-
SELECTION OF MASS UNDERGROUND MINING METHODS Chapter 3 Dennis H.
Laubscher
M i n i n g Geology D i r e c t o r , Shabanie and Mashaba Mines
( P r i v a t e ) L im i ted ,
Bulawayo, Zimbabwe
INTRODUCTION
The s e l e c t i o n o f mass m in ing methods i s t h e s i t
u a t i o n f a c i n g n o t o n l y p l a n n e r s o f new opera
t ions , bu t a l s o those i n v o l v e d w i t h cur- r e n t
opera t ions , i n c l u d i n g t h e e v e n t u a l change from
open-pi t t o underground. I n c r e a s i n g ground c o n t r o l
problems on many mines, p l u s a b e t t e r unders tand ing o f d
i l u t i o n and o r e l o s s problems have meant a rev iew o f c
u r r e n t m i n i n g methods and suppor t techn ioues. Exper
iments a r e o n l y success fu l i f conducted i n t h e r i g h t
min ing environment and on t h e r i g h t sca le . M in ing
personne l have been known f o r t h e i r con- serva t ism and w i
l l tend t o persevere w i t h an es tab l i shed method r a t h e
r t h a n change; o f t e n , h idden c o s t s w i l l obscure t h
e c o r r e c t economic assessment. An e s t a b l i s h e d m i n
i n g sequence i s n o t n e c e s s a r i l y c o r r e c t i n t
h e l o n g term. However, recommendations t o change a re u s u a
l l y met w i t h o p p o s i t i o n because o f inconven ience,
temporary p r o d u c t i o n problems and t h e d e s i r e f o r
a t r o u b l e - f r e e guarantee. I n any p l a n n i n g exerc
ise , care must be taken t h a t t i m e and money a re n o t
wasted i n t r y i n g t o ach ieve t h e una t ta inab le . D u r
i n g t h e dec is ion-mak ing per iod , p r o d u c t i o n pe
rsonne l must bs i n v o l v e d , and, i f poss ib le , t hey shou
ld v i s i t s i m i l a r o m r a t i o n s t o t h a t proposed.
Co-operat ive p r o d u c t i o n pe rsonne l w i l l h e l p
ensure success.
D e s c r i p t i o n s o f m i n i n g methods a r e a v a i l
a b l e i n t e c h n i c a l papers and books, and i t i s n o t i
n tended t o d i s c u s s t h i s aspect , bu t , r a t h e r , t
h e v a r i o u s f a c t o r s wh ich must be recogn ised b e f o
r e a r r i v i n g a t a f i n a l s e l e c t i o n . I n another
paper p resen ted a t t h i s Conference ( ~ e s l o p and
Laubscher, 1981) f a c t o r s a f f e c t i n g o r e recovery and
d i l u t i o n a r e d i scussed i n g r e a t e r d e t a i l f o
r v a r i o u s cave m i n i n g methods.
D e t a i l e d r e f e r e n c e s a r e made t o two ma jo r c
h r y s o t i l e asbestos mines i n Zimbabwe, namely Shabanie N
ine a t Shabani and K i n g Mine a t Mashaba. These d e p o s i t s
a r e l o c a t e d i n com- p l e x g e o l o g i c a l env i
ronments w i t h a wide range o f ground c o n d i t i o n s ( ~ a
u b s c h e r , 1960). The o r i n c i p l e s o u t l i n e d i n
t h i s paper can be a p p l i e d t o any massive d e p o s i t
.
PRELIMINARY CONSIDERATIONS
Comprehensive G e o l o q i c a l Data. G e o l o g i c a l i n
f o r m a t i o n i s a v a i l a b l e d u r i n q t h e e x ~ l o
r -
a t i o n stage, bu t , o f t e n , co re f rom e x p l o r a t
i o n d r i l l i n g i s n o t ana lysed w i t h t h e s e l e c t
i o n o f a m in ing method i n mind. D r i l l i n g c o s t s a r
e h igh , t h e r e f o r e t h e maximum b e n e f i t s must be d
e r i v e d f rom t h i s ope ra t i on . Ho les wh ich have n o t
i n t e r s e c t e d o r e may p r o v i d e d a t a on p o s s i
b l e s h a f t s i t e s , and 35 mm c o l o u r s l i d e s o f
core, sound l o g g i n g and geomechanics c l a s s i f i c a t i
o n , w i l l p r o v i d e a r e c o r d wh ich can be used w i t
h conf idence a t a l a t e r da te . I n t e r s e c t i o n s o f
t h e v a r i o u s r o c k t y p e s shou ld be k e p t f o r f u
t u r e study; f o r example, i s weather ing l i k e l y t o be a
problem, b e a r i n g i n mind t h a t t h i s can be a c c e l e
r a t e d i n t h e warm, humid atmosphere o f an underground
mine.
The accumula t ion o f d a t a r e q u i r e d t o s e l e c t a
m i n i n g method s t a r t s w i t h i n i t i a l g e o l o g i
c a l work i n t h e t a r g e t area, whether t h i s be sur- f
ace mapping o r a diamond d r i l l ho le .
P u b l i c a t i o n s . As t e c h n i c a l papers, r e p o r
t s and books a r e used i n t h e s e l e c t i o n o f m i n i n
g methods, an appea l i s made t o a u t h o r s t o des- c r i b e
t h e g e o l o g i c a l environment so t h a t read- e r s can r
e l a t e t o t h e i r own c o n d i t i o n s . Not enough
emphasis i s g i v e n t o t h e problems exper ienced and t h e
techn iaues employed t o overcome them. Fol low-up papers a r e
seldom w r i t t e n and we shou ld be prepared t o d i s c u s s
our f a i l u r e s . Diagrams a r e o f t e n m is lead ing , as t
h e i d e a l and n o t a c t u a l s i t u a t i o n s a r e dep i
c ted . The caved ore/waste i n t e r f a c e i s n o t a s t r a i
p h t l i n e , and whereas t h i s may n o t be s i g n i f i c a
n t i n a d iagram o f a 200-117 draw h e i g h t , i t c e r t a i
n l y i s i n sub - l eve l c a v i n g w i t h draw zones o n l y
1 - 2 m w i d t h b u t up t o 20 m h igh .
SUMMARY OF FACTORS AFFECTING THE SELECTION OF UNDERGROUND MASS
MINING METHODS
(a ) Reg iona l r o c k s t r e s s e s and m i n i n q
geometry. (b ) The geomechanics r o c k mass c l a s s i f i c a t
i o n
o f t h e orebody and su r round ing r o c k mass.
( c ) R a t i o o f t h e s u r f a c e a rea o f t h e
ore/unpay i n t e r f a c e t o t h e con ta ined o r e wh ich
determines o r e l o s s e s and d i l u t i o n .
(d ) The c a v a b i l i t y and f ragmen ta t i on d a t a o f
t h e o r e and hang ingwa l l .
( e ) The m i n e r a l d i s t r i b u t i o n i n t h e
orebody and i t s d i l u t i o n zone.
23
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MASS UNDERGROUND MINING METHODS ( f ) Location, s t reng th o f
e x t r a c t i o n hor izons
and layou t geometry.
(9) The mining sequence as determined by the e f f e c t o f a
product ion b lock on surround- i n g orebodies and i n s t a l l a
t i o n s .
(h) The need o r no t t o mainta in r e g i o n a l s t a b i l
i t y .
(i) Model studies. ( j) Role o f rock mechanics. (k) Adherence t
o a l o g i c a l p lanning schedule. (1) The degree o f soph i s t
i ca t i on poss ib le i n
t h a t s o c i a l environment, based on ava i la - b i l i t y
o f s p e c i a l i s t s k i l l s o r a predom- inance o f u n s
k i l l e d labour.
REGIONfiL ROCK STRESSES AND NINING GEOMETRY
This i e a f a c t o r o f t e n ignored i n mine plan- ning,
poss ib ly because o f a l a ck o f app rec i a t im o f i t s s i
gn i f i cance and a lso the cost, which i s minor i f compared w i
t h t he cos t o f t he p ro jec t .
Streee measurements are sometimes regarded as an academic exerc
ise by p r a c t i c a l min ing men because the b e n e f i t s
have no t been expla ined o r h igh l ighted. Perhape the blame l i
e s on both sides.
L i ke a l l techniques, once the l i m i t a t i o n 8 are
known, s i g n i f i c a n t b e n e f i t s can be derived. The
magnitude and r a t i o o f s t resses and t h e i r v a r i a t i
o n w i t h depth a re a l l t h a t i s r equ i r ed t o be known.
