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"~

SynopsisC av in g is th e lo west-cost undergroundm in in g m eth od p ro vid edth at th e d rawp oin tsp ac in g, d rawp oin t size ,and ore -hand li ngfacilities are designed tosuit th e ca ve d m ateria l,an d th at th e d raw po in thorizon can bem aln ta ln ed fo r th e lifeqfthe draw. In the nearfu tu re , se ve ra l o pe n-p itm ines that produce m orethan 50 kt per dqy willhave to exam ine thef ea sib ility q f c on ve rtin gto low- co st, la rg e- sc aleunderground oper at ions.S ev era l o th er la rg e-s ca le , l ow- gr adeunderground oper at ionsw ill e xp er ie nc e mq jo rch ang es in th eir m in in gen viro nm ents a s larg ed rop do wns a reimplemented

The se c hang esd em an d a m ore re alistica pp ro ach to m in eplanning than in thep ast, w he re e xistin go pe ration s h ave b eenp rq je cte d to in cre ase dd ep th s w ith littleco nsid era tio n q f thech ang e in m iningen viro nm ent th at w illo cc ur. A s e co nomic sfo rc e th e c on sid er atio nq f u nd erg ro un d m in in gq f la rg e, c ompete nt

. p.a. Box 95 ,Boesmansriviermond6 19 0. C ap e P ro vin ce .

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C a v e m ining-the state of the arto re bo die s b y low- co stm ethods, the role qfcave m ining w ill haveto be dUined In thepast, caving w asgenera llY cons idered forrockm asses that cavea nd fra gm en t re adilY .T he a bility to de fin ec avab il it y a nd fr agmen -t at io n, t he a va il ab il it yq f la rg e, r ob us tl oa d-h au l-d ump u nits ,a b et te r und er st andi ngq f draw-con tro lr equi remen ts , improv edd rilli ng e qu ipme nt fo rse co nda ry b la stin g, a ndreliable cost data havesh ow n th at c om peten to reb od ies w ith c oarse

fragmentation can beexploited by cavem ining at a m uch lowerc ost th an b y d rill-a nd -b la st me thod s.

~ 280

It is com mon to find that old establishedm ines, w hich have developed standards duringthe course of successfully m ining the easytonnage in the upper levels of the orebody, have aresistance to change and do not adjust to theground-control problem s that occur as m iningproceeds to greater depths, or as the rock typesc ha ng e. M in es tha t ha ve ex perien ce d co ntinu ou sproblem s are m ore am enable to adopting newtechniques to cope w ith a changing m iningsitu ation . D etailed k no wle dg e of loc al an dregional structural geology, the use of an acceptedroc km ass classifica tio n to ch arac terize therockm ass, and know ledge of the regional andinduced stress environm ent are prerequisites forgood m ine planning. It is encouraging to note thatthese aspects are receiving m ore and m oreattention.

The Laubscher rockmass c lass if ica ti ons ys tem th at p ro vid es b oth ro ckma ss ra tin gs a ndro ckma ss s tre ng th is n ec es sa ry fo r d es ig np ur po se s. The ro ckma ss ra tin gs (RMR) d efin eth e g eo lo gic al e nv iro nment, a nd th e a dju ste d o rm in in g ro ckma ss ra tin gs (MRMR) con sid er th ee ffe ct o f th e m in in g o pe ra tio n o n th e ro ckmass .The r ati ng s, d eta ils o f th e mi ning env ir onmen t,a nd the w ay in w hich th is a ffe cts th e roc km assa nd g eo lo gic al in te rp re ta tio n a re u se d to d efin eth e cav ab ility , s ub sid en ce ang le s, f ai lu re zones ,f ragmen ta ti on , undercu t- face shape , cave-f ron to ri en ta ti on , undercu tt ing sequence , overal lm in in g s eq uenc e, a nd s up po rt d es ig n.Factors Affec ting Caving Opera tionsTwen ty fiv e p arame te rs th at s ho uld b ec on sid ere d b efo re th e imp lemen ta tio n o f a nyc ave m in ing o pe ra tio n a re se t o ut in T ab le H . T hep arame te rs in c ap ita l le tte rs a re a fu nc tio n o f th eparam eters that follow in the sam e box. M any ofth e p arame te rs are u niq ue ly d efin ed b y th eorebody and the m ining system , and are notd is cu ss ed f ur th er . The par ame te rs con sid er edla te r a re co mmon to a ll ca ve -m ining sy stems a ndneed to be addressed if any form of cave m iningis contemplated.

O CT OBER 1994

CavabilityMonito rin g o f a la rg e n umbe r o f c av in gop eration s h as sh ow n th at tw o ty pes o f c av in gca n o cc ur. T he se ha ve b ee n d efine d a s stre ss a ndsubs idence caving .

