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NG ENG NEERSCALLER NO. D, LITTLETON, COLORADO 80127

HAS BLOCK CAVING A FUTURE I N MIN ING?

C . M. Brinckerhoff

Anaconda CompanyNew York, New York

PREPRlNTNUMBER

F o r p r e se n t a t i o n a t t h e SME-AIME Annual Mee ti ngLos Angeles , Ca l i fo rn ia - February 26-March 1, 1984

Permission is hereby given to publish with appropriate acknowledgments,excerpts or summaries not to exceed one-fourth of the entire text of the paper.Permission to print in more extended form subsequent to publication by the Institutemust be obtained from the Executive Director of the Society of Mining Engineersof AIME.If and when this paper is published by the h i e @ f Mining Engineers of AIME, itmay embody certain changes made by agreement between the Technical PublicationsCommittee and the author, so that the form in which it appears here is not necessarilythat in which i t may be published later.These preprints are available for sale. Mail orders to PREPRINTS, Society of MiningEngineers, Caller No. D, Littleton, Colorado 80127.

PREPRINT AVAILABILITY LIST IS PUBLISHED PERIODICALLY IN

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Has Block Caving a Future in Metal Mining?Much has been written i n the miningtechnical magazines about the caving system.Many of the se a r t ic le s were publ ished i n th eTrans ac t ions o f t he American In s t i t u t e o fMining En gine ers pr io r t o World War 11. In

recen t yea rs 1 t t l e has been w r i t t en on min ingmethods or on the d et a i le d p lanning a lwaysrequ i red .Block caving had primarily been used tomine deep copper ores in Af rica , th e Uni tedS t a t e s and in Ch i l e , a l though o the r o res hadbeen mined by th i s same system. Asb esto s or ehad been caved in Canada, t i n o r e i n B o l i v i a ,molybdenum or e in Colorado, iro n o re inWyoming and Sweden, and limestone inC a l i f o r n i a .The caving of copper ores in the WesternUni t ed S ta t e s , Rhodes i a , an@ i n Chi le wereve ry impor t an t and success fu l ope ra t ions bu tby the end o f t he n ine t een s i x t i e s , co s t s o fp r o d uc t io n s t a r t e d t h e i r u pw ard s p i r a l a nd t h ef u t u r e 1o ok ed u n c e r t a i n . F i n a l l y , t h e e f f o r t st o change th e mining systems t o open p i t andtake advantage of the new equipment which wasthen av ai la bl e fol low ing World War 11, wasmade in several mines in the Uni ted S ta tes .W i t h higher p ric es f o r copper, new mines inth e wes te rn s t a t e s u s ing open p i t methods ,were brought in to product ion and these newerope rat ion s ar e now twenty ye ar s o ld or more .I t i s known th a t a s open p i t mines becomedeeper, the r a t i o of waste rock removed to aton of or e produced, uncontro l 1ablyinc re ases . I t now looks a s i f ou r open p i tm ines even tua l ly w i l l have to ' b i t e t hebullet" and switch to underground mining byt h e e nd o f t h i s c e n t u r y . H ig he r p r i c e s f o rcopper may delay t h i s change tem pora ri ly b ute v e n t u a l l y i t h as t o t a k e p l a ce . T h i s w i l l bethe t ime when block caving, as -3, system incopper mining, wi l l be "born again .For this reason , i t seems wor thwhil e t o ge tsome personal observations and experiences oncav ing i n th e record. Th is may encourageothers to do the same which one day will beapp recia ted by the mining oper ato rs of thef u t u r e .My m in in g e x p er ie n ce s t a r t e d i n t h e e a r l yn ine t een twen t i e s . My exper i ence w i th cav ings t a r t e d i n l a t e 1926 a t I n s p i r a t i o n , A riz on a,The Mine Su perin tende nt t he re a t th e t ime wasF e li x McDonal d, who had been br ou gh t t oInsp i ra t i on , Ar i zona by L.D. Ricke t t s and h i sas so ci a t es in 1912. This was th e same er ath a t Louis Cates was s en t to Ray, Arizona toi n i t i a t e c av in g t he r e .A t I n s p i r a t i o n , my work was a s a S h i f tBoss, Foreman and 1a t e r Genera l Ex t rac t ionForeman. D u r i n g t h i s pe r iod the re was worryover the amount of waste that had been drawnwith t he o re during th e World War I y ea rs .Clyde Weed, Assistant General Manager andMcDonald se l e c t e d me as Genera l E x t rac t ionForeman to work w ith th e D ivis ion Foreman andt o be a p a r t o f th e o p e r a t i n g s t a f f . I n p r i o rye ar s , ex t ra ct io n had been handled by th e MineEnginee ring Department. This change meantth a t both the t onnage p roduced and i t s qu a l i t ywas an o pera t ing responsi b i l i ty . Seven year sl a t e r I was t r a n s f e r r e d t o C h i le a s A s s i s t a n t

