was observed at 2 on 6.52°, 23.03°, 39.42°; likewise, the graph detected montmorillonite in different at 2 position (Figure 3-3). The Ca-bentonite graph showed other minerals such as albite (plagioclase) with 53%, quartz with 4%, calcite 2%, cristobalite, and opaline silica with 13% of total chemical minerals. The flat peak of Ca-bentonite denotes that it is amorphous in nature. The graph displayed multiple number of quartzes, and albite calcite, indicating that it has a favourable source of pozzolanic properties. 3.4 Alkaline activator Sodium hydroxide (NaOH), and sodium silicate (Na2SiO3) were used as an alkaline activator to accelerate the hydration rate and to develop the strength of the stabilized soil. The alkaline activator was enriched with Na and Si. Na2SiO3 was collected in the form of a solution while NaOH was obtained in the form of a pellet with 97% purity and with a specific gravity of 2.4. The Na2SiO3 used in this study was composed of H2O; 66%, Na2O; 8%, and SiO2; 26 %, with a specific gravity of 1.4. 3.5 Summary This chapter briefly explained the geotechnical characteristics of organic soil, PFA, and Ca-bentonite. The microstructural and chemical characteristics of organic soil, PFA, and Ca-bentonite were also presented. Series of geotechnical laboratory tests were carried out to characterize the organic soil, PFA, and Ca-bentonite. The organic soil considered in this study was collected from a bog wetland (locally termed as “muskeg”) situated near Wabasca, which 68 is part of the Northern Alberta lowlands in Canada. The organic soil had a high liquid limit and plastic limit of 104% and 84%, respectively, low unit weight of 1213 kg/m3, and a high natural moisture content of 260±26%. The specific gravity of the soil was measured as 2.17. The PFA was collected from an electrostatic precipitator in a pulp mill located in Kamloops, BC, Canada. The loss on ignition of PFA was 20.6%. The liquid and plastic limits of the PFA was 106.78% and 66%, respectively. The Ca-bentonite was received from Pacific Bentonite Ltd., BC, Canada. The moisture content of Ca-bentonite was 4.91%, and the liquid limit, plastic limit, and plasticity index of Ca-bentonite was 209%, 21%, and 188% respectively. Fines passing from 0.75µm were 97.2% and the specific gravity was 2.49-2.72. 69 Chapter 4: Methodology Chapter 4 presents the procedures for sample mixing, compaction, and curing to prepare the specimen for both the fall cone and 1D consolidation tests. Furthermore, details of the fall cone test, one-dimensional consolidation test, pH test, scanning electron microscope (SEM) with energy dispersive X-ray (EDS), and X-ray diffraction (XRD), are presented in this chapter. 4.1 Mix design One of the principal objectives of this research was to investigate the effects pulp mill fly ash (PFA), alkaline activator (AA), and calcium bentonite (Ca-bentonite) on the strength and compressibility behavior of organic soil. This goal was achieved by conducting a series of fall cone and consolidation tests followed by SEM/EDS and XRD tests through a carefully designed experimental program. The experimental program was composed of three phases, each phase consisting of a series of fall cone and consolidation tests. In addition, pH, SEM/EDS, and XRD tests were conducted on consolidated samples in Phase 1 and Phase 3 of the experimental program. The mix design of stabilized organic soil was formulated in terms of binder composition and dosage. The binder percentage was defined in terms of dry mass of the soil. Phase 1: The principal goal of Phase 1 of the experimental program was to investigate the effect of PFA on the strength and compressibility behavior of organic soil. Seven different mix designs were prepared and stabilized to different treatment levels. The first mix design with no PFA addition was considered a control/reference point for comparison purpose, and 70 the other eight mix designs included an increasing amount of PFA dosages as presented in Table 4.1. Sample ID, PFA dosage, void ratio, and bulk density of the specimens tested in Phase 1 are presented in Table 4.1. Table 4.1. Mix design for sample preparation for the Phase 1. Sample ID PFA % by dry mass Mass of PFA for 1 m3 organic soil ei i (kg/m3) P1 0 0 5.6 1213.9 P2 5 20.1 5.42 1213.3 P3 10 40.2 5.26 1213.3 P4 20 80.4 5.13 1213.7 P5 30 120.6 4.75 1213.2 P6 40 160.8 4.41 1213.2 P7 50 201 4.06 1213.3 The fall cone test was conducted prior to the one-dimensional oedometer test. In total, sixty-three samples were prepared for the fall cone test. Three samples were prepared from each mix design and cured for three different periods, i.e. 0 days, 7 days, and 14 days. The goal of curing the samples for three different periods was to evaluate the effect of the curing period on the soil stabilization. The curing period for the samples for the one-dimensional consolidation tests was determined based on the results from the fall cone test. Seven oedometer tests were conducted simultaneously with different dosages of PFA as shown in Table 4.1. Phase 2: Results from Phase 1 of the experimental program did not show any optimum amount of PFA needed to achieve higher strength and better compressibility behavior of the stabilized soil. According to the literature, the curve of the dosage of PFA vs. undrained shear 71 strength should eventually attain a plateau, although no such trend was observed in the range of PFA contents considered in this study. Phase 2 of the experimental program was designed to use the alkaline activator to accelerate and improve the pozzolanic reaction of PFA to achieve higher strength and better compressibility behavior of the stabilized organic soil. Table 4.2. Mix design for Fall cone tests for Phase 2 Sample ID NaOH concentration AA/PFA ratio PFA % by dry mass Mass of PFA for 1 m3 organic soil Curing days AF1 0 M 0 0 0 0 AF2 0 30 120.6 0 & 7 AF3 8M 0.2 30 120.6 0 & 7 AF4 0.3 30 120.6 0 & 7 AF5 0.4 30 120.6 0 & 7 AF6 0.5 30 120.6 0 & 7 AF7 0.6 30 120.6 0 & 7 AF8 0.7 30 120.6 0 & 7 AF9 0.8 30 120.6 0 & 7 AF10 0.9 30 120.6 0 & 7 AF11 10 M 0.2 30 120.6 0 & 7 AF12 0.3 30 120.6 0, 7, 14 & 28 AF13 0.4 30 120.6 0 & 7 AF14 0.5 30 120.6 0, 7, 14 & 28 AF15 0.6 30 120.6 0 & 7 AF16 0.8 30 120.6 0 & 7 AF17 12 M 0.2 30 120.6 0 & 7 AF18 0.3 30 120.6 0 & 7 AF19 0.4 30 120.6 0 & 7 AF20 0.6 30 120.6 0 & 7 AF21 0.8 30 120.6 0 & 7 After careful review of the literature and considering economical and practical aspects, a constant 30% of PFA by dry mass of organic soil was used in Phase 2 and Phase 3 of the 72 experimental program. A constant 30% PFA for Phase 2 and Phase 3 was decided based on the laboratory test results in Phase 1. A range of PFA amount starting from 5% to 30% at an increment of 5% was tested in Phase 1. Results showed that there is a significant increase in undrained shear strength and decrease in compressibility of organic soil after adding 20% to 30% PFA in Phase 1. Cement induced pre-consolidation pressure appeared at 30% PFA in Phase 1. PFA mixed soil behaved as shale and mudstone (based on C/Cc value) after exceeding 20% PFA in Phase 1. In addition, the pH of the mixed soil should be above 9 for better pozzolanic reaction so pH was greater than 9 when PFA exceeded 20% of soil dry mass in Phase 1. The results from Phase 1 as mentioned above indicated that 30% PFA is crucial for stabilizing the organic soil. Table 4.3. Mix design for one-dimensional oedometer tests. Sample ID PFA % by dry mass Mass of PFA for 1 m3 organic soil NaOH, Concentration AA/PFA ratio ei i (kg/m3) AA1 0 0 0 0 5.6 1213.9 AA2 30 120.6 0 0 4.75 1213.2 AA3 30 120.6 8M 0.4 4.1 1213.4 AA4 30 120.6 10M 0.3 4.01 1213.4 AA5 30 120.6 10M 0.5 3.6 1213.4 AA6 30 120.6 12 M 0.2 4.47 1213.3 AA7 30 120.6 12M 0.8 3.45 1213.3 AA8 30 120.6 12M 0.9 3.61 1213.3 The principal goal of Phase 2 of the experimental program was to investigate the effect of alkaline activated-PFA on the strength and compressibility behavior of organic soil. Twenty-one mix designs were developed for the fall cone test as shown in Table 4.2. The reconstituted 73 organic soil sample (Sample AA1) with 30% PFA was considered as a control/reference sample to evaluate the performance of alkaline activator on undrained shear strength of the stabilized soil. A total of one hundred and thirty-five samples were prepared under this mix design. Three identical samples were prepared for each mix design for each curing period in order to obtain more reliable data (Venda Oliveira et al. 2014). The ratio of Na2SiO3: NAOH was 2:1 for all mix designs. Table 4.4. Mix design for fall cone and oedometer tests Sample ID PFA (%) Mass of PFA for 1 m3 organic soil Ca-bentonite % Mass of Ca-bentonite for 1 m3 organic soil ei i (kg/m3) C1 0 0 0 0 5.60 1213.90 C2 30 120.60 0 0 4.75 1213.20 C3 30 120.60 5 20.10 4.28 1213.33 C4 30 120.60 8 32.16 4.24 1213.36 C51 30 120.60 10 40.20 4.22 1213.37 C6 30 120.60 12 48.24 4.09 1213.25 C7 30 120.60 15 60.30 3.96 1213.29 C81 30 120.60 20 80.40 3.95 1213.37 1Two samples were prepared for one-dimensional consolidation tests from each mixed design; one sample followed by water curing for 7 days and another by air curing for 7 days. Phase 3: The goal of Phase 3 of the experimental program was to investigate the effect of calcium bentonite (Ca-bentonite) on the strength and compressibility behavior of organic soil mixed with a constant PFA of 30% of soil dry mass. As in Phase 1, a series of fall cone and one-dimensional consolidation tests were conducted, followed by pH, SEM/EDS, and XRD tests. Eight mix designs were prepared with a constant amount of 30% PFA and at different dosages of Ca-bentonite varying from 5%, 8%, 10%, 12%, 15%, and 20% of dry mass 74 of the soil (Table 4.4). The reconstituted organic soil sample (Sample C1) without PFA and Ca-bentonite was considered as a control/reference sample to evaluate the performance of alkaline activated PFA. This mix design was used to develop the optimum mixed design for Ca-bentonite and PFA stabilized organic soil. Fall cone tests were conducted prior to a one-dimensional consolidation test. A total of forty-five samples were prepared for the fall cone test. All the fall cone tests were repeated at least three times to obtain more reliable results (Venda Oliveira et al. 2014). Water and air curing methods were considered simultaneously to evaluate the effect of the curing method on soil stabilization. Based on the results from the fall cone test, the curing method for the samples for the one-dimensional consolidation test was determined. 4.2 Sample preparation Similar procedures for sample preparation were adopted for all phases. The remolded soil sample was used to perform the entire series of tests. The mixture of PFA and Ca-bentonite to the soil were quantified by the mass ratio of binder to the dry mass of organic soil. Similarly, the quantity of activator was calculated by mass ratio of activator to the dry mass of the PFA. The activator was prepared according to a Na2SiO3 to NaOH ratio of 2:1. The PFA was grounded and sieved through a 250µm sieve. Sample preparation was carried out in three different steps: (a) mixing, (b) compaction, and (c) curing. 