Groundwater, Pore Pressure and Wall Slope Stability – a model for quantifying pore pressures in current and future mines. A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Engineering Geology in the University of Canterbury by Richard J. Brehaut University of Canterbury 2009 I TABLE OF CONTENTS FRONTISPIECE .................................................................................................................................. XI ACKNOWLEDGEMENTS ............................................................................................................... XII ABSTRACT .......................................................................................................................................XIII CHAPTER 1: INTRODUCTION ............................................................................................... 1 1.1 Preamble ...................................................................................................................... 1 1.2 Thesis Objectives ........................................................................................................ 3 1.3 Research Methods ....................................................................................................... 4 CHAPTER 2: LITERATURE REVIEW ................................................................................... 6 2.1 Introduction ................................................................................................................. 6 2.2 Rock Mass Characteristics .......................................................................................... 6 2.2.1 Defects in Rock Masses ....................................................................................... 7 2.2.2 Effective stress ..................................................................................................... 8 2.2.3 Pore Pressure and Matric Suction ........................................................................ 8 2.3 Slope Design ............................................................................................................. 10 2.3.1 Fundamental Failure Mechanisms ..................................................................... 12 2.4 Groundwater .............................................................................................................. 16 2.4.1 Aquifer structure ................................................................................................ 19 2.4.2 Heterogeneous and Anisotropic flow................................................................. 21 2.4.3 Aquifer Characteristics ...................................................................................... 23 2.4.4 Potentiometric Surface ....................................................................................... 25 2.5 Dewatering and Depressurisation.............................................................................. 25 2.5.1 Site feasibility and primary investigations ......................................................... 29 2.5.2 Dewatering system design ................................................................................. 30 2.5.3 Dewatering methods .......................................................................................... 33 II 2.5.4 Active dewatering: ............................................................................................. 34 2.5.5 Passive dewatering: ............................................................................................ 35 2.5.6 Effects of Horizontal Drain Hole Spacing ......................................................... 36 2.5.7 Monitoring effectiveness of installed dewatering/depressurisation systems. .... 36 2.5.8 Flow Nets ........................................................................................................... 37 2.6 Hydromechanical Coupling....................................................................................... 38 2.7 Numerical Analysis ................................................................................................... 40 CHAPTER 3: GEOLOGY OF THE HAMERSLEY BASIN, WA. ....................................... 44 3.1 Introduction ............................................................................................................... 44 3.2 Ore Genesis and Characterisation ............................................................................. 45 3.3 Stratigraphy of the Hamersley Province ................................................................... 51 3.3.1 Fortescue Group ................................................................................................. 51 3.3.2 Hamersley Group ............................................................................................... 51 3.4 Mount Tom Price Ore Bodies ................................................................................... 56 3.5 Conclusion ................................................................................................................. 58 CHAPTER 4: LOCAL HYDROGEOLOGY .......................................................................... 59 4.1 Introduction ............................................................................................................... 59 4.2 Regional rainfall statistics and catchment details ..................................................... 60 4.3 Aquifer Characteristics .............................................................................................. 60 4.4 Dewatering History ................................................................................................... 63 4.5 Aquifer Performance ................................................................................................. 65 4.6 Monitoring Network .................................................................................................. 67 4.7 Groundwater Management ........................................................................................ 69 4.8 Surface Water Management ...................................................................................... 69 4.9 Conclusion ................................................................................................................. 70 III CHAPTER 5: CASE STUDY -SOUTH EAST PRONGS, MOUNT TOM PRICE. ......... 71 5.1 Introduction ............................................................................................................... 71 5.2 Structural Geology .................................................................................................... 72 5.2.1 South East Prongs Fault Zone ............................................................................ 75 5.3 Hydrogeology ............................................................................................................ 77 5.3.1 Flow Characteristics........................................................................................... 77 5.3.2 Pit Dewatering ................................................................................................... 78 5.3.3 Slope Depressurisation....................................................................................... 80 5.4 Geotechnical .............................................................................................................. 82 5.4.1 Pit slope design philosophy ............................................................................... 82 5.4.2 Rock Mass Characteristics ................................................................................. 82 5.4.3 Failure Mechanisms ........................................................................................... 83 5.4.4 Future Pit Development ..................................................................................... 85 CHAPTER 6: HYDROGEOLOGICAL DRAINAGE MODELLING ................................. 86 6.1 Introduction ............................................................................................................... 86 6.2 Spatial Analysis ......................................................................................................... 86 6.3 Finite Element Numerical Modelling ........................................................................ 88 6.3.1 Model Setup ....................................................................................................... 89 6.3.2 Geometry............................................................................................................ 89 6.3.3 Meshing.............................................................................................................. 94 6.3.4 Hydraulic Characteristics ................................................................................... 95 6.3.5 Boundary Conditions ......................................................................................... 98 6.3.6 Drainholes ........................................................................................................ 100 6.4 Analyses .................................................................................................................. 103 6.4.1 Steady State Analysis ....................................................................................... 103 6.4.2 Transient Analysis ........................................................................................... 104 6.4.3 Breakdown of Analysis Schedule .................................................................... 104 IV 6.5 Outputs and Results ................................................................................................