S tre ss c avin g o ccu rs in v irg in c ave b loc ksw hen the stresses in the cave back exceed thestren gth of the ro ckm ass. C aving c an sto p w he na stable arch develops in the cave back. T heun de rc ut m ust b e in cre ase d in siz e, o r b ou nd aryw ea ken in g must be u nd erta ke n to ind uc e fu rth ercaving.

S ub sid en ce c aving oc curs w he n a djac en tm inin g h as remov ed th e la te ra l re stra in t o n theblo ck b ein g c av ed. T his ca n re sult in ra pidp ropaga tio n o f t he cav e, w it h lim it ed bul ki ng .F ig ur e 1 illu stra te s th e e ffe ct o f removin g th ela te ra l re stra in t from b lo ck 1 6 a t S habani. B lo ck16 had a hydraulic radius of 28 w ith an M RM Rof 6 4 a nd a sta ble , a rch ed ba ck. T he ad ja cen tblock, no. 7, w as caved and resulted in areduction in the M RM R in block 16 to 56, atw hic h sta ge ca vin g o ccu rre d. F or a ra ng e o fMRMR,F igur e 1 illu str ate s wo rl dw id e cav ingand s tab le s it ua ti ons.

T he stresses in the cave back can bem odified to an extent by the shape of the cavef ro nt. Numeric al mod ellin g c an b e a u se fu l to ol,h elp in g th e e ng in ee r to d ete rm in e th e s tre ssp atte rn a ssoc ia ted w ith s ev er al pos si ble min ings eq uenc es . An u nd erc ut fa ce , c on ca ve towa r~ sth e c av ed a re a, p ro vid es b ette r c on tro l o f majo rstruc tu re s. In o re bo die s w ith a ran ge o f M R MR ,the o nse t o f co ntin uo us c av ing w ill be b ase d o nthe lo we r ra tin g z one s if th ese are c ontinu ous inp la n a nd s ec tio n. This is illu stra te d in F ig ure 2B,where the class 5 and 4B zones are shown to bec ontin uo us. In F ig ure 2 A, th e p ods of c la ss 2r ock a re suf fic ientl y la rg e to in flu en ce cav ing,a nd th e c ava bility sh ould b e ba se d on th e ra tin go f th es e p od s.

A ll ro ckm asses w ill c av e. T he m ann er ofth eir c av in g and th e re su lta nt fra gmen ta tio n s iz ed is trib utio n n ee d to b e p re dic te d if c av e m in in gis to b e implemente d s uc ce ss fu lly . The ra te o fc av ing c an be slo we d do wn b y c ontrol o f th ed raw sinc e th e c ave c an p ro pa gate o nly if th ereis space into w hich the rock can m ove. T he rateof caving can be increased by a m ore rapida dva nc e of the u nd erc ut, b ut p rob lems c an a riseif this allow s an air gap to form over a largea re a. In th is s itu atio n, th e in te rs ec tio n o f majo rs tru ctu re s, h ea vy b la stin g, a nd th e in flu x o fwate r c an re su lt in d amagin g a irb la sts . R ap id ,u nc on tro lle d c av in g c an re su lt in a n e arly in flu xof was te d il ut ion .

The r ate o f under cu tt in g (RV) s ho uld b ec on tro lle d s o th at ra te o f c av in g (RC) i s f as te rthan the rate of dam age (RD);

RC>RU>RD.T he J ouma l o f T he S ou th A fr ic an In stitu te o f M in in g a nd M eta liu rg y

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C a v e m ining-the state of the artT ab le 1 1Paramete rs to b e c on sid er ed b efo re th e imp lementa tio n o f c av e m in in g

P RIM AR Y F RA GM EN TA TIO N

R oc km ass stre ng th (M RMR)Geo log ica l s tru ct ure sJo in t/ fr act ure spac ingJ oin t c on di ti on r at in gsS tre ss o r su bsid en ce c av in gI nd uc ed s tr es s

LAYOUTFragmentationDrawpoint spacing and sizeM ethod of draw

U ND ERCU TT IN G S EQ UENCE(pre/advance/post)

Regiona l s tr e ss esR ockma ss s tr en gt hRockbur st po ten ti alR ate o f a dv an ceOre requ ir emen ts

DEVELOPMENTLayoutSequenceProductionDr il li ng and b las ti ng

P RA CT IC AL E XC AV AT IO N SIZ ERockma ss s tr en gt hIn s i tu stressI nd uc ed s tr es sCavi ng s tr e ss esS econ da ry b la st in g

D RA WP OIN T INT ER ACT IO N

D ra wp oin t s pac in gFragmentationT im e fram e o f w orkin g d ra wp oints

SECONDARY BLAST ING /BREAK INGSeconda ry f ragmen ta ti onD ra w m eth odD ra wp oin t s iz eSize o f e quip men t a nd g rizzly sp acing

S UP PORT R EP AIRTonnage drawnPoint and col umn loadi ngB row wearSecondary blast ing

T he J ouma l o f T he S ou th A fric an In stitu te o f M in in g a nd M eta llu rg y OC TO BER 1994 28 1 ....

