than 80 literature case histories of hard rock pillars in room and p i l l a r mining. ..... (figures 1 and 2, note C) and drill drives (figure 1, note. D) or overcuts (figure 2 ... pillar design can seriously affect the recovery of this ore. A pillar that does ..... post-blast clean-up and development rehabilitation, development of blast induced ...
DEVELOPMENT OF E M P I R I C A L R I B P I L L A R DESIGN
CRITERION
FOR OPEN STOPE MINING By MARTIN RAYMOND HUDYMA B.A.Sc,
The U n i v e r s i t y o f B r i t i s h C o l u m b i a ,
1986
A THESIS SUBMITTED I N P A R T I A L FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
MASTER OF A P P L I E D
SCIENCE
in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF MINING AND MINERAL PROCESS We a c c e p t t h i s t h e s i s a s to the
required
conforming
standard
THE UNIVERSITY OF B R I T I S H September
ENGINEERING
COLUMBIA
1988
M a r t i n Raymond Hudyma,
1988
In presenting
this thesis in partial fulfilment
of the
requirements for an advanced
degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department
or
by
his
or
her
representatives.
It
is
understood
that
copying
or
publication of this thesis for financial gain shall not be allowed without my written permission.
Department The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3
DE-6(3/81)
ABSTRACT
The many
design
empirical
verified in
the
rib
with
The
methods,
"pillar
of
determined
the by
and p i l l a r
intact
none
of
has
the
This thesis to
been
done
using
has
been
methods uses data
develop
open, s t o p e m i n i n g .
collected
an
empirical
The m e t h o d
is
graph".
in
the
method
material,
modelling,
The g r a p h has
been
histories
are: the
the
the
average
pillar
pillar
width
r e f i n e d w i t h the of hard
compressive load
and
use
of
rock p i l l a r s
the more
i n room
mining.
to
examine
in
open
s t a b i l i t y g r a p h and t h e p i l l a r
the
stope
rib
strength
useful
under but
pillar
conditions
formulas
and B i e n i a w s k i
(1983)
Guidelines, the
for
by H e d l e y are not
using
temporary
open s t o p e r i b
design.
The
the
design
investigation
open
pillars.
stope
design
(1972),
Obert
of
found
(1980)
open
may be
stope
and D u v a l l
the
rib
(1967)
applicable.
pillar of
the
used
commonly u s e d
c u r v e s d e v e l o p e d b y Hoek a n d Brown
some
for
data base are
a p p l i c a b i l i t y o f e m p i r i c a l methods
pillar
stable
for
pillar
case
pillars
Design Study"
numerical
The p i l l a r
proposed
Mine
stability
literature
pillars
but
variables
height.
80
rib
a design survey.
design
strength
stope
d e s i g n method
c a l l e d the
pillar
open
"Integrated
pillar
than
of
stability
permanent rib
open
pillars,
and
graph stope
method, rib
failing
are
pillars, temporary
iii TABLE OF CONTENTS PAGE ABSTRACT
i i
L I S T OF TABLES
v i i
L I S T OF FIGURES
viii
ACKNOWLEDGEMENT
xiii
CHAPTER 1:
INTRODUCTION
1.1
Contents
1.2
Open S t o p e M i n i n g 1 . 2 . 1 D e f i n i t i o n o f Open S t o p i n g 1 . 2 . 2 A p p l i c a b i l i t y o f t h e Open S t o p i n g 1 . 2 . 3 D e s c r i p t i o n o f T y p i c a l Open S t o p e M i n i n g Methods
2 3 4
R o l e o f R i b P i l l a r s i n Open S t o p e M i n i n g
9
1.3
of the
1 Thesis
CHAPTER 2 : R I B P I L L A R F A I L U R E 2 . 1 F a i l u r e Mechanisms and C h a r a c t e r i s t i c s 2 . 1 . 1 Rock F r a c t u r i n g 2.1.2 P i l l a r Load-Deformation Curve 2 . 1 . 3 Loss o f Load B e a r i n g C a p a c i t y 2.2
S i g n i f i c a n t V a r i a b l e s i n Open S t o p e P i l l a r Stability 2 . 2 . 1 I n t a c t Rock S t r e n g t h 2 . 2 . 2 P i l l a r Load 2 . 2 . 3 P i l l a r Shape and C o n f i n e m e n t 2.2.4 Structural Features i n P i l l a r s 2 . 2 . 5 E f f e c t o f P i l l a r Volume 2.2.6 Effect of B a c k f i l l 2.2.7 Effect of Blasting
2 . 3 Chapter
Summary
CHAPTER 3 : REVIEW OF P I L L A R DESIGN METHODS 3.1 E m p i r i c a l D e s i g n Methods 3.1.1 P i l l a r Strength Determination 3 . 1 . 1 . 1 E m p i r i c a l Strength Formulas 3 . 1 . 1 . 2 Salamon's Formula
1
5
11 11 14 17 19 23 23 23 24 25 26 27 30 31 32 32 34 35 38
iv 3.1.1.3 3.1.1.4 3.1.1.5 3.1.2 P i l l a r 3.1.2.1 3.1.2.2 3.1.3 Safety 3.2
H e d l e y ' s Formula O b e r t a n d D u v a l l Shape E f f e c t F o r m u l a . Hoek a n d Brown P i l l a r S t r e n g t h C u r v e s . Load T r i b u t a r y Area Theory Numerical Modelling Factor
. .
N u m e r i c a l D e s i g n Methods 3 . 2 . 1 Types o f N u m e r i c a l Methods 3 . 2 . 2 I n t e r p r e t a t i o n o f Boundary Element R e s u l t s in Mining 3.2.2.1 Post-Processing Failure C r i t e r i o n . . . . 3.2.2.2 Interactive Failure Criterion 3 . 2 . 2 . 3 P r i n c i p a l S t r e s s Magnitude 3.2.3 L i m i t a t i o n s o f Boundary Element M o d e l l i n g . . . 3 . 2 . 3 . 1 M o d e l l i n g a Rock Mass 3 . 2 . 3 . 2 Computational Assumptions
CHAPTER 4 : OPEN STOPE R I B P I L L A R DATA BASE
40 41 43 45 45 51 51 53 53 57 57 60 63 63 63 66 68
4 . 1 G e n e r a l Data Base I n f o r m a t i o n
68
4.2
Background Data
69
4.3
P i l l a r Assessment
73
CHAPTER 5 : BOUNDARY ELEMENT METHODS I N R I B P I L L A R DESIGN.
.
78
5.1
Boundary Element Codes Used 5 . 1 . 1 BITEM 5 . 1 . 2 MINTAB 5 . 1 . 3 BEAP
79 79 81 84
5.2
Open S t o p e R i b P i l l a r M o d e l l i n g 5 . 2 . 1 D e f i n i n g t h e Open S t o p e G e o m e t r y 5.2.2 D e f i n i n g the Average P i l l a r S t r e s s
84 86 86
5.3
2D M o d e l l i n g o f 3D S t o p e G e o m e t r i e s 5.3.1 Plane S t r a i n S o l u t i o n 5 . 3 . 2 C o m p a r i s o n o f 2D a n d 3D N u m e r i c a l M o d e l l i n g Results
91 92
5.4
5.5
D i s p l a c e m e n t D i s c o n t i n u i t y M o d e l l i n g o f 3D S t o p e Geometries 5 . 4 . 1 Seam T h i c k n e s s L i m i t a t i o n s 5.4.2 Comparison of Displacement D i s c o n t i n u i t y a n d 3D N u m e r i c a l M o d e l l i n g Pillar
Load C a l c u l a t i o n s f o r the
Open
Stope
93 97 97 99
V
Data Base 5.5.1 Assumptions 5.5.2 P i l l a r Load R e s u l t s 5.5.3 Numerical Model Comparison U s i n g the Histories 5.6
102 103 103
Case
107
C h a p t e r Summary
CHAPTER 6 :
110
DEVELOPMENT OF A P I L L A R DESIGN METHOD
6.1
C h o i c e o f V a r i a b l e s f o r Open S t o p e P i l l a r D e s i g n 6 . 1 . 1 A p p l i c a b i l i t y o f S t a t i s t i c a l Methods 6.1.2 Design V a r i a b l e s 6.1.3 Discounted Variables 6 . 1 . 3 . 1 P i l l a r Volume 6.1.3.2 Structural Discontinuities
6.2
Pillar 6.2.1 6.2.2 6.2.3 6.2.4
6.3
Data from L i t e r a t u r e 6 . 3 . 1 D a t a f r o m C a n a d i a n Room a n d P i l l a r M i n i n g . 6 . 3 . 2 D a t a f r o m a B o t s w a n a Room a n d P i l l a r M i n e . 6 . 3 . 3 D a t a f r o m an A u s t r a l i a n Open S t o p e M i n e . . 6 . 3 . 4 Summary o f A l l t h e D a t a
114 .
.
115 115 117 118 119 120
S t a b i l i t y Graph G r a p h i c a l Data A n a l y s i s I n f l u e n c e o f P i l l a r Load A p p r o x i m a t i o n s . . . . Importance o f Y i e l d i n g P i l l a r Case H i s t o r i e s . L i m i t a t i o n s of the P i l l a r S t a b i l i t y Graph. . .
122 122 126 128 130
. . .
. . .
131 131 134 139 143
6.4
Comparison A g a i n s t Other D e s i g n Methods 6.4.1 H e d l e y ' s P i l l a r S t r e n g t h Formula 6 . 4 . 2 Hoek a n d Brown P i l l a r S t r e n g t h C u r v e s 6 . 4 . 3 P i l l a r Shape E f f e c t F o r m u l a s
143 146 151 152
6.5
C h a p t e r Summary
158
CHAPTER 7:
DESIGNING R I B P I L L A R S FOR OPEN STOPE M I N I N G .
.
.
