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Investigation of underground mine pillar design procedures Potvin, Yves 1985

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INVESTIGATION OF UNDERGROUND MINE PILLAR DESIGN PROCEDURES By YVES POTVIN B.Sc, LAVAL University, QUEBEC 1981 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF A«ii«-J MASTER OF^SCIENCE in THE FACULTY OF GRADUATE STUDIES Mining and Mineral Process Engineering We accept t h i s t hesis as conforming tq the required standard THE UNIVERSITY OF BRITISH COLUMBIA March 1985 © Yves Potvin, 1985 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Mining and M i n e r a l Process Engineering The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 DE-6 (3/81) ABSTRACT The p r i n c i p a l f u n c t i o n s of underground mine p i l l a r s are to s t a b i l i z e openings and to c a r r y the loa d of o v e r l y i n g rock s t r a t a . They are o f t e n ( p a r t i a l l y or completely) recovered a t a l a t e r stage when t h e i r s t a b i l i z i n g e f f e c t i s no longer r e q u i r e d . For economic reasons, an optimum-sized p i l l a r i s the smal l e s t one s a t i s f y i n g s a f e t y requirements. Thus the p i l l a r design problem c o n s i s t s of determining the p i l l a r ' s minimum dimensions as the l o a d approaches the u l t i -mate p i l l a r s t r e n g t h . Because the p i l l a r ' s s trength and the l o a d a c t i n g upon i t are both f u n c t i o n s of many i n t e r r e l a t e d f a c t o r s , which may vary as mining progresses, the determination of p i l l a r dimensions i s a complex task. Furthermore, the m u l t i p l i c i t y of p i l l a r shapes, s i z e s , rock m a t e r i a l and f u n c t i o n s add to the designers' problem. Consequently, p i l l a r design programs are s t i l l g e n e r a l l y performed as a t r i a l - a n d - e r r o r process. In order, to improve the present p i l l a r design p r a c t i c e s (1) - A A p i l l a r c l a s s i f i c a t i o n i s proposed t o standardize the design procedure (2) The p r i n c i p a l design methods, d i v i d e d i n t o four groups, are summarized and t h e i r a p p l i c a b i l i t y i s i s de f i n e d (3) A five-phase design procedure with design c h a r t s i s developed (4) The design procedure i s a p p l i e d i n a n a l y s i n g two case h i s t o r i e s TABLE OF CONTENTS CHAPTER 1. Introduction CHAPTER 2. The C l a s s i f i c a t i o n and D e f i n i t i o n of P i l l a r s 2.1 P i l l a r C l a s s i f i c a t i o n 2.2 Category 1: "Plate P i l l a r s " 2.2.1 Description 2.2.2 Definitions 2.3 Category 2: "Separation P i l l a r s " 2.3.1 Description 2.3.2 Definitions 2.4 Category 3: "Stub P i l l a r s " 2.4.1 Description 2.4.2 Definitions 2.5 Category 4: "Inclined P i l l a r s " 2.6 Discussion CHAPTER 3. Review of P i l l a r Design Methods 3.1 Introduction 3.2 Group 1. Experience Methods 3.3 Group 2. Empirical Methods 3.3.1 Empirical strength formulas 3.3.2 Empirical stress formulas 3.3.3 Empirical dimensioning formulas 3.4 Group 3. Theoretical Methods 3.4.1 Theoretical strength formulas 3.4.2 Theoretical stress formulas 3.5 Group 4. Computer Methods CHAPTER 4. P i l l a r Design Procedure Page 4.1 Philosophy of P i l l a r Design 42 4.2 Design Procedure 42 4.2.1 Phase 1. Experience Design 4.2.2 Phase 2. P i l l a r Structural Analysis 4.2.2.1 P i l l a r Transection V e r i f i c a t i o n 4.2.2.2 Shear S t a b i l i t y Analysis 4.2.3 Phase 3. Empirical Design 4.2.4 Phase 4. Theoretical Design 4.2.5 Phase 5. Computer Design 4.3 Design Charts 46 CHAPTER 5. Heath Steele Case History Analysis 5.1 Geology 53 5.1.1 Regional Geology 5.1.2 Mine's Geology 5.1.3 Structural Geology 5.1.4 Jointing 5.2 Mining Method and Underground Structures Dimension 57 5.3 Rock Mechanics Data 59 5.3.1 Rock Strength Parameters 5.3.2 Laboratory Testing 5.3.3 Rock Mass C l a s s i f i c a t i o n 5.3.4 V i r g i n Stress 5.4 P i l l a r C h a r a c t e r i s t i c s 63 5.5 Mining Sequence 63 5.6 Failure History and P i l l a r Geometry 64 5.7 P i l l a r Design Study 5.7.1 Phase 1. Experience Design 5.7.2 Phase 2. P i l l a r Structural Analysis 5.7.3 Phase 3. Empirical Methods 5.7.3.1 Estimation of P i l l a r Load by the Tributary Area Formula 5.7.3.2 Estimation of P i l l a r Strength 5.7.4 Theoretical Methods 5.7.5 Computer Methods 5.8 Discussion of the Results Page 68 90 CHAPTER 6. Geco Case History Analysis 6.1 Geology 9 8 6 . 1 . 1 Regional Geology 6 . 1 . 2 Mine Geology 6.1.3 Structural Geology 6 .2 Mining Method and Underground Structure Dimension 1 02 6.3 Rock Mechanics Data 1 02 6.5.1 Rock Strength Parameters 6.3.2 Laboratory Test 6.3.3 Rock Mass C l a s s i f i c a t i o n 6.3.4 V i r g i n Stress 6.4 P i l l a r C h a r a c t e r i s t i c s 108 6.5 Mining Sequence 108 6.6 Failure Histories and P i l l a r Geometries 1 1 3 6.7 P i l l a r Design Study 1 20 6.7.1 Phase 1 . Experience Design 6.7.2 Phase 2. P i l l a r Structural Analysis VI fa.ge 6.7.3 Phase 3. Empirical Methods 6.7.3.1 Estimation of P i l l a r Load by the Extraction Ratio Formula 6.7.3.2 Estimation of P i l l a r Strength; Hoek's Method. 6.7.4 Theoretical Methods 6.7.5 Computer Methods 6.8 Discussion of Results ^30 CHAPTER 7. Summary and Conclusion 7.1 Design Procedure 1 3 5 7.2 Case Histories I36 7.3 Design Methods 1 3 7 APPENDIX A. Review of Literature APPENDIX B. Determination of the Geco stress regime at  700 f t . depth APPENDIX C. I l l u s t r a t i o n of P i l l a r s APPENDIX D. "BITEM", 2-D, Boundary Element Program v i i LIST OF FIGURES Figure Description Page 1 P i l l a r category 1 "Plate P i l l a r s " 9 2 P i l l a r category 2 "Separation P i l l a r s " 10 3 P i l l a r category 3 "Stub P i l l a r s " 13 4 P i l l a r category 4 "Inclined P i l l a r s " ' 1 5 5 Hard Rock P i l l a r s (Appendix C) 6 Hard Rock P i l l a r s (Appendix C) 7 Soft Rock P i l l a r s (Appendix C) 8 Influence of P i l l a r Width to Height Ratio 2 6 on Average P i l l a r Strength 9 Average V e r t i c a l P i l l a r Stresses i n Typical 28 P i l l a r Layouts. 10 Determination of Load on Chain P i l l a r s by the 2 9 " F i r s t Panel" Load Concept 11 Observed Value of W for Coal Seams 7 f t . 3 2 Thick and having a Crushing Strength in the 3-in. Cube of 3,000 p s i ±10% 12 P i l l a r transected by a single plane of weakness 44 13 Intersecting planes of weakness 44 14 Design chart for "Plate P i l l a r s " , Category 1 47 15 Design chart f o r "Separation P i l l a r s " , ^ Category 2 16 Design chart for "Stub P i l l a r s " , Category 3 ^9 17 Design chart for "Inclined P i l l a r s " , Category 4 5 0 18 Heath Steele Geology 5 4 19 Plan View of Heath Steele Orebody 5 6 20 V i r g i n Stress at Heath Steele 62 21 Longitudinal View of the Investigated Area at 65 Heath Steele v i i i Figure Description Page 22 Extraction Flowchart of 77-89, 77-91, 66 77-93 and 77-95 Stopes 23 Estimated Layout when 77-92 Rib P i l l a r 69 f a i l e d . 24 Estimated Layout when 77-94 Rib P i l l a r 70 f a i l e d 25 S 3 Fracture System 72 26 The Effect of S 3 Fractures on 30 m. (100 f t ) 7 3 wide Rib P i l l a r s 27 The Effect of S 3 Fractures on 60 m. (200 f t ) 7 ^ wide Rib P i l l a r s 28 S 5 Fracture System 75 29 Combined Effect of S 3 and S 5 Fractures 7 6 30 77-90 Rib P i l l a r S l i d i n g 77 31 77-90 Rib P i l l a r Deformation versus Time 78 32 P i l l a r s Extraction Numbers f o r the 77-90 0 1 p i l l a r f a i l u r e geometry. 33 P i l l a r s Extraction Numbers for the 77-92 8 2 p i l l a r f a i l u r e geometry. 34 P i l l a r s Extraction Numbers for the 77-94 8 ^ p i l l a r f a i l u r e geometry 35 The Effect of the Width to Height Ratio on 84 average p i l l a r strength 36 Computer Output of the 77-90 p i l l a r f a i l u r e 87 geometry. (Stress simulation) 37 Computer Output of the 77-92 p i l l a r f a i l u r e 88 geometry. (Stress simulation) 38 Computer Output of the 77-94 p i l l a r f a i l u r e 8 ? geometry. (Stress simulation) 39 P i l l a r Deformation versus Extraction Number (N) ^3 40 Safety Factor versus Extraction Number (N) 9^ 41 Safety Factor versus Extraction Ratio (e) 9 6 42 Schematic Stratigraphic Columns i l l u s t r a t i n g 99 generalized relationships of sulphide zones, Geco Mine i x Figure Description Page 43 Assumed Stress Regime at Geco 1 0 7 44 Longitudinal View of the Investigated Area 1 09 at Geco 45 Extraction Flowchart of 10-19.5, 10-21, 10-22 and 10-23.5 stopes 46 Estimated Layout when 10-21.5 P i l l a r Failed H ? 47 Estimated Layout when 10-23 P i l l a r F a iled H 8 48 Estimated Layout when 10-20 P i l l a r Failed H 9 49 P i l l a r s Extraction Numbers for the 10-21.5 1 2 2 p i l l a r f a i l u r e geometry 50 P i l l a r s Extraction Numbers for the 10-23 1 2 3 p i l l a r f a i l u r e geometry 51 P i l l a r s Extraction Numbers for the 10-20 l 2 l + p i l l a r f a i l u r e geometry 52 Computer Output of the 10-21.5 P i l l a r F ailure i 2 7 Geometry (Stress simulation) 53 Computer Output of the 10-23 P i l l a r Failure i 2 8 Geometry (Stress simulation) 54 Computer Output of the 10-20 P i l l a r F ailure i 2 9 Geometry (Stress simulation) 55 Safety Factor versus Extraction Ratio 1 3 3 56 Comparison of Heath Steele, Geco 1 3 8 Case h i s t o r i e s analysis r e s u l t s X LIST OF TABLES Table Description Page 1 Rock Mechanic Studies i n Noranda Underground 3 Mines 2 P i l l a r and Opening Designing Methods used by 4 Noranda Underground Mines 3 P i l l a r C l a s s i f i c a t i o n Summary 17 4 Constants A and B used i n the "Size Effect Formula" 2 2 5 Constants a and b used i n the "Shape Effect Formula" 2 3 6 Char a c t e r i s t i c Input Data for Computer Methods 38 7 Summary of Computer Methods 39 8 Mining Sequence of the Panel 64 9 Approximate Stope and P i l l a r Dimensions when 6? 77-92 P i l l a r F ailed 10 Approximate Stope and P i l l a r Dimension when 68 77-94 P i l l a r Failed 11 Heath Steele P i l l a r Analysis Results 9 1 12 Summary of the Geology at Geco -'-00 13 Stope 10-19.5 Mining Sequence HO 14 Stope 10-21 Mining Sequence 1 1 0 15 Stope 10-22 Mining Sequence H I 16 Stope 10-23.5 Mining Sequence 1 1 2 17 Approximate Stope and P i l l a r Dimensions when H 5 10-21.5 P i l l a r Failed 18 Approximate Stope and P i l l a r Dimensions when -^6 10-23 P i l l a r F ailed 19 Approximate Stope and P i l l a r Dimensions when 10-20 P i l l a r Failed 20 Geco P i l l a r Analysis Results ' x i ACKNOWLEDGEMENTS The author f i r s t wishes to thank Noranda Research Mining Divi s i o n Group, who have made t h i s project f e a s i b l e by financing the research, pro-viding an impressive amount of information and f o r t h e i r outstanding co-operation. The involvement of the following Noranda mines and the assistance of t h e i r employees was also appreciated: Brunswick Mining and Smelting - Heath Steele Mine - Geco Di v i s i o n Goldstream Mine. As well, special acknowledgements to Dr. H.D.S. M i l l e r for his supervision of the research project, the pertinent advice given, and for his stimulating approach towards rock mechanics. The author wishes to thank the following scholarship funds for f i n a n c i a l support: - Cy and Emerald Keyes Frederick Armand McDiarmid George E. Winkler In addition, thanks to the members of the Department of Mining and Mineral Process Engineering f o r t h e i r helpful attitude, to W. M. Cumming for proof reading, and f i n a l l y to Jacques and Jeannine Potvin f o r th e i r con-tinuous encouragement and support. 1 CHAPTER 1 Introduction 2 The p r i n c i p a l functions of underground mine p i l l a r s are to s t a b i l i z e openings, and to carry the load of overlying s t r a t a . They are often (par-t i a l l y or completely) recovered at a l a t e r stage when t h e i r s t a b i l i z i n g e f f e c t i s no longer necessary. For economic reasons, an optimum-sized p i l l a r i s the smallest one that s a t i s f i e s safety requirements. Thus, the p i l l a r design problem consists of determining the p i l l a r ' s minimum dimension as the load reaches the ultimate p i l l a r strength. Because the p i l l a r ' s strength, and load acting upon i t are both func-tions of many i n t e r r e l a t e d factors, which vary as mining progresses, p i l l a r dimensioning i s a d i f f i c u l t task. Furthermore, the m u l t i p l i c i t y of p i l l a r shapes, s i z e s , rock material and applications add to the designers' confusion. Consequently, p i l l a r design programs are s t i l l generally a t r i a l - a n d -error process. In September 1982, a research project was undertaken, under the super-v i s i o n of Dr. H. D. S. M i l l e r , with the collaboration and f i n a n c i a l support of Noranda Research, Mining Division, to develop a comprehensive p i l l a r design procedure. The project's f i r s t year was e n t i r e l y dedicated to a com-plete review of the p i l l a r design methods available. This i s reproduced i n appendix A. Another of the project tasks was to investigate the current p i l l a r design procedures and the role of rock mechanics techniques i n mine p i l l a r design. To achieve this goal, a questionnaire was mailed to seven Noranda underground operations. The information was completed by v i s i t i n g four mines i n New Brunswick, Quebec, Ontario and B r i t i s h Columbia. Table 1 shows that a f a i r amount of rock mechanic studies had been com-3 T A B L E 1 Rock Mechanic Studies i n Noranda Underground Mines Goldstream Mattabi Matagami Mines Gaspe , Geco Brunswick (BM 5 S) Heath Steele 5. 4. 3. 2. 1. ROCK ROCK MASS STRESS STRENGTH MONITORING CLASSIFICATION LABORATORY TEST INVESTIGATIONS PARAMETER Unit Weight E l a s t i c Mod u Poisson's Ratio ? X X X X X X X X X X X X X 5. 4. 3. 2. 1. ROCK ROCK MASS STRESS STRENGTH MONITORING CLASSIFICATION LABORATORY TEST INVESTIGATIONS PARAMETER I n-Situ Measurement Photo-Elastic Model Computer Modelling X X X X X 5. 4. 3. 2. 1. ROCK ROCK MASS STRESS STRENGTH MONITORING CLASSIFICATION LABORATORY TEST INVESTIGATIONS PARAMETER Compress. Strength o° Tensile Strength •-4 CO T r i a x i a l Strength o I D Shear Strength ^ Failu r e C r i t e r i o n X X X X X X X X X X X X 5. 4. 3. 2. 1. ROCK ROCK MASS STRESS STRENGTH MONITORING CLASSIFICATION LABORATORY TEST INVESTIGATIONS PARAMETER R Q D N G I C S I R Laubscher St r u c t u r a l Mapping XXX X XXX X X X X X X XX X 5. 4. 3. 2. 1. ROCK ROCK MASS STRESS STRENGTH MONITORING CLASSIFICATION LABORATORY TEST INVESTIGATIONS PARAMETER Multi-Wire Extensom. Boroscope Observ. Compression Pad Closure S t a t i o n L e v e l l i n g Survey Station Piezometer X X X X X X X X X X pleted. However, i t must be emphasized that these experiments are related to the operations' s i z e and age, as well as the s t a b i l i t y problems encoun-tered. Table 2 confirms that the mines rely mainly upon previous experience f o r design, leaving the more sophisticated methods to mining consultants. •Table 2 P i l l a r and Opening Designing Methods Used by Noranda Underground Mines Experience Methods Group 1 t/1 M CO OO c •fi c C o Empirical Methods Group 2 in rt in c •H c a. o An a l y t i c a l Methods Group 3 u rt t/1 c •H c o Computer Methods Group 4 in u TO in M C c ID CL, O Goldstream Mattabi Matagarni Mines Gaspe Brunswick Heath Steele Geco M M M M M M M M M M M M M M M M C C c c Note: M - The mine's s t a f f performed the design. C - Consultant performed the design. In order to improve the actual p i l l a r design practices: a p i l l a r c l a s s i f i c a t i o n system i s proposed to standardize the design procedure the p r i n c i p a l design methods are summarized and t h e i r a p p l i c a b i l i t y i s defined a five-phase design procedure with design charts i s developed the procedure i s applied i n analysing two case h i s t o r i e s . 