Rock Mechanics.pdf - Mining and Blasting

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BLOCK CAVING higher levels, the previously cemented joint surfaces were now weakened and more prone to caving. The caveback had intersected this “Gypsum Line”, just prior to the accident. At the time of the accident, the air gap between the estimated top of the caved ore muckpile in the stope above the extraction level and the top of the caveback was a vertical distance of approximately 180 m. On 24 November 1999, the mine was operating a scheduled maintenance shift with approximately 65 persons underground. At about 2:50 p.m. a massive caving event took place over a period of approximately four minutes. This involved the collapse of the majority of the overlying orebody (approximately 14.5 million tonnes) through to surface. 3 This collapse into the air gap below displaced approximately 4.1 million m of air. This air was displaced in a dynamic and extremely powerful and destructive air blast. Some air was displaced through the muckpile onto the extraction level where visibility and ventilation was seriously disrupted, but no injuries or other damage were reported. Some air escaped through to surface. However the vast majority of the air vented through the One Level drive and the decline resulting in the four fatalities.” A different mechanism from those discussed so far is involved in subsidence caving in which a large mass of rock subsides rapidly as a result of shear failure on the vertical or near-vertical boundaries of a block. For this to occur the normal (horizontal) stresses developed on the vertical boundaries of the block, or the shear strength of the interface, must be so low that the total shear resistance developed is unable to resist the vertical force due to the weight of the block. For such a failure to have catastrophic consequences, there would need to exist a large mined-out void into which the caving mass could fall as in the Northparkes incident. This circumstance will not arise in a block or panel cave if the draw control strategy used does not allow a significant air gap to develop below the cave back (see section 15.5.6). 15.5.2 Cavability The discussion of the mechanics of caving given in section 15.5.1 suggests that the major factors likely to influence cavability are discontinuity geometry and strength, rock mass strength, orebody geometry, undercut dimensions, the stresses induced in the crown of the undercut or cave and the presence of any boundary weakening. Although the importance of these factors had been recognised for some time and several attempts had been made to codify their influences, it was not until the development of Laubscher’s caving chart approach in the 1980s (Diering and Laubscher, 1987) that a method of achieving this became widely available. The challenge faced in attempting to develop empirical methods of predicting cavability is to find a means of combining measures of rock mass quality, undercut geometry and induced stresses into one simple and robust tool. Laubscher did this by plotting the value of his Mining Rock Mass Rating, MRMR, against the hydraulic radius, S (area/perimeter), of the undercut which is a measure of the undercut size and shape. Laubscher’s caving chart is divided into three zones – stable, transitional and caving (Laubscher, 1994). If the orebody is elongated in one direction, the question 469
Figure 15.34 Extended Mathews stability graph showing stable and caving lines based on logistic regression (after Mawdesley, 2002). LONGWALL AND CAVING MINING METHODS can arise as to whether or not the minimum dimension (or width) of an elongated undercut can control cavability, irrespective of the value of the hydraulic radius. Data collected by Mawdesley (2002) suggest that the hydraulic radius (or shape factor) is a satisfactory predictor of stability or cavability for aspect ratios of less than about three. It should be noted, however, that the hydraulic radius is calculated from the plan dimensions of the undercut level. It does not permit account to be taken of the curved or arched profile of an arrested cave such as that which developed at Northparkes. Since the 1980s, Laubscher’s caving chart has been the major method used internationally to predict cavability in block and panel caving mines. It has been particularly successful when applied to the weaker and larger orebodies for which it was first developed. However, recent experience suggests that it may not always provide satisfactory results for stronger, smaller and isolated or constrained blocks or orebodies. There may be insufficient case studies available, especially for rock masses having MRMR values of more than 50, to enable the three zones of stability to be delineated with a reasonable degree of accuracy over a wide range of conditions. This is not an unusual occurrence when an attempt is made to extend an empirical method outside the limits of the experience for which it was first developed. Mawdesley et al. (2001) extended the Mathews stability chart method of open stope design introduced in section 9.6 by adding a large number of new data points, particularly from Australian mines, and defining iso-probability contours for stable, failure, major failure and combined failure and major failure cases. Mawdesley (2002) then collected data from caving mines and extended this approach to the assessment of cavability. A logistical regression analysis was used to delineate the caving zone shown in Figure 15.34. It must be emphasised that the line separating major failures from continuous caving in Figure 15.34 does not represent a 100% probability of caving. Insufficient data are available to permit iso-probability contours to be defined accurately. The availability of additional data from well-documented case histories would allow the uncertainty in the design limits to be quantified, giving greater confidence in the use of this technique for predicting cavability. 470
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Rock Mechanics
