Design chart for "Inclined Pillars", Category 4. 50. 18. Heath Steele ...... were also chosen to take advantage of Allcot and Archibald (1981)1 1 pillar design study.
INVESTIGATION OF UNDERGROUND MINE PILLAR DESIGN PROCEDURES By YVES POTVIN B.Sc,
LAVAL U n i v e r s i t y , QUEBEC 1981
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF A«ii«MASTER OF^SCIENCE J
in THE FACULTY OF GRADUATE STUDIES Mining
and M i n e r a l Process
We accept
Engineering
t h i s t h e s i s as conforming
tq the required
standard
THE UNIVERSITY OF BRITISH COLUMBIA March
©
1985
Yves P o t v i n ,
1985
In p r e s e n t i n g
t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the
requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t t h e L i b r a r y s h a l l make it
f r e e l y a v a i l a b l e f o r reference
and study.
I further
agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by t h e head o f my department o r by h i s o r her r e p r e s e n t a t i v e s .
It i s
understood t h a t copying o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l gain
s h a l l n o t be allowed without my w r i t t e n
permission.
Department o f M i n i n g and M i n e r a l P r o c e s s
The U n i v e r s i t y o f B r i t i s h 1956 Main Mall Vancouver, Canada V6T 1Y3
DE-6 (3/81)
Columbia
Engineering
ABSTRACT
The p r i n c i p a l f u n c t i o n s o f underground mine p i l l a r s a r e to
s t a b i l i z e openings and t o c a r r y t h e l o a d o f o v e r l y i n g r o c k
s t r a t a . They a r e o f t e n ( p a r t i a l l y o r c o m p l e t e l y ) r e c o v e r e d a t a l a t e r s t a g e 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 l o n g e r r e q u i r e d . For economic r e a s o n s , an optimum-sized p i l l a r i s t h e s m a l l e s t one s a t i s f y i n g s a f e t y r e q u i r e m e n t s . Thus t h e p i l l a r d e s i g n problem c o n s i s t s o f d e t e r m i n i n g the p i l l a r ' s
minimum dimensions
a s t h e l o a d approaches t h e u l t i -
mate p i l l a r s t r e n g t h . Because t h e p i l l a r ' s
s t r e n g t h and t h e l o a d a c t i n g upon
i t a r e b o t h f u n c t i o n s o f 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 o f p i l l a r
dimensions
i s a complex t a s k . Furthermore,
t h e m u l t i p l i c i t y o f p i l l a r shapes, s i z e s ,
r o c k m a t e r i a l and f u n c t i o n s add t o t h e d e s i g n e r s ' problem. Consequently,
p i l l a r d e s i g n programs a r e s t i l l g e n e r a l l y
performed a s a t r i a l - a n d - e r r o r p r o c e s s . In
o r d e r , t o improve t h e p r e s e n t p i l l a r d e s i g n 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 s t a n d a r d i z e the design (2)
procedure
The p r i n c i p a l d e s i g n methods, d i v i d e d i n t o f o u r groups, a r e summarized and t h e i r
is
i s defined
(3)
A f i v e - p h a s e d e s i g n procedure is
(4)
with design charts
developed
The d e s i g n p r o c e d u r e case
applicability
histories
i s a p p l i e d i n a n a l y s i n g two
TABLE OF CONTENTS
CHAPTER 1. CHAPTER 2.
Introduction 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 o f P i l l a r s
2.1
Pillar
2.2
Category 1:
2.3
Classification
2.2.1
Description
2.2.2
Definitions
Category 2:
2.4
Description
2.3.2
Definitions
Description
2.4.2
Definitions
Category 4:
2.6
Discussion
3.1
"Stub
2.4.1
2.5
Pillars"
"Separation P i l l a r s "
2.3.1
Category 3:
CHAPTER 3.
"Plate
Pillars"
"Inclined
Pillars"
Review o f P i l l a r Design Methods Introduction
3.2
Group 1.
Experience Methods
3.3
Group 2.
Empirical
3.4
3.5
Methods
3.3.1
Empirical
strength
3.3.2
Empirical
s t r e s s formulas
3.3.3
Empirical
dimensioning formulas
Group 3.
Theoretical
formulas
Methods
3.4.1
Theoretical strength
3.4.2
T h e o r e t i c a l s t r e s s formulas
Group 4.
Computer Methods
formulas
Page
CHAPTER 4.
Pillar
Design
Procedure
4.1
Philosophy of P i l l a r Design
42
4.2
Design Procedure
42
4.3 CHAPTER 5. 5.1
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
4.2.3
Phase 3.
Empirical
4.2.4
Phase 4.
Theoretical
4.2.5
Phase 5.
Computer Design
Analysis
Design Design
46
Design Charts Heath S t e e l e Case H i s t o r y A n a l y s i s
53
Geology 5.1.1
Regional
Geology
5.1.2
Mine's
5.1.3
Structural
5.1.4
Jointing
Geology Geology
5.2
Mining Method and Underground S t r u c t u r e s Dimension
57
5.3
Rock Mechanics
59
Data
5.3.1
Rock S t r e n g t h
Parameters
5.3.2
Laboratory T e s t i n g
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 Characteristics
63
5.5
Mining Sequence
63
5.6
F a i l u r e H i s t o r y and P i l l a r
Geometry
64
Page 5.7
5.8
68
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
5.7.3
Phase 3.
E m p i r i c a l Methods
Analysis
5.7.3.1
E s t i m a t i o n of P i l l a r Load by the T r i b u t a r y Area Formula
5.7.3.2
Estimation of P i l l a r
5.7.4
T h e o r e t i c a l Methods
5.7.5
Computer Methods
Strength
90
D i s c u s s i o n o f the R e s u l t s
CHAPTER 6. 6.1
Geco Case H i s t o r y A n a l y s i s 98
Geology 6.1.1
Regional Geology
6.1.2
Mine Geology
6.1.3
Structural
Geology
6.2
Mining Method and Underground
6.3
Rock Mechanics Data
S t r u c t u r e Dimension
102 102
6.5.1
Rock S t r e n g t h Parameters
6.3.2
L a b o r a t o r y Test
6.3.3
Rock Mass C l a s s i f i c a t i o n
6.3.4
Virgin
Stress
6.4
P i l l a r Characteristics
108
6.5
Mining Sequence
108
6.6
F a i l u r e H i s t o r i e s and P i l l a r Geometries
113
6.7
Pillar
Design Study
120
6.7.1
Phase 1 .
Experience Design
6.7.2
Phase 2.
Pillar
Structural
Analysis
VI fa.ge
6.7.3
6.8
Phase 3.
E m p i r i c a l Methods
6.7.3.1
E s t i m a t i o n o f P i l l a r Load by the E x t r a c t i o n Ratio Formula
6.7.3.2
Estimation of P i l l a r Hoek's Method.
6.7.4
T h e o r e t i c a l Methods
6.7.5
Computer Methods
Strength;
Discussion of Results
CHAPTER 7.
^30
Summary and C o n c l u s i o n 135
7.1
Design Procedure
7.2
Case H i s t o r i e s
I36
7.3
Design Methods
137
APPENDIX A.
Review o f L i t e r a t u r e
APPENDIX B.
Determination o f the Geco s t r e s s regime a t 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
vii
LIST OF
Figure
FIGURES
Description
Page 9
1
P i l l a r category
1 "Plate P i l l a r s "
2
P i l l a r category
2 "Separation
3
P i l l a r category
3 "Stub P i l l a r s "
4
P i l l a r category
4 "Inclined P i l l a r s "
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 o f P i l l a r Width to Height Ratio on Average P i l l a r Strength
26
9
Average V e r t i c a l P i l l a r Layouts.
28
Pillars"
P i l l a r Stresses
10 13 '
in Typical
15
10
Determination of Load on Chain P i l l a r s by the " F i r s t P a n e l " Load Concept
29
11
Observed Value o f W f o r Coal Seams 7 f t . Thick and having a Crushing Strength i n the
32
3 - i n . Cube of 3,000 p s i
±10%
12
P i l l a r transected
by a s i n g l e plane o f weakness
13
I n t e r s e c t i n g planes
14
Design c h a r t f o r " P l a t e P i l l a r s " ,
15
Design c h a r t f o r "Separation Category 2
16
Design c h a r t f o r "Stub P i l l a r s " ,
17
Design c h a r t f o r " I n c l i n e d P i l l a r s " ,
18
Heath S t e e l e Geology
54
19
Plan View of Heath S t e e l e Orebody
56
20
V i r g i n S t r e s s at Heath S t e e l e
62
21
L o n g i t u d i n a l View o f the Heath S t e e l e
65
of weakness
44 44
Category 1
Pillars",
Category 3 Category 4
I n v e s t i g a t e d Area at
47 ^
^9 50
viii Figure
Description
Page
22
E x t r a c t i o n Flowchart o f 77-89, 77-91, 77-93 and 77-95 Stopes
66
23
Estimated failed.
Layout when 77-92 Rib P i l l a r
69
24
Estimated failed
Layout when 77-94 Rib P i l l a r
70
25
S
26
The E f f e c t of S F r a c t u r e s on 30 m. wide Rib P i l l a r s
(100 f t )
27
The E f f e c t o f S F r a c t u r e s on 60 m. wide Rib P i l l a r s
(200 f t )
28
S
29
Combined E f f e c t of S
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 E x t r a c t i o n Numbers f o r the 77-90 p i l l a r f a i l u r e geometry.
0
1
33
P i l l a r s E x t r a c t i o n Numbers f o r the 77-92 p i l l a r f a i l u r e geometry.