6 CHAPTER 2 The C l a s s i f i c a t i o n and D e f i n i t i o n of P i l l a r s 7 2.1 P i l l a r C l a s s i f i c a t i o n The l i t e r a t u r e provides no standard d e f i n i t i o n for the term, "under-ground p i l l a r . " I f one attempts to elaborate a general d e f i n i t i o n , i t should be borne i n mind that the p i l l a r may or may not be mineralized, may be permanent or temporary, but i n any event reference must be made to the notion of s t a b i l i t y and security. Regardless of which mining method i s used, every mine must leave p i l -l a r s to s t a b i l i z e underground structures. However, because of the v a r i -able ground conditions, stress, and the multiple p i l l a r applications related to mining methods and orebody geometry, no two p i l l a r s are iden-t i c a l . In the documents reviewed, more than twenty names describing various kinds of p i l l a r s were encountered. This wide variety of p i l l a r s makes the elaboration of a standard design procedure a d i f f i c u l t task. The p i l l a r shape, the load acting on the p i l l a r , and the strength of the p i l l a r meterial are the three most important factors to be considered when designing a p i l l a r . A simple c l a s s i f i c a t i o n (for p i l l a r design purposes) i s suggested, regrouping under the same "category" p i l l a r s of s i m i l a r shape which are submitted to s i m i l a r loading s i t u a t i o n s . In this manner, every p i l l a r i n each category can be designed using i d e n t i c a l equations and a given metho-dology . Because the behaviour of hard rock d i f f e r s greatly from that of soft rock, each category i s broken into two sub-categories: (a) hard rock p i l -l a r s , and (b) soft rock p i l l a r s . Note: The width, height and length of the p i l l a r s may vary greatly within a category, but the general shape must be s i m i l a r . 8 2.2 Category 1. "Plate P i l l a r s " 2.2.1 Description Figure 1 shows that "Plate P i l l a r s " are submitted to a b i a x i a l h o r i -zontal stress f i e l d . The top and the bottom of the p i l l a r s are not loaded. However, t h i s i s not true in the case of surface p i l l a r s , which must bear the v e r t i c a l load due to s u r f i c i a l overburden. The designer should be aware of t h i s fact when dimensioning a surface p i l l a r . No cases of soft rock "Plate P i l l a r s " (Category IB) were found in the l i t e r a t u r e . This i s due p r i n c i p a l l y to the soft rock mining methods which r a r e l y require plate p i l l a r s . 2.2.2 Definitions Category 1A: Hard Rock - Crown P i l l a r s , Roof P i l l a r s , Level P i l l a r s , Strike P i l l a r s , Horizontal P i l l a r s : These are horizontal s l i c e s of varying thickness, l e f t above the excavated area to provide support. They are generally recovered a f t e r t h e i r support function i s no longer required. The term "crown p i l l a r " i s often used to define the shallowest horizontal p i l l a r carrying the overbur-den load (surface p i l l a r ) . - S i l l P i l l a r s : S i l l p i l l a r s are very similar to crown p i l l a r s but they are situated underneath the stopes at each sublevel. 2.3 Category 2. "Separation P i l l a r s " 2.3.1 Description Separation p i l l a r s (Category 2) are subjected to a v e r t i c a l and horizontal load. They are open on t h e i r longitudinal side (Figure 2). It 9 C A T E G O R Y I " p l a t e p i l l a r " c a t e g o r y I Q. ( h a r d r o c k ) c o tego ry I b. I s o f t r o c k ) C R O W N P I L L A R S R O O F P I L L A R S L E V E L P I L L A R S STR IKE P I L L A R S H O R I Z O N T A L P. S I L L P I L L A R S S U R F A C E P I L L A R S FIGURE 1 P i l l a r Category 1 " Plate P i l l a r s " 10 11 C A T E G O R Y 2 s e p a r a t i o n p i l l a r ca tego ry 2 a ( h a r d r o c k c a t e g o r y 2 b u o f t r o c k RIB PILLARS DIP P ILLARS T R A N S V E R S E P I LLARS A B U T M E N T PILLARS BARR IER P ILLARS E N T R Y P ILLARS FIGURE 2 P i l l a r Category 2 "Separation P i l l a r s ' should be noted that the hard rock (Category 2A) and soft rock separation p i l l a r s (Category 2B) do not possess i d e n t i c a l c h a r a c t e r i s t i c shapes, since soft rock p i l l a r s are usually lower and wider (Figure 2). In the case of a very long separation p i l l a r (compared to the other dimensions) the horizontal stress may have a n e g l i g i b l e effect and the prob-lem may be considered to be two dimensional. 2.3.2 Definitions Category 2A: Hard Rock - Rib P i l l a r s , Dip P i l l a r s , Transverse P i l l a r s : A r i b p i l l a r i s a separating wall between two stopes. The length of the r i b i s usually i n the orebody dip d i r e c t i o n and i s continuous. The r i b p i l l a r s transfer the v e r t i c a l load from the roof to the f l o o r , s t a b i l i z i n g the rock overlying the stoped area. They may be recovered at a l a t e r stage of mining. Category 2B: Soft Rock - Barrier P i l l a r s : Barrier p i l l a r s are used to i s o l a t e coal mine panels. They are usually permanent p i l l a r s which control roof s t a b i l i t y and play a major role in v e n t i l a t i o n . - Entry P i l l a r s : These p i l l a r s r e f e r to the longwall mining method. They provide a protection to the panel entries and are recovered during the panel's f i n a l e x p l o i t a t i o n stage. 12 2.4 Category 3. "Stub P i l l a r s " 2.4.1 Description The shape of "stub p i l l a r s " (Category 3) may be square or rectangu-l a r (Figure 3). They are open on the four v e r t i c a l sides and are subjected to a un i a x i a l compressive stress f i e l d . 2.4.2 Definitions Category 3A: Hard Rock - Centre P i l l a r s : These p i l l a r s have the same function as r i b p i l l a r s , but are s i t u -ated in the middle of the stopes. They reduce the span of openings and help to carry the roof load. Contrary to the r i b p i l l a r s which are continuous, the centre p i l l a r s are transected by cross-cuts or d r i f t s . - (Room and P i l l a r ] P i l l a r s , Stub P i l l a r s : The stub p i l l a r s may r e f e r to a uniform room and p i l l a r panel or simply be l e f t randomly wherever s t a b i l i z a t i o n i s needed. Their length, width, height, shape and composition vary according to the s i t e and require-ments. They support the v e r t i c a l load of overlying rock, and may be perma-nent or recoverable. - Post P i l l a r s , Yielding P i l l a r s : These p i l l a r s refer to the "post p i l l a r " mining method. They pro-vide temporary support to the immediate roof. As mining progresses from the bottom up, the post p i l l a r s s t a r t to y i e l d and f i n a l l y collapse "gently" at the bottom, where they are confined by b a c k f i l l . Category 3B: Soft Rock - Panel P i l l a r s : These temporary p i l l a r s are uniformly distributed within a longwall panel. They support the panel's immediate roof and w i l l be removed at a 13 C A T E G O R Y 3 "stub p i l l a r s " H EIGHT c a t ego r y 3 a ( h a r d CENTER PILLARS (ROOM S PILLAR) PILLARS POST PILLARS r o c k ) c a t e g o r y 3 b (soft PANEL PILLARS SPLIT PILLARS REMNANT PILLARS CHAIN PILLARS r o c k ' FIGURE 3 P i l l a r Category 3 "Stub P i l l a r s " l a t e r stage. - S p l i t P i l l a r s : During longwall p i l l a r recovery, the panel p i l l a r s are cut into two s p l i t p i l l a r s . - Remnant P i l l a r s : Remnant p i l l a r s are the residual portion of s p l i t p i l l a r s . As min-ing retreats, they either collapse or are completely recovered. - Chain P i l l a r s : These p i l l a r s play the same role as b a r r i e r p i l l a r s , but they are composed of a series of aligned small p i l l a r s instead of a long, massive, continuous p i l l a r . This provides the highest extraction r a t i o . The p i l l a r s may be designed to y i e l d , permitting the roof to deform. 2.5 Category 4. "Inclined P i l l a r s " 2.5.1 Description Inclined p i l l a r s do not have a p a r t i c u l a r shape or are not sub-mitted to a p a r t i c u l a r loading s i t u a t i o n . However, because they do not f i t into the three preceding categories, and they Tequire special consider-ation for design because of t h e i r i n c l i n a t i o n , i n c l i n e d p i l l a r s form the fourth category of the p i l l a r c l a s s i f i c a t i o n (Figure 4). 2.6 Discussion The author is aware that p i l l a r s i n the forementioned categories are i l l u s t r a t e d with i d e a l i z e d shapes, which i s not the case for real under-ground p i l l a r s . In addition, i t should be realized that the load acting on a p i l l a r i s a function of several factors: - V i r g i n stress - Stress induced by mining 15 C A T E G O R Y 4 inc l i ned p i l lar 11 I HEIGHT c a t e g o r y 4 a ( h a r d r o c k ) c a t e g o r y 4 b (soft rock) FIGURE 4 P i l l a r Category - "Inclined P i l l a r s " - Geological features - P i l l a r shape and orientation - Openings and general mine structures - Ground water. However, i t i s believed that every p i l l a r may f a l l into one of the above four categories, even though the c l a s s i f i c a t i o n oversimplifies the loading mechanism and the p i l l a r geometry. F i n a l l y , because shaft p i l l a r s are fundamentally d i f f e r e n t from other p i l l a r s , they are not included in t h i s c l a s s i f i c a t i o n . Nevertheless, the following d e f i n i t i o n i s proposed: Shaft P i l l a r s : These are permanent p i l l a r s providing protection to the mine shaft system. The shaft and the shaft p i l l a r may be v e r t i c a l or i n c l i n e d . Shaft p i l l a r s become larger with increased depth, and t h e i r shapes are variable. Because the shaft i s a v i t a l component i n underground mines, these p i l l a r s are designed with a high safety factor. Table 3 summarizes the p i l l a r c l a s s i f i c a t i o n . The design methodol-ogy and dimensioning formulas applicable to each category w i l l be developed i n the following chapters. Most of the previous p i l l a r d e f i n i t i o n s were taken from "Roche Mines Associates" (1984)^ as well as Figures 5, 6, and 7 reproduced in Appendix C, which i l l u s t r a t e the d i f f e r e n t kinds of p i l l a r . TABLE 3 PILLAR CLASSIFICATION SUMMARY Category 1 Plate P i l l a r s Category 2 Separation P i l l a r s Category 3 Stub P i l l a r s Category 4 Inclined P i l l a r s A B Hard Rock Soft Rock A B Hard Rock Soft Rock A B Hard Rock Soft Rock A B Hard Rock Soft Rock Crown Roof Level Strike Horizontal S i l l Surface Rib Barrier Dip Entry Transverse Abutment Centre Panel Stub S p l i t " P i l l a r " Remnant (R+P) Chain Post Inclined Inclined 1 —-* / • / + / 4 ^ t t III CHAPTER 3 Review of P i l l a r Design Methods 19 3.1 Introduction The p r i n c i p l e f or designing any underground structure i s simple: strength > ^ stress Thus, a p i l l a r w i l l remain stable i f the load applied i s less than i t s long term load bearing c a p a b i l i t y . D i f f i c u l t i e s a rise i n estimating the p i l l a r ' s ultimate strength as well as the precise load acting upon i t . P i l l a r strength: Because of the rock material's complexity and v a r i a b i l i t y , the evalua-t i o n of rock mass strength i s perplexing. Furthermore, the true strength of a p i l l a r can only be calculated a f t e r considering the strength of the p i l l a r material together with: - The p r o b a b i l i t y of including a weakness zone in the p i l l a r - The deformation and t r i a x i a l strength of the p i l l a r material - The geometry of the p i l l a r - The p i l l a r as part of the general rock structure. Also, environmental factors may cause a time dependent a l t e r a t i o n of the p i l l a r strength. P i l l a r load: As mentioned i n Chapter 2, the load acting on a p i l l a r i s a function of: - The v i r g i n stress - The stress induced by mining - Geological features - P i l l a r shape and orientations - Openings and general mine structure - Ground water. 20 Hence, the stress l e v e l induced i n p i l l a r s ( p i l l a r load), changes as mining progresses. Although several techniques can be used to measure i n s i t u stress, these are expensive, and the re s u l t s are not always r e l i a b l e . Because there are so many factors involved i n the complex mechanism of p i l l a r loading (and deformation) as well as p i l l a r strength, the designer must depend upon numerous methods to account f o r these factors. The following summarizes the most important designing methods. They are divided into four groups, according to t h e i r le v e l of sophistication. Group 1 - Experience Methods Group 2 - Empirical Methods Group 3 - Theoretical Methods Group 4 - Computer Methods. It should be noted that every method, i f used c o r r e c t l y , i s capable of pro-ducing adequately sized p i l l a r s with respect to safety. 3.2 Group 1. Experience Methods This i s by far the most widely used and the least sophisticated method. Based on observations, history, and on the designer's " f e e l i n g " for the rock, i t also relates to sim i l a r work completed i n corresponding geological s i t u a t i o n s . A conservative dimensioning i s f i r s t l a i d out and modifications may have to be made according to the requirements and performance of the de-signed structure. No s p e c i f i c experience method i s proposed, but i t i s strongly recommended that detailed active f i l e s be kept on information concerning the mine s t a b i l -i t y : f a i l u r e s , slabbing, squeezing, caving, convergence, et cetera. This w i l l improve the future experience design and may lead to an empirical approach. 3.3 Group 2. Empirical Methods An empirical method i s the qu a n t i f i c a t i o n of experience into designing formulas or curves. Because most of these methods do not take into account many important factors, one should be aware of the conditions i n which they were developed. While the majority of empirical p i l l a r design methods considers strength and stress separately, some do incorporate strength and stress into a dimensioning formula. The following i s a review of the most important empirical methods. A b r i e f description, the formula(s) and the parameters are given. As well (referring to Chapter 2's p i l l a r c l a s s i f i c a t i o n ) , the p i l l a r categories which can be designed by each method are indicated. 3.3.1 Empirical Strength Formulas. Empirical p i l l a r strength formulas e s s e n t i a l l y involve extrapolating the r e s u l t s of laboratory tests on rock specimens, to f u l l - s i z e mine p i l l a r s . A) Size Effect Formula (Appendix A. Section 3.1) a p = a c [A + B(£)] where: Op = P i l l a r strength (psi) o c = Uniaxial compressive strength of a cube of p i l l a r material W = P i l l a r width h = P i l l a r height A, B = Constants given i n units of p i l l a r strength (Table 4). Description: Rocks have a natural strength anisotropy which i s predominantly due t the presence of d i s c o n t i n u i t i e s ( i . e . j o i n t s , c l e a t s , blast fractures, et cetera) but can also be attributed to variations in rock f a b r i c ( i . e . f o l i a t i o n , bedding planes, et cetera) and mineralogy. As rock samples of constant shape increase i n si z e , the strength of the specimen decreases. Table 4 gives the constants proposed by di f f e r e n t authors to model t h i s be haviour. TABLE 4 CONSTANTS A AND B USED IN THE "SIZE EFFECT FORMULA" SOURCE FORMULA W/H Bunting (1911) 0. .700 + 0. .300 w h 0, .5 - 1 .0 Obert et a l (1960) 0, .778 + 0, .222 w h 0, .5 - 2. .0 Bieniawski (1968) 0, .556 + 0, .444 w h 1 .0 - 3, .1 Van Heerden (1973) 0. .704 + 0, .296 w h 1 .14 - 3 .4 Sorensen 5 Pariseau (1978) 0. .693 + 0. .307 w h 0 .5 - 2, .0 - Applicable to p i l l a r categories: 3. Stub P i l l a r s 4. Inclined P i l l a r s . B) Shape Effect Formula (Appendix A, Section 3.2) where: Op = P i l l a T strength (psi) K = Constant related to the p i l l a r material W = P i l l a r width h = P i l l a r height a, b = Dimensionless constants 2 3 Description: The shape effect denotes a difference i n the unit strength for p i l l a r s of d i f f e r e n t shape but equal cross-section. A change i n mode of f a i l u r e i s one apparent cause of shape e f f e c t . Slender p i l l a r s tend to f a i l by means of a li m i t e d number of fractures. For wide p i l l a r s the p r o b a b i l i t y of developing a single continuous fracture plane i s l e s s . Thus, f a i l u r e of the p i l l a r r e s u l t s from crushing of the p i l l a r material, thereby increasing p i l l a r strength. The t r i a x i a l state of stress i n a squat p i l l a r ' s inner core also contributes to an increase i n p i l l a r strength. Table 5 gives the constants a and b proposed by d i f f e r e n t authors to model t h i s behaviour. TABLE 5 CONSTANTS a AND b USED IN THE "SHAPE EFFECT FORMULA" SOURCE FORMULA a b Streat (1954) kh" 1 . o o w o . 