Mine Superin tende nt a t the P ot re ri l l os Mine ofthe Andes Copper Mining Company, again ac a vi ng o p e r a ti o n . T h i r t e e n y e a r s l a t e r i w ast ransferred to the Chi le Explorat ion Companya t Chuquicamata as General Wanager. Planningwas underway f o r th e development of Chuqui a san underground block caving operation, achange that I was successfu 'l i n r e v e r s i n g i nfavo r of con t inu ing open p i t ope r a t ions f o rano the r f i f t ee n to twenty yea rs when thema t ter coul d be restu died .During the Inspiration-An des perio d, manychanges were made in planning always i n ane f f o r t t o i mp ro ve c o s t s a nd e s p e c i a l l y t oimprove ore drawing. A copy of a short reportwr i t t en in 1928 fo l lows . A t t h a t t ime thechange was made from a f ou r po st s e t loc ate di n t h e r a i s e t o t h e u n d e rc u t f rom t h e g r i z z l yl e v e l t o a c o n tr o l s e t a t t h e g r i z z l y . Theresu l t s o f t he change were be t t e r o re recoveryand a lower per ton co st . This unpublishedr e p o r t w r i t te n i n 1928 fo ll ows:Ore Ex t rac t ion a t t he In sp i r a t i on Conso lida tedCopper Company

"The Inspi ra t ion Consol idated CopperCompany mine consists of three separated i v i s i o n s , e a c h o n e d i f f e r i n g r a d i c a l l y i n t h echaracter of the ground and consequent ly int h e d e t a i l s of c av in g an d o r e e x t r a c t i o n . Inthe 01 de s t d iv i s ion , t he In s p i ra t i o n MainShaft , both oxide and Sulphide ore are drawn.In general , the ground when undercut cavese a s i l y , b u t o t h e r w i se i t s t a n d s we1 1 . A t t h eLive Oak Divis ion th e production i s mostlysulp hide or e a l though on one level bothc l a sse s o f o re a re drawn. In t h i s d iv i s iont h e o r e c av e s e a s i l y a f t e r u n d e rc u t ti n g andpacks rap id ly i f not drawn. The gr iz zl y andhau lage d r i f t s a re ex t remely heavy andd if f ic u l t t o hold open. The Sulphide TunnelDivis ion i s the main sourc e of the oxide orea1 though a smal l qu an t i ty of su lphide i sd rawn . As th i s l a t t e r l i e s below the ox ide ,i t mus t be d rawn ou t f i r s t be fo re any ox idecan be produced. W i t h t he excep t ion of t hea rea c rosse d by a fa u l t , t he g round i n t h i sd i v i s io n s t a nd s w e l l. In t h i s r e s p e c t , i tl i e s between the Live Oak Div ision and theI n s p i r a t i o n D i v i s i o n .The main problems met in the work can bec l a s s e d a s : (1 ) se l ec t ing the mining sys tem,( 2 ) making the ground cave, ( 3 ) main ta in ingthe permanent workings, ( 4 ) recovery of oreand con t ro l o f was t e , (51 sepa ra t in g the twoc l a s s e s of o r e , a n d ( 6 ) t r a n s p o r t i n g o f t h eore . In th e succeed ing pages the se problemsa s r e l a t i n g t o o r e e x t r a c t i o n w i l l b ed i scussed .In gene ra l , t he cha ra c t e r o f t he g roundbeing caved i s ind ic at ed by the area drawn byone drawing po in t. Thus a block of ha rd,coa rse -b reak ing o re has l a d rawilng po in t a t t hece nt er of re ct an gl e 21 by 25 . In the sametype of ground b u t where g r ea t e r con t ro l ofdrawing i s necessary because of t he wasteoverbu rden, t he d rawing po in t i s a t t he cen te ro f a reas 12-1/2 ' by 1 6 ' . In ano ther blockwhich had considerable overburden above theor e, drawing was eff ec te d by means of sq uares e ts from which four f in ge rs were run ou t to