4.2.1 Mixing The dry mixing technique was adopted to prepare the reconstituted soil samples in the laboratory. This method was applied as Aalto (2001) recommended the dry mixing technique for soil having a natural water content greater than 75%. For Phase 1, at first, mass of PFA was 75 calculated by dry mass of organic soil. Visible undecayed leaves, fibers, and grassroots were removed from organic soil, if present. The soil was then thoroughly mixed by hand. Then, the organic soil was mixed thoroughly with the known mass of PFA at a natural water content. For Phase 2, the calculated amount of PFA and soil were first mixed homogeneously. Then calculated amount of alkaline activator was added to the homogenously mixed PFA and organic soil. For Phase 3 of the experimental program, a known mass of Ca-bentonite and PFA were mixed thoroughly to create a homogenous mixture before mixing with the organic soil. The soil was mixed carefully by hand until a uniform mixture was obtained. The procedure took 6-8 minutes for homogenous mixing, and precautions were taken to ensure minimal moisture loss during the process. The mixed samples were sealed in a plastic bag for 24 hours and kept in a humidity room before compaction. Hand mixing was preferred rather than using a mechanical mixing (i.e. kitchen mixer). 4.2.2 Compaction The mixed soil was compacted in a stainless container and consolidation ring for the fall cone and consolidation test, respectively (Figure 4.1). The mass of mixed soil required to fill the container or consolidation ring was calculated. The mixed soil sample was divided into 3-4 different layers for uniform compaction. The mixed soil was compacted by hand tamping to obtain the targeted density close to the in-situ density of the collected organic soil (i.e. 1213 kg/m3). The same compaction effort was used to compact each layer. For consistency, the initial density of all samples was maintained equally as the in-situ density of the organic soil in order to investigate the performance of PFA, alkaline activator, and Ca-bentonite on the strength and compressibility of the soil. 76 4.2.3 Curing Two curing techniques, namely water curing and air curing, were considered in this research. In the water curing method, the compacted samples were kept at room temperature for an hour prior to being fully submerged in water. The sample compacted in the oedometer ring or small container was submerged in distilled water in a large container and cured for 7 days at room temperature (Figures 4.2a and b). For the air curing method, the compacted sample was sealed in a plastic bag and then placed in a humidity room for 6 days (Figure 4.2c). The air-cured sample was fully submerged in distilled water for 24 hours before the fall cone and 1D consolidation test. Figure 4.1. Compacted (a) oedometer sample and (b) samples for fall cone test. Phase 1: Water curing Fall cone test: curing period (in days): 0, 7, and 14 (to investigate the effect of curing period on stabilizing the organic soil using PFA). 1D consolidation test: curing period (in days): 7 Phase 2: Water curing (a) (b)77 Fall cone test: all the specimens were cured for 0 and 7 days. The curing period was extended for few specimens to 14, and 28 days (to investigate effect of curing period on stabilizing the organic soil with the activated PFA). Consolidation test: water curing (in days): 7 Phase 3: Water and air curing Fall cone test: both water and air curing methods were adopted to evaluate the effect of the curing method on the strength of the stabilized soil. Consolidation: two set of specimens (at 10% Ca-bentonite and 20% Ca-bentonite) were cured by water and air. The rest of the specimens were cured by water. All samples were cured for 7 days. Figure 4.2. (a) Water curing for oedometer test (b) water curing for fall cone test, and (c) air curing. (a)(b) (c)78 4.3 Experimental phases 4.3.1 Fall cone test Fall cone tests (BS 1377 1990) are widely used to estimate the undrained shear strength (su) of a fine-grained soil based on the cone factor, K (Zentar et al. 2009; Tanaka et al. 2012; Llano-Serna et al. 2018). Wang et al. (2013) and Zhang et al. (2018) conducted a series of fall cone tests to investigate the effect of binding materials such as cement and lime on the strength of the silty soil and Dunkirk sediments respectively. Wang et al. (2013) also reported that the undrained shear strength measured by the fall cone tests is comparable to the strength determined by the vane tests. Fall cone tests were conducted on natural and stabilized organic soil samples to calculate su values according to BS 1377 (BS 1377 1990). Results of the tests show the effect of the binding materials on the soil’s strength. The impact of the curing period and technique on the stabilization of organic soil with binders (i.e. PFA and Ca-bentonite) was also investigated using the fall cone test. A cone made of stainless-steel with a mass of 80 g and a 30 tip angle was used in this test. Equation 1 was used to estimate the undrained shear strength of the tested soil. The cone factor, K= 0.8, was considered for the calculation (Zentar et al. 2009; Llano-Serna et al. 2018). The fall cone specimen was prepared in a sample cup of 55-mm diameter and 40-mm height. The tip of the cone was set touching the top of the specimen and the cone was then released. The depth of the specimen penetrated by the cone was measured after 5 seconds (Figure 4.3). All the tests were repeated three times (Venda Oliveira et al. 2014) and the average cone penetration depth was calculated to estimate the su of the untreated and treated organic soil, 79 𝑠𝑢 = 𝐾𝑄ℎ2 [4.1] where, su = undrained shear strength, K = cone factor, Q = total cone weight, and h = cone penetration depth. Figure 4.3. (a) Set up for fall cone test and (b) penetration of cone into soil after release from its initial position. 4.3.2 Oedometer test The compressibility behavior of treated organic soil was evaluated by a one-dimensional consolidation test using an oedometer apparatus (Bowles 1992). The size of the specimen used for the consolidation test was 63.24 mm in diameter and 20.71 mm in height. The soil compressions in each specimen were recorded under predefined normal stress at every 5 s. Both primary incremental loading (primary consolidation) and a creep stage loading (secondary consolidation) were considered in the one-dimensional consolidation test. Primary consolidation was comprised of five incremental load stages i.e. 6.25 kPa, 12.5 kPa, 25 kPa, 50 kPa, and 100 kPa, where each load stage lasted for 24 hours (Table 4.5). Secondary compression under a creep load of 100 kPa was applied for a period of 14 days. The setup for the oedometer test and samples after the consolidation test are shown in Figure 4.4. 80 Figure 4.4. (a) Setup for oedometer test, (b) consolidated sample in oedometer ring, and (c) sample after consolidation test. Table 4.5. Loading scheme for consolidation test. Load (kPa) Loading time (hr) 6.25 24 12.5 24 25 24 50 24 100 336 4.3.3 pH tests The pH of the organic and treated organic soil was measured. Organic soil contains mainly low pH which adversely affects the bonding between the particles. Mixing the organic soil with binders can increase the pH, and thereby creates a favorable environment by neutralizing the humic acid to complete the pozzolanic reaction (Janz and Johansson 2002; Venda Oliveira et al. 2012). A series of pH tests (BS1377: 1990) using a pH meter were carried out to investigate the degree of alkalinity of the stabilized organic soil after mixing. (b) (c)(b)81 4.3.4 SEM/EDS and XRD The microstructure and chemical composition of the organic soil and binder materials were characterized by SEM/EDS and XRD testing. These tests were conducted on both treated and untreated soil to determine the cementation mechanism and to evaluate the morphological evolution of the stabilized soil. A Tescan MIRA3 SEM instrument was used to visualize particle shape, morphological evolution, state of raw materials, and microstructure of the hydrated paste. The post-consolidation samples were kept in a refrigerator for 48 hours for pre-freezing, and were later freeze-dried at -50°C for 48 hours. Specimens for SEM/EDS analysis were obtained from the mid-section of the post-consolidated samples placed on the surface of an aluminum stub using adhesive carbon tape. The samples were then coated with palladium and platinum at a ratio of 10/80 under higher vacuum pressure until the desired thickness of 10 nm was achieved. Resolutions of 500, 1000, 2000, 5000, and 10,000 magnifications were used to capture the SEM images, providing information about the soil particles morphology, shape, size, and orientation. Energy Dispersive Spectroscopy (EDS) analysis was performed on organic soil and stabilized organic soil samples to investigate changes in proportion of major chemical elements. The EDS test primarily focused on investigating the existence of major elements (i.e. Ca, Si, and Al) which play a crucial role in developing cementitious bonds between soil particles through the pozzolanic reaction. A semi-quantitative estimation based on peak height and area spectra for non-clay minerals was considered to calculate the percentage weight (wt%) of different elements present in the samples (Pierce and Siegel 1969). 82 The XRD test was carried out to determine the crystalline components present in the soil, PFA, and Ca-bentonite. The test sample was ground to 10 m grain in a vibratory McCrone micronizing mill. Continuous-scan XRD data were obtained over a range of 3-80°2 with CoKα radiation on a Bruker D8 Advance Bragg-Brentano diffractometer. The Bruker D8 Advance Bragg-Brentano diffractometer was equipped with a Fe filter foil, 0.6 mm (0.3°) divergence slit, incident- and diffracted-beam Soller slits and a LynxEye-XE detector. The long fine-focus Co X-ray tube was run at 35 kV and 40 mA, with a take-off angle of 6°. 