160
7.1
Permanent P i l l a r s
162
7.2
Temporary P i l l a r s 7 . 2 . 1 S t a b l e Temporary 7 . 2 . 2 F a i l e d Temporary
163 165 166
7.3
Pillars Pillars
Case Example: T r a n s v e r s e R i b P i l l a r s a t N o r i t a . 7 . 3 . 1 G e o l o g y and M i n i n g M e t h o d 7 . 3 . 2 Back A n a l y s i s U s i n g the P i l l a r S t a b i l i t y Graph 7 . 3 . 3 Comments C o n c e r n i n g t h e P i l l a r D e s i g n
.
.
167 167 170 173
vi CHAPTER 8: SUMMARY AND CONCLUSIONS
174
8.1 Summary 8.1.1 Open Stope R i b P i l l a r F a i l u r e 8.1.2 C u r r e n t P i l l a r Design Methods 8.1.3 I d e n t i f i c a t i o n and Q u a n t i f i c a t i o n o f t h e Design V a r a i b l e s 8.1.4 Development o f t h e P i l l a r S t a b i l i t y Graph. . .
174 174 175
8.2 C o n c l u s i o n s 8.2.1 A p p l i c a b i l i t y o f t h e Method 8.2.2 L i m i t a t i o n s o f t h e Method 8.2.3 Design o f Open Stope R i b P i l l a r s
179 179 179 180
8.3 F u t u r e Work
181
176 177
REFERENCES
183
APPENDIX 1
190
vii L I S T OF TABLES PAGE TABLE 1. Constants proposed by v a r i o u s authors f o r t h e s i z e e f f e c t formula ( a f t e r Babcock, Morgan and Haramy 1981).
36
TABLE 2. Constants proposed by v a r i o u s authors f o r t h e shape e f f e c t formula ( a f t e r Babcock, Morgan and Haramy 1981).
37
TABLE 3. Constants proposed by v a r i o u s authors f o r t h e shape e f f e c t formula ( a f t e r Babcock, Morgan and Haramy 1981).
37
TABLE 4. The s a f e t y f a c t o r s proposed by v a r i o u s authors f o r e m p i r i c a l p i l l a r d e s i g n i n e n t r y mining methods.
52
TABLE 5. Background data f o r a l l t h e p i l l a r case histories.
70
TABLE 6. Comparison o f BEAP and BITEM f o r f o u r s e t s o f d i f f e r e n t orebody geometries.
94
TABLE 7. Comparison of BEAP and MINTAB f o r t h e f o u r different tests.
98
TABLE 8. P i l l a r l o a d i n f o r m a t i o n f o r a l l t h e open stope r i b p i l l a r case h i s t o r i e s u s i n g BITEM, MINTAB and t h e T r i b u t a r y Area Theory.
105
TABLE 9. Comparison o f MINTAB and BITEM r e s u l t s , when both programs l i m i t a t i o n s a r e s a t i s f i e d .
107
TABLE 10. Comparison o f BITEM and MINTAB, when t h e MINTAB 108 l i m i t a t i o n i s met, but the BITEM l i m i t a t i o n i s not met. TABLE 11. Comparison between good BITEM and poor MINTAB 111 geometries shows t h e average p i l l a r s t r e s s v a r y i n g up t o ± 25%. TABLE 12. Data used by Von Kimmelmann e t a l . (1984) i n the development o f a p i l l a r f a i l u r e c r i t e r i o n .
136
TABLE 13. Comparison o f t h e v a l u e o f ore f o r mines u s i n g 161 temporary p i l l a r s a g a i n s t mines u s i n g permanent p i l l a r s .
viii L I S T OF FIGURES PAGE FIGURE 1. The elements o f an i d e a l i z e d l o n g i t u d i n a l longhole open s t o p i n g method showing t h e b l a s t i n g , mucking and b a c k f i l l i n g o p e r a t i o n s .
6
FIGURE 2. The elements o f an i d e a l i z e d t r a n s v e r s e b l a s t h o l e open s t o p i n g method showing t h e d r i l l i n g , b l a s t i n g , mucking and b a c k f i l l i n g o p e r a t i o n s .
7
FIGURE 3a. P a r a l l e l f r a c t u r i n g and s p a l l i n g due t o a l a c k of confinement a t the p i l l a r w a l l s .
16
FIGURE 3b. I n t e r n a l s p l i t t i n g and a x i a l c r a c k i n g o f a p i l l a r due t o deformable p i l l a r l a y e r s o r t h e propagation of p a r a l l e l wall f r a c t u r e s .
16
FIGURE 3c. Diagonal c r u s h i n g f r a c t u r e s may occur i n c o n f i n e d o r massive p i l l a r s .
16
FIGURE 4. A h y p o t h e t i c a l l o a d - d e f o r m a t i o n curve can be used t o d e s c r i b e t h e s t r e s s - s t r a i n c h a r a c t e r i s t i c s o f a pillar.
18
FIGURE 5. Wagner (1974) d i d a s e r i e s o f i n s i t u l o a d deformation t e s t s on c o a l p i l l a r s u s i n g h y d r a u l i c j a c k s . The graph on t h e t o p shows t h e l o a d - d e f o r m a t i o n c h a r a c t e r i s t i c s o f the p i l l a r i n g e n e r a l . The o b l i q u e diagrams g i v e t h e r e l a t i v e l o a d on each o f t h e 25 j a c k s at f o u r stages o f p i l l a r compression.
20
FIGURE 6. The s t r e s s - s t r a i n curves f o r l a b o r a t o r y specimens loaded under i n c r e a s i n g c o n f i n i n g p r e s s u r e s show an i n c r e a s e i n peak l o a d and an i n c r e a s e i n t h e post-peak l o a d b e a r i n g c a p a c i t y .
22
FIGURE 7. There i s a very l a r g e i n f l u e n c e o f specimen s i z e on t h e s t r e n g t h o f i n t a c t rock, f o r s m a l l specimen diameters.
28
FIGURE 8. S t r e n g t h t e s t i n g o f samples o f i n c r e a s i n g specimen l e n g t h shows a d e c r e a s i n g i n f l u e n c e o f s i z e .
28
FIGURE 9. Histogram o f t h e s a f e t y f a c t o r s f o r s t a b l e and f a i l e d p i l l a r case h i s t o r i e s i n South A f r i c a n bord and p i l l a r c o a l mining.
39
ix FIGURE 10. The estimated s t r e s s and s t r e n g t h f o r case h i s t o r i e s o f p i l l a r s i n room and p i l l a r mining i n t h e E l l i o t l a k e uranium mining d i s t r i c t .
42
FIGURE 11. Hoek and Brown (1980) proposed a s e r i e s o f p i l l a r s t r e n g t h curves based on t h e t h e o r e t i c a l d i s t r i b u t i o n o f rock mass f a i l u r e i n a p i l l a r .
44
FIGURE 12. The analogy o f s t r e a m l i n e s i n a smoothly f l o w i n g stream o b s t r u c t e d by b r i d g e p i e r s i s o f t e n used to d e s c r i b e t h e c o n c e n t r a t i o n o f s t r e s s i n p i l l a r s .
47
FIGURE 13. The t r i b u t a r y area theory, f o r average p i l l a r load c a l c u l a t i o n , applied t o several d i f f e r e n t p i l l a r layouts.
47
FIGURE 14. Salamon (1974) showed t h e v a r i a t i o n i n p i l l a r s t r e s s caused by i n c r e a s i n g t h e number o f p i l l a r s (N) i n a mining p a n e l . The graph shows a d i s t i n c t i n f l u e n c e o f the l o c a t i o n o f a p i l l a r and t h e number o f p i l l a r s on the s t r e s s induced.
49
FIGURE 15. A study u s i n g two dimensional boundary element numerical m o d e l l i n g shows t h e i n f l u e n c e o f p i l l a r shape and t h e number o f p i l l a r s on t h e average s t r e s s .
50
FIGURE 16. An i d e a l i z e d s k e t c h showing t h e p r i n c i p l e o f numerical m o d e l l i n g o f underground e x c a v a t i o n s .
54
FIGURE 17. An e m p i r i c a l f a i l u r e c r i t e r i o n has been a p p l i e d t o t h e two dimensional s t r e s s d i s t r i b u t i o n o f a s t a b l e open stope r i b p i l l a r .
59
FIGURE 18. The t h e o r e t i c a l d i s t r i b u t i o n o f f a i l e d r o c k i s much g r e a t e r i n t h i s p i l l a r .
59
FIGURE 19. The peak s t r e n g t h , deformation c h a r a c t e r i s t i c s , and e f f e c t o f l o c a t i o n used f o r i n v e s t i g a t i n g a p i l l a r case h i s t o r y w i t h a displacement d i s c o n t i n u i t y program.
61
FIGURE 20. The normal s t r e s s and t h e f a i l e d r e g i o n s e s t i m a t e d w i t h t h e displacement d i s c o n t i n u i t y program f o r a s i l l p i l l a r case h i s t o r y .
61
FIGURE 21. The d i s t r i b u t i o n o f normal s t r e s s i n a mining b l o c k was e s t i m a t e d f o r two d i f f e r e n t mining sequences t o determine t h e b e s t stope e x t r a c t i o n sequence.
64
FIGURE 22. T h i s f i g u r e shows t h e g e o m e t r i c a l d e f i n i t i o n f o r t h e stope and p i l l a r dimensions used i n t h i s t h e s i s .
72
X
FIGURE 23. I s o m e t r i c view o f an opening t h a t i s l o n g i n one d i r e c t i o n and t h e d i s c r e t i z a t i o n o f t h e boundary used i n two d i m e n s i o n a l m o d e l l i n g .
80
FIGURE 24. O b l i q u e view o f t h e MINTAB seam geometry and the s t r e s s a p p l i e d l o c a l l y on each element i n t h e r e e f .
83
FIGURE 25. A t y p i c a l BEAP geometry showing t h e boundary of t h e e x c a v a t i o n s d e f i n e d by two d i m e n s i o n a l q u a d r a t i c , non-conforming elements i n a t h r e e d i m e n s i o n a l s t r e s s field.