8
2
34
P i l l a r s E x t r a c t i o n Numbers f o r the 77-94 p i l l a r f a i l u r e geometry
35
The E f f e c t of the Width to Height average p i l l a r s t r e n g t h
36
Computer Output of the 77-90 p i l l a r f a i l u r e geometry. (Stress simulation)
87
37
Computer Output o f the 77-92 p i l l a r f a i l u r e geometry. (Stress simulation)
88
38
Computer Output of the 77-94 p i l l a r f a i l u r e geometry. (Stress simulation)
8
39
P i l l a r Deformation versus E x t r a c t i o n Number (N)
^3
40
S a f e t y F a c t o r versus
9^
41
S a f e t y F a c t o r versus E x t r a c t i o n Ratio
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 a l i z e d r e l a t i o n s h i p s of s u l p h i d e zones, Geco Mine
3
72
F r a c t u r e System 3
3
5
73
^
7
75
F r a c t u r e System 3
and
S
5
76
Fractures
Ratio on
E x t r a c t i o n Number (N) (e)
^
8
84
?
96 99
ix
Figure
Description
Page 107
43
Assumed S t r e s s Regime at Geco
44
Longitudinal at Geco
45
E x t r a c t i o n Flowchart o f 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
47
Estimated Layout when 10-23 P i l l a r F a i l e d
H8
48
Estimated Layout when 10-20 P i l l a r F a i l e d
H9
49
P i l l a r s E x t r a c t i o n Numbers f o r the 10-21.5 p i l l a r f a i l u r e geometry
122
50
P i l l a r s E x t r a c t i o n Numbers f o r the 10-23 p i l l a r f a i l u r e geometry
123
51
P i l l a r s E x t r a c t i o n Numbers f o r the 10-20 p i l l a r f a i l u r e geometry
l
52
Computer Output o f the 10-21.5 P i l l a r F a i l u r e Geometry ( S t r e s s s i m u l a t i o n )
53
Computer Output o f the 10-23 P i l l a r Geometry ( S t r e s s s i m u l a t i o n )
Failure
54
Computer Output o f the 10-20 P i l l a r Geometry (Stress s i m u l a t i o n )
Failure
55
Safety
56
Comparison o f Heath S t e e l e , Geco Case h i s t o r i e s a n a l y s i s r e s u l t s
View o f the I n v e s t i g a t e d
Area
109
Failed
F a c t o r versus E x t r a c t i o n Ratio
H?
i
7
2
i
i
2 l +
2
2
8
9
133 138
X
LIST OF TABLES Table
Description
Page
1
Rock Mechanic S t u d i e s i n Noranda Underground Mines
3
2
P i l l a r and Opening D e s i g n i n g Methods used by Noranda Underground Mines
4
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 " S i z e E f f e c t Formula"
22
5
Constants a and b used i n the "Shape E f f e c t Formula"
23
6
Characteristic
38
7
Summary o f Computer Methods
39
8
Mining Sequence
of the Panel
64
9
Approximate Stope and P i l l a r Dimensions when 77-92 P i l l a r F a i l e d
6?
10
Approximate Stope and P i l l a r Dimension when 77-94 P i l l a r F a i l e d
68
11
Heath S t e e l e P i l l a r A n a l y s i s R e s u l t s
91
12
Summary o f the Geology at Geco
13
Stope 10-19.5 Mining Sequence
14
Stope 10-21 Mining Sequence
15
Stope 10-22 Mining Sequence
16
Stope 10-23.5 Mining Sequence
17
Approximate Stope and P i l l a r Dimensions when 10-21.5 P i l l a r F a i l e d
18
Approximate Stope and P i l l a r 10-23 P i l l a r F a i l e d
19
Approximate Stope and P i l l a r Dimensions when 10-20 P i l l a r F a i l e d
20
Geco P i l l a r A n a l y s i s R e s u l t s
Input Data f o r Computer Methods
-'-00 HO 1
1
0
HI 112
Dimensions when
'
H5 -^6
xi
ACKNOWLEDGEMENTS The author f i r s t Group, who
wishes to thank Noranda Research
Mining
Division
have made t h i s p r o j e c t f e a s i b l e by f i n a n c i n g the r e s e a r c h , pro-
v i d i n g an impressive amount of i n f o r m a t i o n and f o r t h e i r o u t s t a n d i n g cooperation.
The
involvement
of the f o l l o w i n g Noranda mines and
a s s i s t a n c e of t h e i r employees was
also appreciated:
Brunswick Mining and -
Heath S t e e l e Mine
-
Geco D i v i s i o n Goldstream
the
Smelting
Mine.
As w e l l , s p e c i a l acknowledgements to Dr. H.D.S. M i l l e r f o r h i s s u p e r v i s i o n of the r e s e a r c h p r o j e c t , the p e r t i n e n t advice given, and f o r h i s s t i m u l a t i n g approach towards rock mechanics. The author wishes to thank the f o l l o w i n g s c h o l a r s h i p funds f o r financial
support: -
Cy and
Emerald
Keyes
F r e d e r i c k Armand McDiarmid George E. In a d d i t i o n , thanks
Winkler to the members o f the Department of Mining
and
M i n e r a l Process E n g i n e e r i n g f o r t h e i r h e l p f u l a t t i t u d e , to W. M. Cumming f o r p r o o f r e a d i n g , and f i n a l l y t o Jacques and Jeannine tinuous encouragement and
support.
Potvin f o r their
con-
1
CHAPTER
1
Introduction
2
The p r i n c i p a l f u n c t i o n s o f underground mine p i l l a r s openings,
and t o c a r r y the l o a d o f o v e r l y i n g s t r a t a .
t i a l l y o r completely)
are t o s t a b i l i z e
They are o f t e n (par-
recovered a t a l a t e r stage when t h e i r
stabilizing
e f f e c t i s no l o n g e r necessary. For economic reasons, an optimum-sized p i l l a r i s the s m a l l e s t one t h a t s a t i s f i e s s a f e t y requirements. Thus, t h e p i l l a r design problem c o n s i s t s o f determining the p i l l a r ' s minimum dimension
as the l o a d reaches
Because t h e p i l l a r ' s
the u l t i m a t e p i l l a r s t r e n g t h .
s t r e n g t h , and l o a d a c t i n g upon i t are both
t i o n s o f many i n t e r r e l a t e d f a c t o r s , which vary as mining p r o g r e s s e s , dimensioning
is a difficult
Furthermore,
funcpillar
task.
the m u l t i p l i c i t y o f p i l l a r shapes, s i z e s , rock m a t e r i a l
and a p p l i c a t i o n s add t o the d e s i g n e r s ' c o n f u s i o n . 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 p r o j e c t was undertaken,
under the super-
v i s i o n o f Dr. H. D. S. M i l l e r , with the c o l l a b o r a t i o n and f i n a n c i a l o f Noranda Research, design procedure.
Mining D i v i s i o n , t o develop a comprehensive
support
pillar
The p r o j e c t ' s f i r s t y e a r was e n t i r e l y d e d i c a t e d t o a com-
p l e t e review o f the p i l l a r design methods a v a i l a b l e . T h i s i s reproduced
i n appendix A.
Another o f the p r o j e c t design procedures
tasks was to i n v e s t i g a t e the c u r r e n t p i l l a r
and the r o l e o f rock mechanics techniques i n mine p i l l a r
design. To achieve t h i s g o a l , a q u e s t i o n n a i r e was mailed to seven Noranda underground o p e r a t i o n s .
The i n f o r m a t i o n was completed
mines i n New Brunswick, Quebec, O n t a r i o and B r i t i s h
by v i s i t i n g
four
Columbia.
Table 1 shows t h a t a f a i r amount o f rock mechanic s t u d i e s had been com-
3
X
X
X
i n Noranda Underground
PARAMETER STRENGTH INVESTIGATIONS STRESS LABORATORY TEST
Matagami
Mattabi
u
X
X
X
X
Poisson's Ratio
?
X
X
X
X
In-Situ
X
Measurement
P h o t o - E l a s t i c Model Computer M o d e l l i n g
X
Compress. S t r e n g t h
o°
X
X
X
Tensile Strength
•-4 C O T r i a x i a l Strength
o ID
Shear S t r e n g t h
X
X
X
X
X
X
^
X
X
X
X
X
X
N G I
X
X
Criterion
X
X
X
X
X
X
C S I R
X
Laubscher
Extensom.
X
X
X
Compression Pad Closure S t a t i o n
X
X
X
L e v e l l i n g Survey S t a t i o n
X
Piezometer
X
X
Multi-Wire
X
Mapping
X
Structural
X
MONITORING
X
X
Boroscope Observ.
5.
X
E l a s t i c Mod
Failure R Q D ROCK MASS CLASSIFICATION
X
U n i t Weight
X
4.
3.
2.
1.
ROCK
Goldstream
Mines
Heath Steele
Brunswick (BM 5 S)
Rock Mechanic S t u d i e s
Geco
1
Mines Gaspe ,
T A B L E
X
X
pleted. to
However, i t must be emphasized t h a t these experiments
are
related
the o p e r a t i o n s ' s i z e and age, as w e l l as the s t a b i l i t y problems encoun-
tered. T a b l e 2 confirms that the mines r e l y mainly upon p r e v i o u s for
experience
design, l e a v i n g the more s o p h i s t i c a t e d methods to mining c o n s u l t a n t s .
•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
Empirical Methods Group 2
OO
in
•fi c c
rt
C
o
M
Analytical Methods Group 3
in
c
•H
t/1
u
rt
c a. o M
c •H
Computer Methods Group 4 in
u
TO
in
M C
c
ID
c
CL,
O
o
Goldstream
M
Mattabi
M
M
Matagarni
M
M
Mines Gaspe
M
M
C
c
Brunswick
M
M
C
c
Heath S t e e l e
M
M
Geco
M
M
Note:
M
M
-
The mine's s t a f f performed
C
-
C o n s u l t a n t performed
the d e s i g n .
the d e s i g n .