L i t h o l o g i c a l changes and s t ruc - tu res w i l l a f f
e c t the magnitude and d i r e c t i o n o f readings. W i t h i n
a geo log i ca l compart- ment, v a r i a t i o n s can be
expected, f o r , i f compressional o r t ens i ona l cond i t i
ons were the r e s u l t o f the t e c t o n i c cycle, t he r e s
u l t a n t imp r i n t s can be measured (Fig. 1). There are c e r
t a i n s i t u a t i o n s where t h e r e g i o n a l s t resses
w i l l be i n s i g n i f i c a n t i n r e l a t i o n t o o t he
r factors.
I n s t a r t i n g a new mine, s i t e s may n o t be ava i lab
le t o measure t h e r e g i o n a l stresses. I n t h i s s i t ua
t i on , i n f o rma t i on a v a i l a b l e i n t he d i s t r i
c t and an i n t e r p r e t a t i o n o f t h e t e c t o n i c
cyc les ehould enable one t o assess whether t h e h o r i z o n t
a l o r v e r t i c a l s t r ess i s dominant. Stress measurements
have been done i n many qa r t s o f t he wor ld and i t w i l l be
noted tha t , i n s h i e l d areas and f o l ded be l t s , h o r
i z o n t a l stresses are dominant. Thus, where t he geology shows
h i gh l a t e r a l stresses, t h e odds are t h a t h o r i z o n
t a l s t resses are dominant. I n Zimbabwe, s t r ess measurements
conducted a t lihabanie Nine (asbestos), Ga thG Nine (asbestos) and
Dalny Mine (go ld ) have shown s i m i l a r r e s u l t s ( ~ i g
. 2).
M)RIZONTAL STRESS (HI S SHABANIE MlNE G GATHS MlNE D DALNY
MlNE
-AFFECTED BY STOANG
FIGURE 2 STRESS DEPTH RELATIONSHIPS
The e f f e c t o f h i gh h o r i z o n t a l s t resses on
mine design are i l l u s t r a t e d by t he f o l l o w i n g two
examples a t Shabanie.
Block 16. Th is orebody, o f f i v e m i l l i o n tonnes, w i t
h dimensions o f 350 m x 110 m, was planned as a sub-level caving
operat ion. Based on experience on t h e upper l e ve l s , i t was
considered t h a t an area o f 80 m x 60 n would have t o be mined
by h o r i z o n t a l c u t s i n order t o i n i t i a t e t he
caving o f the competent ( c l a s s 2 ) , p a r t i a l l y se
rpen t in i sed dun i te hangingwal l . Caving f i n a l l y took p
lace when an ad jacent b lock caved, by which t ime the area mined
was 140 m x 110 m. The h igh h o r i z o n t a l s t resses r esu l
t ed i n increased f r ic t ion/compress ion on t he v e r t i c a
l j o i n t s and a s t a b i l i s a t i o n o f t h e back i n an
arch shape. The removal o f t he east-west h o r i z o n t a l s t
r ess component by t h e caving o f t h e b lock t o t h e west r a
s u l t e d i n t h ~ o r o ~ a g a t i o n o f t he cave. S im i l
a r s t a b l e s i t - ua t ions occurred i n b lock caving opera
t ions a t Lrad endri rick, 1970) and R i o Blanco (ca rpen te r
and Woolfe, 1972) both environments e f h i g h h o r i z o n t a l
s t ress.
Block 52. The dec i s i on t o mine t h i s b lock
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DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES i l t
h open stones and pos t f ' i l l i n q , was aased gn [ i o t o r
~ t i a l Door orebody f ragmenta t ion , a com- n e t e n t
hancjinowall, h i q h h o r i z o n t a l s t r e s s e s an? exper
ience elsewhere where v o i d s o f 80 m x 150 m were s t i l l i n
e x i s t e n c e a f t e r 25 years. The o n l y d i f f e r e n c
e i n t h e m i n i n g method was t h a t , whereas, p r e v i o u
s l y , m i n i n g had been by h o r i z o n t a l s l i c e s , i
n t h i s case a no r th -sou th v e r t i c a l s l o t , 60 m l o
n g x 10 m wide x 48 m - 60 m h i g h was c u t w i t h a p lanned
r e t r e a t eas t and west. When planned, t h e s t r e s s
magnitude o f 17 MPa i n t h e s t r i k e d i r e c t i o n had
been e x t r a p o l a t e d f rom da ta on t h e upper l e v e l s
, and i t was assumed t h a t a s u i t a b l e s t r e s s
environment e x i s t e d . Subsequent s t r e s s meas- urements
done i n t h e v i c i n i t y showed t h e v a l u e t o be 25
NPa, an i n c r e a s e o f 47%. The s t r e s s c o n c e n t r a
t i n g f a c t o r s i n t h e back o f t h e v e r t i c a l s l
o t meant va lues o f 60 MPa t o 80 NPa, r e s u l t i n g i n f a
i l u r e o f t h e rock mass, p a r t i c u - l a r l y where a
dominant s t r u c t u r e i n t e r s e c t e d t h e back. Wi th
t h e loosened r o c k s f a l l i n g o u t under g r a v i t y ,
t h e f a i l u r e zone propagated up- wards, i n c r e a s i n g
t h e h e i g h t o f t h e s l o t and t h e adverse aspect r a t
i o . I t was o n l y when t h e upper l e v e l was r a p i d l y
r e t r e a t e d westwards and eastwards t h a t t h e geometry
was changed and some degree o f s t a b i l i t y occur red.
However, e x t e n s i v e d i l u t i o n had occu r red and p r o
d u c t i o n problems were exper ienced. The o r i e n t a t i o n
o f t h e s l o t w i t h r e s p e c t t o a h i g h h o r i z o n
t a l s t r e s s , t h e presence o f a s teep-d ipp ing s t r u c
- t u r e n o t l o c a t e d by t h e v e r t i c a l hang ingwa l
l bo reho les and r a p i d wea the r i ng o f t h e hanging- w a l
l dun i t e , were t h e c o n t r i b u t o r y f a c t o r s
.
GEONECHANICS ROCK MASS CLASSIFICATION
I t i s s u r p r i s i n g t h a t r o c k mass c l a s s i f i
- c a t i o n s have n o t been u n i v e r s a l l y accepted by t
h e g e o l o g i c a l p r o f e s s i o n o r by m i n i n g com-
pan ies . I n f a c t , i t was c i v i l eng ineers who saw t h e
need f o r a means o f communication w i t h c o n t r a c t o r s
who s t i m u l a t e d t h e development o f c l a s s i f i c a t
i o n systems. The need f o r a means o f communication and an
unders tandab le des- c r i p t i o n o f rock mass p r o p e r t i
e s is presen t i n t h e m in ing i n d u s t r y . Vague d e s c
r i p t i v e terms, such as "good", " f a i r " and "poor" serve
no purpose because f a i r ground on a mine w i t h ground problems
would be poor ground on a mine where rock b o l t s a r e a r a r i
t y . We f i n d so o f t e n t h a t sound g e o l o g i c a l
work i s n o t recog- n i s e d by m in ing pe rsonne l because t h
e in form- a t i o n cannot be passed on w i t h o u t t hose pe
rsonne l hav ing d e t a i l e d g e o l o g i c a l knowledge.
What we r e q u i r e i s t h a t t h e m i n i n g g e o l o g i s
t assembles a l l h i s f a c t s and p resen ts them i n a form
accep tab le t o a l l concerned w i t h t h e m i n i n g ope ra t
i on .
A r ock mass c l a s s i f i c a t i o n system must r e c o g n
i s e a l l t h e g e o l o g i c a l f a c t o r s wh ich a f f e
c t t h e i n - s i t u s t r e n g t h o f t h e r o c k mass. A c
l a s s i f i c a t i o n techn ique must be s t r a i g h t - f o
rward so t h a t i t forms p a r t o f normal mine o e o l o q i c
e l i n v e s t i g a t i o n s . H i g h l y sophis-
t i c a t e d techn iques a r e time-consuming and most mines
cannot a f f o r d t h e l a r g e s t a f f r e q u i r e d t o p
r o v i d e complex d a t a o f d o u b t f u l p r a c t i c a l b
e n e f i t . The approach adopted i s t h a t t he r o c k mass i
s assigned an i n - s i t u va lue regard- l e s s o f i t s p o s
i t i o n i n space. To decide how t h e rock mass w i l l behave
du r i ng mining, t h e r a t i n g s a r e a d j u s t e d f o r
weathering, f i e l d and induced s t resses, changes i n s t r e s
s caused by mining, t h e o r i e n t a t i o n and t y p e o f
excavat ion and t h e e f f e c t o f b l a s t i n g (~aubsche r ,
1977).