85
FIGURE 26. T h i s f i g u r e d e f i n e s t h e dimensions f o r stopes and p i l l a r s , and t h e o r i e n t a t i o n f o r t h e i n s i t u s t r e s s regime f o r t h i s t h e s i s .
87
FIGURE 27a. A r i b p i l l a r i n a h o r i z o n t a l seam loaded by the weight o f the overburden.
88
FIGURE 27b. The d i r e c t i o n o f l o a d i n g on a p i l l a r i n a v e r t i c a l orebody.
88
FIGURE 28. The mid-height p l a n e and c e n t e r l i n e f o r t a l l open stope geometries.
90
FIGURE 29. The shaded p l a n e has t h e g r e a t e s t i n f l u e n c e on the mid-height a s t r e s s .
94
FIGURE 30. O v e r e s t i m a t i o n o f average p i l l a r l o a d by t h e 2D "BITEM" boundary element method f o r t h e 12 runs i n the f o u r t e s t s .
96
FIGURE 31. The dimensions and geometry comparison t e s t s .
98
v
o f t h e MINTAB/BEAP
FIGURE 32. The d i f f e r e n c e between t h e average p i l l a r 101 s t r e s s p r e d i c t e d by MINTAB and t h e average p i l l a r s t r e s s p r e d i c t e d by BEAP f o r t h e comparison t e s t s . FIGURE 33. O v e r e s t i m a t i o n o f average p i l l a r l o a d by t h e 2D "BITEM" boundary element method f o r t h e comparison t e s t s and 3 case h i s t o r i e s .
109
FIGURE 34. The d i f f e r e n c e between t h e average p i l l a r 112 s t r e s s p r e d i c t e d by MINTAB and t h e average p i l l a r s t r e s s p r e d i c t e d by BEAP f o r t h e comparison t e s t s and 13 case histories. FIGURE 35. The p i l l a r s t a b i l i t y graph showing t h e open stope r i b p i l l a r data base.
123
FIGURE 36. The p i l l a r s t a b i l i t y graph showing t h e s t a b l e and f a i l e d zones and t h e t r a n s i t i o n a r e a .
125
FIGURE 37. The p i l l a r s t a b i l i t y graph w i t h t h e p i l l a r l o a d reduced f o r a l l t h e data p o i n t s by t h e maximum amount l i s t e d i n T a b l e 8.
127
FIGURE 38. The p i l l a r s t a b i l i t y .graph w i t h a l l t h e case h i s t o r i e s o f t h e 13 y i e l d i n g p i l l a r s j o i n e d by s o l i d lines.
129
FIGURE 39. The p i l l a r s t a b i l i t y graph showing t h e d a t a from room and p i l l a r mining p u b l i s h e d by Hedley and Grant (1972) i n t h e i r study on t h e development o f a p i l l a r s t r e n g t h formula.
133
FIGURE 40. A p l a n view o f room and p i l l a r mining a t BCL L i m i t e d , showing t h e use o f l o n g p i l l a r s and square pillars.
137
FIGURE 41. The p i l l a r s t a b i l i t y graph showing t h e l o n g p i l l a r data p r e s e n t e d by Von Kimmelmann e t a l . (1984).
138
FIGURE 42. The square p i l l a r data p r e s e n t e d by Von Kimmelmann e t a l . (1984) i s p l o t t e d on t h e s t a b i l i t y graph u s i n g an e f f e c t i v e width i n t h e H/W r a t i o .
140
FIGURE 43. The f i v e stages o f t h e S86 p i l l a r i n an open stope p i l l a r t e s t a t Mt. I s a ( a f t e r Brady 1977).
142
FIGURE 44. The t h i r d , fourth, and f i f t h s t a g e s o f t h e S86 open stope r i b p i l l a r , p r e s e n t e d by Brady (1977), a r e shown on t h e p i l l a r s t a b i l i t y graph.
144
FIGURE 45. The p i l l a r s t a b i l i t y graph showing t h e open stope r i b p i l l a r data and t h e l i t e r a t u r e d a t a .
145
FIGURE 46. The range o f r i b p i l l a r dimensions seen i n 17 Canadian open stope mines. FIGURE 47. Comparison o f t h e p i l l a r s t a b i l i t y Hedley's formula f o r two s a f e t y f a c t o r s .
148
graph and
150
FIGURE 48. Three o f the Hoek and Brown (1980) p i l l a r s t r e n g t h curves p l o t t e d on t h e p i l l a r s t a b i l i t y graph.
153
FIGURE 49. Comparison between t h e p i l l a r s t a b i l i t y graph and t h e Obert and Duval1 (1967) shape e f f e c t formula a p p l i e d w i t h a s a f e t y f a c t o r o f 1.0.
155
xii FIGURE 5 0 . The s h a p e e f f e c t f o r m u l a p r o p o s e d b y B i e n i a w s k i (1983) a p p l i e d w i t h t h r e e d i f f e r e n t s a f e t y f a c t o r s i s compared a g a i n s t t h e p i l l a r s t a b i l i t y g r a p h .
157
FIGURE 5 1 . The r a n g e o f t e m p o r a r y r i b p i l l a r u s e d i n 14 C a n a d i a n o p e n s t o p e m i n e s .
164
FIGURE 5 2 . Isometric view of transverse stoping at N o r i t a .
dimensions
b l a s t h o l e open
168
FIGURE 5 3 . A l o n g i t u d i n a l s e c t i o n o f t h e b l a s t h o l e open s t o p i n g b l o c k at N o r i t a showing the p i l l a r case h i s t o r i e s ( 1 0 - 6 , 1 0 - 7 , and 10-8) u s e d i n t h i s c a s e history analysis.
171
FIGURE 5 4 . The p i l l a r s t a b i l i t y g r a p h s h o w i n g t h e l o c a t i o n o f t h e s t a b l e and f a i l e d t r a n s v e r s e p i l l a r case h i s t o r i e s at N o r i t a .
172
ACKNOWLEDGEMENT The author wishes t o acknowledge Noranda Research, F a l c o n b r i d g e L i m i t e d , t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g Research C o u n c i l and t h e Cy and Emerald Keyes s c h o l a r s h i p fund f o r f i n a n c i a l support d u r i n g t h e p r o j e c t . Thanks a r e extended t o t h e employees o f t h e mines and groups which p r o v i d e d time and i n f o r m a t i o n t o t h e study: - Algoma S t e e l Corp. L i m i t e d - G.W. Macleod Mine - B a r r i c k Resources - Camflo Mine - BP Canada Inc. - Mines S e l b a i e - Cambior - Niobec Mine - C o r p o r a t i o n o f F a l c o n b r i d g e Copper - Corbet Mine, Lac S h o r t t Mine - Dome Mines L i m i t e d - F a l c o n b r i d g e L i m i t e d - E a s t Mine, F r a s e r Mine, Kidd Creek, Lockerby Mine, Mining Technology D i v i s i o n , Onaping Mine, S t r a t h c o n a Mine - Hudson Bay Mining and Smelting - C e n t e n n i a l Mine, C h i s e l Lake Mine, F l i n F l o n Mine, Spruce P o i n t Mine - Inco L i m i t e d - L i t t l e S t o b i e Mine, Mine Research D i v i s i o n , S t o b i e Mine, Thompson D i v i s i o n - Kiena Gold Mines - Noranda M i n e r a l s I n c . - Brunswick M i n i n g and Smelting, Chadbourne Mine, Geco Mine, Golden G i a n t Mine, Lyon Lake Mine, M a t t a b i Mine, Mattagami Lake Mine, Mines Gaspe, Mining Technology D i v i s i o n , N o r i t a Mine - Pamour Porcupine Mines L i m i t e d - Ross Mine, No. 1 Mine - S h e r r i t t Gordon - Ruttan Mine - Westmin Resources L i m i t e d . A l s o , thanks t o Dr. H.D.S. M i l l e r f o r h i s e f f o r t s i n s e t t i n g up the I n t e g r a t e d Mine Design P r o j e c t . his the for
S i n c e r e g r a t i t u d e i s expressed t o P r o f e s s o r A l a n Reed f o r comments and h e l p i n w r i t i n g t h e t h e s i s and t h e members o f Department o f Mining and M i n e r a l Process E n g i n e e r i n g a t UBC h e l p and support d u r i n g t h e p r o j e c t .
S p e c i a l thanks t o my p a r t n e r Mr. Yves P o t v i n . His technical c o n t r i b u t i o n s and a d v i c e have had an immeasurable i n f l u e n c e on t h i s t h e s i s and my understanding o f mining and rock mechanics. F i n a l l y , and most o f a l l , I wish t o express my thanks t o Harry and N e l l i e Hudyma f o r t h e i r continuous encouragement and support d u r i n g a l l my endeavors.
1 CHAPTER 1 INTRODUCTION
Open
stope
1930's.
mining
The d e s i g n
has been p r a c t i c e d o f open
determining the l a r g e s t pillars.
Systematic
separating
stable
methods
"rib"pillars
Canadian
open
Sciences
and
Research
stope
"Integrated University
mining
Design
of B r i t i s h
H.D.S. M i l l e r .
t o design
i s centered
open
not been
conditions. Research
Falconbridge
Mine
mines
since the
stopes and t h e optimum
have
Engineering
and
stope
i n Canada
Columbia
confirmed
a
under
size for and t h e i r
i n typical
In 1986, t h e N a t u r a l
Council
Limited
Project",
stopes
around
(NSERC),
agreed
to
research
Noranda
sponsor
project
the
a t the
t h e s u p e r v i s i o n o f Dr.
The g o a l o f t h e study was t o i n v e s t i g a t e
open
stope mine d e s i g n methods by c o n f i r m i n g t h e v a l i d i t y o f e x i s t i n g stope
and
rib pillar
e m p i r i c a l methods.
design
methods
or
by
developing
new
T h i s t h e s i s i s a c o m p i l a t i o n and a n a l y s i s o f
the i n f o r m a t i o n and data c o l l e c t e d f o r t h e d e s i g n o f r i b p i l l a r s i n open stope mining. The
first
contents
describing
1.1
of
of the t h e s i s .
introduce
pillars
section
t h e problem open
stope
this
chapter
The
remainder
o f d e s i g n i n g open mining,
i n open stope mining.