4.4 Summary This chapter presented the experimental program carried out on organic soil stabilized with PFA, alkaline activator, and Ca-bentonite. The experimental program was divided into three phases based on the binder materials. For Phase 1, PFA was used as a binder material to stabilize the organic soil. For Phase 2, PFA and alkaline activator were mixed and used as binder materials. Ca-bentonite and PFA were mixed to prepare binder materials for Phase 3 of the experimental program. Sample preparation was carried out in three different steps, i.e. mixing, compaction, and curing. The dry mixing technique was adopted to prepare the reconstituted soil samples in the laboratory. Series of fall cone and 1D consolidation tests were conducted to evaluate the undrained shear strength and compressibility behavior of the stabilized soil. SEM, EDS, XRD, and pH tests were performed to understand the morphological and chemical evolution of the stabilized soil. 83 Chapter 5: Performance of Pulp Mill Fly Ash, and Alkaline Activator This section presents the results from Phase 1 and Phase 2 experimental programs. Phase 1 experimental program consisted of series of tests on untreated and PFA treated organic soil. Similarly, Phase 2 experimental program comprised series of test on untreated and alkaline activated PFA treated organic soil. The organic soil was collected from Wabasca lowlands, Northern Alberta, Canada, contained 26% of organic matter. The PFA was obtained from a pulp mill located at Kamloops, BC, Canada. To develop the optimum mix design for PFA stabilized organic soil necessary to achieve the desired engineering behavior, different PFA- organic soil mixtures with varying PFA amounts of the dry mass of organic soil were considered. Series of fall cone and 1D consolidation tests were conducted on untreated and treated soil specimen. In addition to fall cone and oedometer tests, scannig electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS) and X-Ray diffraction (XRD) tests were carried to investigate chemical interactions between PFA and soil components. 5.1 Effect of pulp mill fly ash 5.1.1 Undrained shear strength The undrained shear strength of the PFA-treated organic soil estimated using depth of fall cone penetration is plotted against PFA content in Figure 5.1. The treated soil samples were tested after 0, 7, and 14 days curing, and all tests were carried out under fully saturated conditions. The value of su of PFA mixed soil was normalized by su of the untreated soil. For PFA-soil 84 mixtures, strength development after 7 days and 14 days curing might be attributed to the development of cementitious gel that glue soil particles together and fill the pores of organic soil. However, the results show no considerable difference in su values between 7 days and 14 days curing (Figure 5.1). The results showed an increase in su, as expected, in order of increasing PFA dosage and curing time. The rate of increase was low at 5% and 10% PFA and later intensifies when PFA exceeded 20% of dry mass of soil. According to the literature (e.g. Tastan et al. 2011), the curve should eventually attain a plateau, although no such trend was observed in the range of PFA contents considered in this research. The value of su increased by 2.0 and 3.25 times as compared to untreated organic soil when PFA content was 20% and 50% of dry soil mass, respectively (Figure 5.1). These results are promising for adopting PFA stabilization for improving the strength behavior of organic soil. Figure 5.1. Effect of PFA content on the shear strength of organic soil. Organic soil is rich in humic compounds. The soil particles are coated with humic compound which creates barrier between soil particles during pozzolanic reaction (Venda 85 Oliveira et al. 2012). The amount of PFA must exceed a certain threshold to neutralize the humic compound before significant strength is obtained (Janz and Johansson 2002). This might be the reason for an abrupt increase in su of PFA-treated soil when PFA was 20%. Figure 5.1 reveals that the effect of organic matter is normalized when 20% PFA was added to the organic soil. 5.1.2 Compressibility behaviour This section intends to evaluate how the amount of PFA and applied stress ('v) altered the compressibility behavior of the PFA-treated organic soil when compared with the untreated soil. Specifically, change in the pre-consolidation pressure ('p), void ratio (e), compression index (Cc), coefficient of volume compressibility (mv), Coefficient of consolidation (Cv), and hydraulic conductivity are discussed in this section. 5.1.2.1 Stress-strain behaviour Compression curve (e-log 'v) obtained from 1D consolidation tests conducted on organic and PFA-treated organic soil after a week of water curing is presented in Figure 5.2. The testing of PFA treated soil samples intended to evaluate the degree of improvement of compressibility behavior influenced by the stabilization of organic soil with PFA. All e-log 'v curves of the PFA treated organic soil fairly coincided with each other at the end of 100 kPa loading (Figure 5.2). The characteristics of the compression curve obtained for organic soil considered in this study were similar to previous studies on other highly organic soil (Venda Oliveira et al. 2012; Sobhan et al. 2012). A considerable impact of PFA on the compression behavior of organic soil was the increase in 'p with an increase in the amount of PFA (Figure 5.2). The value of 'p increased 86 by about 400% when the amount of PFA increased from 30% to 50% of dry soil mass (from an approximate value of 12.5 kPa at 30% PFA to 50 kPa at 50% PFA). As a result of this PFA-induced 'p was developed by the cementation process, and e-log 'v curve of the PFA stabilized organic soil shifted to higher 'v. This shifting of e-log 'v towards higher 'v indicated that stabilized soil was able to carry higher stress maintained by the normal organic soil at an equal void ratio (Figure 5.2). These observations resembled the findings reported by Tremblay et al. (2002), Tastan et al. (2011), and Venda Oliveira et al. (2012). They concluded that the treated organic soil could sustain a higher load than the untreated soil at the same value of e. Figure 5.2. Influence of PFA on compression behavior of organic soil. Figure 5.2 shows the post incremental loading (i.e. v =100 kPa) void ratio. The void ratio of stabilized organic soil was comparatively more than that of organic soil, although ei of stabilized soil was less than that of organic soil. The higher post incremental loading void ratio 87 might be attributed to the stabilized soil structure formed by the addition of PFA (Hebib and Farrell 2003; Bobet et al. 2011). Figure 5.3. Effect of surcharge load and PFA content on the change in void ratio of organic soil. 5.1.2.2 Change in void ratio Figure 5.3 presents a change in void ratio (e) value after each load increment for all soil samples treated with different PFA contents. Change in e decreased at higher PFA dosage; however, it increased at higher 'v. The value of e was obtained by subtracting e value after each load increment from the void ratio (ei) before loading; it could be used as an index to quantify the effect of PFA on the compressibility behavior of soil. Previous research also highlighted the same trend of e with an increase in a binding agent (Sobhan et al. 2012 Latifi et al. 2017). The skeleton of the soil structures became more stable as the PFA content 88 increased. The soil particles could not move or roll down easily even at higher 'v because the pozzolanic reaction induced formation and deposition of cementitious gel welded the soil particles together (Tremblay et al. 2001). The decrement of e with higher PFA dosage indicated the improvement of compressibility of organic soil when treated with a higher amount of PFA. Hence, the oedometer test results also revealed the potential use of the PFA technique for stabilizing the organic soil. 5.1.2.3 Coefficient of consolidation and compression index Variation of Cv and Cc with respect to 'v and PFA content are presented in Figure 5.4. The value of Cv and Cc was calculated for each loading stage. The value of Cv was obtained using logarithm-of-time method (Casagrande and Fadum 1940), while Cc was calculated from e-log 'v curve, using non-linear consolidation theory. The pattern of Cv of stabilized organic soil with respect to ’v was similar regardless of PFA content. The value of Cv for untreated soil at the beginning of the consolidation test i.e. at lower ’v was very high as compared to PFA-treated soil. The value of Cv decreased significantly at the beginning with an increase in normal 'v and remained almost constant at higher 'v as shown in Figure 5.4a. The rapid decrease of Cv in the lower stress range was related to the decrease of hydraulic conductivity with an increase in normal 'v, which will be discussed later. Similar to most of the soft soil, Cv was observed to increase slightly or remain constant in the higher stress range (Terzaghi et al. 1996; Bobet et al. 2011; Sobhan et al. 2012). The value of Cv decreased from about 60 mm2/min for untreated soil to 1 mm2/min - 7 mm2/mm for treated soil at 6.25 kPa as shown in Figure 5.4a. 89 Figure 5.4. (a) Variation of Cv with respect to normal stress and PFA content, and (b) Variation of Cc with respect to normal stress and PFA content. Figure 5.4b presents the relationship between Cc and 'v for both untreated and PFA-treated samples. The value of Cc at lower stress level decreased significantly with an increase in PFA-content. The value of Cc of the untreated soil when 'v was 12.5 kPa is about 3.1 and this value was about 0.1 at 40% and 50% PFA at the same stress level. The value of Cc at 40% and 50% PFA increased abruptly after 25 kPa and 50 kPa respectively which might be attributed to the breakage of bond formed by PFA or structural breakdown of treated soil at that corresponding stress level. The higher value of Cc at the beginning of consolidation might be attributed to higher ei of untreated soil and soil with low PFA. 90 Figure 5.5. Variation of mv with respect to PFA and normal stress, ’v. Figure 5.5 presents mv vs. 'v relationship for all PFA content. The value of mv at lower 'v was very sparse; however, mv at higher 'v (i.e. 100 kPa) converged to a similar value regardless of PFA content. Similar to Cc, the value of mv at 40% and 50% PFA was considerably smaller as compared to untreated soil and other composition of PFA as shown in Figure 5.5. Figure 5.6. Effect of PFA on (a) compression index (Cc) and (b) coefficient of volume compressibility (mv). 91 The value of Cc and mv presented in Figure 5.6 were calculated considering all the loading stages in e-'v curve as shown in Figure 5.2. The slope of the linear portion of e-log'v curve was taken as a Cc of the soil. Similarly, mv was obtained from e vs. 'v curve by calculating volume decrease per unit volume of soil per unit increase of effective stress. The value of Cc and mv of the untreated soil were considerably higher than that of the treated ones. The results showed decrease in both Cc and mv with increase in PFA content. The value of Cc decreased from 2.03 to 1.40 when 10% PFA was added to organic soil, while Cc remained constant or decreased slightly when PFA increased from 10% to 30%. The value of Cc further decreased from 1.30 to 0.6 with increase in PFA from 30% to 50%. The value of mv decreased abruptly when the amount of PFA exceeded 20% as shown in Figure 5.6b. The trend of mv decrement was similar to the increment of su as shown in Figure 5.1 which indicated that the effect of PFA in stabilizing the organic soil is noticeable when PFA is higher than 20%. 