Numerical Modelling of Shaft Lining Stability Prepared by Yudan Jia BEng in Civil Engineering MSc in Structural Engineering Thesis submitted to The University of Nottingham for the degree of Doctor of Philosophy October 2010 This thesis is dedicated to my parents, sisters, brother, my husband and our baby. Abstract Thesis The University of Nottingham ______________________________________________________________________ ABSTRACT This research project focuses on the application of numerical modelling methods to rock mechanics problems, combining theoretical, experimental and numerical modelling work. Specifically, practical finite difference modelling approach for analysing shaft lining stability through the Marl and Potash strata at Boulby mine UK has been developed using the commercially available software FLAC2D/FLAC3D (ITASCA, 2008). A soft rock Marl occurs close to the bottom of the two deep shafts at the mine. Both shafts concrete linings through this stratum have suffered considerable pressure, which has caused gradual failure of the shaft lining. So far, both shaft linings through the Marl stratum have been restored twice after sunk in 1970s and a further third relining is now required and being planned. The in situ observations, the rock engineers’ experience, and the available in situ measurements at the mine have been significantly helpful in the validation of the numerical modelling. Many factors at the mine site have, however, made this numerical modelling research challenging, including complicated lining structures, complex lining failure conditions and the scarcity of laboratory test data for the weakest rock material - the Marl, which easily weathers on exposure. Based on a comprehensive literature review, a database of materials properties relevant to this research has been produced. The methodology of obtaining appropriate rock mass input material properties to use in numerical modelling based on laboratory test data has been studied. In three-dimensional models in this research, two modelling methods have been developed to simulate each stage in the shaft linings: the continuous model for all shaft linings and independent models for each shaft lining. The numerical modelling results imply that: Firstly, in the independent three-dimensional models, the modelling results were difficult to understand due to the complexity of the structures ______________________________________________________________________ i Abstract Thesis The University of Nottingham ______________________________________________________________________ representing the shaft relining systems and difficulty in defining appropriate properties for the interface elements. Therefore, the continuous three-dimensional model that gives the analysable modelling results is recommended by the author for this research. By this method, the effect of the historic changes in the stress field on each shaft lining’s stability can be investigated from initial shaft construction to subsequent relining phases. Secondly, the weak rock Marl should not be the only reason for the shaft linings’ failure through this stratum. The roadway approximately 10 m beneath the Marl stratum was also a key factor for the stability of the shaft linings. The weak Marl cannot carry the stress redistribution around the shaft caused by the roadway excavation, which was an uneven loading acting on the circular shaft linings. This uneven loading introduced high shear and tensile stresses which threatened the stability of the circular concrete structures. Thirdly, the interface materials between high strength concrete blocks in shaft relinings improved the flexibility of the lining systems successfully, but decreased the strength of the whole lining systems as weak “joints”. In addition, the single ring concrete blocks (the first and third relinings) are a more effective lining than the double rings (the second relining), and the third relining would perform better than the previous ones. As a recommendation for the further simulation, it is worth attempting to simulate the longer term deformation and stress conditions of the shaft concrete lining systems using the Creep model built in FLAC2D/FLAC3D codes. Additionally, deeper research work combined with in situ investigation can be done to decrease the uncertainty of the input material properties to make the numerical models as close to the real engineering situation as possible. ______________________________________________________________________ ii Acknowledgements Thesis The University of Nottingham ______________________________________________________________________ ACKNOWLEDGEMENTS First of all I would like to express my sincere thanks and gratitude to my supervisors Dr. David Reddish and Dr. Rod Stace for