In o r d e r to improve the a c t u a l p i l l a r design p r a c t i c e s : a pillar classification design
system i s proposed
t o s t a n d a r d i z e the
procedure
the p r i n c i p a l design methods are summarized and t h e i r i s defined
applicability
a f i v e - p h a s e design procedure with design charts i s developed the 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 .
6
CHAPTER 2 The
C l a s s i f i c a t i o n and
Definition
of
Pillars
7 2.1
Pillar The
Classification
l i t e r a t u r e p r o v i d e s no standard d e f i n i t i o n f o r the term, "under-
ground p i l l a r . "
I f one
attempts to e l a b o r a t e a general d e f i n i t i o n , i t
should be borne i n mind t h a t the p i l l a r may be permanent o r temporary, but i n any notion o f s t a b i l i t y Regardless
and
o r may
not be m i n e r a l i z e d ,
may
event r e f e r e n c e must be made to the
security.
o f which mining method i s used, every mine must leave
l a r s to s t a b i l i z e underground s t r u c t u r e s . However, because o f the
pil-
vari-
able ground c o n d i t i o n s , s t r e s s , and the m u l t i p l e p i l l a r a p p l i c a t i o n s r e l a t e d to mining methods and orebody geometry, no two
pillars
are i d e n -
tical . In the documents reviewed, kinds o f p i l l a r s
more than twenty names d e s c r i b i n g v a r i o u s
were encountered.
T h i s wide v a r i e t y o f p i l l a r s
e l a b o r a t i o n o f a standard design procedure
a difficult
The p i l l a r shape, the l o a d a c t i n g on the p i l l a r , the p i l l a r m e t e r i a l are the three most important
makes the
task. and the s t r e n g t h o f
f a c t o r s to be
considered
when d e s i g n i n g a p i l l a r . A simple c l a s s i f i c a t i o n
( f o r p i l l a r design purposes) i s
regrouping under the same " c a t e g o r y " p i l l a r s submitted
to s i m i l a r l o a d i n g s i t u a t i o n s .
suggested,
of s i m i l a r shape which are
In t h i s manner, every p i l l a r i n
each category can be designed u s i n g i d e n t i c a l equations
and a given metho-
dology . Because the behaviour
o f hard rock d i f f e r s g r e a t l y from t h a t o f s o f t
rock, each category i s broken i n t o two l a r s , and Note:
(b) s o f t rock
sub-categories:
(a) hard rock
pil-
pillars.
The width, h e i g h t and l e n g t h o f the p i l l a r s
w i t h i n a category, but the general shape must be
may
similar.
vary g r e a t l y
8
2.2
Category 1. 2.2.1
"Plate
Pillars"
Description
Figure
1 shows that " P l a t e P i l l a r s " are submitted t o a b i a x i a l h o r i -
zontal stress f i e l d . However, t h i s i s not
The
top and
the bottom o f the p i l l a r s
to s u r f i c i a l
overburden.
of t h i s f a c t when dimensioning a s u r f a c e
T h i s i s due
which r a r e l y r e q u i r e p l a t e
2.2.2
The
designer
should be aware
pillar.
cases of s o f t rock " P l a t e P i l l a r s "
the l i t e r a t u r e .
(Category IB) were found i n
p r i n c i p a l l y to the s o f t rock mining methods
pillars.
Definitions Category 1A:
Hard Rock
- Crown P i l l a r s , Roof P i l l a r s , Horizontal P i l l a r s :
Level P i l l a r s ,
Strike
These are h o r i z o n t a l s l i c e s of v a r y i n g t h i c k n e s s , the excavated area
to p r o v i d e
t h e i r support f u n c t i o n i s no o f t e n used to d e f i n e the den
load
support. longer
shallowest
Pillars,
l e f t above
They are g e n e r a l l y recovered
required.
The
after
term "crown p i l l a r " i s
h o r i z o n t a l p i l l a r c a r r y i n g the
overbur-
(surface p i l l a r ) . - Sill Sill
Pillars:
pillars
s i t u a t e d underneath the
2.3
loaded.
t r u e i n the case of s u r f a c e p i l l a r s , which must bear
the v e r t i c a l load due
No
are not
Category 2. 2.3.1
are very
s i m i l a r to crown p i l l a r s but they are
stopes at each s u b l e v e l .
"Separation
Pillars"
Description
Separation h o r i z o n t a l load.
pillars
(Category 2) are subjected
to a v e r t i c a l
They are open on t h e i r l o n g i t u d i n a l s i d e
and
(Figure 2).
It
9
CATEGORY "plate
category CROWN ROOF
I
(hard rock)
pillar"
cotegory
PILLARS PILLARS
LEVEL
PILLARS
STRIKE
PILLARS
HORIZONTAL SILL
Q.
P.
PILLARS
SURFACE
FIGURE 1
PILLARS
Pillar
Category
1
" Plate
I
Pillars"
I b.
Isoft
rock)
10
CATEGORY 11
category 2 a
separation
(hard
rock
2
pillar category
RIB
PILLARS
BARRIER
DIP
PILLARS
ENTRY
TRANSVERSE ABUTMENT
FIGURE 2
PILLARS PILLARS
P i l l a r Category 2
"Separation P i l l a r s '
2b
PILLARS PILLARS
uoft
rock
should be noted t h a t the hard rock pillars
(Category 2A) and
(Category 2B) do not possess
s o f t rock p i l l a r s
s o f t rock s e p a r a t i o n
i d e n t i c a l c h a r a c t e r i s t i c shapes, s i n c e
are u s u a l l y lower and wider
(Figure 2).
In the case of a v e r y long s e p a r a t i o n p i l l a r dimensions) lem may
the h o r i z o n t a l s t r e s s may
be c o n s i d e r e d to be two
2.3.2
(compared to the other
have a n e g l i g i b l e e f f e c t and the
dimensional.
Definitions Category
2A:
Hard Rock
- Rib P i l l a r s ,
Dip P i l l a r s ,
Transverse
Pillars:
A r i b p i l l a r i s a s e p a r a t i n g w a l l between two the r i b i s u s u a l l y i n the orebody d i p d i r e c t i o n and pillars
t r a n s f e r the v e r t i c a l
They may
The
l e n g t h of
i s continuous.
The r i b
stabilizing
be recovered at a l a t e r
stage
mining. Category
2B:
- Barrier
S o f t Rock
Pillars:
Barrier p i l l a r s
are used t o i s o l a t e c o a l mine p a n e l s .
u s u a l l y permanent p i l l a r s which c o n t r o l r o o f s t a b i l i t y in
stopes.
load from the r o o f to the f l o o r ,
the rock o v e r l y i n g the stoped area. of
prob-
They are
and p l a y a major r o l e
ventilation. - Entry These p i l l a r s
Pillars: r e f e r to the l o n g w a l l mining method.
They p r o v i d e a
p r o t e c t i o n t o the panel e n t r i e s and are recovered d u r i n g the panel's e x p l o i t a t i o n stage.
final
12 2.4
Category 2.4.1 The
lar to
3. "Stub
Pillars"
Description shape o f "stub p i l l a r s "
(Figure 3).
They are open on the f o u r v e r t i c a l
a u n i a x i a l compressive 2.4.2
stress
- Centre These p i l l a r s
s i d e s and are s u b j e c t e d
field.
3A:
Hard Rock
Pillars: have the same f u n c t i o n as r i b p i l l a r s , but are
ated i n the middle of the stopes. c a r r y the r o o f l o a d .
the c e n t r e p i l l a r s
They reduce the span of openings
C o n t r a r y to the r i b p i l l a r s which are
are t r a n s e c t e d by c r o s s - c u t s or
- (Room and The
be square or rectangu-
Definitions Category
to
(Category 3) may
stub p i l l a r s
Pillar] may
Pillars,
Stub
and
help
continuous,
drifts.
Pillars:
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, h e i g h t , shape and composition vary a c c o r d i n g t o the s i t e ments.
situ-
They support the v e r t i c a l
and r e q u i r e -
l o a d o f o v e r l y i n g rock, and may
be perma-
nent or r e c o v e r a b l e . - Post P i l l a r s , These p i l l a r s
Yielding
Pillars:
r e f e r to the "post p i l l a r " mining method.
v i d e temporary support to the immediate r o o f . bottom up, the post p i l l a r s
s t a r t to y i e l d
the bottom, where they are c o n f i n e d by Category - Panel
3B:
As mining progresses from
the
and f i n a l l y c o l l a p s e " g e n t l y " at
backfill.
S o f t Rock
Pillars:
These temporary p i l l a r s panel.
They pro-
are u n i f o r m l y d i s t r i b u t e d w i t h i n a longwall
They support the panel's immediate r o o f and w i l l be removed at a
13
CATEGORY
3
"stub p i l l a r s "
H EIGHT
category CENTER
3a
PILLARS
(ROOM S PILLAR) POST
category
(hard roc k )
PANEL PILLARS
SPLIT
PILLARS
FIGURE 3
P i l l a r Category 3
PILLARS PILLARS
REMNANT CHAIN
"Stub
Pillars"
3 b
PILLARS
PILLARS
(soft
rock'
later
stage. - Split
Pillars:
During longwall split
are the r e s i d u a l p o r t i o n o f s p l i t p i l l a r s .
r e t r e a t s , they e i t h e r c o l l a p s e or are completely - Chain
continuous p i l l a r . pillars
may
This provides
"Inclined
i n s t e a d of a long, massive,
the highest
extraction ratio.