5.1.2.4 Hydraulic conductivity Figure 5.7 shows a change in hydraulic conductivity (k) with 'v and e for various PFA content. The results showed that the decrease in the value of k occurred with an increase in applied 'v. In general, k decreased sharply at lower stress range and moderately at higher stress range. Other researchers (Bobet et al. 2011; Venda Oliveira et al. 2012) were also observed similar trends for k values. The value of k of untreated soil decreased significantly from the initial value of 6.1×10-6 m/min at 'v = 6.25 kPa (& e = 5.2) to 2.3×10-9 m/min at 'v = 100.0 kPa (& e = 2.5). These phenomena occurred because of the reduction of e with the increment of stress as shown in Figure 5.2. The pattern of k curve is similar to the e-log'v curve as shown in Figure 5.2. The value of k of the organic soil was about ten times higher than that of the 92 stabilized organic soil at low 'v range; however, at the end of 100 kPa loading, k of the stabilized organic soil exceeded k of the organic soil. Although ei decreased gradually with increasing PFA dosage, k of the treated soil at the end of primary consolidation increased moderately with PFA percentage at any given e. These results were similar to previous studies (Hebib and Farrell 2003; Venda Oliveira et al. 2012). Hebib and Farrell (2003) and Venda Oliveira et al. (2012) claimed that the increase in k with the addition of cementing agent was attributed to the higher e at the end of consolidation and the change in internal structure (i.e. fabric) of the treated soil occurred as a result of the pozzolanic reactions. However, some other previous studies (e.g. Sakr et al. 2009; Deng et al. 2015) reported contrary results that k decreased when the soil was stabilized using chemical additives such as cement, gypsum, and fly ash. Similarly, others reported that k increased for the addition of cementing agents to a certain amount and after which it decreased (McCallister and Petry 1992; Jha and Sivapullaiah 2015). Figure 5.7. Variation of k value with applied pressure and PFA content. 93 As expected, the value of k decreased as e decreased (shown in Figure 5.7b). Previous studies reported that k value of a treated soil largely depended on the inter-aggregate pores, not intra-aggregate pores. The hydraulic conductivity at 40% and 50% PFA at a void ratio from 3.0 to 4.0 were smaller than the untreated soil and treated soil with 5%, 10%, 20%, and 30% PFA. As the hydrated PFA or pozzolanic gel began to fill the pores between soil aggregates when PFA was more than 40% of dry of mass soil and therefore, k started to decrease even under identical e conditions. Figure 5.8. (a) Change in void ratio during creep with PFA content, and (b) variation of Cα with PFA content. 5.1.2.5 Secondary compression index Creep test (i.e. secondary consolidation) was conducted at the highest 'v (= 100 kPa) of primary consolidation for 14 days to evaluate the creep behavior of the organic soil treated with PFA. The effect of PFA on the secondary compression index (C) will be discussed in this section. The effect PFA on the creep behavior of organic soil is illustrated in Figure 5.8. 94 The figure shows that at 100 kPa stress, C of untreated organic soil was about 0.14. The value of C of stabilized organic soil at 5% and 50% PFA were about 0.062 and 0.021 respectively (Figure 5.8). The decrease in C with the addition of PFA was encouraging result which implied successful adoption of the PFA to control potential creep deformation of any infrastructure constructed in organic soil (Sobhan et al. 2012; Venda Oliveira et al. 2014). 5.1.2.6 Evaluation of Cα/Cc ratio Figure 5.9 shows the trend of Cα/Cc with respect to the amount of PFA mixed with organic soil. The concept of Cα/Cc to evaluate the compressibility of soil was proposed by Mesri et al. (1997) and has been applied by several researchers (Hwang et al. 2005; Sobhan et al. 2007 and 2012; Sarsour 2014; Talib et al. 2015) on organic soil from various origins. Mesri et al. (1997) defined the type of geomaterial based on the value of Cα/Cc as shown in Figure 5.9. Peat material has the highest value of Cα/Cc, while the least value of Cα/Cc is related to granular material (Santagata et al. 2008). As the value of Cα/Cc decreases, engineering behavior of soil is considered to change from that of weak organic soil to mineral soil and ultimately to a stiff granular material (Hwang et al. 2005; Sobhan et al. 2007). Based on the description above, the alternation of organic soil treated with PFA to a granular material is shown in Figure 5.9. A favorable value of Cα/Cc was reached at PFA of 20%. With the further addition of PFA, no considerable improvements were observed. Compared to untreated organic soil which contributed to Cα/Cc =0.068, treated organic soil with 30%, 40% and 50% PFA showed better Cα/Cc with 0.02926, 0.02524 and 0.02477 individually. Results in Figure 5.9 indicate that less creep settlements developed when the organic soil was mixed with a higher dosage of PFA. For comparison purposes, the pattern of Cα/Cc of the cement-treated soil with organic content between 50%-60% and moisture content 240%-289% is also shown in Figure 5.9; these data 95 are from the preceding literature such as Hwang et al. (2005), Sobhan et al. (2012) and Sarsour (2014). Figure 5.9. Variation of C/Cc with PFA or cement content. 5.1.3 Microstructural analysis and chemical analysis 5.1.3.1 SEM analysis Figure 5.10 presents some typical images of the soil specimens after 7 days curing and consolidation test for 18 days. SEM image of the untreated soil specimen was characterized by fibers and soil particles in loose conditions (Figure 5.10a). Elongated fibrous structures were common in the organic soil sample (Figure 5.10a). In case of an untreated soil specimen, a random geomechanical arrangement with no definite orientation of microstructure could be observed. Contrary to untreated soil, as expected, a regular and smooth surface having definite 96 orientation of microstructure was observed in the PFA treated organic soil as shown in Figures 5.10 b-5.1f. Figure 5.10. SEM image of the organic soil after consolidation test (a) untreated soil, (b) with 5% PFA, (c)10% PFA, (d) 20% PFA, (e) 30% PFA, (f) 40% PFA, and (g) 50% PFA. Voids(a) (b) (c)(d) (e) (f)Fly ashBuilding thinner Skeleton structureLoosely bonded and layered soil particlesBuilding more thicker non-destructive structureFly ash coating outside layerFly ash blended cementitious gelEttringite needles like structure creating denser binder materials(g)Strong non-destructive structure97 Visible fibers in Figure 11b indicated that there was an insufficient pozzolanic reaction, and hence, the fibers were not fully coated by the new gel produced by the pozzolanic reaction and insufficient cementitious bond developed between the soil particles when PFA was 5% of dry soil mass (Figure 11b). Figure 11c showed a thin skeleton-like structure, which was a sign of forming cementitious bonds between the soil particles. With increasing PFA content, a thicker skeleton structure was seen in Figures 11d-11e, an indication of the presence of cementitious gel between the soil structure. White lumps, a product from PFA induced pozzolanic reactions (Jha and Sivapullaiah 2015; Latifi et al. 2016), were observed in the pores of organic soil. The higher amount of PFA resulted in the larger white lumps as shown in Figures 11b-11g. With an increase in PFA, it was seen from Figure 11c that there was more cementitious gel and coating of soil particles by cementitious products. Figure 11f showed that there was an ettringite needle-like structure that tends to create denser cementing materials. The needles like structure produced more cementitious gel and develop a stronger bond. 5.1.3.2 Energy Dispersive Spectroscopy (EDS) analysis EDS test was conducted to evaluate the chemical elements of the PFA-treated organic soil. Aluminum, Silica and Calcium components are more important to develop the cementitious bond between the soil particles. The weight percentages of three major components and the proportion of the Al:Si, Al:Ca and Ca:Si is presented in Figure 5.11 and Table 5 with increment in the percentage of PFA at 7 days curing. The increased amount of Al, Si and Ca in the samples indicates newly formed cementitious materials after adding PFA to organic soil. It was observed that the percentage weight of Al, Si, and Ca increased with an increase in PFA. Leong et al. (2012) and Hassan et al. (2019) presented similar chemical compositions of untreated and treated organic soil. 98 Figure 5.11. EDS spectrum of untreated and treated organic soil (a) 0% PFA-untreated soil, (b) 5% PFA, (c) 10 PFA, (d) 20% PFA, (e) 30% PFA, (f) 40% PFA, and (g) 50% PFA. 99 A considerable amount of Ca in a sample with higher PFA indicated the availability of extra PFA for pozzolanic reaction (Figure 12). This might be attributed to the shorter curing period i.e. 7 days. Better improvement in terms of compressibility and strength of the PFA-treated organic soil could be achieved if the curing period was prolonged. Table 5.1. pH value and EDS analysis of soil treated with different % of PFA at 7 days curing periods % of PFA pH AL (wt%) Si (wt%) Ca (wt%) Al:Si Al:Ca Ca:Si 0 7.19 1.9 10.42 2.99 0.18 0.64 0.29 5 7.25 2.12 12.36 2.8 0.17 0.76 0.23 10 8.97 2.5 13.65 3.68 0.18 0.68 0.27 20 9.7 2.77 14.68 4.28 0.19 0.65 0.29 30 10.07 3.76 14.9 5.1 0.25 0.74 0.34 40 10.41 4.03 16.1 9.08 0.25 0.44 0.56 50 10.84 5.03 17.12 16.43 0.29 0.31 0.96 5.1.3.3 Variation of soil pH with PFA treatment The pH of the organic and PFA-treated organic soil was measured. Normally, the pozzolanic reactions are poor in acidic conditions and more active in the alkaline environment (Tremblay et al. 2002; Wong et al. 2013). Organic soil contains mainly low pH which adversely affects the bonding between the particles. Mixing the organic soil with fly ash can increase the pH, which is enough to produce cementitious bonds by neutralizing the acid (Janz and Johansson 2002; Venda Oliveira et al. 2012). Results of pH tests of all PFA-treated organic soil specimens were in between 7 to 11 (Table 5). The pH value of the sample with 20% PFA was greater than 9 which indicated that the humic compound in the organic soil was neutralized by PFA and resulting in considerable increment of su when PFA was 20% or more as shown in Figure 5.1. 