METHOD SELECTION

-A

NUMERICAL APPROACH

Chapter 4

David E. Nicholas
Vice-President
C a l l & Nicholas, Inc.
Tucson, Arizona

INTRODUCTION

I n t h i s paper, a numerical p r o c e s s f o r
s e l e c t i n g a mining method, with t h e emphasis
on underground mass mining t e c h n i q u e s , such
a s caving, induced caving, and s t o p i n g , i s
proposed.
I n t h e p a s t , s e l e c t i o n of a mining method
f o r a new property was based p r i m a r i l y on
o p e r a t i n g experience a t s i m i l a r type d e p o s i t s
and on methods a l r e a d y i n use i n t h e d i s t r i c t
of t h e deposit.
Then, t h e chosen method was
modified during t h e e a r l y y e a r s of mining a s
ground c o n d i t i o n s and o r e c h a r a c t e r were b e t t e r understood.
Today, however, t h e l a r g e
c a p i t a l investment r e q u i r e d t o open a new mine
o r change an e x i s t i n g mining system make it
imperative t h a t t h e mining methods examined
during t h e f e a s i b i l i t y s t u d i e s and t h e method
a c t u a l l y s e l e c t e d have a high p r o b a b i l i t y o f
attaining the projected production r a t e s .
Although experience and e n g i n e e r i n g judgment
s t i l l provide major i n p u t i n t o t h e s e l e c t i o n of
a mining method, s u b t l e d i f f e r e n c e s i n t h e
c h a r a c t e r i s t i c s of each d e p o s i t , which may
a f f e c t t h e method chosen o r t h e mine d e s i g n ,
can u s u a l l y be p e r c e i v e d o n l y through a n a l y s i s
of measured c h a r a c t e r i s t i c s .
The parameters t h a t must be examined when
choosing a mining method i n c l u d e :
1) geometry and grade d i s t r i b u t i o n o f t h e
deposit;
2) rock mass s t r e n g t h f o r t h e o r e zone, t h e
hanging w a l l , and t h e f o o t w a l l ;
3 ) mining c o s t s and c a p i t a l i z a t i o n r e q u i r e ments;
4) mining r a t e ;
5) type and a v a i l a b i l i t y of l a b o r ;
6 ) environmental concerns; and
7) o t h e r s i t e - s p e c i f i c c o n s i d e r a t i o n s .
T h i s paper encompasses a d e t a i l e d look a t t h e
f i r s t two parameters s i n c e t h e y , p l u s mining
c o s t s , have t h e g r e a t e s t impact on t h e s e l e c t i o n of a mining method.
The proposed method s e l e c t i o n p r o c e s s i s
f o r a p r o j e c t where d r i l l i n g h a s d e f i n e d s u f f i c i e n t geologic r e s e r v e s , b u t l i t t l e o r no
underground development h a s been done.
Since each d e p o s i t has i t s own c h a r a c t e r i s t i c geanetry/grade d i s t r i b u t i o n , and rock

mechanics p r o p e r t i e s , mining method s e l e c t i o n

should be a t l e a s t a two-stage p r o c e s s .
I n Stage 1 , t h e d e p o s i t i s d e s c r i b e d i n
terms of geometry, grade d i s t r i b u t i o n , and
rock mechanics p r o p e r t i e s . Using t h e s e parame t e r s , t h e mining methods can be ranked t o
determine which a r e most a p p l i c a b l e ; they can
then be considered i n g e n e r a l terms o f mining
and c a p i t a l i z a t i o n c o s t , mining r a t e , type
and a v a i l a b i l i t y of p e r s o n n e l , environmental
concerns, and o t h e r s i t e - s p e c i f i c considerations.
I n Stage 2, t h e most l i k e l y mining methods
a r e c o s t e d o u t , based on a g e n e r a l mine p l a n .
Mining and c a p i t a l i z a t i o n c o s t s a r e used t o
determine a cut-off grade f r a n which a minable
r e s e r v e can be c a l c u l a t e d ; economic comparisons
can t h e n be made t o determine t h e optimum mini n g method and economic f e a s i b i l i t y .
During t h e mine planning phase of Stage 2,
rock mechanics information would be used t o
provide r e a l i s t i c e s t i m a t e s of underground
opening s i z e , amount of s u p p o r t , o r i e n t a t i o n
of openings, and caving c h a r a c t e r i s t i c s , and
open p i t s l o p e a n g l e s . I f ground c o n t r o l o r
o p e r a t i o n a l problems should be encountered w i t h
t h e methods being c o n s i d e r e d , m o d i f i c a t i o n s
could be made. Although planning on paper
extends s t a r t - u p time, it i s cheaper t o e r r
on paper than t o f i n d t h e e r r o r a f t e r mining
has begun.
METHOD SELECTION

STAGE 1

The main purpose of Stage 1 i s t o s e l e c t

those mining methods which should be considered
i n g r e a t e r d e t a i l . The s i m p l e s t way t o do t h i s
i s by d e f i n i n g those c h a r a c t e r i s t i c s r e q u i r e d
f o r each mining method and then d e t e r m i n i n g
whether t h e c h a r a c t e r i s t i c s of t h e d e p o s i t a r e
s u i t a b l e . However, no one mining method i s s o
r e s t r i c t i v e t h a t it can be used f o r o n l y one
s e t of c h a r a c t e r i s t i c s , a s i n d i c a t e d by t h e
c l a s s i f i c a t i o n system proposed by Boshkov and
Wright (1973). I n t h e mining method s e l e c t i o n
proposed, geometry, grade d i s t r i b u t i o n , and
rock mechanics c h a r a c t e r i s t i c s a r e ranked
according t o t h e i r a c c e p t a b i l i t y f o r t e n
g e n e r a l mining methods.

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

Data Required
The most important d a t a r e q u i r e d f o r s e l e c t i o n of a mining method and i n i t i a l mine layout
a r e geologic s e c t i o n s and l e v e l maps, a grade
model of t h e d e p o s i t , and rock mechanics chara c t e r i s t i c s o f t h e d e p o s i t , f o o t w a l l , and hanging w a l l . Much o f t h i s d a t a can be o b t a i n e d
from d r i l l c o r e , and, i f it i s not c o l l e c t e d
during t h e i n i t i a l core logging o r a s s a y i n g ,
it w i l l be l o s t .
Geology. Basic geology i n t e r p r e t a t i o n i s of
major importance i n any mineral e v a l u a t i o n .
Geologic s e c t i o n s and l e v e l maps which show
major rock t y p e s , a l t e r a t i o n zones, and major
s t r u c t u r e s , such a s f a u l t s , v e i n s , and f o l d
I t may be a d v i s a b l e
a x e s , should be prepared.
t o d e f i n e t h e a l t e r a t i o n zones on a s e p a r a t e
s e t o f maps, which can then be o v e r l a i n onto
t h e rock type geology maps. These g e o l o g i c
s e c t i o n s and l e v e l maps should be prepared a t
t h e same s c a l e a s w i l l be used f o r mine planning.
S e c t i o n s should be drawn t o t r u e s c a l e ,
without any v e r t i c a l e x a g g e r a t i o n , because it
makes it e a s i e r t o v i s u a l i z e t h e r e l a t i v e layo u t of mine workings. The a r e a included on
t h e maps should extend h o r i z o n t a l l y i n a l l
d i r e c t i o n s 1.75 times t h e depth beyond t h e
l i m i t of t h e orebody. Although an a r e a t h i s
s i z e may seem e x c e s s i v e , it w i l l ensure t h a t
t h e r e i s s u f f i c i e n t information f o r e v a l u a t i n g
t h e l i m i t of ground s u r f a c e movement due t o
mining: t h i s information i s needed t o l o c a t e
s h a f t s , a d i t s , and b u i l d i n g s , e t c .
The importance of a complete s e t o f i n t e r p r e t e d s e c t i o n s and l e v e l maps cannot be overs t a t e d . They a r e necessary f o r d e f i n i n g grade
d i s t r i b u t i o n , a s w e l l a s u n i t s of s i m i l a r rock
mechanics c h a r a c t e r i s t i c s .
Geometry of Deposit and Grade D i s t r i b u t i o n .
During Stage 1 of t h e method s e l e c t i o n p r o c e s s ,
geometry and grade d i s t r i b u t i o n a r e d e f i n e d .
The geometry of t h e d e p o s i t i s d e f i n e d i n terms
of g e n e r a l shape, o r e t h i c k n e s s , plunge, and
depth (Table 1 ) . Grade d i s t r i b u t i o n i s d e f i n e d
a s uniform, g r a d a t i o n a l , o r e r r a t i c (Table 1 ) .
Defining t h e geometry and grade d i s t r i b u t i o n
of a d e p o s i t r e q u i r e s development o f a grade
model. The t y p e of model c o n s t r u c t e d w i l l depend on t h e complexity of t h e geology and how
w e l l it i s understood, a s w e l l a s on t h e d r i l l
hole spacing. The grade model should be p u t on
s e c t i o n s and l e v e l maps a t t h e same s c a l e a s
t h e geology maps and should be contoured by
g r a d e , o r t h e blocks should be c o l o r e d by grade
categories.
These contoured o r c o l o r e d grade
s e c t i o n s and l e v e l maps, when o v e r l a i n o n t o
t h e g e o l o g i c s e c t i o n s and l e v e l maps, w i l l
i n d i c a t e t h e dominant rock t y p e s , a s w e l l a s
t h e i r s p a t i a l r e l a t i o n s h i p s t o t h e orebody.

