the development ofrock-burst control strategies ... · the development ofrock-burst control...

The development of rock-burst control strategiesfor South African gold mines

by R. HEUNIS*. Pr. Eng.. M.S.A.I.M.M.

SYNOPSISThe hazard of rock bursts and rock falls as experienced in South African gold mines is examined in the light of

recent research findings. The problem areas include high fatality rates, losses in production and equipment, and adetrimental influence on the recruitment of labour. The nature of the hazard is analysed, and it is shown that rockbursts are strongly related to the spatial rate of energy release by mining but that rock falls are not. The role ofblasting and geological discontinuities is discussed. Practical rock-burst control strategies are compared, includingthe use of stabilizing pillars, back-filling and, in particular, techniques for reducing the frequency and severity ofaccidents resulting from large rock bursts. The latest stope-support experiments and their potential implicationsto mining are consid~red. It is concluded that a better understanding of the rock-burst and rock-fall hazard hasresulted from the industry's research efforts. The findings can be applied towards obtaining an improved under-ground environment for men and machines.

SAMEVATTINGDie rotsbarsting- en rotsstortingsgevaar in Suid-Afrikaanse goudmyne word geanaliseer aan die hand van onlangse

navorsingsbevindinge. Probleemgebiede sluit in hoe sterftesyfers, produksieverliese, beskadiging van toerustingen 'n nadelige invloed op arbeidswerwing. Die aard van die probleem word ondersoek en daar wc>rd aangetoondat rotsbarstings sterk verband hou met die ruimtelike tempo van energievrystelling as gevolg van mynbou, maarrotsstortings nie. Die rol wat skietwerk en geologiese diskontinu"iteite speel ten opsigte van die rotsbarstings-probleem, word beskou. Verskeie praktiese beheermaatreels vir die bekamping van die probleem word met mekaarvergelyk, insluitende die gebruik van strekkingspilare en terugvulling van die gemynde gebied, en in die besonder,tegnieke vir die vermindering van die voorkoms en skade as gevolg van ongelukke veroorsaak deur groot rots-barstings. Die mees onlangse afboubestutting eksperimente en die potensiele uitwerking daarvan cp toekomstigemynboupraktyk word bespreek. Die slotsom is dat die bedryf se navorsingspoging tot op hede gEOlelhet tot 'n beterbegrip van die rotsbarsting- en rotsstortingsprobleem en dat die bevindinge aangewend ':an word ten einde 'nverbeterde werksomgewing vir ondergrondse werkers en masjinerie te verkry.

Introduction

The Rock Burst Research Project was instigated in1973 by the Anglo American Corporation in conjunctionwith the Mining Operations Laboratory of the Chamberof Mines of South Africa. Its aim is a study of all aspectsof the rock-burst problem as encountered in deep-levelSouth African gold mines, and the recommendation ofsolutions to the problem. The research forms part of anintensified industry-wide programme on the improve-ment of productivity in the gold-mining industry.

The practical significance of the research completedso far is proving to be considerable. Not only is it alreadyassisting in providing clearer guidelines for the industrybut, perhaps even more important, it will also allowfuture deep-level mining to be planned with greaterconfidence.

The ideas advanced in this paper are essentially of astrategic nature; while they are generally applicable,the greatest benefits will be derived from their utiliza-tion in the planning stages of mines.

For the sake of clarity, the terms rock burst and rockfall as used here are defined as follows:

rock burst - a seismic event radiating sufficientlyintense shock waves to cause visibledamage to an excavation

rock fall - a fall of rock fragment(s) from the roofor sides of an excavation

Mine Accidents

The improvement in safety standards in South African

*Anglo American Corporation, Gold and Uranium Division,Johannesburg.

gold mines is reflected in Fig. 1, which shows the ratesof fatal accidents since 1906.

Despite the rapid increase in the average depth ofmining over these years, the average number of fatalaccidents due to 'fall of ground' (i.e., combined rockburst and rock fall) has been gradually reduced andmaintained at a more or less constant rate of 0,7 perthousand employees per annum. However, the combinedcontribution of rock-burst and rock.fall accidents tothe overall accident rate has risen from about 20 percent to more than 50 per cent over the same period,making it now the dominant cause of fatal accidentsin the industry. In deep-level mines, the contributionis typically 70 per cent.

A more detailed analysis, distinguishing betweenrock-burst and rock-fall accidents, revealed that therelative contribution of the rock-burst component alsogradually rose as the average depth of mining increased.

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Fig. I-Accident fatality rate in gold mines per thousandpersons in service per annl,lm, allocated under causes1

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1980

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139

80 79.8%

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Fig. 2 shows a breakdown into causes of fatal accidentsat Western Deep Levels, a typical deep-level mine, overa recent three year period. The combined fatality ratedue to rock bursts and rock falls was 1,75 per thousandemployees per annum, or 80 per cent of the total forthe mine. The rock-burst contribution equalled 1,0 perthousand per annum, or 46 per ccnt of the total. Com-pared with the corresponding estimates of 0,73 perthousand per annum or 55 per cent of thc total, and0,19 per thousand per annum or 15 per-cent of the totalrespectively for five typical shallcw and medium-depthgold mines, it is clear that, as the average depth ofmining increases in the future, rock bursts and rockfalls will tend to dominate increasingly as the majorcause of fatal accidents, the relative importance shiftingfrom rock-fall to rock-burst accidents.