The accuracy o f t h e geomechanics c l a s s i f i - c a t i o
n depends on t h e sampl ing o f t h e area b e i n g i n v e s t i
g a t e d . A s imple statement, b u t so o f t e n i g n o r e d
when expend i tu re on geo- t e c h n i c a l i n v e s t i g a t i
o n s i s kep t t o a minimum. C l a s s i f i c a t i o n d a t a
must be prov ided a t an e a r l y s tage t o ensure c o r r e c t
dec i s i ons on m i n i n g methods, l a y o u t and suppor t
requirements. D u r i n g e x p l o r a t i o n , development i s l
i m i t e d and bo reho les a r e t h e main source o f i n fo rma
t i on . Boreho les d r i l l e d f o r v a l u a t i o n purposes
may n o t p r o v i d e s u f f i c i e n t coverage f o r s t r u
c t u r a l i n t e r p r e t a t i o n and r o c k mass c l a s s
i f i c a t i o n . D e t a i l e d g e o l o g i c a l knowledge o
f t h e area, bo th on s u r f a c e and underground, d e f i n e s
t h e s t ruc- t u r a l u n i t s , which, combined w i t h p r o
p e r l y s i t e d boreholes, p r o v i d e s t h e da ta f o r
mine p lann ing .
I n t h e geomechanics c l a s s i f i c a t i o n developed by
Laubscher (1977), a v a l u e r a t i n g o f 0 - 100 i s used t o
cover a l l v a r i a t i o n s i n j o i n t e d rock masses f rom
ve ry good t o ve ry poor. The c l a s s i f i c a t i o n i s d i
v i d e d i n t o f i v e classes, w i t h v a l u e r a t i n g s
o f 20 per c lass , and each c l a s s i s sub-div ided i n t o an
A and B sub-class w i t h a 10-po in t r a t i n g ( ~ i g .
3).
The accuracy o f a c l a s s i f i c a t i o n system must be v
iewed w i t h r e s p e c t t o t he prec iseness o f m i n i n g
methods and suppo r t systems. The i n - s i t u r a t i n g s
measured by competent personnel show v e r y l i t t l e v a r i a
t i o n . C r i t i c i s m i s sometimes l e v e l l e d a t t h e
accuracy o f t h e ad jus tments . The i m p o r t a n t p o i n t
w i t h t h e ad jus tments i s t h a t i t makes personne l t h i
n k i n t e rms o f what t h e excava t i on o r min ing oper- a t
i o n w i l l do t o t h a t r o c k mass. Adjustments a r e
summarised i n Tab le I, which a l s o i l l u s - t r a t e s an
example where these adjustments are a p p l i e d t o an i n - s i
t u r a t i n g o f 60 by two p l a n n i n g o f f i c i a l s , A
and B.
Both A and B recogn ised t h a t t h e rock mass, which, i n i n
i t i a l exposures, would be c lassed as good ground, would d e t
e r i o r a t e w i t h mining, and, t h e r e f o r e , suppo r t
should be i n s t a l l e d a t an e a r l y date. A would
recommend pa t te rned g rou ted b o l t s a t 1 m spac ing w i t h
50 mm mesh r e i n f o r c e d sho tc re te , whereas t h e more
con- s e r v a t i v e 0 would recommend 0,75 m b o l t spac ing
and 75 mm sho tc re te . The d i f f e r e n c e i n c o s t would
n o t be s i g n i f i c a n t , b u t what i s i m p o r t a n t i
s t h a t t h e recommended support would do i t s work i n . t h e
c o r r e c t environment and n o t be i n s t a l l e d i n t o a
f a i l e d rock mass
-
MASS UNDERGROUND MINING METHODS
B. BASIS OF THE CLASSIFICATION
D. ASSESSMENT OF JOINT CONDITIONS C. RATINGS FOR
MULTI JOINT SYSTEMS
MAXIMUM SPACING EXAMPLES W C I N G S A:a;ZmB~Q45mC;0,5m D:lO E
& 7 U T M G S A.19 ABr13 ABC-5 A B D d ME-13
t GNORE TMS MCTOR FOR STRAICHT,KLISHED OR STRAIGHT SMOOTH J O R
m
FIGURE 3 THE GEOME CHANICS CLASSIFICATON OF JOINTED ROCK
MASSES
A. MEANING OF THE RATINGS
CLASS
RATING(x~-L OF Bl
DESC RIPTION
1
A B
Y)O - 81
VERY GOOD
5
A B
20 - 0
VERY POOR
2 4
A B A B
8 0 - 61
GOOD
60 - 41
FA1 R
40 - 21
POOR
-
0 H)
(Lo
50
100 m
1
I I
I I
J SCALE
ESMC
LASS
I A,B
PZ
l CLA
SS 2
AB
)+CLA
SS
3 A,
B a
cL
A~
s
4 AP
CL
ASS
5 A,
B I i
-
MASS UNDERGROUND MINING METHODS and thereby be cons ide rab l y
l e s s e f f e c t i v e . be capable o f suppo r t i ng wedges o
f 1 000 tonnes
o r more.
TABLE I
Weathering F i e l d and Induced
St resses
Changes i n S t r e s s S t r i k e and O ip B l a s t i n g
Average
Examples o f i n - s i t u r a t i n g s f o r d i f f e r e n t
o rebod ies a r e shown i n F ig . 4. I n t h e Shabanie orebody, t
h e b u l k o f t h e r o c k mass ranges from c l a s s 3A t o c l
a s s 28, i .e . r a t i n g s between 5 1 and 70 - range 19. I n t
h e K i n g orebody, t h e r o c k mass ranges f rom minor c l a s
s 5 t o 3A, i .e. r a t i n g s between 10 and 60 - a range o f 50.
Both d e p o s i t s were p lanned f o r b lock cav ing w i t h a c
o n v e n t i o n a l h o r i z o n t a l g r i z z l y l a y o u t
. There i s a s i g n i f i c a n t d i f f e r e n c e i n t h e
magni tude and range o f t h e r a t i n g s between t h e two d e
p o s i t s and t h i s would i n d i c a t e a pronounced d i f f
e r e n c e i n behav iour d u r i n g t h e min ing . A t Shabanie
Nine, cav ing took p l a c e o n l y a f t e r a l a r g e a rea
had been undercut , f r agmen ta t i on was poor and no suppo r t
problems were exper ienced on t h e e x t r a c t i o n l e v e l .
A t K i n g Nine, t h e orebody caved r e a d i l y and t h e
inhomogenui ty (10 - 60 r a t i n g s ) meant good f ragmen ta t i
on i n c l a s s e s 5 and 4 m a t e r i a l b u t poor f r a g m e
n t a t i o n i n t h e c l a s s 3. Severe suppor t problems were
exper ienced on t h e e x t r a c t i o n h o r i z o n because o f
t h e c l a s s e s 5 and 48 zones and l a r g e wedge f a i l u r
e s . As t h e p r e d i c t i o n s based on t h e r o c k mass c
l a s s i f i - c a t i o n s were proved i n p r a c t i c e ,
subsequent m in ing o p e r a t i o n s were changed, based on c l
a s s i f i c a t i o n data.
J o i n t spac ing and j o i n t c o n d i t i o n a r e s ig -
n i f i c a n t f a c t o r s i n d e s i g n i n g open s topes o
r c u t - a n d - f i l l stopes. The o r i e n t a t i o n and
spac ing o f t h e j o i n t s w i l l de te rm ine t h e s i z e o
f t h e p o t e n t i a l l y u n s t a b l e wedge which r e q u i
r e s suppo r t i n t h e back o f t h e s tope o r whose f a i l u
r e may a l t e r t h e shape o f t h e p i l l a r s . Whether t h
e wedge w i l l d i s s o c i a t e f rom i t s h o s t r o c k
mass w i l l be i n f l u e n c e d by t h e c o n d i t i o n o f
t h e j o i n t s . Ground w i t h w i d e l y spaced j o i n t s w
i l l have a h i g h i n - s i t u c l a s s i f i c a t i o n r a
t i n g , b u t i f t h e j o i n t o r i e n t - a t i o n i s un
favou rab le i n t h e back o f a s tope and t h e c o n d i t i o
n r a t i n g s a r e low, t h e n t h e suppor t o f t hese l a r
g e , p o t e n t i a l l y u n s t a b l e wedges becomes c o s t
l y , as t h e c a b l e b o l t s must
Adjusted Rat ings: - 60 x 0,63 = 38 (4A)
B - 60 x 0,50 = 30 (48) j A
T o t a l P o s s i b l e Adjustment
75% 120 - 76%
120 - 60%
70%
80%
Assessed, b u t n o t measured, c l a s s i f i c a t i o n r a
t i n g s made by t h e au tho r o f exposures seen on t h e u s u
a l b r i e f v i s i t s t o m in ing operati-ons, wh ich shou ld
be regarded as " b a l l pa rk " f i g u r e 9 a r e g i v e n
below.
