GEOTECHNICAL STUDIES OF RETREAT PILLAR COAL MINING AT SHALLOW DEPTH By Michael Cullen B.Eng., McGill University, 1986 M.Eng., McGill University, 1988 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In THE FACULTY OF GRADUATE STUDIES (Department of Mining and Mineral Process Engineering) We accept this thejsi^as confornjiflg to thejequired standard THE UNIVERSJTY/OF BRITISH COLUMBIA March 2002 © Michael Cullen 2002 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of A*^p M,toft£>i Pfc^v &i£ g-u^/o WUL\ o ^_ The University of British Columbia Vancouver, Canada Date GEOTECHNICAL STUDIES OF RETREAT PILLAR COAL MINING AT SHALLOW DEPTH ABSTRACT This thesis presents the results of research into geotechnical aspects of retreat pillar coal mining at shallow depth (less than 100m). The fieldwork component was carried out over a four-year period at the Quinsam Coal Mine. The geotechnical aspects investigated were excavation stability, excavation support, pillar design, gob cave prediction, and subsidence. These were all considered critical aspects of safe and cost effective mining. Geotechnical design at the Quinsam Coal Mine was initially carried out using existing design tools. For the most part these tools were developed at moderate to deep depth mines (greater than 100m). The suitability of these tools at shallow depth mines was previously not known; as such the primary objectives of this research were: 1) to determine if the existing geotechnical design tools were applicable at shallow depth, 2) to develop new design methods where the existing tools were found to not be suitable, 3) to improve safety, productivity and costs at shallow depth retreat pillar coal mines. Suitable existing tools where found for design of ground support, pillar size, excavation size, and gob cave prediction. Existing tools where not found to be suitable for prediction of caving and surface subsidence. The cave height at shallow depth was determined to extend much higher than predicted by existing methods. The extent of the ground surface affected by subsidence was determined to be much less than that predicted by existing methods. New tools for predicting caving and subsidence have been developed based on measurements at the Quinsam Coal Mine supplemented with numerical modelling. The distinct response of the rock mass to mining at shallow depth is attributed to the magnitude and orientation of the induced stresses. At shallow depth low compressive and tensile stresses are more likely to occur, and more likely to extend for a greater distance into the rock mass above the excavation. Although this work is based on studies carried out at the Quinsam Coal Mine it is believed that the findings can be applied to other shallow depth coal mines in similar geological environments. Since adopting many of the recommendations and design procedures developed through this work, the Quinsam Coal Mine has achieved significant improvements in safety, productivity, and costs. ii TABLE OF CONTENTS ABSTRACT ii