Contents o f t h e T h e s i s
is a
o f the
of the chapter stope
and d i s c u s s i n g f
summary
r i b pillars the r o l e
will by
of r i b
2 This role
study
of
rib
begins
pillars
characteristics the
factors
Chapter
3
that
collected
element open
a
rib
base p i l l a r s i s
the
In
2,
the
discussed
and
s t a b i l i t y are
the
empirical
rib pillars.
Integrated
Mine
and
Design
methods
to
determine
pillars.
The
load
estimated
in this
the
average
induced
on
is
boundary
stress
of
the
in data
the
S t a b i l i t y G r a p h " , based on g r a p h i c a l a n a l y s i s o f t h e
rib
of
a new e m p i r i c a l
and d a t a
from
section.
all
pillar
Project
Chapter 5 d i s c u s s e s the use o f
pillar
literature.
It
Chapter
7 briefly
discusses
A summary a n d c o n c l u s i o n o f t h e t h e s i s
Open S t o p e Open
a l s o compares t h e
the
mining
is
a
general
v a r i e d m i n i n g method. up
features. applicability, from
new
f o r open s t o p e
rib
application of
the
rib
pillars.
i s found i n Chapter
8.
Mining
stope
make
largely
numerical
The r i b
s t a b i l i t y g r a p h f o r t h e d e s i g n o f open s t o p e
highly
identified.
d e s i g n method c a l l e d
pillars.
that
the
Chapter
f a i l u r e are
method w i t h e x i s t i n g e m p i r i c a l d e s i g n methods
1.2
and
the
data
pillar
mining
C h a p t e r 6 shows
development
pillar
of
stope
mining.
pillar
review
i n Chapter 4.
stope
stope
f o r open s t o p e
in
numerical
"Pillar
open
influence r i b p i l l a r
contains
presented
in
d e s c r i b i n g open
of progressive
d e s i g n methods u s e d data
by
the The and
method,
name
There are
and
following description
many
variations
an u n p u b l i s h e d paper
open
to
describe
many i m p o r t a n t
discussion of
used
of stope
on o p e n s t o p e
on the
a
features
each
of
the
definition,
mining mining
is
taken
methods,
3
w r i t t e n a t U.B.C. (Hudyma 1988a).
1.2.1
D e f i n i t i o n o f Open S t o p i n g
Three
characteristics,
common t o a l l open s t o p i n g methods,
make i t d i s t i n c t from o t h e r mining methods. i ) Open
stoping
is a
non
e n t r y mining
method.
Once
stope
p r o d u c t i o n has s t a r t e d , a l l a c t i v i t i e s r e q u i r i n g miners done from the p e r i p h e r y of the stope.
The
are
open stope does
not need t o be entered and a t no time are miners exposed t o the p r o d u c t i o n f a c e , ii)
It
is
generally
(although
a
naturally
some a r t i f i c i a l
supported
support
mining
i s occasionally
N a t u r a l l y supported means t h a t displacement and of
method used).
deformation
the rock mass i s l i m i t e d t o e l a s t i c o r d e r s of magnitude.
The
underground
stable
and
methods) . unstable
structures
created
self-supporting Mining
r e l e a s e of
is
done
energy
(in in
a
due
are
designed
opposition manner
t o mining
to
to
to
be
caving
ensure
does not
that occur
(from Brady 1981). iii)
Stopes
are
opened
stabilizing f i l l
These
three
to
to
and
enter
full
dimensions
before
a
i s introduced.
characteristics
a l l o t h e r underground methods. pillar
their
distinguish Cut and f i l l ,
open
stoping
from
l o n g w a l l , room and
shrinkage are a l l e n t r y methods t h a t r e q u i r e workers the
production
f a c e of the
stope.
Block
caving
and
4
sublevel
caving
induce
large,
unstable
movements
of
rock
and
i n c l u d e the c o n t i n u a l d i s s i p a t i o n of energy
as mining
proceeds,
so
supported
methods.
they
can
Methods such to
not
be
considered
as AVOCA, which i n t r o d u c e s f i l l
prevent stope i n s t a b i l i t y ,
the
stope
naturally
full
of
broken
extraction
or shrinkage s t o p i n g , which keeps
ore,
are
because the stope i s never f u l l y
1.2.2
during
excluded
from
open s t o p i n g
open.
A p p l i c a b i l i t y of Open S t o p i n g
There
are
some orebody
and
a p p l i c a t i o n o f open s t o p i n g .
geological
limitations
to
the
M o d i f i c a t i o n s o f open s t o p i n g can
be made t o mine a wide v a r i e t y of o r e b o d i e s , but some c o n d i t i o n s present d i f f i c u l t Open
stoping
dipping.
Stopes
angle of repose gravity be
30°)
about
i s best
suited
to
orebodies
but
that
are
steep
i n the orebody must d i p s u f f i c i e n t l y above the of the broken
ore
(above
50°
t o 55°)
flow of the ore t o the stope bottom.
successful
than
problems.
i n shallow d i p p i n g o r e b o d i e s the
orebody
must
be
15 metres i n t r u e t h i c k n e s s ) .
t o permit
Open s t o p i n g can
(approximately
quite thick
less
( g r e a t e r than
I f an orebody i s not steep
d i p p i n g o r t h i c k and f l a t , open s t o p i n g can not be
used.
For mining a steep d i p p i n g orebody, the orebody o u t l i n e must be f a i r l y 5
r e g u l a r and the orebody needs t o be g r e a t e r than
metres
in
delineate
and
width. mine.
w a l l r o c k d i l u t i o n due
Irregular
orebodies
G e n e r a l l y , a t widths to d r i l l
are
less
about
difficult than
5
to
metres,
h o l e d e v i a t i o n and b l a s t damage
5
becomes t o o g r e a t t o use open s t o p i n g e f f e c t i v e l y . The
r o c k mass s t r e n g t h of the
country
rock
i s very
the rock, the
important
orebody
and
the
i n open s t o p i n g .
l a r g e r the stopes can be made, and
surrounding The
stronger
consequently,
the more p r o d u c t i v e the method w i l l be.
At the l e a s t ,
fair
mass s t r e n g t h i s needed
w a l l rock t o
guarantee
i n the ore and
rock
t h a t t h e open stopes w i l l be n a t u r a l l y s u p p o r t i n g . A
final
restriction
reasonably l a r g e . (because
open
advantage justify
of
on
open s t o p i n g i s the orebody must
T h i s i s necessary t o get a few working
stoping the
is
large
often
scale
a of
cyclical the
method),
mining
faces
to
method,
be
take
and
to
the c o s t of the development a s s o c i a t e d w i t h open stope
mining.
1.2.3
D e s c r i p t i o n of T y p i c a l Open Stope M i n i n g Methods
Open s t o p i n g methods are so dependent on the orebody shape, size
and
Most
open
basic
orientation
that
stope mining
stages:
s t o p i n g has
a large
two
activities
pre-mining
development u s u a l l y - sublevel
no
(figures
be
exactly
the
same.
generalized into
and
production.
development.
two Open
Typical
includes: such
horizon
1 and
are
amount of pre-mining
accesses
drilling
can
development
as ramps, man-way r a i s e s
note A), and s u b l e v e l d r i f t s - a
mines
which
2, note C)
(figure
1,
( f i g u r e s 1 and 2, note B), includes
and d r i l l
D) or o v e r c u t s ( f i g u r e 2, note E ) ,
stope drives
access
drifts
( f i g u r e 1,
note
3
LEGEND A - MAN WAY-RAISE - SUBLEVEL DRIFT STOPE ACCESS DRIFT C D - DRILL DRIFTS
B
F H I L
-
FOOTWALL HAULAGE DRIFT DRAWPOINT COLLECTION CONE RING DRILL PATTERN
FIGURE 1. The elements o f an I d e a l i z e d l o n g i t u d i n a l l o n g h o l e open s t o p i n g method showing t h e b l a s t i n g , mucking and b a c k f i l l i n g o p e r a t i o n s ( a f t e r Hudyma 1988a).
LEGEND B C E F
-
SUBLEVEL DRIFT STOPE ACCESS DRIFT FULL STOPE OVERCUT FOOTWALL HAULAGE DRIFT
G H J K
-
FULL STOPE UNDERCUT DRAWPOINT SLOT RAISE PARALLEL DRILL HOLES
FIGURE 2. The elements o f an i d e a l i z e d t r a n s v e r s e b l a s t h o l e open s t o p i n g method showing t h e d r i l l i n g , b l a s t i n g , mucking and b a c k f i l l i n g o p e r a t i o n s ( a f t e r Hudyma 1988a).
8 - a mucking h o r i z o n , which may i n c l u d e : - a f o o t w a l l haulage d r i f t
( f i g u r e s 1 and 2, note F ) ,
- stope access undercuts ( f i g u r e 2, note G) o r drawpoints
( f i g u r e s 1 and 2, note H),
- stope undercut scrams, V - c u t s o r c o l l e c t i o n
cones
( f i g u r e 1, note I ) , - t h e opening
of a s l o t
raise
( f i g u r e 2, note J) by s t a g i n g ,
drop r a i s i n g , Alimak r a i s e c l i m b e r o r by r a i s e b o r e r . P r o d u c t i o n mining
involves:
- using p a r a l l e l to
d r i l l holes t o slash ore into the s l o t
form an expansion s l o t which
the
raise
i s opened t h e f u l l width o f
stope,
- drilling ring
production holes i n p a r a l l e l
patterns
(figure
1, note
b l a s t o r e i n t o t h e expansion Generally,
t h e expansion s l o t
and
ore i s slashed
the
production face.
the
orebody,
The h o l e s a r e used t o
slot.
i s opened a t one end o f t h e stope
into the s l o t This
L) .
