.-..Synopsis

With the advent in recentyears qf mechanizationin deep-level gold andplatinum mines, largequantities qfrubberwes are being used onboth prodJJction andsupport vehicles.Conditions in these typesqfmines are extremelyharsh and, as a result,we perfonnance isslgn!ficant{y lower thanin other types qfminingsuch as open-cast base-metal mining, wheremorefavourableconditions prevail.Various options areavailable to the user toreduce we costs,including the use qfretreads, the.filling qfwes, and the repair qfdamaged wes. Whilethese methods mqy begenerallY succesdUl inopen-cast conditions,their use in deep-levelmining needs to becartdUl{y evaluatedforeach particular set qf

conditions. A basicrequirement for «1!Y

evaluation qf we costsis the availabiliO' qf

reliable data on fYreperformance and,for this

. JohannesburgConsolidated InvestmentCompal!)', Limited, P.O.Box 590, Johannesburg2000.

@ The South 4frican

Institute if Mining andMetallurgy, 1994. SAISSN 0038-223X/3.00

+ 0.00. Paper receivedMqy. 1994; revised Jul.

1994.

Economic considerations in the use ofrubber tyres .in mechanized deep-level

. .mInIngby R.D.C. Andrew*

Introduction

Although pneumatic rubber tyres have beenwidely used in other types of mining operationssuch as open-cast base-metal and coal mining,as well as in a variety of earthmoving activities,it is only relatively recently that tyres are findingapplication in deep-level mining. Unlike condi-tions in other types of mining, those in deep-level mining are not conducive to good tyreperformance, and tyre costs are consequently farmore significant in deep-level mining than inother types of mining.

The tyres used in deep-level mining aresimilar to those used elsewhere, but have impor-tant differences to cater for extremely high wearrates, short tramming distances, slow travellingspeeds, and heavy loading requirements, all ofwhich are special features of the productionvehicles used in mechanized deep-level mines.For maximum traction and to minimize cuttingand tearing, no tread pattern is used with thistype of tyre, which is commonly referred to as aslick.

Tyre costs can be reduced by improved road-way conditions and enhanced driving skills, andby the promotion of good tyre monitoring andmanagement practices, all of which are generallyunder the control of the user. Other methods,which fall more into the domain of the tyresupplier, are the use of retreads, the filling oftyres, and the repair of damaged tyres. All theseoptions have significant economic implicationsand should be evaluated constantly. Of particularimportance to the user is an ability to evaluatethe cost of retreads and filling, and to assesswhether the repair of a damaged tyre is morecost-effective than the purchase of a new tyre.

The different conditions encountered indeep-level mining and the different circum-stances, e.g. difficult accessibility, must beconsidered when cost evaluations are under-taken, and comparisons with tyres used in other,more favourable mining conditions should becarried out with due caution.

The Joumal of The South African Institute of Mining and Metallurgy

Design and Construction of Tyres

A section of a typical earthmover tyre commonlyused in deep-level mining is given in Figure 1.The smooth or slick tread is used, instead of ablock or lugged tread pattern, on tyres fitted toproduction vehicles such as load-haul-dumpers(LHDs) and front-end loaders, to offer a maxi-mum surface contact area and to minimizecutting and tearing. Many dump trucks also usethis type of tyre under harsh roadway conditions.The maximum available depth of tread is used(commonly referred to as the L5 tread pattern),and rubber compounds with the highest degreeof cut and abrasion resistance are employed.

Figure 1-Cross-section through a tyre (slick)

OCTOBER 1994 271 ...

The use of rubber tyres in mechanized deep-level mining

reason, it is essentialthat proper (Vre recordsare maintainedThis paper reviews theperformance qf (Vres indeep-level gold andplatinum mines inJohannesburgConsolidated InvestmentCompatlY, Limited (fCJ)over a period qf about 8

years, and shows MWthe costs qfretreadingand (Vrejilling can beevaluated in practice. Italso discusses the costimplications qf repairingdamaged (Vres.