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7 0

~ 6 0:: E~ : : E

5 0

4 0

3 0

2 0

1 0

A I0 1 0 2 0 5 0ClASS lA ,B ClASS 2A,B ClASS 3A,B~~11

Cavem ining-the state of the art10 0

90

8 0

0I0

3 0I

1 0 0 15 04 0 6 0

I2 00

7 00 2 0 50I

2 50 feet0

Figure 1-A s tabi li ty d ia gram for var io us m ines wo rldw ideHYDRAULIC RADIUS =Area/perimeter

CRO SS SEC TIO N B LOCK 7/2

STABLE: Only lo ca l s up po rt r eq uir ed .TRANSITIONAL: S up po rta ble in u ppe r b and , w ith a rc hin gin m iddle band and interm ittent caving/arching in low erband depending on outside influences such as blasting,water.CA V ING : P rogressive caving of cave back or w alls intop re vio us ly c av ed a re as .C av ab ility , f un ctio n o f:R o ckmas s s tr en gt hGeol og ic al s tr uc tu reI n s itu stressWaterI nd uc ed s tr es s ]

MRMR

0 Stable~ T ran sition al. Caving

80metres

CROSS SE CTION BLOCK WEST/2

~F igure 2 -Geomechan ic s c la ss if ic at io n dat a

O CT OBER 199482

.'., ..'; .

Il O O mC lA SS 4A ,B C lA SS 5A ,BI:;;~"~',":;;':"I ~

T he J ou rn al o f T he S ou th A fric an In stitu te o f M in in g a nd M eta llu rg y

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C a v e m ining-the state of the art

INPUT DATA =

CAVINGOPERATION =

G o o d geotechnicaIinformation,as well asmon ito rin g o f th e ra te o f c av in g and ro ckma ssd amage , is n eeded to f in e- tu ne th is r ela tio nship .F r a g m e n t a t i o nIn c av in g o pe ra tio ns, fra gmen ta tio n h as ab ea rin g on th e f oll ow ing:> - D rawp oin t spa cin g>- D ilution entry into the draw colum n>- D raw control> - Drawpo in t p roductiv ity> - S ec on da ry b la stin g/b re ak in g cos ts> - S ec on da ry blastin g d am ag e.

T he in put da ta ne ed ed for the ca lcu la tion ofth e p rimary fra gmen ta tio n a nd th e fa cto rs th atd ete rm in e th e s ec on da ry fra gmen ta tio n a s afu nc tio n o f the ca vin g o pe ration a re shown inF igur e 3 .Cav ing r esults i n p rima ry f ragmenta tio n,w hich ca n b e d efin ed a s th e siz e distrib utio n ofth e p artic le s th at se pa ra te from th e c av e b ac kan d e nte r th e d raw c olumn. T he p rim aryfr agmen ta tio n f rom s tr es s cav ing is g en er allyfin er th an th at from sub sid en ce c av in g owin g toth e ra pid p ro paga tio n o f c av in g in th e la tte r c as e,w it h d is in tegr atio n o f th e r ockmass , p rima ril ya long f avou rably o riented jo in t s et s, and li ttlesh ea rin g o f in ta ct ro ck . The o rie nta tio n o f th eca ve fro nt or ba ck w ith re sp ec t to the jo int se tsan d direc tion o f prin cip al stre ss ca n h av e as ign if icant eff ec t on the p rimary f ragmen ta ti on .

S ec on da ry fra gmen ta tio n is th e re du ctio nin siz e o f th e o rig in al p artic le th at e nte rs th edraw colum n as it m oves through the drawcolumn . The p ro ce sse s to whic h p artic le s a resub jected d ete rm in e th e f ragmenta ti on s iz ed is trib utio n in th e p artic le s th at re po rt to th ed rawpo in ts . A s tr ong, we ll- jo in ted ma te ria l c anre sult in a sta ble pa rticle sh ap e at a low drawheight. Figure 4 shows the decreasein f ragmen-ta tio n f or d if fe rent d raw heights and le ss -j oin ted( co ars e) to we ll- jo in ted (f in e) r ockmass es . Ara ng e in roc km ass ratings w ill result in a w idera ng e in fra gmen ta tio n s iz e d is trib utio n a sc ompa red w ith tha t pro duc ed by ro ck w ith as in gle ra tin g, s in ce th e fin e mate ria l p ro du ce d b yth e fo rm er te nds to c ushio n th e large r blo ck s an dp re ve nts fu rth er a ttritio n o f th ese b lo ck s. This isillustra ted in F igu re 2 B, in w hic h cla ss 5 a ndc la ss 4 m ate rial is shown to c ushio n the larg erp rim ary fra gm ents from c lass 3 . A slow ra te o fd raw a llows a h ig he r p ro bability o f time-d ep en de nt failure as the ca ving stre sse s w ork onp artic le s in th e d raw column .