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Rock Mechanics for underground mini
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Contents Preface to the third editi
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CONTENTS 9 Excavation design in blo
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CONTENTS ix Appendix A Basic constr
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PREFACE TO THE THIRD EDITION Mining
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PREFACE TO THE SECOND EDITION In th
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PREFACE TO THE FIRST EDITION design
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ACKNOWLEDGEMENTS Safety in Mines Re
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Figure 1.1 (a) Pre-mining condition
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ROCK MECHANICS AND MINING ENGINEERI
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Figure 1.4 Principal features of a
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10 Figure 1.5 Definition of activit
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Figure 1.7 Components and logic of
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Figure 2.1 (a) A finite body subjec
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Figure 2.2 Free-body diagram for es
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STRESS AND INFINITESIMAL STRAIN As
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STRESS AND INFINITESIMAL STRAIN In
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Figure 2.3 Free-body diagram for de
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Figure 2.5 Problem geometry for det
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Figure 2.7 Rigid-body rotation of a
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STRESS AND INFINITESIMAL STRAIN the
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STRESS AND INFINITESIMAL STRAIN str
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⎡ ⎢ ⎣ xx yy zz xy yz zx STRES
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Figure 2.11 Cylindrical polar coord
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STRESS AND INFINITESIMAL STRAIN fre
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Figure 2.13 Construction of a Mohr
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STRESS AND INFINITESIMAL STRAIN fun
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3 Rock Figure 3.1 Sidewall failure
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Figure 3.2 Jointing in a folded str
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Figure 3.5 Diagrammatic longitudina
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Figure 3.7 Discontinuity spacing hi
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Figure 3.9 Illustration of persiste
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Figure 3.11 Typical roughness profi
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ROCK MASS STRUCTURE AND CHARACTERIS
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Figure 3.17 Sample number vs. preci
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Figure 3.19 Diagrammatic illustrati
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Figure 3.20 Computerised depiction
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Figure 3.23 Stereographic projectio
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Figure 3.26 Polar stereographic net
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Figure 3.28 Contours of pole concen
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Figure 3.30 Geological Strength Ind
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Figure 4.1 Idealised illustration o
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ROCK STRENGTH AND DEFORMABILITY wit
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Figure 4.4 Influence of end restrai
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ROCK STRENGTH AND DEFORMABILITY whe
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Figure 4.8 Principle of closed-loop
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Figure 4.12 Two classes of stress-
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Figure 4.14 Point load test apparat
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Figure 4.15 Biaxial compression tes
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Figure 4.18 Results of triaxial com
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ROCK STRENGTH AND DEFORMABILITY was
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Figure 4.23 Coulomb strength envelo
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Figure 4.25 Extension of a preexist
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Figure 4.29 The three basic modes o
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Figure 4.30 Normalised peak strengt
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ROCK STRENGTH AND DEFORMABILITY Tab
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Figure 4.32 The normality condition
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Figure 4.33 Variation of peak princ
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Figure 4.35 Direct shear test confi
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Figure 4.37 Shear stress-shear disp
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Figure 4.40 Peak and residual effec
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Figure 4.43 Effect of shearing dire
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Figure 4.45 Relations between norma
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Figure 4.47 Coulomb friction, linea
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ROCK STRENGTH AND DEFORMABILITY whe
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Figure 4.49 Composite peak strength
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Figure 4.50 Hoek-Brown peak strengt
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Figure 4.52 Determination of the Yo