Table 1:

D e f i n i t i o n of Deposit Geometry and

Grade D i s t r i b u t i o n
Geometry of Deposit

1) General shape
equi-dimensional:
platey

tabular:

a l l dimensions a r e on the
same order of magnitude
two dimensions a r e many
times t h e t h i c k n e s s ,
which does not usually
exceed 100 m (325 f t )
dimensions vary over
short distances

irregular:

2) Ore t h i c k n e s s
narrow:

<10 m (<30 f t )

intermediate:

10 m

30 m (30 f t

thick:

30 m

100 m (100 f t - 325 f t )

very t h i c k :

>I00 m (>325 f t )

100 f t )

3) Plunge
flat:

<20

intermediate:

20

steep :

>55"

55'

4 ) Depth below s u r f a c e

provide a c t u a l depth
5) Grade d i s t r i b u t i o n
uniform
t h e grade a t any p o i n t i n t h e d e p o s i t does
n o t vary s i g n f i c a n t l y from t h e mean grade
for t h a t deposit
gradational
grade v a l u e s have zonal c h a r a c t e r i s t i c s ,
and t h e grades change gradually from one
t o another
erratic
grade v a l u e s change r a d i c a l l y over s h o r t
d i s t a n c e s and do n o t e x h i b i t any d i s c e r n i b l e p a t t e r n i n t h e i r changes
Rock Mechanics C h a r a c t e r i z a t i o n . I n Stage 1
t h e rock p r o p e r t i e s need t o be c l a s s i f i e d so
t h a t an o v e r a l l rock mechanics p i c t u r e of t h e
d e p o s i t i s provided. A number of c l a s s i f i c a t i o n systems have been presented (Deere, 1968;
Coates, 1970; Bieniawski, 1973; Barton e t a l . ,
1974; and Laubscher, 1977). A l l t h e s e systems
include t h e b a s i c measurements of rock subs t a n c e ( i n t a c t . rock) s t r e n g t h , some measurement o f t h e f r a c t u r e i n t e n s i t y , and some measurement of t h e f r a c t u r e s t r e n g t h . The c l a s s i f i c a t i o n systems of Bieniawski, Barton e t a l . ,
and Laubscher use i n d i v i d u a l parameters t o
c a l c u l a t e an o v e r a l l rock mass q u a l i t y . The

METHOD SELECTION
d e f i n i t i o n of rock substance s t r e n g t h , f r a c t u r e
spacing, and f r a c t u r e shear s t r e n g t h used i n
t h e method s e l e c t i o n i s presented i n Table 2.
Table 2:

1) Rock Substance Strength

(uniaxial strength[Pa]/overburden pressure

[Pal
<8
8 - 15
>15

Fractures/m
>16
10
16
3 - 10
3

Method Selection Process

Ten b a s i c mining methods, not including
hydraulic o r s o l u t i o n mining, should be considered i n any s e l e c t i o n process:
1) Open p i t
a method where mining s t a r t s
a t t h e surface and waste i s removed t o
uncover t h e ore; includes s t r i p mining
and quarrying
2) Block caving
a method i n which columns
of rock a r e undercut and cave under t h e i r
own weight; t h e roof material i s expected
t o cave a s well; includes panel and continuous caving.
3) Sublevel stoping - a method of stoping i n
which t h e ore i s b l a s t e d by benching,
r i n g d r i l l i n g , o r long hole; most of t h e
o r e i s drawn o f f a s it i s b l a s t e d , leavi n g an open stope.
4) Sublevel caving
an induced caving
method i n which t h e ore i s b l a s t e d by
r i n g d r i l l i n g from d r i f t s ; overlying
rock i s expected t o cave a s t h e ore i s
drawn.
5) Longwall - a method i n which t h e d e p o s i t ,
usually a coal seam, i s removed i n a continuous operation along a long working
face; using an extensive s e r i e s of props
over t h e face and working a r e a s ; mined
o u t a r e a s usually cave.
a method i n which a
6) Room-and-pillar
g r i d of rooms i s developed, leaving p i l l a r s , u s u a l l y of uniform s i z e , t o support
t h e roof; t h e p i l l a r s may o r may not be
removed a t a l a t e r time;
a stoping method i n
7) Shrinkage stoping
which most of t h e b l a s t e d ore i s l e f t t o
accumulate i n t h e stope u n t i l t h e stope
i s completely mined. The broken o r e i s
then drawn off a l l a t once.
8 ) Cut-and-fill
a stoping method i n which
each s l i c e of rock i s removed a f t e r
b l a s t i n g and i s then replaced with some
type of f i l l m a t e r i a l , leaving space t o
mine t h e next s l i c e .
9) Top s l i c i n g
a method i n which staggered
h o r i z o n t a l l i f t s a r e mined; t h e overlying
rock i s supported by a timber mat and t h e
overlying rock i s expected t o cave.
10) Square-set
a method i n which timber
squares a r e formed t o replace t h e rock
mined and t o support t h e surrounding
rock; includes o t h e r timbered stoping
methods, such a s s t u l l stoping.

2) Fracture Spacing

very close:
close :
wide :
very wide:

strength. These maps, when overlain onto t h e

geology and grade o u t l i n e , w i l l s p a t i a l l y def i n e rock mechanics c h a r a c t e r i s t i c s .
The use of any of t h e e x i s t i n g c l a s s i f i c a t i o n systems w i l l a l s o provide t h e data t o
determine t h e c l a s s e s defined i n Table 2 .

Rock Mechanics ~ h a r a c t e r i s t i c s

weak:
moderate:
strong:

- A NUMERICAL APPROACH

(f t )

% RQD

(>5)
(3 - 5)
(1 - 3)
(<I)

0
20
40
70

20
40
70
100

3) Fracture Shear Strength

weak :

clean j o i n t with a smooth surface

o r f i l l with m a t e r i a l whose
s t r e n g t h i s l e s s than rock substance s t r e n g t h

moderate:

clean j o i n t with a rough surface

strong:

j o i n t i s f i l l e d with a m a t e r i a l
t h a t i s equal t o o r stronger
than rock substance s t r e n g t h

Rock substance s t r e n g t h i s t h e r a t i o of t h e
uniaxial compression s t r e n g t h t o t h e overburden
s t r e s s . The u n i a x i a l compression s t r e n g t h can
be estimated using t h e method o r i g i n a l l y presented by Terzaghi and Peck (1967), which was
then modified by Deere (1968), Jennings and
However,
Robertson (1960) , and P i t e a u (1970)
a b e t t e r estimate of t h e u n i a x i a l compression
strength could be obtained r e l a t i v e l y inexpens i v e l y by using a point load t e s t i n g machine.
The overburden s t r e s s i s determined from t h e
depth and density of rock.