F ra gmen ta tio n is th e ma jo r fa cto rde termining drawpo in t p roducti vi ty . Experi encehas show n that 2 m 3 is the largest size of blockthat can be m oved by a 6 yd LHD and still allowan a cc ep ta ble ra te o f p ro du ctio n to b emain ta in ed . In F ig ure 5 th e p ro du ctiv ity o f alayout using 3,5 yd, 6 yd, and 8 yd LHDs and ag riz zly a re re la te d to th e p erc en ta ge o f fra gmen tsla rge r tha n 2 m3. T h e usa ge of se co nd arye xp losiv es is ba se d on the amou nt o f ove rsizethat cannot be handled by a 6 yd LHD.

Orientation-CaveFront/Joints

PRODUCTION = Seconda ry B l as ti ngD am age a nd C osts

Op er at io na l C os ts a nd Ov er al l P ro du ct iv it y

F igure ~nput data for the calculation of fragm entation

The Joum al of T he South A frican Institute of M ining and M etallurgy O CTO BER 1994 283 ....

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C a v e mining-the state of the art350mm14"Gr

5 0 0 m m20' Gr 3yd IJID 6yd lJID

10 0

""

90.S 80f)(f)'"

70

'"" 80~ 50~ 40".. 30"'" 20100,1 0.01. 1 5% Dr il le d a nd B l as te d

= 8% o f O p er at in g C os ls

0. 1SIZE m 3 * 1 0 5 m DrawHeightPrimary Fragmentation- - - - S ec on da ry F ra gm en ta ti on

1.0 2.0

F ig ur e 4 -S iz e d is tr ib utio n o f c av e fr agme nta tio n

1400130012001100100090 0

-- ':a BO O[; 7000..'" 60 0 50 00E- 40 0

30 020 010 00

""'"

""....

0,1 1 5 10 2030 5070P e r c e n t a g e + 2m 3 - S econdary F ragm entation

10.0 100.0

35 0 "" .

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iPlX DII.IJBlU. APD: DRAIBlU.

I T - - T I ~

C a v e m ining-the state of the artDrawbell

DranoneDrawpoinl

18m 22m 24m11 1411.1 11111"11.111111"'11.111jl12mljjl Illllljl8m lljlljl~m llI I I 11 I I I j I I I I I I I I I I I j I I I1 /1?~II /I?~jjlljlllll~~lj~~1j '11 '1IIIlllljllfl\III(l\1I} \ riJ24m ll I I I Iloml...J2om ll I} \ [12mW18l.JI I \ I I L..-~ I j I \ I j L ~ I I L ~ I I L ~ j

~~..l :( n ~1 en ) W d W " W d ~ t j :.. ..30m ProduclionLevelF ig ure & -Max imum and m in im um d rawzon e s pa cin g (is ola te d d rawzon e = 1 0m a re a o f in flu en ce = 225 m ')

11

Lo rPressureZone

Vedl e. 1 S tr es sInere.sed by. dj .eent wo rk ingdrorpoint

- - Vedle.tR.dl.1

A . D IPs @ 2.2 X IDZ DIAM.D IL . ENTRY 15%

B. DIPs @ 1.5 X IDZ DIAM.D IL . ENTRY 6 0%

-- -

c. DIPs @ 1.1 X IDZ DIAM.D IL . ENTRY 8 5% D. DPs@ 1.1 X IDZ WORKED

IN ISOLATION DIL. ENTRY 25% IT - -TI~Figure 7 - The r esul ts o f t hr ee -d imens iona l sand-mode l e xper iment s F ig ure 8 -F low lin es a nd in fe rre d s tre ss es b etw ee nad jacen t work ing operat ionsT he J ouma l o f T he S ou th A fr ic an In stitu te o f M in in g a nd M eta llu rg y O CT OBER 1 994 285 ....

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Loading Width I !a ximum /l li nimum Spa ci ng o f D r aYZon es5m = 24/14m4m = 15/Bm 20/llm 22/13m3m = 10/5m 13/7m IB/IOm 21/12m2m = 9/4m 12/6m 16/9m

Areaofionuencem' - -~ 95 - - ~160 - -~ 290 --~360

C a v e mining-the state of the artR o c k m a s s ClassF F / mR o c k S iz e R an geL oa di ng W id th5m =4m =3m =2m =

550-70.01-0.3m

420-1.50.1-2m

35-0.40.4-5m

Iso la ted Drawzone Di ame te r

21.5-0.21.5-9m

6.5m6m

9ma.5mam

13m12.5m12m

Figure 9-M axim um /m inim um spacing of draw zones based on isolated draw zone diam eter

INPUT DATA =

EFFECT OFLAYOUT =

PRODUCTIONINFLUENCE =

F ig ur e 1 o- Draw- co ntr ol re qu ir em en ts

Ore Recovered + Dilution = P ro du ctio n T on nag e a nd G ra de

~ 28 6

It ha s be en e sta blishe d tha t th e d raw w illa ng le towa rd s le ss d en se a re as . This p rin cip lec an be used to mov e th e m ateria l o ve rlyin g th em ajo r a pe x by the diffe ren tia l draw o f line s ofdrawbe lls so tha t zo ne s o f va ry ing de nsity arecreated.