~ 272

-_u

The smooth tyre tread includes a cut-out atequally spaced positions around the circum-ference of the tyre to provide the user with themeans of monitoring tread wear during service.This can also be used to indicate the point atwhich the tyre should be removed from servicefor retreading. Since the retreading processrequires a certain amount of the original tread tobe present, the tyre must be removed before thefull depth of tread has been consumed. In deep-level mining this point is normally reached when80 per cent of the tread has been consumed (orwith 20 per cent of the tread depth remaining(TDR)). If the roadway conditions are such thatthey produce a smooth profile of tread wear,which is normally the case in open-cast miningbut not in deep-level mining, retreading a tyrewith a 10 per cent TDR is possible. If the tyre isnot removed at this critical point, retreading willnot be possible and the tyre will have to bescrapped.

Since the side-walls of tyres are particularlyvulnerable to damage, many manufacturers nowinclude an extra buffer section of rubber foradditional protection. The cross-ply type of tyreis most commonly used, although radial tyresare also available. The larger contact surfacearea (footprint) of a radial tyre is an advantagein that it provides better traction, but the suscep-tibility of the radial side-wall to damage is a dis-advantage. With few exceptions, most tyres usedin deep-level mining are of the tubeless type.

Performance of Tyres in Deep-level Mining

Since the inception of mechanized mining in itsgold and platinum mines, Johannesburg Consoli-dated Investment Company, Limited (lCI) hasadopted a rigorous computerized system tomonitor and record tyre performance. In thissystem, the date on which a tyre is fitted to aparticular vehicle is recorded, together with allother relevant data such as project number, tyreposition on vehicle, tyre brand and serialnumber, new or retreaded tyre, and the hour-meter reading of the vehicle. When the tyre hasbeen removed from service, the hour-meterreading is again recorded and the reason forremoval is stated. A tyre may be removedbecause it is to be retreaded, if it has failedprematurely, for later matching purposes onanother vehicle if there has been uneven wearon the tyres or because the full tread of the tyrehas been consumed. When analysed, these dataprovide a wide range of information on tyreperformance, including the average tyre life foreach project, for each different brand and type oftyre used, for different types of vehicles, and forretreads.

From a considerable amount of data collectedover a period of 8 years at JCI's gold and plati-num mines, the following conclusions on tyreperformance can be stated.

~ There is a wide variation in tyre life and,for a typical mining project, a distributionhistogram similar to that given in Figure 2

[ - Gold TM3

'J, ot Tyres120

~ Platinum TM3[

60

1800X25 L5 suers FlrTED ON LHDI

40

20

0400 600 800 1000 1200 1400 1600

Tyre Lite (In Excess ot-) (Hours)

Figure 2- Typical distribution of tyre life

OCTOBER 1994

1800 2000

The Journal of The South African Institute of Mining and Metallurgy

The use of rubber tyres in mechanized deep-level mining

is common.~ The figure shows that a substantial num-

ber of tyres in a single project (20 to 30per cent) fail prematurely. Since thesetyres are usually severely damaged, theycannot be retreaded and have no scrapvalue. Prematurely failed tyres represent asource of high losses, and in February1994 cost about R100 per hour in the caseof the commonly used 1800X25 L5 slickused in gold-mining conditions!.

~ For LHDs, the average tyre life, where thefull tread depth has been consumed (run-out), is approximately 800 hours in goldmines and 1000 hours in platinum mines.Dump trucks, which are not normally usedin loading operations, showed a 20 to 30per cent improvement in tyre lifel.

~ When a deliberate retreading policy wasimplemented, it was found that tyres wereremoved at an average TDR of 22 per centand, for the LHDs used in gold mining,this corresponded to a new tyre life ofapproximately 500 hours2.