When the above fatality rates are compared with thecorresponding national average mortality rate of around0,8 per thousand per annum for males aged 30 to 40,it is clear that, in deep-level mines, the additional riskposed by rock bursts and rock falls is indeed significant.

Apart from average fatality rates, the temporaldistribution of accidents of a given size, expressed interms of number of fatalities per accident, was alsoconsidered. Based on an analysis of accident data per-taining to Western Deep Levels, the mean time T(n)between accidents resulting in approximately n fatalitiesamong E employees was found to be represented by

24 000T(n)-

EnI,5 months.

For example, on a mine employing 16000 people, anaccident involving approximately 10 fatalities is ex-pected to occur once every 47,4 months or approximately4 years.

. . . . . .

Fig. 2- The relative contributions to the accident fatalityrate on a typical deep-level mine, allocated under various

causes and taken over a recent three-year period

140 APRIL 1980

[/)

l-Ll.

:r:[/)

80

70

Fig. 3- The relative contributions to the rate of loss Inworking shifts as a result of accidents on a typical deep-levelmine, allocated under various causes and taken over a recent

three-year period

(1)

A recent study2 by the South African Chamber ofMines of the geographical distribution of 350 fatal rock-burst and rock-fall accidents in gold mines on the WestRand indicated that 60 per cent of fatal accidents occureither in strike gullies within 6 m of stope faces or inthe stope face itself. It was also found that approxi-mately 74 per cent of all fatal accidents occur nearthe stope face. A similar detailed investigation forWestern Deep Levels confirmed that about 73 per centof all rock-burst and rock-fall fatalities occur in ornear the stope face.

An assessment was made of the significance of non-fatal rock-burst and rock-fall accidents from an analysisof man-shifts lost due to accidents at Western DeepLevels over a three-year period. The results are sum-marized in Fig. 3. A comparison of these non-fatallosses with the fatality losses depicted in Fig. 2 showsthat rock-burst and rock-fall accidents are generallymore severe than other accidents, resulting in 80 percent of the fatalities but only about 44 per cent (2,42per thousand shifts worked) of the accident shifts lost(5,5 per thousand shifts worked).

Worker Response

Information concerning the attitude of in-service andpotential mine labour towards the risk of rock burstsand rock falls is relatively scanty. So that at least someidea could be gained of worker responses, interviewswere conducted with first-line personnel officers toestablish the attitude of workers to the rock-bursthazard. In combination with results derived elsewherein the industry 3, a somewhat unexpected pictureeme~g3d. It app3ars that the rock-burst hazard doesnot affect labour availability and morale as much as iscommonly assumed. An analysis of absenteeism and

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Total No. of Monthly turnover expressed as ano. of accidents percentage of average monthly turn-

fatalities analysed over in the affected section of the mineper

Months after occurrence of a~cidentaccident-1 0 1 2 3

- - - --2 10 97 105 97 81 121

3 to 5 7 88 132 103 88 8810 to 40 4 103 72 92 137 91

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TABLE I

l

ANALYSIS OF LABOUR TURNOVER TRENDS IN AN AFFECTED SECTIONAFTER A FATAL ROCK-BURST OR ROCK-FALL ACCIDENT

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turnover shows that only a small proportion of the totallabour force is noticeably affected by rock-burst androck-fall accidents or incidents. A consideration, forexample, of the worst case situations (namely, the turn-over of White labour in affected mine overseers' sectionson four different mines before and after the occurrenceof large ac:Jidents) showed no significant trends. Table Ishows the responses; it is clear that the fluctuations inlabour turnover in the affected sections were well withinnormal limits.

The absence of significant fluctuatio:J.s in turnoverwhen and where the impact of major accidents shouldbe at a maximum, makes the existence of significantlonger-term effects unlikely.

A similar comparison of absenteeism in the affectedsections following particularly severe rock-burst acci-dents at Western Deep Levels, resulting in the loss of atleast 42 panel shifts, is shown in Table lI.

lt is clear that, in the month after the accident,absenteeism rose to about 15 per cent, compared withthe usual average level of 4 per cent, but soon returnedto normal. Only 3 of these catastrophic events occurredon the mine during 5 years of mining, contributing only2 to 3 per cent of the total number of man-shifts lostdue to absenteeism. Similar comparisons for smalleraccidents, in which the number of panel shifts lost peraccident was less than 10 and which constitute approxi-mately 95 per cent of all rock-burst accidents, fail toshow any consistent rise in absenteeism in the affectedsections after such accidents.

Although similar information pertaining to Blacklabour is not available, there are indications that theeffects of the rock-burst hazard on morale are even less.For example, a 1978 survey 3 conducted by the Chamberof Mines on Black novice labour entering the miningindustry failed to show safety as a major factor affectingthe attitude of Black labourers once employed on themines. Risk was regarded as being the least importantunfavourable factor associated with their new jobs.