100 5.2 Effect of Alkaline Activator (AA) This section presents the results form Phase 2 experimental program. Phase 2 experimental program investigated the performance of alkaline activator on stabilization of organic soil. The alkaline activator solution used was a combination of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). NaOH of different concentrations of 8, 10, and 12 molal were prepared before testing. The value AA/PFA ratio, concentration of NaOH and curing period were varied systematically. The ratio of Na2SiO3 to NaOH solution by mass was maintained as 2:1. The undrained shear strength and compressibility of the stabilized organic soil were investigated using fall cone and 1D consolidation test. 5.2.1 Undrained shear strength The undrained shear strength of the PFA-treated organic soil estimated using depth of fall cone penetration is plotted against AA/PFA ratio in Figure 5.12. The treated soil samples were tested 7 days curing. The value of su of alkaline activated PFA mixed soil was normalized by su of the soil treated with 30% PFA (specimen without alkaline activator). To evaluate the effect of the alkaline AA/PFA ratio on su, eight different alkaline AA/PFA ratios were used. Moreover, NaOH of different concentrations of 8, 10, and 12 molal were considered. Overall, the alkaline activator exhibited adverse impact on su of the treated organic soil. Decreased in su might be attributed to disintegration of the soil matrix due to alkaline activator (Olivia 2016). In general, the ratio of AA/PFA exhibited a more profound effect on undrained shear strength of the treated organic soil as compared to the concentration of NaOH. The treated specimen with AA/PFA ratio of 0.2 and 0.3 exhibited lowest strength (Figure 5.12). The treated specimen with AA/PFA ratio of 0.5 and 10M NaOH concentration exhibited higher strength 101 among the tested specimens. However, the undrained shear strength of the specimen with AA/PFA ratio of 0.5 and 10M NaOH concentration was marginally higher than undrained shear strength of the specimen with 30% PFA and without alkaline activator. Alkaline activator induced low strength and compressibility might be attributed to the alkali environment of the stabilized, low curing temperature, and short curing period (Olivia 2016; Nath et al. 2016). Figure 5.12. Normalized undrained shear strength for various AA/PFA and NaOH concentration. Figure 5.13 shows normalized undrained shear strength of the treated soil obtained after 0, 7 and, 14, and 28 days curing. The value of su of alkaline activated PFA mixed soil was normalized by su of the soil treated with 30% PFA (specimen without alkaline activator). No distinct correlation was observed between curing period and su. There was a significant decrease in su of the stabilized specimen with alkaline activator/PFA ratio 0.5 from 14 days to 28 days of curing. Decrease in su with curing period might be attributed to recrystallization of the CSH to zeolites (Brough and Atkinson 2002). 102 Figure 5.13. Effect of curing period on undrained shear strength. 5.2.2 Compressibility behaviour Figure 5.14 compares the consolidation settlement of the untreated and treated specimens after 7 days of curing. The consolidation settlement is normalized with the initial height of the specimen. The consolidation test results revealed that the use of alkaline activator did not have any improvement in compressibility behaviour of the treated organic soil as compared to the treated organic soil with 30% PFA. For untreated specimen under effective stress of 100 kPa, normalized consolidation settlement was equal to 0.47, which indicates a total strain of 47% at an effective stress of 100 kPa. The normalized consolidation settlement reduced to 0.32 at 30% of PFA which means almost 32% of consolidation settlement was prevented. Addition of alkaline activator with 30% PFA did not exhibit reduction in consolidation settlement compared to the treated specimen with 30% PFA. Out of six test results of alkaline activated specimens presented in Figure 5.14, the treated specimen with AA/PFA ratio of 0.3 and 10M NaOH concentration 103 demonstrated the lowest consolidation settlement. However, the consolidation settlement of the specimen with AA/PFA ratio of 0.3 and 10M NaOH concentration was marginally lower than the specimen with 30% PFA and without alkaline activator. This might be attributed to the improvement of compressibility behaviour at that particular quantity and concentration of alkaline activator. Figure 5.14. Consolidation settlement of the samples treated different concentration and dosage of alkalinity. Therefore, it seems the organic soil specimen prepared with alkaline activator within scope of this research is relatively unstable and disintegrate the soil matrix into compressible structures. Various combinations of alkaline activator and PFA need to be investigated to obtain the most effective combination of alkaline activator and PFA. As the effect of alkaline activator on stabilization of the organic soil was not positive, further analysis to obtained 104 consolidation parameters were not carried out. Similarly, SEM/EDS and XRD test were not conducted on post consolidated sample. 5.3 Summary This chapter presented the results from Phase 1 and Phase 2 of experimental program. The effect of PFA on strength, compressibility and microstructural behaviour of the organic soil was investigated in phase 1. Similarly, the performance of alkaline activator on strength and compressibility behaviour of the organic soil was investigated in Phase 2. Series of fall cone and 1D consolidation tests were conducted in both Phase 1 and Phase 2. In addition to fall cone and oedometer tests, SEM/EDS and XRD tests were carried to investigate the microstructural behaviour of PFA treated organic soil in phase 1. The results suggested that when the organic soil was treated with PFA, its undrained shear strength and compressibility behavior were improved. Moreover, a potential large settlement of organic soil can be reduced by adding PFA to organic soil. The value of Cc and Cα was decreased significantly when compared to untreated organic soil, indicating that the PFA stabilizer can effectively improve the creep behavior of organic soil. This study implies that beneficial utilization of PFA for stabilization of organic soil could be a practical step towards environmentally sound management of this waste by-product. Contrary to PFA, alkaline activator did not exhibit any improvement on strength and compressibility of the organic soil. 105 Chapter 6: Performance of Calcium Bentonite and Pulp Mill Fly Ash This section presents the results from Phase 3 experimental programs. Phase 3 experimental program consisted of series of tests on untreated and calcium bentonite (Ca-bentonite) and pulp mill fly ash (PFA) treated organic soil. A various mix design was prepared for the laboratory tests with the addition of 5%, 8%, 10%, 12%, 15%, and 20% Ca-bentonite with constant 30% PFA to develop the optimum mix design for organic soil necessary to achieve the desired engineering behavior. Fall cone and one-dimensional oedometer tests were carried out to evaluate the undrained shear strength and compressibility behavior of the stabilized organic soil. Additional fall cone tests were conducted on the treated specimen to investigate the effect of curing period on shear strength of the treated soil. A series of scanning electron microscope (SEM) tests with energy dispersive X-ray (EDS) and X-ray diffraction (XRD) experiments were conducted to investigate the microstructural and chemical aspects of the organic soil treated with Ca-bentonite and PFA. 6.1 Undrained shear strength Undrained shear strength (su) of the treated and untreated soil was estimated using the depth of penetration of the fall cone after 7 days of curing (Equation 4.1). Figure 6.1 shows the variation of su of the treated soil with varying Ca-bentonite dosage and constant 30% PFA. The undrained shear strength of organic soil treated with Ca-bentonite and 30% PFA binders was normalized by the su value of the untreated soil. 106 Figure 6.1. Variation of undrained shear strength (su) with % of Ca-bentonite. Results suggest that the value of su increased to almost 2.5 times that of organic soil when 30% of PFA was added to soil without any Ca-bentonite. The fall cone test results revealed that Ca-bentonite dosage had a significant influence on strength development (Figure 6.1) regardless of the curing method. The value of su was observed to increase with an increase in Ca-bentonite dosage up to 10% but then decreased at 12% Ca-bentonite. The increase in su upon addition of Ca-bentonite can be attributed to the presence of a certain amount of Si, Al, and Ca within bentonite which enhanced the pozzolanic reaction between the soil, PFA and Ca-bentonite during the curing period. Results from Phase 1 experimental program that su increased by 2.5 and 3.25 times as compared to untreated organic soil when PFA content was 40% and 50% of dry soil mass, respectively. The value of su at 10% of Ca-bentonite and 30% of PFA was about 2.2 and 1.7 times of the su achieved at 40% and 50% PFA. The fall cone test results clearly indicate the 107 benefit of using two binders i.e. PFA and Ca-bentonite. This beneficial effect might be attributed to the transformation of the soil due to Ca-bentonite that allows better distribution of PFA and increases its effectiveness. The fines in Ca-bentonite works as a filler to reduce the voids of the stabilized soil (Cristelo et al. 2012). Figure 6.2. Comparison of consolidation settlement of the specimens cured by water and air. 6.2 Effect of curing method Figure 6.1 also illustrates the effects of the curing method on su of the soil treated with Ca-bentonite and constant 30% PFA. The trend of su with varying Ca-bentonite dosage regardless of the curing method is consistent, as shown in Figure 6.1. There was no significant difference between su of water-cured and air-cured specimens for all tests. For example, in the case of the air-cured specimen with 10% Ca-bentonite, the normalized su of the specimen was 5.8, an almost similar normalized su of 5.6 achieved using the water curing method. Figure 6.2 compares the consolidation settlement of the specimens cured by water and air. The 108 consolidation settlement is normalized with the initial height of the specimen. No significant difference was observed in the consolidation settlement upon both incremental loading and creep loading due to the curing method. 6.3 Compressibility behavior 6.3.1 Consolidation settlement Figure 6.3 shows that the primary consolidation behavior of stabilized organic soil was strongly affected by the Ca-bentonite dosage under incremental loading. Normalized consolidation settlement was obtained by dividing the consolidation settlement by the initial height of the sample. Even at a lower normal stress of 12.5 and 25 kPa, the organic soil showed a significant volumetric compression. The consolidation test results revealed that the addition of different dosages of Ca-bentonite and 30% of PFA controlled the large settlement of the soil. The specimen with Ca-bentonite and 30% PFA did not exhibit any significant deformation until the applied load was 12.5 kPa. For the untreated specimen under effective stress of 100 kPa, the normalized consolidation settlement was equal to 0.47 at an effective stress of 100 kPa. Normalized consolidation settlement was reduced by 55% at 10% Ca-bentonite and 30% PFA as compared to untreated organic soil. Of the various percentages of Ca-bentonite with 30% PFA, the amount of 10% Ca-bentonite exhibited the best result, although other mix designs produced favorable results when compared to untreated organic soil. It is important to note that any increase in the amount of Ca-bentonite beyond 10% did not have a major impact on reducing soil settlement. 