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the undercut lev e l . The area over the squares et was 25' by 21' and t h a t drawn by eachf ing er was 12-1/2 ' by 10-1/2 '. I n s im i l a rground a second method was fou nd t o have avery great advantage over the square setsystem, I f the ground does take weight, i t i smuch eas ier t o keep the fewer drawing po in tsw hic h ar e l o c a te d on th e g r i z z l y d r i f t i n ar u n n in g co n d i t i o n t h an t o ha ve t o kee p i nr e p a i r t h e g r i z z l y d r i f t , s qu are s e t a nd th ema in ra i se se rv ing it. I n the weak groundencoun tered i n the L i v e Oak D iv i s io n , squaresets have been la rg e ly re p la ced as a drawingco n t r o l se t i n f a vo r o f a system o f d r a w in gp o i n t s so c lo s e t o t h e g r i z z l y d r i f t . Thema i n t r o u b l e s w i t h t h e sq u a r e se t s we r e t h a t(1 ) the rep a i r s were excess ive and (2 ) thebacks o f the f i n ge r r a i s es wou ld cave and thesp ac in g o f d r a w in g p o i n t s c o u l d n o t bema inta ined . Bo th o f these fac t o r s caused poore x t r a c ti o n . T he re a re i n o p e r a t io n i n t h i sweak g round severa l b locks us ing the g r i z z l yl e v e l c o n t r o l b u t t h e a c t ua l d e t a i l s i n eachcase a r e s l i g h t l y d i f f e r e n t , The sys te m mo stf av o re d a t t h e p re s en t ti m e has t h e g ~ i z r l yl o c a t e d a t t h e c e n te r s f r e c t a n g le s 2 5' by15 ' . Va rs'a ti ons o f t h i s sys tem have beenm a in ly i n t h e s pa cin g o f g r i z z l i e s a nd g r i z z l yd r i f t s . G r i z z l i e s a t t h e c e n te r o f a re as 2 0'by 20 ' 20 ' by 15 ' , 20 ' by 12- l /Z1 , 25 ' by 75 'and 2 by 12 -1 /2', have been t r i e d w i t hs l i g h t a d v a n t a g e t o t h e 25-y 15 ' spacing, AV -set co n s t r u c te d on b o th s i d e s o f t h e g r i z z l yi n p l a ce o f t h e a dva nce se t s was t r i e d b u t t h el a t t e r i s s im p le r i n c o ns tr uc ti on . T h is1a t t e r i s an e xc e ed in g ly i m p o r t a nt p o i n t i nt h i s ground which " fun nels " and packs. It i se v i d e n t t h a t by s t a g ge r in g t h e g r i z z l i e s i none d r i f t w i t h r e s p ec t t o t ho se i n th e n e x td r i f t , a more s a t i s f a c t o r y d i s t r i b u t i o n o fdrawing po in ts would be made, There ar ec e r t a i n d i f f i e u l i e s i n d ev elo pin g a b l o ckl i k e t h i s w h ic h have u p t o t h e p r es e n tp r eve n te d i t s a d op ti on .The most imp or tan t o pe ra t i o n i n mak ing theg round cave, rega rd less o f the sys tem, i s theundercu t t i ng . I n the ve ry weak g round i t i so f t e n d i f f i c u l t t o h o l d open t h e w o rk in gs o nthe undercu t l e ve l l on g enough t o make athorou gh undercut, However, i f t h i s i s d oneand a good unde rcut i s made, th e ground cavese a s i l y and w i t h o u t f u r t h e r he lp , I n t h es t ronger g round it i s necessary t o weaken thes i de s o f t h e b l ock . Th i s i s a cco mpl ish ed byv e r t i c a l r a i s e s on t h e c o r ne r s o f t h e b lo c kw i t h p e r ip h e r a l d r i f t s r u n n i n g a ro un d t h eb l oc k c o n n e ct in g t h e v e r t i c a l r a i s e s . It wasfound that the system promoted caving andr e ta r d e d t h e t en de ncy o f t h e b l o ck t o sh e a ra long i t s boundar ies as migh t happen i f t h i nsh ri n ka g e s to p es were u sed t o c u t o f f t h eb lock, It a l so h e lp s t o f l a t t e n o u t t h ena tu ra l a rch fo rmed du r ing the cav ing p rocess ,and w i t h t h e v e r t i c a l r a i s e s t o p re v en tr o u n d in g o f t h e cor n e r s o f t h e b l o ck . B o th o ft he se t h i n g s c o n t r i b u t e t o p o or e x t r a c t i o n i ft h e b l o ck i s ov er la in by easy- runn ing waste ,A f t e r an area has been undercut , whether o rn o t t h e c h u te s a r e k e p t i n w o rk i ng c o n d i t i o na f f e c t s t h e e x t r a c t i o n . I f t h e p i l l a r b etweent h e u n d e r c u t t i n g l e v e l and t h e g r i z z l y l e v e lhas been too th in , the we igh t may c rush i n the