Fracture spacing can be defined i n terms of

f r a c t u r e s per meter o r RQD, Rock Quality Designation (Table 2 ) . RQD i s t h e sum length of a l l
pieces of core g r e a t e r than o r equal t o two
times t h e core diameter divided by the t o t a l
length of a d r i l l run. However, I believe t h e
f r a c t u r e s per meter measurement i s b e t t e r
because it provides a more q u a n t i t a t i v e
description of t h e rock fragment s i z e . Fract u r e shear s t r e n g t h i s determined by observat i o n (Table 2 ) .
As p a r t of t h e geologic log, one should
estimate o r measure t h e u n i a x i a l compression
strength and t h e f r a c t u r e s per meter, o r RQD
measurement, and the f r a c t u r e shear s t r e n g t h .
This d a t a can then be i n t e r p r e t e d on s e c t i o n s
and l e v e l maps a t t h e same s c a l e a s the geologic maps. The cumulative sum technique
(Piteau and Russell, 1972) can be used t o help
define zones of s i m i l a r rock substance
s t r e n g t h , f r a c t u r e spacing, and f r a c t u r e

Boshkov and Wright (1973) , Morrison (1976) ,

Laubscher (1977) , and Tymshare, Inc. (1981)
have presented schemes f o r s e l e c t i n g mining
methods. Boshkov and Wright (1973) l i s t e d t h e

42

DESIGN A N D OPERATION O F CAVING AND SUBLEVEL STOPING MINES

n ~ i n i n qmethods p o s s i b l e f o r c e r t a i n combinat i o n s of o r e w i d t h , plunge o f o r e , and s t r e n g t h

o f o r e . Morrison (1976) c l a s s i f i e d t h e mining
methods i n t o t h r e e b a s i c g r o u p s , r i g i d p i l l a r
s u p p o r t , c o n t r o l l e d s u b s i d e n c e , and c a v i n g ; he
t h e n used g e n e r a l d e f i n i t i o n s of o r e w i d t h ,
s u p p o r t t y p e , and s t r a i n energy accumulation
a s t h e c h a r a c t e r i s t i c s f o r d e t e r m i n i n g mining
method ( F i g u r e 1 ) . Laubscher (1977) developed
a d e t a i l e d r o c k mechanics c l a s s i f i c a t i o n from
which c a v a b i l i t y , f e a s i b i l i t y of open s t o p i n g
o r room and p i l l a r m i n i n g , s l o p e a n g l e s , and
g e n e r a l s u p p o r t r e q u i r e m e n t s c o u l d be d e t e r mined.
Tymshare, I n c . (1981) developed a
numerical a n a l y s i s t h a t d e t e r m i n e s one o f f i v e
mining methods, (1) open p i t , ( 2 ) n a t u r a l cavi n g , ( 3 ) induced c a v i n g , ( 4 ) s e l f - s u p p o r t i n g ,
and ( 5 ) a r t i f i c i a l l y s u p p o r t i n g , and c a l c u l a t e s
t h e tonnage and g r a d e f o r t h e t y p e o f d e p o s i t
d e s c r i b e d . T h i s method i s meant t o be used a s
a pre-feasibility t o o l for geologists.

I propose a s e l e c t i o n method which combines

p o r t i o n s of a l l t h e above methods. The s e l e c t i o n p r o c e s s h a s two s t e p s :
(1) determine t h e
c h a r a c t e r i s t i c s of t h e d e p o s i t , a s d e f i n e d i n
T a b l e s 1 and 2 ; and ( 2 ) f o r each mining method,
add up t h e v a l u e s from T a b l e s 3 and 4 f o r t h e
combination of c h a r a c t e r i s t i c s d e f i n e d i n S t e p
1.

Each mining method h a s been ranked a s t o t h e

s u i t a b i l i t y of i t s geometry/grade d i s t r i b u t i o n
(Table 3 ) , and o r e zone (Table 4 a ) , hanging
w a l l ( o v e r l y i n g w a l l r o c k ) (Table 4 b ) , and
f o o t w a l l ( u n d e r l y i n g r o c k s ) rock mechanics
c h a r a c t e r i s t i c s (Table 4 c ) . There a r e f o u r
ranks :
preferred:

t h e characteristic i s preferred
f o r t h e mining method;

probable:

i f the characteristic exists, the

mining method can be used;

unlikely:

i f t h e c h a r a c t e r i s t i c e x i s t s , it
i s u n l i k e l y t h a t t h e mining
method would be a p p l i e d , b u t
d o e s n o t completely r u l e o u t t h e
method; and

0-30m (0-100ft)

rl

x o r r o v

lo

W l d e

1 I1 I

O r e

eliminated:

i f the characteristic exists,

t h e n t h e mining method could
n o t be used.
The v a l u e s used f o r e a c h rank a r e l i s t e d i n
T a b l e 5. Values f o r t h e e l i m i n a t e d rank were
chosen s o t h a t i f t h e sum of t h e c h a r a c t e r i s t i c v a l u e s e q u a l l e d a n e g a t i v e number, t h e
A z e r o v a l u e was
method would b e e l i m i n a t e d .
chosen f o r t h e u n l i k e l y rank because it does
n o t add t o t h e chance o f u s i n g t h e method, b u t
n e i t h e r d o e s it e l i m i n a t e t h e method. The
v a l u e s u s e d f o r p r o b a b l e and p r e f e r r e d were
chosen s o t h a t t h e c h a r a c t e r i s t i c s f o r one
p a r a m e t e r c o u l d b e ranked w i t h i n a mining
method and between mining methods.
T a b l e 5:

Invormobly

W i d e

O r e

+30m(+100ft)

F i g u r e 1:

A Method S e l e c t i o n Scheme

( a f t e r M o r r i s o n , 1976)

Rank Value

Ranking

Value

preferred
probable
unlikely
eliminated

3 - 4
1 - 2
0
-4 9

An example i s p r o v i d e d t o i l l u s t r a t e t h e
s t e p s i n u s i n g t h i s s e l e c t i o n system and t o
p o i n t o u t problems w i t h t h e system. The f i r s t
s t e p i s t o l i s t t h e geometryjgrade d i s t r i b u t i o n
and r o c k mechanics c h a r a c t e r i s t i c s of t h e
d e p o s i t ( T a b l e 6 , column 1 ) . The c h a r a c t e r i s t i c columns i n T a b l e s 3 and 4 a r e t h e n i d e n t i f i e d f o r t h e d e p o s i t , and t h e v a l u e s added up
f o r t h e geometry/grade d i s t r i b u t i o n , o r e zone
r o c k mechanics, h a n g i n g w a l l rock mechanics,
and f o o t w a l l r o c k mechanics f o r each mining
method ( T a b l e 6 , columns 2 and 3 ) .

METHOD SELECTION - A NUMERICAL APPROACH

Table 3: Ranking of Geome.try/Grade
General
Shape

D i s t r i b u t i o n f o r D i f f e r e n t Mining Methods

Ore Thickness

Mining Method

Ore Plunge
F

Grade
Distribution
U

Open P i t
Block Caving
Sublevel S t o p i n g
Sublevel Caving
Longwall
Room

&

Pillar

Shrinkage S t o p i n g
Cut

&

Fill

Top S l i c i n g
Square S e t

M = Massive
T/P = Tabular o r
Platy
I = Irregular

N = Narrow
I = Intermediate

T = Thick
VT = Very Thick

The t h r e e g r o u p s o f rock mechanics c h a r a c t e r i s t i c s s h o u l d be t o t a l e d .