LIST OF TABLES vi LIST OF FIGURES vii LIST OF PHOTOS ix ACKNOWLEDGMENTS x CHAPTER 1 INTRODUCTION 1 CHAPTER 2 THE QUINSAM COAL MINE 8 2.0 INTRODUCTION 8 2.1 GENERAL GEOLOGY OF THE QUINSAM COAL AREA 8 2.2 MINING AT THE QUINSAM COAL MINE 11 CHAPTER 3 SITE CHARACTERIZATION 21 3.0 INTRODUCTION 21 3.1 GEOLOGIC DISCONTINUITY DATA 21 3.2 ROCK MECHANICAL PROPERTIES 24 3.3 ROCK MASS CLASSIFICATION 27 3.4 IN SITU STRESS 28 3.5 IMPLICATIONS FOR MINING 31 3.5.1 Geologic Structure 31 3.5.2 Mechanical Properties and Rock Mass Classification 33 3.5.3 Mining Induced Stresses 33 3.6 CONCLUSIONS 40 CHAPTER 4 EXCAVATION STABILITY 43 4.0 INTRODUCTION 43 4.1 LITERATURE REVIEW 43 4.2 EXCAVATION CLOSURE MONITORING PROGRAM 46 4.2.1 Mechanical Borehole Extensometers 46 4.2.2 Rod Extensometer 48 4.2.3 Magnetic Multipoint Borehole Extensometer 50 4.3 EXCAVATION STABILITY VERSUS ROCK MASS CLASSIFICATION 53 4.4 CONCLUSIONS 55 CHAPTER 5 GROUND SUPPORT 58 5.0 INTRODUCTION 58 5.1 ROCK BOLTS 59 5.1.1 Visual Observations of Performance 61 5.1.2 Rock Bolt Pull Tests 61 5.1.3 Drill Hole Size 65 5.1.4 Rock Bolt Load 67 5.1.5 Full Column Resin Rock Bolts 68 5.2 EXISTING SUPPORT DESIGN METHODS 68 5.2.1 Rock Mass Support Design Methods 69 5.2.1.1 US Corps of Engineers 69 iii GEOTECHNICAL STUDIES OF RETREAT PILLAR COAL MINING AT SHALLOW DEPTH 5.2.1.2 Farmer and Shelton ! 70 5.2.1.3 French Matrix System 72 5.2:1.4 "Q" Classification System 72 5.2.1.5 "RMR" Classification System 75 5.2.1.6 CMRS Classification System 76 5.2.2 Analytical Support Design Methods 76 5.2.2.1 Beam Building and Arching 76 5.2.2.2 Dead Weight Load 80 5.3 COMPARISON OF THE EXISTING SUPPORT DESIGN METHODS TO EXPERIENCE AT THE QUINSAM COAL MINE 84 5.4 CONCLUSIONS 85 CHAPTER 6 PILLAR DESIGN 88 6.0 INTRODUCTION 88 6.1 LITERATURE REVIEW 89 6.1.1 Pillar Stress 89 6.1.2 Pillar Strength 94 6.1.3 Factor of Safety 97 6.1.4 Pillar Failure Modes 98 6.2 PILLAR PERFORMANCE AT THE QUINSAM COAL MINE 99 6.2.1 Coal Strength 99 6.2.2 Visual Pillar Performance 100 6.3 PILLAR DESIGN METHOD 105 6.3.1 Method Verification 106 6.4 RECOMMENDATIONS FOR PILLAR DESIGN AT SHALLOW DEPTH RETREAT PILLAR COAL MINES 108 6.4.1 Barrier Pillars '. 108 6.4.2 Roadway or Mains Pillars 109 6.4.3 Panel Pillars 109 6.4.4 Remnant Pillars 110 6.5 CONCLUSIONS 110 CHAPTER 7 GOB CAVE PREDICTION 113 7.0 INTRODUCTION 113 7.1 LITERATURE REVIEW 113 7.2 GOB CAVE STUDIES AT THE QUINSAM COAL MINE 115 7.2 GOB CAVE MODEL 121 7.4 CONCLUSIONS 124 CHAPTER 8 SUBSIDENCE 126 8.0 INTRODUCTION 126 8.1 SUBSIDENCE DEFINITIONS 128 8.2 LITERATURE REVIEW 133 8.2.1 Sub-Surface Ground Movements 135 8.2.2 Angle of Draw 137 8.2.2.1 Angle of Draw Prediction 139 8.2.3 Cave and Fractured Zone Height 141 8.2.3.1 Predicting the Caved Zone Height 143 8.3 CAVING AND SUBSIDENCE AT THE QUINSAM COAL MINE 146 8.4 COMPARISON OF THE EXISTING SUBSIDENCE PREDICTION METHODS TO EXPERIENCE AT THE QUINSAM COAL MINE 155 8.5 NUMERICAL MODELING OF SUBSIDENCE 158 8.6 INVESTIGATION OF STRESSES AROUND THE CAVING FRONT 171 8.7 CONCLUSIONS 174 iv