~ On average, in gold-mining conditions, aretreadability rate of 30 per cent wasachieved, although 60 per cent of the tyreshad been deliberately removed forretreading. This indicated that half thetyres removed for retreading could not beretreaded on account of excessive damagedetected during the retreading process.Since these tyres had sacrificed an average22 per cent of their first life capability,their failure to return to service as retreads

Table I

Typical modes of tyre failure in deep-level mining

Failure mode

100

Percentage

Bead damageCasing fatigueCasing separation/disintegrationCrown cut failureTread cracking, lifting, disintegratingCut separationDeflation damageDelaminationInner-liner failureInjuries over retreading limitsCrown impact damageLoose bead wiresMechanical damageOil saturationCrown penetrationRepair failureSidewall cutsShoulder separationSidewall impactsSidewall separationTyres used to run-out

20,2

1125

0,45

0,60,6

24

0,20,4

712

180,40,2

124

Total tyres damaged 82

Tyres suitable for retreading 18

Total tyres

The Journal of The South African institute of Mining and Metallurgy

was a source of major losses2.~ The life achieved with retreaded tyres was

equally as variable as that with new tyresbut, on average, was between 70 and 90per cent of the average new tyre life2.

~ Less than 5 per cent of tyres were capableof being retreaded twice, and virtually notyres were retreaded more than twice. Thisis a major difference compared with open-cast mining, where multiple retreading iscommon3, and is indicative of the harshconditions in deep-level mining.

~ The typical average cost per hour for anew 1800X25 L5 slick on an LHDwasR15 to R25 per hour for gold mining andRIO to R15 per hour for platinum mining(February 1994)4. This implies that thecost of tyres for an operating LHD wasbetween R40 and R1O0 per hour, whichrepresents a substantial portion of anLHD's total operating costs,

Analysis of Scrap Tyres

Apart from records of tyre life, tyre performancecan be gauged from an examination of usedtyres to assess their mode of failure. Table Igives a typical analysis of tyre failures. Thisanalysis was carried out for 471 used tyres ofdifferent types and sizes collected over a periodof 3 months from }CI'sgold and platinum mines5.This analysis shows that only 85 tyres (18 percent) were suitable for retreading, while 272tyres (58 per cent) were damaged and had to bescrapped. Various modes of failure were noted,with cuts, deflation damage (under-inflatedtyres), and oil saturation the major areas ofconcern. Oil saturation is particularly harmfulsince, not only does it initiate other forms ofdamage, but it also precludes retreading.

The high losses that can arise from prematurefailure are shown in Table H, which analyses thelosses from 347 prematurely failed tyres basedon wasted residual tread5. Since these failuresoccurred over a period of 3 months, the loss ofapproximately R473 000 represents an annualloss of approximately R2 million, which at thetime was equal to about 10 per cent of the totalannual tyre account for }CI'smechanized goldand platinum mines6.

The examination of scrap tyres can also formthe basis for possible claims against manufac-turers and retreaders, since many of the failureswere shown to have been initiated by inherentprocessing defects. It is of concern that, based on}CIexperience, approximately 10 per cent of allnew tyres and 5 per cent of retreads are subjectto claims6. This is believed to be considerablyhigher than is experienced in other areas, e.g.open-cast mining.

OCTOBER 1994 273 <Ill

The use of rubber tyres in mechanized deep-levelmining

Tab/ell

Estimated losses from premature failure of tyres

Number

472911

Loss (Rands)t

8452

4196

84791542

116101265551024723988

53471871105400040322

38675454

347

. Tread depth remaining at time of failuret Losses are calculated on proportional costs of TOR using new and retread prices as at

February 1994

RUN-OUT

RETREAD ONCE

(IOS TDR) m Rands

RETREAD TWICE

(IOS TDR)

Retreading

The main reason for the use retreads is to reducecosts. However, retreading can also have impor-

I

tant ancillary benefits for conserving tyres andfor promoting good tyre-management practices.