T h i s t o t a l should
t h e n be added t o t h e geometry/grade d i s t r i b u t i o n sum (Table 7 ) . Using t h e above t y p e o f
c h a r a c t e r i s t i c g r o u p i n g , one can s e e which
g r o u p i n g ( s ) r e d u c e t h e chance o f u s i n g a p a r t i c u l a r mining method, o r , f o r c a s e s where t h e
t o t a l sum i s n e a r l y e q u a l , one c a n d e t e r m i n e
which c h a r a c t e r i s t i c s a r e t h e most s u i t a b l e
f o r t h e mining method.
A f t e r t h e mining methods have been ranked
(Table 8 ) , based on geometry/grade d i s t r i b u t i o n and rock mechanics c h a r a c t e r i s t i c s , t h e r e
may be a number o f methods which a p p e a r
suitable.
I n o u r example, t h e open p i t method i s t h e
obvious c h o i c e from a geometry and r o c k
mechanics c h a r a c t e r i s t i c s p o i n t o f view.
The
n e x t f o u r methods, b l o c k c a v i n g , t o p s l i c i n g ,
s q u a r e - s e t , and c u t - a n d - f i l l , a r e grouped
t o g e t h e r . I t i s worthwhile a t t h i s t i m e t o
look a t t h e r a n k i n g o f a l l t h e mining methods
by i n d i v i d u a l c h a r a c t e r i s t i c s ( T a b l e 9 ) .
Examination of T a b l e 9 r e v e a l s t h a t t h e c h o i c e
of a mining method i n v o l v e s compromise.
For
example, c u t - a n d - f i l l would be a good method
from t h e r o c k mechanics p o i n t o f view, b u t it
h a s t h e w o r s t geometry/grade d i s t r i b u t i o n
c h a r a c t e r i s t i c s , whereas t o p s l i c i n g h a s one
of t h e w o r s t rock mechanics c h a r a c t e r i s t i c s ,
b u t i t s geometry/grade d i s t r i b u t i o n c h a r a c t e r i s t i c s are considered t h e b e s t .

F = Flat
I = Intermediate
S = Steep

U = Uniform
G = Gradational
E = Erratic

I t would n o t b e r e a s o n a b l e t o move d i r e c t l y
t o S t a g e 2 a t t h i s p o i n t , s i n c e p r e p a r i n g det a i l e d mine p l a n s - f o r a l l a p p l i c a b l e methods
d e l i n e a t e d i n S t a g e 1 would b e e x t r e m e l y timeconsuming and c o s t l y .

C o n t i n u i n g w i t h o u r example, t h e f i v e
methods w i t h s i m i l a r t o t a l v a l u e s s h o u l d b e
examined g e n e r a l l y i n t e r m s o f mining c o s t s .
Although a l l f i v e methods were r a n k e d a s
a p p l i c a b l e , mining c o s t s may be s i g n i f i c a n t l y
d i f f e r e n t f o r e a c h method.
Morrison (1976)
h a s ranked t h e mining methods by i n c r e a s i n g
u n i t mining c o s t , which I have m o d i f i e d
s l i g h t l y , a s follows:
1) open p i t
6 ) room-and-pillar
2) b l o c k c a v i n g
7) s h r i n k a g e s t o p i n g
3) s u b l e v e l s t o p i n g 8 ) cut-and-f ill
4) s u b l e v e l c a v i n g
9) t o p s l i c i n g
5) l o n g w a l l
10) s q u a r e - s e t
On t h e b a s i s o f r e l a t i v e o p e r a t i n g c o s t , t h e
methods would b e ranked a s f o l l o w s :
1) open p i t
4) t o p s l i c i n g
2) b l o c k c a v i n g
5) square-set
3) c u t - a n d - f i l l
Based on t h i s s i m p l i f i e d r a n k i n g b y mining c o s t ,
I would e v a l u a t e open p i t and b l o c k c a v i n g
f i r s t . C u t - a n d - f i l l would t h e n be c o n s i d e r e d
if n e i t h e r o f t h e s e two methods proved f e a s i ble.

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

44

4 b : Hanging W a l l

T a b l e 4 : Ranking o f Rock Mechanics

Characteristics f o r Different
Mining Methods
Key:

Mining
Method

Rock
Substance
Strength

Fracture
Strength

Fracture
Spacing
W V W

-49

1 4

Room &
Pillar

Shrinkage
Stoping

Cut

TOP
Slicing

Square S e t

V C C

Open P i t

Block
Caving

Sublevel
Stoping

-49

Sublevel
Caving

Longwall

Rock S u b s t a n c e S t r e n g t h
W = Weak
M = Moderate
S = Strong

F r a c t u r e Spacing
VC = Very C l o s e
C = Close
W = Weak
VW = Very Weak
Fracture Strength
W = Weak
M = Moderate
S = Strong

4 a : Ore Zone

&

Fill

4c: Footwall

Mining
Method

Rock
Substance
Strength

Fracture
Strength

Fracture
Spacing

W M S

VCC

Open P i t

Block
Caving

Sublevel
Stoping

Sublevel
Caving

Longwall

W V W

Room &
Pillar

Shrinkage
Stoping

Cut

TOP
Slicing

Square S e t

&

Fill

Table 6: Example of Numerical Method S e l e c t i o n Process

Geometry/Grade
Distribution

(Column 1)

General shape:

(Column 2)

etc.

block caving
open p i t
( v a l u e s from Table 3 )

tabular o r
platey
very t h i c k
flat
uniform
130 m (425 f t )

Ore t h i c k n e s s :
Ore plunge:
Grade d i s t r i b u t i o n :
depth (used l a t e r ) :

(Column 3)

Rock Mechanics C h a r a c t e r i s t i c s

( v a l u e s from Table 4)

Ore Zone
Rock substance
strength:
F r a c t u r e spacing:
Fracture strength:

moderate
close
moderate

4
2
3
-

1
4
3
-

4
4
3
-

1
3
2

11

4
2
2
-

3
3
1
-

Hanging Wall
Rock substance
strength :
F r a c t u r e spacing:
Fracture strength:

strong
wide
moderate

Footwall
Rock substance
strength :
F r a c t u r e spacing:
Fracture strength :

Table 7: Example

moderate
close
weak

C h a r a c t e r i s t i c s Values Totaled f o r D i f f e r e n t Mining Methods

Rock Mechanics C h a r a c t e r i s t i c s

Geometry/Grade
Distribution

Ore

HW

F
W

Total

Grand
Total

Open P i t

12

11

28

40

Block Caving

13

21

34

Sublevel Stoping

10

14

24

Mining Method

Sublevel Caving

16

29

-37

19

-18

-38

18

-20

Shrinkage Stoping

10

20

30

Cut

10

25

32

Longwall
Room

&

&

Pillar

Fill

13

Top S l i c i n g

15

19

34

Square S e t

10

25

33

46

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

'Table 8 :

Ranklnq R e s u l t s

Total Polnts

method t h a t i s h i g h l y mechanical o r t e c h n i c a l
and r e q u i r e s s k i l l e d personnel should not be
chosen, of course. Environmental concerns a r e
more and more becoming a c o n t r o l l i n g f a c t o r i n
method s e l e c t i o n . Also, t h e environmental cond i t i o n s underground must be considered.
Whether o r not subsidence i s permitted can
determine what methods a r e f e a s i b l e .

Met hod
open p i t
block caving
top s l i c i n g
square-set
cut-and-fill
shrinkage s t o p i n g
s u b l e v e l caving
sublevel stoping
room-and-pillar
longwall

Remember, t h e purpose of t h i s numerical

method s e l e c t i o n system i s not t o choose t h e
f i n a l mining method.
I t i s intended t o i n d i c a t e those methods t h a t w i l l be most e f f e c t i v e
given t h e geometry/grade d i s t r i b u t i o n and rock
mechanics c h a r a c t e r s i t i c s , and which w i l l r e If
q u i r e more d e t a i l e d study i n Stage 2 .
nothing e l s e , t h i s s e l e c t i o n system w i l l allow
miners/engineers t o consider what c h a r a c t e r i s t i c s a r e important f o r t h e mining methods
being considered.

Having narrowed t h e p r e f e r r e d mining methods t o two, each should now be g e n e r a l l y examined i n terms of mining r a t e , l a b o r a v a i l a b i l i t y , environmental concerns, and o t h e r
site-specific considerations, i n order t o
determine whether t h e s e parameters w i l l
e l i m i n a t e any method from f u r t h e r consideration.

METHOD SELECTION

STAGE 2

Mining r a t e should be d i c t a t e d by t h e mining

method chosen and t h e s i z e of t h e d e p o s i t .
However, i n i n s t a n c e s where a m i l l a l r e a d y
e x i s t s i n t h e area, a production r a t e t h a t i s
perhaps higher o r lower t h a n t h a t d i c t a t e d by
t h e l e a s t c o s t l y mining method may be r e q u i r e d .
T h e r e f o r e , a compromise must be made.