A
95 / 90 95 j 90
i 80 i 75 90 85
I N i n e 1 Orebody ! I
' 60-80 , K i d d Creek I
/ Na t tagami j 60-80; minor BO- q90 and 40-60
J e f f r e y 10-10
Cass iar* 10-30
I I FOX 70-90 I I CreightOn 70-90 I Cl imax 30-60
97 - -
63%
RATIO OF SURFACE AREA OF THE ORE/UNPAY INTERFACE TO THE
CONTAINED ORE,WHICH DETERMINES ORE LOSSES AND DILUTION
97
50%
/ Lakeshore , 1 20-60 i i I San Nanue l , 15-10 i 1 mount I s a
.
I t i s a t t h e c o n t a c t between o r e and waste t h a t
o r e l o s s e s and d i l u t i o n occur. The r a t i o o f t h
e con ta ined o re t o t h e ore/waste i n t e r f a c e i s r e l
a t e d t o v a r i a t i o n s i n shape and s i z e o f o rebod
ies and w i l l g i v e an i n d i c a t i o n o f t h e magni tude
o f o r e l o s s and d i l u t i o n . If we cons ide r t h e
shapes A and B ( ~ i g . 5 ) , t h e n t h e r a t i o o f con ta
ined o r e (s.G. = 2,8) t o a square met re o f s u r f a c e a rea
i s A = 65 t / m 2 and 8 = 140 t / m z . T h i s does n o t mean t
h a t , i n t h e case o f A, d i l u t i o n and o r e l o s s w i
l l be 2,15 t i m e s t h a t o f 8, b u t t h a t , i f caved, A w
i l l n o t be as v i a b l e as 8. I f t h e draw h e i g h t i s
reduced t o 50 m i n A and 100 m i n 8, t h e n t h e r a t i o s a
r e A = 34,5 t /mz and 0 = 93,3 t / m 2 and a f a c t o r o f 2,7.
I n t h i s case, i n - s i t u o r e losses, which a re r e l a t
e d t o l a y o u t and shape r e g a r d l e s s o f draw h e i g
h t , wou ld be
P e r i p h e r a l
70-90; 1
i 70-90 1 S t r a t h c o n a 50-70 / B e l l Nine* ' 10-60 *
Measured ,
g r e a t e r and d i l u t i o n would be h i g h e r because
of t h e tendency t o overdraw l i m i t e d draw h e i g h t s . I
n f a c t , i n t h e second case o f A, a method
m ino r 50-60
F.W. 6-15 H.W. 30-50
I F.W. 70-90 H.W. 40-90
F.W. 80-90
I
I 10-70
o t h e r t h a n c a v i n g may be economica l l y more
sound.
4
I f t h e o n l y m i n i n g method s e l e c t i o n tech- n i
q u e used was t o employ. t h e same method a s your ne ighbour
because bo th d e p o s i t s were
-
CAVE
D GR
OUND
MA
SS
MOV
EMEN
T ON
/'
HORI
ZONT
rU,
STRE
SS
REM
OVED
c
SHEA
R A
S TO
E RE
DUCE
D
ORE
MIG
RATI
ON
GOOD
FR
AGME
NTAT
ION
UNDE
RCUT
AD
VANC
E (R
U)
RELA
TED
TO
RATE
OF
A
ND
PO
INT
LOAD
S.
CAVI
NG I
RC)
AND
DR
IFT
INCR
EASE
D VE
RTIC
AL
FAIL
UR
E (O
F)
RC
(RU)
DF
-
MASS UNDERGROUND MINING METHODS
l a r g e and c o n t a i n e d t h e same m i n e r a l , t h e
n t h e above t e c h n i q u e would, a t l e a s t , i n d i c a
t e whether s i m i l a r r e s u l t s c o u l d b e e x p e c t e
d i f . e l l o t h e r f a c t o r s were common.
Area = 40 000 m2 P e r i m e t e r = 1 720 m Ore = 112 000 t / m
s t r i k e . . . Ore
- 1 1 2 000 = 6 5 t/",2 mZ s u r f a c e a r e a - 1 720
Area = 40 000 m2 P e r i m e t e r = 800 m Ore = 112 000 t / m s
t r i k e . . Ore
-
m2 s u r f a c e a r e a = 140 t / m
- 800
FIGURE 5
CAVABILITY AND FRAGMENTATION DATA OF OREBODY AND H A N G I N G W
A L L
Block c a v i n g o r p a n e l r e t r e a t c a v i n g is t h
e l o w e s t - c o s t underground min ing method, p r o v i d e d
t h a t d r a w p o i n t s i z e and h a n d l i n g f a c i l i t
i e s a r e t a i l o r e d t o s u i t t h e caved m a t e r i a l
, and t h e e x t r a c t i o n h o r i z o n c a n b e e c o n o m
i c a l l y main- t a i n e d f o r t h e l i f e o f draw. I n t h
e c a s e o f i s o l a t e d o r e b o d i e s , n o t o n l y
must t h e u n d e r c u t a r e a b e s u c h t h a t a n orebody
c a v e i s i n i t i a t e d , b u t a l s o t h a t t h e h a n g
i n g w a l l c a v e s a t t h e r e w i r e d r a t e . I n s u b
- l e v e l c a v i n g o p e r a t i o n s , t h e c a v e must f
o l l o w t h e r e t r e a t o r t h e hangirq- w a l l h a s t o
be u n d e r c u t s o t h a t t h e p r o d u c t i o n a r e a i
s o v e r l a i n by caved ground .
Two f o r m s o f c a v i n g a r e r e c o g n i s e d : S t r
e s s c a v i n g o c c u r s when s l o u g h i n g from t h e
back t a k e s p l a c e and t h e c a v e p r o g r e s s e s
upwards. I n t h i s c a s e , t h e s i z e o f t h e a r e a u n
d e r c u t r e q u i r e d t o i n i t i a t e t h e c a v e i s d
e p e n d e n t on t h e r a t i o o f s t r e s s , t h e r o c k
mass s t r e n g t h and t h e o r i e n t a t i o n o f j o i n t
s . S u b s i d e n c e c a v i n g o c c u r s ( a ) when p r e v
i o u s min ing h a s removed l a t e r a l r e s t r a i n t and t
h e r e is a r a p i d f a l l o f b l o c k s w i t h l i m i t e
d b u l k i n g , o r ( b ) when t h e r a t e o f u n d e r c u t
t i n g e x c e e d s t h e r a t e o f f a i l - u r e o f t h e
back u n t i l t h i s f a i l s e n masse w i t h t h e p o s s i
b i l i t y o f an a i r b l a s t . The r a t e o f u n d e r c u
t t i n g s h o u l d be such t h a t i t i s s l o w e r t h a n t
h e f a i l u r e o f t h e back, b u t f a s t e r t h a n t h e f
a i l u r e o f t h e e x t r a c t i o n h o r i z o n c a u s e d
by h i g h a b u t m e n t s t r e s s e s . The s t r e s s c h a
n g e s t h a t a n e x t r a c t i o n h o r i z o n i s s u b j e
c t e d t o i n a c a v i n g o p e r a t i o n a r e i l l u s t r
a t e d i n F i g . 6.
The r o c k mass s u r r o u n d i n g t h e e x t r a c t i o n
o p e n i n g s i s s u b j e c t e d t o f o u r s t r e s s c y c
l e s i n a l l c a v i n g s i t u a t i o n s and a f i f t h c y
c l e i f t h e r e t r e a t i s t o w a r d s an u n f a v o u r
a b l e m a j o r g e o l - o g i c a l s t r u c t u r e . The s t
a g e s a r e -
( 1 ) a d j u s t m e n t o f t h e r o c k mass t o t h e o p e
n i n g s ;
( 2 ) a b u t m e n t s t r e s s e s ahead o f t h e u n d e r
c u t , and r e - d i s t r i b u t i o n o f s t r e s s e s a r o
u n d t h e caved a r e a ;
( 3 ) u p l i f t a f t e r t h e u n d e r c u t i s c o m p l
e t e , w i t h t h e removal o f t h e v e r t i c a l s t r e s s
e s ;
( 4 ) v e r t i c a l l o a d i n g on t h e a p e x e s from p
o i n t l o a d s and a n i n c r e a s i n g column o f caved m a
t e r i e l ; and
( 5 ) h i g h t o e s t r e s s e s i f wedge f a i l u r e o c
c u r s a g a i n s t a s t r u c t u r a l f e a t u r e .