The Mechanics of Retreading

Unlike in other areas where retreaded tyres arefreely available, e.g. in the passenger-car andheavy-duty vehicle industries, retreads in deep-level mining have to be generated by the user.New tyres must first be used and carefullymonitored so that they can be removed at thecorrect point before the tread has been fullyconsumed. This sacrifice of first life, as well asthe additional cost of retreading, must be shownto be economically justified compared with thealternative of using the tyre to destruction. As

Cost/Hour

Lro

m/ire

Lrt

O,9m/Lro

Y' O,Sm/Lro

0 :2 3

~ 274

Figure 3- The mechanics and cost structure of retreads

OCTOBER 1994

--

mentioned earlier, indicators of tread wear canbe used to show the correct time at which thetyre must be removed. In deep-level mining, thisis normally at 20 per cent TDR, although the 10per cent TDR criterion can be used if conditionshave produced a uniform pattern of wear.

Figure 3 shows the mechanics of retreading,together with simple mathematical expressionsthat can be developed for the cost evaluation ofretreads in particular situations. The followingsymbols are used:

m =the cost of a new tyre, in Randsn = the cost of retreading, in Rands

Lra = the average life of a new tyre if used todestruction (run-out life), in hours

Lrt = the average life of the new tyre to thepoint at which it is removed forretreading, in hours

Y = the average life of the first retread, inhours

Y' = the average life of the second retread,

in hours.Then, fromFigure3,

Cost per hour of the new tyre to run-out =m/Ira

Cost per hour of the new tyre plus retread =m+n/Lrt+Y

Cost per hour of the new tyre plus tworetreads =m + 2n/Lrt+ Y + Y'.

If it can be assumed that the retread price isequal to half the price of a new tyre, which isoften the case, and if the first retread life is equalto 80 per cent of the run-out life and the secondretread life is equal to 80 per cent of the firstretread life, which are often used as the criteriafor the acceptability of retreads, then

where Lrt =80 per cent of Lra (20 per centTDR):

Cost per hour for one retread = 0,93 m/IraCost per hour for two retreads = 0,88 m/Ira;

where Lrt= 90 per cent of Lra (10 per centTDR):

Cost per hour for one retread = 0,80 m/IraCost per hour for two retreads = 0,76 m/Ira.

Since the cost of using a new tyre to destruc-tion is given by m/Ira, the analysis shows that,in the idealized case where all tyres are removedand retreaded, retreading once can producesavings of 7 to 20 per cent, depending on thepoint at which the tyre is removed, and retread-ing twice can produce savings of 12 to 24 percent.

Figure 3 also shows the conservation aspectof retreading in that a new tyre that has beenretreaded once is equivalent to 1,5 new tyresand, if retreaded twice, to 2,1 new tyres.

The Journal of The South African Institute of Mining and Metallurgy

The use of rubber tyres in mechanized deep-level mining

Practical Constraints in Retreading

The previous section considered an idealized

situation in which all tyres were assumed to beretreaded. In the more realistic situation, mainlyon account of the severity of deep-level miningconditions, the number of retreads that canactually be used is restricted by the following.

~ Sometyres are used to destruction (run-out) because they were not removedtimeously or because they were judged to

be unsuitable for retreading.~ Others, which were removed at the correct

point for retreading, are subsequently

found to be unsuitable for retreadingduring the retreading process and arescrapped.

~ Still others fail prematurely and arescrapped.

In the practical situation, these various cate-gories of tyres will co-exist with retreaded tyres,and the overall cost per hour will depend on thenumber of tyres in each category and on theaverage tyre life achieved by each category.

In the analysis of actual tyre costs in thepractical situation,

a ==the proportion of tyres that are used todestruction and achieve their run-out lifevalue (Lro)

b==

the proportion of tyres that are removedat the correct point for retreading (Lrt)and that return to service as retreads

c==

the proportion of tyres that are removedat the correct point for retreading (Lrt)

but that do not return to serviced

==the proportion of tyres that failprematurely at an average life of p hours

and are scrapped.