2 84-29d r i f t s and case spo t ty d rawing . Th is we igh tp o ss i b l y may b e t r a n s f e r r e d t o t h e h au la ged r i f t , w hi ch c o m p li c at e s t h e t r o u b le , I npa r t s o f th e mine where a weak su lph ide orel i e s b el ow th e o x i de , i t has been no t i ce d th a tt h e we i g h t i s h e a v i e s t as l o n g as su l p h i d e o r ei s d raw n, a nd t h a t t h e k e y i n g a c t i o n o f t h eo x id e b ou ld e rs p r o te c t s t h e g r i z z l y d r i f t s .Usu a l l y f ewe r r e p a i r s a r e n ece ssa ry i n t h osed r i f t s w hic h ar e c lo s e t o t h e p e ri p he r y o f t h eb lock , Th is i s undoub ted ly due t o thes u p p o r t in g a c t i o n o f t h e b ou nd ary p i 11ars.One hears o f drawing an area t o reduce t hew e ig h t; a c t u a l l y t h i s i s i mp ro ba bl e. I nc a v i n g b l o c k s o f o r e i t i s n o t p o s si bl e t oease the we igh t by drawing heav i ly , Drawingsma ll a mo un ts a t f r e q u e n t i n t e r v a l s may h e l pbu t un usua l ly heavy drawing i s bad a1 though i tdoes sho r ten the t ime tha t the a rea must bemainta ined,E x t r a c t i o n i s o f t e n c o n si d er e d a p r ob le m o fc on tr o l le d drawing. However, i t i s more t h a nj u s t t h i s , o t h er f a c to r s en t e r i n t o th eproblem and i f t h e y a r e n o t h a n d l e d co r r e c t l y ,c o n t r o l l e d d ra win g i s l i t t l e h elp , As n e a rl yas poss ib le , th e human fa c to r shou ld beremoved f rom ex t rae t i on. Bl ocks should bep l an n e d w i t h e ve r y p o ss i b l e p r e ca u t i o n t osecure cav ing and t o keep ou t and na tu ra l l yc o n t r o l t h e w aste. I n a l l b u t t h e w ea ke stg ro un d, as i n p a r t s o f t h e L i v e Oak D i v i s i o n ,p i l l a r s s ho uld be l e f t on a11 s id e s o f t h eb lock . The b lo ck should be weakened f rom th i sp i l l a r by t h e u su al m ethod o f v e r t i c a l r a i s e sa nd p e r i p he r a l d r i f t s . The s i z e o f th e b l oc kshou ld be such tha t i t can be drawn rap id lya f t e r c a vi ng s t a r t s . A v e r t i c a l h e i g h t o f 3 'p e r 24 ho u rs o ve r t h e b l o ck i s a s r a p i d a snorma l l y adv i sab le . F req uen t l y th e hau l aged r i f t c ap ac it y i s t he l i m i t i n g f a c t o r i ndrawing a b lock . B locks 150 ' x 150' have beens a t i s f a c t o r y i n s iz e. I n a l l ex ce pt t hes t r o n g e s t a nd mo st r e s i s t a n t - t o - ca v i n g b l o cks ,open ing up, un der cu t t i ng . and d rawing i sp rog ress i ve ,As waste appears i n chu tes where ex t r ac t i oni s 80 % o r g r e a te r, a nd th e g ra de o f t h e o r edoes n o t improve on pack ing and reopening, th eback o f t h e d r i f t i s g l or y -h o le d . O ve rd ra win gf r e q u e n t l y b r i n g s i n t h e w as te b e f o r e t h e o r ei n t h e b l o ck i s more t h a n 3 0% e x t r a c te d ,Sl ower drawing , a1 though appear ing t o re qu i ret h e d r i f t s t o b e m a in t ai n ed l o n g e r t h an w ou ldb e d e s ir e d, p e r m i t s a g r e a te r a r e a t o b edrawn. F r e q u e n t ly th e sh o r t p e r i o d o f p a ck in gbetween s h i f t s i s s u f f i c i e n t t o r e q u i re asimp1 e r e -u n d e r cu tt i n g . Th i s a c tu a l l y i s goo dprocedure and keeps the drawing area la rg e rt h a n wo u l d b e p o ss i b l e i f the chu tes ranf r e e l y a n d f u n n e l l dn a t u r a l y.O cca s i o n a ll y t h e wa ste r u n s i n u n d er t h eo r e b e fo r e t h e l a t t e r ha s caved. Th i s i su s u a l ly due t o an i n s u f f i c i e n t p i l l a r o rh a v i n g a vo i d t o o c l o se t o t h e wa s tea d ja c en t, I n t h e l a t t e r c as e t h e bo un da ryp i l l a r b reaks up and the was te sweeps i n underthe ore .I n o r de r t o d raw e v e n ly a n d t o t h e b e s ti n t e r e s t o f e x t r a c ti o n , a s p ec i al o r g a n i z a t io nwas developed. Th is inc lu des a Ch ie fEx t ra c t i on Foreman, rep resen ted a t eachd i v i s i o n b y a d i v i s i o n e x t r a c t i o n f orem an.