5 0, .5 1. 00 Holland-Gaddy (1962) kh' 1 . o o w o . 5 0, .5 1. 00 Greenwald et a l (1939) kh-° . 6 3 w 0 . 5 0, .5 0. 833 Hedley 6 Grant (1972) kh-° . 7 5 w 0 - S 0, .5 0. 75 Salamon § Munro (1967) kh" 0 . 6 6 w 0 . » S 0. .46 0. 66 Bieniawski (1968) kh-° • 5 5 w ° - 1 6 0. .16 0. ,55 Morrison et a l kh"° .5 w 0 . 5 0. .5 0. 5 Zern (1926) kh"° .5 w 0 . 5 0. .5 0. ,5 Hazen d, A r t i e r (1976) kh"° •5 w 0 . 5 0, .5 0. .5 Holland (1956) k h - 0 .5 w 0 . 5 0. .5 0. .5 - Applicable to p i l l a r categories: 3. Stub P i l l a r s 4. Inclined P i l l a r s C) Salamon "Modified" Shape Effect Formula (Appendix A, Section 3.3.4) o p = K — £ • , where We = /Wa .W2 h where: Op = P i l l a r strength (psi) K = Constant related to the p i l l a r material compressive strength Wi,W2 = Cross-section sides of the p i l l a r s We = The equivalent width f o r a rectangular p i l l a r h = P i l l a r height a,b = Dimensionless constants Description: 2 The results of underground tests (Wagner, 1974) on coal p i l l a r s have shown that p i l l a r s of rectangular cross-sections are about 40% stronger than square p i l l a r s of the same width and height. A reasonably good estimate of the strength of rectangular p i l l a r s can be obtained by substituting the square root of the cross-sectional area of the p i l l a r for W, in the shape effect formula. - Applicable to p i l l a r categories: 3. Stub P i l l a r s 4. Inclined P i l l a r s D) Sheorey and Singh "Modified" Shape Effect Formula (Appendix A, Section 3.3.4) °P • .b h whe re: Op = P i l l a r strength (psi) K = Constant related to the axi a l compressive strength of the p i l l a r material Wi,W2 = Cross-section sides of the p i l l a r h = P i l l a r height a,b = Dimensionless constants (Table 5) Description: This method as the Salamon modified formula uses the concept of an equivalent width. However, Sheorey and Singh recommend using the average value of the rectangular cross-section sides as equivalent width. - Applicable to p i l l a r categories: 3. Stub P i l l a r s 4. Inclined P i l l a r s E) Heek and Brown Curves (Appendix A, Section 3.3.7) Ci = a 3 + v/maca3 + sa£ where: 0\ = Major p r i n c i p a l stress at f a i l u r e o 3 = Minor p r i n c i p a l stress at f a i l u r e o"c = The un i a x i a l compressive strength of intact rock material m and s are constants which depend upon the properties of the rock and upon the extent to which i t has been broken be-fore being subjected to the stresses o*i and a 3 . Description: 3 The Hoek and Brown curves were developed based on the assumption that the o v e r a l l strength of a p i l l a r i s approximately equal to the average strength across the centre of the p i l l a r . Figure 8 shows the re s u l t s of a series of calc u l a t i o n s using stress d i s t r i b u t i o n from computer modelling, together with Hoek's f a i l u r e c r i t e r i o n . Once the rock mass quality i s de-fined, one may determine the p i l l a r strength f o r d i f f e r e n t p i l l a r dimensions. - Applicable to p i l l a r categories: 2. Separation P i l l a r s 3. Stub P i l l a r s 4. Incl-ined P i l l a r s . 26 -p so E (D U +» W > -H W W <D U e o o x rt 3 +> bD P w ft rt > 3.0r Intact samples of fin e grained igneous c r y s t a l l i n e rock m=l? , s=l Very good quality rock mass m=8.5 , s=0.1 Good quality rock mass m=1.7 , s=0.004 F a i r quality rock mass m=0.34 , s=0.0001 Poor qu a l i t y rock mass m=0.09 , s=0.00001 P i l l a r width/height ; W^ /h FIGURE 8 Influence of P i l l a r Width to Height r a t i o on Average P i l l a r Strength. After Hoek and Brown ( 1 9 8 0 ) ^ 27 3.3.2 Empirical Stress Formulas A) The Extraction Ratio Formula or Tributary Area (Appendix A, Section 1.3.4) _ TH (W+BHL+B) °p - i n n — where: a p = P i l l a r load Y = Unit weight of the rock H = Depth below surface B = Width of the opening L = P i l l a r length W = P i l l a r width. Description: If a large area i s mined out with a reasonably uniform pattern of p i l l a r s , i t can be said that nearly the whole weight of the overburden w i l l be carried by the p i l l a r s in equal proportions. Figure 9, Hoek and Brown (1980)^ gives the extraction r a t i o formula f o r d i f f e r e n t p i l l a r shapes. It should be noted that the t r i b u t a r y area theory represents the upper l i m i t of the average p i l l a r stress. (Overestimates the load on p i l l a r s by about 40%). 4 Bieniawski (1983) . The tr i b u t a r y area does not take into account the arch-ing e f f e c t , or any other mechanical behaviour of the overlying strata. - Applicable to p i l l a r categories: 2. Separation P i l l a r s 3. Stub P i l l a r s . B) Chain P i l l a r Formula (Appendix A, Section 1.3.8) Swilski (1983) 5 a = -1 2 v v h  UP yH ' (Lp+S)(Wp+2Wp+3S) where: Op = P i l l a r load (psi) Y = Unit weight of the rock RIB PILLARS SQUARE PILLARS crp- = / M l + W o/w } crp =y z { l + w V w p ) FIGURE 9 Average V e r t i c a l P i l l a r Stresses i n Typical P i l l a r Layouts. I l l u s t r a t i o n s are a l l plan views. After Hoek and Brown (1980)- 3 29 H = Depth below surface wp = P i l l a r width LP " P i l l a r length S = Spacing between chain p i l l a r s wF = Width of the face. COAL RIBSIDE y Z T - J IL -J-s a r e a o f I P strata load ' I \ i COAL FACE w COAL PANEL Ch§in p i l l d t s ¥ \ T a i l Entry FIGURE 10. Determination of Load on Chain P i l l a r s by the " f i r s t panel" Load Concept. After Szwilski (1983) 5 Description: The chain p i l l a r formula i s based on the extraction r a t i o formula but i t considers the extra load acting on the chain p i l l a r s by the cantilever action of the immediate roof. However, th i s s i m p l i f i e d procedure ignores the effect of the gob support, creating a pressure arch from the compacted gob to the nearest s o l i d coal panel. - Applicable to p i l l a r category: 3B - Chain P i l l a r s . C) Subsidence Formula (Appendix A, Section 10.3) Whittaker and Singh (1981) 6 0 p = imrp2 ( p + w ) • D • 1 / 4 w 2 + c o t * -p For W/D < 2 tan <f> and a p = 9.81 y PfP.D + D2tancf0 For W/D > 2 tan <f> where: o"p = P i l l a r load (psi) Y = Average density of the overburden <j> = Angle of shear of roof strata at edge of long-wall extraction and measured to v e r t i c a l P = Width of b a r r i e r p i l l a r W = Width of longwall extraction D = Depth below surface. Description: The subsidence theory has been applied to the b a r r i e r p i l l a r s i t u a t i o n to ascertain the extent of str a t a pressure arching across a longwall ex-t r a c t i o n to produce loading of the adjacent b a r r i e r p i l l a r s . B a s i c a l l y , t h i s approach assumes that the goaf area behind the longwall i s loaded by a triangular roof mass which shears at an angle 4> to the v e r t i c a l . The loading developed by the mass of roof strata outside the triangular region i s presumed to be transferred to the b a r r i e r p i l l a r s . - Applicable to p i l l a r category: 2B - Barrier P i l l a r s 3.3.3 Empirical Dimensioning Formulas. Other empirical formulas do not consider stress and strength separately P i l l a r dimensioning formulas are often used to design coal b a r r i e r p i l l a r s . 3 A) Mines' Inspector Formula (Appendix A, Section 10.2) Ashley (1930) W = 20 + 4T + 0.1D where: W = Width of p i l l a r (feet) T = Bed Thickness (feet) D = Thickness of the overburden (feet) Description: The Ashley formula was developed from experiments in the Pennsylvania coal f i e l d s . It i s based on the conservative assumption that an arch of height equal to ha l f the panel width w i l l s t a b i l i z e . Simple hand c a l c u l a -tions based on the above assumption r e s u l t i n p i l l a r sizes with width to height r a t i o s of approximately three to f i v e depending upon depth, p i l l a r height and panel width. - Applicable to p i l l a r category: 2B - Barrier P i l l a r s B) Holland Formula (Appendix A, Section 10.2) D = 1ST or = ±%L™JL K log e where: D = Width of Barrier P i l l a r (feet) T = Thickness of p i l l a r (feet) W2 = The estimated convergence on the high stress side of the p i l l a r (mm). (W2 may be estimated with Fig. 11) K = Constant = 0.09 i f caving following mining i s permitted = 0.08 i f s t r i p packs are b u i l t = 0.07 i f hydraulic stowage i s carried out. 32 D e s c r i p t i o n : The Holland formula i s based on the convergence studies by B e l i n s k i and Bore c k i (1964) . Compared with Ashley's formula, i t gives a more r e a l i s t i c p i l l a r width and considers p i l l a r t h i c k n e s s , as w e l l as other p e r t i n e n t f a c -t o r s . Holland's formula, however, i s incomplete i n that i t disregards the pr o p e r t i e s of the p i l l a r rock. Consequently, t h i s method should be a p p l i e d only i n c o n d i t i o n s s i m i l a r to those i n which Holland experimented. (Figure l l ) 1000 700 5 00 400 300 200 100 6 0 40 30 20 0 0 0 400 800 1200 1600 2000 Thickness of Overburden ( F t . ) HS: H y d r a u l i c Stowage C : Caved SP: S t r i p Packed PJ?s Room & P i l l a r L : Longwall 2400 2800 FIGURE 11 Observed Value of W2 f o r Coal seams 7 f t . Thick and Having a Crushing Strength i n the 3 i n . Cube of 3000 p s i . + 10% - A p p l i c a b l e to p i l l a r category: 2b - B a r r i e r P i l l a r s . 33 C) Morrison, Corlett and Rice. (Appendix A, Section 10.2) W = j D for D < 4000 feet where: W = Width of p i l l a r (feet) D = Depth below surface (feet) Description: The two previous formulas were developed s p e c i f i c a l l y f o r coal. The Morrison, Corlett and Rice formula gives s a t i s f a c t o r y r e s u l t s in most kinds of rock. Nonetheless, i t oversimplifies the problem and should be used as a guide or preliminary estimation only. - Applicable to p i l l a r categories: 2a) - Abutment P i l l a r s b) Barrier P i l l a r s Note Not applicable to Rib Entry and Dip P i l l a r s . D) Barrier P i l l a r Formula (Appendix A, Section 10.2) W = I i D + 15 where: W = Width of p i l l a r (feet) D = Depth below surface (feet) Description: This formula i s cited i n the l i t e r a t u r e as a t r a d i t i o n a l rule of thumb approach to designing b a r r i e r p i l l a r s . Again i t i s oversimplified and should be used as a rough estimation only. - Applicable to p i l l a r category: 2B - Barrier P i l l a r s 34 3.4 Group 3. Theoretical Methods The t h e o r e t i c a l methods attempt to evaluate mathematically the p r i n c i p a l factors a f f e c t i n g the stress and strength of p i l l a r s . A more r e a l i s t i c model i s then proposed. However, the behaviours of p i l l a r s are very complicated and to be consistent with the theory, the methods need a f a i r number of i n -put parameters. C o l l e c t i n g data in a mining environment (especially at the early stage of a mine's l i f e ) i s not an easy task, and often the techniques are too expensive or not adequately advanced to provide accurate data. Because the t h e o r e t i c a l methods are complex and d i f f i c u l t to apply, the re-su l t s are often not r e l i a b l e . They are useful in further comprehending the mechanism involved i n p i l l a r design. 3.4.1 Theoretical Strength Formulas At least four t h e o r e t i c a l methods have been reviewed i n the l i t e r a t u r e research: Appendix A - Coates (Section 3.3.3) - Grobbelaar (Section 3.5.1) - Wilson (Section 3.5.2) - Panek (Section 3.5.3) It was noted that only Wilson's method has been used by designers, and a b r i e f description of t h i s method i s given below. A) Confined Core Method (Wilson) Y 1 i _ c^ Jo h " (tan B) u- = u (tan B-l) ' n a. where: y = The depth of y i e l d zone from the r i b s i d e (feet) h = Seam height (feet) Ov = The maximum p i l l a r stress (psi) (situated at the y i e l d zone/confined core interface) Oo = Unconfined compressive strength (psi) •Tan 3 = T r i a x i a l stress c o e f f i c i e n t 1 + s i n (j) 1 - s i n $ <}) i s .the angle of i n t e r n a l f r i c t i o n of the c o a l . Description: This concept recognizes that a " y i e l d " or " f r a c t u r e " zone develops around the periphery of a p i l l a r which confines a c e n t r a l e l a s t i c core. Because of t h i s confinement the inner core i s subjected to t r i a x i a l stress conditions. The l i m i t of the average core stress i s reasoned to be equal to the p i l l a r peak abutment s t r e s s , which i s located at the y i e l d zone/confined core i n t e r f a c e . Based on t h i s assumption, p i l l a r strengths can be c a l -culated . - Applicable to p i l l a r category: 2B. Ba r r i e r P i l l a r s 3.4.2 T h e o r e t i c a l Stress Formula Five t h e o r e t i c a l methods to evaluate the stress a c t i n g on a p i l l a r have been reviewed i n the l i t e r a t u r e research. Appendix A - Beam and Plate Theory (Section 1.3.5) - Wall d e f l e c t i o n theory (Section 1.3.6) - Photoelastic data (Section 1.3.7) - Pariseau (Section 9.2) - Hedley (Section 9.3) The wall d e f l e c t i o n formula and the photoelastic technique were r e l a -t i v e l y popular i n the past but they are no longer widely used. Thus they w i l l not be reviewed.Although they played an important r o l e i n the early development of rock mechanics, they can now be replaced by more e f f i c i e n t techniques. A) Beam and Plate Theory Method Description: A number of equations were derived from C i v i l engineering beam theory. Some of them may be used to design p i l l a r s i f they r e a l i s t i c a l l y describe the i n s i t u underground s i t u a t i o n . A complete understanding of the theory as well as the implications of the input parameters are e s s e n t i a l . - Applicable to p i l l a r categories: 1. Plate P i l l a r s 2. Separation P i l l a r s B) Pariseau Inclined P i l l a r Formulas SP Y h ( 1 * Kp)+ (1 - Kg) cos 2* 1 - R y h Cl-Ko) sin 2cQ 1 - R where: Sp = Average p i l l a r stress in the normal d i r e c t i o n tp = Average p i l l a r shear stress Y = Unit weight of the rock h = Depth below surface KQ = Ratio of horizontal over v e r t i c a l v i r g i n stress R = Extraction r a t i o « = I n c l i n a t i o n of the seam Description: Pariseau proposed an extension of the a p p l i c a b i l i t y of the extraction r a t i o (or t r i b u t a r y area) formula to i n c l i n e d seams of a r b i t r a r y dip. The shear forces caused by the seam's i n c l i n a t i o n i s accounted f o r . - Applicable to p i l l a r category: 4. Inclined P i l l a r s 3? C) Hedley's Modified Formula for Inclined P i l l a r s yh c o s 2 ^ + OH s i n 2 * P 1 - R where: a-P Average p i l l a r stress i n the normal d i r e c t i o n Y Unit weight of the rock h Depth below surface R Extraction r a t i o CC Horizontal v i r g i n stress I n c l i n a t i o n of the orebody Description: The pre-mining stress f i e l d and extraction r a t i o are the two p r i n c i p a l factors a f f e c t i n g p i l l a r stress. For i n c l i n e d workings Hedley stated that the normal stress acting on the seam i s a combination of the components of v e r t i c a l stress and horizontal stress. This combination i s used i n the ex-t r a c t i o n r a t i o formula to determine the average p i l l a r stress. - Applicable to p i l l a r category: 4. Inclined P i l l a r s 3.5 Group 4. Computer Methods The computer methods are v e r s a t i l e and may be adapted to every p i l l a r category. Also, the use of d i g i t a l computers i n underground mine design i s , from the mathematical point of view, the most precise method. However, the accuracy of the r e s u l t s i s related d i r e c t l y to the quality of the input data. Table 6 gives the c h a r a c t e r i s t i c input data required by computer models. Numerous computer programs are used by rock mechanic s p e c i a l i s t s . The methods are summarized in Table 7 and are divided into two groups: - integral methods - derivative methods At present, very few Canadian mines have t h e i r own computer models. Mine designers generally prefer to r e l y on consultants' expertise for the sophisticated methods. More information on "BITEM", the boundary element gram used i n t h i s study i s available i n APPENDIX D . TABLE 6 CHARACTERISTIC INPUT DATA FOR COMPUTER METHODS 1. Rock(s) Strength Parameters: - Uniaxial compressive strength ( a c ) - Unit weight (y) 2. V i r g i n Stress: V e r t i c a l Stress (a ) v Horizontal stress (a„) n 3. General Mine Geology 4. General Structure and Geometry of the Mine 5. Rock Deformation Indices A. E l a s t i c : - E l a s t i c Modulus (E) - Poisson's Ratio (v) B. P l a s t i c - Creep Constants - V i s c o s i t y constants 6. Others - m and s Indices (Hoek c r i t e r i a ) - F r i c t i o n angle. 