Then, using the same symbols as before,noting that by definition a + b + c + d == 1, and

assuming only first retreads, one obtains thefollowing:

Overall cost per hour =am + b(m + n) + cm + dm

aLro + (b + c)Lrt + bY + dp

which simplifies to

m + bnOverall cost per hour = .[1]

aLro + (b + c)Lrt + bY + dp

Expression [1] can be used for the calculation

of the overall tyre costs in any practical situation.

This is shown here for the data given in Table Ill.New tyre price

==RIO SOO (1800X2S LS slick)

Retread price==

RSOOO (February 1994)Retread life

== 640 h.Then from [1],Costperhour using retreads==RlS,86

Cost per hour with no retreads ==R14, 19.

The Journal of The South African Institute of Mining and Metallurgy

Table 11/

Data used in the calculation of costs

Category

800640640200

Average life, h

Run-out (a)

Retreaded (b)

Not retreaded (e)

Premature failure (d)

In this case, the use of retreads has no costbenefits even though a retread rate of 40 percent was used.It should be noted that premature failures

occur5 both when retreads are used and when noretreads are used and, for practical purposes,premature failure can be ignored in comparisonsof retreading versus run-out costs. However, thecost of premature failure in a particular situationcan be calculated from expression [1].

Sensitivity Analysis

The effects of different factors when retreads areused, e.g. the proportions of various categoriesof tyres, the life of new tyres to the point forretreading, the run-out life of new tyres, and thelife of retreads can be estimated from expression[2] by way of a sensitivity analysis. In thefollowing example, the effect of the retread rateis studied.

From [1], and assuming there is nopremature failure (Le.d ==0),

m + bnCost per hour =

aLro + (b + c)Lrt + bY

If it can be assumed that the retread price isequal to half the new tyre price (Le. n ==

O,S

m), the life of the new tyre to the retreading

point is equal to 80 per cent of the run-out

life (Le. Lrt ==0,8 Lro) , and the life of the

retread is equal to 80 per cent of the run-out

life (Le. Y ==0,8 Lro). When only first

retreads are considered, then from [1],

Cost per hour ==X . m/Lro,

1 + 0,5bwhere X =

a + 1, 6b + 0,8c[2]

Since the cost per hour of the run-outcondition (no retreads) is given by m/Lro, thevalue of X, for a given set of values for a, b, andc, will indicate whether or not retreading is cost-effective. If X is greater than 1, the use ofretreads will not be cost-effective, while a valueof X less than 1 will show that retreads are cost-effective.

OCTOBER 1994 275 ....

The use of rubber tyres in mechanized deep-level mining

1.3 noQ.5m : Lrl-o.8Lro : Y-D.8Lro

LOSS (X)1)

a-No. ot Run-out !)"ms

0.9

0.8

0.7

'jI, RETREADS

Figure <!-Sensitivity analysis: retread rate

Figure 4 shows expression [2] plotted forvarious values of a, b, and c. It can be seen that,in this case, the potential for producing a loss bythe use of retreads is greater than the potentialfor savings, since the maximum possible savingis only 7 per cent while the maximum loss withretreads is about 20 per cent. Figure 4 alsoindicates practical limits if retreads are used, e.g.if 40 per cent of the tyres are used to their run-out value, a minimum retread rate of 40 per centis required and, since there is a fIXednumber oftyres being used, a maximum of 20 per cent ofthe tyres can be scrapped.

Lrl-D.8Lro

Y-D.8Lro1.0

Loss

Xl 1.0

0.9

.5

Retread price (n)

Figure 5-Sensitivity analysis: retread price

~ 276 OCTOBER 1994

--

Expression [1] can also be used to determinethe way in which the retread price (n) influencestyre costs at different retread rates (b). If the lifeof a new tyre to the retreading point is againassumed to be equal to 80 per cent of the run-out life (Le. LTt =0,8 Lro) , and if 40 per cent ofthe tyres attain their run-out life (Le. a =0,4),

then from [1]Cost per hour = X'

. m/Lro,-0

1 + bn-o.~ whereX' =

a + 1,6b + 0,8c[3]

-0.4

DD

Expression [3] is plotted in Figure 5 forvarious values of the retread rate (b) and fordifferent values of the retreading price (n),where the retreading price is expressed as afraction of the new tyre price (m). In this case, toachieve a 5 per cent saving by the use ofretreads, the retread price must be 35 to 43 percent of the new tyre price. If the retread price isclose to half the new tyre price (as is often thecase), there are no cost benefits with a retreadrate of 40 per cent, and only marginal savings(about 2 per cent) with a retread rate of 50 percent.

