The Southern African Institute of Mining and Metallurgy Founded in 1894 SPECIAL PUBLICATIONS SERIES 8 SECOND EDITION OF ROCK ENGINEERING of UNDERGROUND COAL MINING J. Nielen van der Merwe Bernard J. Madden SECOND EDITION OF ROCK ENGINEERING FOR UNDERGROUND COAL MINING SAIMM PUBLICATIONS THE MONOGRAPH SERIES Ml Lognormal-De Wijsian Geostatistics for Ore Evaluation (2nd ed 1981) D.G. Krige M2 An Introduction to Geostatistical Methods of Mineral Evaluation (2nd ed 1981) J.-M.M. Rendu M3 Principles of Flotation (1982) (3rd imp. 1986) Edited by R.P. King M4 Increased Underground Extraction of Coal (1982) Edited by C.J. Fauconnier and R.W.O. Kersten M5 Rock Mechanics in Mining Practice (1983) (3rd imp. 1986) Edited by S. Budavari M6 Assay and Analytical Practice in the South African Mining Industry (1986) W.C. Lenahan and R. de L. Murray-Smith M7 The Extractive Metallurgy of Gold in South Africa, 2 volumes (1987) Edited by G.G Stanley M8 Mineral and Metal Extraction — An Overview (1994) L.C. Woollacott and R.H. Eric M9 Rock Fracture and Rockbursts—an illustrative study (1997) Edited by W.D. Ortlepp THE SPECIAL PUBLICATIONS SERIES SPI Proceedings, Underground Transport Symposium (1986) Edited by R.C.R. Edgar SP2 Backfill in South African Mines(1988) SP3 Treatment and Re-use of Water in the Minerals Industry(1989) SP4 COREX Symposium 1990 (1990) Edited by H.M.W. Delport and P.J. Holaschke SP5 Measurement, Control, and Optimization in Mineral Processing (1994) Edited by H.W. Glen SP6 Handbook on Hard-rock Strata Control (1994) A.J.S. Spearing SP7 Rock Engineering for underground coal mining (2002) J. Nielen van der Merwe and Bernard J. Madden SUPPLEMENT TO THE SAIMM JOURNAL The Metals and Minerals Industry in South Africa - Part 1 (1989) Edited by H.W. Glen THE SYMPOSIUM SERIES S1 Mathematical Statistics and Computer Applications in Ore Valuation(1966) S2 Planning Open Pit Mines(1970) (4th imp.) Edited by P.W.J. van Rensburg S3 Application of Computer Methods in the Mineral Industry (APCOM 1973) Edited by M.D.G. Salamon S4 Infacon1974 Edited by H.W. Glen S5 Proceedings of the 12th CMMI Congress2 volumes (1982) Edited by H.W. Glen S6 Rockbursts and Seismicity in Mines(1984) Edited by N.C. Gay and E.H. Wainwright S7 The Planning and Operation of Open Pit and Strip Mines(1986) Edited by J.P. Deetlefs S8 GOLD 100: Proceedings of the International Conference on Gold (1986) Volume 1: Gold Mining TechnologyEdited by H. Wagner and R.P. King Volume 2: Extractive Metallurgy of GoldEdited by C.E. Fivaz and R.P. King Volume 3: Industrial Uses of GoldEdited by G. Gafner and R.P. King S9 APCOM 87: Proceedings of the Twentieth International Symposium on the Application of Computers and Mathematics in the Mineral Industries (1987) Volume 1: MiningEdited by L. Wade, R.W.O. Kersten and J.R. Cutland Volume 2: MetallurgyEdited by R.P. King and I.J. Barker Volume 3: GeostatisticsEdited by I.C. Lemmer, H. Schaum and F.A.G.M. Camisani-Calzolari S10 International Deep Mining Conference (1990) Volume 1: Innovations in Metallurgical PlantEdited by G.A. Brown and P. Smith and Application of Materials Engineering in the Mining IndustryEdited by B. Metcalfe Volume 2: Technical Challenges in Deep Level MiningEdited by D.A.J. Ross-Watt and P.D.K. Robinson S11 Infacon 6 ( Incorporating Incsac) (1992)Edited by H.W. Glen S12 Massmin 92 Edited by H.W. Glen (out of print) S13 Minefill 93 Edited by H.W. Glen S14 XVth CMMI Congress Publications (1994) Volume 1: MiningEdited by H.W. Glen Volume 2: Metals Technology and Extractive Metallurgy (1994)Edited by H.W. Glen S15 Surface Mining 1996 Edited by H.W. Glen S16 Hidden Wealth (1996) Edited by H.W. Glen S17 Heavy Minerals 1997 Edited by R.E. Robinson S18 The 8th International Platinum Symposium (1998) S19 Mining in Africa ’98 S20 Extraction Metallurgy Africa ’98 S21 Sixth International Symposium for Rock Fragmentation by Blasting (1999) S22 Metallurgy Africa ’99 S23 Heavy Minerals 1999 Edited by R.G. Stimson S24 Tunnels under Pressure Technical Editor T.R. Stacey S25 Mine Hoisting 2000 S26 Coal–The Future (2000) S27 The Fifth International Symposium on Rockburst and Seismicity in Mines (RaSim 5) (2001) S28 6th International Symposium on Mine Mechanization and Automation (2001) S29 XIV International Coal Preparation Congress and Exhibition S30 Surface Mining 2002—Modern Developments