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Chute checkers working w i t h t h e d i v i s i o next ract ion foremen operate i n t he b lockssup ervis ing th e drawing and handing in a t theen d o f t h e s h i f t a d i s t r i b u t i o n r e p o r t o f t h eore drawn. S h i f t bosses and foremen ar efurnished weekly w i t h a record of every chutei n t h e i r b l o c k s , t h e t o n s es t i m a t e d , t o n sdrawn, and th e remaining . Th is same rec or d i spos ted in t he b lock a t each g r i zz ly . S pec i a le f f o r t i s made t o t r a i n t h e c h u t e c h ec k er s i njudging ore. They tak e da il y samples andbecause of th i s work ar e i n a good posi t ion tojudge the m ateria l drawn. A keen sense ofr i v a l r y e x i s t s i n t h i s j ud g in g of o r e , w hichbenef i t s t he sys t em.In ce r t a in pa r t s o f t he mine the su lph ideo re i s ove r l a in by ox ide o re . A sepa ra t ion o fthese two c l a s se s i s requ i red by th e reduc t ionp l a n t s . T h i s is largely accompl ished by thechu te checker who rece ives i n s t ru c t io ns a s t owhat i s oxide and sulphide. In drawing ab lock of t h i s m ixed o re t he re a re f i v es t a g e s : ( 1 t h e s u l ph i d e s t a g e , ( 2 ) t h e m ixedox ide and su lph ide s t age , ( 3 ) t he ox ide s t age ,( 4 ) the oxide and waste s ta ge , and (5 ) waste.Approximately 70% of the su lph ide ore i sremoved i n t h e f i r s t st a g e, t h e r e s t i s drawnin the second s t age a l t hough o f t en due to t hebreaking up of a p i l l a r a chute of mixed orew i l l r e v e r t back t o s u lp h i de . E f f o r t s a r ed i rec t ed towards keep ing the chu tes i n t hesame cl as s of ore . This makes the sep ara t ione a s i e r . The a c t u a l s e p a r a t i o n o f t h e o r e i smade in th e con tro l s e t by drawing only onecl as s of ore a t a t ime. The usual method i sto draw a pres cribe d number of t r a in s of onec l ass o f o re and then change to t he o the rc l a ss . Th i s sys tem i s simp le and genera l lys u c c e s s f u l . In c l o s i n g o u t t h e b l o ck , g l o r yho l ing ope ra t ions u sua l ly b r ing the su lph idee x t r a c t i o n t o a b o u t 9 0 % .T he s e r v i c e a b i l i t y o f t h e r a i s e s f ro m t h eg r i z z l y t o t h e ha ul ag e l e v e l a f f e c t t h ee x t r a c t i o n i n th e sto pe . Rais es which hang upf r e qu e n tl y , o r a r e to o f l a t , i n t e r f e r e w i t hth e production and th e drawing schedule. Formuck which breaks f i n e and has a tendenc y t opack , s i x t y deg ree ra i se s were found to bem o r e s a t i s f a c t o r y t h a n i f t h e y w e r e f l a t t e r .In running much of th is type, especia l lythrough 1ong ra i ses , i t o c c a si o n al 1 i snecessary t o draw by te lephone, running themuck wi thou t s topp ing i t , d i re c t ly i n t o t hec a r s . Muck o f t h i s ty p e i s l e f t i n t h e s er a i s e s f o r a s h o r t t im e r e q u i r e s f r e q u en tb l a s t i n g b e f o r e i t runs ag ain . Coarse muckcan be hand led th rough ra i se s a s f l a t a s 50".I found tha t 54' ra is es were th e be st and gavel e s s t r o u b l e t ha n a ny o t h e r s .