The
Pillars"
Description
Inclined p i l l a r s mitted
small p i l l a r s
but they are
be designed to y i e l d , p e r m i t t i n g the r o o f to deform.
Category 4. 2.5.1
As min-
recovered.
p l a y the same r o l e as b a r r i e r p i l l a r s ,
composed o f a s e r i e s o f a l i g n e d
do not have a p a r t i c u l a r shape or are not
to a p a r t i c u l a r l o a d i n g s i t u a t i o n .
i n t o the three preceding
a t i o n f o r design
sub-
However, because they do
c a t e g o r i e s , and
they T e q u i r e
because of t h e i r i n c l i n a t i o n ,
f o u r t h category o f the p i l l a r c l a s s i f i c a t i o n
2.6
two
Pillars:
These p i l l a r s
fit
are cut i n t o
Pillars:
Remnant p i l l a r s
2.5
the panel p i l l a r s
pillars. - Remnant
ing
p i l l a r recovery,
special
inclined pillars
not
consider-
form
the
(Figure 4 ) .
Discussion The
illustrated
author i s aware t h a t p i l l a r s
i n the forementioned c a t e g o r i e s
with i d e a l i z e d shapes, which i s not
ground p i l l a r s .
In a d d i t i o n , i t should
the case f o r r e a l under-
be r e a l i z e d that the
a p i l l a r i s a f u n c t i o n of s e v e r a l f a c t o r s : - Virgin stress - S t r e s s induced by mining
are
load a c t i n g on
15
CATEGORY inclined
4
pillar
11
I HEIGHT
category
FIGURE 4
4a
(hard
rock)
P i l l a r Category -
"Inclined
category
Pillars"
4b
(soft
rock)
- Geological - Pillar
features
shape and
- Openings and
orientation
general
mine s t r u c t u r e s
- Ground water. However, i t i s b e l i e v e d that every p i l l a r may above f o u r c a t e g o r i e s , l o a d i n g mechanism and
i n t o one
of
the
even though the c l a s s i f i c a t i o n o v e r s i m p l i f i e s the the p i l l a r geometry.
F i n a l l y , because s h a f t p i l l a r s other p i l l a r s ,
fall
they are not
included
are fundamentally d i f f e r e n t from
in this classification.
Nevertheless,
the f o l l o w i n g d e f i n i t i o n i s proposed: Shaft
Pillars:
These are permanent p i l l a r s system. pillars
The
s h a f t and
the
s h a f t p i l l a r may
become l a r g e r with i n c r e a s e d
Because the
shaft i s a v i t a l
are designed with a high
p r o v i d i n g p r o t e c t i o n to the mine s h a f t be v e r t i c a l or i n c l i n e d .
depth, and
t h e i r shapes are v a r i a b l e .
component i n underground mines, these
and
pillars
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 . ogy
Shaft
design
methodol-
dimensioning formulas a p p l i c a b l e to each category w i l l be
developed
i n the f o l l o w i n g
chapters.
Most o f the p r e v i o u s Mines A s s o c i a t e s "
The
p i l l a r d e f i n i t i o n s were taken from "Roche
(1984)^ as w e l l as F i g u r e s
5,
Appendix C, which i l l u s t r a t e the d i f f e r e n t kinds
6, and
7 reproduced i n
of p i l l a r .
TABLE 3 PILLAR CLASSIFICATION SUMMARY Category 1 Plate P i l l a r s A Hard Rock
B Soft Rock
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 Hard Rock
B Soft Rock
A Hard Rock
B S o f t Rock
A Hard Rock
B S o f t Rock
Crown
Rib
Barrier
Centre
Panel
Inclined
Inclined
Roof
Dip
Entry
Stub
Split
Level
Transverse
"Pillar"
Remnant
Strike
(R+P)
Abutment
Horizontal
Chain
Post
Sill Surface
/
4 ^
•
—-* t
1
/
+
/ 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 o r d e s i g n i n g any underground s t r u c t u r e i s strength stress Thus, a p i l l a r w i l l
long term load b e a r i n g pillar's
simple:
^
>
remain s t a b l e i f the load a p p l i e d i s l e s s than i t s
capability.
Difficulties
a r i s e i n estimating
the
u l t i m a t e s t r e n g t h as w e l l as the p r e c i s e load a c t i n g upon i t .
Pillar
strength:
Because of the rock m a t e r i a l ' s complexity t i o n o f rock mass s t r e n g t h i s p e r p l e x i n g .
and
variability,
the
evalua-
Furthermore, the t r u e s t r e n g t h
a p i l l a r can o n l y be c a l c u l a t e d a f t e r c o n s i d e r i n g the s t r e n g t h o f the m a t e r i a l together
p r o b a b i l i t y o f i n c l u d i n g a weakness zone
i n the - The
pillar
deformation and
pillar
triaxial
s t r e n g t h of
the
material
- The
geometry of the
- The
p i l l a r as p a r t o f the general
A l s o , environmental f a c t o r s may pillar
pillar
with:
- The
pillar rock
structure.
cause a time dependent a l t e r a t i o n of the
strength.
Pillar
load:
As mentioned i n Chapter 2, the load a c t i n g on a p i l l a r of: - The
v i r g i n stress
- The
s t r e s s induced
- Geological - Pillar
by mining
features
shape and
- Openings and - Ground water.
orientations
general mine s t r u c t u r e
of
i s a function
20 Hence, the s t r e s s l e v e l induced i n p i l l a r s
( p i l l a r l o a d ) , changes as mining
progresses. Although s e v e r a l techniques can be used t o measure i n s i t u
stress,
these a r e expensive, and the r e s u l t s a r e not always r e l i a b l e . Because t h e r e are so many f a c t o r s i n v o l v e d i n the complex mechanism o f p i l l a r l o a d i n g (and deformation) as w e l l as p i l l a r
s t r e n g t h , the d e s i g n e r
must depend upon numerous methods t o account f o r these
factors.
The f o l l o w i n g summarizes the most important d e s i g n i n g methods. are d i v i d e d
i n t o f o u r groups,
They
according to t h e i r l e v e l o f s o p h i s t i c a t i o n .
Group 1 - Experience Methods Group 2 - E m p i r i c a l Methods Group 3 - T h e o r e t i c a l 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 o f producing adequately s i z e d p i l l a r s with r e s p e c t t o s a f e t y .
3.2
Group 1.
Experience Methods
T h i s i s by f a r the most widely used and the l e a s t s o p h i s t i c a t e d method. Based on o b s e r v a t i o n s , h i s t o r y , and on the d e s i g n e r ' s " f e e l i n g " f o r the rock, i t a l s o r e l a t e s to s i m i l a r work completed situations.
A c o n s e r v a t i v e dimensioning
i n corresponding geological is first
may have t o be made a c c o r d i n g t o the requirements signed
l a i d out and m o d i f i c a t i o n s and performance
o f the de-
structure.
No s p e c i f i c
experience method i s proposed,
but i t i s s t r o n g l y
recommended
that d e t a i l e d a c t i v e f i l e s be kept on i n f o r m a t i o n concerning the mine ity:
failures,
s l a b b i n g , squeezing, c a v i n g , convergence,
et c e t e r a .
stabil-
This w i l l empirical
3.3
improve the f u t u r e experience
design and may lead to an
approach.
Group 2.
E m p i r i c a l Methods
An e m p i r i c a l method i s the q u a n t i f i c a t i o n o f experience formulas or curves.
into designing
Because most o f these methods do not take i n t o account
many important f a c t o r s , one should be aware o f the c o n d i t i o n s i n which they were developed. While the m a j o r i t y o f e m p i r i c a l p i l l a r design methods
considers
s t r e n g t h and s t r e s s s e p a r a t e l y , some do i n c o r p o r a t e s t r e n g t h and s t r e s s i n t o a dimensioning
formula.
The f o l l o w i n g i s a review o f the most important e m p i r i c a l methods. b r i e f d e s c r i p t i o n , the formula(s)
and the parameters are given.
( r e f e r r i n g 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
As w e l l
categories
which can be designed by each method a r e i n d i c a t e d .
3.3.1
E m p i r i c a l Strength
Formulas.
E m p i r i c a l p i l l a r s t r e n g t h formulas e s s e n t i a l l y i n v o l v e e x t r a p o l a t i n g the r e s u l t s o f l a b o r a t o r y t e s t s on rock
specimens, t o f u l l - s i z e mine
pillars. A)
S i z e E f f e c t Formula
a
p
= a
where:
(Appendix A.
S e c t i o n 3.1)
[A + B(£)]
c
Op
=
Pillar
o
=
U n i a x i a l compressive s t r e n g t h o f a cube o f p i l l a r m a t e r i a l
W
=
Pillar
width
h
=
Pillar
height
A, B
=
Constants given i n u n i t s o f p i l l a r s t r e n g t h (Table 4 ) .
c
strength (psi)
A
Description: Rocks have a n a t u r a l s t r e n g t h a n i s o t r o p y which i s predominantly due t the
presence o f 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 , b l a s t f r a c t u r e s , et
c e t e r a ) but can a l s o be a t t r i b u t e d to v a r i a t i o n s i n rock f a b r i c ( i . e . f o l i a t i o n , bedding p l a n e s , et c e t e r a ) and mineralogy.
As rock samples o f
constant shape i n c r e a s e i n s i z e , the s t r e n g t h o f the specimen decreases. T a b l e 4 g i v e s the c o n s t a n t s proposed by d i f f e r e n t authors t o model t h i s be haviour.
TABLE 4 CONSTANTS A AND B USED IN THE "SIZE EFFECT FORMULA"
SOURCE Bunting
FORMULA
(1911)
+ 0,.222
Obert et a l (1960)
0,.778
Bieniawski
0,.556 + 0,.444
(1968)
Van Heerden (1973)
0..704
Sorensen 5 P a r i s e a u (1978)
0..693 + 0..307
- Applicable to p i l l a r
B)
0..700 + 0..300
categories:
Shape E f f e c t Formula
where:
+ 0,.296
W/H w h w h w h w h w h
3.