With r e f e r e n c e t o ( 4 ) above , i t s h o u l d b e n o
t e d t h a t t h e a v e r a g e v e r t i c a l s t r e s s on t
h e a p e x e s ( e x t r a c t i o n l e v e l p i l l a r s ) i s
r e l a t e d t o t h e draw a r e a and t h e h e i g h t o f c a
v e . Cave model s t u d i e s ( ~ e s l o ~ and l e u b s c h e r
, 1 9 8 1 ) h a v e shown t h a t , f o r dynamic c o n d i t i o n
s , w i t h h e i g h t t o b a s e r a t i o s o f 1:1, 2:1, 3:1,
4 : l and 5:1, t h e a v e r a g e v e r t i c a l s t r e s s on t
h e b a s e i s a p p r o x i m a t e l y 535, 30%, 22% 17% and 14%
r e s p e c t i v e l y o f t h e mass o f t h e caved g r o u n d
.
-
DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES
u n i f o r m ( ~ e s l o ~ and Laubscher, 1981). The
Average V e r t i c a l S t r e s s i n WPa on E x t r a c t i o
n L e v e l s w i t h D iameters o f -
I I
TABLE II
The i n c r e a s e i n v e r t i c a l s t r e s s w i t h s t
a t i c c o n d i t i o n s w i l l be app rox ima te l y 10% and s
t r e s s e s i n t h e c e n t r e o f t h e a rea under draw w i
l l be h i g h e r t han a t t h e s ides . Smal l , h i o h l y p
r o d u c t i v e areas a r e p r e f e r a b l e t o large, l ow-p
roduc t i ve areas. Hung-up areas w i t h w e l l developed arches,
l a r g e wedges s u p p o r t i n g c o l - umns o f caved m a t e
r i a l and c o n s o l i d a t e d mat- e r i a l w i l l concen t
ra te t h e v e r t i c a l s t r e s s . I n t h e case o f a
200-m-diameter e x t r a c t i o n l e v e l w i t h a 5 : l r a t
i o a able 11) p o i n t l o a d s o f 100 MPa (14 500 p s i ) o r
h i g h e r , c o u l d be expected. The chances o f h i g h s t r
e s s concen- t r a t i o n s a r e reduced i f t h e f r a g m e n
t a t i o n i s u n i f o r m .
The a r c h i n g s t r e s s e s a re imposed on t h e sur-
rounding r o c k mass o r p r e v i o u s l y caved areas, r e s u
l t i n g i n an i nc reased v e r t i c a l s t r e s s on t h e i
r bases. T h i s i s one o f t h e reasons why t h e "checker
board" cave l a y o u t was n o t success fu l . The a r c h i n g
s t r e s s e s i n c r e a s e t h e anq le o f f r i c t i o n on
j o i n t s , and, t h e r e f o r e , t h e ang le o f cave can va
ry f rom depth t o sur- face. Near su r face , where l a t e r a l
r e s t r a i n t i s l i m i t e d , t o p p i n g o f b l o c k s
i s common w i t h low cave ana les ( ~ e s l o ~ , 1974).
The e f f e c t o f deeth on c a v i n g must be re - l a t e d
more t o t h e geology, p o t e n t i a l mass move- ments and m i
n i n g induced s t r e s s e s t h a n t h e mass o f t h e caved
m a t e r i a l .
I f t h e orebody i s p r i m a r i l y broken, as i n sub - l
eve l cav ing o r a sh r i nkage o p e r a t i o n w i t h a f o l
l o w i n g cave, t hen h a n g i n g w a l l f ragment- a t i o n
w i l l have an e f f e c t on d i l u t i o n . I n s ~ c h cases,
a f i n e l y f ragmented h a n g i n g w a l l w i l l i n c r e a
s e t h e d i l u t i o n percentage.
I n b l o c k o r pane l r e t r e a t cav ing , f r a g - m e n
t a t i o n has a b e a r i n g on p r o d u c t i v i t y and, t h
e r e f o r e , i n f l u e n c e s des ign parameters and c o s t
o f , and damage from, secondary b l a s t i n g , as w e l l as
damage t o t h e major apex.
Un i fo rm, good f ragmen ta t i on and poor heteroqeneous f
ragmen ta t i on can be c l a s s e d as t h e end members o f a s
e r i e s d e p i c t i n g l ow t o n i g h d i l u t i o n , p
rov ided t h e r e i s i n t e r a c t i o n between t h e drawpoin
ts , and draw r a t e s a r e
cav ing o f an orebody w i t h good f ragmen ta t i on l e a d s
t o s i g n i f i c a n t bu l k i ng , w i t h as much as 30% o f
t h e o r e drawn b e f o r e the ore/waste i n t e r f a c e i s a
f f e c t e d , and d i l u t i o n w i l l r e p o r t i n t h e d
rawpo in t a f t e r 70% o f t h e o re has been drawn. A t t h e o
t h e r end o f t h e scale, b u l k i n g i s l i m i t e d ,
channelways r a p i d l y develop as f i n e r m a t e r i a l i s
drawn o f f and f i n e d i l u t i o n can e n t e r t h e
drawpoin t a f t e r 40% o f t h e o r e has been drawn.
Caving r e s u l t s i n pr imary f ragmenta t ion , which i s t
h e p a r t i c l e s i z e developed i n t h e f a i l u r e zone
o f t h e advancing cave, end second- a r y f ragmenta t ion ,
which occurs i n t h e draw column. Pr imary f ragmenta t ion i s
determined by t h e s t r e s s e s i n t h e cave back and t h e s
t r e n g t h and o r i e n t a t i o n o f t h e j o i n t s w i t
h r e s p e c t t o t hose s t resses. S t r e s s cav ing w i l l
r e s u l t i n b e t t e r f ragmenta t ion than subsidence c a v
i n g where t h e r e i s a r a p i d s e t t l i n g o f m a t e r
l a l w i t h l i t t l e bu l k i ng . Secondary f r a g m e n t a
t i o n occu rs i n t h e draw column; however, as s t r e s s e s
i n moving m a t e r i a l a re n o t h igh , secondary f ragmen ta
t i on i s n o t as pro- nounced as g e n e r a l l y be l ieved.
As a r c h i n g deve lops above t h e e x t r a c t i o n hor
izon, second- a r y f r a q m e n t a t i o n can occur, p rov ided
t h e apexes a r e s t r o n g e r t h a n t h e caved m a t e r i
a l . An a rch o f c l a s s 2 rock aga ins t a c l a s s 4 ma jo r
apex w i l l r e s u l t i n f a i l u r e o f t h e apex u n l e s
s t h e a rch i s des t royed o r j o i n t s a r e now o r i e n t
a t e d a t a more favou rab le ang le f o r shear f a i l u r e .
Fragmenta t ion has been s t u d i e d a t d rawpo in t s by v i s
u a l obse rva t i ons and by diamond d r i l l i n g l a r g e b l
o c k s up t o 10 m x 7 m x 5 m which had moved 100 m i n a drew
column. J o i n t spac ing d a t a i n d i c a t e d t h a t b l o
c k s of t h i s magni tude shou ld n o t occur i f f a i l u r e i
s r e l a t e d t o t h e o v e r a l l j o i n t e d spacing. How-
ever, t h e diamond d r i l l i n g showed t h a t j o i n t s w i
t h h i g h c o n d i t i o n r a t i n g s were present ; t h e r
e f o r e , t h e b l o c k s were de f i ned by t h e spac ing o f
t h e j o i n t s w i t h l owes t c o n d i t i o n r a t i n g s
(weakest ) .
R.Q.D. has been used by o t h e r s t o determine c a v a b i l
i t y and f ragmenta t ion , b u t t h i s i s an i n a c c u r a t
e method because o f t h e 100 mm l i m i t - a t i o n . 0,s m b l
o c k s would g i v e h i g h R.P.D. va lues , b u t anyone i n v o
l v e d i n cav ing would be happy t o have t h a t k i n d o f f
ragmenta t ion .
H y d r a u l i c r a d i u s , which i s area d i v i d e d by
p e r i m e t e r i s o f t e n used t o d e f i n e t h e undercut
area, and, i n t h e f o l l o w i n g t a b l e (III), i t s r e l
a t i o n s h i p t o t h e ad jus ted r a t i n g s i s shown.
I n F i g . 7, T a y l o r has shown t h a t , by apply- i n g
ad jus tmen ts t o t h e i n - s i t u r a t i n g s , t h e
assessment o f c a v a b i l i t y of t h e b lock can be e s t a b
l i s h e d f o r i t s m i n i n g l i f e ( ~ a y l o r ,
1980).