~

TABLE IIANALYSIS OF WHITE LABOUR ABSENTEEISM TRENDS IN AN

AFFECTED SECTION AFTER A CATASTROPHIC ROCK-BURST OR ROCK-

FALL ACCIDENT

No. of panel shiftslost per accident

Percentage of absenteeism in affectedsection

--~-Months after occurrence of accident

-1 0 1 2 3

42~50

76

243

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY APRIL 1980 141

151

131713

2150

However, before entering the mine labour market, riskto safety was regarded as being the most importantunfavourable factor.

Losses in Production

Production losses attributable to rock bursts androck falls over a four-year p.3riod on Western DeepLevels are shown in Figs. 4 and 5. Losses directlyattributable to rock bursts and rock falls are expressedas percentages of total production. The combined lossesdue to rock bursts and rock falls averaged 11,5 per cent,as can be seen from the two diagrams. Although theaverage of the rock-burst losses is considerably higherthan the 2 to 3 per cent claimed for the industry, amuch more disturbing figure is the short average periodbetween damaging rock bursts or rock falls in a particu-lar panel, which constitutes a face length of approxi-mately 35 m. An analysis of rock-burst data pertainingto 25D panels over a three-year period showed the

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Fig. 4-Production losses due to rock bursts for a typicaldeep-level mine, expressed as a percentage of actual pro.

duction over a period of four years

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Fig. S-Production losses due to rock falls for a typical deep-level mine, expressed as a percentage of actual production

over a period of four years

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Leader Reef

50 60 70

ENERGY RELEASE RATE (MJ/m2)

Fig. 6- The relationship between production losses due torock bursts and the energy release rate for two differentreefs. The average number of panel shifts lost per damagingrock burst equalled 4,5 independent of energy release rate

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ENERGY RELEASE RATE (MJ/ m2)

Fig. 7- The relationship between production losses due torock falls and the energy release rate for two different reefs

80

interval between damaging rock bursts in any particularpanel to average approximately 100 days. This almostcertainly would become an important factor for con-sideration when capital-intensive equipment is intro-duced in the stopes of deep-level mines.

Nature of the Hazards

The findings of the Rock Burst Research Projecthighlighted a basic difference between the nature of therock-burst and the rock-fall hazards4, 5.

It was found that the most suitable measure fordescribing the relative difficulty of mining on a particularreef or in a particular area is the spatial rate of energyrelease (ERR) due to mining. This single scalar quantityexpresses the combined effect of the mining geometry,depth of mining, stoping width, and elastic propertiesof the rock. Methods for the calculation of ERR havebeen reported extensively6-8. Figs. 6 and 7 summarizethe results, based on two years of production data, forthe Ventersdorp Contact and Carbon Leader Reefs.Production losses are expressed in terms of panel shifts,i.e. the number of lost blasts on a 35 m panel. One panelshift produces approximately 70 t of ore.

142 APRIL 1980

~

80

It is clear from these diagrams that a sympatheticrelationship exists between production losses throughrock bursts and ERR, and that rock"fall losses werehighly independent of ERR. It is also clear that theactual levels for a given ERR differed markedly for thetwo reefs, probably owing to different country rock.Scanty data at high ERR on the Ventersdorp ContactReef reduces the significance of the slight negativeslope of the curve for rock-fall loss versus ERR. Littleerror is introduced if behaviour similar to that shownby the well-established curve for the Carbon Leaderis assumed, but at a higher average level of 4,5 panelshifts lost per thousand centares (ca). The relationshipbetween accident man-shifts lost versus ERR was foundto be less predictable, probably also because of insuffi-cient data.

Similar, supporting results were obtained throughindependent investigations conducted by the Chambaof Mines Research Laboratories at Harmony Gold Mine9and at the East Rand Proprietary MineslO. Althoughthe results obtained in these three instances need notall be directly applicable to other mines, it is never-theless felt that the key concepts can generally beapplied to the control of rock-burst and rock-fall hazards,or to a comparison of the potential effectiveness ofvarious options available to a mine in adopting a rock-burst and rock-fall control strategy.

From the above it appears that the rock-burst hazardhas an almost linear relationship to ERR in a particulararea, whilst the overall rock-fall hazard is apparentlynot strongly related to ERR and is more likely to berelated to a combination of quality of support, stopingwidth, mining method employed, and geological struc-tures in the vicinity of the reef excavations. The strategiccontrol of rock bursts can therefore be effected onlythrough a reduction of the average ERR, while rockfalls, given a particular reef and mining method, canbe reduced only by an improvement in the quality ofsupport.