Stub
4.
Inclined
0,.5
- 1 .0
0,.5
- 2..0
1 .0
- 3,.1
1 .14 - 3 .4 0 .5
- 2,.0
Pillars Pillars.
(Appendix A, S e c t i o n 3.2)
Op
=
P i l l a T strength (psi)
K
=
Constant r e l a t e d t o the p i l l a r
W
=
Pillar
h
=
P i l l a r height
a, b
=
Dimensionless c o n s t a n t s
width
material
23
Description: The shape e f f e c t denotes a d i f f e r e n c e i n the u n i t strength o f d i f f e r e n t shape but equal c r o s s - s e c t i o n . one apparent cause o f shape e f f e c t . of a l i m i t e d number o f f r a c t u r e s .
A change i n mode o f f a i l u r e i s
Slender p i l l a r s tend t o f a i l by means For wide p i l l a r s the p r o b a b i l i t y o f
d e v e l o p i n g a s i n g l e continuous f r a c t u r e plane i s l e s s . p i l l a r r e s u l t s from c r u s h i n g pillar
strength.
core a l s o c o n t r i b u t e s
Thus, f a i l u r e o f the
of the p i l l a r m a t e r i a l , thereby
The t r i a x i a l
for pillars
increasing
s t a t e o f s t r e s s i n a squat p i l l a r ' s
t o an i n c r e a s e
i n p i l l a r strength.
inner
Table 5 gives the
c o n s t a n t s 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 Streat
FORMULA
(1954)
Holland-Gaddy
(1962)
kh"
1
. o o
kh'
1
.oo o.
w
a
o . 5
w
b
0,.5
1. 00
5
0,.5
1. 00
Greenwald et a l (1939)
kh-°
. 6 3
5
0,.5
0.833
Hedley 6 Grant
kh-°
.75 0- S
0,.5
0.75
Salamon § Munro (1967)
kh"
.66 0. » S
0..46
0.66
Bieniawski
kh-° • w ° -
16
0..16
0.,55
(1972)
(1968)
0
w
0 .
w
w
5 5
M o r r i s o n 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
kh
.5
w
0 . 5
0..5
0..5
(1956)
- Applicable
- 0
to p i l l a r categories:
3.
Stub
Pillars
4.
Inclined
Pillars
C)
Salamon " M o d i f i e d " Shape E f f e c t Formula
o
p
=
K —£• , h
where:
where We
(Appendix A, S e c t i o n 3.3.4)
= /W .W a
2
Op
=
Pillar
K
=
Constant r e l a t e d t o the p i l l a r compressive s t r e n g t h
Wi,W
=
C r o s s - s e c t i o n s i d e s o f the p i l l a r s
We
=
The e q u i v a l e n t pillar
h
=
Pillar
a,b
=
Dimensionless
2
strength ( p s i ) material
width f o r a r e c t a n g u l a r
height constants
Description: 2 The
r e s u l t s o f underground t e s t s (Wagner, 1974)
shown that p i l l a r s square p i l l a r s
on c o a l p i l l a r s
have
o f r e c t a n g u l a r c r o s s - s e c t i o n s a r e about 40% stronger
o f the same width and h e i g h t .
the s t r e n g t h o f r e c t a n g u l a r p i l l a r s
A reasonably
can be obtained
than
good estimate o f
by s u b s t i t u t i n g the
square r o o t o f the c r o s s - s e c t i o n a l area o f the p i l l a r f o r W, i n t h e shape effect
formula.
- Applicable to p i l l a r categories:
D)
3.
Stub
4.
Inclined
Sheorey and Singh " M o d i f i e d " Shape E f f e c t Formula
°P whe r e :
•
Pillars Pillars
(Appendix A, S e c t i o n 3.3.4)
.b h Op
=
Pillar
K
=
Constant r e l a t e d t o the a x i a l compressive s t r e n g t h o f the p i l l a r m a t e r i a l
=
C r o s s - s e c t i o n s i d e s o f the p i l l a r
=
Pillar
Wi,W
2
h
strength (psi)
height
a,b
=
Dimensionless constants
(Table 5)
Description: This method as the Salamon m o d i f i e d e q u i v a l e n t width.
formula uses the concept o f an
However, Sheorey and Singh recommend u s i n g the average
value o f the r e c t a n g u l a r c r o s s - s e c t i o n s i d e s as e q u i v a l e n t - A p p l i c a b l e to p i l l a r c a t e g o r i e s :
E)
3.
Stub
4.
Inclined P i l l a r s
Heek and Brown Curves (Appendix A, S e c t i o n
Ci
=
a
+ v ma a /
3
c
3
o
3.3.7)
stress at f a i l u r e
= Minor p r i n c i p a l s t r e s s at f a i l u r e
3
o"
c
= The u n i a x i a l compressive strength o f i n t a c t rock m a t e r i a l
m and s a r e constants
which depend upon the p r o p e r t i e s o f
the rock and upon the extent f o r e being
Pillars
+ sa£
0\ = Major p r i n c i p a l
where:
width.
subjected
t o which i t has been broken be-
t o the s t r e s s e s o*i and a . 3
Description: 3 The
Hoek and Brown
the o v e r a l l
curves were developed based on the assumption that
strength o f a p i l l a r
s t r e n g t h across
i s approximately equal
the c e n t r e o f the p i l l a r .
t o the average
Figure 8 shows the r e s u l t s o f a
s e r i e s o f c a l c u l a t i o n s u s i n g s t r e s s d i s t r i b u t i o n from computer together
with Hoek's f a i l u r e c r i t e r i o n .
f i n e d , one may determine the p i l l a r
modelling,
Once the rock mass q u a l i t y i s de-
strength f o r d i f f e r e n t p i l l a r
- Applicable to p i l l a r categories:
2.
Separation
3.
Stub P i l l a r s
4.
Incl-ined P i l l a r s .
dimensions.
Pillars
26
-p
3.0r
so
E (D U +»
I n t a c t samples o f f i n e g r a i n e d igneous c r y s t a l l i n e rock m=l? , s = l
W
> -H W W bD P
Good q u a l i t y rock mass m=1.7 , s=0.004
w
F a i r q u a l i t y rock mass m=0.34 , s=0.0001
ft
Poor q u a l i t y rock mass m=0.09 , s=0.00001
rt >
Pillar
FIGURE 8
width/height
;
W^/h
I n f l u e n c e o f P i l l a r Width t o Height r a t i o on Average P i l l a r
Strength.
A f t e r Hoek and Brown
(1980)^
27
3.3.2 A)
E m p i r i c a l S t r e s s Formulas
The E x t r a c t i o n Ratio Formula o r T r i b u t a r y Area
_
°p -
(Appendix A, S e c t i o n 1.3.4)
TH (W+BHL+B)
inn—
where:
a
=
Pillar
Y
=
U n i t weight o f the rock
H
=
Depth below s u r f a c e
B
=
Width o f the opening
L
=
Pillar
length
W
=
Pillar
width.
p
load
Description: I f a l a r g e area i s mined out with a reasonably pillars,
uniform p a t t e r n o f
i t can be s a i d t h a t n e a r l y the whole weight o f the overburden
be c a r r i e d by the p i l l a r s
i n equal p r o p o r t i o n s .
(1980)^ g i v e s the e x t r a c t i o n r a t i o formula
F i g u r e 9, Hoek and Brown
f o r d i f f e r e n t p i l l a r shapes.
should be noted t h a t the t r i b u t a r y area theory r e p r e s e n t s the average p i l l a r
stress.
(Overestimates
will
It
the upper l i m i t o f
the load on p i l l a r s
by about 4 0 % ) .
4 Bieniawski ing
(1983) .
The t r i b u t a r y area does not take i n t o account the a r c h -
e f f e c t , o r any other mechanical behaviour o f the o v e r l y i n g s t r a t a . - Applicable to p i l l a r categories:
B)
Chain P i l l a r Formula
U
= -1
P
where:
Separation
3.
Stub
(Appendix A, S e c t i o n Swilski
a
2.
2
yH
v
(1983)
1.3.8)
5
vh
' (Lp+S)(Wp+2W +3S) p
Op
=
Pillar
load ( p s i )
Y
=
Unit weight o f the rock
Pillars
Pillars.
RIB
PILLARS
crp- = / M l +
FIGURE 9
SQUARE PILLARS W o
/w
}
Average V e r t i c a l P i l l a r Layouts. After
crp =y z { l +
w
Vw )
Stresses i n Typical
I l l u s t r a t i o n s are a l l p l a n views.
Hoek and Brown (1980)-
3
p
Pillar
29
H
=
w = p
L
P "
S
=
w = F
Depth below s u r f a c e P i l l a r width Pillar
length
Spacing
between chain
Width o f the f a c e .
Z
y
IL T -J-s
COAL
I P
RIBSIDE
pillars
I a r e a
o f
strata load
- J
i
\
'
\
COAL FACE
COAL PANEL w Ch§in p i l l d t s
FIGURE 10.
¥
Tail
Entry
Determination o f Load on Chain P i l l a r s by the " f i r s t p a n e l " Load Concept. A f t e r S z w i l s k i (1983)
5
Description: The it
c h a i n p i l l a r formula
i s based on the e x t r a c t i o n r a t i o formula but
c o n s i d e r s the e x t r a load a c t i n g on the c h a i n p i l l a r s by the c a n t i l e v e r
a c t i o n o f the immediate r o o f .