39 TABLE 7 SUMMARY OF COMPUTER METHODS Integral Methods - Boundary elements - 2 dimensional - 3 dimensional - Displacement d i s c o n t i n u i t i e s - 2 dimensional - 3 dimensional Derivative Methods - F i n i t e elements ' - 2 dimensional - 3 dimensional - F i n i t e difference - 2 dimensional - 3 dimensional Hybrid Methods - Mixed Boundary and F i n i t e elements Programs have been developed recently. F i n a l l y , a summary i s given below of the p r i n c i p a l design methods reviewed i n t h i s chapter. Further information on these methods, formulas and curves are available in the l i t e r a t u r e review, Appendix A. GROUP 1. EXPERIENCE METHODS GROUP 2. EMPIRICAL METHODS STRENGTH PILLAR CATEGORY STRESS PILLAR CATEGORY DIMENSIONING PILLAR CATEGORY a) Size Effect (3,4) a) Extraction r a t i o (tributary area) (2,3) a) Ashley (2b) b) Shape Effect (3,4) b) Chain P i l l a r (3b) b) Holland (2b) c) Salamon Modi-fi e d (3b,4) c) Subsidence (2b) c) Morrison, Co r l e t t , Rice (Abutment P i l l a r ) d) Sheorey, Singh (3b,4) d) Barrier P i l l a r (2b) e) Hoek curves (2,3,4) GROUP 3. THEORETICAL METHODS STRENGTH PILLAR CATEGORY STRESS PILLAR CATEGORY a) Wilson (2b) a) Beam Theory (1,2) * Grobbelaar b) Pariseau (4) * Coates c) Hedley (4) * Panek * Photoelastic analysis * Wall de f l e c t i o n (Coates) GROUP 4. COMPUTER METHODS * Are no longer widely used. CHAPTER 4 PILLAR DESIGN PROCEDURE 42 4.1 Philosophy of P i l l a r Design Hoek and Brown (1980) 3 have described the philosophy of underground structure design as follows: "The basic aim of any underground structure design should he to u t i l i z e the rock i t s e l f as the principal structural material, creating as l i t t l e disturbance as possible during the excavation process and add-ing as l i t t l e as possible in the way of concrete or steel support. The extent to which this design aim can be met depends upon the geolo-gical conditions existing on site and the extent of the designer's awareness and consideration of these conditions. "A good engineering design is one of balance and one in which all factors interact. Designers must consider even those elements which cannot be quantified. " A good p i l l a r design i s one properly sized f o r both safety and e f f i -ciency. An optimum sized p i l l a r might be defined as the smallest one that s a t i s f i e s safety requirements. 4.2 Design Procedure Many p i l l a r design methods, formulas and curves have been reviewed in Chapter three, but none of these i s completely independent. In the following f i v e phase design procedure, the designer uses several methods which become more sophisticated as experience with the rock material is gained. Also, design charts are included to help select suitable methods fo r each type of p i l l a r . 4.2.1 Phase 1. Experience Design. I n i t i a l l y a f a i r amount of uncertainty exists concerning the mechanical behaviour of rock on a large scale, and on the location, attitude, and pro-perties of f a u l t s or j o i n t s . Hence, at t h i s stage, only a conservative preliminary design i s possible, using the designer's experience and the study of similar case h i s t o r i e s . Also during t h i s phase, the c o l l e c t i o n of rock mechanics data should be undertaken to prepare f o r the following phases of design, which employ more sophisticated methods. 4.2.2 Phase 2. P i l l a r Structural Analysis The second phase objective i s to determine whether plane(s) of weakness (faults or major d i s c o n t i n u i t i e s ) control the p i l l a r ' s s t a b i l i t y . These d i s c o n t i n u i t i e s a f f e c t the p i l l a r strength because they reduce the resistance to s l i d i n g (shear f a i l u r e ) . This can occur i n two ways: 1. By a single plane and movement that takes place along the plane (Figure 12) 2. By inte r s e c t i n g planes (Figure 13). The movement may be in the d i r e c t i o n of the trend and plunge of t h e i r intersections, or along one of the single planes. 4.2.2.1 P i l l a r Transection V e r i f i c a t i o n For very simple cases, a scale drawing may be s u f f i c i e n t to determine whether the p i l l a r f a i l u r e may be s t r u c t u r a l l y controlled. However, for more complicated situations a sterographic method w i l l be required. Com-prehensive instructions for using the stereographic technique i s reproduced g in Appendix VI of the l i t e r a t u r e review, J . A. Tousseuil . Because the plane must intersect both sides of the p i l l a r and be con-tinuous over i t s entire length, p i l l a r s having a high width to height r a t i o are not l i k e l y to f a i l by s l i d i n g . FIGURE 12 P i l l a r Transected by a Single Plane of Weakness. A f t e r Touseull 9 45 4.2.2.2 Shear S t a b i l i t y Analysis If transection occurred, Hoek and Brown (1980) 3 suggest evaluating the shear s t a b i l i t y along a f a u l t or major discontinuity using the following technique: - estimate these parameters on several points along the f a u l t d : major p r i n c i p a l stress 0 3 : minor p r i n c i p a l stress 6 : angle between the f a u l t and O j Assume that Shear Stress: T = 1/2 (o"i - a 3) sin 2B (1) Normal Stress: a - 1/2 (Ci+a 3) - ( a i - a 3 ) cos 2B (2) and the shear strength TS of the f a u l t i s defined by: xs = c + a Tan <j> (3) where: c i s the cohesion <j> i s the angle of f r i c t i o n o i s the normal stress - Equation 3 in Equation 2 TS = c + l/2((ai+o 3) - (Oi-a 3) cos 26) tan cf> - Then, a factor of safety (TS/ t) can be calculated along the f a u l t and gives an indi c a t i o n of the potential f o r s l i p on the f a u l t . This analysis should be used i n conjunction with a structural analysis to ensure that wedges which are free to f a l l or s l i d e are not formed by the f a u l t and other f a u l t s or j o i n t s . 4.2.3 Phase 3. Empirical Design The rock mechanics data c o l l e c t i o n program should now be adequately ad-vanced to provide the input parameters required by the empirical methods, which may be selected using the charts (Figures 14, 15, 16, 17). 46 If none of the empirical formulas reviewed are applicable to a p a r t i c u -l a r s i t u a t i o n , the designer may attempt to develop his own curves or formulas, adapted to h i s conditions by monitoring, many observations and good engineer-ing judgement. 4.2.4 Phase 4. Theoretical Design The r e a l aim of the o r e t i c a l design methods i s to aid i n understanding the complexity of the problem and to provide a mathematical model for rock behaviours. However, because i t requires advanced mathematics as well as a considerable amount of input data, only a few theoretical methods have been adapted to mine design. In any case, i t i s a valuable exercise to "play" with a theoretical method at t h i s stage of design. 4.2.5 Phase 5. Computer Design During t h i s phase the p i l l a r dimensions w i l l be optimized. A computer model w i l l be "adapted" to the mine's p i l l a r s . F i r s t , i t should be used to analyze case h i s t o r i e s i n order to gain confidence i n the model and to i n -vestigate the rock mass behaviour. F i n a l l y , careful underground observations, monitoring and measurements should provide feedback on each computer design. 4.3 Design Charts The charts (Figures 14 to 17) summarize the preceding f i v e phase design procedure. A chart representing each p i l l a r category ((1) Plate, (2) Separ-ation, (3) Stub, (4) Inclined) indicates the methods, relationships and formulas that should be used in the f i v e phase procedure. FIGURE lk P I L L A R CATEGORY I "plate pillars" category la . (hord r o c k ) PHASE I PHASE 2 PHASE 3 PHASE 4 PHASE 5 SURFACE PILLAR CROWN PILLAR LEVEL PILLAR STRIKE PILLAR HORIZONTAL P. ROOF PILLAR SILL PILLAR E X P E R I E N C E M E T H O D S T R U C T U R A L A N A L Y S I S E M P I R I C A L M E T H O D no melhods\ ovoiloble / T H E O R E T I C A L M E T H O D beam theory C O M P U T E R M E T H O D FIGURE 13 P I LLAR CATEGORY 2 "separation p i l l a r s " category 2a ( hard rock) PHASE I PHASE 2 PHASE 3 PHASE 4 PHASE 5 RIB P I L L A R OIP P I L L A R T R A N S V E R S E A B U T M E N T -E X P E R I E N C E METHOD E X P E R I E N C E M E T H O D S T R U C T U R A L ANALYS IS E M P I R I C A L M E T H O D T H E O R E T I C A L M E T H O D Hoek's curves t r ibutary a r e a Morr i son ,c . ,R . beam t h e o r y COMPUTER M E T H O D category 2 b (soft rock) B A R R I E R P. E N T R Y P I L LAR E X P E R I E N C E M E T H O D S T R U C T U R A L A N A L Y S I S E M P I R I C A L M E T H O D T H E O R E T I C A l M E T H O D IM C O M P U T E R M E T H O D Hoek's curves t r i bu ta ry oreo s u b s i d e n c e Ashley H o l l a n d bar rier p i l l a r b e o m t h e o r y W i l s o n FIGURE 1^ P I LLAR CATEGORY 3 "stub pi l lar" category 3a ( h a r d r o c k ) PHASE PHASE 2 P H A S E 3 PHASE 4 PHASE 5 C E N T E R PILLAF ROOM & PILLAR P O S T P I L L A R E X P E R I E N C E M E T H O D S T R U C T U R A L A N A L Y S I S E M P I R I C A L M E T H O D T H E O R E T I C A L M E T H O D t r i b u l o r y a r e o H o e k ' s c u r v e s s h a p e e f f e c t s i z e e f f e c t T I (no m e t h o d s ) C O M P U T E R M E T H O O category 3b (soft r o c k ) P A N E L P I L L A R S P L I T P I L L A R R E M N A N T P. CHAIN P I L L A R • q u a r e p l l l p r E X P E R I E N C E M E T H O D S T R U C T U R A L A N A L Y S I S E M P I R I C A L M E T H O D t r i b u t a r y a r e a H o e k ' s c u r v e s s h a p e e f f e c t s i z e e f f e c t i r e c l a n g u l a r p i l l a r I E M P I R I C A L M E T H O D T H E O R E T I C A L M E T H O D ( n o m e t h o d s ) > t r i b u t a r y a r e a 1 | H o e k ' s c u r v e s j S a l o m o n m o d i f i e d ' S h e o r e y ft S i n g h ' I > c h a i n p i l l a r ->-C O M P U T E R M E T H O D F I G U R E 1 ? PILLAR C A T E G O R Y 4 inclined pillar I I category 4 PHASE PHASE 2 PHASE 3 PHASE 4 PHASE 5 INCLINED P. E X P E R I E N C E M E T H O D s q u o r e p i l lar S T R U C T U R A L A N A L Y S I S E M P I R I C A L M E T H O D Hoek's curves shape e f f e c t —J J s i z e e f f e c t I rectongular pil)ar E M P I R I C A L M E T H O D T H E O R E T I C A L M E T H O D P a n s e a u H e d l e y Hoek's c u r v e s Salomon modified I Sheorey 8 S ingh C O M P U T E R M E T H O D CHAPTER 5 HEATH STEELE CASE HISTORY ANALYSES INTRODUCTION During the summer of 1984, four Noranda underground mines were v i s i t e d seeking p i l l a r f a i l u r e case h i s t o r i e s . The Geco "B-Block", mined out i n the early 60's, was selected because the f a i l u r e s were well documented by Bray (1967) 1 0. The 77-92 and 77-94 r i b p i l l a r f a i l u r e s at Heath Steele were also chosen to take advantage of A l l c o t and Archibald (1981) 1 1 p i l l a r design study. The examination of case h i s t o r i e s may generate pertinent and useful information f or future designs. Geco (Chapter 6) and Heath Steele (Chap-ter 5) case h i s t o r i e s are analyzed using the following procedure: Review of General Information 1. Geology 1.1 Regional geology 1.2 Mine geology 1.3 Structural geology 2. Mining method and underground structures dimensions. 3. Rock Mechanics Data 3.1 Rock strength parameters 3.2 Laboratory tests 3.3 Rock Mass C l a s s i f i c a t i o n 3.4 V i r g i n stress Review of P i l l a r Information 4. P i l l a r C h a r a c t e r i s t i c s 5. Mining sequence 6. Failure history and p i l l a r geometry 5 3 P i l l a r Design Study 7 .1 Phase 1 Experience method 7 .2 Phase 2 P i l l a r s t ructural analysis 7 .3 Phase 3 Empirical methods 7 .4 Phase 4 Theoretical methods 7 .5 Phase 5 Computer methods 5.1 Geology (After A l l c o t t and Archibald (1981) 1 1) 5.1.1 Regional Geology The massive sulphide stratiform deposits of northern New Brunswick are hosted by the Tetagouche rock group. This rock group i s highly folded, middle Ordovician in age and covers a c i r c u l a r area approximately 56 km. (35 miles) in diameter. The Tetagouche rock group i s broken into three l i t h o l o g i c a l u n its: Sedimentary, Metabasalt, and R h y o l i t i c . 5.1.2 Mine Geology The massive sulphide deposits l i e within the r h y o l i t e unit i n close proximity to the quartz feldspar c r y s t a l t u f f , which i s also known as Augen Schist and Porphyry. The stratigraphic rock units in the ore zone area top towards the north, which i s indicated by the metal zoning i n the sulphides and graded bedding i n the sediments. These units l i s t e d from youngest to oldest are as follows: (Figure 18) 1. Banded Quartz Feldspar Crystal Tuff This rock unit i s banded i n places with 5-10 cm. bands, i n t e r l a i d with varying grain size and proportions of quartz and feldspar phenocrysts. 7800 LEVEL 5. Acid Tuff 6. Sediments FIGURE 18 Heath Steele Geology Banded Quartz Crystal Tuff The quartz c r y s t a l t u f f occurs as a 9 to 15 m. (30-50 f t . ) thick bed on the hanging wall side of the massive sulphides. The por-phyry i s quite competent and fresh i n appearance, with a com-pressive strength of 56.5 MPa (8200 p s i ) . However, the fracturing tends to be blocky when exposed on the stope's wall. Iron Formation This zone i s present as a discontinuous t h i n band along the upper margin of the massive sulphide formation. However, i t also occurs i n small patches along the footwall contact and within the sulphide zone. This i s a competent bed, but i s too thin and discontinuous to be r e l i e d upon as a s t a b i l i z i n g unit. Massive Sulphides The massive sulphides are very f i n e grained, with a compressive strength of 177 MPa (22,917 p s i ) , and form the most competent rock unit i n the mine. Very l i t t l e sloughing occurs where the walls of the stopes consist of massive sulphides. Acid Tuff This i s the least competent rock unit i n the mine, and tends to be soft and sloughs r e a d i l y when exposed on the walls of stopes. It forms a 1.5 to 21 m. (5-70 f t . ) thick bed on the footwall of the sulphides and becomes discontinuous i n places. C l a s t i c Sedimentary Rocks The sedimentary rocks i n the footwall below the acid t u f f are i n t e r -calated with quartz feldspar c r y s t a l t u f f s and form a band approxi-mately 366 m. (1200 f t . ) thick. 