for the New Millennium S31 IFSA 2002, Industrial Fluidization South Africa S31A APCOM 2003—Application of Computers and Operations Research in the Minerals Industries S32 ISSA/Chamber of Mines Conference—Mines and Quarries: Prevention of Occupational Injury and Disease S33 ISRM—Technology Roadmap for Rock Mechanics S34 Heavy Minerals Conference 2003 S35 Safety in Mines Research Institutes (2003) S36 VII International Conference on Molten Slags, Fluxes & Salts (2004) Tenth International Ferroalloys Congress INFACONX 2004 S37 Deep and high stress mining 2004 S38 ‘Platinum adding value’ 2004 S39 Base Metals—Southern Africa’s response to changing global base metals market dynamics 2005 S40 Strategic versus Tactical approaches in mining 2005 S41 Best practices in rock engineering SARES 2005 S42 IFSA 2005 (Mintek) S43 Southern African Pyrometallurgy 2006 International Conference S44 Stability of rock slopes in open pit mining and civil engineering situations S45 ‘Platinum Surges Ahead’ 2006 S46 Hydraulic Transport of Solids - Hydrotransport 17 (2007) S47 Fourth Southern African Base Metals Conference ‘Africa’s base metals resurgence’ S48 Heavy Minerals Conference, 2007 S49 Cave Mining Conference, 2007 S50 International Symposium on Lead and Zinc processing –Lead & Zinc 2008 S51 Surface Mining 2008 S52 Platinum in transformation 2008 S53 IFSA 2008 S54 Hydrometallurgy Conference 2009 S55 Shotcrete for Africa 2009 S56 Base Metals Conference 2009 S57 Heavy Minerals Conference, 2009 S58 Hard Rock Safe—Safety Conference 2009 S59 Sampling and Blending 2009 S60 World Gold 2009 Conference S61 Physical Beneficiation 2010 Conference Published by The Southern African Institute of Mining and Metallurgy Fifth Floor, Chamber of Mines Building, 5 Hollard Street, Johannesburg, 2107 Republic of South Africa © The Southern African Institute of Mining and Metallurgy, 2010 ISBN 978-1-920410-06-3 The papers in this volume have been for the most part prepared from files supplied by the authors, with additional typesetting and formatting by The Southern African Institute of Mining and Metallurgy. Printed by Camera Press, Johannesburg THE SOUTHERN AFRICAN INSTITUTE OF MINING AND METALLURGY SPECIAL PUBLICATIONS SERIES 8 (SP8) SECOND EDITION OF ROCK ENGINEERING FOR UNDERGROUND COAL MINING A Practical Guide for Supervisors at all levels, Mine Planners and Students J. Nielen van der Merwe Professor Rock Engineering, Department of Mining Engineering, University of the Witwatersrand and Bernard J. Madden G-Ro, Geotechnical Services (Pty) Ltd. ABOUT THE AUTHORS Prof. J. Nielen van der Merwe, Pr. Eng., has almost 30 years’ experience of practical rock engineering on mines; consulting, researching and, recently, teaching. His initial rock engineering work was done on the deep level gold mines, after which he became involved with platinum, chrome, asbestos, etc. His coal rock engineering experience was gained on mines in the Vaal Basin, KwaZulu-Natal, the Highveld and Witbank. He has published more than 40 papers in several countries of the world and is a past president of The South African National Institute of Rock Engineering. He was the initiator and first President of the International Interest Group on Mining Rock Engineering and is President Elect of the International Society for Rock Mechanics. He is currently Head of the Department of Mining Engineering at the University of Pretoria. Dr Bernard J. Madden has worked in research and consulting to the mining industry, particularly in underground coal mining, in Australia and South Africa for the past 20 years. He has worked for the research institutions of the Chamber of Mines Research Organization (COMRO), CSIR Miningtek, and ACIRL, and currently operates a consultancy to the mining industry. He has published over 30 papers. Bernard Madden and Nielen van der Merwe ABOUT THE INSTITUTE The Southern African Institute of Mining and Metallurgy is a dynamic organization attending to the professional needs of its members since its inception in 1894. There are approximately 3 800 members and the Institute’s membership continues to grow. A key service by the Institute to its members, is the promotion of the transfer of technology and scientific knowledge, relevant to the sustainable development of the minerals and metals section of the South African economy and its stakeholders. PREFACE