However, t h i s s i m p l i f i e d procedure
ignores
the e f f e c t o f the gob support, gob
t o the nearest -
C)
solid
creating a pressure
coal panel.
A p p l i c a b l e t o p i l l a r category:
Subsidence Formula
arch from the compacted
3B
-
(Appendix A, S e c t i o n 10.3)
Chain
Pillars.
Whittaker and Singh (1981) 6
0
p
imrp
=
For W/D and
a
= 9.81 y
p
(
p
+
w
)
•
D
•
1
/
4
w
2
+
c
o
t
*
-
p
< 2 t a n
For W/D where:
2
PfP.D +
Dtancf0 2
> 2 t a n
o"p
=
Pillar
load (psi)
Y
=
Average d e n s i t y o f the overburden
=
Angle o f shear o f r o o f s t r a t a at edge o f longw a l l e x t r a c t i o n and measured t o v e r t i c a l
P
=
Width o f b a r r i e r
W
=
Width o f longwall e x t r a c t i o n
D
=
Depth below s u r f a c e .
pillar
Description: The to
subsidence theory has been a p p l i e d t o the b a r r i e r p i l l a r
a s c e r t a i n the extent
of s t r a t a pressure
t r a c t i o n t o produce l o a d i n g o f the adjacent
situation
a r c h i n g a c r o s s a longwall exbarrier
pillars.
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 t r i a n g u l a r roof mass which shears at an angle 4> t o the vertical.
The l o a d i n g developed by the mass o f r o o f s t r a t a o u t s i d e the
t r i a n g u l a r r e g i o n i s presumed t o be t r a n s f e r r e d t o the b a r r i e r -
A p p l i c a b l e t o p i l l a r category:
2B - B a r r i e r
pillars.
Pillars
3.3.3
E m p i r i c a l Dimensioning
Formulas.
Other e m p i r i c a l formulas do not c o n s i d e r s t r e s s and s t r e n g t h P i l l a r dimensioning formulas are o f t e n used to d e s i g n c o a l b a r r i e r
A)
Mines' I n s p e c t o r Formula
W
=
(Appendix A, S e c t i o n 10.2)
20 + 4T +
where:
separately pillars.
A s h l e y (1930)
3
0.1D
W
=
Width o f p i l l a r
(feet)
T
=
Bed Thickness
(feet)
D
=
Thickness o f the overburden
(feet)
Description: The Ashley formula was developed from experiments i n the P e n n s y l v a n i a coal f i e l d s .
It i s based on the c o n s e r v a t i v e assumption that an a r c h o f
height equal to h a l f the panel width w i l l t i o n s based on the above assumption r e s u l t
stabilize.
Simple hand c a l c u l a -
in pillar
s i z e s with width to
height r a t i o s of approximately three t o f i v e depending upon depth,
pillar
height and panel width. - A p p l i c a b l e to p i l l a r
B)
H o l l a n d Formula
D
category:
(Appendix A, S e c t i o n
=
1ST
or
=
2B
-
Barrier
Pillars
10.2)
±%L™JL
K log e where:
D
=
Width o f B a r r i e r P i l l a r
(feet)
T
=
Thickness o f p i l l a r
W2
=
The estimated convergence on the h i g h s t r e s s s i d e o f the p i l l a r (mm). (W may be estimated with F i g . 11)
(feet)
2
K
=
Constant = 0.09 i f c a v i n g f o l l o w i n g mining i s permitted = 0.08 i f s t r i p packs are b u i l t = 0.07 i f h y d r a u l i c stowage i s c a r r i e d out.
32 Description: The H o l l a n d f o r m u l a i s based on t h e convergence s t u d i e s by B e l i n s k i and B o r e c k i (1964) . Compared w i t h A s h l e y ' s f o r m u l a , i t g i v e s a more r e a l i s t i c p i l l a r w i d t h and c o n s i d e r s p i l l a r t h i c k n e s s , a s w e l l a s o t h e r p e r t i n e n t f a c t o r s . H o l l a n d ' s f o r m u l a , however, i s i n c o m p l e t e i n t h a t i t d i s r e g a r d s t h e p r o p e r t i e s o f the p i l l a r r o c k . Consequently,
t h i s method s h o u l d be a p p l i e d
o n l y i n c o n d i t i o n s s i m i l a r t o those i n which H o l l a n d
experimented.
(Figure l l )
HS: H y d r a u l i c Stowage
1000 700 5 00 400 300
C : Caved SP: S t r i p Packed
0
PJ?s Room & P i l l a r
200
L : Longwall
100 60
40 30 20 0 0
400 800 1200 1600 2000 T h i c k n e s s o f Overburden ( F t . )
FIGURE 11
Observed Value o f W
2
2400
2800
f o r C o a l seams
7 f t . T h i c k and Having a C r u s h i n g S t r e n g t h i n the 3 i n . Cube o f 3000 p s i . + 10%
- Applicable to p i l l a r
category:
2b - B a r r i e r
Pillars.
33 C)
Morrison,
C o r l e t t and W
=
Rice.
j D
where:
(Appendix A, S e c t i o n
10.2)
f o r D < 4000 f e e t
W
=
Width of p i l l a r
(feet)
D
=
Depth below s u r f a c e
(feet)
Description: The
two
previous
formulas were developed s p e c i f i c a l l y f o r c o a l .
Morrison,
C o r l e t t and
o f rock.
Nonetheless, i t o v e r s i m p l i f i e s the problem and
Rice formula g i v e s s a t i s f a c t o r y r e s u l t s i n most kinds
a guide or p r e l i m i n a r y e s t i m a t i o n
2a)
-
b)
D)
Not
a p p l i c a b l e to Rib E n t r y and
B a r r i e r P i l l a r Formula W
=
I
i
where:
D
+
should
be used
as
only.
- A p p l i c a b l e to p i l l a r c a t e g o r i e s :
Note
The
(Appendix A,
Dip
Section
Abutment Barrier
Pillars Pillars
Pillars.
10.2)
15
W
=
Width of p i l l a r
(feet)
D
=
Depth below s u r f a c e
(feet)
Description: T h i s formula i s c i t e d
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 r u l e of thumb
approach to d e s i g n i n g b a r r i e r p i l l a r s . should -
be used as a rough e s t i m a t i o n
Again i t i s o v e r s i m p l i f i e d and
only.
A p p l i c a b l e to p i l l a r category:
2B
- Barrier
Pillars
34 3.4
Group 3.
T h e o r e t i c a l Methods
The t h e o r e t i c a l methods attempt
to evaluate mathematically
f a c t o r s a f f e c t i n g the s t r e s s and s t r e n g t h o f p i l l a r s . i s then proposed.
the p r i n c i p a l
A more r e a l i s t i c
However, the behaviours of p i l l a r s
are very
model
complicated
and t o be c o n s i s t e n t with the theory, the methods need a f a i r number o f i n put parameters.
C o l l e c t i n g data i n a mining
e a r l y stage o f a mine's l i f e ) are too expensive
environment
( e s p e c i a l l y at the
i s not an easy t a s k , and o f t e n the techniques
or not adequately advanced to p r o v i d e a c c u r a t e 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 s u l t s are o f t e n not r e l i a b l e .
They are u s e f u l
to apply, the r e -
i n f u r t h e r comprehending the
mechanism i n v o l v e d i n p i l l a r d e s i g n . 3.4.1
T h e o r e t i c a l S t r e n g t h Formulas
At l e a s t f o u r t h e o r e t i c a l methods have been reviewed
i n the
literature
research: Appendix A
It was
- Coates
( S e c t i o n 3.3.3)
- Grobbelaar
( S e c t i o n 3.5.1)
- Wilson
( S e c t i o n 3.5.2)
- Panek
( S e c t i o n 3.5.3)
noted t h a t o n l y Wilson's method has been used by designers, and
a b r i e f d e s c r i p t i o n of t h i s method i s g i v e n below. A)
Confined Core Method Y h
"
(Wilson)
1 (tan B ) - = (tan B - l ) u
y =
where:
u
i _
'
n
c^ a. o J
The depth o f y i e l d zone from r i b s i d e (feet)
the
h
=
Seam h e i g h t
(feet)
Ov
=
The maximum p i l l a r s t r e s s ( p s i ) ( s i t u a t e d at the y i e l d zone/confined core i n t e r f a c e )
Oo
=
Unconfined
compressive
strength (psi)
•Tan 3 =
Triaxial
stress coefficient
1 + s i n (j) 1 - sin $
I
M
E
T
H
O
tributary
H
O
area
1
Hoek's
curves
j
modified
'
S h e o r e y ft S i n g h
pillar
D
methods)
D
Salomon
chain
T
>
E M P I R I C A L
REMNANT
E
'
->-
C
O
R D
FIGURE
PILLAR
1?
CATEGORY
inclined category 4 P H A S E
PHASE
pillar
2
squore
4
II
PHASE 3
pillar
PHASE
4
PHASE 5
EMPIRICAL METHOD
INCLINED
P.
EXPERIENCE
STRUCTURAL
METHOD
ANALYSIS
—J J
I
Hoek's curves shape effect size effect
pil)ar
COMPUTE R
METHOD
METHOD
Panseau
Hedley
EMPIRICAL rectongular
THEORETICAL
METHOD Hoek's curves Salomon modified Sheorey 8 S i n g h
I
CHAPTER 5 HEATH STEELE CASE HISTORY ANALYSES
INTRODUCTION During the summer o f 1984,
f o u r Noranda underground
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 Bray
e a r l y 60's, was (1967) . 1 0
s e l e c t e d because
mines were v i s i t e d
The Geco "B-Block", mined out i n
the f a i l u r e s were w e l l documented by
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
were a l s o chosen to take advantage
o f A l l c o t and A r c h i b a l d
Steele
(1981)
11
pillar
d e s i g n study. The examination of case h i s t o r i e s may information f o r future designs. ter
generate p e r t i n e n t and
Geco (Chapter 6) and Heath
5) case h i s t o r i e s are analyzed u s i n g the f o l l o w i n g procedure: Review of General Information
1.