56 5.1.3 Structural Geology The B Zone i s a tabular shaped v e r t i c a l or steep northerly dipping massive sulphide body, which s t r i k e s at N 73°E. The massive sulphides have a s t r i k e length of approximately 1150 m. (3,800 f t . ) , vary i n thickness from a few centimeters to 75 meters (250 f t . ) and have been traced to a depth of 1097 m. (3,600 f t . ) . Folding i s the primary s t r u c t u r a l control and although there i s minor f a u l t i n g , f a u l t s have had no major influence on the shape of displacement of the ore zone. The orebody has undergone f i v e periods of fo l d i n g which are numbered one to f i v e i n time sequence as they occurred. (Figure 19) FIGURE 19. .Diagramatical Plan View of Heath Steele Orebody, Showing Orientation of Folding Si: The f i r s t period of f o l d i n g l e f t very l i t t l e i f any imprint on the massive sulphide. The only r e a l evidence for t h i s period of fo l d i n g i s a few f l a t l y i n g r e l i c t cleavage planes i n the host rocks. S2: The second period of f o l d i n g had the greatest effect on the shape of the orebody. This period has shaped the orebody into a number of shaped i s o c l i n a l f o l ds which plunge at approximately 60° i n a S 73° W d i r e c t i o n . S 3 : The t h i r d period of deformation produced open or tight concentric folds which plunged steeply northwest or southeast. This folding-leads to some d i l u t i o n problems i n mining as the folds are d i f f i -c u l t to define with the normal f i f t e e n meters (SO f t . ) spaced d e f i n i t i o n diamond d r i l l i n g . S 4 : The fourth period of deformation resulted i n a series of open, con-ce n t r i c folds, which plunged to the northwest and appeared as not more than gentle warps. S j : The f i f t h period of f o l d i n g produced open folds which plunged 70° i n a northeasterly d i r e c t i o n . This period of deformation produced only rare folds i n the mine area. - JOINTING Golder Associates (1981) 1 2 There are two major j o i n t sets evident throughout the mine. Both are steeply dipping, with one set approximately p a r a l l e l to the strike of the orebody and one approximately transverse to the s t r i k e . A t h i r d set of near horizontal j o i n t s appeared to be more prominent i n the sulphides. The j o i n t set p a r a l l e l to the s t r i k e indicated a spread i n strike d i r e c t i o n and i s probably a combination of two j o i n t sets. Joints are usually planar or s l i g h t l y undulating and spaced at about 1 B . to 3 a . (3.3 - 9.8 f t . ) . 5.2 Mining Method and Underground Structures Dimension After A l l c o t t and Archibald (1981) 1 1 Mining of Heath Steele B Zone orebody proceeded with blast hole open stoping method, with l a t e r selected area f i l l i n g . Extraction progressed from the upper levels to the lower levels and east to west with production maintained on two to three levels simultaneously. The production rate was 3000 tons per day from 1970 to 1976, increased to 3,500 tons per day in 1977, and f i n a l l y reached 4,200 tons per day in 1982. Mining of 8600 production l e v e l , 110 m. (360 f t . ) below surface, was completed without any serious s t a b i l i t y problems and a d i l u t i o n factor of 10% was adequate compensation for overbreak or minor f a l l s of waste. Five centimeters (2 inches) diameter blasthole rings were used, and underground ore haulage was by Track equipment. Stope dimensions were generally about 30 m. (100 f t . ) on st r i k e and up to 45 m. (150 f t . ) high. Twelve meters (40 f t . ) r i b p i l l a r s were l e f t to separate the stopes. Mining advanced to the 8300 production l e v e l , at 200 m. (650 f t . ) be-low surface, using the same method. Stope height was increased to 60 m. (200 f t . ) and trackless load-haul-dump was introduced. Although i t had been easy to mine adjacent footwall and hanging wall stopes with an intervening 15 m. (50 f t . ) p i l l a r of low grade sulphides and the 10% d i l u t i o n was s t i l l s a t i s f a c t o r y , at t h i s stage small scale sloughing started from the west side of 83-78 r i b p i l l a r . The cause was ascribed to the in t e r s e c t i o n of j o i n t s at the face of the p i l l a r , but not to loading. Mining started on 8050 production l e v e l , 275 m (900 f t . ) from surface. It was decided to remove s i l l p i l l a r s under 8300 level so that the new stope heights would be increased to 140 m. (450 f t . ) . At t h i s time the st r i k e length was 45 m. (150 f t . ) and the r i b p i l l a r s were 15 m. (50 f t . ) wide. Real s t a b i l i t y problems occurred when recovering r i b p i l l a r s between primary stopes. In the f i r s t instance a r i b p i l l a r that was instantaneously blasted caused the adjacent r i b p i l l a r to burst and i n i t i a t e d a cave i n the back extending over a 150 m. (500 f t . ) s t r i k e length. After t h i s , stope lengths were limited to 43 m. (140 f t . ) , and 85 m. (280 f t . ) height. The r i b p i l l a r lengths were increased to 18 m. (60 f t . ) Ground problems were encountered with increasing frequency as the depth from surface increased. The l a s t procedure u t i l i z e d b a c k f i l l i n g the c r i t i c a l area and removing r i b p i l l a r s only between f i l l e d stopes. At l e v e l 7430 the stope dimensions were presumed to be 30 m. (100 f t . ) long, 60 m. (200 f t . ) high and separated by 30 m. (100 f t . ) r i b p i l l a r s . 5.3 Rock Mechanics Data 5.3.1 Rock Strength Parameters - Density Ore: y = 4581 kg/ m3 (286 |p-) Waste: y = 2883 kg/ m3(179 i | f ) - E l a s t i c Modulus F-W Chlorite Tuff E = 68,536 MPa (9.9 M. psi) Ore Massive Sulphide E =119,284 MPa (17.3M. psi) H-W Qtz. Porphyry E = 68,743 MPa (9.9 M. psi) - Poisson's Ratio F-W Chlorite Tuff v = 0.25 Ore Massive Sulphide v = 0.24 H-W Qtz. Porphyry v = 0.19 5.3.2 Laboratory Testing - Unconfined Compressive Strength F-W Chlorite Tuff o c = 84 MPa (12,182 psi) Ore Massive Sulphide 176.5 MPa (25,598 psi) H-W Qtz. Porphyry o c = 91 MPa (13,198 psi) 60 5.3.3 Rock Mass C l a s s i f i c a t i o n The footwall, hanging wall and orebody rocks were c l a s s i f i e d by Golder 12 Associates (1981) using the NGI system. Results are given below as well as an estimated CSIR rating for comparison purposes. 77-92 Cross-.Cut Footwall - C h l o r i t e Tuff NGI RQD 95 90 Jn 3 6 Jr 2 2 Ja 0. .75 0. .75 Jw 1. .0 1. .0 SRF 1. .0 1. .0 Q 84 40 CSIR Intact Strength 7 RQD 20 Spacing of Joints 25 Condition of Joints 6 Ground Water 1_0 68 77-92 Cross-Cut Sulphides NGI RQD 85 95 Jn 6 6 Jr 1 1 Ja 0.75 0.75 Jw 1.0 1.0 SRF 1.0 1.0 Q 18.9 21 CSIR Intact Strength 12 RQD 17 Spacing Joints 25 Condition of Joints 6 Ground Water 7 67 I 6 1 7 7 - 9 2 Gross-Gut Hangingwall Porphyry NGI RQD 9 5 9 5 Jn 6 3 . 0 J r 2 . 0 1 . 0 Ja 0 . 7 5 0 . 7 5 Jw 1 . 0 1 . 0 SRF 1 . 0 1 . 0 42 42 GSIR I n t a c t Strength 7 RQD 2 0 Spacing of J o i n t s 2 5 Condition of J o i n t s 6 Ground Water 10 w 5.3«4 V i r g i n S t r e s s No v i r g i n s t r e s s measurements have been made at the mine. Measurements have been achieved at Brunswick Mining, which i s located, i n the same rock formation about 5 0 km ( 3 0 miles) away. The r e s u l t s at 7 00 m. ( 2 3 0 0 f t . ) depth are as f o l l o w s : (Figure 2 0 ) - To determine the v i r g i n s t r e s s at Heath Steele, the f o l l o w i n g assump-t i o n has to be made: "The r a t i o of st r e s s e s ( v e r t i c a l and h o r i z o n t a l s ) a t Brunswick i s com-parable with the r a t i o of s t r e s s e s a t Heath S t e e l e " . Thus, the s t r e s s regime 3 0 0 m. ( 1 0 0 0 f t . ) below surface at Heath St e e l e o v = a, = v ^ x (depth below surface) v 1 waste r = 2883 kg/m 3 x 305 m = 8.79 x 10 5 kg/m 2 o 3 = 8.79 x 10 5 kg/m 2 = 8.62 MP a (180 KPSF)* Note: KPSF = k i l o pounds per square foot 62 FIGURE 20 V i r g i n Stress at Heath Steele 63 °H (north-south) = ° ' = 2 x ° ' = 2 x 8.62 MPa a'i = 17.24 MPa (360 KPSF) °H (east-west) 0 2 = 1 - S x a 8 = 1 . 5 x 8 . 6 2 MPa 0 2 = 12.93 MPa (272 KPSF) 5.4 P i l l a r C h a r a c t e r i s t i c s - Ribs: O r i g i n a l l y 12 m. (40 f t . ) x 61 m. (200 f t . ) high x ore width L a t e r a l l y 27 m. (90 f t . ) x 76 m. (250 f t . ) high x ore width - Sills/Crowns: Production l e v e l up contains cones or trough usually extending up 15 meters (50 f t . ) above l e v e l . Below l e v e l usually 15 to 23 meters (50 to 75 feet) depending on ore widths and loca l geometry. - P i l l a r Support: Generally no systematic support i s given. In s p e c i f i c problem areas some cable b o l t i n g has been used. In some small p i l l a r s in the room and p i l l a r overcuts, some perimeter strapping has been done i n isolated problem areas. Otherwise local support of development within p i l l a r areas has been the use of standard rock bolts ( 5 or 8 f t . ) 1.5 to 2.5 m. Early practice was to use mechanical bolts and straps or plates. L a t t e r l y r e s i n anchored rebar pins are almost exclusively used. 5.5 Mining Sequence The investigated area consists of four open stopes (77-95, 77-93, 77-91, 77-89) separated by three r i b p i l l a r s (77-94, 77-92, 77-90). The depth varies from 245 m. (800 f t . ) to 366 m. (1200 f t . ) below surface, and a 300 m. (1000 f t . ) depth was assumed for c a l c u l a t i o n purposes. Figure 21 i s a longitudinal view of the s t o p e / p i l l a r panel layout, and Table 8 summarizes the mining sequence (from Blasting Record). TABLE 8 MINING SEQUENCE OF THE PANEL STOPE START DATE FINISH DATE 77-95 May, 1977 Nov. 1978 77-93 Dec. 1976 May, 1978 77-91 Apr. 1976 Apr. 1978 77-89 May, 1975 Dec. 1977 77-92 ( P i l l a r Recovery) Apr. 1978 Apr. 1978 5.6 Failure History and P i l l a r Geometry - November 1977: 77-92 Rib P i l l a r It was discovered that there was excessive wedge type sloughing from the walls of t h i s p i l l a r into the stopes on each side to the extent that i t was considered not to be providing any support. Accordingly, i t was decided to blast out t h i s p i l l a r and to stop further mining west i n 77-93 stope to increase the 77-94 p i l l a r . - A p r i l 1978: The 77-92 r i b was blasted, and recovered. - September, 1978: 77-94 Rib P i l l a r After the 77-92 r i b was blasted, mining continued i n 77-95 stope. On September 23/78, a blast i n 77-95 stope appeared to have triggered a violent reaction i n 77-94 r i b causing seismicity and buckling of track r a i l s i n t h i s p i l l a r on both 7950 and 7800 l e v e l s . - July 1980: 77-96 Rib P i l l a r , (supplementary information) A c t i v i t y i n t h i s r i b on 7700 le v e l developed i n July 1980 and was 65 FIGURE 21 Schematic Longitudinal View of the Investigated Area at Heath Steele. (Mining method: Blast hole Open stoping) Scale: 1 in. = 100 f t . 8200 66 w <u ft o -p C O » A O N I <S-c O N I o-H O N I O N 00 £>-o -p u d x: o o c o •H o rtJ -p CM CM 3 O I—I - 7 7 - 9 ^ P i l l a r d e teriorates 7 7 - 9 2 P i l l a r recovered 7 7 - 9 2 P i l l a r f a i l e d 6 ? associated with the b l a s t i n g of 74-97 stope. From the Figure 22 Extraction Flowchart the stope and p i l l a r geometries and dimensions were estimated: a) when the 77-92 r i b p i l l a r f a i l e d i n November 1977 (Figure 23, Table 9) b) when the 77-94 p i l l a r collapsed i n September 1978 (Figure 24, Table 10). The design study w i l l concentrate on these two p a r t i c u l a r cases, and because the major p r i n c i p a l stress i s horizontal i n the N-S axis d i r e c t i o n , only the plan view needs to be considered. TABLE 9 APPROXIMATE STOPE + PILLAR DIMENSIONS WHEN 77-92 PILLAR FAILED (NOVEMBER, 1977) Length Width Height Stopes 77-89 43m. (140 f t ) 15m. (50 f t ) 110m. (360 f t ) 77-91 30m. (100 f t ) 40m. (130 f t ) 120m. (395 f t ) 77-93 27m. ( 90 f t ) 40m. (130 f t ) 120m. (395 f t ) 77-95 18m. ( 60 f t ) 40m. (130 f t ) 90m. (300 f t ) P i l l a r s 77-90 15m. ( 50 f t ) 15m. ( 50 f t ) 117m. (385 f t ) 77-92 27m. ( 90 f t ) 40m. (130 f t ) 122m. (400 f t ) 77-94 30m. (100 f t ) 40m. (130 f t ) 107m. (350 f t ) 68 TABLE 10 APPROXIMATE STOPE AND PILLAR DIMENSIONS WHEN 77-94 PILLAR FAILED (SEPTEMBER, 1978) Length Width Height Stopes 77-89 43m. . (140 f t ) 15m. . ( 50 f t ) 137m. (450 f t ) caved 77-91) ) 77-93) 98m. . (320 f t ) 40m. . (130 f t ) 168m. (550 f t ) 77-95 43m. (140 f t ) 40m. . (130 f t ) 90m. (300 f t ) P i l l a r s 77-90 15m. ( 50 f t ) 15m. , ( 50 f t ) 122m. (400 f t ) 77-92 77-94 P A T i c n 18m. ( 60 f t ) r t\ x 40m. . (130 f t ) 122m. (400 f t ) 5.7 P i l l a r Design Analysis Every p i l l a r involved i n the Heath Steele case h i s t o r y may be c l a s s i -f i e d as "separation p i l l a r s " (Category 2 of the p i l l a r c l a s s i f i c a t i o n ) . According to the design charts of Chapter three, the following design-ing methods should be used. 5.7.1 Phase 1. Experience Design From the description of the mining method (Section 5.2), the most re-cent practice at Heath Steele was b a c k f i l l i n g the c r i t i c a l area and recover-ing r i b p i l l a r s only between primary f i l l e d stopes, which were estimated to be 30m. (100 f t ) long, 60m. (200 f t ) high and separated by 30m. (100 f t ) r i b p i l l a r s . FIGURE 2 3 Estimated Layout When 7 7 - 9 2 Rib P i l l a r F a i l e d FIGURE 24 Estimated Layout When 77-94 Rib P i l l a r F a i l e d 71 5.7.2 Phase 2. P i l l a r Structural Analysis After A l l c o t t and Archibald (1981) 1 1 The p i l l a r s consist of massive sulphides which have a compressive strength of 176.5 MPa (25598 p s i ) , the strongest unit i n the geological sequence. The most important factor a f f e c t i n g the strength of the p i l l a r s i s the fracture systems, more s p e c i f i c a l l y S 3 and S 5. The S 3 fractures s t r i k e at N 30°W, which i s approximately 15° to the N-S axis of the r i b p i l l a r s and the d i r e c t i o n of the compressive forces acting on the p i l l a r s (Figure 25). Note that p i l l a r s with a north-south axis of 30m. (100 f t ) and east-west axis of 18m (60 f t ) have the S 3 fractures supported on both hanging wall and footwall over a s t r i k e length of 10m (33 f t ) (Figure 26). How-ever, a p i l l a r with a north-south axis of 60 m. (200 f t ) and an east-west axis of 18m. (60 f t ) has the S 3 fractures supported on both the hanging wall and footwall over a s t r i k e length of only 2m. (7 f t ) (Figure 27). In the present cases, the p i l l a r lengths (N-S axis) never exceed 40m. (130 f t ) (Tables 9 and 10). Thus, the S 3 fractures alone should not pro-voke s l i d i n g i n the p i l l a r s . The Ss (N 40°E) fractures s t r i k e at about 60° to the north-south axis of the r i b p i l l a r s and dip steeply to the northwest. (Figure 28). They are much less well developed than the S5 fractures and are at a f l a t t e r angle to the d i r e c t i o n of the compressive forces. The i n t e r s e c t i o n of the S 3 and S 5 fractures near the sides of the r i b p i l l a r s sometimes re s u l t i n s p a l l i n g and deterioration of the p i l l a r s . (Figure 29). P I L L A R P L A N V I E W C O M P R E S S I V E F O R C E S 1 C O M P R E S S I V E FIGURE 2 5 . S Fracture System F O R C E S PILLAR PLAN VIEW 73 i A/ H A N G I N G W A L L S T O P E |* 60'- *| F O O T W A L L FIGURE 2 6 The E f f e c t of S 3 Fractures on 3 0 m. ( 1 0 0 f t . ) Wide Rib P i l l a r s . ( A f t e r A l l c o t t & Archibald) 1 1 PILLAR PLAN VIEW HANGING WALL STO 6 0 ' 1 FOOTWALL The E f f e c t of Fractures on 60 m. ( 2 0 0 f t . ) Wide Rib P i l l a r s ( A f t e r A l l c o t t & Archibald) 1 1 P I L L A R P L A N VIEW C O M P R E S S I V E F O R C E S C O M P R E S S I V E F O R C E S FIGURE 28. S 5 Fracture System P I L L A R P L A N VIEW C O M P R E S S I V E F O R C E S C O M P R E S S I V E F O R C E S FIGURE 29 Combined Ef f e c t of S- and S- Fractures 7 7 L I N K A G E O F S 3 S L I P S I 50 ft 1 PLAN VIEW FIGURE 3 0 7 7 - 9 0 Rib P i l l a r S l i d i n g . (After A l l c o t t & Archibald) fi CO 4 0 0 3 0 0 c o m co t q FIGURE 31 (After A l l c o t t & Archibald) 1 : L E-i £5 CO s; 6 0 0 6 5 0 0 5 s t e s s f a c t o r a s t r a i n vs. t i m e ( 7 7 - 9 0 r i b ) 2 0 0 2 1 0 0 I o o ro O O o o in 8 DAYS g (0 o o no o o C T ) a o o o o o o CM Because the 77-90 p i l l a r i s only 15m. (50 f t ) wide (east-west a x i s ) , these wedge f a i l u r e s may induce major s t a b i l i t y problems, and provoke p i l l a r s l i d i n g (Figure 30). The deformation of the 77-90 r i b p i l l a r was monitored and Figure 31 c l e a r l y shows that the p i l l a r s l i d i n g described previously, occurred. 