This second edition of the book contains certain extensions to the first edition: firstly, to stay abreast of developments in the field of underground coal mining rock engineering and, secondly, to satisfy the needs of the industry in the current milieu. For the immediate future, the challenge for South African mines will be to extend the lives of mines in the Witbank coalfield as far as possible. This requires safe secondary mining of old and often small pillars at shallow depth, which were not intended for further mining at the time of their development. A chapter on shallow mining has therefore been added as have methods to deal with time-related scaling of pillars. The appendices have been reviewed and more worked examples, to further explain the various methodologies, have been added. Where it was attempted with the first edition to simplify the mathematics as far as possible by substituting certain constants with acceptable numbers, the more correct fundamental equations have now been retained in parallel to the simplified ones. Finally, a few minor corrections were made to the script of the first edition; the need for this is inevitable, no matter how carefully a script is checked prior to publication. The work took longer than Bernard and I anticipated as there was less time available for it than we expected. This is a sign of the times, professional people being busier than ever before. It also indicates the seriousness with which mine owners and operators now approach rock-related stability issues on the mines. We hope that this second edition will contribute in some small way to the safe and profitable operation of our coal mines. Nielen van der Merwe May 2010 Contents Page No. CHAPTER 1: FUNDAMENTALS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 FIRST LEVEL OF FUNDAMENTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 SECOND LEVEL OF FUNDAMENTALS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 BEAM BEHAVIOUR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 FRICTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 STATE OF STRESS IN THE COAL MINING ENVIRONMENT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 MECHANICAL PROPERTIES OF ROCK. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 CHAPTER 2: GEOTECHNICAL CLASIFICATION INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 ROCK MASS CLASSIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 PHYSICAL RISK AND PERFORMANCE RATING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 SHAFTS AND INCLINES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 COAL ROOF CLASSIFICATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 CHAPTER 3: ROOF AND SIDEWALL STABILITY INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 PRE-MINING STATES OF STRESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 EFFECTS OF CREATING A ROADWAY ON THE STRESS ENVIRONMENT . . . . . . . . . . . . . . . . . . . 28 FAILURE MODES OF THE ROOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 EFFECTS OF DISCONTINUITIES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 CONTROLLABLE PARAMETERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 DIFFERENT SUPPORT SYSTEMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 SYSTEM SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 POSITION OF BOLTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 LENGTH OF BOLTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 INSTALLATION OF A NORMAL RESIN ANCHOR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 CABLE ANCHORS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 JOINT SUPPORT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 SUPPORT OF A VERY WEAK ROOF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 SUPPORTING THE COMPETENT LAYER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 INSPECTING AND MAKING SAFE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 RIBSIDE SUPPORT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 BREAKER LINE SUPPORTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 PROBABILISTIC DESIGN METHODS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 CHAPTER 4: PILLAR DESIGN INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 BASIC MINING METHODS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 HISTORY OF PILLAR DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 DESIGN OF SQUAT PILLARS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 COMPARISON OF DIFFERENT STRENGTH FORMULAE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 CORRECTION FOR PARALLELOGRAM SHAPED PILLARS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 CORRECTION FOR CONTINUOUS MINERS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 BARRIER PILLAR STRENGTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 EFFECTS OF DISCONTINUITIES ON STRENGTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 COLLAPSES SINCE 1966. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 PREVENTION OF PILLAR FAILURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 ROLE OF THE OVERBURDEN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 HOLISTIC APPROACH TO PILLAR SYSTEM DESIGN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Contents (continued) CHAPTER 5: PILLAR EXTRACTION INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 ROCK MECHANICS PRINCIPLES RELEVANT TO PILLAR EXTRACTION. . . . . . . . . . . . . . . . . . . . 69 THE EXTRACTION SAFETY FACTOR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 PILLAR AND SYSTEM STIFFNESS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 DIRECTION OF STOOPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 THE ROLE OF SNOOKS IN STOOPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 SIZING OF SNOOKS USING FUNDAMENTAL PROCEDURES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 DIRECTION OF SPLITTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 SEQUENCE OFFENDER EXTRACTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 SUPPORT DURING STOOPING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 DETERMINING PILLAR SIZES FOR STOOPING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 RATE OF EXTRACTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 INFLUENCE OF THE FACE ADVANCE ON THE STRESS REGIME. . . . . . . . . . . . . . . . . . . . . . . . . . . 83 PARTIAL PILLAR EXTRACTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 EVALUATING OLD PILLARS FOR EXTRACTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 GENERAL REMARKS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 CHAPTER 6: LONGWALLING INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 STRESS HISTORY OF A LONGWALL PANEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 INTER PANEL PILLAR DESIGN AND LONGWALL DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . 90 MINING INTER PANEL PILLARS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 PANEL ORIENTATION AND FACE SHAPE. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 FACE BREAKS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 FACE MOVES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 NEGOTIATING DYKES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 FACE LENGTH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 GOAFING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 MULTIPLE SEAM LONGWALLING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 CHAPTER 7: MULTIPLE SEAM MINING INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 FACTORS INFLUENCING INTERACTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 BORD-AND-PILLAR MINING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 HIGH EXTRACTION OVER BORD-AND-PILLAR WORKINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 LONGWALLING UNDERNEATH LONGWALLING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 LONGWALLING UNDERNEATH STOOPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 MINING OVER GOAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 SIMULTANEOUS MINING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 DESIGN FLOWCHART . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 CHAPTER 8: SHALLOW WORKINGS INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 UNDERGROUND MINING FROM A HIGHWALL. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 SINKHOLE FORMATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 DETERMINATION OF MINEABLE DEPTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 BORD AND PILLAR MINING AT SHALLOW DEPTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 HIGH EXTRACTION MINING AT SHALLOW DEPTH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 PRACTICAL CONSIDERATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120