Steele
Geology 1.1
Regional geology
1.2
Mine geology
1.3
Structural
geology
2.
Mining method and underground
3.
Rock Mechanics
structures
dimensions.
Data
3.1
Rock s t r e n g t h
parameters
3.2
Laboratory t e s t s
3.3
Rock Mass C l a s s i f i c a t i o n
3.4
Virgin
stress
Review o f P i l l a r Information 4.
Pillar
Characteristics
5.
Mining
sequence
6.
Failure
h i s t o r y and p i l l a r
geometry
useful (Chap-
53 P i l l a r Design 7 .1
Phase 1
Experience method
7 .2
Phase 2
P i l l a r structural
7 .3
Phase 3
E m p i r i c a l methods
7 .4
Phase 4
T h e o r e t i c a l methods
7 .5
Phase 5
Computer methods
5.1
Study
analysis
Geology ( A f t e r A l l c o t t and A r c h i b a l d ( 1 9 8 1 ) ) 11
5.1.1
Regional
Geology
The massive s u l p h i d e s t r a t i f o r m d e p o s i t s of n o r t h e r n New hosted by the Tetagouche rock group.
Brunswick are
T h i s rock group i s h i g h l y f o l d e d ,
middle O r d o v i c i a n i n age and covers a c i r c u l a r area approximately
56
km.
(35 m i l e s ) i n diameter. The Tetagouche rock group i s broken i n t o three l i t h o l o g i c a l Sedimentary, Metabasalt, and
5.1.2
units:
Rhyolitic.
Mine Geology
The massive s u l p h i d e d e p o s i t s l i e w i t h i n the r h y o l i t e u n i t i n c l o s e p r o x i m i t y to the quartz f e l d s p a r c r y s t a l t u f f , which i s a l s o known as Augen S c h i s t and The
Porphyry.
s t r a t i g r a p h i c rock u n i t s i n the ore zone area top towards the
n o r t h , which i s i n d i c a t e d by the metal bedding
i n the sediments.
as f o l l o w s : 1.
zoning i n the s u l p h i d e s and
These u n i t s l i s t e d from youngest
graded
to o l d e s t are
( F i g u r e 18)
Banded Quartz
Feldspar C r y s t a l Tuff
T h i s rock u n i t i s banded i n p l a c e s with 5-10
cm.
bands,
interlaid
with v a r y i n g g r a i n s i z e and p r o p o r t i o n s o f quartz and f e l d s p a r phenocrysts.
7800 LEVEL
5. A c i d T u f f 6. Sediments
FIGURE 18
Heath S t e e l e 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.
bed
on the hanging w a l l s i d e o f the massive s u l p h i d e s .
phyry
The por-
i s q u i t e competent and f r e s h i n appearance, with a com-
p r e s s i v e s t r e n g t h o f 56.5 MPa (8200 p s i ) .
However, the f r a c t u r i n g
tends t o be b l o c k y when exposed on the stope's
Iron
(30-50 f t . ) t h i c k
wall.
Formation
T h i s zone i s present as a d i s c o n t i n u o u s t h i n band along the upper margin o f the massive s u l p h i d e f o r m a t i o n . i n small patches zone.
along the f o o t w a l l c o n t a c t and w i t h i n the s u l p h i d e
T h i s i s a competent bed, but i s too t h i n and d i s c o n t i n u o u s
to be r e l i e d upon as a s t a b i l i z i n g
Massive
However, i t a l s o occurs
unit.
Sulphides
The massive s u l p h i d e s a r e v e r y f i n e g r a i n e d , with a
compressive
s t r e n g t h o f 177 MPa (22,917 p s i ) , and form the most competent u n i t i n the mine.
Very l i t t l e
rock
sloughing occurs where the w a l l s o f
the stopes c o n s i s t o f massive s u l p h i d e s .
Acid Tuff T h i s i s the l e a s t competent rock u n i t
i n the mine, and tends to be
s o f t and sloughs r e a d i l y when exposed on the w a l l s o f stopes.
It
forms a 1.5 t o 21 m. (5-70 f t . ) t h i c k bed on the f o o t w a l l o f the s u l p h i d e s and becomes d i s c o n t i n u o u s i n p l a c e s .
C l a s t i c Sedimentary Rocks The
sedimentary
rocks i n the f o o t w a l l below the a c i d t u f f are i n t e r -
c a l a t e d with quartz f e l d s p a r c r y s t a l t u f f s and form a band a p p r o x i mately 366 m. (1200 f t . ) t h i c k .
56 5.1.3
S t r u c t u r a l Geology
The B Zone i s a t a b u l a r shaped v e r t i c a l or steep n o r t h e r l y d i p p i n g massive s u l p h i d e body, which s t r i k e s a t N 73°E. a s t r i k e l e n g t h o f a p p r o x i m a t e l y 1150 m. from a few c e n t i m e t e r s depth o f 1097 m.
to 75 meters
The massive s u l p h i d e s
have
(3,800 f t . ) , v a r y i n t h i c k n e s s
(250 f t . ) and have been t r a c e d to a
(3,600 f t . ) .
F o l d i n g i s the primary s t r u c t u r a l c o n t r o l and although there
i s minor
f a u l t i n g , f a u l t s have had no major i n f l u e n c e on the shape o f displacement o f the ore zone.
The orebody
has undergone
f i v e p e r i o d s o f f o l d i n g which a r e
numbered one to f i v e i n time sequence as they o c c u r r e d .
(Figure
19)
FIGURE 19. .Diagramatical P l a n View o f Heath S t e e l e Orebody, Si:
Showing O r i e n t a t i o n o f F o l d i n g
The f i r s t p e r i o d o f f o l d i n g l e f t the massive s u l p h i d e . f o l d i n g i s a few f l a t
very l i t t l e
i f any imprint
on
The o n l y r e a l evidence f o r t h i s p e r i o d o f lying relict
cleavage planes
i n the host
rocks. S2:
The second p e r i o d o f f o l d i n g had the g r e a t e s t e f f e c t on the shape of the orebody.
T h i s p e r i o d has shaped the orebody i n t o a number
o f shaped i s o c l i n a l f o l d s which plunge at approximately 60° i n a S 73° W d i r e c t i o n .
S3:
The t h i r d p e r i o d o f deformation produced f o l d s which plunged
s t e e p l y northwest
open or t i g h t c o n c e n t r i c
or southeast.
This folding-
leads to some d i l u t i o n problems i n mining as the f o l d s are d i f f i c u l t to d e f i n e 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 S4:
drilling.
The f o u r t h p e r i o d o f deformation r e s u l t e d i n a s e r i e s of open, conc e n t r i c f o l d s , which plunged t o the northwest
and appeared
as not
more than g e n t l e warps. Sj:
The f i f t h p e r i o d of f o l d i n g produced in a northeasterly direction.
open f o l d s which plunged
T h i s p e r i o d o f deformation
70°
produced
o n l y r a r e f o l d s i n the mine a r e a . - JOINTING
Golder A s s o c i a t e s
There are two major j o i n t s t e e p l y d i p p i n g , with one
(1981)
1 2
s e t s evident throughout
h o r i z o n t a l j o i n t s appeared
d i r e c t i o n and
Both are
set approximately p a r a l l e l t o the s t r i k e o f the
orebody and one approximately t r a n s v e r s e to the s t r i k e .
The j o i n t
the mine.
set p a r a l l e l
to be more prominent
A t h i r d set o f near
i n the s u l p h i d e s .
to the s t r i k e i n d i c a t e d a spread i n s t r i k e
i s p r o b a b l y a combination
o f two
joint
sets.
J o i n t s are u s u a l l y p l a n a r or s l i g h t l y u n d u l a t i n g and spaced at about 1 B . to 3 a .
5.2
(3.3 - 9.8
ft.).
Mining Method and Underground S t r u c t u r e s Dimension A f t e r A l l c o t t and A r c h i b a l d
(1981)
11
Mining o f Heath S t e e l e B Zone orebody proceeded s t o p i n g method, with l a t e r s e l e c t e d area f i l l i n g . from the upper l e v e l s t o the lower maintained
on two
with b l a s t hole open
E x t r a c t i o n progressed
l e v e l s and east t o west with p r o d u c t i o n
to three l e v e l s s i m u l t a n e o u s l y .
The p r o d u c t i o n r a t e
was
3000 tons per day from 1970 to 1976, i n c r e a s e d to 3,500 tons per day i n 1977,
and f i n a l l y reached 4,200 tons per day i n 1982. Mining
of 8600 p r o d u c t i o n
completed without
level,
110 m.
(360 f t . ) below s u r f a c e , was
any s e r i o u s s t a b i l i t y problems and a d i l u t i o n f a c t o r o f
10% was adequate compensation f o r overbreak or minor f a l l s o f waste. centimeters
(2 inches) diameter b l a s t h o l e r i n g s were used, and underground
ore haulage was by Track equipment. 30 m.
(40 f t . ) r i b p i l l a r s were l e f t
low
Stope dimensions were g e n e r a l l y about
(100 f t . ) on s t r i k e and up t o 45 m.
Mining
(150 f t . ) h i g h .
to separate
advanced to the 8300 p r o d u c t i o n
s u r f a c e , u s i n g the same method.
l e v e l , a t 200 m.
the 10% d i l u t i o n was s t i l l
sloughing
(650 f t . ) be-
Stope height was i n c r e a s e d to 60 m. introduced.