( f i g u r e 2, note K) o r
causing a gradual r e t r e a t of
retreat
may be l o n g i t u d i n a l
(along
as i n f i g u r e 1) o r t r a n s v e r s e (across t h e orebody,
as i n f i g u r e 2 ) . As a stope i s b l a s t e d , o r e i s removed from t h e bottom stope. trackless system. or
The
o r e i s almost
load-haul-dump There
always
equipment,
removed
with
and taken
of the
t h e use o f
t o an
orepass
a r e a few mines u s i n g s l u s h e r / s c r a p e r equipment
c o n t i n u o u s mining
equipment t o move t h e muck t o an orepass,
but t h e s e o p e r a t i o n s a r e q u i t e r a r e .
The o r e pass system moves
9 the muck t o a c e n t r a l c o l l e c t i o n p o i n t f o r t r a n s p o r t out of the mine.
When the stope
i s completely b l a s t e d ,
w i t h waste r o c k o r c l a s s i f i e d of
pillars
filling
1.3
left
mill
between stopes
i t may
be
t a i l i n g s t o permit
(both f i g u r e s
1 and
filled
recovery
2 show the
of s t o p e s ) .
R o l e o f R i b P i l l a r s i n Open Stope M i n i n g The
entire full
most
economic
orebody
i n one
lens
mining
open
stope
longitudinal
creates
major
backfill
l i k e l y be needed.
stope
mining
is
to
stope
the
instability, will
method stope.
I f the use
potential
support
provide
i n v o l v e s mining
such
for as
of
serious
rib
to
a
this stope
pillars
The r o l e of r i b p i l l a r s
stability
the
mining
and
i n open
block
by
l i m i t i n g r o c k mass displacements and r e s t r i c t i n g the exposure
of
the r o c k mass i n the stope back and w a l l s . In had
the p a s t ,
t o be
left
improvements
i f full
to maintain
i n mining
the sequencing
l e n s mining was overall
technology
mine s t a b i l i t y . have caused
of e x t r a c t i o n so t h a t p i l l a r s
even i n v e r y l a r g e o r e b o d i e s .
pillars
to
separate
stopes
v a r i e d i n s i z e from about 2000 m factors
pillars
Recently,
a t r e n d towards
are never c r e a t e d ,
However, o f the 34 Canadian
stope mines i n v e s t i g a t e d i n t h i s study rib
not p o s s i b l e ,
(from 1986-1988), 27
i n the orebody. 3
These
up t o 150,000 m , 3
open used
pillars
depending
on
such as: the orebody geometry, the type of open s t o p i n g
method, and
the mining
sequence.
The dimensions
i n the data base are g i v e n i n Chapter 4.1
o f the
pillars
(Table 5, page 70).
10 It role.
is
important
that
rib pillars
Mines u s i n g r i b p i l l a r s
may
d e s i g n can s e r i o u s l y
- l o s s of p i l l a r - the
need
consequences of poor A
cause:
sloughing,
pillar
recovery,
access,
for
remedial
r e h a b i l i t a t i o n or a r t i f i c i a l - low
The
i t s intended r o l e may
- e x c e s s i v e stope or p i l l a r and expensive
designed
a f f e c t the r e c o v e r y of t h i s ore.
p i l l a r t h a t does not perform
- difficult
their
l e a v e as much as h a l f of the
orebody r e s e r v e s i n temporary p i l l a r s . pillar
perform
productivity,
- or t h e l o s s of ore r e s e r v e s .
measures support,
such
as
development
11 CHAPTER 2 RIB PILLAR FAILURE
The f i r s t pillar
step
stability
i n quantifying the v a r i a b l e s that i s t o describe
pillar
failure.
stope r i b p i l l a r f a i l u r e has n o t been deeply the
principles
rock
masses
objective istics and
of f a i l u r e
i n intact
are applicable
of t h i s
chapter
While
researched, rock,
stope
soft
open
some of rock and
rib pillars.
The
i s to b r i e f l y discuss the character-
of p i l l a r i n s t a b i l i t y
documentation o f f a i l u r e
these
t o open
hard
influence
and compare i n open stope
them
to
observations
rib pillars.
Using
i d e a s about p i l l a r f a i l u r e , t h e f a c t o r s t h a t i n f l u e n c e the
s t a b i l i t y o f open stope p i l l a r s w i l l be
identified.
2.1 F a i l u r e Mechanisms and C h a r a c t e r i s t i c s Rib
pillar
failure
progressive
(stable)
Progressive
failure
mass
i n a slow,
violent rock.
can be
failure
broken
and b u r s t i n g
r e f e r s t o gradual
non-violent
into
manner.
two
basic
(unstable)
modes: failure.
d e t e r i o r a t i o n of a Bursting
failure
r e l e a s e o f energy c a u s i n g t h e instantaneous
rock
i s the
fracture of
Although t h e c o n d i t i o n s a s s o c i a t e d w i t h each may be very
different,
both modes o f f a i l u r e c r e a t e s e r i o u s d i f f i c u l t i e s f o r
mining. T h i s t h e s i s w i l l d e s c r i b e and q u a n t i f y p r o g r e s s i v e
failure.
P r o g r e s s i v e f a i l u r e i s r e l a t e d t o t h e i n s i t u rock p r o p e r t i e s o f the
p i l l a r and mine,
and t h e s t a t i c
underground
stress
field.
12 Both of these f a c t o r s are q u a n t i f i a b l e w i t h r e a s o n a b l e accuracy. Bursting
failure
i s also
related
to
in situ
rock
properties.
However, i t i s a l s o dependent upon f a c t o r s such as l o c a l concentration, changes
the
i n the
investigate
and
the
unstable
dynamic
these
technology reason,
energy
released
stress
factors
budget
thesis
as
they
not
the
I t i s not
are
for
attempt
to
not
this to
mining
and
intended
to
quantifiable study.
describe
with
For or
this
quantify
failure.
Although
rib pillar
failure
uncommon, i t i s r a r e l y w e l l of
field.
available
will
due
stress
documentation
pillars
is
i s that
difficult
in
i n open
stope
documented.
visual open
A reason
observation stope
mining
mining
and
i s not
f o r the
lack
monitoring
and
there
is
of no
t universal
method t o d e s c r i b e
rib p i l l a r failure. documentation considered
Another p o t e n t i a l reason f o r the absence of
i s that
an
methods
using
pillar
failure
often
serious
Consequently, not
be
the
immediate
mining
are
enough
failure
backfill. does to
of
problem,
not
until
rib pillars
especially In
cause
warrant
the o p e r a t i o n a l
experienced
the c h a r a c t e r i s t i c s and e f f e c t s of
the
with
primary
operational
changing
i s often
the
mining
rib
problems
that
mining
starts.
stope
mining,
e f f e c t s of r i b p i l l a r
pillar
open
not
sequence.
failure This
may
failure
o f t e n r e s u l t s i n low p r o d u c t i v i t y , waste d i l u t i o n , h i g h e r mining c o s t s and p o s s i b l y l o s t ore. Several
signs
indicating p i l l a r
stope r i b s have been i d e n t i f i e d .
stability
problems i n open
These s i g n s of p i l l a r d i s t r e s s
13 are: - c r a c k i n g and s p a l l i n g o f rock i n r i b p i l l a r development and
raises,
- a u d i b l e n o i s e heard i n t h e p i l l a r s
or microseismic
events
l o c a t e d w i t h m o n i t o r i n g systems, - deformed o r plugged d r i l l
holes causing d r i l l
rods t o be
s t u c k and c a u s i n g problems i n l o a d i n g h o l e s , - overdraw from primary stopes w i t h t h e " f r e e " muck b e i n g u n b l a s t e d , o v e r s i z e m a t e r i a l from p i l l a r w a l l s , - s t r e s s r e d i s t r i b u t i o n from r i b p i l l a r s pillars
a f f e c t i n g nearby
and hanging w a l l and f o o t w a l l d r i f t s
- h o u r g l a s s i n g and c r a c k i n g o f p i l l a r s
seen
and r a i s e s ,
from
development, - major displacements
and changes i n s t r e s s shown by
instrumented m o n i t o r i n g systems such as s t r e s s meters and No
single
sign
sloughmeters.
necessarily
denotes
pillar
s i g n s a r e commonly r e p o r t e d d u r i n g p i l l a r Progressive p i l l a r may be minor a t f i r s t , and
deterioration
existing
structural
structurally influence
controlled
of geological
predominant. related
can
extensometers,
failure
failure,
failure.
i s a gradual process.
but g e t worse w i t h time. occur
but these
through
intact
discontinuities. failures
occur
Problems
Pillar rock
and
Although
in pillars,
f r a c t u r e s appears
along purely
the o v e r a l l
s t r u c t u r e i n open stope p i l l a r s
Stress, p i l l a r
damage
i s not
l o a d i n g and development o f s t r e s s
t o be predominant.
Consequently,
the
14 d i s c u s s i o n of r i b p i l l a r pillar
loading,
and
the
failure will
focus
on r o c k f r a c t u r i n g ,
subsequent l o s s of p i l l a r
load
bearing
ability.
2.1.1
Rock F r a c t u r i n g Rock f r a c t u r i n g i s a primary i n d i c a t o r of p i l l a r
i s the u l t i m a t e pillar
reason f o r the l o s s of l o a d b e a r i n g
disintegration.
fracturing
as
"...
the
rock m a t e r i a l .
new
surfaces."
the
Brady formation
I t involves
the
As
pillar
and
(1985)
define
separation
of bonds t o
central
parts
of
and
Soder
p i l l a r mines.
walls
the
pillar
and
the
size
and and
increases.