Di lu ti on En tr yThe p erc en ta ge o f d ilu tio n e ntry is th epe rce nta ge of the o re co lumn tha t ha s b ee nd rawn b efo re d ilu tio n a pp ea rs in th e d rawpoin t,and is a function of the am ount of m ixing thatoccurs in the draw colum ns. T he m ixing is afuncti on of the fol lowing :

O CTO BER 1994 T he J ouma l o f T he S ou th A fd ca n In stItu te o f M in in g a nd M eta llu rg y

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0 002 0.20.4 0.40.6 0.60.8 1.61.2 2.01.6 1.22.0 0.8

20% 40%[ 1[ 1V a l u e of A 1.9% 1.4%

%Dra . B 0.9% 1.4%

,,C :-A-, B4180 60 40 20

RMRRMR Range Curves Examples Ratings0 - 14 NO.l A 50 - 6015 - 29 NO.230 - 49 No.3+5 0 No.4 B 5 - 60

20 40 60 80 90 120 140HEIGHT O F IN TE RACT IONZONE IllZ

Range D.Z.Spacing H.I.Z10 21m 45m55 21m 90m

C a v e mining-the state of the art

ApplicationCa lcu la ti on of t onnageR ec ord in g o f to nn ag es p ro du ce dCon tr ol li ng t he d raw

A

U n p a y

7 0 % 1 0 0 %1 % O ra . [I 11 .0 % 0 .7 %1 .4 % 1 .1 %

1 2 8 % 1 5 0 %11 O r e R e c o ve r y0 .5 % 8 5 %

0 .7 % 9 6 %

2 0 0D r a w p o i n t s

A ve ra ge O re G ra de = 1 . 4 % S hu t o ff G ra de = 0 . 7 %F ig u re 1 1 -G r a de analysis

~ Ore draw height~ Range in fragm entation of both ore and

waste~ Drawzone spacing~ R ange in tonnages draw n from draw points.

B

The r ange in f ragmenta tio n s iz e d is tr ib uti ona nd the m in im um d rawz one sp ac in g a cross th ema jo r a pe x w ill g iv e th e h eig ht o f th e in te ra ctio nz on e (H IZ ). This is illu stra te d in F ig ure 1 2.T here is a volum e increase as the cavep rop aga te s, so th at a ce rta in amou nt of m ate ria lis d rawn b efo re th e c av e re ac he s th e d ilu tio nz on e. The v olume in cre as e o r swe ll fa cto rs a reb ase d on th e fragme nta tion an d a re ap plie d tocolumn height. The f ol low ing a re typi ca l swel lfa cto rs : fin e fra gmen ta tio n 1 ,1 6, med ium 1 ,1 2,coa rs e 1 ,08.

A draw -c ontrol fa cto r is ba se d o n thev ar ia tio n in tonnages f rom wo rk ing d rawpo in ts .Th is is ill us tr ated in F igur e 13. I f p roduct io nd ata a re n ot a va ila ble , th e d raw- co ntro l e ng in ee rmu st p re dic t a lik ely d raw p atte rn . A fo rmulab ase d on th e ab ove fa ctors h as be en d ev elo pe dto d et erm in e th e d il uti on entr y p er centage :-

Di lut ion entry = (A -B)/A x C x 100,where

A =Draw-column hei gh t x swe ll f ac to rB = Heigh t o f int erac ti onC= Draw-control factor.The g ra ph fo r d ilu tio n e ntry was o rig in ally

d rawn a s a s tra ig ht lin e, b ut u nd erg ro un do bs erv atio ns sh ow th at, whe re th ere is e ar lyd ilu tio n, th e ra te o f in flu x fo llows a c urv ed lin ew ith a long ore 'tail', as show n in Figure 14.F ig ure 1 5 sh ow s tha t d ilu tio n e ntry is a lsoa ffe cte d b y th e a ttitu de o f th e d rawzon e, whic hc an ang le towa rd s h ig he r o ve rb urd en lo ad s.