GEOTECHNICAL STUDIES OF RETREAT PILLAR COAL MINING AT SHALLOW DEPTH CHAPTER 9 CONCLUSIONS AND RECOMMENDATIONS 179 9.0 INTRODUCTION 179 9.1 MINING INDUCED STRESSES 180 9.2 EXCAVATION STABILITY 181 9.3 EXCAVATION SUPPORT DESIGN 183 9.4 GROUND SUPPORT WITH ROCK BOLTS 184 9.5 PILLAR DESIGN 185 9.6 GOB CAVE PREDICTION 186 9.7 CAVING AND SUBSIDENCE 187 9.8 CONTRIBUTIONS TO THE ADVANCEMENT OF KNOWLEDGE 189 9.9 RECOMMENDATIONS FOR FURTHER WORK 192 REFERENCES 195 APPENDIX 1 QUINSAM COAL MINE SITE CHARACTERIZATION 207 FAULT AND JOINT MEASUREMENTS FROM THE 2N MINE 208 UNIAXIAL COMPRESSIVE STRENGTH TESTS 215 POINT LOAD INDEX TESTING 238 DIRECT SHEAR STRENGTH TESTS 242 SLAKE DURABILITY TESTS 245 MOISTURE CONTENT TESTS 246 DENSITY TESTS 246 Q ROCK MASS CLASSIFICATION PARAMETERS 247 RMR ROCK MASS CLASSIFICATION PARAMETERS 247 CMRR ROCK MASS CLASSIFICATION PARAMETERS 247 APPENDIX 2 ROOF CONVERGENCE MEASUREMENT DATA 248 ROD EXTENSOMETER DATA 249 APPENDIX 3 ROCK BOLT PULL TEST DATA 250 RESULTS OF PULL TESTS ON ROOFBOLTS 251 APPENDIX 4 PILLAR CLASSIFICATION RECORDS AND FACTOR OF SAFETY VALUES 253 PILLAR CLASSIFICATION RECORDS 254 APPENDIX 5 SURFACE SUBSIDENCE 260 SURFACE SUBSIDENCE STATION LOCATION, INITIAL COORDINATES, AND FINAL ELEVATIONS 261 SURFACE SUBSIDENCE LEVEL SURVEY DATA: STATION NUMBER, DATE OF SURVEY, AND ELEVATIONS 265 SURFACE SUBSIDENCE: SUMMARY OF NUMERICAL MODELING RESULTS \ 271 v GEOTECHNICAL STUDIES OF RETREAT PILLAR COAL MINING AT SHALLOW DEPTH LIST OF TABLES Table 3.1: Mechanical Properties of Rocks 26 Table 3.2: Point Load Strength and Estimate of UCS 26 Table 3.3: Typical Rock Mass Classification Values 27 Table 3.4: Estimated In Situ Stress For k = 1.0 29 Table 3.5: Mining Induced Stresses Determined from Computer Simulations 37 Table 4.1: Roof Convergence Data from the Literature 45 Table 5.1: Average Finished Drill Hole Diameter 66 Table 5.2: Pull Tests Results For Different Drill Hole Size and Resin Cartridge Size 66 Table 5.3: Support Design Recommendations of the US Corps of Engineers 70 Table 5.4: Support Design Recommendations of Farmer and Shelton 71 Table 5.5: Support Design Recommendations of French Matrix System 73 Table 5.6: Support Design for 6m Wide Excavation in Competent Siltstone 85 Table 6.1: Empirical Values Used in Pillar Strength Formulas 95 Table 7.1: Gob Stability AND Convergence Data from the Literature 115 Table 7.2: Roof Convergence at Time of Gob Cave 117 Table 7.3: Gob Cave Data, Quinsam Coal Mine 119 Table 8.1: Subsidence Data From the Quinsam Coal Mine 150 Table 8.2: Surface Fractures Over 2S Mine Sections 101 153 Table 8.3: Prediction of the Height of the Caved Zone and the Fractured Zone 155 Table 8.4: Prediction of the Angle of Draw 156 Table 8.5: Material Properties Used in Numerical ModelS 161 Table 8.6: Summary of Numerical Model Results 164 Table 8.7: Induced Stresses Above the Caved Zone Determined from Computer Simulations 171 vi GEOTECHNICAL STUDIES OF RETREAT PILLAR COAL MINING AT SHALLOW DEPTH LIST OF FIGURES Figure 2.1: General