5.7.3 Phase 3: Empirical Methods Empirical methods may be used to estimate the load acting on a p i l l a r , and i t s ultimate strength at f a i l u r e . Since a) 77-92 and b) 77-94 p i l l a r s were reported f a i l e d (Section 5.6), and c) 77-90 suffered structural f a i l -ure (Section 5.7.2), the empirical methods w i l l be applied to these three cases. 5.7.3.1 Estimation of P i l l a r Load by the Extraction Ratio Formula The stress can be calculated for each p i l l a r , using the following re-lationship : a p = 0"i . N where * N = Extraction number (Figures 32, 33, 34) Op = Average p i l l a r stress Sum of 1/2 s t r i k e length of each _ Adjacent stopes + s t r i k e length of p i l l a r Strike Length of P i l l a r Oi = 17.24 MPa (360 KPSF) (Section 5.3.4) A) 77-90 P i l l a r F a i l u r e Geometry (Figure 32) P i l l a r N Oi(MPa) a p (MPa) 77-90 3.4 17.24 58.48 77-92 2.1 17.24 36.20 77-94 1.9 17.24 32.76 80 B) 77-92 P i l l a r Failure Geometry (Figure 35) P i l l a r N gr (MPa) (MPa) 77_90 F A I L E D ---77-92 3.1 17.24 53.44 77-94 1.9 17.24 32.76 C) 77-94 P i l l a r Failure Geometry (Figure 34) P i l l a r N Oi(MPa) a p(MPa) 77-90 F A I L E D 77-92 -- Failed and Recovered 77-94 6.3 17.24 108.61 5.7.3.2 Estimation of P i l l a r Strength; Hoek's Method The p i l l a r strength may be estimated using the curves developed by 3 Hoek S Brown (1980) (Figure 35). The p i l l a r material was c l a s s i f i e d by 12 Golder Associates (1981) as a good qu a l i t y rock mass (Q - 20), and the uniaxi a l compressive strength i s : a c = 176.5 MPa (25598 psi) A) 77-90 P i l l a r Failure Geometry D , , , „ w / u P i l l a r Strength/a c P i l l a r Strength P i l l a r W / H ( M P A ) 77-90 1.0 0.3 52.95 77-92 0.75 0.25 44.13 77-94 0.75 0.25 44.13 FIGURE 32 P i l l a r s Extraction Numbers for the 7 7 - 9 0 P i l l a r Failure Geometry. a , = PLAN VIEW 17.2 MPa FIGURE 33 P i l l a r s Extraction Numbers | PLAN VIEW for the 77-92 P i l l a r Failure Geometry• O] = I 7.2 M P , co ro FIGURE 34 P i l l a r s Extraction Numbers for the ?7-94 P i l l a r Failure Geometry. PLAN VIEW CT. = 17.2 M Pa 84 Intact samples of fine grained igneous crystalline rock m=17 , s=l Very good quality rock mass m=8.5 , s=0.1 Good quality rock mass m=1.7 , s=0.004 Fair quality rock mass m=0.> , s=0.0001 Poor quality rock mass m=0.09 , s=0.00001 P i l l a r width/height i W^ /h FIGURE 35 The E f f e c t of the Width to Height Ratio on. Average P i l l a r Strength. 85 B) 77-92 P i l l a r Failure Geometry P i l l a r W / H P i l l a r Strength/a c P i l l a r Strength 77-90 F A I L E D 77-92 0.75 0.25 44.13 77-94 0.75 0.25 44.13 C) 77-94 P i l l a r Failure Geometry p . 1 1 a T . w / P i l l a r Strength/a. P i l l a r Strength P l l l a r W / H (MPa) 77-90 F A I L E D 77-92 FAILED AND RECOVERED --77-94 0.5 0.15 26.48 5.7.4 Theoretical Methods It was stated i n Chapter 3 that the t h e o r e t i c a l methods are useful for better understanding the mechanism involved in p i l l a r design. However, the complexity and the great amount of data required make them impractical, d i f f i c u l t to apply, and inaccurate. Thus, no theoretical methods have been used for the Heath Steele case history analysis. 5.7.5 Phase 5. Computer Methods The computer stress analysis program used was "BITEM", a two-dimension-al boundary element program developed by the C.S.I.R.O. This program has been modified and adapted to the U.B.C. main frame I.B.M. computer by R. Pakalnis. Again, the three situations modelled were the p i l l a r f a i l u r e geometries of 77-90, 77-92 and 77-94. The p i l l a r stress i s taken as the 86 average value of 30 nodal stresses d i s t r i b u t e d within each p i l l a r . N.B. The stress units of the computer output are i n k i l o pounds per square foot (KPSF). Figures 36, 37, 38. A) 77-90 P i l l a r Failure Geometry (Figure 36) P i l l a r Op Average P i l l a r Stress 77-90 43.09 MPa 900 (KPSF) 77-92 27.77 MPa 580 (KPSF) 77-94 28.73 MPa 600 (KPSF) B) 77-92 P i l l a r F ailure Geometry (Figure 37) P i l l a r . „.?P Average P i l l a r Stress 77-90 Failed 77-92 38.30 MPa 800 (KPSF) 77-94 32.85 MPa 687 (KPSF) C) 77-94 P i l l a r F ailure Geometry (Figure 38) P i l l a r . „.?P Average P i l l a r Stress 77-90 Failed ---77-92 Recovered-77-94 57.46 MPa 1200 (KPSF) PLAN VIEW 36 Computer Output of the 77-90 P i l l a r Failure Geometry. Pri n c i p a l Major Stress Contour. co PLAN VIEW FIGURE 37 Computer Output of the 7 7 - 9 2 P i l l a r Failure Geometry. P r i n c i p a l Major Stress Contour. PLAN VIEW Computer Output of the 77-94 P i l l a r Failure Geometry. P r i n c i p a l Major Stress Contour. 9 0 5.8 Discussion of the Results Before discussing the rock mechanics re s u l t s of A] 77-90, B) 77-92 and C) 77-94 p i l l a r f a i l u r e s , the required assumptions must be reviewed. 1. The v i r g i n stress was estimated from measurements at Brunswick Mining and Smelting. 2. For every p i l l a r f a i l u r e case reported, stopes and p i l l a r s ' dimensions were assessed. For each s i t u a t i o n the p i l l a r loads were calculated using two d i f f e r e n t methods, t r i b u t a r y area and computer simulation. The r e s u l t s of both methods corroborate very well (Table 11). The p i l l a r strengths were estimated using 3 Hoek and Brown (1980) curves. A so-called "safety f a c t o r " which i s the r a t i o of the p i l l a r strength over the p i l l a r load at a given time was also calculated. A safety factor of 1 means that the load acting on the p i l l a r equals i t s ultimate strength and f a i l u r e i s imminent. The f a i l u r e h i s t o r y can then be reconstructed using Table 11: A) 77-90 P i l l a r F ailure Geometry (Nov. 1977) The f i r s t p i l l a r to collapse was 77-90 (S.F. = 1.04) (November, 1977). The f a i l u r e was not documented, probably because i t was progressive, non-violent, and caused by s l i d i n g along the S 3 fracture (Figures 30, 31). B) 77-92 P i l l a r F a i l u r e Geometry (Nov. 1977) After the 77-90 p i l l a r p a r t i a l l y l o s t i t s bearing c a p a b i l i t y , a part of the load was r e d i s t r i b u t e d causing the 77-92 p i l l a r to f a i l . (S.F. = 0.96). C) 77-94 P i l l a r F ailure Geometry (Sept. 1978) According to the actual geometry of the panel, Table 11 shows that the 77-94 p i l l a r had previously f a i l e d . (S.F. = 0.46). TABLE 11 HEATH STEELE PILLAR ANALYSIS RESULTS P i l l a r Extraction Tributary Computer Mean (1) IV/H P i l l a r Safety Extraction Number Area Stress Stress (Plan View) Strength Factor Ratio Remarks (MPa) (MPa) (MPa) (MPa) % A) 77-90 P i l l a r Failure Geometry, November 1977 77-90 3.4 58.48 43.09 50.79 1 52.95 1.04 71 P i l l a r Slide 77-92 2.1 36.20 27.77 x 31.99 0.75 44.13 1.38 51 Stable 77-94 1.9 32.76 28.73 30.75 0.75 44.13 1.44 49 Stable B) 77-92 P i l l a r Failure Geometry, November 1977 77-90 F A I L E D 77-92 3.1 53.44 38.30 45.87 0.75 44.13 0.96 68 P i l l a r Failed 77-94 1.9 32.76 32.85 32.80 0.75 44.13 1.35 49 Stable C) Actual Geometry (77-94 Failed) September 1978 77-90 F A I L E D 77-92 F A I L E D and R E C O V E R E D 77-94 6.3 108.61(-2') 57.46 57.46 0.5 26.48 0.46 84 P i l l a r Failed (1) The mean stress i s assumed to be the average value of computer and tributa r y area methods. Only the computer stress was considered in C) Actual Geometry 77-94, the tributa r y area value was judged irrelevant. (2) Irrelevant value. These conclusions from the r e s u l t s of computational design methods (Table 11) are in agreement with the i n s t a b i l i t y events experienced at Heath Steele (according to the documentation) which suggest that the dimensions and stress values assumed were correct. Furthermore, A l l c o t t and Archibald (1981) 1 1 attempted to elaborate an empirical design curve for Heath Steele p i l l a r s , using bore-hole extensome-ter s ' deformation records. From the observations of f a i l i n g p i l l a r s , four stages of deterioration have been defined. Stage 1: Intact P i l l a r No v i s i b l e or audible evidence of movement, although extensometers may r e g i s t e r convergence. Stage 2: P i l l a r F a i l u r e Sound and movement are observed. It i s s t i l l possible to d r i l l , blast and muck the p i l l a r material, but at times continuous movement w i l l prevent t h i s . After s t a b i l i z a t i o n recovery can begin again. Stage 3: Post Fa i l u r e Manageable recovery i s no longer possible, but s t a b i l i z a t i o n w i l l allow retention of f i l l or through access. Stage 4: No r e l i a b l e use remains in the p i l l a r . Combining these q u a l i t a t i v e observations with the deformation versus extraction numb er curve (Figure 3 9), A l l c o t t and Archibald notices that p i l -l a r s f a i l e d at Stage 2 of deterioration corresponding to an extraction number (N) of 3.3. Table 11 r e s u l t s confirm A l l c o t t and Archibald's observations. Figure 40 depicts a plot of the extraction number versus safety factor, showing that p i l l a r f a i l u r e (S.F. = 1) e f f e c t i v e l y occurred around an extrac-ti o n number N = 3.3. €6 FIGURE 40 EXTRACTION NUMBER VS SAFETY FACTOR i CD CD CD - P cn _c +-> fd cu. IE l_ cn in cu C3 OJ _ j CO 1 cn I cn 1 LE RB r\- m r-CE r- NS X + < cn ZD OJ in U l 95 Because s e v e r a l methods have been employed which give s u b s t a n t i a l l y the same r e s u l t as the A l l c o t t and A r c h i b a l d experiments, the p i l l a r design pro-cedure and imput parameters can be considered c a l i b r a t e d a t Heath S t e e l e . F i n a l l y , Figure 41 represents a p l o t of the l o c a l e x t r a c t i o n r a t i o "e" versus the s a f e t y f a c t o r of each p i l l a r a t d i f f e r e n t stages of e x t r a c t i o n . Note: e = 1 0 0 x ( s u m °f l / ^ width of each adjacent stope) sum of l / 2 width of each adjacent stope + the width of p i l l a r Figure 41 i n d i c a t e s that a t 3 0 0 m ( 1 0 0 0 f t . ) depth, i n s t a b i l i t y i s i n i t i a t e d when the e x t r a c t i o n r a t i o exceeds 65 - 70%. Thus i t i s suggested to l i m i t primary e x t r a c t i o n to 65% a t t h i s depth. This w i l l minimize- s t a -b i l i t y problems t h a t have caused e x t r a support costs, mining delay, l o s s of ore reserves as w e l l as making p i l l a r s recovery very d i f f i c u l t . Curves s i m i l a r to Figure 41 should be developed i n order to determine the optimum percentage of e x t r a c t i o n a t d i f f e r e n t depths ( 6 0 0 m, 9 0 0 m ,1200 m) at Heath S t e e l e . CHAPTER 6 GECO CASE HISTORY ANALYSIS 98 6.1 Geology (after Bray (1967) 1 0 6.1.1 Regional Geology The Manitouwadge Syncline i s a broad easterly plunging syncline of meta-sediments and metavolcanics, The surrounding country rocks are mainly granite and trondjemite, showing evidence of g r a n i t i z a t i o n near the syncline i n the form of gneissic granite and migmatite. The metasediments consist of quartz feldspar b i o t i t e gneiss, quartzites with varying amounts of b i o t i t e , iron formation and the quartz muscovite group which i s host rock for the Geco orebody. The metavolcanics are older than the metasediments and contain horn-blende schist and amphibolite. 6.1.2 Mine Geology The Geco orebody i s located in the s e r i c i t e schist group of rocks on the south limb of the Manitouwadge syncline. A large open drag f o l d approximate-l y 760 m. (2500 f t ) long and plunging 35° to the east i s the host structure for the orebody. ' The orebody i s large and steeply dipping with a core of massive sulphides and a surrounding envelope of disseminated sulphides. The major economic minerals mined are chalcopyrite, s p h a l e r i t e and galena. A cross-Section from south to north across the Geco orebody shows the following sequence of formations: - g r e y gneiss (including b i o t i t i c quartzite) - s e r i c i t e s c h i s t (containing the orebody) - b i o t i t e amphibole garnet gneiss. Intrusive into these formations are basic dykes, granite, pegmatite dykes and diabase dykes. Table 12 and Figure 42 summarize the Geco geology. 99 FIGURE 42 Schematic S t r a t i g r a p h i c Columns I l l u s t r a t i n g G e n e r alized R e l a t i o n s h i p s o f Sulphide Zones, Geco Mine 100 The orebody forms a tabular mass lying more or less v e r t i c a l , and rak-ing eastward at from 20 degrees to 30 degrees. In cross-section, the ore-body has the shape of an onion, with the bulbous bottom portions conforming to the curvature of the dragfold. The massive sulphide core (orebody) varies i n thickness from a few inches to about 45 m. (150 f t ) with an average thickness of 12 m. (40 f t ) . The grade of the ore averages better than 2 percent copper, 4 percent zinc and 2 oz/ton of s i l v e r . TABLE 12 SUMMARY OF THE GEOLOGY AT GECO Rock Types GRANITE GROUP 1) Granite 2) B i o t i t e garnet gneiss 3) B i o t i t e s i l l i m a n i t e gneiss SERICITE SCHIST GROUP 4) Quartz b i o t i t e anthophyllite hornfels 5) S e r i c i t e schist 6) Chalcopyrite 7) Sphalerite 8) Pyrite, pyrrhotite, sphalerite, chalcopyrite GREY GNEISS GROUP 9) Quartz feldspar b i o t i t e gneiss 10) B i o t i t i c quartzite 11) Iron formation INTRUSIVES 12) Diabase 13) Pegmatite 14) Quartz d i o r i t e - 101 6.1.3 Structural Geology Multiple folding in the ore bearing sch i s t , transverse to the main dragfolding, aggravates the ground weaknesses induced by fa u l t i n g and f r a c -turing, and in some places increases the tendency to slough. As a ru l e , the pegmatite dykes are not mineralized, except where i n • contact with the massive sulphide core. They are, therefore, r a r e l y i n -cluded i n a stope, but may form a stope wall. Since the large dykes (over 1 m. (3 f t ) are extensively fractured, they tend to slab and break off when exposed over a wide surface. By contrast, the massive sulphide core of the orebody i s r e l a t i v e l y free from j o i n t s and fractures and has been observed standing s o l i d l y over horizontal lengths of 21 m. (70 f t ) and v e r t i c a l heights of over 90 m. (300 f t ) . Ground control in the mine i s also adversely affected, es p e c i a l l y when we have folding i n the area of the top of the stope. Because the folded schist i s not standing v e r t i c a l l y , i t w i l l not support the same v e r t i c a l load as steeply dipping schists. This problem w i l l increase with depth and more intense f o l d i n g . Thus, the s t r u c t u r a l weaknesses of the ore-bearing formation consists of: (a) f o l i a t i o n and some f a u l t i n g i n an east-west d i r e c t i o n . (b) j o i n t i n g and minor f a u l t i n g i n a north-south d i r e c t i o n . (c) weak contacts along diabase dykes and along quartz diorite/quartz muscovite schist contacts. (d) regional, drag and cross f o l d i n g . (e) irregular fractures and j o i n t s in broad pegmatites. 12 - Jointing. After Golder Associates (1981) There are two steeply dipping j o i n t sets. The f i r s t one has an east-west s t r i k e , the other st r i k e s roughly north-south. No persistent near 1 0 2 horizontal j o i n t sets were found at Geco. 6.2 Mining Method and Underground Structure Dimension The p r i n c i p a l mining method at Geco i s blasthole mining, but where the ore narrows to 8 m (25 f t ) or less i t i s necessary to use a cut and f i l l method. The rocks at Geco are not the best suited to open stoping as they w i l l slough r e a d i l y when exposed in the large areas of a stope wall. Geco have overcome the problem by modifying the mining method by introducing f i l l as the ore i s drawn; thus keeping the stopes f u l l at a l l times. To prevent i n -s t a b i l i t y in the backs, they are cable bolted using tensioned 9.1 m long cable bolts. The orebody i s divided into blocks for convenience of i d e n t i f i c a t i o n . Mining started i n 1957 at the west extremity of the orebody (Block A) and progressed eastward. A t y p i c a l block i s about 150 to 180 m. (500 f t ) high, consisting of three 21 m (70 f t ) wide primary stopes separated by two 37 m (120 f t ) p i l l a r s and flanked by two boundary p i l l a r s 46 m (150 f t ) wide. The primary stopes are mined f i r s t and drawn under rock f i l l and then consolidated with the introduction of cemented hydraulic f i l l . The two 37 m (120 f t ) p i l l a r s are then removed between the f i l l e d stopes. These p i l l a r s are usually mined in 60 to 90 m (200 to 300 f t ) l i f t s to minimize d i l u t i o n from the f i l l walls. To date most of the primary stoping i s completed and p i l l a r mining con-tri b u t e s a large share of the production. 