(1987)
The
in
form
f r a c t u r e s propagate
defined
6
f a i l u r e based on v i s u a l o b s e r v a t i o n
i n room and
ability
t o the l a c k of confinement of
f a i l u r e progresses,
i n t e n s i t y of e x i s t i n g f r a c t u r e s Krauland
breaking
and
F r a c t u r i n g g e n e r a l l y s t a r t s a t the p i l l a r
p i l l a r material. in
Brown
of p l a n e s of
the
where the r o c k mass i s weakest due
develop
and
failure
stages
to
classify
of p i l l a r f r a c t u r i n g
stages d e f i n e d
are:
"0) No f r a c t u r e s . 1) S l i g h t s p a l l i n g of p i l l a r c o r n e r s and p i l l a r w a l l s , w i t h s h o r t f r a c t u r e lengths i n r e l a t i o n t o p i l l a r h e i g h t , s u b p a r a l l e l to p i l l a r walls. 2) One or a few f r a c t u r e s near s u r f a c e , d i s t i n c t s p a l l i n g . 3) F r a c t u r e s appear a l s o i n c e n t r a l p a r t s o f the p i l l a r . 4) One or a few f r a c t u r e s occur through c e n t r a l p a r t s of the p i l l a r , d i v i d i n g i t i n t o two or s e v e r a l p a r t s , w i t h rock f a l l s from the p i l l a r . F r a c t u r e s may be p a r a l l e l t o p i l l a r w a l l s or d i a g o n a l , i n d i c a t i n g emergence of an hour-glass-shaped p i l l a r . 5) D i s i n t e g r a t i o n of the p i l l a r . Major b l o c k s f a l l out and/or the p i l l a r i s cut o f f by w e l l d e f i n e d f r a c t u r e s . A l t e r n a t i v e l y , a w e l l developed h o u r - g l a s s shape may emerge, w i t h c e n t r a l p a r t s completely crushed."
15 Krauland
and Soder a l s o
pillar
failure
was
inhomogeneities,
the
remained best
noted highly
basic
that
although
variable pattern
due
of
f a i l u r e mechanism. approach
to
and
definition
to
failure
constant f o r progressive f a i l u r e .
documentation
t h e appearance o f
o f an
geological propagation
T h i s i s perhaps the actual
mine
pillar
Use o f t h e Krauland and Soder o b s e r v a t i o n a l
classify
open
stope
pillars
p o s s i b l e due t o t h e l a c k o f v i s u a l a c c e s s .
i s not g e n e r a l l y However, t h e mode o f
f a i l u r e d e s c r i b e d above i s s i m i l a r t o t h a t seen by t h e author i n several mines
open stope mines and i s documented i n a few open
(Falmagne 1986; Bray
was a v a i l a b l e .
1967) where s u f f i c i e n t v i s u a l
stope access
The o n l y o b s e r v a t i o n o f Krauland and Soder t h a t
t h i s author has not seen i n open stope mining i s t h e d i v i s i o n o f pillars
into
distinct
r e g i o n s due t o f r a c t u r i n g .
the mechanism i s not l i k e l y potential
f o r a fracture
This part of
t o occur i n open stope p i l l a r s .
t o completely
sever a p i l l a r
The
i s much
lower i n open stope mining than i n room and p i l l a r mining due t o the l a r g e r s c a l e o f open stope p i l l a r s .
F r a c t u r e s would have t o
be v e r y continuous, f l a t and p l a n a r t o t r a n s e c t and d i v i d e open stope
pillars.
From p e r s o n a l o b s e r v a t i o n and l i t e r a t u r e d e s c r i p t i o n s , of
t h e most
common types
of fracturing
found
i n mine
some
pillars
are: i) the 1987)
surface fracturing
first
location
and
often a
and s p a l l i n g
(figure
o f f r a c t u r e development r e s u l t of
lack of
3a) i s u s u a l l y
(Krauland and Soder
p i l l a r wall
confinement
original pillar surface
FIGURE 3a. P a r a l l e l f r a c t u r i n g and s p a l l i n g due t o a l a c k o f confinement a t t h e p i l l a r w a l l s ( a f t e r Brady and Brown 1985). -soft partings
- i n t e r n a l splitting
FIGURE 3b. I n t e r n a l s p l i t t i n g and a x i a l c r a c k i n g o f a p i l l a r due t o deformable p i l l a r l a y e r s o r the p r o p a g a t i o n o f p a r a l l e l w a l l f r a c t u r e s ( a f t e r Brady and Brown 1 9 8 5 ) .
FIGURE 3c. Diagonal c r u s h i n g f r a c t u r e s may occur i n c o n f i n e d o r massive p i l l a r s ( a f t e r Brady and Brown 1985)
17 ( F a i r h u r s t and Cook 1966). ii)
internal axial
highly wall
deformable
rock
cracking
layers
(Brady
( f i g u r e 3b) may be caused by
between
and Brown
the p i l l a r
1985) o r may
and t h e
adjacent
be p a r a l l e l
surface
f r a c t u r e s t h a t propagate o r develop i n t h e c e n t r e (Agapito
1974).
iii)
diagonal
in confined
2.1.2
of the p i l l a r
crushing
f r a c t u r e s ( f i g u r e 3c) a r e o f t e n found
o r massive p i l l a r s
(Coates 1981).
P i l l a r Load-deformation Curve P i l l a r l o a d i n g can be h y p o t h e t i c a l l y d e s c r i b e d u s i n g a l o a d -
deformation is p
( s t r e s s - s t r a i n ) curve
loaded,
max'
t
n
i t compresses a c c o r d i n g maximum
e
Beyond t h i s
point,
pillar
point
"...
of f a i l u r e
load.
capacity
bearing
This
At a load
i s reached.
constant
will
Bieniawski
will
be taken as
(1987)
states,
i s a s t a t e a t which t h e r o c k specimen
changes from a g r a d u a l l y
to a
OA.
capacity
peak l o a d
in a pillar.
the ultimate strength
or t h e p i l l a r
load
t o the l i n e
As a p i l l a r
p o s t - f a i l u r e deformation o f t h e p i l l a r
occur but a t a reduced the
(see f i g u r e 4) .
or gradually
increasing decreasing
load-bearing load-bearing
capacity." Determining t h e a c t u a l load-deformation c h a r a c t e r i s t i c s o f a hard
r o c k mine p i l l a r
rock l a b o r a t o r y small
i n situ
Bieniawski
and
i s not p o s s i b l e .
specimens a r e e a s i l y coal
pillars
Curves f o r s m a l l
determined
have been
Van Heerden 1975), but
developed
hard
and curves f o r (Wagner 1974;
i t i s not e x p e r i m e n t a l l y
18
FIGURE 4 . A h y p o t h e t i c a l load-deformation curve can be used to d e s c r i b e the s t r e s s - s t r a i n c h a r a c t e r i s t i c s o f a p i l l a r . The p i l l a r e x h i b i t s l i n e a r e l a s t i c deformation (along l i n e OA) u n t i l the maximum l o a d i s reached ( P )• Pillar deformation c o n t i n u e s (along l i n e AB), but w i t h a d e c r e a s i n g l o a d b e a r i n g c a p a c i t y ( a f t e r S t a r f i e l d and F a i r h u r s t 1968). m a x
19 practical
t o conduct load-deformation t e s t s on
jointed
rock
deformation concept, and
(Brady
curve
i t is a
1977).
of
a hard
pillar does
rock
load bearing not
Agapito
(1974),
the
load-
a theoretical pillar
failure
capacity.
Capacity
capacity.
study
main reason f o r l o s s of
However, the
signify
in his
as
describe
f r a c t u r i n g i s the
necessarily
leaves
rock mine p i l l a r
load bearing
Loss o f Load Bearing Ultimately,
this
convenient method t o
the l o s s o f p i l l a r
2.1.3
While
l a r g e samples of
that of
the
onset of f r a c t u r i n g pillar
o i l shale
has
pillars,
failed.
found
that
f r a c t u r i n g s t a r t e d as minor s p a l l i n g i n the p i l l a r p e r i m e t e r occurred
a t s t r e s s l e v e l s w e l l below the u l t i m a t e
of a p i l l a r . outer
He
shell
of
a l s o noted t h a t as the
concentrations
built
monitored
in
the
pillar, up
situ
in
stress
capacity
f r a c t u r i n g occurred
monitoring the
load
pillar
showed core.
in
that
more
the
stress
Wagner
distribution in
and
(1974)
than
30
underground c o a l p i l l a r s u s i n g a s e r i e s of h y d r a u l i c j a c k s .
He
found
of
the
that
at
pillar
central
carried
core
the
load
the
pillar
of the
bearing and
by the p i l l a r After surface
several
stages
of
relatively pillar
capacity
compression, little
stress
( f i g u r e 5) . of
a pillar
the
He
perimeter
compared
to
noted t h a t most
i s found
i n the
i s l a r g e l y dependent on the confinement
core
the of of
provided
shell.
failure
of
fracturing),
the
pillar
(due
to
serious
internal
Wagner (1974) found t h a t a c o n f i n e d
and
pillar
Pillar compression (mm)
2
FIGURE 5 . Wagner (1974) d i d a s e r i e s of i n s i t u l o a d deformation t e s t s on coal p i l l a r s using hydraulic jacks. For t h i s case, 2 5 jacks were put i n a 5X5 pattern i n a square p i l l a r . The graph on the top shows the l o a d deformation c h a r a c t e r i s t i c s o f the p i l l a r i n general. The oblique diagrams give the r e l a t i v e load on each o f the 25 jacks a t four stages of p i l l a r compression. The diagrams show that with i n c r e a s i n g compression and i n c r e a s i n g average p i l l a r s t r e s s , the core of the p i l l a r c a r r i e s an i n c r e a s i n g percentage of the load, while the unconfined periphery of the p i l l a r c a r r i e s l e s s load. Diagram four shows that the p i l l a r core c a r r i e s a s i g n i f i c a n t load despite the f a c t that the p i l l a r i s l o s i n g i t s o v e r a l l load bearing capacity (redrawn from Wagner 1974).
core
had
Soder post
a
considerable
(1987) wrote failure
slenderness also
range of of
the
supported
demonstrated
is
bearing
pillar
the
that
loading
and
the
laboratory
if
the
depends l a r g e l y upon
the
presence of
t e s t i n g of
confining
capacity
i n open
will
peel
fill.
rock
and
This
is
specimens
in
Fairhurst
pressure
on
a
capacity
on
the p o s t
detached
failure
prevent
from
backfill,
pillar
starts,
of
walls
the
the can
fractured
confinement
the
leaving
p o s s i b l e to provide has
considerable core had
fractured
pillar.
installation
and
of
the
seen
open
wall
be
very
wall
pillar
These
of
material
methods
artificial
stopes
full
of
broken
some confinement t o the
several
load bearing
examples o f capacity.
failed
by b a c k f i l l
from the p i l l a r
walls.
before
i t had
large.
core,
and
There
are
becoming
the
such ore
core.
material
from
include
support
pillars
pillar
use
as as
long
rib pillars
In t h e s e cases,
the o p p o r t u n i t y
with
the
of
cable
p i l l a r walls.
remained c o n f i n e d because the f r a c t u r e d p i l l a r
confined
is
failure
i n open stope r i b
confinement
stope mining
o f f , preventing
to
(1968)
sample
r e s u l t i n g i n complete p i l l a r d i s i n t e g r a t i o n .
methods
and
in
Starfield
dependent
progressive
finally
load
Krauland
c a p a c i t y i s g r e a t l y enhanced (see f i g u r e 6).