The purpose of Stage 2 i n t h e method s e l e c t i o n p r o c e s s i s t o l a y o u t g e n e r a l mining p l a n s

f o r t h o s e methods d e l i n e a t e d i n Stage 1, d e t e r mine c u t - o f f g r a d e s , and then c a l c u l a t e minable
r e s e r v e s s o t h a t economic analyses can be made
i n o r d e r t o determine which mining method w i l l
provide t h e g r e a t e s t r e t u r n on investment.

Other f a c t o r s a f f e c t i n g t h e mining method

s e l e c t e d would be t h e market f o r t h e r e s o u r c e
b e i n g mined and t h e a v a i l a b l e l a b o r pool.
If
t h e l a b o r pool i s l a r g e and u n s k i l l e d , a

I t i s n o t t h e purpose of t h i s paper t o d i s cuss d e t e r m i n a t i o n of cut-off grade o r minable

reserves.
Rock mechanics c h a r a c t e r i s t i c s of
t h e d e p o s i t t h a t a r e c r i t i c a l f o r mine planning

Table 9 : Ranking of Mining Methods by Each C h a r a c t e r i s t i c

Geometry/Grade
Distribution

t o p = 15
bcv = 13
s c v = 13
p i t = 12
sst = 10
s h s = 10
8
sqs =
c&f =
7
lng = -37
r & p = -38

Ore

pit =
bcv =
lng =
c&f =
sqs =
scv =
r&p=
shs =
top =
sst =

HW

9
8
8
8
8
7
7
6
6
5

pit =
r&p =
sst =
c&f =
sqs =
bcv =
scv =
shs =
top =
lng =

Rock
Mechanics
Total

FW

11
8
7
7
7
6
6
6
6
5

c&f =
sqs =
pit =
shs =
bcv =
top =
lng =
scv =
r&p =

sst

10
10
8
8
7
7
6
3
3
2

pit =
c&f =
sqs =
bcv =
shs =
lng =
top =
r&p=
scv =
sst =

28
25
25
21
20
19
19
18
16
14

Grand
Total

pit =
bcv =
top =
sqs =
c&f =
shs =
scv =
sst =
lng =
r&p=

-- .

p i t = open p i t
bcv = block caving
s s t = sublevel stoping

scv =
lng =
r&p =
shs =

s u b l e v e l caving
longwall
room & p i l l a r
shrinkage stoping

c&f = cut & f i l l

top = top s l i c i n g
s q s = square s e t

METHOD SELECTION - A NUMERICAL APPROACH

and s e l e c t i n g a mass mining method w i l l be d i s cussed.
Rock Mechanics Data
In order t o estimate c a v a b i l i t y of a d e p o s i t ,
stope widths, p i l l a r s i z e s , and slope angles,
more rock mechanics d a t a i s required f o r Stage
2 than f o r Stage 1. Most of t h i s a d d i t i o n a l
d a t a should have been c o l l e c t e d a t t h e same
time a s the d a t a f o r Stage 1. Design of p i t
slopes and underground openings depends l a r g e l y
on the geology of t h e a r e a , t h e s t r e n g t h of t h e
rock mass, and the pre-mine s t r e s s . Strength
of the rock mass i s a function of the strength
of the i n t a c t rock, t h e s t r e n g t h of t h e geologic s t r u c t u r e s ( j o i n t s , f a u l t s , e t c . ) , and
the c h a r a c t e r i s t i c s of t h e geologic s t r u c t u r e
( o r i e n t a t i o n , length, spacing, e t c . ) . Once t h e
geologic s t r u c t u r e d a t a a r e a v a i l a b l e , potent i a l f a i l u r e geometries can be defined and s t a b i l i t y analyses can be made using the s t r e n g t h
properties.
Strength Properties. Basic s t r e n g t h p r o p e r t i e s
needed f o r Stage 2 of t h e method s e l e c t i o n
process are uniaxial compression s t r e n g t h ,
s t i f f n e s s (Young's Modulus), Poisson's r a t i o ,
t e n s i l e strength, i n t a c t rock shear s t r e n g t h ,
n a t u r a l f r a c t u r e shear s t r e n g t h , and f a u l t
gouge shear strength. Rock u n i t s , such a s
s a l t , s h a l e s , e t c . , may r e q u i r e creep t e s t i n g
under controlled temperature and humidity.
A l l the s t r e n g t h p r o p e r t i e s , except perhaps
the f a u l t gouge s t r e n g t h , can be measured using
u n s p l i t d r i l l core specimens. The number of
specimens required f o r r e p r e s e n t a t i v e t e s t i n g
depends somewhat on v a r i a b i l i t y of t h e rock
u n i t ; however, t h r e e t o s i x samples per rock
type per t e s t type should be s u f f i c i e n t f o r
Stage 2. During d r i l l i n g , u n s p l i t core samp l e s must be saved f o r rock t e s t i n g . W
e
recommend c o l l e c t i n g t h r e e samples per rock
type per t e s t type per d r i l l hole ( C a l l , 1979).
By sampling each hole, a c o l l e c t i o n of samples
w i l l be b u i l t up, from which samples f o r t e s t ing can be selected.

Geologic Structure. Rock mass s t r e n g t h a l s o

depends l a r g- e l y on t h e c h a r a c t e r i s t i c s of t h e
geologic s t r u c t u r e s , o r i e n t a t i o n , spacing,
length, strength, e t c . Fracture shear s t r e n g t h
has already been discussed i n t h e rock s t r e n g t h
section. For Stage 2 of t h e method s e l e c t i o n ,
areas with similar j o i n t o r i e n t a t i o n s a r e def i n e d a s s t r u c t u r a l domains; d i s t r i b u t i o n of
the f r a c t u r e s e t c h a r a c t e r i s t i c s and p o t e n t i a l
f a i l u r e paths a r e defined f o r each domain.
-

Geologic s t r u c t u r e s a r e divided i n t o two

categories: major s t r u c t u r e s and rock f a b r i c .
Major s t r u c t u r e s a r e f a u l t s , f o l d s , d i k e s ,
e t c . , which have lengths on t h e order of t h e
deposit s i z e and a r e usually considered i n d i vidually i n design. Rock f a b r i c i s predominantly j o i n t s and f a u l t s t h a t have a high

frequency of occurrence and a r e not continuous.