A sho r t ve r t i ca l sec t ion immed ia t e ly be lowthe g r i zz ly p reven t s a hang-up ju s t be low theg r i z z l y a f t e r t h e r a i s e h as been f i l l e d . I ta ls o helps speed up muck pa ssi ng thro ugh ar a i s e f l a t t e r t h an d e s i r ab l e .In s h o r t r a i s e s , g r i z z l y d r i f t s s ho ul d r u np a r a l l e l t o t h e ha ul ag e d r i f t . G r e a t e r s pe edand superv i s ion in d rawing i s poss ib l e t han i fg r i z z l y d r i f t s r u n a t r i g h t a n g le s t o t h ehau lage d r i f t . In ra i se s o f 10 ca rs , , o r morec a p a c i t y t h i s is o f l i t t l e im p or ta nc e.

W hile I was a t I n s p i r a t i o n , a s e r i o u s l e g a l

conflict developed between two miningcompanies in Nevada. While at te nd ing th a tcase, I made the acquaintance of many miningpeople who were interested i n caving. Thesein cl ud e Louis Ca tes , Will iam Boyd, WalterLarsh and John Kinnear Sr ., Dean Pr ob er t ofthe Universi ty of Cal i fornia and many others .Three of us rode w i t h Cates every day of thelong drawn-out t r ia l and enjoyed the informalta lk s we had drivi ng from Reno to Carson City.In t he fa l l o f 1935 , I was t ran s fe r re d toPotreri l los , Chi le , and had only a few monthsto break in before the f ine MineSuperintendent Bob Van B u s k i r k r e t i r e d . I.L.Greninger was then Assistant General Manager.Greninger had previous mine experience and hada good understanding of the problems his staffwere facing.Opera tions a t t he P o t re r i l l o s Mine werest ra ig h t forward and wi thout much t ro ub le fromweight and crushing. A t I n s p i r a t i o n t h e r ewere areas of the Live Oak Division on the1200 lev el in which one had t o craw l.Squeezing ground would c lose in grizzlyd r i f t s , r e s t r i c t i n g t h e p a s sa g e o f men a ndsu pp li es . The weight was even a problem on he1200 hau lage l eve l , and requ i red fou r t o s ixr e p a i r c re ws a s h i f t t o s e t back p o s t s a ndra is e caps in the haul age 1e v e l . On t h e u pp er1evel s , however, th is problem d id not e xi s t .As mentione d above, ground we igh t was no t as e r i o u s p ro ble m a t t h e P o t r e r i l l o s M ine,excep t i n one ar ea . During the World War I1ye a rs , we mined ox ide o re , su lph ide o re andp y r i t e a s o u r so u rc e of s u l f u r f o r t h e a c i dp l a n t .I hav e bee n as ke d many tim e how we knewwhether th e ground wool d cave well f o rmining. . Professor Phil ip Buckey of Columbiaalways wanted to d iscuss that problem.Ac tua lly one does not know and hi s judgment i sbased on h is experiences.After two episodes wi th b locks that d id notcave as expected , you approach th is problemca re fu l ly . The ore body as well as the rockabove i t must be examined fo r s l i p o r f r ac t u r ep l a ne s . I o c ca s i o n a l ly c a r r i e d a h a m e r s o Ic ou ld t a p t h e s i d e s of t h e d r i f t s o r a t t i m es ,even s t r i ke a rock w i t h a 4-pound hammer. I fthe sound was d u l l , i t probably would cave.However, i f the b low re su l te d in a r i ng ingnois e , I would have doubts about i t s cavingq u a l i t i e s .While a t Ins pi ra tio n we developed a blocki n t h e P i n t o a r e a , u n de r cu t i t an d th e n w a i te df o r i t to s t a r t caving. There was no weightand only occasionally would you hear any rockf a l l f rom th e back of th e undercut zone. Wfi na l l y en1 a rged the a rea undercu t byundercu t t i ng 20% o f t he ad j acen t b lock .Several weeks la ter , I received a phone cal lfrom Bob Watts, the Sh if t Boss. I t was 2 a.m.and Bob to ld me th a t t h e re was cons t an tcrac king n ois es in th e block and he had takenthe men out of t he are a but had l e f t theequipment. He was to ld to get everyone awayfrom th e area . Within two hours the e n ti reblock had sheared on i t s f our s id es andcrushed out every opening in b lock--haulaged r i f t s , m ain r a i s e s and g r i z z l y l e v e l . Wnever at tempted t o open up and re pa ir t heblock.