Another important factor emerging from the analysiswas that the total number of panel shifts lost per thou-sand centares, i.e. due to rock burst, rock fall, and othercauses, showed only a slight dependence on ERR. Inthe Carbon Leader Reef, for instance, the followingrelationship was found to closely describe the actualtotal loss rate Lt:

Lt=45+0,333 X ERR(MJ 1m2) panel shifts perthousand centares (2)

From Figs. 6 and 7, the combined rock-burst and rock-fall component, Lr, for the same reef is given approxi-mately by

Lr=2+0,018 xERR(MJ/m2) panel shifts perthousand centares

""""'"

(3)If the average ERR of the mine is reduced fourfold,from say 40 MJ/m2 to 10 MJ/m2, the rock-burst lossrate would be reduced to 25 per cent, the combinedrock-burst and rock.fallloss rate to 80 per cent, and theoverall loss rate to 83 per cent of the original levels. Avery significant reduction in average ERR would there-fore lead to a significant reduction in the rock-bursthazard, but only a relatively small improvement inproduction.

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JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

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The above findings are of a general nature and arenot intended for use in the analysis of specific isolatedareas prone to rock bursts over a short period of time,but should prove valuable in the making of strategicdecisions concerning the evaluation of mining layouts.

Geological Discontinuities

It is generally known that most mining-inducedseismic events are associated with geological discon-tinuities in the surrounding rock mass, including dykes,faults, joints, and bedding planes. The discontinuitiesrepresent planes of weakness and are therefore likely tofail before mining-induced stresses reach the highlevels required for the fracturing of intact rock. Recentlyconsiderable effort in the modelling of the behaviour ofdiscontinuities has led to a better understanding of theproblemll, 12.

A practical example of the influence of a regionalpredominant joint system on seismicity along longwallsis shown in Fig. 8. The left side of Fig. 8 shows the cross-sectional distribution of seismic events taken over atwo-year period around an east advancing longwallat Western Deep Levels, while the right side shows thedistribution for a nearby west advancing longwall.The directions of the longwall and predominant jointsare respectively approximately perpendicular andparallel in the two cases. The difference in seismicbehaviour may have a bearing on the rock-burst androck-fall hazardl3, and is currently under investigation.

Recent findings of the Chamber of Mines SeismicResearch Project in the Klerksdorp area included theinteresting observation that the maximum magnitudeof seismic events originating on faults is strongly relatedto the displacement on the fault. The following expressionwas quotedl4:

Mmax=0,32+1,88 log D, ..., (4)where Mmax=maximum Richter magnitude, and

D=fault displacement.From the above it appears that, in large displacementfaulting, very large rock bursts are possible.

Large displacement faulting was not considered inthe studies at Western Deep Levels since the maximumthrow encountered on faults in the vicinity of mining

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43 EAST lONGWAll0 ..JQQ.2l..

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Fig. 8- The distribution I?atterns .of s,:lsml~ events aroundtwo longwalls advancing In opposite directions, taken over

two years

.2

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14 18 2016

Fig. 9-The diurnal distribution of seismic events on theCarbon Leader Reef, normalized to the daily blasting time

operations rarely exceeds4 40 m. Instead, an attemptwas made to quantify the strategic importance of thedykes and small faults abounding in the area in termsof their contribution to the overall rock-burst and rock-fall hazard. It was found that the effect of any dykeor fault intersecting, or within about one panel length(35 to 40 m) of, a p!1nel, could be accounted for by theaddition of 30 MJ/m2 to the calculated ERR for thatpanel. The proportional correction (ERR+30 MJ/m2)/ERR, if multiplied by the production losses predictedin terms of the theoretical ERR as in Figs. 6 and 7,would then be a good estimate of the loss rates due torock bursts and rock falls in an affected panel.

A situation often encountered, in which the longwallshape is not maintained in the immediate vicinity ofdykes striking perpendicular to the direction of longwalladvance, could be explained quite effectively by theinclusion of the 30 MJ/m2 term. A rule of thumb wasfound; if dyke panels were planned to achieve 2tadditional blasts per month, the longwall shape wouldbe more or less maintained over a period of time.

Since Western Deep Levels has approximately 240panels with an average ERR of 40 MJ/m2, approxi-mately 45 of which intersect dykes or faults at anyonetime, the contribution of the geologically affected areasto the overall rock-burst and rock-fall hazard shouldbe 45x70/(195x40+45x70), i.e. approximately 30per cent. This is considerably less than the estimatedaverage figure2 for the industry of 60 per cent, iHustra-ting that the influence of geological discontinuity be-comes less as the average ERR increases.

A further important observation concerning dyke-dyke, dyke-fault, and fault-fault intersectionsl3, 15 wasmade at Western Deep Levels. The intersections actas stress concentrators, generating clusters of smallseismic events in their immediate vicinity. The positionsof the intersections can be determined from a study ofthe location of these seismic-event clusters. The implica-tion is that this phenomenon, which should manifestitself most strongly during the very early stages ofmining when the original stress field is perturbed forthe first time, could possibly be used to delineate dykesand faults long before mining approaches them. Theapproach of longwalls to major dykes and faults could


then be planned timeously to minimize the risk ofrock burst. Placement of critical excavations such asreplacement haulages or inclined shafts could also beplanned to avoid major dykes and faults.

Seismicity and Blastin~

A detailed study was made of the relationship be-tween blasting and seismicity on the Ventersdorp Con-tact and Carbon Leader Reefs at Western DeepLevels16, 17. A large number of diverse conclusionsresulted from the analysis, the most important beingthe following:(1) A significant portion of the total number of seismic

events on a mine occurs contemporaneously withenlargement of the excavation. The size of thisportion may vary from reef to reef, e.g. 25 per centon the Carbon Leader Reef versus 8 per cent on theVentersdorp Contact Reef, as can be seen fromFigs. 9 to 11.