, R M R O F A L L M AT E R IA L IN T H E P O T E N T IA L D R A W C O L U M N T O B E U S E D IN C A L C U L A T IO N A S F IN E S F L O W M UC H F U R T H E R T H A N C O A R S E

M I N J} ,{ il l ! S P A C IN G O F D R A 1 fZ O N E SA CR O SS T HE M A lO R A PE X IBm15m -- --

12m9m

V E R T IC A L L IN E " ( ! ' L O C A T E D A T H IG H E S T R A T IN G O F M A T E R IA L IN D R A W C O L U M NFigure 12-Height of the interaction zone (H IZ)

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C a v e mining-the state of the art1.00.90.80.70.60.50.40.30.20.10

2 ID 11

Standard Deviation x 100 of Tonnage of W orking Drawpoints

DiPsMonthlyTonnage

Wll2000

E/41800

Ell80 0

W12 E/21000 2500

w/3600

E/3 w/41500 800

Mean = 1375 Standard D eviation = 682/100 = 7D raw Con tro l F ac to r = 0.6

F ig ure 1 3- T he dra w-c on tro l fa cto r

A-B x C x 100AA = O re D raw C olumn H eig ht x Swe ll F ac to r = 168mB = Height o f I nteraction Zone = 90mC = D raw-control Factor = 0.6

= D ilu tio n E ntr y

168 - 90 x 0.6 x 100168 = 28%0100

C 90 80M 70P 600S 50I 40T 30I 200N 100

20 0 25 0

D IL U T IO N /W A S T E

0 20 25 00 60 80 100Percentage Draw

20 0

F igu re 1 4-C alc ula tio n of dilu tio n e ntry

~ 28 8 OCTOBER 1 9 9 4

L a y o u t sA fa c to r tha t needs to be resolved i s the co rr ec tshape of the m ajor apex. It is considered that as ha ped p illa r w ill a ssis t in th e re co ve ry o f fin eo re . Wh ere c oa rs e ma te ria l re su lts in majo ra rc hin g, a sq ua re -to pp ed majo r a pe x (p illa r) isp re fe ra ble in te rm s o f a rc h fa ilu re a nd b roww ear, as show n in Figure 16. The m ain area ofb row wea r is immed ia te ly a bo ve th e d rawpoin t.If the v ertica l he ig ht o f pilla r ab ov e the b row issm all, a s sh ow n in F ig ure 16A, fa ilu re o f th e to ps ec tio n w ill re du ce th e s tre ng th o f th e lowe rs ec tio n a nd re su lt in a gg ra va te d b row wea r.

M ore thought m ust be given to the design ofLHD l ay ou ts in o rd er to p ro vid e th e maximumamou nt of m ano eu vring sp ace for the m in im umsiz e o f drift o pe ning so tha t larg er m ac hin es c anb e u se d w ith in th e o ptimum d rawzon e s pa cin gs .Ano th er a sp ec t th at n ee ds a tte ntio n is th e d es ig no f LHD s to re du ce th e le ng th a nd in crea se th ew id th . Whilst the use o f la rge m ac hin es m ig ht b ea n a ttrac tio n, it is re commend ed th at ca utio n b ee xe rc ise d a nd th at a d ec isio n o n m ach in e size beb ase d o n the c orre ct a ssessm ent o f the re qu ire dd rawzon e s pa cin g in te rm s o f fra gmen ta tio n. Theloss o f rev enu e th at ca n resu lt from h igh d ilu tio na nd o re lo ss fa r e xc ee ds th e lowe r o pe ra tin gcos ts a ssoc ia ted w ith l ar ge r mach in es .

E ight d if fe rent hor izon ta l LHDl ayouts andtw o in clin ed d rawpo int LHD la yo uts are in use a tv ariou s op erating m in es. A n e xample o f anin clin ed LHD l ayo ut is sh ow n in F igu re 1 7.UndercuttingUnd erc uttin g is o ne o f th e mos t impo rta nta spe cts o f ca ve m ining sinc e, n ot o nly is acomp le te under cu t n eces sa ry to induce cav ing,b ut the u nd ercu t m eth od c an re du ce thed amagi ng e ff ec t o f in duced s tr es se s.

The normal under cu ttin g s equence i s t od ev elo p th e d rawb ell a nd th en to b re ak th eu nd erc ut in to th e d rawb ell, a s s hown in F ig ure 1 8.In e nv iro nmen ts o f h ig h s tre ss , th e p illa rs a ndb row s a re damag ed by th e a dva nc in g a butm ents tre ss es . The Hen de rs on M in e te ch niq ue o fd ev elo pin g th e d rawb ell w ith lo ng h ole s from th eu nd erc ut le ve l re du ce s th e time in te rv al a ndexte nt o f d amag e a ss oc ia te d w ith p os tunder cu tti ng . I n o rd er t o p re se rv e s tabili ty ,H end erson M in e ha s also fo un d it n ece ssa ry tod ela y th e d ev elo pment o f th e d rawb ell d rift u ntilthe be ll h as to be b lasted (F ig ure 1 9).

The d amag e c au se d to p illa rs a ro un d d riftsa nd d raw be lls by a bu tm en t stresse s iss ig nific an t, b ein g th e ma jo r fa cto r in b row wea rand excavatio n collaps e. Rockbu rs ts a re a ls oloc ated in th ese a re as. T he so lution is toc omple te th e u nd erc ut b efo re th e d ev elo pment o fth e d rawpoin ts a nd d rawb ells . T he 'a dv an ce du nd ercu t' te chn iqu e is sh ow n in F igu re 20 .