The i n t r o d u c t i o n o f l a r g e L.H.D. equipment t o
underground m i n i n g opera t ions , and improve- ments i n suppo
r t techniques, have meant t h a t
-
DESIGN AND OPERATION OF CAVING AND SUBLEVEL STOPING MINES ; a t
i s f a c t o r y i n t h e u p p e r l e v e l s o f t h e m i n e
, THE MINING SEQUENCE I S DETERMINED BY THE ~t g r a a t e r d e p
t h s w i t h i n c r e a s e d s t r e s s e s , a EFFECT OF A
PRODUCTION BLOCK O N c h a n q e may b e n e c e s s a r y .
SURROUNDING OREBODIES A N D INSTALLATIONS
Orebody s h a p e and d i p w i l l d e t e r m i n e w h e t h
e r A m i n i n g s e q u e n c e w i l l be r e q u i r e d w h e
t h e r a h o r i z o n t a l o r i n c l i n e d f o o t w a l l d
r a w p o i n t t h e d e p o s i t c o n s i s t s o f a s i n g l
e m a s s i v e o r e - l a y o u t o r a c o m b i n a t i o n o f
t h e two i s u s e d . I n c o m p a r i n g t h e two l a y o u t
s f o r , s a y , a n o r e - body d i p p i n g a t l e s s t h a
n 5 0 a n d h a v i n g a t h i c k n e s s n o t e x c e e d i n g
100 m , t h e n t h e h o r i z - o n t a l w i l l b e more p r o
d u c t i v e i n t h e e a r l y s t a g e s , b u t u l t i m a t
e o r e l o s s e s w i l l b e h i g h e r .
The o r i e n t a t i o n o f t h e e x t r a c t i o n d r i f
t , w i t h r e s p e c t t o t h e m a j o r g e o l o g i c a l s
t r u c - t u r e s , p a r t i c u l a r l y s h e a r z o n e s ,
m u s t t a k e p r e c e d e n c e o v e r t h a d r a w p o i n t
o r i e n t a t i o n . The s i z e o f d r i f t s and d r a w p o
i n t o p e n i n g s w i l l b e i n f l u e n c e d by t h e s t
r e n g t h o f t h e r o c k m a s s i n w h i c h t h e y a r e b
e i n q d e v e l o p e d a n d t h e e x p e c t e d f r a g m e n
t a t i o n . D r a w p o i n t l i f e d e p e n d s on t h e h e
i g h t o f d r a w a n d p r o d u c t i o n p o t e n t i a l , w
h i c h l a t t e r i s d i c t a t e d by f r a g - m e n t a t i
o n , s i z e o f d r a w p o i n t o p e n i n g a n d l a s h i n
g t e c h n i q u e . The p r o d u c t i o n r e q u i r e - men t
f r o m a d r a w p o i n t c a n n o t e x c e e d t h e p r o d u
c t i o n p o t e n t i a l and w i l l , t h e r e f o r e , d i c
t a t e t h e number o f w o r k i n g d r a w p o i n t s r e q u
i r e d . The v a l u e d i s t r i b u t i o n m u s t b e
body o r a s e r i e s o f d i s c o n n e c t e d o r e b o d i
e s a s a t S h a b a n i e Mine ( ~ i g . 1 ) . The s e q u e n c
e t h a t i s a d o p t e d mus t b e s u c h t h a t s u b s e q u
e n t o p e r - a t i o n s a r e n o t p r e j u d i c e d .
I n t h e c a s e o f a m a s s i v e d e p o s i t wh ich w i l
l b e cave-mined, t h e h e i g h t o f draw w i l l h a v e t o b
e e s t a b l i s h e d , a n d , t h e r e f o r e , t h e v e r t
i c a l i n t e r v a l o f t h e e x t r a c t i o n h o r i z o n
s . Wore t h o u g h t i s r e q u i r e d i n t h e s e l e c t i
o n o f t h e e x t r a c t i o n h o r i z o n i n t e r v a l t h
a n r e l y i n g on t h e p r s m i s e - t h a t b e c a u s e
Company ' X ' u s e s 1 0 0 m o r 200 m t h i s i s c o r r e c t .
T h e r e mus t b e c o n t i n - u i t y o f p r o d u c t i o n ,
and t h e c a v i n g o f t h e l o w e r h o r i z o n m u s t n o
t a f f e c t t h e u p p e r h o r i z o n . I f t h e o r s b o d
y i s l a r g e i n t h a t t h e h o r i z o n t a l a x e s are
more t h a n t w i c e t h e p r o p o s e d h e i g h t , t h e n
s e q u e n c e p r o b l e m s are n o t l i k e l y . How- e v e
r , i n o r e b o d i e s w i t h s m a l l e r p l a n d i m e n s
i o n s , s e q u e n c e p r o b l e m s d o e x i s t , and sound
geo log- i c a l k n o w l e d g e a n d a n a s s e s s m e n t o
f t h e p o t e n t i a l f a i l u r e p a t t e r n i s r e q u i
r e d . The i d e a l d r a w h e i g h t w i l l b e b a s e d on
o r e b o d y
t a k e n i n t o c o n s i d e r a t i o n , a s h i g h - v a
l u e , l ong- v a l u e , c o s t o f e x t r a c t i o n h o r i
z o n , economic l i f e d r a w c o l u m n s w i l l p e r m i t
g r e a t e r e x p e n d i - l i f e o f e x t r a c t i o n h o r
i z o n and p r o d u c t i o n t u r e on t h e e x t r a c t i o
n l e v e l and d r a w p o i n t s p o t e n t i a l p e r d r a w
p o i n t . F o r example , t h e t h a n t h o s e w i t h l o w -
v a l u e , s h o r t - l i f e d r a w i d e a l d r a w h e i g h
t may b e 150 m, b u t i f t h e c o l u m n s . o r e b o d y p l
a n d i m e n s i o n s a r e 200 rn x 200 m w i t h
t w o m a j o r j o i n t s e t s a t r i g h t a n g l e s and
An i m p o r t a n t d e s i g n f e a t u r e i s t h e b a s i c
d i p p i n g a t 45O, a n d f a i l u r e c a n b e e x p e c t e
d
s t r e n g t h o f t h e e x t r a c t i o n s t r u c t u r e
: t h e s i z e a l o n g t h e s e j o i n t s , t h e n i t w i l
l n o t b e poss- o f t h e o p e n i n g s , t h e s p a c i n g o
f t h e d r a w p o i n t s , i b l e t o s tar t p r o d u c t i o
n on t h e l o w e r e x t r a c t - t h e s i z e o f t h e p i l
l a r s ( a p e x e s ) a n d t h e s u r - i o n h o r i z o n i f
t h e p l a n n e d p r o d u c t i o n r a t e is, f a c e a r e a
o f e x p o s e d r o c k f a c e . When a n s a y , 200 0 0 0 t o
n n e s p e r month. T h i s i s b a s e d L.H.D. l a y o u t was p
r o p o s e d f o r B e l l Mine , on t h e p r o d u c t i o n a r
e a b e i n g o f s u f f i c i e n t Q u e b e c , t o r e p l a c
e t h e g r i z z l y l a y o u t , a m a j o r s i z e t o a l l o
w f o r u n d e r c u t t i n g , c o m m i s s i o n i n g o b j e
c t i o n was t h e s i z e o f t h e L.H.D. d r i f t s i n o f d
r a w p o i n t s , p r o d u c i n g d r a w p o i n t s , hung-up
a r o c k m a s s w h i c h had p r e s e n t e d s u p p o r t d r
a w p o i n t s a n d m a i n t e n a n c e o f d r a w p o i n t s
. p r o b l e m s . However , t h e s u r f a c e a r e a o f E i t
h e r t h e p r o d u c t i o n r a t e i s r e d u c e d o r t h e
e x p o s e d r o c k i n t h e q r i z z l y l a y o u t , i n c l
u d i n g d raw h e i g h t i s r e d u c e d . I f t h e j o i n t
s e t s t w o metres o f o r e p a s s , was g r e a t e r t h a n
f o r t h e d i p a t 45O i n o n e d i r e c t i o n and 6S0 i n t
h e L.H.D. l a y o u t . T h i s mean t t h a t d e v e l o p m e n
t , o t h e r , c h a n g i n g t h e d i r e c t i o n o f m i n i
n g c o u l d c o n i n g a n d u n d e r c u t b l a s t damage w