(2) If it is assumed that most of the mining-inducedseismicity is closely coupled to the actual enlarge-ment of the stoping excavations, then continuousmining will distribute seismicity more uniformly in

.03

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~ .02;z;::g

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Fig. ID-The diurnal distribution of seismic events in thehangingwall (lava) of the Ventersdorp Contact Reef,

normalized to the daily blasting time

.03

P-1

~ .02Z

~

2

AFTER

Fig. 11- The diurnal distribution of seismic events in thefootwall (quartzite) of the Ventersdorp Contact Reef, nor-

malized to the daily blasting time

.\

100

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HOURS AFTER BLAST

1816 20

Fig. 12- The re~ative rock-burst exposure versus re-entryperiod following blasting, for the Carbon Leader and Venters-

dorp Contact Reefs

20

time, resluting in a likely o:l-shift seismicity increaseof about 200 per cent.

(3) Blasting in one area of a mine was found to increasethe probability of rock bursts elsewhere by up to500 per cent, depending on the distance and amountof blasting. This is of importance where adjacentmines blast at different times, and would complicatethe use of scattered blasting methods for reducingore-surge capacity requirements in deep-level mines.

(4) The exact sequence of blasting between varioussections using a centralized blasting system doesnot appear to affect later on-shift seismicity sig-nificantly. For instance, synchronized blasting isneither expected to aggravate nor alleviate thelater on-shift rock-burst safety hazard. The im-portant factor is to minimize the exposure of peoplein stopes while blasting takes place in the vicinity.

(5) Optimum re-entry periods vary from reef to reef.Fig. 12 shows the rock-burst exposure at WesternDeep Levels if the re-entry period after blasting isvaried on the Ventersdorp Contact and CarbonLeader Reefs. The rock-burst exposure is normalizedto the exposure associated with re-entry immedi-ately after the blast, on the assumption of an 8-hourunderground shift.

Control of Rock Bursts

\

Two important objectives in the development of aneffective strategy for rock-burst control are, firstly, areduction in the average rock-burst fatality rate perthousand employees per annum of a mine if this exceedsthe average for the industry and, secondly, a limitationof the incidence and effects of single, very large rock-burst accidents.

As indicated earlier, it appears from the findings of theRock Burst Research Project and other studics that astatistical reduction of the rock-burst hazard can beachieved through a reduction in the energy releascd bymining, which in turn can be achieved by the practice ofpartial extraction, by the back-filling of mined-out areaswith waste rock, slimes, sand, etc., or by a reduction inthe stoping width if total closure occurs in back areas.The installation of timber or prop support does little toreduce the incidence of rock bursts, as already stated.

144 APRIL 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

Irrespective of the method used, the effectiveness ofreducing the energy released by mining depends strictlyon the prevention of elastic closure and ride in the mined-out area.

Design criteria, and the advantages and disadvantagesof various mining methods using partial extraction havebeen dealt with in considerable detaiI6-8,l°. In deep-levelmines practising longwall mining, the introduction ofon-reef strike pillars, spaced regularly along the entirelength of the longwalls, appears at this stage to be themost attractive method for reducing the energy releasedby mining, for the following reasons.(a) Maximum protection is afforded at the face where

and while people are exposed. The protectionremains very constant throughout the life of thelongwall, which is not the case for dip pillars.

(b) Strike-stabilizing pillars do not normally seriouslyinterfere with the development of footwall drives,and only minor modifications to existing layoutswould be required.

(c) The life of overstoped or understoped access excava-tions should increase considerably since they wouldremain destressed through the life of the mine.Replacement development should therefore bereduced.

(d) Segmenting of longwalls allows for greater flexibilityof mining when major unplanned stoppages occur,e.g. due to fires or rock damage in stopes and haul-ages.

It was found18 that properly designed pillars would inpractice typically result in a sixfold to eightfold reduc-tion of the energy released by mining and, hence, anexpected proportional reduction of the rock-bursthazard. The combined rock-burst and rock-fall fatalityrate would then drop to 50 per cent of its previous levelin the case of Western Deep Levels, since the rock-fallcomponent is not expected to be significantly influencedby the introduction of pillars. Although approximately15 to 18 per cent of the reef would be permanentlylocked up in a well-designed strike-pillar system, thefinal overall extraction ratio at the end of the life of amine should not be much reduced because the use ofstability pillars will reduce the size of the final remnants,which would otherwise not be extracted. From equation(2) relating total production losses to ERR, it is alsoclear that some reduction can be expected in productionlosses if the average ERR is reduced by a factor of 6 to8. With Western Deep Levels as an example, it isexpected that the introduction of a strike-pillar systemon the Carbon Leader Reef would result in a 7 to 9 percent improvement in production per metre of availableface.