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ROCK STRENGTH AND DEFORMABILITY 4 A
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5 Pre-mining Figure 5.1 Method of s
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Figure 5.2 The effect of irregular
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PRE-MINING STATE OF STRESS surround
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PRE-MINING STATE OF STRESS induced
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Figure 5.5 (a) Definition of hole l
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Figure 5.6 (a) Core drilling a slot
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Figure 5.7 Principles of stress mea
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PRE-MINING STATE OF STRESS strength
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PRE-MINING STATE OF STRESS A second
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PRE-MINING STATE OF STRESS by the e
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PRE-MINING STATE OF STRESS extend i
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PRE-MINING STATE OF STRESS (d) Dete
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METHODS OF STRESS ANALYSIS quantita
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METHODS OF STRESS ANALYSIS It is in
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Figure 6.2 A thick-walled cylinder
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METHODS OF STRESS ANALYSIS For the
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Figure 6.3 Problem geometry, coordi
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Figure 6.4 Problem geometry, coordi
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METHODS OF STRESS ANALYSIS When the
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Figure 6.5 Superposition scheme dem
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METHODS OF STRESS ANALYSIS The disc
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Figure 6.7 Development of a finite
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METHODS OF STRESS ANALYSIS Solution
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Figure 6.8 A simple finite element
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Figure 6.9 A schematic representati
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METHODS OF STRESS ANALYSIS block ce
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METHODS OF STRESS ANALYSIS where ˚
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METHODS OF STRESS ANALYSIS The prin
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EXCAVATION DESIGN IN MASSIVE ELASTI
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Figure 7.2 A logical framework for
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Figure 7.3 (a) Axisymmetric stress
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Figure 7.6 A plane of weakness, ori
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Figure 7.8 A flat-lying plane of we
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Figure 7.10 Shear stress/normal str
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Figure 7.12 Ovaloidal opening in a
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Figure 7.15 States of stress at sel
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Figure 7.16 Prediction of the exten
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Figure 7.18 Contour plots of princi
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Figure 7.19 Problem geometry for de
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8 Excavation Figure 8.1 An excavati
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EXCAVATION DESIGN IN STRATIFIED ROC
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Figure 8.4 Experimental apparatus f
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Figure 8.7 Free body diagrams and n
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Figure 8.8 Assumed distributions of
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Figure 8.9 Flow chart for the deter
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Figure 8.10 Normalised arch thickne
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EXCAVATION DESIGN IN STRATIFIED ROC
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Figure 8.11 Normalised deflection a
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9 Excavation Figure 9.1 Generation
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Figure 9.3 (a) A finite, non-tapere
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(a) (b) Figure 9.4 (a) Vertical cro
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(a) (b) (c) EP EP Reference circle
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Figure 9.10 JP 100 is the only JP w
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Figure 9.12 Traces of the views of
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EXCAVATION DESIGN IN BLOCKY ROCK In
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Figure 9.14 Free-body diagrams of a
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EXCAVATION DESIGN IN BLOCKY ROCK di
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Figure 9.16 Symmetrical wedge in th
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Figure 9.17 (a) Geometry for determ
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Figure 9.18 Problem geometry demons
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Figure 9.20 Cut-and-fill stope mine
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Figure 9.22 Chart to determine fact
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EXCAVATION DESIGN IN BLOCKY ROCK Th
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Figure 10.1 (a) Pre-mining state of
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Figure 10.3 (a) Dynamic loading of
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Figure 10.5 (a) Pre-mining and (b)
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Figure 10.6 Problem definition and