The Journal of The South African Institute of Mining and Metallurgy

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C a v e mining-the state of the artA. A v e r a g e L o a d in g A b o v e D ra w p o in t s

t t t t ~ t

f.n.. . ~ ::::

FOOTWALL V unD RA W PO IN TS

B . H ig h e r L o a d in g T o S id e O f D ra w p o in t sSIMULATED HILL

FEATURE ffiGHERO V E R B U R D E N LOADING

~ ~M O D E LC O R N E RE F F E C T

F ig ure 1 5-ln clin ed dra wpo in t la you t sh ow in g th e e ffec t of d iffere nt o ve rbu rd en loa din g (thre e-dim ensio nal sa nd -m od el e xp erim en ts)

M O R E S T A B L EA R C H E S

F A IR F IN E SD R A W

S T A B L E A R C H E SH I G H E R

F ig ure 1 6-S ha pes o f m ajo r a pe x/p illa rs

T he J ouma l o f T he S ou th A fric an In stitu te o f M in in g a nd M eta llu rg y

P O O R F IN E SD R A W

B E T T E RA R C H F A IL U R E

O CTOBER 1994 289 ...

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C a v e m ining-the state of the art

Inclination - 4 0 'S p a c i n g . D IP

1 21 51 8

S T R I K E1 01 2

1 2 / 1 5

V E R T I C A L1 0

1 2 . 51 5

F i g u r e 17-An ex am ple o f th e lay ou t o f a n in clin ed d raw po in t

A. Production Leve l Layout De ta il

I~-EI~ ,I II II I--~--r r-~---_ _ ~ _ _ L L_~---

E!

B . C ross S ectionF ig u re 1 8 -L H D l ayouta t E lTeniente

~ 29 0 OC TOBER 1994

C . Isom etric V iew ofB lo ck caving w ith LHD

The Joum al of The South Afncan Insm ute of M ining and M etallurgy

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-----C . M e d iu m /L o w~Stress Weak G r o u n d . ! ;. : !P ro d u c t io n D r i f t c ;f ; ' jD r a w p o in t a n d

D r a w b e l l D r i f t ~ F u l ly ~ T e m p o r a r y --- Full y - - -S u p p o r t e d S u p p o r t e d S u p p o r t e d

C a v e m in in g - t h e s t a t e o f t h e a r tI n th e p a s t i t w a s c o n s id e r e d th a t th e h e ig h t

o f t h e u n d e r c u t h a d a s ig n i f i c a n t in f lu e n c e o nc a v in g a n d , p o s s ib ly , th e f lo w o f o r e . T h ea s b e s t o s m in e s in Z im b a b w e h a d u n o e r c u ts o f3 0 m w it h n o r e s u l ta n t im p r o v e m en t in c a v in g o rf r a g m e n ta t io n . T h e lo n g t im e in v o lv e d inc o m p le t in g t h e u n d e r c u t o f te n le d to g ro un d -c o n t r o l p r o b le m s . G o o d re su lt s a re o b ta in e d w it hu n d e r c u t s o f m in im u m h e ig h t p r o v id e d th a tc o m p le te u nd e r c u tt in g is a ch ie ve d . W h e r eg ra v i t y is n e e d e d f o r t h e f lo w o f b la s t e d u n d e r c u to r e , th e u n d e r c u t h e ig h t n e e d s t o b e o n ly h a lft h e w id t h o f t h e m a jo r a p e x . T h is r e s u l ts in a na n g le o f r e p o s e o f 4 5 d e g r e e s a n d a l lo w s th e o r et o f lo w f r e e ly .Suppor t Requ ir ementsI n a r e a s o f h ig h s t r e s s , w e a k r o c k w il l d e f o rmp la st ic a ll y a nd s t r o ng r o ck w il l e xh ib it b ri t t l e ,o ft e n v io le n t , f a ilu re . I f th e r e is a la rg e d if fe re n c eb e tw ee n th e R M R a n d M R M R , y ie ld in g s u p p o r ts y s t e m s a r e re q u ir e d . T h is is e x p la in e d inF ig u re 2 1 .

F ig u r e 1 9 -1 s o m et r ic v ie w o f a p a n e l- c a v e o p e ra t io n

B la s t e d H o r . H o le s - - -A . Very H i g h StressProduction D r i f tO n l y

~ - = - m - - - ~ = - ~ -.~ ~

~ ly S u p p o r t e ~

N a r r o wU n d e r c u t

P r o d u c t i o nL e v e l

B . H ig h StressP r o d u c t io n D r i f ta n d D r a w p o in t

' - - = m ~ - - -

~ F u l l yS u p p o r t e dN a r r o wU n d e r c u t

P r o d u c t i o nL e v e l

F ig ur e ~ iff er en t s eq ue nc es o f a d va nc ed u nd erc uttin gT he Joum al of T he South African Institute of M in ing and M etallurgy O CT OBER 1994

N a r r o wU n d e r c u t

291 ....