e r e more mean a h i g h e r d r a w h e i g h t . e x t e n s i v
e i n r e l a t i o n t o t h e vo lume o f r o c k i n t h e p i l
l a r s , a n d t h e s u p p o r t r e q u i r e m e n t s w e r e
g r e a t e r . W h i l s t t h e d r i f t s p a n s w e r e l a r
g e r i n t h e L.H.D. l a y o u t , t h i s w a s o f f s e t by t
h e b i g q e r vo lume o f r o c k i n t h e a p e x e s ( p i l l
a r s ) i n r e l a t i o n t o t h e vo lume o f o p e n i n g . T
h e o b j e c t i s t o d e s i g n a s t r u c t u r e w h i c h w
i l l g i v e minimum p r o b l e m s ( l o w c o s t s ) a n d opt
imum o r e e x t r a c t i o n . I t i s p o i n t l e s s t o d e
s i g n f o r maximum e x t r a c t i o n when c o n t i n u e d c
o l l a p s e
I n t h e c a s e o f d i s c o n n e c t e d o r e b o d i e s
, t h e m i n i n q o f t h e i n d i v i d u a l o rebody may n o
t p r e s e n t a p r o b l e m , b u t i t i s t h e i n f l u e n
c e on a d j a c e n t o r e b o d i e s and i n s t a l l a t i o
n s t h a t must b e a s s e s s e d . H e r e , m a j o r g e o l
o g i c a l s t r u c t u r e s a n d / o r l i t h o l o g i c a l
c h a n g e s c a n p l a y a s i g n i f - i c a n t p a r t . I n
a m a j o r chrome m i n i n g o p e r a t i o n i n Zimbabwe, t h
e ch rome o r e o c c u r s i n p o d s o f v a r y i n g s i z e a
n d h a s b e e n s u c c e s s f u l l y mined
a n d r e p a i r work mean i t i s o n l y p o s s i b l e t o
by o p e n s t o p i n g f o r many y e a r s . However, e x t r a
c t h a l f t h e o r e a t a c o s t h i g h e r t h a n t h e t h
e b o d i e s a r e i n c r e a s i n g i n s i z e , t h e i n t e
r - v a l u e o f t h e o r e . The s t r o n g e s t d e s i g n w
i l l j a c e n t p i l l a r s a r e d e c r e a s i n g and t h e
s u r r o u n d - b e when t h e o p e n i n g i s s u r r o u n d
e d by m o s t r o c k ; i n g c o u n t r y r o c k i s becoming l
e s s c o m p e t e n t , t h a t is , w i t h s i n g l e - s i d
e d f o o t w a l l d r a w p o i n t q and management is, t h e r
e f o r e , c o n c e r n e d a b o u t a n d t h e w e a k e s t d
e s i g n when t h e a p e x i s c u t on t h e r e g i o n a l s t
a b i l i t y . W i l l t h e h a n g i n g w a l l b o t h s i d e
s down t o f l o o r l e v e l w i t h l a r g e , f a i l ? W i l
l t h e r e b e i n t e r a c t i o n b e t w e e n c l o s e l y -
s p a c e d o p p o s i t e d r a w p o i n t s a n d i n d i v - c
a v i t i e s , a n d w h a t w i l l t h e e f f e c t b e on i d
u a l d r a w p o i n t o r e p a s s e s . s h a f t s ? I n a c o
m p l e x g e o l o g i c a l e n v i r o n m e n t ,
-
MASS UNDERGROUND MINING METHODS
TABLE I V
i I
Ad jus ted Class Depth # I r
t h e r e g i o n a l geology must be known and t h e d a t a
ope ra t i on . Both o rebod ies are l a rge , a t p l o t t e d on
t r a n v e r s e and l o n g i t u d i n a l sec t i ons , depths
o f 600 and 1 000 met res r e s p e c t i v e l y as w e l l as th
ree-d imens iona l models, which w i l l below sur face. cover t h
e area o f m i n i n g and s h a f t s .
1 I
100 m Angle 75O t o o f
500 m cave 1 750 - 650 Approximate e x t e n t o f f a i l u r e
zone
1
Cave angles a r e o f t e n used t o d e f i n e t h e l i m i t
o f ground f a i l u r e . T h i s i s n o t c o r r e c t because
a f a i l u r e zone ex tends beyond t h e cave boundary, and t h i
s zone i s n o t d e f i n e d by ang les b u t by d i s t a n c e
s a long t h e weakest r o c k mass i n t h e a rea and t h e ad
jus tment o f t h e sur round ing r o c k mass t o t h e s t r e s
s e s induced by t h e min ing ope ra t i on . P rov ided a l l ma
jor weak s t ruc tu res , t h e r e g i o n a l s t r e s s e s and
t h e e x t e n t o f t he m i n i n g o p e r a t i o n s have
been recognised, t a b l e I V can serve as a rough gu ide f o r bo
th angle o f cave and t h e e x t e n t o f t h e f a i l u r e
zone, and i s based on m o n i t o r i n g sever- a l min ing opera
t ions .
THE NEED OR NOT TO MAINTAIN REGIONAL STABILITY
5
45' - 35'
35 O
W h i l s t cave m i n i n g may be t h e obv ious method a t t
h e s t a r t o f an underground o p e r a t i o n i s i t t h e c
o r r e c t one f o r t h e whole d e p o s i t ? Does another
method have t o be used f o r t h e l i f e o f t h e depos i t , o
r can a change be made a t a l a t e r stage? The l a t t e r would
be p r e f e r a b l e because a background o f knowledge on ground
behaviour i n t h a t s t r e s s environment would be b u i l t
up.
4
55O - 45O
45' - 35"
2
750 - 650
65' - 55'
Whether t h e r e i s a need t o m a i n t a i n r e g i o n a l
i s n o t o n l y w i t h r e f e r e n c e t o t h e e f f e c t o
f caved ground on s u r f a c e o r i n s t a l l a t i o n s , b u
t a r e imposs ib le m i n i n g c o n d i t i o n s go ing t o be
c rea ted by cave m in ing? Mass movements cannot be c o n t r o l
l e d . I n c e r t a i n complex env i ron- ments o f d i p p i n
g d i s c r e t e orebod ies , incompet- e n t f o o t w a l l s
and h i g h r e g i o n a l s t resses, t h e n mass movement o f t
h e ground between caved ore- bod ies w i l l mean i n t o l e r a
b l e s t r e s s e s i n t h e f o o t w a l l e x t r a c t i o n
open ings o f t h e down-dip orebodies.
3
650 - 550
55' - 45'
The reasons f o r t h e s e l e c t i o n o f m i n i n g
methods f o r t h e Nt . I s a 1100 Orebody and Henderson orebod
ies wou ld make an i n t e r e s t i n g comparison. I n t h e case
o f Nt . I sa , r e g i o n a l s t a b i l i t y i s be ing m a i
n t a i n e d by f i l l i n g l a r g e stopes, and Henderson i s
a b l o c k c a v i n g
100 m 1 200 m 1
30 m
Another f a c t o r wh ich has r e c e i v e d g r e a t e r
emphasis i n r e c e n t y e a r s i s t h e p r o t e c t i o n o
f t h e environment and t h e d i s p o s a l o f t o x i c t a i l
i n g s as fill i n stopes. W h i l s t a caved landscape may be u
n a t t r a c t i v e i n t h e s h o r t term, i n a r i d c l i m
a t e s t h e long-term b e n e f i t s would be water s to rage
and improv ing ground water resources.
50 m
MODEL STUDIES
The b e n e f i t s o f mathemat ica l model i n v e s t i - g a
t i o n s a r e d i r e c t l y p r o p o r t i o n a l t o t h e
sim- p l i c i t y o f t h e orebody geometry and t h e geolog- i c
a l environment. I f assumpt ions a r e made, and i t must never be
f o r g o t t e n t h a t t h e y were made, t h e n these s t r e
s s a n a l y s i s programmes can p r o v i d e some u s e f u l i
n f o r m a t i o n on m i n i n g induced s t resses.