S t r u c t u r a l data can be obtained by using det a i l l i n e mapping (Call e t a l . , 19761, c e l l
mapping, o r o r i e n t e d core mapping. D e t a i l l i n e
mapping i s a technique t h a t involves the measurements of f r a c t u r e c h a r a c t e r i s t i c s of a l l
j o i n t s which i n t e r s e c t a l i n e . This mapping
technique i s a spot sample within a s t r u c t u r a l
domain; it provides t h e d a t a f o r determining
d i s t r i b u t i o n of j o i n t s e t c h a r a c t e r i s t i c s on a
joint-by-joint b a s i s . C e l l mapping, which
involves measuring t h e mean o r i e n t a t i o n and
f r a c t u r e c h a r a c t e r i s t i c s f o r each f r a c t u r e s e t
within a 10 m t o 15 m (30 f t t o 50 f t ) wide
c e l l , can be done by t h e geologist during h i s
mapping of surface and underground rock exposures. This method provides t h e d a t a needed
t o evaluate v a r i a b i l i t y i n geologic s t r u c t u r e
on an a r e a l b a s i s and i s , thus, a means of
d e l i n e a t i n g s t r u c t u r a l domains.
C e l l mapping and d e t a i l l i n e mapping a r e
used i n those instances where some type of rock
exposure e x i s t s . However, i n cases i n which
s t r u c t u r e d a t a can be obtained only from d r i l l
core, a few o r i e n t e d core holes should be included i n t h e d r i l l i n g program. Oriented core
holes provide t h e same information a s d e t a i l
l i n e mapping, except t h a t oriented core d a t a
w i l l not provide j o i n t length c h a r a c t e r i s t i c s .
The o r i e n t e d core d a t a can, a l s o , a i d t h e
geologist i n h i s i n t e r p r e t a t i o n of t h e geology.
Pre-mine S t r e s s . Pre-mine s t r e s s i s one of t h e
most d i f f i c u l t parameters t o determine.
Because of t h e complex t e c t o n i c s a s s o c i a t e d
with many mineral d e p o s i t s , the s t r e s s f i e l d
w i l l probably be v a r i a b l e , depending on proximity t o t h e nearest major geologic s t r u c t u r e .
Techniques such a s s t r e s s - r e l i e f overcoring
and hydrofracturing a r e a v a i l a b l e , but they
a r e generally expensive and d i f f c u l t t o j u s t i f y
u n t i l t h e f e a s i b i l i t y of mining t h e d e p o s i t has
been e s t a b l i s h e d . The pre-mine s t r e s s f i e l d
can be estimated using t h e geologic h i s t o r y ,
o r i e n t a t i o n of geologic s t r u c t u r e s , and type
of f a u l t movement (Abel, personal communicat i o n ) . Although t h i s method i s i n d i r e c t and
could be misleading about t h e pre-mine s t r e s s
f i e l d , it i s probably b e t t e r t o use i t o r
assume a h y d r o s t a t i c s t r e s s f i e l d than t o
assume t h e e l a s t i c theory.
Hydrology. Hydrologic conditions can a f f e c t
s t r e n g t h p r o p e r t i e s of t h e rock, a s well a s t h e
c o s t of mining.
Information needed includes a
water t a b l e map, l o c a t i o n of water sources, and
l o c a t i o n s of geologic s t r u c t u r e s t h a t would be
water-bearing.
Because a pump t e s t would provide a q u a n t i t a t i v e estimate of t h e pumping r e quirements necessary during mining, one should
be made.

48

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

Rock Mechanics I n p u t f o r S e l e c t i o n
~f Mass Mining Methods
I f t h e e n g i n e e r h a s t h e n e c e s s a r y informat i o n , a s d i s c u s s e d above, he can p r o v i d e
r e a l i s t i c e s t i m a t e s on s i z e o f o p e n i n g s , supp o r t r e q u i r e m e n t s , c a v a b i l i t y , and s l o p e a n g l e s
f o r s e l e c t i n g a m i n i n g method. A t t e m p t i n g t o
determine t h e s e parameters w i l l enable t h e
e n g i n e e r t o s e e which d a t a i s c r i t i c a l i n t h e
a n a l y s i s o r i s l a c k i n g ; t h e r e f o r e , when d e v e l opment s t a r t s o r f u r t h e r e x p l o r a t i o n i s i n
p r o g r e s s , t h e d a t a c o l l e c t i o n program c a n be
properly set-up.
Open p i t . Although t h i s symposium i s c o n c e r n e d
p r i m a r i l y w i t h underground mass m i n i n g methods,
t h e open p i t method s h o u l d be c o n s i d e r e d d u r i n g
A t what d e p t h o f o v e r t h e method s e l e c t i o n .
b u r d e n t o g o underground i s p r i m a r i l y a funct i o n o f t h e m i n e r a l v a l u e and t h e s t r i p p i n g
ratio.
Using a method s i m i l a r t o t h a t p r e s e n t e d by S o d e r b e r g ( 1 9 6 8 ) , a n e s t i m a t e o f t h e
maximum s t r i p p i n g r a t i o f o r a g i v e n m i n e r a l
v a l u e was c a l c u l a t e d ( F i g u r e 2 ) . The m i n e r a l
v a l u e i s a f u n c t i o n o f t h e m a r k e t p r i c e and
t h e cut-off grade.
I n order t o estimate s t r i p p i n g r a t i o , t h e s l o p e a n g l e and t h e l i m i t o f
t h e o r e zone i n s e c t i o n a r e needed ( S o d e r b e r g ,
1 9 6 8 ) . S l o p e a n g l e c a n have m a j o r i m p a c t o n
t h e s t r i p p i n g r a t i o ; consequently, r a t h e r than
s i m p l y u s i n g a 45" s l o p e a n g l e , t h e most
r e a l i s t i c s l o p e a n g l e s h o u l d be d e t e r m i n e d
from t h e a v a i l a b l e d a t a . An a s s e s s m e n t o f t h e
f i n a l s l o p e a n g l e s c a n be made by d e f i n i n g
p o t e n t i a l f a i l u r e g e o m e t r i e s from t h e o r i e n t a t i o n o f t h e g e o l o g i c s t r u c t u r e s and t h e n choosi n g a s l o p e a n g l e t h a t m i n i m i z e s t h e number o f
daylighted structures.
I f shear strength,
l e n g t h , and s p a c i n g d a t a a r e a v a i l a b l e , a s t a b i l i t y a n a l y s i s c a n be made. With t h e e s t i m a t e s o f t h e m i n e r a l v a l u e and t h e s t r i p p i n g
r a t i o , w h e t h e r a n open p i t method s h o u l d be
c o n s i d e r e d can be d e t e r m i n e d ( F i g u r e 2 ) .

Mineral Value @/Ton ore)

Figure 2:

S t r i p p i n g R a t i o v s . M i n e r a l Value.

attrition.
However, by comparing f r a g m e n t s i z e
d i s t r i b u t i o n w i t h e x i s t i n g caving d e p o s i t s ,
u s i n g t h e same f r a g m e n t a t i o n a n a l y s i s ( F i g u r e
3 ) , c a v a b i l i t y o f t h e d e p o s i t b e i n g examined
can be determined.
The fragment s i z e d i s t r i b u t i o n c u r v e c a n b e g e n e r a t e d from d e t a i l l i n e
d a t a o r from f r a c t u r e p e r f o o t d a t a ( T a b l e 1 0 ) .
D e t a i l s o f t h e a n a l y s i s c a n b e found i n White
( 1 9 7 7 ) . Because t h e a n a l y s i s i s two-dimens i o n a l , o r i e n t a t i o n of t h e d r i l l h o l e s o r
c r o s s - s e c t i o n s a n a l y z e d s h o u l d be c o n s i d e r e d .
The f r a g m e n t a t i o n c a n a l s o b e e v a l u a t e d u s i n g
RQD and t h e c a v a b i l i t y i n d e x ( F i g u r e 4) o r
L a u b s c h e r ' s Rock Mass Reading System ( 1 9 7 7 ) .

Block caving.
During Stage 2, t h e c a v a b i l i t y
o f t h e d e p o s i t s h o u l d be examined i n g r e a t e r
d e t a i l t h a n d u r i n g S t a g e 1. Once t h e c a v a b i l i t y i s d e t e r m i n e d , t h e minimum d r a w p o i n t spaci n g , s u p p o r t a b l e d r i f t s i z e , and s u b s i d e n c e
l i m i t s h o u l d a l s o be d e t e r m i n e d .

fragment s i z e a n a l y s i s
was d e v e l o p e d by W h i t e , N i c h o l a s & Marek
( 1 9 7 7 ) . The a n a l y s i s r e s u l t s i n a d i s t r i b u t i o n o f fragment s i z e based on f r a c t u r e spaci n g , b u t it d o e s n o t i n c l u d e t h e e f f e c t s o f

C~ISpl

Inspbollon IThm1.n

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Y

d I21111

4. W h - 0
5. lakI.hm.

The c a v a b i l i t y o f a d e p o s i t i s d e t e r m i n e d
by t h e f r a g m e n t s i z e d i s t r i b u t i o n a t t h e drawp o i n t and t h e u n d e r c u t w i d t h r e q u i r e d t o s u s t a i n a cave.
I f t h e fragment s i z e i s c o a r s e ,
t h e u n d e r c u t w i d t h may be g r e a t e r t h a n t h e
width o f t h e d e p o s i t , o r t h e drawpoints w i l l
be p l u g g e d much o f t h e t i m e , t h e r e b y r e d u c i n g
m i n i n g r a t e and i n c r e a s i n g s e c o n d a r y b l a s t i n g
cost.
A two-dimensional

SIEVE

SIZE

11100 L..Il

*dl"

P ~ l l0 . 0 ~ 1 1 ~
Yushw

S l h

IN1

F r a g n e n t S i z e D i s t r i b u t i o n Curves o f
some E x i s t i n g Block Caving Mines
( a f t e r White, 1977)
D a t a a r e from l i m i t e d a r e a s and d o n o t
n e c e s s a r i l y r e p r e s e n t a n average f o r
t h a t mine.