6.3 Rock Mechanics Data 6.3.1 Rock Strength Parameters - Density Ore Massive Sulphide = 5334 kg/m3 (333 l b s / f t 3 ) Disseminated ore = 3204 kg/m3 (200 l b s / f t 3 ) Waste: Hanging wall = 2666 kg/m3 (166 l b s / f t 3 ) and footwall - E l a s t i c Modulus and Poisson's Ratio Ore: E = 103425 MPa (15M. psi) v = 0.31 Waste: E = 105340 MPa* v = 0.2* 12 6.3.2 Laboratory Test. After Golder Associates (1981) - Unconfined Compressive Strength Ore: a c = 100 MPa (psi) (MPa) Quartz b i o t i t e schist 6,000 41 Quartzite 23,000 159 S e r i c i t e 4,000 27 Quartz b i o t i t e gneiss 7,500 52 Quartz b i o t i t e muscovite 15,500 107 Hornblende b i o t i t e qtz schist 7,000 48 Granite (gneiss) 9,000 62 Granite b i o t i t e 10,500 72 Quartz b i o t i t e 27,000 186 Quartz muscovite schists 13,500 93 - Tensile Strength Ore: a t = 8 MPa - T r i a x i a l Compressive Strength Ore: 0 3 = 6.9 MPa = 150 MPa a 3 = 13.8 MPa Oi = 232 MPa a 3 = 20.7 MPa • al = 307 MPa Estimated from the t y p i c a l value of E l a s t i c Modulus and Poisson Ratio f o r gneiss rock. (Hoek and Brown (1980) 3, p. 262,267) 104 6.3.3 Rock Mass C l a s s i f i c a t i o n The footwall, hanging wall and orebody rocks were c l a s s i f i e d by Golder 12 Associates (1980) 2850 l e v e l 28-54.5 Cross-cut S e r i c i t e Schist (Two Ratings) NGI NGI RQD 60 50 Jn 4 6 Jr 2 2 Ja 0.75 1.0 Jw 1.0 1.0 SRF 2 1.0 Q = 20 16.7 Intact Strength RQD Spacing of Joints Condition of Joints Ground Water CSIR 7 13 10 12 10 52 Hangingwall Ramp Below 2850 L B i o t i t e Garnet Gneiss (Two Ratings) NGI NGI RQD 60 90 Jn 6 4 Jr 3 2 Ja 1 0.75 Jw 1 1 SRF 2.5 1 Q = 12 60 Intact Strength RQD Spacing of Joints Condition of Joints Ground Water CSIR 7 13 20 6 10 56 1850 Level Hangingwall D r i f t o f f 18-36 Cross-Cut Hangingwall Schist (Two Ratings) NGI NGI RQD 90 60 Jn 2 3 Jr 1 1.5 Ja 1 2 Jw 1 1 SRF 2.5 2.5 Q 18 6 CSIR Intact Strength 7 RQD 20 Spacing of Joints 20 Condition of Joints 6 Ground Water 10_ 63 1850 Level i n Footwall of 19-40 P i l l a r Stope Footwall S e r i c i t e Schist (Two Ratings) NGI NGI RQD 75 60 Jn 4 3 Jr 1 2 Ja 4 2 Jw 1 1 SRF 2.5 2.5 Q = 1.9 8 CSIR Intact Strength 7 RQD 13 Spacing of Joints 20 Condition of Joints 6 Ground Water 10 56 1 0 6 1850 Level 44 P i l l a r S i l l Massive Sulphides (Two Ratings) NGI NGI RQD 60 80 Jn 9 9 J r 1 1.5 Ja 2.0 0.75 Jw 1.0 1 SRF 4.0 1 Q = 0.8 17.8 CSIR Intact Strength 7 RQD 13 Spacing of Joints 20 Condition of Joints 6 Ground Water 10 56 2250 Level 27-61 Stope Footwall Schist (Two Rating NGI NGI RQD 50 70 Jn 4 3 Jr 1 2 Ja 4 3 SRF 2.5 2.5 Q = 1.3 6.2 CSIR Intact Strength 7 RQD 13 Spacing of Joints 20 Condition of Joints 6 Ground Water 10 56 Values obtained for the footwall schist varied from 1.3 to 20 (poor to good rock); 0.8 to 17 for the sulphides (poor to good rock); and from 6 to 60 f o r the hangingwall schist (poor to very good rock). 107 6.3.4 V i r g i n Stress No stress measurements have been performed at Geco. However, the v e r t i c a l stress i s assumed to be equal to the weight of the overlying rock. c v = yh = a 3 where: y - density of the waste rock h = depth below surface The major p r i n c i p a l stress a\ and intermediate p r i n c i p a l stress o 2, both horizontal, are estimated using two d i f f e r e n t sources, described i n Appendix B. The mean values are: a 1 = 2.6 a v a 2 = 2.1 o v fsee Figure 431 FIGURE 43 Assumed Stress Regime at Geco 108 At 215 metres (700 f t ) depth, the stress regime i s : a 3 «= y.h «= 2660 kg/m3 x 215 m «= 5.72 x 10 5 kg/m2 a 3 «= 5.72 x 10 5 kg/m2 «= 5.66 MPa (116 KPSF) 01 «= 2.6a3 «= 14.73 MPa (302 KPSF) 02 « 2.1o3 « 11.89 MPa (244 KPSF) 6.4 P i l l a r Characteristics Six stopes with intervening p i l l a r s were used to mine the 'B' Block. The stopes' i n i t i a l dimensions were 21m. (70 f t ) long and up to 150 m. (500 f t ) high ( v e r t i c a l ) . The r i b p i l l a r s , designed to be recovered at a subsequent stage, were 37 m. (12 f t ) long. The p i l l a r material i s a massive sulphide which i s r e l a t i v e l y strong (a c «= 100 MPa) . 6.5 Mining Sequence The investigated area consists of four open stopes (10 - 19.5, 10 - 21, 10 - 22 and 10 - 23.5) separated by three r i b p i l l a r s (10 - 20, 10 - 21.5, 10 - 23). The depth varies from 150 m. (500 f t ) to 320 m. (1050 f t ) below surface, and a 215 m (700 f t ) depth was assumed for c a l c u l a t i o n purposes. Figure 44 i s a longitudinal view of the stope/pillar/panel layout and Tables 13 to 16 summarize the mining sequence. FIGURE 44 L o n g i t u d i n a l View of the Investigated Area at Geco . TABLE 13 STOPE 10-19.5 MINING SEQUENCE Date Broken Ore Total Tons Remarks Feb. 1960 8,050 March 1960 20,150 28,200 A p r i l 1960 60,030 88,230 Sept. 1960 10,000 98,230 Oct. 1960 105,630 203,860 Dec. 1960 6,820 210,680 Sept. 1961 1 1 Sloughing TABLE 14 STOPE 10-21 MINING SEQUENCE Date Broken Ore Total Tons Remarks Nov. 1959 3,890 3,890 Dec. 1959 11,500 15,390 Jan. 1960 19,903 35,293 Feb. 1960 18,678 53,971 Mar. 1960 67,027 120,998 Sept. 1960 20,000 140,998 Oct. 1960 5,000 145,998 Small amount of sloughing from north side Nov. 1960 15,230 161,228 10-21.5 p i l l a r cracked Dec. 1960 32,390 193,618 10-21.5 p i l l a r f a i l e d 10--21 SOUTH STOPE March 1961 27,677 221,295 A p r i l 1961 3,100 224,395 May 1961 9,400 233,795 June 1961 24,640 258,435 TABLE 15 STOPE 10-22 MINING SEQUENCE Date Broken Ore Total Tons Remarks March 1960 1,930 1,930 June 1960 3,250 5,180 July 1960 7,500 12,680 August 1960 15,000 27,680 Sept. 1960 48,350 76,030 Nov. 1960 15,000 91,030 Dec. 1960 35,800 126,830 10-23 P i l l a r Failed March 1961 3,074 129,904 'Caving' 10-22 SOUTH STOPE June 1961 6,990 136,894 July, 1961 14,340 151,234 August 1961 50,220 201,454 10-21.5 PILLAR Dec. 1960 50,000 50,000 A p r i l 1961 10,000 60,000 10-21.5 p i l l a r recovery June 1961 30,000 90,000 112 TABLE 16 STOPE 10-23.5 MINING SEQUENCE Date Broken Ore Total Tons Remarks May- 1960 250 250 June 1960 2,285 2,535 Aug. 1960 10,600 13,135 Sept. 1960 16,300 29,435 Oct. 1960 38,945 68,380 Dec. 1960 19,140 87,520 10-23 P i l l a r Failed Jan. 1961 13,550 101,070 March 1961 12,830 113,900 May 1961 26,455 140,355 July 1961 22,619 162,974 Aug. 1961 407 163,381 Over Break Sept. 1961 1,395 164,776 Over Break 10--23 PILLAR Dec . 1960 5,000 5,000 Feb. 1961 20,000 25,000 March 1961 17,889 42,889 A p r i l 1961 10,000 52,889 6.6 Failure Histories and P i l l a r Geometries (After Bray 1967) 1 0 - October, 1960: sloughing started i n 10-21 stope. - November, 1960: extensive cracking of: . 10-21.5 p i l l a r . 10-23 p i l l a r 850 l e v e l , major remedial work on: . 10-21.5 n i l l a r December, 1960: - January, 1961: - March, 1961: - September, 1961: - A p r i l , 1962: - October, 1963: 10-21.5 p i l l a r collapsed from the 7A to 5A sublevel. (The upper h a l f of the p i l l a r ) . 650 l e v e l , 10-23 p i l l a r showed extensive sloughing. A f u l l r a i s e was driven to surface to back-f i l l 10-22 stope. 10-22 stope cave to the elevation of the 450 l e v e l . The f i l l r a i s e acted as a s l o t . The west side of the 10-20.5 p i l l a r suffered sloughing. 10-19.5 stope cave to the elevation of 450 l e v e l . Caving reached the 250 le v e l cross-cut. From Figure 45 extraction flowchart, the stope and p i l l a r geometries and dimensions were estimated: a) when 10-21.5 r i b p i l l a r f a i l e d , in November, 1960 (Figure 46, Table 17). b) when 10-23 r i b p i l l a r f a i l e d , i n November, 1960 (Figure 47, Table 18). c) when 10-20 r i b p i l l a r f a i l e d , i n August, 1961 (Figure 48, Table 19). This design study w i l l concentrate on these three p a r t i c u l a r cases, and because the major p r i n c i p a l stress i s horizontal i n the north-south d i r e c -t i o n , only the plan view needs to be considered. 114 o -p w 10-20.5 P i l l a r deteriorates 10-22 Stope cave to 450 l e v e l 10-21.5 P i l l a r recovered 10-21.5 & 10-23 f a i l e d 115 TABLE 17 APPROXIMATE STOPE AND PILLAR DIMENSIONS WHEN 10-21.5 PILLAR FAILED (November, 1960) Stopes Length Width Height 10-19.5 21 m. ( 70 f t . .) 20 m. ( 65 f t , •) 150 m. (500 f t . • ) 10-21 30 m. (100 f t . •) 20 m. ( 65 f t . •) 150 m. (500 f t . • ) 10-22 21 m. ( 70 f t . 0 17 m. ( 55 f t . 0 150 m. (500 f t . 0 10-23.5 24 m. ( 80 f t . • ) 14 m. ( 45 f t , •) 150 m. (500 f t . .) P i l l a r s 10-20 21 m. ( 70 f t . •) 21 m. ( 70 f t , •) 150 m. (500 f t . .) 10-21.5 9 m. ( 30 f t , •) 20 m. ( 65 f t , •) 150 m. (500 f t . • ) 10-23 15 m. ( 50 f t . •) 18 m. ( 60 f t . 0 150 m. (500 f t . • ) TABLE 18 APPROXIMATE STOPE AND PILLAR DIMENSIONS WHEN 10-23 PILLAR FAILED (November, 1960) Stopes Length Width Height 10-19.5 21 m. ( 70 f t . ) 20 m. (65 f t . ) 150 m. (500 f t . ) 1 0 - 2 1 -1 61 m. (200 f t . ) 18 m. (60 f t . ) 150 m. (500 f t . ) 10-22 ) 10-23.5 24 m. ( 80 f t . ) 14 m. (45 f t . ) 150 m. (500 f t . ) P i l l a r s 10-20 21 m. ( 70 f t . ) 21 m. (70 f t . ) 150 m. (500 f t . ) 10-21.5 F a i l e d - -10-23 15 m. ( 50 f t . ) 18 m. (60 f t . ) 150 m. (500 f t . ) TABLE 19 APPROXIMATE STOPE AND PILLAR DIMENSIONS WHEN 10-20 PILLAR FAILED (August, 1961) P i l l a r s Length Width Height 10-20 15 m. (50 f t . ) 30 m. (100 f t . ) 150 m. (500 f t . ) 10-21.5 --- F a i 1 e d 10-23 F a i l e d Stopes 1 0 - 1 9 . 5 24m. (80 f t . ) 2 0 m. ( 6 5 f t . ) 1 50 m. ( 5 0 0 f t . ) 10-21 1 0_22 100 m. ( 3 3 3 f t . ) 3 0 m. (100 f t . ) 240 m. (80 f t . ) 1 0 - 2 3 . 5 Scale: 1 i n . = 100 f t . PLAN VIEW FIGURE 46 Estimated Layout When 10-21.5 P i l l a r Failed. Estimated Layout When 10-20 P i l l a r F ailed 120 6.7 P i l l a r Design Study The three r i b p i l l a r s involved i n the present study (10-20, 10-21.5, 10-23) may be c l a s s i f i e d as "separation p i l l a r s ' (Category two). According to the design charts, the following design methods should be applied. 6.7.1 Phase 1. Experience Design From the description of the mining method (Section 6.2) a t y p i c a l "block" i s mined using three 21 m. (70 f t . ) wide primary stopes separated by 37 m. (120 f t . ) p i l l a r s and flanked by two boundary p i l l a r s 46 m. (150 f t . ) wide. The primary stopes are mined f i r s t and drawn under rock f i l l and then consolidated with the introduct ion of hydraulic f i l l . The two 37 m. (120 f t . ) p i l l a r s are then removed between the f i l l e d stopes. 6.7.2 Phase 2. P i l l a r Structural Analysis The r i b p i l l a r s are located mostly in the massive sulphide core which i s r e l a t i v e l y free from j o i n t s and fractures (Section 6.1.3). Thus, i t is believed that the structures (fa u l t s , j o i n t s and di s c o n t i n u i t i e s ) do not play an important role in p i l l a r s t a b i l i t y at Geco. 6.7.3 Phase 3. Empirical Methods. Estimation of p i l l a r load by the extraction r a t i o formula (Tributary area) o p «= o i . N where: N «= extraction number Op «= average p i l l a r stress Oi •= 14.74 MPa (302 KPSF) (Section 6.3.4) 12 A) 10-21.5 P i l l a r Failure Geometry (Figure 49) P i l l a r N Oi (MPa) a p (MPa) 10-20 2.4 14.74 35.38 10-21.5 3.8 14.74 56.01 10-23 2.5 14.74 36.85 B) 10-23 P i l l a r F a i l u r e Geometry (Figure 50) P i l l a r N Oi(MPa) a p(MPa) 10-20 2.9 14.74 42.75 10-21.5 Failed and Recovered 10-23 3.8 14.74 56.01 C) 10-20 P i l l a r Failure Geometry (Figure 51) P i l l a r N oi (MPa) a p (MPa) 10-20 5 14.74 73.70 10-21.5 Failed and Recovered 10-23 Failed N = 2.4 10-20 N = 3.8 4— N = 2.5 1-10-23 FIGURE 49 O; = 14.7 MPa P i l l a r s Extraction Numbers for the 10-21.5 P i l l a r F ailure Geometry. P L A N VIEW ro ro O; = 14.7 MPa FIGURE 5 0 P i l l a r s Extraction Numbers for the 1 0 - 2 3 P i l l a r Failure Geometry. PLAN VIEW ro « — 1 1 1 0 - 2 0 ,1 ft ' ' .''''V'-.'•. .' 1 G[ - 17.4 M Pa FIGURE 5 1 P i l l a r s Extraction Numbers for the 1 0 - 2 0 P i l l a r Failure Geometry. PLAN VIEW 1 2 5 6.7.3.2 Estimation of P i l l a r Strength; Hoek's Method The p i l l a r s ' strength can be estimated using Hoek and Brown (1980) curves (Figure 8). The p i l l a r material was c l a s s i f i e d (Section 6.3.3) and the Rock Quality Index varies from Q « 0.8 to Q « 17.8. A good quality rock mass i s then assumed, and the unia x i a l compressive strength i s o*c= 100 MPa (2105 KPSF). A) 10-21.5 P i l l a r Failure Geometry P i l l a r W/H P i l l a r Strength/o c P i l l a r Strength (MPa) 10-20 1 0.3 30 10-21.5 0.5 0.2 20 10-23 0.8 0.25 25 B) 10-23 P i l l a r F a i l u r e Geometry P i l l a r W/H P i l l a r Strength/a c P i l l a r Strength (MPa) 10-20 1 0.3 30 10-21.5 Failed and Recovered 10-23 0.8 0.25 25 C) 10-20 P i l l a r Failure Geometry P i l l a r W/H P i l l a r Strength/a c P i l l a r Strength (MPa) 10-20 0.5 0.2 20 10-21.5 Failed and Recovered 10-23 Failed 126 6.7.4 Theoretical Methods As i n Heath Steele's case history, no th e o r e t i c a l methods have been used for the Geco p i l l a r f a i l u r e analysis. 6.7.5 Computer Methods The two-dimensional boundary elements program "BITEM" was used again to model the three si t u a t i o n s : (a) 10-21.5, (b) 10-23 and (c) 10-20 p i l l a r f a i l u r e geometry. (Figure 52,53,54). A) 10-21.5 P i l l a r Failure Geometry (Figure 52) P i l l a r op P i l l a r Load (MPa) 10-20 26.33 (550 KPSF) 10-21.5 38.30 (800 KPSF) 10-23 31.12 (650 KPSF) B) 10-23 P i l l a r Failure Geometry (Figure 53) P i l l a r Op P i l l a r Load (MPa) 10-20 31.12 (650 KPSF) 10-21.5 Failed 10-23 39.84 (832 KPSF) C) 10-20 P i l l a r Failure Geometry (Figure 54) P i l l a r Op P i l l a r Load (MPa) 10-20 38.30 (800 KPSF) 10-21.5 Failed 10-23 Failed FIGURE 5 2 COMPUTER OUTPUT OF THE 1 0 - 2 1 . 