THE ROLE OF ROCK MECHANICS
So o f t e n r o c k mechanics i s i n t r o d u c e d as a c
rash programme when t h e mine expe r iences severe ground problems
which a f f e c t p r o d u c t i o n , and answers a r e expected
o v e r n i g h t a f t e r l o c a l pe rsonne l admi t d e f e a
t . A s imp le low-cost m o n i t o r i n g and o b s e r v a t i o
n programme r e l a t e d t o sound g e o l o g i c a l and c l a s
s i f i c a t i o n d a t a can be s e t up as a r o u t i n e i n
v e s t i g a t i o n by mine s t a f f . T h i s w i l l p r o v i
d e a background o f d a t a on wh ich sound d e c i s i o n s can
be made by o p e r a t i n g p e r s o w n e l o r w i t h t h e a
s s i s t a n c e o f c o n s u l t a n t s . The r e l u c t a n c
e t o m o n i t o r ground behav iou r w h i l s t e v e r y t h i
n g i s o o i n g w e l l must be a men ta l a t t i t u d e o f
"it won' t happen here" o r " w e ' l l c r o s s t h a t b r i d g
e when we reach it". T a l k r o c k mechanics t o some o p e r a t
o r s and t h e y immed ia te l y t h i n k o f c o n s u l t a n t
' s f e e s and expens ive i n s t r u m e n t a t i o n , and w i
l l g l a d l y quote examples o f where i t has n o t been o f b e
n e f i t . Parker (1973), i n h i s e x c e l l e n t s e r i e s
o f a r t i c l e s , has shown t h e b e n e f i t s o f s imp le
m o n i t o r i n g . The au tho r has been i n v o l v e d i n r o
c k mechanics i n v e s t i g a t i o n s on h i s Company's mines
s i n c e 1964, and t h e d a t a acou i red has been i n v a l u a
b l e i n making
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MASS UNDERGROUND MINING METHODS
dec i s ions t o change m i n i n g methods.. The t h i n g t o
guard aga ins t i s ove r -e labo ra t i on ; s imple, e f f e c t
i v e m o n i t o r i n g t o p r o v i d e background d a t a can
be done on any mine w i t h o u t n e c e s s a r i l y an i nc
rease i n s t a f f .
ADHERENCE TO A LOGICAL PLANNING SCHEDULE
Successfu l mine p l a n n i n g depends on adher- ence t o a
schedule based on t h e c a p a b i l i t i e s o f t h e o rgan i
sa t i on . F i g . 8 i n d i c a t e s departmen- t a l r o l e s
and t h e sequence o f even ts l e a d i n g t o underground s x p
l o i t a t i o n . Our exper ience has shown t h a t p r o d u c t
i o n problems occur when t h e r e i s a depar ture f rom these
procedures and expediency i s a l l o w e d t o p l a y an i m p o
r t a n t par t . The t i m i n g o f t h e schedule i s based on t
h e commissioning o f m i n i n g b l o c k s i n a complex g e o l
o g i c a l environment i n t h e c o r r e c t p roduct - i o n
sequence, w i t h t h e work done by t h e perm- anent mine s t a f
f . The p e r i o d between e x p l o r a t i o n and p r o d u c t
i o n can be reduced i f t h e geology i s s imp le and/or more e f
f o r t and money a re p u t i n t o o b t a i n i n g t h e i n f
o r m a t i o n and i n c r e a s i n g development r a t e s .
DEGREE OF SOPHISTICATION POSSIBLE
Wining o p e r a t i o n s a r e world-wide, b u t t h e s e l e
c t i o n o f a m i n i n g method must recogn ise t h e s o c i a
l environment. I n i n d u s t r i a l i s e d count- r i e s w i t
h h i g h wages and a r e l u c t a n c e f o r people t o make m i
n i n g a career , i n c r e a s e d mechanisat ion has meant g r e
a t e r e f f i c i e n c i e s , l a b o u r s a t i s f a c t i o
n and con t i nued economic opera t ions . The techn iques
developed i n these areas a re o f t e n a p p l i e d i n deve lop
ing c o u n t r i e s where l a b o u r - i n t e n s i v e methods
would be more s u i t a b l e because o f t h e l a r g e numbers n
o t g a i n f u l l y employed. However, once mech- a n i s a t i o
n i s i n t r o d u c e d , a l b e i t on a s m a l l scale,
manual l a b o u r no l o n g e r becomes a t t r a c t i v e t o t
h e l o c a l s . Wechanisat ion i s increased, bu t t h e l a b o
u r f o r c e s remain l a r g e because wastage i s l i m i t e d
and Governments a r e a g a i n s t i nc reases i n unemployment.
The ma jo r problem i s t h e maintenance o f t h e equipment i n c
o u n t r i e s w i t h a predominance o f u n s k i l l e d labour
, and " l o c a l i s a t i o n " i s a ~ o l i t i c a l p la t f
o rm. S k i l l s have t o be impor ted, l o c a l personne l have
t o be employed a t wages wh ich may be low by wes te rn standards,
bu t , because o f t h e numbers i n v o l v e d , t h e wage b i l
l , i n c l u d i n g t h e p r o v i s i o n o f s o c i a l se rv
i ces , i s ex t remely h igh . "Cheap l a b o u r " i s a misnomer
today because wages a r e b e i n g f o r c e d up b y minimum wage
l a w s a t r a t e s f a r i n excess o f i nc reases i n p r o d
u c t p r i c e s , w i t h l i t t l e o r no compensatory i n c r
e a s e i n e f f i c i e n c y . Therefore, i n a c o u n t r y w
i t h "cheap labour" , t h e c o s t pe r tonne mined c o u l d be
h i g h e r t h a n i n t h e U n i t e d S t a t e s o r Canada.
The remote- ness o f t h e o p e r a t i o n s f rom t h e source o
f equipment and spa res i s a problem. Nanu- f s c t u r e r s w i
l l n o t p r o v i d e t h e necessary back- up s e r v i c e u n
l e s s s u f f i c i e n t u n i t s a r e i n use,
and companies w i l l n o t buy t h e u n i t s u n l e s s t h
e r e i s a back-up se rv i ce .
M i n i n g pe rsonne l t r a i n e d i n i n d u s t r i a l i
s e d c o u n t r i e s w i l l n a t u r a l l y i n t r o d u c e
techn iques w i t h which t h e y a r e f a m i l i a r . The b u l
k o f t e c h n i c a l j o u r n a l s o r i g i n a t e i n i n d
u s t r i a l i s e d c o u n t r i e s and a l l a r t i c l e s a
r e d i r e c t e d t o i nc reased tonnages w i t h reduced l abou
r ; nobody e x t o l s t h e " v i r t u e s " o f manually-worked
g r i z z l y drawpoin ts , o r hand- lash ing development
ends.
M i n i n g companies a r e always regarded as be ing weal thy,
and, thus , t h e u s u a l a t t i t u d e i s t h a t a l a r g e
percentage o f t h e o r o f i t must be r e t a i n e d by t h e
Sta te ; t h e r e f o r e , t b e planned p r o f i t marg in may
have t o be t w i c e t h a t acceptab le i n t h e Company's home
coun t r y . T h i s means t h a t t h e method s e l e c t i o n p
rocess must be comprehensive, as t h e r e c o u l d be l i t t l e
room f o r manoeuvre once m in ing has s t a r t e d .
CONCLUSIONS
An a t temp t has been made t o p r o v i d e some g u i d e l i
n e s i n t h e s e l e c t i o n o f mass m i n i n g methods, and
i t i s concluded t h a t , i f t h e c o r r e c t d a t a i s ob
ta ined and p r o p e r l y analysed, t h e c o r r e c t s e l e c
t i o n can be made. The co r - r e c t s e l e c t i o n means t h
e h i g h e s t p r o d u c t i v i t y a t t h e l owes t cos t ,
t h e h i g h e s t m i n e r a l r ecove ry and t h e minimum
problems.
ACKNOWLEDGEMENTS
The au tho r w ishes t o express h i s g r a t i t u d e t o
Nessrs. T. G. Hes lop and 0. 3. C a t h e r a l l f o r t h e i r c
o n s t r u c t i v e c r i t i c i s m and t o Mrs. C. W. Jansen f
o r t y p i n g t h i s paper.
REFERENCES
Carpenter, L.R. and Woolfe, B.R., 1972, "R io Blanco", N i n i n
q Waqazine, Way, 1972, pp. 333-339.
Heslop, T.G., 1974, " F a i l u r e by O v e r t u r n i n g i n
Ground Ad jacent t o Cave Wining a t Havelock Wine", Proc. T h i r
d Congress, x. Soc. Rock Wech., Denver, 1974, Vol . 2, P a r t B,
pp. 1085-1089.
Heslop, T.G. and Laubscher, D.H., 1981, "Draw C o n t r o l i n
Caving Opera t i ons on Southern A f r i c a n C h r y s o t i l e
Asbestos Nines", s. Conf. on Cavinq and Sub-Level S top inq, S.M.E.
- A.I.W.E., Denver, Nov., 1981.
Kendr ick , R., 1970, " I n d u c t i o n Caving o f t h e Urad
Nine", W in inq Conq. J., Vol. 56, Oct., 1970, pp. 39-44.
Laubscher, D.H., 1968, "The O r i g i n and Occur- rence o f C h
r y s o t i l e Asbestos i n t h e Shabanie and Washaba Areas o f
Rhodesia". Svm~os ium on
. . .
t h e Rhodesian Basement Complex. Trans. Geol. Soc. o f S.A.,
Annex. t o Vo l . LXXI, -. -
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