Once
was
of
machines.
highly
However,
author
loss
l o s s of l o a d b e a r i n g
also
bolts,
capacity.
the peak l o a d c a p a c i t y i n c r e a s e s and
load bearing The
bearing
pillars
by
"stiff-testing"
increased,
that
load
as The a
pillar
material t o slough
22
FIGURE 6. The s t r e s s - s t r a i n c u r v e s f o r l a b o r a t o r y specimens l o a d e d under i n c r e a s i n g c o n f i n i n g pressures show an i n c r e a s e i n peak l o a d and an i n c r e a s e i n t h e post-peak l o a d bearing c a p a c i t y ( a f t e r S t a r f i e l d and F a i r h u r s t 1968) .
23 2.2
S i g n i f i c a n t V a r i a b l e s i n Open Stope P i l l a r
Stability
Based on t h e f a i l u r e c h a r a c t e r i s t i c s d e s c r i b e d are
several variables that
rib pillars. potential
2.2.1
of p i l l a r s ,
Strength
fracturing playing
i s an important
most common index is
the
uniaxial
strength
standardized
diameter
conditions.
sample core).
found
i n a report
in pillar
compressive
(UCS) drill
i s the
core
strength.
The
o f d i f f e r e n t rock
test.
The
maximum
load
can s u s t a i n
under
uniaxial that
a
uniaxial
The UCS i s v a r i a b l e upon specimen s i z e , so
diameter
drill
factor
f o r comparing t h e s t r e n g t h
compressive
the
a large r o l e i n the s t a b i l i t y
the resistance of the p i l l a r material t o f r a c t u r i n g
crushing
loading
i n t h e d e s i g n of
influence.
I n t a c t Rock
types
there
T h i s s e c t i o n w i l l d e s c r i b e t h e v a r i a b l e s and t h e i r
With r o c k
and
c o u l d be important
above,
i s standardized
Further
information
t o about
54 mm
(NX s i z e
about t h e u n i a x i a l t e s t can be
by an I n t e r n a t i o n a l
Commission
on standard-
i z a t i o n o f l a b o r a t o r y t e s t s (ISRM Commission 1979).
2.2.2
Pillar Pillar
fracturing pillar
may
Load
load
i s a primary f a c t o r i n p i l l a r deformation,
and p i l l a r have
a
failure.
to
determine
stress
The d i s t r i b u t i o n o f s t r e s s i n a
significant
s t a b i l i t y of the p i l l a r .
rock
effect
on t h e performance and
However, t h e r e i s no c o n c l u s i v e method
in a pillar
and t h e r e
i s no s i n g l e
value
24 that
can
used
to
describe
state
of
stress
the
complete l o a d i n g
condition
of
a
pillar. The
the
stress
a p p l i e d t o the p i l l a r as w e l l as the l o c a t i o n i n s i d e the
pillar.
The
stress
field and
and
applied the
to
a
pillar
a p i l l a r varies
s i z e and
other p i l l a r s .
in
The
stress
p r o x i m i t y of e x c a v a t i o n s and points
stress
in
kept
a
in
pillar
on
upon
the
pre-mining
stress
l o c a t i o n of stopes, underground workings i n s i d e the
upon areas of weakness such as
these
varies
the
mind,
with
geological
i s dependent
discontinuities,
f r a c t u r i n g i n the p i l l a r .
determining
a
pillar
high
degree
the
the With
distribution
of
precision
to
find
of
is
not
possible. For
this
represent
the
thesis,
it
load
a
on
average
stress
height
centerline,
techniques.
The
normal s t r e s s e s first
found
necessary
pillar.
The
several
i s that
This
using
was
along
a
value
taken
the
numerical
i s frequently
to
as
pillar
t h i s l o c a t i o n has
i n the p i l l a r , and
area of f a i l u r e .
load
points
determined
reason
w i l l be d i s c u s s e d
2.2.3
at
was
the mid-
modelling the
highest
observed as
c h o i c e of s t r e s s a n a l y s i s
the
location
i n more d e t a i l i n Chapter 5.2.2.
P i l l a r Shape Chapter
stability
2.1.3
and
the
described load
the
bearing
role
of
confinement
capacity.
Pillar
huge i n f l u e n c e on confinement o f the p i l l a r c o r e . - the
load-convergence
c h a r a c t e r i s t i c s of
in
pillar
shape has
a
It affects:
pillars
at
failure
25 (Hudson e t a l . 1971; S t a r f i e l d and F a i r h u r s t 1968), - t h e p o s t - f a i l u r e deformation modulus o f p i l l a r s al.
(Hudson e t
1971; Wagner 1974),
- the s t r e s s d i s t r i b u t i o n i n a p i l l a r
( S t a r f i e l d and F a i r h u r s t
1968; Wagner 1974), - and
the
effect
of geological
p i l l a r s t i f f n e s s and f a i l u r e This
confirms
pillar
2.2.4
pillar
and
fracturing
on
significant variable
in
(Sarkka 1984).
shape
as a
stability.
Structural Discontinuities i n P i l l a r s
The
effect
upon whether as
that
structure
of g e o l o g i c a l
the structure
f a u l t s and
structure
involves
on r i b p i l l a r s
depends
major d i s c o n t i n u i t i e s such
shear zones o r minor d i s c o n t i n u i t i e s l i k e
joint
sets.
P i l l a r s i n t e r s e c t e d by a major s t r u c t u r e must be analyzed
based
on
strength
the
o f the major
stability. structure
However,
structure i n open
The
will
stoping,
orientation
play
and
a dominant
shear
role in
i n t e r s e c t i o n o f a major
i s not a common problem and d e s i g n o f such p i l l a r s i s
an e x c e p t i o n rib
specific situation.
rather
pillars
are
than a r e g u l a r located
to
occurrence.
avoid
When p o s s i b l e ,
intersection
by
major
geological discontinuities. Less
prominent d i s c o n t i n u i t i e s such as
fracturing, The
a r e a much more common problem
influence
o f minor
upon t h e o r i e n t a t i o n ,
j o i n t i n g and
local
in pillar
design.
d i s c o n t i n u i t i e s on r i b p i l l a r s
depends
continuity,
frequency and shear
strength
of
the
structures.
At
the
minor d i s c o n t i n u i t i e s on triaxial
state
joints.
of
Archibald
stability
i s small
e f f e c t of the
confinement prevents rock movement a l o n g
the
Geological
pillar
the
because
d i s c o n t i n u i t i e s have
e f f e c t on i n s t a b i l i t y and
p i l l a r c e n t r a l core,
i n unconfined r e g i o n s
(1981),
Page
and
a
more s i g n i f i c a n t
of p i l l a r s .
Brennan
(1981),
Allcott and
Von
Kimmelmann (1984) mention s t r u c t u r a l l y c o n t r o l l e d wedge f a i l u r e s from
pillar
walls.
confinement influence
the
of
stability analysis
of
One
expect
to
find
rock near p i l l a r w a l l s .
structure
analyses. is
would
An
described
by
is
best
Potvin
method
et
al.
or
no
Consequently,
accounted
excellent
little
for
for
using
wall
(1988a).
the wall
stability The
method
q u a n t i f i e s the i n f l u e n c e of g e o l o g i c a l s t r u c t u r e , mining induced stress,
and
surface
of
pillar
dimensions t o
open stope. the
predict
When the
the
of
analysis predicts a
e f f e c t o f minor s t r u c t u r e
r i b p i l l a r s w i l l be
stability
on
the
each
stable
stability
of
separated
by
small.
E f f e c t o f P i l l a r Volume Pillars
n a t u r a l and pillar
the
are
made
of
blocks
of
intact
mining induced d i s c o n t i n u i t i e s .
volume
variables: and
an
wall,
unfractured
2.2.5
stope
the
on
stability
is
volume e f f e c t on
influence
of
the
number
really the of
rock So the
a
strength
influence
function of
of
of two
i n t a c t rock,
s t r u c t u r a l defects
in
the
pillar. L a b o r a t o r y compressive t e s t i n g o f s m a l l samples has
shown an
i n f l u e n c e o f specimen s i z e on t h e compressive s t r e n g t h o f i n t a c t rock
(see f i g u r e
7) .
However,
testing
of
large
intact
rock
specimens has found t h a t above a " c r i t i c a l " volume, t h e s t r e n g t h does not decrease s i g n i f i c a n t l y asymptotic Herget
(see f i g u r e 8 ) .
T h i s concept of
specimen s t r e n g t h i s r e p o r t e d by B i e n i a w s k i
e t a l . (1984),
and P r a t t
e t a l . (1972).
(1975) ,
These authors
found t h e c r i t i c a l volume t o be l e s s than one c u b i c metre. the volume larger
o f b l o c k s i n open stope p i l l a r s
than
this
critical
volume,
there
With
u s u a l l y b e i n g much is a
very
limited
i n f l u e n c e o f the volume e f f e c t o f i n t a c t rock. The depend
number upon
of s t r u c t u r a l
the volume
discontinuities
o f the p i l l a r .
in a pillar
Hoek and Brown
will (1980)
suggest t h a t t h i s i n f l u e n c e can be q u a n t i f i e d through t h e use o f rock
mass
classification
Stephansson
(1985),
correction pillar
methods.
and
factors
to
other
account
authors
with
open
stope
and
Both
Agapito
have
for pillar
strength determination.
investigated
Hardy
of
rib pillar
(1977),
suggested
volume these case
be
that
used
in
ideas w i l l
be
histories
in
Chapter 6.1.3.