5 PILLAR FAILURE GEOMETRY MAJOR PRINCIPAL STRESS CONTOUR 5 3 COMPUTER OUTPUT OF THE 1 0 - 2 3 PILLAR FAILURE GEOMETRY MAJOR PRINCIPAL STRESS CONTOUR FIGURE 5 4 COMPUTER OUTPUT OF THE 1 0 - 2 0 PILLAR FAILURE GEOMETRY MAJOR PRINCIPAL STRESS CONTOUR 1 3 0 6.8 Discussion of the Results Before discussing the rock mechanics results of A) 10-20, B) 10-21.5 and C) 10-23 p i l l a r f a i l u r e s , we must review the assumptions required. - The v i r g i n stress was estimated following the procedure described i n Appendix B. - E l a s t i c Modulus and Poisson Ratio were selected using Hoek and 3 Brown (1980) t y p i c a l values for gneiss rock i n t h i s area. - Each time a p i l l a r was reported f a i l e d , stope and p i l l a r dimensions were assessed. The p i l l a r load was calculated using tributary area and computer simu-l a t i o n . The load values determined by the t r i b u t a r y area are about 30% higher than those from computer simulation. Since the t r i b u t a r y area over-s i m p l i f i e s the problem and represents the upper l i m i t of the average p i l l a r stress, the load values from computer simulation are judged more r e a l i s t i c and kept as mean values. The p i l l a r strength was estimated using Hoek and 3 Brown (1980) curves, and each p i l l a r ' s safety factor was determined. The sequence of f a i l u r e events, according to the computational methods summarized i n Table 20 are as follows: A) 10-21.5 P i l l a r Failure Geometry (November 1960) According to the documentation (Bray (1967)) 1 0, both 10-21.5 and 10-23 p i l l a r s f a i l e d i n November, 1960. Table 20 shows that 10-21.5 was the f i r s t p i l l a r to f a i l (S.F. «= 0.53). This low safety factor indicates that the geo-metry assumed at f a i l u r e was not exact (S.F. should be around 1). Neverthe-les s , the r e s u l t s are s t i l l capable of reconstructing the history of the f a i l u r e s . TABLE 20 GECO PILLAR ANALYSIS RESULTS P i l l a r Extraction Tributary Number "N" Area (MPa) Computer Stress (MPa) Mean Stress (MPa) W/H (Plan View) P i l l a r Strength (MPa) Safety Factor Extraction Ratio % Remarks A) 21.5 P i l l a r Failure Geometry, November 1960 10-20 2.4 35.38 26.33 26.33 1 30 1.15 55 Stable 10-21.5 3.8 56.01 38.30 38.30 0.5 20 0.53 74 P i l l a r f a i l e d 10-23 2.5 36.85 31 .12 31.12 0.8 25 0.81 60 Failure i n i t i a t e d B) 23 P i l l a r Failure Geometry, November 1960 10-20 2.9 42.75 31.12 31.12 1 30 0.97 66 Stable 10-21.5 F a i l e d and R e c o v e r e d • 10-23 3.8 56.01 39.84 39.84 0.8 25 0.63 74 P i l l a r f a i l e d C) 10-20 P i l l a r Failure Geometry, August 1961 10-20 5 73.70 38.30 38.30 0.5 20 0.53 80 P i l l a r f a i l e d 1 3 2 B) 10-23 P i l l a r Failure Geometry (November 1960) Just a f t e r the 10-21.5 p i l l a r collapsed, the stress r e d i s t r i b u t i o n caused the complete f a i l u r e of 10-23 p i l l a r (S.F. = 0.63), and the 10-20 p i l l a r probably started to show some i n s t a b i l i t y (S.F. = 0.97) C) 10-20 P i l l a r F a i l u r e Geometry (August 1961) The 10-20 p i l l a r safety factor of 0.53 indicates that i n August 1961, the load had already largely exceeded the bearing c a p a b i l i t y of the p i l l a r . Although these results are not as precise as those of Heath Steele's case history, they are consistent with the f a i l u r e events at Geco. While the accuracy of the design procedure and input data must s t i l l be improved, the res u l t s are valuable as a sta r t i n g point for future designs. Figure 55 i s a plot of the l o c a l extraction r a t i o "e"* versus the safety factor of each p i l l a r at d i f f e r e n t stages of extraction. It can be observed that at t h i s depth (±300 m) s t a b i l i t y problems begin when the ex-tr a c t i o n r a t i o exceeds 55%. Permissible extraction r a t i o can be determined at d i f f e r e n t depths using the same procedure. Geco ac t u a l l y l i m i t s primary mining to an extraction r a t i o of 37%. It should be noticed that a lack of points i n the upper part of the curve (Figure 55) i s due to the inaccurate estimation of the stope and p i l l a r dimensions i n s i t u a t i o n A: (10-21.5 P i l l a r Failure Geometry). * Defined i n Section 5.8 3 Geco x 10-20 2 . 5 + 10-2 1.5 A 10-23 2 r-O 4-> U rd L_ >, <U <+-fd LO 1 . 5 5 r-0 100 E x t r a c t i o n R a t i o C4) CHAPTER 7 SUMMARY AND CONCLUSIONS 1 3 5 7.1 Design Procedure In the context of the North American mining industry, most underground p i l l a r s are s t i l l designed using a t r i a l and error process. Three major obstacles i n designing p i l l a r s are responsible for t h i s s i t u a t i o n . Accurate estimation of p i l l a r strength Evaluation of p i l l a r load The m u l t i p l i c i t y of p i l l a r s . This study aims to improve the actual p i l l a r design practices. - A c l a s s i f i c a t i o n system was f i r s t proposed which divides p i l l a r s into four categories: Category 1. Plate P i l l a r s Category 2. Separation P i l l a r s Category 3. Stub P i l l a r s Category 4. Inclined P i l l a r s . This c l a s s i f i c a t i o n resolves the problem of the m u l t i p l i c i t y of p i l l a r s and allows standardization of the p i l l a r design procedure. - A f i v e phase design procedure was developed. It suggests that every suitable designing method should be used, becoming more sophisticated as ex-perience i s gained with the rock material. Also, design charts provide a guideline for the selection of the pertinent methods. This procedure per-mits : i ) A standard design process for a l l p i l l a r types. i i ) A more accurate estimation of p i l l a r load and strength by using several methods which take into account d i f f e r e n t factors. i i i ) An optimization of the rock mechanics data employed as a design t o o l . Thus, the procedure helps to overcome the three major obstacles i n de-signing p i l l a r s . Also, i t i s simple to apply and minimizes the p o s s i b i l i t y of misconstruing the r e s u l t s . 7.2 Case Histories This p i l l a r design procedure was applied i n back-analysing p i l l a r f a i l -ure at Heath Steele andGeco Divi s i o n . Because both case h i s t o r i e s were well documented and involved simple geometry, the input parameters affecting the design of p i l l a r s could be understood, controlled and adjusted. A plot depicting extraction r a t i o versus "safety f a c t o r " for each p i l l a r at d i f f e r e n t stages of extraction appears to be an e f f i c i e n t manner of synthesizing the analysis r e s u l t s . As well, the curves indicate the l i m i t of extraction permissible at a given depth. Both mines--Heath Steele and Geco--had experienced costly p i l l a r f a i l -ures r e s u l t i n g i n production delays, extra ground support and loss of ore reserves before they were able to determine a safe extraction r a t i o . Also, because i t i s based on experience only, there i s no indication whether the p i l l a r s are overdesigned, or to which depth t h i s extraction r a t i o (50% at Heath Steele and 37% at Geco) w i l l remain safe f o r primary extraction. Although further research i s required i n order to obtain a wider va r i e t y of p i l l a r s and rock mass q u a l i t i e s , the extraction r a t i o versus safety factor curves represent a method of optimizing primary extraction, avoiding major s t a b i l i t y problems. The curves take the following factors into account: - v i r g i n stress - stress induced by mining • - strength of p i l l a r material 1 3 ? - rock mass quality - structural d i s c o n t i n u i t i e s - percentage of extraction - effect of adjacent openings - ov e r a l l geometry and orientation of the underground structures - p i l l a r width to height r a t i o - depth below surface. However, damage created by blas t i n g , as well as groundwater effects were ignored because they are d i f f i c u l t to quantify. They may play an important part in p i l l a r s t a b i l i t y . The use of rock mass c l a s s i f i c a t i o n allows r e s u l t s from d i f f e r e n t s i t e s to be compared. Figure 56 combines the curves from both case h i s t o r i e s . The rock mass quality i s indicated f or each case. 7.3 Design Methods A review of the p r i n c i p a l p i l l a r design methods i s given in Chapter 3. They are subdivided into four groups according to t h e i r le v e l of s o p h i s t i -cation, and t h i s study makes the following conclusions: Group 1. Experience Methods - Most mines s t i l l r e l y p r i n c i p a l l y upon experience design. - Keeping detailed f i l e s on a l l information concerning the mine s t a b i l i t y such as f a i l u r e , slabbing, squeezing, caving, conver-gence w i l l improve the experience design. Group 2. Empirical Methods - Because empirical methods ignore many factors influencing p i l l a r s t a b i l i t y , the knowledge of the conditions in which they were developed i s e s s e n t i a l . . 5 H e a t h S t e e 1 e & Ge c 0 x 77-90 o 10-20 + 77-92 • 10-2 1 . 5 A 77-94 o 10-23 — RMR = 68 -Heath S t e e l e STRBLE RMR = 5 6 UNSTABLE Geco ... i i i 1 1 — 1 L_ 1 . 0 0 10 20 30 40 50 E x t r a c t i o n R a t i o 60 70 80 90 100 Group 3. Theoretical Methods - The t h e o r e t i c a l and a n a l y t i c a l methods are complex and d i f f i c u l t to apply, and t h e i r r e s u l t s are often not r e l i a b l e . They are use-f u l i n further comprehending the mechanism involved i n p i l l a r design. - To determine t h e o r e t i c a l l y p i l l a r strengths; only Wilson's formula has been widely used. - The Coates' wall d e f l e c t i o n formula and the photoelastic technique to determine p i l l a r load were r e l a t i v e l y popular in the past but are no longer employed. If the p i l l a r ' s structure can be r e a l i s t i c a l l y represented by the beam or plate theory, i t i s a well-accepted method of designing p i l l a r s . Group 4. Computer Methods - The computer methods are v e r s a t i l e and may be adapted to every category of p i l l a r . - Although they are mathematically precise, the accuracy of the r e s u l t s is. related to the quality of the input data and designer's s k i l l s . Finally,- i t i s important to remember that designing underground p i l l a r s i s a progressive task. The accuracy and the designer's confidence in the r e s u l t s w i l l improve concurrently with the continuing application of the design procedure. Careful underground observations, monitoring and measure-ments should provide feedback on each design. 140 REFERENCES 1 . Roche Mines A s s o c i l s l t e e ( 1 9 8 4 ) ; Etat de l a Question sur le Dimensionnement et l a S t a b i l i t y des P i l l i e r s de surface. Energy Mines and Resources Canada, Fevr i e r 1 9 8 4 2 . WAGNER, H. (197^) ; Determination of the Complete Load-Deformation C h a r a c t i r i s t i c s of Coal P i l l a r s , Advances i n Rock Mech., Vol. 113., International Society of Rock Mech., pp. 1 0 7 6 -1 0 8 1 , 19?4 3. HOEK, E. f BROWN, E.T. ( 1 9 8 0 ) 5 Underground Excavation i n Rock , Institut e of Mining and Metallurgy, London, 5 2 7 p. 4 . BIENIAWSKI , Z.T. ( 1 9 8 3 ) i Improve Design of Room and P i l l a r Coal Mines for U.S. Conditions , S t a b i l i t y i n Underground Mining, Society of Mining Eng. of AIME, I 9 8 3 5. SZWILSKI, T.B. ( 1 9 8 3 ) ; Sizing of Chain P i l l a r , S t a b i l i t y i n Under-Mining, Chapt. 2 5 , Society of Mining Eng. of AIME, 1 9 8 3 , PP. 5 3 9 - 5 5 7 6. WITTAKER, B.N., SINGH, R.N. ( 1 9 8 1 ) ; S t a b i l i t y of Longwall Mining Gate Roadways i n Relation to Rib P i l l a r Size , Int. Journal Rock Mech., Min. S c i . and Geomech. abstr., Vol. 18, pp. 3 3 1 - 3 3 4 7. ASHLEY, G.H. ( 1 9 3 0 ) ; Ba r r i e r P i l l a r L e g i s l a t i o n i n Pennsylvania Trans. AIME, Coal Div., 1 9 3 0 , pp. 7 6 - 9 6 8 . BELINSKI, A., BORECKI, M. ( 1 9 6 4 ) j Results of Investigation on Rock Pressure by the Longwall System of Coal Mining i n the Upper S i l e s i a n Coal F i e l d , Proceeding 4 t h . Int. Conference on Strata Control and Rock Mech., Columbia University, 1 9 6 4 , pp.85 -88 9. TOUSEULL, J.A., ; Stereographic Method of Determining Wether Planes of Weakness Transect P i l l a r s , U.S. Bureau of Mines, Denver, Colorado. 1 0 . BRAY, R.C.E. ( 1 9 6 7 ) ; Control of Ground Movement at the Geco Mine , Annual General Meeting, Noranda Mines, Geco Div. Geology Dept., Ottawa 1 9 6 7 141 1 1 . ALLCOTT, G.A., ARCHIBALD, TJ.E. ( l 9 8 l ) ; Description of P i l l a r Behaviour at Heath Steele Mines, CIM B u l l e t i n , Oct. 1 9 8 1 , pp. 8 0 - 8 7 1 2 . Golder Associates ( 1 9 8 1 ) ; Prediction of Stable Excavation Spans fo r Mining at Depth Below 1 0 0 0 Meters i n Hard Rock, Canada Center f o r Mineral and Energy Technology, Canmet Contract no. 80-00081, A p r i l 1981, 1 3 4 p. 1 3 . COATES, D.F. ( 1 9 6 6 ) ; P i l l a r Loading: II Model Studies, Mines Branch Research Report R - 170 , Queen's Printer, Ottawa 1 9 6 6 142 APPENDIX A REVIEW OF LITERATURE (See "LITERATURE RESEARCH REPORT" June 1984) 1 4 3 APPENDIX B Determination of the Geco Stress Regime at 700 f t . Depth No stress investigations have been performed at Geco. Both horizontal stresses are estimated using the following methods: 1. From Herget (1983) Results from groundstress determinations i n the Canadian Shield are analyzed i n regard to change with depth of the r a t i o of maximum horizontal stress to measured v e r t i c a l stress (o^ max/ av) a n <3 minimum horizontal stress to measured v e r t i c a l stress (o^ min/^v)-253.87 C°h max/°v) « depth (m) frr /rr 1 279.82 lo-h min/<V = depth (m) + 1.45 + 0.88 at 215 m. (700 f t . ) depth £L c gHfinaxl _ 253.87 0-3 depth + 1.45 2.64 0_2_ O-3 H(min) = 279.82 o v depth + 0.88 «= 2.20 2. From Hoek and Brown (1980) In s i t u stress measurements have been done at Wawa Mine, not far from Geco, and the shallow depth r e s u l t s (^300 m.) tend to confirm stress values determined by Herget's formulas. G.W. MacLeod Mine, Wawa, O n t a r i o G.W. MacLeod Mine, Wawa, O n t a r i o S i der i t e T u f f Depth (m.) 370 1Z°_ a h Vcr 1 6 . 1 '5-1 1.29 ref. 81 AL G.W. MacLeod Mine, Wawa, O n t a r i o Tuff G.W. MacLeod Mine, Wawa, O n t a r i o T u f f G.W. MacLeod Mine, Wawa, O n t a r i o M e t a - d i o r i t e G.W. MacLeod Mine, Wawa, O n t a r i o Chert Wawa, O n t a r i o G r a n i t e E l l i o t Lake, Ontar i o 5 7 5 575 1*80 575 3 ^ 5 21.5 1A. 6 1 8 . 7 26.6 20.0 1.23 1.25 1 . 5 2 2 . 5 0 81 81 81 81 82 S a n d s t o n e El 1 i o t Lake, O n t a r i o E l 1 l o t Lake, O n t a r i o Quartz i te Diabase dyke TTB ( 1 1.0)* 2.56 rrrrr 7 S T 1(00 1 7 . 2 T77o-1 . 9 0 8 3 "ST 8^ 144 APPENDIX C ILLUSTRATION OF PILLARS (after Roche Mines Associates (1984) 1) lk-5 MAIN SHAFT SURFACE PILLAR, RIB PILLAR -, CENTER PILLAR, ROOF PILLAR, SILL PILLAR OVERBURDEN -, MINERALIZED ZONE SHAFT PILLAR -, I I IP in FIGURE 5 HARD ROCK PILLAR 146 RIB PILLAR STOPE FIGURE 6 • HARD ROCK PILLARS FIGURE 7 SOFT ROCK PILLARS 148 APPENDIX D "BITEM", 2-D Boundary Element Program DESCRIPTION OF BITEM Program BITEM i s based on program BITE, which was developed by P.C. Riccardella during PhD studies at Carnegie-Mellon University and has been released through CSIRO. BITE performs elasticity analyses for homogeneous solids only; BITEM has been developed at CSIRO to analyse systems consisting of a number of regions with different material properties. The boundary integral technique uses only information relating to the boundary surface to enable an analysis of the whole solid. The net effect i s a reduction i n the dimension of the problem posed. As applied in BITEM the boundary integral equation technique enables a two-dimen-sional analysis of plane strain (or stress) linear elasticity problems given only a description of the (one-dimensional)boundary surfaces of the so l i d . Advantages of this technique over other available stress analysis methods, which require as input data a specification of the whole body, are as followst 1. Reduction i n volume of input data, and thus greater ease modelling problems 2. Savings in computer time and storage PROGRAM CAPABILITY Program BITEM solves two-dimensional elasticity problems for a piece-wise homogeneous isotropic linearly elastic material, using the bounda-ry integral equation technique. Data required by the program include the elastic•properties of each individual homogeneous domain, a description of the geometry of the boundary surface of each such domain, and some of the displacement and t r a c t i o n boundary conditions along these boun-daries. Such boundary condition s p e c i f i c a t i o n s need only be made on those surfaces which do not interfaces between adjoining regions, e.g. on excavation boundaries i n mining a p p l i c a t i o n s . The program i s capable of generating the remaining t r a c t i o n and displacement unknowns on a l l boundaries of the s o l i d , interface or otherwise, together with stress c a l c u l a t i o n s on the boundaries and stress and displacement solutions f o r s p e c i f i e d locations within the s o l i d . L i n e a r l y varying (rather than constant value) displacements and t r a c t i o n s are assumed over the d i s -c r e t i z e d segments of the boundaries. This l i n e a r boundary value approach i s more accurate than the constant boundary value approach, while re-qu i r i n g l i t t l e or no increase i n computer running times and storage requirements. In addition to i t s a p p l i c a b i l i t y i n normal geomechanics problems, BITEM i s able to solve several inclusion-type problems; f o r example, the problem of an i n c l u s i o n which has been stressed p r i o r to i t s inser-t i o n i n a s o l i d . 

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