The problems associated with the introduction ofstrike pillars on existing longwalls arise mainly fromthe development of ventilation raises or cross-cutsthrough the pillars for single-district ventilation of thelongwall. Since the development is always close to or onthe reef plane, the pillar has to be regularly slotted on-dipin order to destress the intersecting raises or cross-cuts.In the past many problems have been encountered withthe slotting operation.

If the position of the face immediately down-dip of the

pillar is allowed to lead that of the face immediatelyup-dip of the pillar by a distance equal to the pillarwidth, the stress environment for the on-reef slottingoperation will be lower than the average for the long-wall, provided the slotting operation is completed beforethe face has advanced a further pillar width. Fig. 13shows how the maximum ERR increases if the operationis delayed unduly. Although the ERR does not exceed50 MJjm2 in: the typical example shown, the pillarwould normally be highly cross-fractured by a combina-tion of a strike-fracture system due to the pillar andpossibly a dip-fracture system parallel to the longwalland face directions. The necessity fQr an early completionof the slotting operation, while stress levels and ERRremain relatively low, has been borne out by experienceat Western Deep Levels.

The effectiveness of back-filling in reducing ERR, andhence the rock-burst hazard, was modelled for varioustypical mining situations, using a MINSIM-type com-puter programme modified to model non-linear back-fillbehaviour. The results were compared with those ob-tained from the modelling of strike pillars19, 2°.

The model involved the mining of a hypothetical3 km by 3 km area with a 1 km by 1 km shaft pillarat the centre through the life of the area. Mining wassimulated as breast longwalling with advance on strike.The reef was given an average depth of 3200 m, a dip of21 degrees, and a stoping width of 1 m. The followingcases were compared:

(i) Complete extraction.(ii) 20 m wide on-strike stabilizing pillars spaced 140 m

apart, i.e. one pillar per three 40 m panels, giving85 per cent extraction.

(iii) 40 m wide on-strike waste pillars, spaced 120 mapart, i.e. 33 per cent of the area, or everyone inthree 40 m panels, stowed. Ultimate compactions to50, 60, and 70 per cent of initial stoping width wereassumed.

TOip PLAN

Fig. IJ-The maximum energy-release rate that is encoun-tered with the on-reef destressing of strike pillars increases

rapidly if this lags behind the longwall


TABLE IIICOMPARISON OF AVERAGE ENERGY RELEASE RATE (}IJ/m2) FOR

STABILIZING PILLARS VERSUS !BACK-FILLING FOR THE HYPO-

THETICAL EXAMPLE

Complete Stabilizingextraction pillarsBack-filling

--~

-- ~-- -----Ultimate

compaction 50% 60% 70%~~

Area stowed33%50%

100%

434037

363329

282522

1168

(iv) Same as (iii) but 50 per cent of the area, or one illevery two panels, stowed.

(v) Same as (iii) but 100 per cent of area, i.e. all panels,fully stowed.

The results are summarized in Table Ill.The amount of energy absorbed by typical practical

fill materials was calculated to be between I and 4 MJ/m2in the above mining situation. As can be seen from TableIll, the fill effectiveness is determined largely by theultimate compaction, rather than by its ability toabsorb energy.

It appears that strike pillars are generally much moreeffective than back-filling in reducing the ERR. Thisapplies particularly to situations in which the closurewithout support would be small compared with thestoping width.

The frequency and effects of large single rock-burstaccidents can be reduced as follows.(I) A reduction of the overall energy released by mining

should also reduce the incidence of large rock bursts.(2) Segmentation of longwalls by strike pillars should

usually confine rock-burst damage to the facesbetween two adjacent pillars. From this point ofview, slender pillars would be sufficient, e.g. 20 mpillars spaced 140 m apart with a I m stopingwidth, to yield 85 per cent extraction.

(3) Segmentation of longwalls by allowing, for example,one half of the longwall to advance ahead of theother half by a distance of 100 m or more. MINSIM-type simulations show that the two parts willbehave almost independently, and experience atWestern Deep Levels has also shown that they areunlikely to be affected simultaneously by a singlelarge rock burst.

(4) Segmentation of longwalls into short panels of20 to 40 m, each leading its upper or lower neigh-bour by about 10 m, results in a similar effect. Apartfrom being a desirable layout for cleaning purposes,where dip and strike scrapers are used, and allowingfor some production 'slackness' in the system tocope with unplanned stoppages, the short leads tendto confine rock-burst damage to a very limited num-ber of panels, each lead acting as a damage 'barrier'.

The segmenting of longwalls by the introduction ofvery long leads has been practised at Western DeepLevels for the past ten years, but it was found that thismethod, though effective in reducing the extent of thedamage due to large events, introduced other undesirablefeatures. The trailing panels immediately adjacent tothe long lead will always be subject to extremely high

146 APRIL 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

energy-release rates and intensive cross-fracturing pat-terns, caused by the acute intersection between the longlead and the face directions. It was also found that10ngwaII segmentation into panels, though effective incontrolling damage due to small and medium rock bursts,was not effective in controlling damage resulting fromvery large events. Over the same ten-year period, therewere many instances in which more than five contiguouspanels were simultaneously affected by a single event.Also, the presence of a 10 m lead between panels issufficient to result in cross-fracturing of the hangingwalland, since this zone usually coincides with the positionof the scraper strike gully, it is probably the main causeof the high accident rates associated with these gullies.The argument is supported by the generally loweraccident rates exhibited by the strike or diagonal gulliesof breast-mined longwalls.