2.2.6 E f f e c t o f B a c k f i l l The mining
use
fill
methods.
(Campbell use
of
1987)
cemented
purpose o f
A
survey
found t h a t
fill
fill
i s very
is
to
important
by
the O n t a r i o M i n i s t r y
almost
stope
o f Labour
a l l O n t a r i o open stope mines
aid in pillar
used t o
i n c u r r e n t open
provide
recovery. overall
mine
The
general
stability,
Sp*>ol Book
T—t*d by
Harbta Umastona Granlta Basalt Basalt-andaslta Cabbro ftarbla Norita Cranlta Quartz d l o r l t a
O O V A
( e/«cS0) . o
H09I'" KolfMn " Bureharti at a l ' " Kolfa»n'" Nalakldla " llnlckaya " llnickaya " Blantawtkl'* Hosklns t H o r l n o P r a t t at a l ' * ' 1
lava
1
1
1
7
1 7 0
(50/d) -" 0
0.3
150
Specifwn
dlamtar
200
250
d
FIGURE 7. There i s a v e r y l a r g e i n f l u e n c e of specimen s i z e on t h e s t r e n g t h o f i n t a c t rock, f o r small specimen diameters ( a f t e r Hoek and Brown 1980). 150 100 70 50
Ix , • Iron ore
r
*
Johns (1966)
Oiorite
° Prott ^ ft ft W (D 25 H•0 H H> >-
20* H
ffi
Q hf UJ fl> I3 3 O S 8 Q a ft lit C C ^ tr cr O © ft (D a. O a t» (D UJ n < O ft» © (D zLd H n ft 3 0£ (J 3 * p. 0)
B
t-
-30*
CO
ft 0)
n
SEAM THICKNESS RATIO (LENGTH:BREADTH) • 3D TESTS
102 - using
the
average
of
several
elements
to
determine
average p i l l a r s t r e s s has the e f f e c t o f "smoothing
the
out" l a r g e
d i f f e r e n c e s a t i n d i v i d u a l elements i n the p i l l a r , - o r the open tests
are
modelling
stope r i b p i l l a r
much
simpler
than
the
and
geometries more
excavation
v e r i f i c a t i o n s by Crouch and
a n a l y z e d i n the
amenable
to
geometries
DD
numerical
analyzed
in
o f the comparison
r a t i o of three w i l l BEAP
for
influence
open
suggest
that
using a
the
seam t h i c k n e s s
g i v e v e r y good agreement between MINTAB and
stope
o f the
the
Brady.
While complex mining geometries have not been i n v e s t i g a t e d , results
12
rib
pillars.
Further
seam t h i c k n e s s r a t i o
will
be
checks
of
the
done i n Chapter
5.5 u s i n g case h i s t o r i e s from the d a t a base.
5.5 P i l l a r Load C a l c u l a t i o n s f o r t h e Open Stope Data Base There stress
i s no
or
Chapter
load
3,
elastic
can determine the
As
numerical
deteriorating
For
pillars
condition,
m o d e l l i n g may
load be
average
d i s c u s s e d above, m o d e l l i n g can
approximations o f the p r e - f a i l u r e
pillars.
numerical
i n a mine p i l l a r .
linear
consistent mine
a b s o l u t e method t h a t
that
give
load
i n hard
rock
a
sloughing
by
linear
a considerable overestimate.
the
r o c k f r a c t u r i n g and p i l l a r deformation. l i n e a r e l a s t i c load w i l l
conditions.
A failed
For f a i l e d
or
elastic
can be a t t r i b u t e d t o the l o c a l l o s s o f l o a d b e a r i n g c a p a c i t y to
in
often
have
determined
and
This due
pillars,
not be r e p r e s e n t a t i v e o f the s t r e s s
pillar will
have l o s t some, o r n e a r l y a l l
103 of i t s l o a d b e a r i n g into
nearby
linear
competent p i l l a r s
elastic
sloughing
capacity, r e s u l t i n g i n stress r e d i s t r i b u t i o n
modelling
o r abutments.
The i n a b i l i t y o f
t o determine an approximate
and e s p e c i a l l y f a i l e d p i l l a r s p r e s e n t s
developing
load f o r
difficulties in
a r e l i a b l e method o f p r e d i c t i n g p i l l a r
failure.
5.5.1 Assumptions In o r d e r conditions pillars
t o s e t a c o n s i s t e n t method f o r d e t e r m i n i n g
for a l l pillar are
infinitely
characteristics. load
bearing
assumption geometrical
elastic
i t will in
regardless
will
permit
conditions
will
of t h e i r
t e c h n i c a l l y accurate
deformation
not l o o s e
physical
their
condition.
t o t h e a c t u a l problem, t h i s
the i n v e s t i g a t i o n that
be assumed t h a t
their
T h i s means t h a t p i l l a r s
capacity
While n o t b e i n g
assessments,
loading
existed
of
before
the stress failure
and
and
a
rudimentary look a t t h e c o n d i t i o n s t h a t have r e s u l t e d i n f a i l u r e of for
open
stope p i l l a r s .
predicting
Ultimately,
conditions
that
i twill
provide
are
associated
and MINTAB
t o model
the basis
with
pillar
failure.
5.5.2 P i l l a r Load The
ability
Results of
BITEM
geometry i n t h e data base was e v a l u a t e d . adequately
account
f o r the excavations
conditions of the p i l l a r , situation
occurred
each
problem
I f a program c o u l d not affecting
the stress
numerical a n a l y s i s was n o t done.
f o r BITEM
when
t h e geometries
This
o f a l l the
104 significant the
excavations
problem.
pillar
c o u l d n o t be i n c l u d e d i n t h e plane o f
MINTAB was n o t used
geometry when en-echelon
to investigate
a stope and
stopes were p a r t o f t h e problem
geometry, o r t h e orebody had s i g n i f i c a n t changes i n t h i c k n e s s o r significant
changes i n d i r e c t i o n .
F o r each case h i s t o r y ,
Table
8 shows: - t h e pre-mining - the
limiting
applicability (the
s t r e s s normal t o t h e orebody, geometrical o f MINTAB
ratios
associated
with
the
(the seam t h i c k n e s s r a t i o ) and BITEM
stope h e i g h t t o l e n g t h r a t i o ) ,
- t h e average
s t r e s s p r e d i c t e d f o r t h e p i l l a r by each numerical
method and t h e b e s t estimate o f t h e average - the estimated
error
associated with
pillar
the best
stress,
load
due t o
assumptions a s s o c i a t e d w i t h m o d e l l i n g t h r e e d i m e n s i o n a l and
pillar
geometries
with
numerical
methods
that
stope
a r e not
three dimensional, - t h e average theory
pillar
load calculated
i n the t r i b u t a r y
n u m e r i c a l l y determined
based
area
(chapter 3.1.2.1),
- and t h e e r r o r
The
using the t r i b u t a r y
best
estimate
on t h e l i m i t i n g
area
load
compared t o t h e
load.
o f t h e average ratios
pillar
l o a d was
f o r BITEM and MINTAB.
chosen
I f a case
h i s t o r y had a h i g h stope l e n g t h t o stope width r a t i o ,
t h e BITEM
load
thickness
was used.
I f a case
history
r a t i o , t h e MINTAB l o a d was used.
had a h i g h
seam
I f t h e stope geometry d i d not
105
•
•"
PUBMINING STRESS (MPa)
PILLAR NUMBER
2 3 7 8 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61
.
! !
39 39 46 46 14 14 16 40 40 17 17 17 17 17 17 12 12 12 12 12 12 15 15 15 15 15 15 55 55 55 55 23 23 23 23 15 15 15 23 18 18 30 30 30 30 30 35
BITEM HEIGHT: LOAD LENGTH (MPa) RATIO 1.4 1.4 4.5 4.5 2.6 1.8 4.0 2.0 1.7 2.9 4.0 3.5 1.4 3.5 0.9 3.0 3.0 NA 1.8 NA 0.9 5.0 5.0 6.3 2.5 1.5 2.5 5.0 5.0 5.0 5.0 2.1 2.1 1.5 1.5 NA NA NA NA 5.6 3.4 1.1 5.8 4.4 0.8 0.6 5.0
,
,
!
!
!
51 64 55 69 28 29 29 90 91 43 28 29 38 33 57 29 44 NA 26 NA 60 26 38 31 31 38 40 99 75 76 102 30 32 41 49 NA NA NA NA 43 44 59 38 40 72 82 70
MINTAB SEAM ! THICK, j LOAD RATIO | (MPa) 0.3 ; 0.3 j 0.6 | 0.6 | 1.5 | 1.7 ! 0.9 ! 3.0 | 3.3 ! 0.8 ! 0.7 j 0.7 j
1
0.7 | 0.8 ! 1.8 J 3.8 ! 1.1 ! 0.8 ; 1.7 | 3.8 ! 1.0 | 1.1 ! 1.2 !
i.o !
1.3 0.7 0.5 0.5 0.7 0.6 NA NA NA NA 3-3 7.0 4.6 0.7 3.0 3.0 0.4 0.2 0.2 0.2
! ', | j [ ; ; ! ; ; j ! ! ! ! i | ] ! !
N0.7 A
;
1
47 55 60 83 24 24 24 66 63 41 28 26 31 27 30 24 33 28 21 31 37 28 38 30 30 32 35 78 60 59 83 NA NA NA NA 31 39 48 36 46 46 48 46 45 54 53 NA
ESTIMATED AVERAGE PILLAR LOAD j Z ERROR (MPa) 51 ! 25-45Z 64 | 25-45Z 1.0 but
