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

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

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

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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

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t h e Rhodesian Basement Complex. Trans. Geol. Soc. o f S.A., Annex. t o Vo l . LXXI, -. -

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