their academic guidance, encouragement and financial support throughout the PhD course. I would also like to thank Prof. Hai-Sui Yu and Prof. Yang Ju for their supervisions, help and encouragement during this research. Acknowledgements are also given to Mr. Allan Williams and Mr. Mike Keen, the rock engineers of Boulby mine, Cleveland Potash Ltd for their support during the progress of the research project. Thanks must go to Dr. Philip Rowsell, Mr. Mark Dale and Mr. Craig Cox for their generous help and collaboration during the laboratory tests for this research. I am thankful to all friends at the Nottingham Centre for Geomechanics with whom I have had a wonderful time throughout three years’ PhD study. Sincerely thanks must go to my parents, my husband Yate, my sisters and brother without whose support and encouragement I would not have been what I am now; especially to my husband Yate, who I believe is a great husband and will be a great father. The research work outlined in this thesis was largely funded by Cleveland Potash Ltd. Part-funding was also gratefully received from the National Basic Research Project of China (Grant No. 2010CB226804, 2002CB412705), the New Century Excellent Talents Program of the Ministry of Education of China (Grant No. NCET-05-0215) and Beijing Key Laboratory Research Project (Grant No. JD102900671), who have provided financial support with part of the tuition fees. Finally, I would like to thank the Institute of Materials, Minerals and Mining (IOM3) and South Midlands Mining and Minerals Institute (SMMMI) for the awards that provided part of my research and living expenses. __________________________________________________________ iii List of Contents Thesis The University of Nottingham ______________________________________________________________________ LIST OF CONTENTS Abstract i Acknowledgements iii List of Contents iv List of Figures viii List of Tables xiv Notation xvi CHAPTER 1 INTRODUCTION 1.1 Introduction 1 1.2 Problem Definition 2 1.3 Aims and Objectives 6 1.4 Technical Challenges 7 1.5 Research Outline 9 CHAPTER 2 GENERAL LITERATURE REVIEW 2.1 In Situ State of Ground Stress 12 2.2 Stress Distribution around Excavations 20 2.3 Rock Mass Classification Systems 23 2.3.1 Introduction 23 2.3.2 Rock Quality Designation (RQD) 25 2.3.3 Rock Mass Rating (RMR) System 26 2.3.4 Coal Mine Classification Rating (CMCR) 29 2.3.5 Rock Tunnelling Quality Index, Q 32 2.3.6 Conclusions 37 2.4 Shaft Stability Problems 38 2.5 Previous Rock Mechanics Research at Boulby Mine 42 2.6 History of Shafts at Boulby Mine 48 2.6.1 The Design for the Original Lining of the Shafts 48 2.6.2 The Design for the First Relining of the Shafts 51 2.6.3 The Design for the Second Relining of the Shafts 54 2.6.4 The Design for the Third Relining of the Shafts 58 2.6.5 Conclusions and Assumptions 60 2.7 In Situ Stress Measurements at Boulby Mine 65 __________________________________________________________ iv List of Contents Thesis The University of Nottingham ______________________________________________________________________ 2.8 In Situ Deformation Measurements at Boulby Mine 67 2.9 Chapter Summary 69 CHAPTER 3 LABORATORY DETERMINATION OF GEOTECHNICAL PARAMETERS 3.1 Determination of Rock Mass Strength 70 3.1.1 Introduction 70 3.1.2 Geological Strength Index (GSI) 71 3.1.3 Hoek-Brown Failure Criterion 75 3.1.4 RocLab Software 76 3.2 Rock Materials from Boulby Mine 82 3.2.1 Laboratory Tests Data Collection 82 3.2.2 Materials Properties Used in Modelling 85 3.3 Laboratory Tests for Concretes used at Boulby Mine 85 3.3.1 Introduction 85 3.3.2 Laboratory Tests Results and Analysis 88 3.3.3 Material Properties Used in Modelling 90 3.4 Interface Problems in the Shaft Lining Modelling 93 3.4.1 Introduction 93 3.4.2 Interfaces between Epoxy Resin and Concrete 96 3.4.3 Interfaces between Cement Mortar and Concrete 98 3.4.4 Interfaces between Plywood Pack and Concrete 100 3.5 Other Parameters used in the Shaft Lining Modelling 107 3.5.1 Material Properties of Polyurethane and Vermiculite 107 3.5.2 Material Properties of Cement Grout 108 3.6 Chapter Summary 109 CHAPTER 4 INTRODUCTION OF FLAC2D/FLAC3D 4.1 Introduction 110 4.2 Fields of Application 110 4.3 Fundamental Components of a Problem 112 4.3.1 Finite Difference Grid 113 4.3.2 Boundary Conditions 117 4.3.3 Initial Stress Conditions 119 4.3.4 Constitutive Models 119 __________________________________________________________ v List of Contents Thesis The University of Nottingham ______________________________________________________________________ 4.3.5 Material Properties 122 4.4 Chapter Summary 123 CHAPTER 5 TWO-DIMENSIONAL NUMERICAL MODELLING OF SHAFTS’ LINING SYSTEMS 5.1 Introduction 124 5.2 Parametric Study 126 5.2.1 Geometry of the Model and Mesh Definition