109 Figure 6.3. Effect of Ca-bentonite dosage on the primary consolidation behavior of treated and untreated organic soil. 6.3.2 Stress-strain behavior Figure 6.4a shows the stress-strain curves (compression curve) resulting from the 1D consolidation tests plotted in the form of void ratio (e) vs. logarithm of the effective stress (σv). No pre-consolidation stress (stress history) was apparent in the e vs. log σv plot for the untreated organic soil as a decrease in e was fairly linear with σv (Figure 6.4a). However, pre-consolidation stress ('p) appeared to be observed in the case of the stabilized soil. The value of p was found as 25 kPa at 10% Ca-bentonite and 30% of PFA treated soil as shown in Figure 6.4a. The development of p was attributed to the Ca-bentonite and PFA-induced cementing process between soil particles. The stress-strain curve (e vs. log σv) of the treated organic soil shifted to a higher σv, indicating that treated soil was able to carry higher stress sustained by the untreated organic soil at a given e (Figure 6.4a). Figure 6.4a shows the higher cementation-induced pre-consolidation stress at 10% Ca-bentonite and 30% PFA. The higher 110 pre-consolidation stress at 10% Ca-bentonite and 30% PFA reconfirms the optimum dosage of Ca-bentonite as 10%, as was also observed in fall cone test. Though ei of the treated soil was considerably less than the untreated soil, the post-primary consolidation (i.e. v =100 kPa) void ratio was more than that of untreated organic soil (Figure 6.4a). The higher post-primary consolidation void ratio might be attributed to the formation of a stabilized soil structure by cementing process (Bobet et al. 2011; Tastan et al. 2011). Figure 6.4. (a) Compression curve from 1D consolidation test and (b) void ratio after each loading increment. 6.3.3 Change in void ratio Figure 6.4b shows the change in void ratio (e) at each loading stage. The difference between the initial void ratio before loading (ei) and void ratio following each load increment was considered as e. The change in e of soil after each loading increment decreased significantly for treated soil. The value of e was observed to be minimum at 10% Ca-bentonite and 30% PFA. However, when the Ca-bentonite dosage exceeded 10% of the soil dry mass, e increased 111 slightly with increased Ca-bentonite dosage. Figure 6.4b confirms that Ca-bentonite and PFA binders resulted in the soil becoming less compressible. The cementitious gel produced by the pozzolanic reaction developed a more stabilized soil structure and caused a minimal change in the void ratio compared to the untreated organic soil under similar stress conditions (Hebib and Farrell 2003, Kang 2011; Bobet et al. 2011). Figure 6.5. Variation of Cc with normal stress and Ca-bentonite dosage. 6.3.4 Primary compression index, coefficient of consolidation and constrained modulus The variation of primary compression index (Cc) with respect to v and Ca-bentonite dosage are presented in Figure 6.5. The value of Cc was calculated from the slope of the e vs. log v curve (Figure 6.4a) for each load increment. The value of Cc increased slightly with an increase in v, as shown in Figure 6.5, indicating that the compression behavior of the organic soil might be affected by higher v. As the Ca-bentonite dosage increased, Cc decreased at the same stress condition, and the change in Cc reduced significantly with an increase in v. The 112 increment of Cc was minimal when 'v increased from 50 kPa to 100 kPa as shown in Figure 6.5. The value of Cc of the untreated specimen at 'v =12.5 kPa was 3.1 and decreased to 0.1 when 10% Ca-bentonite was added as a binder on top of 30% PFA. The results indicate that the potential for large primary consolidation settlement of organic soil can be prevented by adding Ca-bentonite and PFA. The coefficient of consolidation (Cv) was obtained using the logarithm-of-time method (Casagrande and Fadum 1940) for each loading increment. The increment of Cv was minimal when 'v increased from 50 kPa to 100 kPa as shown in Figure 6.6a. The value of Cv was observed to increase with an increase in Ca-bentonite dosage up to 10%, but then decreased at 12%. The value of Cv at 100 kPa increased by about 15 times from its untreated one when 10% Ca-bentonite and 30% PFA were added to the soil. Figure 6.6. Variation of (a) coefficient of consolidation and (b) constrained modulus with normal stress and Ca-bentonite dosage. 113 The variation of constrained modulus (M) with effective stress and Ca-bentonite dosage with a constant 30% PFA is presented in Figure 6b. Constrained modulus (M) is defined as the inverse of the coefficient of volume compressibility (mv). The value of M increased as v increased. The observed response is expected considering cumulative compaction (increase in density) of the specimen under each loading increment. As expected, the treated soil exhibited a stiffer response with higher M. In general, the soil treated with 10% Ca-bentonite and 30% PFA showed a higher value of M at all loading stages. The values of Cc and M presented in Figure 6.7 were calculated from the entire range of the e vs. 'v curve shown in Figure 6.7a. The value of Cc reduced from 2.05 to 1.3 at 30% PFA and a further reduction in Cc was observed with the addition of Ca-bentonite (Figure 6.7a). The value of Cc at 10% Ca-bentonite and 30% PFA decreased by 52% as compared to untreated organic soil, reaching to about 1.0 as shown in Figure 6.7a. The optimum dosage of Ca-bentonite with 30% PFA achieving minimum Cc was observed as 10% of the soil dry mass. The value of Cc began to increase when the Ca-bentonite exceeded 10% of the soil dry mass. The value of M for the untreated organic soil was observed to be about 90 kPa (Figure 6.7b). The value of M increased by about 80% when 30% PFA was added to the organic soil, as compared to untreated organic soil. Overall, the treated specimen showed a stiffer response with Ca-bentonite. The value of M ramped up 10% with an increase in Ca-bentonite dosage. However, the value of M started to decrease at 12% Ca-bentonite. The value of M at 10% Ca-bentonite and 30% PFA increased by 3.2 and 1.9 times as compared to untreated organic soil and organic soil treated with 30% PFA, respectively. 114 Figure 6.7. Variation of (a) constrained modulus and (b) primary compression index with Ca-bentonite dosage. 6.3.5 Hydraulic conductivity Hydraulic conductivity of the specimen following each load increment was estimated by the coefficient of permeability (k) using the results from 1D consolidation tests on both treated and untreated specimens. Hong et al. (2012) reported that k measured in a flexible-wall cell correlates well with k measured in fixed-ring oedometer cell at the same stress condition. The variation of k with σv and e is presented in Figure 6.8. As illustrated in Figure 6.8a, the measured k decreased with an increase in σv, as expected based on the inverse relationship between v and e (Figure 6.4a). For an example, the value of k of untreated specimen decreased by 103 times from the initial value of 6.1×10-6 m/min at 'v = 6.25 kPa (& e = 5.2) to 2.3×10-9 m/min at 'v = 100.0 kPa (& e = 2.5). The hydraulic conductivity showed an abrupt decrease at lower stress range and moderately at a higher stress range. The hydraulic conductivity decreased with a decrease in e due to an increase in effective stress (Figure 6.8b). 115 Figure 6.8. Hydraulic conductivity of the samples as a function of (a) normal stress and (b) void ratio at the end of each loading stage. The hydraulic conductivity of the stabilized organic soil specimen at 6.25 kPa effective stress was observed to be reduced by up to 102 times in comparison to that of the untreated specimen under the same stress condition. However, the value of k of the treated specimen was greater than that of the untreated specimen at 100 kPa effective stress. This behavior was expected as the change in void ratio for untreated soil was significantly greater than treated soil samples as shown in (Figure 6.4b). The increase in k might be attributed to an increase in void ratio cased by an increase in micropores of stabilized soil and change in soil fabric. Micropores were developed due to pozzolanic reaction with Ca-bentonite and PFA and served as the main channels for flow (Bobet et al. 2011). The increase in k with Ca-bentonite is in good agreement with several previous studies (Cortellazzo et al. 1999; Hebib and Farrell 2003; Hwang et al. 2005). 116 6.3.6 Creep Deformation Previous studies showed that creep due to secondary consolidation can contribute to half of the total consolidation settlement in organic soil. Creep behavior of the stabilized soil is a crucial parameter for evaluating the effectiveness of binder material on soil stabilization (Mesri et al. 1997; O’Kelly 2005; Venda Oliveira 2012). The influence of the Ca-bentonite dosage on creep deformation at 100 kPa vertical stress is illustrated in Figure 6.9. The creep deformation was reduced significantly for the specimen with 30% PFA. Minimal creep deformation was noted at 10% Ca-bentonite and 30% PFA, then increased slightly at 12% Ca-bentonite and 30% PFA. As compared to untreated organic soil, the creep deformation reduced by 56% when 10% Ca-bentonite was added to the soil mixed with 30% PFA. The current results are promising for adopting Ca-bentonite and PFA binders on improving the compressibility behavior of organic soil. Figure 6.9. Influence of Ca-bentonite content on one-dimensional creep. 117 Figure 6.10. Variation of Cα/Cc with Ca-bentonite dosage and compassion of Cα/Cc with previous research. Figure 6.10 presents the variation of Cα/Cc with Ca-bentonite dosage. The value of Cα/Cc has been used to characterize or define the type of soil (Figure 6.10), was initially proposed by Mesri et al. (1997), and has since been applied by several researchers (Hwang et al. 2005; Sobhan et al. 2007 and 2012; Sarsour 2014; Talib et al. 2015). The value of Cα (= Δe/Δlogt) was calculated from the slope of the e-logt curves for 14 days at 100 kPa effective stress. Material such as peat shows considerable creep under constant loading, and has the highest value of Cα/Cc. As the creep of granular material is not significant, granular material has the lowest value of Cα/Cc. As the value of Cα/Cc decreases, creep behavior of soil is considered to change from that of weak organic soil to mineral soil and ultimately to a stiff granular material (Hwang et al. 2005; Sobhan et al. 2007; Talib et al. 2015). Previous studies by Hwang et al. (2005) and Sobhan et al. (2012) showed that the value of Cα/Cc decreased with an increase in the dosage of binder material (Figure 6.10). The value of Cα/Cc of untreated 118 organic soil was about 0.06 which reduced to 0.029 with the addition of 30% PFA. With the addition of 5% Ca-bentonite, the value of Cα/Cc reduced to about 0.020. Less creep settlements developed when the organic soil was stabilized with Ca-bentonite. No considerable improvements were observed with further addition of Ca-bentonite. 