F i g u r e 3:

NOTE:

17th L...(l

Y u h

METHOD SELECTION

- A NUMERICAL APPROACH

Table 10:

a r c h ( F i g u r e 5a) h a s t h e p o t e n t i a l f o r caving.
However, t h e maximum t r a n s f e r d i s t a n c e can be
reduced by some t y p e of boundary weakening.

vf
Percent r e t a i n e d a t s i z e X = Vt
6N
where V = t o t a l volume = t
~3
V

49

= volume g r e a t e r t h a n s i z e X

-.

B = ( l / f r a c t u r e spacing) *
6
;
N = number o f fragments i n sample; and
X = fragment s i z e t o be analyzed.

Data Pointm
rrm c l i m a x
and u r a d

-.-.
..
..

TRANSPER I
DISTANCE
a . s t ~ p . w i d t h i n 2~ maxim?
tranmfar dimtance

PILWLR

PIWAR
TRANSPER
I DISTANCE

TRANSFER
I DISTANCE

'

b . S t o p e w i d t h im g r e a t e r t h a n
231 maximum t r a n m f e r d i a t a n c e

F i g u r e 5:

P r e s s u r e Arch Concept.

CAVABILITX INDEX ( C I )

F i g u r e 4:

RQD v s . C a v a b i l i t y Index

( a f t e r McMahon and Kendrick, 1959)

Undercut w i d t h r e q u i r e d t o s u s t a i n a cave i s
most c r i t i c a l f o r t h o s e d e p o s i t s where t h e f r a g mentation i s c o a r s e and t h e a v e r a g e u n d e r c u t
width of t h e d e p c s i t is l e s s t h a n approximately
150 m (500 f t ) . Using L a u b s c h e r ' s c l a s s i f i c a t i o n (1977) o r t h e p r e s s u r e a r c h concept
(Alder e t a l . , 1951) , t h e u n d e r c u t w i d t h
r e q u i r e d t o s u s t a i n a cave can be e s t i m a t e d .
Laubscher p r o v i d e s a n h y d r a u l i c r a d i u s , a r e a /
p e r i m e t e r , f o r h i s f i v e c l a s s e s of rock.
In
t h e pressure arch concept, t h e rock i s conside r e d t o have a maximum d i s t a n c e t h a t it can
t r a n s f e r t h e l o a d ( F i g u r e 5 ) . The a b i l i t y o f
t h e rock t o t r a n s f e r a v e r t i c a l s t r e s s i n a
l a t e r a l d i r e c t i o n o v e r an underground opening
depends on t h e s h e a r s t r e n g t h of t h e r o c k , t h e
h o r i z o n t a l s t r e s s , and t h e s t r e n g t h o f t h e
rock p i l l a r s . Although e a c h d e p o s i t h a s i t s
own maximum t r a n s f e r d i s t a n c e , a c o r r e l a t i o n
between depth and maximum t r a n s f e r d i s t a n c e
has been determined ( F i g u r e 6 )
Based on t h e
p r e s s u r e a r c h c o n c e p t , i f t h e u n d e r c u t width
does n o t exceed t w i c e t h e maximum t r a n s f e r
d i s t a n c e then o n l y t h e r o c k under t h e p r e s s u r e

DEPTH

Figure 6:

(10

T r a n s f e r D i s t a n c e v s . Depth.

Once it h a s been determined t h a t t h e d e p o s i t

i s c a v a b l e , drawpoint s p a c i n g and g a t h e r i n g
d r i f t s i z e should be determined f o r t h e g e n e r a l
mine d e s i g n .
Drawpoint s p a c i n g i s p r i m a r i l y a f u n c t i o n of
t h e o r e and o v e r l y i n g w a s t e fragment s i z e d i s t r i b u t i o n and t h e p i l l a r s t r e n g t h . The g e n e r a l
consensus h a s been t h a t t h e s m a l l e r t h e f r a g ment s i z e t h e narrower t h e width of draw, cons e q u e n t l y , t h e c l o s e r t h e drawpoint s p a c i n g .
Also, when t h e o v e r l y i n g m a t e r i a l i s more f r a g mented t h a n t h e o r e , t h e drawpoint s p a c i n g

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

shculd bc; cl.oser- i n :,rder tcj miriirnize di.Lutiori.
However, comparison of e x i s t i n g p r o p e r t i e s i n d i
c a t e s t h e c o r r e l a t i o n between fragment s i z e and
draw width a r e a i s weak (Figure 7) ; e s p e c i a l l y
c o n s i d e r i n g t h e i n d i c a t i o n s from t h e Henderson
Mine where t h e o r e i s moderately t o w e l l f r a g mented, t h e drawpoint spacing i s wide, 1 2 . 2 m
X 1 2 . 2 m (40 f t X 40 f t ) , and t h e o r e recovery
appears t o be good. The ground between t h e
drawpoints can be considered a p i l l a r (Figure
8 ) , and, i f analyzed a s such, it can be used
t o determine t h e minimum drawpoint spacing.
The load on t h e p i l l a r i s t h e most d i f f i c u l t
parameter t o determine. The worst loading cond i t i o n occurs when t h e undercut i s w i t h i n 100
f t of t h e p i l l a r and t h e rock i s being loaded
by t h e abutment s t r e s s e s . Kendorski (1975)
e s t i m a t e s t h a t t h i s abutment loading i s two
t i m e s t h e overburden s t r e s s , while Panek
(1978) e s t i m a t e s t h a t i t i s t h r e e t i m e s t h e
overburden s t r e s s . Using t h r e e t i m e s t h e
t r i b u t a r y - a r e a - l o a d t o determine load on t h e
p i l l a r and W i l s o n ' s (1972) p i l l a r a n a l y s i s t o
determine load c a r r y i n g c a p a c i t y , a minimum
drawpoint s p a c i n g can be e s t i m a t e d . Using t h e
fragmentation curves and t h e graph i n F i g u r e 7
and t h e p i l l a r a n a l y s i s , an e s t i m a t e of t h e
drawpoint s p a c i n g can be made.

Pillar

CRE IGHTON

Figure 8:

DRAW AREA (M')

Figure 7:

A r e a loading pllla:

Fragment S i z e v s . Draw Area

( a f t e r White, 1979) .

The o r e g a t h e r i n g d r i f t s i z e and support

r e q u i r e d a r e important i n e s t i m a t i n g c o s t of
t h e mining method.
The d r i f t s should be
o r i e n t e d s o a s t o minimize p o t e n t i a l f a i l u r e
g e o m e t r i e s , which a r e u s u a l l y normal t o t h e
s t r i k e of t h e predominant s t r u c t u r e s . Laubs c h e r (1977) and Barton and Lunde (1974) have
c o r r e l a t e d t h e i r rock c l a s s i f i c a t i o n s t o supp o r t requirements.
Because B a r t o n ' s work was
p r i m a r i l y on t u n n e l s , which g e n e r a l l y have
more s u p p o r t t h a n a d r i f t i n a mining operat i o n , h i s work may not be a p p l i c a b l e t o

D e f i n i t i o n of P i l l a r between
Drawpoints.

determining s u p p o r t requirements f o r a mine.