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ENERGY, MINE STABILITY, MINE SEISMI
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Figure 10.9 Force and stress compon
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Figure 10.12 Distribution of radial
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Figure 10.15 Problem geometry for d
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Figure 10.17 (a) Schematic represen
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Figure 10.20 Elastic/post-peak stif
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Figure 10.24 Relation between frequ
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Figure 10.28 Six possible ways that
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Figure 10.29 First motions for P an
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11Rock support and reinforcement 11
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Figure 11.1 (a) Hypothetical exampl
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Figure 11.4 Non-linear support reac
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Figure 11.5 Idealised elastic-britt
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Figure 11.6 Calculated required sup
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ROCK SUPPORT AND REINFORCEMENT The
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Figure 11.9 Ground reaction curves
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Figure 11.12 Use of grouted reinfor
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ROCK SUPPORT AND REINFORCEMENT If,
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Figure 11.16 Local reinforcement ac
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Figure 11.18 Typical working sketch
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Figure 11.19 Permanent support and
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Figure 11.22 Basis of natural coord
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Figure 11.24 Distributions of (a) s
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Figure 11.26 Resin grouted rockbolt
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Figure 11.28 Alternative methods of
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ROCK SUPPORT AND REINFORCEMENT Tabl
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Figure 11.31 Toussaint-Heintzmann y
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MINING METHODS AND METHOD SELECTION
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Figure 12.2 Elements of a supported
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Figure 12.6 Schematic layout for bi
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Figure 12.8 Layout for shrink stopi
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Figure 12.9 Schematic layout for VC
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Figure 12.11 Key elements of longwa
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Figure 12.13 Mining layout for tran
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13 Figure 13.1 Schematic illustrati
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Figure 13.3 Layout of barrier pilla
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Figure 13.5 Principal modes of defo
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Figure 13.8 Geometry for tributary
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PILLAR SUPPORTED MINING METHODS str
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Figure 13.10 Distribution of vertic
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Figure 13.12 Pillar behaviour domai
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PILLAR SUPPORTED MINING METHODS Lun
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Figure 13.15 Options in the design
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Figure 13.17 Relation between yield
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Figure 13.19 Model of yield of coun
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Figure 13.20 North-south vertical c
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Figure 13.23 Stope-and-pillar layou
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Figure 13.25 Calibrated stability c
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PILLAR SUPPORTED MINING METHODS wor
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Figure 13.28 Pillar performance, de
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Figure 13.29 (a) Stope and pillar l
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Figure 13.31 (a) Plane strain analy
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PILLAR SUPPORTED MINING METHODS Pan
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14 Artificially supported mining me
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ARTIFICIALLY SUPPORTED MINING METHO
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Figure 14.2 Simplified view of stru
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Page 435 and 436: Figure 14.5 Confined block model fo
Page 437 and 438: Figure 14.7 Crown and sidewall stre
Page 439 and 440: ARTIFICIALLY SUPPORTED MINING METHO
Page 441 and 442: ARTIFICIALLY SUPPORTED MINING METHO
Page 443 and 444: Figure 14.10 Sublevel open stoping
Page 445 and 446: Figure 14.12 Some applications of c
Page 447 and 448: 15 Longwall and caving mining metho
Page 449 and 450: Figure 15.2 Shear stress drop in th
Page 451 and 452: LONGWALL AND CAVING MINING METHODS
Page 455 and 456: Figure 15.6 Hydraulic prop reaction
Page 457 and 458: Figure 15.7 Development and extract
Page 459 and 460: Figure 15.8 Vertical stress redistr
Page 461 and 462: Figure 15.11 Distribution of observ
Page 463 and 464: Figure 15.13 Plan view of microseis
Page 465 and 466: Figure 15.16 Ground-support interac
Page 467 and 468: Figure 15.18 Roadway support and re
Page 475 and 476: Figure 15.25 Comparison of isolated
Page 477 and 478: Figure 15.26 Geometry of a sublevel
Page 479 and 480: Figure 15.28 Theoretical determinat