The strain energy stored in the rock surrounding alongwall supported by stabilizing pillars was shown tobe much lower than that without piIlars21. Evidence todate also indicates that strike pillars do not increase therock-burst hazard in any way, provided their width issufficient to inhibit pillar failure and provided the aver-age pillar stress is sufficiently Iow not to cause foundationfailure.

It was concluded that strike pillars appear to presentthe best solution to confining rock-burst damage to arelatively small face length, in addition to reducing theenergy released by mining. If at all possible, longwallsbetween two pillars should not be further segmented bythe introduction of large inter-panel leads since thiswould tend to aggravate gully conditions.

Control of Rock Falls

The findings of the Rock Burst Research Projectindicate that the hazard of rock falls is not strongly

MI~ED UNMINLD

"T1

"'

'\;rObliQlle X-fracture ~on short lead ;;

~ C"

~ ;:;

>('

o?

""~-7/>-

,.00

~<).

"(,

Fig. I4-Plan view of a typical long and short lead panel,showing the interaction of the longwall fractures with facefractures and the resulting hangingwall conditions in the

strike gully for the two cases

III

. !

related to the energy released by mining, but rather tothe mining method and quality of installed support forany given reef. The average rock-fall hazard certainlyvaries widely from one reef to another, as illustrated inFig. 7 for the Ventersdorp Contact and Carbon LeaderReefs on Western Deep Levels.

The importance of the interaction of various mininginduced fracture systems with one another, or withpre-existing geological joint systems, has been high-lighted by recent research findingsl3, 15. Although theirimplications to the rock-fall hazard appear complexand are not well understood at this stage, certain generalbut useful observations have been made.

The most undesirable situation from the point of viewof rock falls appears to be the presence of a cross-fracturepattern, i.e. two fracture systems intersecting at anacute angle, in the hanging wall where people are regu-larly exposed. Such a condition was often found toexist near or in strike barrier and boundary pilhrs, andalso in the strike gullies of stope panels having excessivelylong lags or leads, as is often found in overhand orunderhand longwalls. Fig. 14 shows how 10ngwa.Jl andface fracture systems are formed, and how they tend toform cross-fracture patterns in the vicinity of strikegullies as the lead between stope panels is increased.Two distinct fracture systems, longwall and face frac-tures, were found to exist almost without exception inabout a hundred panels that were mapped intensively.Fig. 15 shows a typical distribution of fracture orienta-tions in one of the measured panels. It appeared thatleads of even 5 m were sufficient to result in a prominentcross-fracture pattern, adversely affecting strike-gullyconditions in the vicinity of the face.

The Rock Burst Research Project attempted to assistin defining the requirements of good or 'optimum' in-stalled stope support for deep-level mines having stopingwidths of not more than 1,5 m, and to evaluate the in situ

NPanel: 96W2Date: 1-12-75Number: 200 fractures

NORMAL TOFACE

DIRECTION

NORMAL TOLONGWALLDIRECTION

Fig. IS-lower-hemisphere projection of the distribution ofnormals of 200 hangingwall fracture orientations in a typicalbreast-mined panel within an overhanded longwall configura-tion. For comparison, the normals to the actual face and

longwall directions are also shown

performance of existing support types. Firstly, it wasfound that support should be provided as close to the faceas possible; secondly, that only actIve support was capableof providing meaningful support load close to the face22.

'";;;!!! Sandwichz pack2

~

f::: ~z~

<:> ,,/" <:>In""

In\",,/

V')/

",,""/"

/'/

,"/"'/'."/../....

'"cc"-

C>

"a<0--'

:; 10 15" 5 10

CLOSURE (cm)

'" R.Y. H. prop

'"2

Press

- In~;;----

15 5 10 /5

Fig. 16-Comparison between the press-tested versus in situload-compression performances of a standard 60 cm by 60 cmsandwich pack, a 20 cm diameter resinated pipe-stick tele-scopic prop, and a 400 kN rapid-yielding hydraulic prop

(initial stoping width I m)

~

I

l ~; ;~----g~ E rh--- ~----lJ..

""... t:::I Lj

~N

~j----------------------14---3m ~ -Z,2m-

'3m~ 20~

~ 10c:0f-

6

~ r ~t T;5~----~~ E L l-----~~ ~

~J_---------------------14-- 3m >lC 2.2m-.j

~ 20.::..§. 10c:0f-

4 6

IT -25t--251"- 25'-25t-----

"~ L-l--1--!-----

u .