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C a v e mining-the state of the art00 200

20

M 30R

40MR 50

60

70

80

90

R M R30 70 800 600

SYSTEMS

Failure

. RIGID SUPPORT SYSTEM S. ..i'. .

10 0

Figu re 21 -Suppo rt r equ ir ement s f or c av ing operat ions

AcknowledgementsThis p a p e r p re se nt s a n u pd at eof the technology of cavemining. It is not possible toquote references since theb ulk o f th e d ata s up po rtin gthe contents of this paperhave not been published andthe basic concepts are know nt o m in in g e ng in ee rs .H ow ever, it is appropriate toa ck now le dg e t he c on tr ib ut io nsfrom discussions w ith thefo llo win g p eo ple in c an ad a,C hile, S ou th A frica, a ndZ im babw e: R . A lvarez, P .).B artle tt, N .).W . B ell, T . C are w,A .R . G ue st, C . P ag e,D . S ta cey , an d A . S us aeta.Thanks are due toP.). B artlett for assisting in thefin al p re paratio n of th e p ap er .T he sim ulation program forthe calculation of prim ary andse conda ry f r agment at ionreferred to in the text w aswrittenbyG .5 . E ste rh uiz en at P reto riaUniversity.

~ 29 2

Table IVSupport techniques

90 100

tHighstress

= 1 m + (0,33 W x F)=1 m= R ig id r eb ar

0,5 mm x 100 mm apertureCables = 1 m + 1,5 W

Mes h- re in fo rc ed s ho tc re teR ig id s te el a rc he sMass iv e conc re teL ar ge w as he rs ( tr ia ng le s)T en do n s tr ap s25 m m rope-cable slings

P re -s tr es sed c ab le s have l it tl e app li ca ti on i nu nd erg ro un d situ atio ns u nle ss it is to st ab ili zefra ctu re d ro ck i n a low-s tre ss e nv iro nm en t. T hen ee d fo r e ffe ctiv e la te ra l c on stra in t o f th e ro cka nd o f lin in g su rface s su ch as co ncrete c an no t b eto o h ig hly empha si ze d. S uppo rt t ec hn iq ue s a rei ll us tr at ed i n Tab le IV.Conclusions~ C av ability can be assessed provid ed

a ccur at e geo te chn ic al d at a a re ava il ab lea nd th e g eo lo gic al v aria tio ns a rere co gn iz ed . T he m in in g ro ckmass ra tin g(MRMR) sys tem prov ides t he nec es sa ryd ata fo r th e emp iric al d efin iti on o f th eu nd erc ut d im en sio n in term s o f th ehydrau lic r ad ius .

~ N um erical m odelling can assist thee ng in ee r in u nd ers ta nd in g a nd d efin in gt he s tre ss env ir onmen t.

~ Fragmentation is a m ajor factor in ana ss es sm en t o f t he fe as ib ility o f c av em ining i n l ar ge compet en t o rebodi es .P ro grams a re b ein g d ev elo pe d fo r t hed ete rm in at io n o f fra gm en ta tio n. a nd e ve nthe l es s s ophi st ic at ed p rog rams p rov ideg ood d esig n d at a. T he e co nom ic v ia bil ityo f c av in g in c ompete nt o re bo die s isd ete rm in ed by LHDp ro du ctiv ity a nd th ecos t o f b re ak ing l ar ge f ra gment s.

Low s tr es s

Bird cag e cables from un de rcut le ve lIn clin ed p ip esGroutingE xt ra bo lt s and cab lesP la tes , s traps , and arches

W is th e s pa n o f t he tu nn el a nd F is b ase d o nMRMR = 0-20: F = 1,4 MRMR = 21-30: F = 1,30M RM R=41-50: F= 1,1 MRMR=51-60: F= 1,05O CTOBER 1994

MRMR = 31-40: F = 1,2MRMR > 61 : F = 1,0The Joumal of The Sout h A fr ic an I ns ti tu te of Mini ng and Me ta ll ur gy

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C a v e mining-the state of the art~ D r a w p o i n U d r a w z o n e spacings fo r coarserm aterial need to be exam ined in term s ofr ecov er y and impr ov ed m ininge nv iro nmen ts . S pa cin gs mus t n ot b ein cre ase d to lowe r o pe ra tin g c ost s a t th eexp en se o f o re r ecov er y.

~ The interactive theory of draw and thediam ete r of the isola te d d raw zo ne ca n b eu se d in the de sign of drawzo ne spa cing .~ C om plications occur w hen the draw zones pa ci ng is d es ig ne d o n th e p rim aryfra gmen ta ti on a nd t he s ec on da ry fra gmen -ta tio n i s s ig nific an tly d iffe re nt. .