Summary

A retreading policy that deliberately removestyres from service before the full tread has beenconsumed is at best only marginally cost-effective in practice and has to be implementedwithin very strict limits:

~ 30 to 40 per cent of the tyres must be usedto destruction.

~ 20 to 30 per cent of the tyres must beretreaded.

~ The life of a new tyre to the point at whichit is removed for retreading must be 90 percent of the life of a new tyre.

~ The life of a retread must be at least 80 percent of the run-out life of a new tyre.

~ The price of the retread must not begreater than 46 per cent of the price of anew tyre.

In practical deep-level mining situations,many of the limits given here are difficult tosatisfy on account of the harsh miningconditions, which promote severe tyre damageand premature failure. It is also important tonote that retreading has other hidden costs, suchas the need for a vehicle to be taken out ofservice more frequently.

The Joumal of The South African Institute of Mining and Metallurgy

The use of rubber tyres in mechanized deep-level mining

When tyres are removed for reasons otherthan for deliberate retreading, e.g. if removed formatching purposes on other vehicles, the costimplications of retreading are considerably lessonerous on the user, since there is no sacrifice oftyre life. The use of retreading in this type ofsituation will result in the conversion of goodused tyres (which must satisfY retreadingrequirements for minimum residual tread depth)to tyres that should be able to achieve at least 80per cent of the life of an equivalent new tyre at asubstantially lower cost. In deep-level mining,this strategy, Le. the use of new tyres andretreading only when good used tyres areavailable, will produce the most favourableresults.

Tyre Filling

Filling can be used to eliminate or reduce thepremature failure of tyres and thereby increasethe overall average life, as well as avoiding pro-duction losses. Filling can also increase the run-out life of a tyre since a filled tyre can often beused past the normal point at which a pneumatictyre is considered to be unsafe.

Tyre-filling material is normally a two-component iso-cyanate polyurethane, which ispumped into a tyre and which, after curing,achieves a resilience approaching that of a con-ventional pneumatic tyre. The substitution of airby elastomeric polyurethane implies that the tyrecannot deflate when the casing or tread isdamaged.

For the purpose of the following costanalysis, it was assumed that filling eliminatespremature failure and serves to increase the run-out life by 10 per cent. With the same symbolsas used previously and if

... Addl! anal costs

60

50 COlt at decrease in ran-Oat We

4050S z n

30

40S z nNot cost et/ectJve Cost et/ectJve

20

30S z

(b)

10

(d) 20S z

0

0 10 20 30 40

... Decrease In run-out 11/e

Figure ~ost evaluation of tyre filling

The Joumal of The South African Institute of Mining and Metallurgy

k :::: the percentage decrease in the averageoverall run-out life (Lro) as aconsequence of premature failure, and

x::::the cost of filling expressed as a fractionof the new tyre price,

Cost per hour with no filling::::m/ kLro [4]

Cost per hour with filling::::m(l + x)/l, 1 Lro. [5]

For filling to be cost-effective, the cost perhour with filling must be less than the cost perhour without filling, or

m(l + x)/l,l Lro must be less than m/kLro,

or 1 + x/l, 1 must be less than l/k. [6]

Figure 6 shows the inequality in [6] plottedfor different values of x and k, and from this thedata of Table IV can be deduced.

Table IV

Data from Figure 6

Cost of filling(vs new tyre)

%

50403020

In February 1994, the price of filling rangedfrom 40 to 50 per cent of the new tyre price. Atthese prices, it is cost-effective to fill tyres only ifit can be shown that any premature failure servesto lower the average run-out life by no less than20 to 26 per cent.