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Ten years l a t e r a block of ox ide ore a tTunnel 16 a t Andes was devef oped, and the nunde rcut. There was no si gn of cav ing , now ei gh t a t a l l b u t suddenly a t 10 a.m. onemorning, th er e were heavy sounds of rockdropping from the back i n th e undercut , Themen were sen t ou t t o the surfa ce to e a t anea rl y lunch, Suddenly th e whole back broke upand dropped the 15 to 20 fe e t to the undercutl eve l and i t s f inge r ra i s es . The suddeness oft he co l lap se fo rced t he a i r ou t o f eve ry r a i s eb u t did no damage. One man who was s i t t i n g ona r a i l ou t s i de the po r ta l o f t he a d i t wasbadly h u r t by an or e c a r which was pushed o u tof t he ad i t a t a h igh speed by t he a i r b l a s t .Two en gi ne er s who had remained in th e minemanaged to cl i ng t o t imbers and pipe un t i l thepressure dropped to normal.A t th e r equ est of Pat ino management a tS i g l o Veinto in Bol iv i a , I went to th e i r majormine to give an opinion as to whether theyshould change t h e i r mining system t o BlockCaving. Af ter a week of examin ation Ireported that caving would work except i n onear ea ca ll e d Las Animas. Kanagement decide d t ochange to caving. I have heard sin ce tha tcaving worked well in every p a r t of th e mineexcept the Las Animas area in which a blockfa i l ed t o cave .The best dimensions for a block which wasto be undercut and allowed to cave depended ont he physi cal cha r ac t e r i s t i c s of t he rock . Inweak ground, th e be s t si z e might be 100' x10 0' . Draw po in t spacing would be clo se andthe r a te of draw 3 ' a day o r even more.. Thel i f e of t he b lock would be re l a t i ve ly s hor t .For medium to strong rock the block wouldbe developed w i t h an area 150' by 150' or evenas large as 180' by 180' and the rate of draww ould s t a r t a t 8 t o 10 i nches a day. A f t e r i twas certain that caving was proceeding wellthe ra t e of draw could be s t ea d i l y increased ,The area of the caving blosks was alwaysrec tang ula r. I f any weakening of the so l i dsides of the ore blocks was consideredadvisable, caving could be speeded up byweakening the block co rn er s. Th is was done byd r i f t s , r a i s e s , o r even c ar r y i n g s h o r tshr inkage s topes f rom the ver t i ca l cornerra is es to the sid es of the block. However toomuch block weakening might ca us e a block t oshea r and se t t l e as happened i n t he ca se spreviously ment ioned.Although i t has not been t r ied, making oreb locks c i rc u l ar i n s t ea d of square seems tohave some me rit . I t would eli m ina te the blockcor ner s which always ar e a source of worry andi t should e l imina te th e co s t o f weakening theso l i d s i de s of t he b l ock , The i r r e gu l a rpi l 1ars be tween the c i rcu lar bT osks coul d berecovered as a secondary operat ion,Blocks that have been observed, almosti n v a r ia b l y br ea k t o t h e s u r f ac e f i r s t a s around hole , about twenty or t h i r t y f ee t i ndiameter. I t would be months befo re th e ful lr ec t angu l a r shape of t he b lock i s ev i den t a tthe surface above the block.Tom Dudley, w i t h a l o t of cav ing exper iencebehind h i m , sugges t s t h a t i n the fu tur e whenmining i n rough t e r r a i n t ha t c av ing be s t a r t edunder the h igh poin t s of t he surfa ce . I t ishis th roug ht th a t many of th e surges o f weight