Page 481 and 482: Figure 15.31 Deterioration of a cro
Page 483 and 484: Figure 15.32 Distinct element simul
Page 485: LONGWALL AND CAVING MINING METHODS
Page 489 and 490: Figure 15.36 Comparison of postand
Page 493 and 494: Figure 15.39 Idealised plan illustr
Page 495 and 496: Figure 15.41 Idealised vertical sec
Page 497 and 498: Figure 15.42 Vertical slice through
Page 501 and 502: 16 Figure 16.1 Trough subsidence ov
Page 503 and 504: MINING-INDUCED SURFACE SUBSIDENCE c
Page 505 and 506: Figure 16.4 North-south section, At
Page 507 and 508: Figure 16.6 (a) Rectangular block g
Page 509 and 510: MINING-INDUCED SURFACE SUBSIDENCE f
Page 511 and 512: Figure 16.8 Relation between stope
Page 513 and 514: MINING-INDUCED SURFACE SUBSIDENCE M
Page 515 and 516: MINING-INDUCED SURFACE SUBSIDENCE
Page 517 and 518: Figure 16.14 Chart developed to est
Page 519 and 520: Figure 16.16 Progressive hangingwal
Page 521 and 522: Figure 16.19 Idealised model used i
Page 523 and 524: Figure 16.21 Longitudinal section,
Page 525 and 526: MINING-INDUCED SURFACE SUBSIDENCE t
Page 527 and 528: MINING-INDUCED SURFACE SUBSIDENCE w
Page 529 and 530: MINING-INDUCED SURFACE SUBSIDENCE F
Page 531 and 532: Figure 16.25 Subsidence troughs pre
Page 533 and 534: Figure 16.28 Predicted and measured
Page 535 and 536: 17 Blasting mechanics 17.1 Blasting
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Figure 17.1 An empirical matching o
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Figure 17.2 A finite difference mod
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Figure 17.4 Reflection of a cylindr
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BLASTING MECHANICS means that no ci
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Figure 17.8 Layout of blast holes i
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Figure 17.9 Influence of field stat
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Figure 17.11 Generation of surface
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BLASTING MECHANICS The components o
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BLASTING MECHANICS amplitudes of th
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BLASTING MECHANICS 17.9 Evaluation
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Figure 17.15 (a) Schematic cross se
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BLASTING MECHANICS in Figure 17.17,
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MONITORING ROCK MASS PERFORMANCE (a
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MONITORING ROCK MASS PERFORMANCE su
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MONITORING ROCK MASS PERFORMANCE Ta
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Figure 18.2 The Distometer ISETH, a
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Figure 18.5 Self-inductance multipl
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MONITORING ROCK MASS PERFORMANCE is
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Figure 18.9 Biaxial vibrating wire
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MONITORING ROCK MASS PERFORMANCE me
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Figure 18.12 Cross section at 6650N
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Figure 18.13 Examples of convergenc
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Figure 18.15 Longitudinal section l
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Figure 18.16 (Cont.) MONITORING ROC
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Appendix A Basic constructions usin
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Figure A.3 Determining the angle be
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APPENDIX A USE OF HEMISPHERICAL PRO
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APPENDIX B STRESSES AND DISPLACEMEN
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Figure A.6 Axisymmetric tunnel prob
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Figure A.9 Bolt load-extension curv
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APPENDIX D LIMITING EQUILIBRIUM ANA
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ANSWERS TO PROBLEMS 2 (a) 0.087 - 0
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ANSWERS TO PROBLEMS 3 wp = 38.6 m,
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REFERENCES Symp. & 17th Tunn. Assn
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REFERENCES Brady, B. H. G. and Bray
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REFERENCES Collier, P. A. (1993) De
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REFERENCES Drescher, A. and Vardoul
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REFERENCES Gustafsson, P. (1998) Wa
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REFERENCES Hood, M. and Brown, E. T
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REFERENCES Kaiser, P. K. and Tannan
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REFERENCES Lorig, L. J. and Brady,
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REFERENCES Ortlepp, W. D. (1994) Gr
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REFERENCES Rojas, E., Molina, R. an
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REFERENCES Spottiswoode, S. M. and
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REFERENCES Villaescusa, E., Windsor
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Index Page numbers appearing in bol
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INDEX Coulomb (cont.) parameters 96
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INDEX Excavation (cont.) support ra
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INDEX Jaeger’s plane of weakness
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INDEX Panel caving 470-2, 473, 474,
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INDEX Seismic (cont.) moment 306, 3
Page 643 and 644:
INDEX Strength (cont.) residual 86,
Page 645:
INDEX United States (USA) 395, 396,
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