I f one of t h e s e c l a s s i f i c a t i o n systems i s used,
t h e rock c l a s s e s can be i n t e r p r e t e d on t h e
l e v e l maps, where t h e g a t h e r and haulage d r i f t s
a r e shown. From t h e s e l e v e l maps, t h e percent
of a r e a t h a t t h e d i f f e r e n t support systems w i l l
be r e q u i r e d can be determined and t h e support
c o s t estimated.
I f n e i t h e r of t h e s e systems
have been used b u t information on t h e o r i e n t a t i o n , s p a c i n g , s h e a r s t r e n g t h , and p o s s i b l y
l e n g t h of t h e j o i n t s e t s and f a u l t systems i s
a v a i l a b l e , t h e s u p p o r t required f o r t h e d r i f t s
can be determined by (1) i d e n t i f y i n g p o t e n t i a l
f a i l u r e g e o m e t r i e s , ( 2 ) determining t h e load a t
t h e edge of t h e opening, and ( 3 ) determining
which support s y s t e m ( s ) can c a r r y t h e load
calculated i n s t e p 2.
The subsidence l i m i t should be d e f i n e d f o r
l o c a t i n g b u i l d i n g s and s h a f t s which a r e t o l a s t
the l i f e of t h e deposit.
In t h e absence of a
major g e o l o g i c s t r u c t u r e , a 45" angle p r o j e c t e d
o n t o t h e s u r f a c e from t h e bottom of t h e ore
zone i s u s u a l l y considered t h e c l o s e s t t o t h e
d e p o s i t one should l o c a t e long-term f a c i l i t i e s .
However, most a c t u a l ground movement t a k e s

METHOD SELECTION

- A NUMERICAL APPROACH

p l a c e w i t h i n a 60" a n g l e from t h e d e p o s i t . I f
a major f a u l t e x i s t s , it w i l l p r o b a b l y c o n t r o l
t h e l i m i t of s u b s i d e n c e .
Stoping. The two i m p o r t a n t p a r a m e t e r s i n t h e
economics of a s t o p i n g method f o r which a r o c k
mechanics s t u d y can p r o v i d e e s t i m a t e s a r e t h e
width of t h e s t o p e s and t h e s i z e o f t h e p i l lars.
I n sublevel stoping, t h e width of a stope
i s a f u n c t i o n o f t h e immediate and i n t e r m e d i a t e
The immediate r o o f
roof (Alder and Sun, 1968)
i s c h a r a c t e r i z e d by t h e p r e s s u r e a r c h c o n c e p t
already discussed.
The maximum s t o p e w i d t h i s
t w i c e t h e maximum p r e s s u r e a r c h . P i l l a r s
spaced t h i s d i s t a n c e must be a b l e t o c a r r y
tributary-area-load.
The immediate r o o f i s
t h a t ground under t h e p r e s s u r e a r c h which w i l l
behave a s beam, p l a t e , o r a r c h . J o i n t o r i e n t a t i o n , spacing, and l e n g t h can be used t o d e f i n e
I n many i n s t a n c e s , t h e beam
t h e s t o p e width.
developed by b o l t i n g can be used. The p i l l a r s
w i t h i n t w i c e t h e maximum t r a n s f e r d i s t a n c e d o
n o t have t o c a r r y t r i b u t a r y - a r e a - l o a d , b u t
r a t h e r t h e l o a d under t h e p r e s s u r e a r c h , h a l f way t o t h e n e x t s u p p o r t . Using Wilson's p i l l a r
a n a l y s i s (1972) and t h e p o t e n t i a l f a i l u r e
geometries through t h e p i l l a r , t h e p i l l a r l o a d
c a r r y i n g c a p a c i t y can be d e t e r m i n e d ( N i c h o l a s ,
1976).

F i g u r e 9:

E c c e n t r i c i t y v s . Height o f Draw
E l l i p s o i d ( a f t e r J a n e l i d and K v a p i l ,
1966).

SUBLEVEL INTERVAL

SUBLEVEL IMERVAL

For s h r i n k a g e s t o p i n g , t h e same t y p e o f
a n a l y s i s needs t o be made a s f o r s u b l e v e l
stoping, except t h a t t h e c a v a b i l i t y o f t h e
o v e r l y i n g rock h a s t o b e e v a l u a t e d .
Support
requirements can b e e s t i m a t e d , a s d i s c u s s e d
under block caving.
S u b l e v e l caving. F o r s u b l e v e l c a v i n g , r o c k
mechanics d a t a on t h e c a v a b i l i t y o f t h e hanging wall, the sublevel d r i f t s i z e , t h e support
needed, and t h e s p a c i n g between t h e s u b l e v e l
J a n e l i d and Kvapil (1966)
d r i f t s is required.
have p r e s e n t e d g u i d e l i n e s f o r t h e l a y o u t o f a
s u b l e v e l mine. The hanging w a l l must come in
behind t h e o r e zone; o t h e r w i s e s u b l e v e l c a v i n g
w i l l n o t work. Using a n a l y s e s s i m i l a r t o t h o s e
i n block caving w i l l p r o v i d e an e s t i m a t e o f t h e
dimension needed t o i n i t i a t e t h e cave and t h e
fragment s i z e d i s t r i b u t i o n . J a n e l i d and Kvapil
a l s o r e l a t e d d r i f t s i z e t o t h e required width
of draw. Another a s p e c t o f a s u b l e v e l d e s i g n
is t h e support required f o r t h e s e d r i f t s .
If
e x t e n s i v e s u p p o r t i s r e q u i r e d , t h e method may
n o t be f e a s i b l e .
Support requirements can be
estimated, a s discussed previously.
V e r t i c a l s p a c i n g of d r i f t s i s mainly a
f u n c t i o n o f equipment, b u t t h e h o r i z o n t a l spaci n g between d r i f t s i s d e t e r m i n e d by t h e w i d t h
of t h e draw e l l i p s o i d and t h e s t a b i l i t y of t h e
rock. J a n e l i d and K v a p i l r e l a t e d d r i f t spaci n g t o t h e d i s t a n c e between s u b l e v e l s and
t h e e c c e n t r i c i t y of t h e e l l i p s o i d (Figure 9 ) .
The ground between t h e d r i f t s can be c o n s i d e r e d p i l l a r s ( F i g u r e 1 0 ) and a n a l y z e d a s such.

I
DRIFT SPACING

F i g u r e 10:

DRIFT
HEIGHT
%RIFT
WIDTH

S u b l e v e l Caving Geometry.

The w o r s t l o a d c o n d i t i o n
n e a r e s t t h e cave. T h e r e
i n g o c c u r r i n g , which c a n
s t a b i l i t y of p i l l a r s c a n

o c c u r s f o r t h e ground
i s some abutment l o a d b e e s t i m a t e d , and t h e
be determined.

Concluding Comments
Mining method s e l e c t i o n s h o u l d be based p r i m a r i l y on t h e geometry and g r a d e d i s t r i b u t i o n
o f t h e d e p o s i t , t h e r o c k mechanics c h a r a c t e r i s t i c s of t h e o r e z o n e , hanging w a l l and f o o t w a l l , and on t h e mining and c a p i t a l i z a t i o n
c o s t , with f i r s t p r i o r i t y given t o t h e rock
mechanics c h a r a c t e r i s t i c s .
S e l e c t i o n of t h e
mining method s h o u l d o c c u r i n two s t a g e s .
S t a g e 1:

D e f i n e t h e geometry/grade d i s t r i b u t i o n and r o c k mechanics c h a r a c t e r i s t i c s o f t h e d e p o s i t and r a n k

t h e mining methods a c c o r d i n g t o
t h e i r a b i l i t y t o accommodate t h e s e
characteristics.

52

DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES

S t a g e 2:

Develop a n i n i t i a l mine p l a n o f
t h e two o r t h r e e h i g h e s t r a n k i n g
m i n i n g methods t o p r o v i d e a b e t t e r
e s t i m a t e o f t h e mining and c a p i t a l i z a t i o n c o s t and t o d e t e r m i n e
c u t - o f f g r a d e and m i n a b l e r e s e r v e s .
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ACKNOWLEDGMENTS
The a u t h o r w i s h e s t o e x p r e s s h i s g r a t i t u d e
t o Susan Jones and Lynn McLean f o r t h e i r e d i t o r i a l review o f t h i s p a p e r .
P o r t i o n s of t h i s p a p e r a r e t a k e n from "The
F e a s i b i l i t y Study - S e l e c t i o n o f a Mining
Method I n t e g r a t i n g Rock Mechanics and Mine
Planning" (NichoJas and Marek, 1981) , p r e s e n t e d a t t h e 1981 Rapid Excavation and
Tunneling Conference