The example of Table V shows how thisanalysis can be used in practice to determinewhether the filling of 1800X25 15 slick tyres isjustified in a given set of conditions. In this case,the cost of filling tyres is not justified since itwould increase the tyre cost by 10 per cent.

Table V

Costs of filling tyres

New tyre priceFilling price

Average run-out life(excluding prematurefailed tyres)

Average run-out life(including premature

failureDecrease in run-out life

caused by prematurefailure

Cost of decrease in run-outlife

Cost of filling

OCTOBER 1994 277 "ill

"

~

Synopsis

Caving is the lowest-cost undergroundmining method providedthat the drawpointspacing, drawpoint size,and ore-handling

facilities are designed tosuit the caved material,and that the drawpointhorizon can bemalntalnedfor the lifeqfthe draw. In the near

future, several open-pitmines that produce morethan 50 kt per dqy willhave to examine the

feasibility qf convertingto low-cost, large-scaleunderground operations.Several other large-scale, low-gradeunderground operationswill experience mqjorchanges in their miningenvironments as largedropdowns areimplemented

These changesdemand a more realisticapproach to mineplanning than in thepast, where existingoperations have beenprqjected to increaseddepths with littleconsideration qf thechange in miningenvironment that willoccur. As economics

force the considerationqf underground miningqf large, competent

. p.a. Box 95,

Boesmansriviermond

6190. Cape Province.

<9 The South 4fricanInstitute if Mining andMetallurgy, 1994. SA

ISBN 0038-223X/3.00 +

0.00. Paper received,Sept 1993; revised paperreceivedAzg. 1994.

Cave mining-the state of the artby D.H. Laubscher*

Introduction

Cave mining refers to all mining operations in

which the orebody caves naturally after

undercutting and the caved material is recovered

through drawpoints. This includes block caving,panel caving, inclined-drawpoint caving, and

front caving. Caving is the lowest-cost

underground mining method provided that the

drawpoint size and handling facilities are

tailored to suit the caved material and that the

extraction horizon can be maintained for the life

of the draw.

The daily production from cave-mining

operations throughout the world is approximately

370 kt per day, with the following breakdown

from different layouts:

GrizzlySlusherLHD*

Total

kt

9035

245

370.Load-haul-dumpunits

By comparison, South African gold mines

produce 350 kt per day.

In the near future, several mines thatcurrently produce in excess of 50 kt per day from

open-pit mines will have to examine the

feasibility of implementing low-cost, large-scale

underground mining methods. Several cave mines

that produce high tonnages from underground areplanning to implement dropdowns of 200 m or

more. This will result in a considerable change intheir mining environments. These changes will

necessitate detailed mine planning, rather than

the simple projection of current mining methods

to greater depths.

The Journal of The South African Institute of Mining and Metallurgy

As more attention is directed to the miningoflarge, competent orebodies by low-costunderground methods, it is necessary to derIDethe role of cave mining. In the past, caving hasbeen considered for rockmasses that cave andfragment readily. The ability to better assess thecavability and fragmentation of orebodies, andthe availability of robust LHDs, anunderstanding of the draw-control process,suitable equipment for secondary drilling andblasting, and reliable cost data have shown thatcompetent orebodies with coarse fragmentationcan be cave-mined at a much lower cost thanwith drill-and-blast methods. However, once acave layout has been developed, there is littlescope for change.

Aspects that have to be addressed arecavability, fragmentation, draw patterns fordifferent types of ore, drawpoint or drawzonespacing, layout design, undercutting sequence,,md support design.

Table I shows that there are significantanomalies in the quoted performance of differentcave operations.

Explanation

Under-evaluation of the orebody and dilution zone

A case of highly irregular draw and under-evaluation of thedilution

Drawpoints being drawn in isolation

A large range in fragmentation. and irregular, high values inthe dilution zone

OCTOBER 1994 279 ....