i n a block come as a r e s u l t of ground cra cki ngabove and ou tsi de of a block. This wouldr e s u l t i n t h e g r i z z l y 1evel of t he blockcarry ing not on ly the weight d i re c t l y above i tb u t s u pp o rt in g a ls o a t l e a s t p a r t i a l l y t h eweight of the large rock mass that has crackedloose f rom a h igh poin t and i s s l id i ng overand down toward the block being drawn.There were never any weight problems i n th eoxide blocks. Cracking and subsid ence abovethe sulphide blocks did extend outward fromthem a t an angle as f l a t as 20 t o 25 degrees .The question has been asked whether therei s a maximum height of ore which can bedraw n. W m ined ou r py r i t i c b locks i n onel i f t of more than 350 f e e t with no problem.The gr i zz ly l eve l s tood u p well and we neverhad any problems w i t h cav ing i t o r d rawingi t . T h is i s a ques t ion o f w ea r i n he r a i s e sand on t he g r i zz l y s e t s . I f t h e g r i zz l y l eve lhad been d i f f i cu l t t o m ai n ta i n f o r o redrawing, there were t imes when the chance toge t below th e block on a deeper lev el lookedpre t ty a t t ra c t i ve . However i n good ground,the he ight of t he ore b locks could beincreased and 400 t o 600 f e e t l i f t s s ho ul d n o tbe avoided.Di lut ion : Di lu t ion by su rf ac e waste i n asulphid e block o ften shows up a f te r a blockhas been drawn 25% t o 301. The s l i g h t changei n c o l o r, t o i nc lu d e a l i t t l e brown, i s t h eonly ear ly ev idence th a t cav ing i s above th eor e zone. Ore drawing pr op er ly done, i s t h ekey t o good re su l t s such a s completely drawinga block w i t h 110% of th e tonnage and 90% ofthe copper .Using the caving system in mining i s verychal l enging . There i s always the ques t ion ofwhether the block wi ll cav e well and what r a teof production would p roduce t he be s t r e su l t s .I s th e ore body wide enough? I s i tf ra c tur ed enough t o cave wel l ? 1s themater ial above the ore body well fr ac tur ed ?I s the mater i a l above the o re body sand orgravel, which doul d bad l y d i l u t e t he o re?What would be the best spacing of thedrawpoints i n t he b lock? Will the rock belowt he o re body suppo r t t he d r i f t s and r a i s e swhich w ill be needed the re ? In mines where Ihave worked the rock seems to be lessf r a c t u r e d i n t he deeper l ev e l s . This d id nothold t r u e a t th e Live Oak Division ofIn sp ir at io n, where th e 1200 lev el had some oft e most severe maintenance c ond i t ion s of myexperience.These are some of the subjects which mustbe considered i n the pianning for a new BlockCaving mining op era tion . Ne ver the less , BlockCaving i s a marve lous mining sys tem i f i t sunderground orga niza t ion i s exper ienced . I tis my b e l i e f t h a t i n a new o p e ra t i on a t l e a s ttwo of the senior employees of the mine shouldhave had previous experience i n severa ldi ff er en t block caving mining oper at ions. Them ost c r i t i c a l pe r i od i s t he unde rcu tt i ng of anew block which must be done completely andrapid ly . I f i t is too slow, the block cans t a r t t o t a k e w e ig h t, c r u s h in g p i l l a r s anddr i l l ho les and even crush ing the gr i z z ly1evel .There have been many improvements i n miningd e t a i l s i n t he pas t twenty yea rs . Gr i zz ly

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