Although i t had been easy to mine adjacent stopes with an i n t e r v e n i n g 15 m.
Twelve meters
the stopes.
(200 f t . ) and t r a c k l e s s load-haul-dump was
and
Five
f o o t w a l l and hanging w a l l
(50 f t . ) p i l l a r of low grade s u l p h i d e s
s a t i s f a c t o r y , at t h i s stage
s t a r t e d from the west s i d e o f 83-78 r i b p i l l a r .
small s c a l e The cause was
a s c r i b e d to the i n t e r s e c t i o n o f j o i n t s at the face o f the p i l l a r , but not to
loading. Mining
from s u r f a c e .
s t a r t e d on 8050 p r o d u c t i o n
I t was decided
to remove s i l l
level,
p i l l a r s under 8300 l e v e l so
t h a t the new stope h e i g h t s would be i n c r e a s e d t o 140 m. time the s t r i k e l e n g t h was 45 m.
275 m (900 f t . )
(450 f t . ) .
At t h i s
(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 r e c o v e r i n g r i b p i l l a r s between
primary stopes.
In the f i r s t
b l a s t e d caused the adjacent back extending
over a 150 m.
i n s t a n c e a r i b p i l l a r t h a t was
instantaneously
r i b p i l l a r to b u r s t and i n i t i a t e d a cave i n the (500 f t . ) s t r i k e l e n g t h .
After this,
stope
lengths were l i m i t e d to 43 m. rib pillar
(140 f t . ) , and 85 m.
l e n g t h s were i n c r e a s e d to 18 m.
(280 f t . ) h e i g h t .
The
(60 f t . )
Ground problems were encountered with i n c r e a s i n g frequency as the depth from s u r f a c e
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 o n l y between f i l l e d stopes. At
l e v e l 7430 the stope dimensions were presumed to be 30 m.
l o n g , 60 m.
5.3
(200 f t . ) h i g h and separated by 30 m.
Rock Mechanics 5.3.1
(100 f t . )
(100 f t . ) r i b p i l l a r s .
Data
Rock S t r e n g t h Parameters - Density
Ore:
Waste:
y
= 4581
k g / 3 (286
|p-)
y = 2883 k g / 3 ( 1 7 9
i|f)
m
m
- E l a s t i c Modulus F-W C h l o r i t e T u f f
E = 68,536 MPa
(9.9 M. p s i )
Ore Massive Sulphide E =119,284 MPa
(17.3M. p s i )
H-W
(9.9 M. p s i )
Q t z . Porphyry
E = 68,743 MPa
- Poisson's Ratio v =
0.25
Ore Massive S u l p h i d e v =
0.24
v =
0.19
F-W C h l o r i t e T u f f
H-W
5.3.2
Q t z . Porphyry
Laboratory T e s t i n g - Unconfined Compressive S t r e n g t h F-W C h l o r i t e T u f f
o
c
=
Ore Massive Sulphide H-W
Qtz. Porphyry
o
84 MPa 176.5 MPa
c
=
91 MPa
(12,182 p s i ) (25,598 p s i ) (13,198 p s i )
60
5.3.3
Rock Mass C l a s s i f i c a t i o n
The f o o t w a l l , hanging wall and orebody rocks were c l a s s i f i e d by
Golder
12 A s s o c i a t e s (1981)
u s i n g the NGI
system.
R e s u l t s are given below as well
as an estimated CSIR r a t i n g f o r comparison purposes. 77-92 Cross-.Cut Footwall - C h l o r i t e T u f f NGI RQD Jn Jr Ja Jw SRF Q
95 3 2 0..75 1..0 1..0
90 6 2 0..75 1..0 1..0
84
40
CSIR I n t a c t Strength RQD Spacing o f J o i n t s Condition of Joints Ground Water
7 20 25 6 1_0 68
77-92 Cross-Cut
Sulphides NGI
RQD Jn Jr Ja Jw SRF
85 6 1 0.75 1.0 1.0
95 6 1 0.75 1.0 1.0
Q
18.9
21 CSIR
Intact Strength RQD Spacing J o i n t s C o n d i t i o n of J o i n t s Ground Water
12 17 25 6 7 67
61
I
77-92
Gross-Gut H a n g i n g w a l l P o r p h y r y NGI RQD Jn Jr Ja Jw SRF
95 6
95 3.0 1.0
2.0 0.75 1.0 1.0
0.75 1.0 1.0
42
42 GSIR 7
Intact Strength RQD Spacing o f J o i n t s Condition o f J o i n t s Ground Water
5.3«4
Virgin
20
25 6 10
w
Stress
No v i r g i n s t r e s s measurements have been made a t t h e mine.
Measurements
have been a c h i e v e d a t Brunswick M i n i n g , which i s located, i n t h e same r o c k f o r m a t i o n about
50
km
depth a r e a s f o l l o w s :
( 3 0 miles)
away. The r e s u l t s a t
7 0 0 m. ( 2 3 0 0
ft.)
(Figure 2 0 )
- To determine t h e v i r g i n s t r e s s a t Heath S t e e l e , t h e f o l l o w i n g assumpt i o n has t o be made: "The r a t i o o f s t 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 comp a r a b l e w i t h the r a t i o o f s t r e s s e s a t Heath Thus, the s t r e s s regime o
vv
= a, = v ^ waste 1
= o
Note:
3
2883
= 8.79
KPSF = k i l o
Steele".
3 0 0 m. ( 1 0 0 0 f t . )
x
(depth below r
kg/m x 10
5
3
below s u r f a c e a t Heath
surface)
x 305 m = 8.79 x 1 0 kg/m
2
= 8.62 MP a
pounds p e r square f o o t
5
kg/m
2
(180 KPSF)*
Steele
62
FIGURE 20
V i r g i n S t r e s s at Heath S t e e l e
63
°H
(north-south)
° '
=
a'i = °H
(east-west)
0 2
0
5.4
Pillar
2
=
2
17.24 MPa 1-Sxa
=
=
° '
x
8
12.93 MPa
=
2 x 8.62 MPa (360 KPSF)
= 1 . 5 x 8 . 6 2 MPa (272 KPSF)
Characteristics
- Ribs: O r i g i n a l l y 12 m.
(40 f t . ) x 61 m. (200 f t . ) h i g h x ore width
Laterally
(90 f t . ) x 76 m.
27 m.
(250 f t . ) h i g h x ore width
- Sills/Crowns: P r o d u c t i o n l e v e l up c o n t a i n s cones o r trough u s u a l l y extending up
15 meters
(50 f t . ) above l e v e l .
Below l e v e l u s u a l l y 15 t o
23 meters (50 to 75 f e e t ) depending on ore widths and l o c a l geometry. - Pillar
Support: G e n e r a l l y no systematic support
i s given.
areas some c a b l e b o l t i n g has been used.
In s p e c i f i c problem In some small
pillars
i n the room and p i l l a r o v e r c u t s , some perimeter s t r a p p i n g has been done i n i s o l a t e d problem areas. of
( 5 or 8 f t . ) 1.5 t o 2.5 m.
use mechanical
The
E a r l y p r a c t i c e was to
b o l t s and s t r a p s o r p l a t e s .
anchored r e b a r p i n s a r e almost
Mining
support
development w i t h i n p i l l a r areas has been the use o f standard
rock b o l t s
5.5
Otherwise l o c a l
Latterly
resin
e x c l u s i v e l y used.
Sequence
i n v e s t i g a t e d area c o n s i s t s o f f o u r open stopes
77-89) separated by three r i b p i l l a r s from 245 m. (800 f t . ) t o 366 m.
(77-95, 77-93, 77-91,
(77-94, 77-92, 77-90).
The depth v a r i e s
(1200 f t . ) below s u r f a c e , and a 300 m.
(1000
f t . ) depth was assumed f o r c a l c u l a t i o n
purposes.
F i g u r e 21 i s a l o n g i t u d i n a l view o f the s t o p e / p i l l a r panel l a y o u t , and Table 8 summarizes the mining
sequence (from B l a s t i n g
Record).
TABLE 8 MINING SEQUENCE OF THE PANEL STOPE
5.6
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
F a i l u r e H i s t o r y and P i l l a r Geometry - November 1977:
77-92 Rib P i l l a r
It was d i s c o v e r e d t h a t t h e r e was e x c e s s i v e wedge type sloughing from the w a l l s o f t h i s p i l l a r
i n t o the stopes on each s i d e t o the extent
that i t was c o n s i d e r e d not t o be p r o v i d i n g
any support.
Accordingly, i t
was decided t o b l a s t out t h i s p i l l a r and t o stop f u r t h e r mining west i n 77-93 stope to i n c r e a s e the 77-94 p i l l a r . - A p r i l 1978:
The 77-92 r i b was b l a s t e d , and r e c o v e r e d .
- September, 1978:
77-94 Rib P i l l a r
A f t e r the 77-92 r i b was b l a s t e d , mining continued i n 77-95 stope. On September 23/78, a b l a s t i n 77-95 stope appeared
t o have t r i g g e r e d a
v i o l e n t r e a c t i o n i n 77-94 r i b c a u s i n g s e i s m i c i t y and b u c k l i n g o f t r a c k rails
i n t h i s p i l l a r on both 7950 and 7800 l e v e l s . - J u l y 1980:
77-96 Rib P i l l a r ,
(supplementary
A c t i v i t y i n t h i s r i b on 7700 l e v e l developed
information) i n J u l y 1980 and was
65
FIGURE 21
Schematic Longitudinal View of the Investigated Area at Heath Steele. (Mining method: Blast hole Open stoping)
Scale: 1 i n . = 100 f t .
8200
66
- 77-9^ P i l l a r deteriorates
77-92
Pillar
recovered
w