6.4 Morphological and chemical analysis 6.4.1 SEM/EDS analysis The morphology and chemical composition were compared between the untreated and treated specimens. The micrograph of the untreated and treated specimens obtained from SEM image are presented in Figures 6.11-6.14. The chemical composition of the untreated and treated organic soil obtained from the EDS analysis are presented in Table 6.1. The SEM image of the untreated organic soil (Figure 6.11) shows a highly porous structure with coarse organic particles. Figure 6.11. Micrograph and chemical composition of untreated organic soil. 119 Figure 6.12 shows the SEM image of organic soil treated with 30% PFA. The change in microstructures, such as a less porous structure with white lumps produced on the surface, can be observed in Figure 6.12. The lumps are new cementing materials such as calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH) (Jha and Sivapullaiah 2018). The EDS analysis also revealed the formation of new cementing materials (Figure 6.12) after addition of 30% PFA. The SEM image of the soil treated with 10% Ca-bentonite and 30% PFA (Figure 6.13) illustrates the compact and dense microstructure with heavily formed ettringite crystals (ET) and large calcium silicate aluminate hydrate (CASH) materials. The ettringite crystals play an important role in gaining soil strength. The chemical composition of heavily formed ET and CASH is shown in Figure 6.13. Increased percentage weight of Si, Al, and Ca in a Ca-bentonite treated soil specimen indicates an additional supplement of ions, compared to untreated soil. Dissolved Ca++ and liberation of (OH-) ions help to increase the pH of soil and produces cementing materials such as CSH, CAH, CASH, and ET. Figure 6.12. Micrograph and chemical composition of new materials of 30% PFA treated soil. 120 Figure 6.13. Micrograph and chemical composition CASH and ET materials from soil treated with 10% Ca-bentonite with 30% PFA. Figure 6.14. Micrograph and chemical composition soil treated with 15% Ca-bentonite with 30% PFA. The EDS test results in Table 6.1 show that Si, Al, and Ca were the dominant chemical elements in the stabilized soil. The percentage of Si, Al, and Ca continuously increased with an increase in Ca-bentonite dosage. All three elements play a crucial role in the pozzolanic 121 reaction to form the cementitious materials. Table 6.1 shows that the treated soil at 10% Ca-bentonite and 30% PFA had the lowest ratio of Ca:Si. The decrease in Ca:Si ratio can be related to the development of strength with the addition of 10% Ca-bentonite with 30% PFA (Kunther et al. 2017). The lowest value of Ca:Si indicates the formation of CASH and CSH materials due to the dissolution of Si and Al from the soil with addition of Ca-bentonite. Kunther et al. (2017) reported that a decrease in Ca:Si and an increase in Al:Si and Al:Ca result in a higher compressive strength due to the formation of CASH and CSH. The lowest value of Ca:Si at 10% Ca-bentonite and 30% PFA indicated the optimum dosage of Ca- bentonite in terms of hydration. Table 6.1. EDS analysis of soil treated with different % of Ca-bentonite + 30% PFA. Binder composition pH AL (wt%) Si (wt%) Ca (wt%) Al:Si Al:Ca Ca:Si Untreated 7.19 1.9 10.42 2.99 0.18 0.64 0.29 0% Ca-bentonite + 30% PFA 10.07 3.76 14.9 5.1 0.25 0.74 0.34 5% Ca-bentonite + 30% PFA 10.47 4.3 15.02 8.2 0.29 0.52 0.55 8% Ca-bentonite + 30% PFA 10.05 4.35 15.66 8.35 0.28 0.52 0.53 10% Ca-bentonite + 30% PFA 10.59 5.09 16.33 8.38 0.31 0.61 0.51 12% Ca-bentonite + 30% PFA 10.62 5.11 16.37 8.75 0.31 0.58 0.53 15% Ca-bentonite + 30% PFA 10.68 5.51 17 11.81 0.32 0.47 0.69 20% Ca-bentonite + 30% PFA 10.79 5.94 18.6 13.8 0.32 0.43 0.74 Hydrated Ca-bentonite remained unreactive with soil particles with addition of excessive Ca-bentonite (>10%). Figure 6.14 shows the soil layer covered with hydrated Ca-bentonite and PFA with addition of 15% Ca-bentonite with 30% PFA. The unreactive Ca-bentonite acts as a lubricant and can fill the pores of soil particles but does not have any 122 cementing properties. The unreactive hydrated paste is weak in undrained shear strength and highly compressible and might be responsible for degrading the strength and compressibility behavior of the treated specimen when Ca-bentonite exceeded 10% of the dry mass of soil. 6.4.2 X-ray diffraction (XRD) analysis Figure 6.15 shows a typical XRD diffraction spectrum for untreated organic soil, soil treated with 30% PFA, and soil treated with 10% Ca-bentonite and 30% PFA. The physicochemical reaction occurs between the binders and soil during the curing period resulting in phase changes that reflect the formation of cementing materials (Yi et al. 2014). Figure 6.15. X-ray diffraction spectrum for untreated soil, soil treated with 30% PFA and soil treated with 10% Ca-bentonite and 30% PFA. 123 The XRD graphs presented in Figure 6.15 illustrate the formation of CASH, CSH, and ET by hydration with the addition of Ca-bentonite and PFA in organic soil. A peak of clinochlore was seen at 2 7.21° on untreated organic soil, while in treated soil, the peak was eliminated, as shown in Figure 6.15. The depletion of the peak was observed after adding the binder materials due to the physicochemical reaction between the binder materials and the organic soil during the curing period. A phase change in mineralogical soil was observed, reflecting the formation of new cementing materials (Yi et al. 2014). A peak of montmorillonite at 2 of 6.52° observed in Ca-bentonite (Figure 3.3) was reduced by further dissolution during the curing process and formed CSH at 2 of 6.92°. ET was observed at 2 of 13.50°- 48.98. Likewise, dissolution of opaline silica, calcite, and cristobalite and formation of new sharp peaks of CSH and CASH were observed. The CSH formed at 2 of 7.33°-76.03°. CASH formed with multiple diffractions at 2 of 34.15° as shown in Figure 6.15. Previous research reported that a reduction of sharp peaks of montmorillonite, opaline silica, calcite, and cristobalite during the curing process was associated with strengthening of the treated soil (Jha and Sivapullaiah 2018; Muhammad and Siddiqua 2019). The newly formed sharp peaks in the stabilized soil resulted due to alternation of minerals and indicate the formation of new crystal materials in stabilized soil. Moreover, an enlarged view of the diffraction pattern shows the formation of CASH, CSH, and ET more precisely (Figure 6.15). The formation of new crystal minerals in treated organic soil at 10% Ca-bentonite and 30% PFA resulted in low compressibility and higher undrained shear strength. XRD analysis revealed that PFA and Ca-bentonite can be effective binders for organic soil stabilization since Ca-bentonite and PFA enabled the formation of CSH, CASH, and ET when mixed with organic soil. 124 6.4.3 Variation of soil pH with Ca-bentonite The pH of the organic and treated organic soil was measured. Normally, the pozzolanic reactions are active in the alkaline environment rather than in acidic conditions (Tremblay et al. 2002; Wong et al. 2013). Organic soil is acidic in nature which adversely affects the pozzolanic reactions. Mixing the organic soil with Ca-bentonite and fly ash can increase the pH, resulting in favorable environment to develop cementitious bonds by neutralizing the acid (Janz and Johansson 2002; Venda Oliveira et al. 2012). Results of pH tests of treated organic soil specimens were in between 7 to 11 (Table 6.1). The pH value of the specimen with 5% Ca-bentonite and 30% PFA was observed greater than 9 indicates that the humic compound in the organic soil was neutralized by Ca-bentonite and PFA and causing in considerable increment of su. 6.5 Summary This chapter presented results the effect of Ca-bentonite and PFA on the strength, compressibility behavior, and microstructures of organic soil. A mix design was prepared for the laboratory tests with various dosages of Ca-bentonite with a constant amount of 30% PFA. Fall cone and one-dimensional oedometer tests were carried out to evaluate the undrained shear strength and compressibility behavior of the stabilized organic soil. Similarly, a series of SEM/EDS and XRD experiments were conducted to investigate the microstructural and chemical aspects of the organic soil treated with Ca-bentonite and PFA. A mixture of 10% Ca-bentonite with 30% PFA was found as the optimum dosage for improving the undrained shear strength and compressibility of organic soil regardless of the curing method. Change in the mineralogical and morphological contents of the stabilized organic soil were observed due to 125 the pozzolanic reaction. The results showed that Ca-bentonite and PFA were able to considerably improve the strength and compressibility of the selected organic soil. 126 Chapter 7: Conclusions and Recommendations 7.1 Summary Soil with higher organic content is typically weak and highly compressible, making construction of roads and pipelines over organic soils problematic. Engineering properties of organic soils are not well documented and study on the stabilization of organic soil is very sparse. Therefore, a well-developed methodology for the stabilization technique of organic soil is essential to prevent failures of infrastructure and to minimize damage to infrastructure. A common practice for building infrastructure on organic soil involves preloading of soil or excavation and replacement of organic soil with granular materials. These approaches are time-consuming and costly, while the chemical stabilization technique is considered as a more viable option in terms of cost and time (Kazemian 2017). The aim of this research was to asses the effectiveness of PFA, alkaline activator, and Ca-bentonite on the strength and compressibility behavior of a natural soil having 26% organic content (typically found in the Wabasca wetlands, Northern Alberta, Canada). This goal was achieved by conducting a series of fall cone and 1D consolidation tests followed by microstructural and morphological tests through a carefully designed experimental program. The experimental program consisted of three phases: Phase 1, Phase 2, and Phase 3. The principal goal of Phase 1 was to investigate the effect of PFA on the strength and compressibility behavior of organic soil. Phase 2 of the experimental program was designed to use the alkaline activator to accelerate and improve the pozzolanic reaction of PFA in order to achieve higher strength and better compressibility behavior of the stabilized soil. Phase 3 of 127 the experimental program was designed to investigate the effect of Ca-bentonite on the strength and compressibility behavior of organic soil mixed with a constant amount of PFA of 30% of the soil dry mass. 7.2 Scientific contributions There exists limited information on the stabilization of organic soil in the literature, and expert knowledge and advanced testing methods are required for optimizing the stabilization techniques. In this research, an effort was taken to minimize this gap in knowledge while also generating new experimental data. The extensive study on the stabilization of organic soil was carried out through laboratory tests and microstructural and chemical analysis and made a significant contribution to the body of knowledge of chemical stabilization techniques for organic soil. A significant contribution of this research is the potential use of PFA and Ca-bentonite as new binders to stabilize the organic soil. This research was a first attempt in stabilizing the organic soil using Ca-bentonite. The output of the research can be applied in developing standard design guidelines and laboratory procedures for the performance evaluation of stabilized organic soil. 7.3 Major findings The effect of PFA, alkaline activator, and Ca-bentonite on the strength and compressibility behavior of organic soil were investigated through a series of laboratory tests and microstructural and chemical analysis. The experimental program was comprised of fall cone tests, one-dimensional consolidation tests, and SEM/EDS and XRD tests for monitoring microstructural and chemical evolution. The results obtained were analyzed in terms of the undrained shear strength, primary and secondary (creep) compression index, coefficient of 128 consolidation, ratio of secondary compression index to primary compression index, and the change in microstructure and chemical composition of the stabilized soil. 7.3.1 PFA stabilization Organic soil was mixed with different dosages of PFA varying from 5%, 10%, 20%, 30%, 40%, and 50% of the soil dry mass. The following conclusions were drawn from PFA stabilization. 1. Higher undrained shear strength was achieved with an increase in PFA dosage. The value of su was 3 times that of organic soil when mixed with 50% PFA. From this observation, it can be concluded that PFA stabilization of organic soil offers an environmentally friendly alternative to more traditional cement and lime stabilization. No significant difference in su was noted between the organic soil mixed with PFA cured for 7 days and 14 days. 2. PFA-treated soil shifted to higher 'v at an equal void ratio, indicating the formation of stable soil structures. A promising effect of PFA stabilization on the compressibility of the organic soil was the increase in the PFA-induced pre-consolidation pressure, 'p. 3. The value of k and Cv of the soil increased with an increase in PFA dosage. Higher void ratio and change in the soil fabric resulted in a higher k. The value of Cc and mv of the treated soil with 50% of PFA decreased by approximately 4.0 and 5.0 times compared to the Cc and mv of the untreated soil. 4. The secondary compression index C decreased with an increase in PFA content by as much as a factor of 4 at 50% PFA. The value of C/Cc also decreased with the addition of PFA. The C/Cc went from 0.06 to 0.02713 on the untreated organic soil with 50% 129 PFA. Stabilized organic soil with 20% of PFA showed inorganic or granular soil type behavior. 5. The microstructural study revealed that the newly formed cementitious materials bonded the soil particles together, resulting in the improvement of strength and compressibility of treated soil. Gel-like cementitious products were visualized in the SEM image. The SEM image of the untreated soil was characterized by fibers and soil particles in loose conditions with a more discontinuous structure. The treated soil samples exhibited elongated fibrous structures with white lumps of cementitious product. 6. A considerable amount of Ca in the sample with higher PFA indicated the availability of extra PFA for pozzolanic reactions even after 7 days curing and 14 days of consolidation testing. Better improvement in terms of compressibility and strength of the PFA-treated organic soil could be achieved if the curing period was prolonged. Based on the results from the series of tests performed on the PFA stabilized organic soil, it can be concluded that improved strength and compressibility of organic soil can be achieved by using PFA, however the degree of improvement depends on the amount of organic content in the soil, characteristics of organic soil, and PFA as well as the amount of PFA. 7.3.2 Alkaline activator stabilization A various mix design at different AA/PFA ratios, molarity, and Na2SiO3/NaOH ratios were prepared and cured for 7 and 28 days. The following conclusions were drawn from the test on the alkaline activated fly ash-treated organic soil. 1. Addition of alkaline activators to PFA-treated organic soil induced low strengths after 7 days curing due to the low reactivity of silica and alumina present in the organic soil. 130 2. Within the scope of this work, no improvement was observed in the strength and compressibility behavior of PFA-treated soil with the addition of alkaline activator. 3. The alkaline activator-induced low strength and compressibility might be attributed to the alkali environment, low curing temperature, and short curing period. 7.3.3 Ca-bentonite and PFA stabilization The various mix design was prepared for the laboratory tests with the addition of 5%, 8%, 10%, 12%, 15%, and 20% Ca-bentonite with constant 30% PFA. The following conclusions were drawn from the test on Ca-bentonite and PFA-treated organic soil. 1. Addition of Ca-bentonite with 30% PFA to organic soil acted as a binder and increased the undrained shear strength. The undrained shear strength of stabilized soil increased by 5.8 times when organic soil was mixed with 10% Ca-bentonite and 30% PFA. The undrained shear strength increased with an increase in Ca-bentonite dosage with 30% PFA and with optimum Ca-bentonite 10% of soil dry mass. 2. No significant effect of the curing method was observed on both the undrained shear strength and compression behavior. This lack might be attributed to the higher natural water content of the organic soil. 3. Both the primary compression index (Cc) and the secondary compression index (Cα) were significantly reduced by Ca-bentonite and PFA stabilization. The value of Cc at 10% Ca-bentonite and 30% PFA decreased by 55% as compared to untreated organic soil. The creep deformation reduced by 56% when 10% Ca-bentonite and 30% PFA were added to the soil. 131 4. The value of k and Cv of the soil increased with an increase in PFA dosage. Higher void ratio and change in the soil fabric resulted in a higher k. The value of M at 10% Ca-bentonite and 30% PFA increased by 3.2 times as compared to untreated organic soil. 5. The optimum content of Ca-bentonite with 30% PFA for the maximum undrained shear strength and minimum compression index of stabilized soil was found to be 10% of the soil dry mass. A compact and dense microstructure with fewer pores was observed at 10% Ca-bentonite and 30% PFA. 6. The lower Ca:Si rato in the specimen treated with 10% Ca-bentonite and 30% PFA could be attributed to further pozzolanic reactions occurring when Ca-bentonite was added to the organic soil. Microstructural analysis confirmed that Si and Ca available in Ca-bentonite contributed to increase the undrained shear strength and to reduce the compressibility. 7. The XRD analysis showed the formation of calcium aluminate silicate hydrate (CASH), calcium silicate hydrate (CSH), and ettringite (ET) by hydration with addition of Ca-bentonite and PFA in organic soil. The CASH, CSH, and ET each contributed to improve the undrained shear strength and compressibility of the organic soil. Based on the results from the series of tests conducted on Ca-bentonite and PFA stabilized organic soil, it can be concluded that the Ca-bentonite and PFA can be utilized to stabilize the organic soil which covers significant land in Canada and USA. The use of Ca-bentonite and PFA in soil stabilization can be a sustainable approach to address the financial and ecological concerns regarding the use of OPC and lime. Moreover, the utilization of PFA for stabilization of organic soil could be a practical step towards environmentally sound management of this waste by-product. 132 7.4 Limitations and future works Organic soil is known to be a problematic soil in the geotechnical industry. Several difficulties were faced in this study as well, while testing the organic soil. A description of some of these limitations and recommendations for further studies are as follows: 1. Alkaline-activated fly ash samples were cured at room temperature. A range of curing temperatures could be tested to obtain a better understanding of the strength and compressibility behavior of the organic soil. 2. The maximum curing period considered in this research was 28 days. Most of the tests were conducted after 7 days of curing. Since the PFA and alkaline activator induced shear strength and compressibility are time dependent, the effect of the curing period should be investigated considering a longer curing period. 3. Sample was compacted to in-situ density at natural water content. Samples could be compacted to various densities at different water contents to determine the effect of density and initial water content on the strength and compressibility behavior of the soil. 4. The optimum amount of Ca-bentonite with constant 30% PFA was found in this study. Various combinations of Ca-bentonite and PFA need to be investigated to determine the most effective combination of Ca-bentonite and PFA. 5. The organic content of the soil used in this study was 26%. The behavior of a broader range of organic soil was not evaluated in this study. A comprehensive study could be performed to investigate the effects of organic content on the undrained shear strength and compressibility behaviour of the stabilized soil. 133 6. No improvement was observed in the strength and compressibility behavior of PFA-treated soil with the addition of alkaline activator. The ratio of Na2SiO3: NaOH was 2:1 for all mix designs. A few trials of the laboratory tests were conducted on alkaline activator-stabilized soil. However, various ratios of alkaline activator solution could be added at a constant percentage of PFA to understand the effect of alkaline activator. Similarly, varying ratios of alkaline activator to PFA and concentrations of NaOH could be systematically varied to activate PFA effectively. 7. 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Thesis/Dissertation
eng
Civil Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Effect of pulp mill fly ash and calcium bentonite on the mechanical and microstructural behavior of organic soil from Alberta, Canada
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