~~ 1-----------------------Z.2m---lm, lm.lm-.f

"20

~t 10c:0

4

DISTANCE FROM FACE (metres)

.6 X.6 m STANDARDSANDWICH PACK

R3-94/ca

.20m TELESCOPICPIPE/ STICK PROP

R4-54/ca

400kN R.Y.H.PROP

R5-53 / ca

Fig. 17-An in situ load performance versus cost comparisonof three stope-support systems. The distances from the faceto the first support line are averages obtained from actual

measurements


Fig. 16 shows the measured laboratory and in situload-compression curves for three popular types of stopesupport. The measured in situ performance of the samesupport types in the vicinity of the face are shown inFig. 17, together with 1979 cost estimates for eachsupport type. It is clear that rapid-yielding hydraulicprops are superior in providing good support close tothe face, and at reasonable cost. It should be noted thatneither the telescopic pipe-timber props nor the con-crete-timber sandwich packs, both of which are sup-posed to be stiff support elements, afford good pro-tection until considerable compression has occurred,at which time the face normally would already haveadvanced by about 12 m.

After the low initial stiffness characteristics of passive(installed with minimum prestressing) stope-supportelements had been determined, and after a preliminaryexperiment by the Chamber of Mines had been com-pleted at the Blyvooruitzicht Gold Mining Company,the concept of concentrated active (installed withsignificant prestressing) stope support was introducedat Western Deep Levels13

'22. The closely supervised

experiment was conducted initially on two stope panelsover a period of more than one year, and has sincebeen considerably extended. Three rows of 400 kNrapid-yielding hydraulic props were installed at a

density of 1,5 props per metre of face, with the firstrow approximately 2 m from the face. A fourth row,consisting of 1600 kN rapid-yielding hydraulic propscalled 'barrier props', was installed 1 m behind the thirdrow of props at a density of 0,4 props per metre of face.Fig. 18 depicts a typical layout. No permanent stopesupport was installed other than the usual solid timbergully support for the local protection of strike and dipgullies. The initial aim of the experiment was to provethat stope support in the back area of deep-level mines, i.e.v;here the ratio of depth below surface to maximum openspan exceeds 5, did little to improve conditions at the face,where most of the workers are exposed. The observedstability of the method is attributed to the narrownessof the hangingwall vertical tensile zone in the vicinityof the face of deep-level stopes. Theory predicts that the

tensile zone extends typically to not more than about1,5 m into the hangingwall at distances up to 20 mbehind the face, so that massive caving is not expectedto occur immediately behind the last row of props. Anumber of fairly large seismic events occurred within100 m of the experimental panels, yet little rock-falldamage was observed following the events, indicatinga stable support system. All in all, the experimentappeared to be highly successful from the point of viewof strata control, and should, after further refinement,

Fig. IS-The experimental concentrated stope-support system, viewed from the back area towards the face. Notethe condition of the unsupported hangingwall in the foreground behind the last support line

148 APRIL 1980 JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY,-\

result in an improved environment for people workingin stopes.

On the basis ofthe above findings it was concluded thata reduction of the rock-fall hazard, which remains acomplex problem, could best be achieved at this stageby an improvement of the stope support in the immedi-ate vicinity of the face, and by avoidance of cross-fracture patterns in the hangingwall above where peopleare exposed. The use of concentrated active stope sup-port and the elimination or reduction of leads betweenstope panels on longwalls appear to offer significantadvantages in the control of the strata in narrow stopes.

Progress, which in the foreseeable future should leadto a better understanding of the rock-fall problem, isalso being made in the investigation of the stability andbehaviour of fractured ground.

Productivity and Rock Bursts

From the above description of some of the recentfindings on the nature and extent of rock bursts androck falls, it is clear that the problem is very much amining problem in the full sense. It is integrated withmany facets of the underground operations such asplanning, method of mining, blasting, cleaning, materialhandling, support, ventilation, mechanization, andhuman factors. Attempts at improving productivity ona mine should therefore incorporate these rock-burstand rock-fall considerations.

Future Research

The South African gold-mining industry is currentlyinvesting an estimated one million rands per annum inrock-burst and rock-fall research alone, indicating itscommitment to finding ways of improving the under-ground environment. It is by nature difficult to predictthe directions that future research may take, but goodprogress can be expected in the development of thefollowing.(a) The prediction of rock bursts, to allow impending

rock-burst sites to be identified and evacuatedbefore the event takes place23, 24.

(b) An understanding of the interaction betweeninduced and geological discontinuities, and itsrelationship to the stability of the fracture zonesurrounding underground excavations.

(c) Seismic methods for the delineation of dykes andfaults ahead of mining excavations, to facilitatelong-term planning.

(d) An understanding of the violentfaults and dykes subjected tostresses.

failure of largemining-induced

Conclusions

The rock-burst and rock-fall hazard is an importantsafety problem for some gold mines, contributing asmuch as 80 per cent to the overall accident-fatality rate.The extent of the problem is also increasing on anindustry-wide scale, as evidenced by the statistics of thePrevention of Accidents Committee pertaining to SouthAfrican gold mines.

Accidents resulting in a large number of fatalitiesreceive wide publicity in the media, none of which canbe described as beneficial to the industry. Though

already-employed labour does not at present seem to bemuch influenced by such events, potential labourapparently is. Also, in both instances, the situation maybe expected to become more fluid in future, as theaverage standard of living and education rises. Sufficientknowledge exists concerning the problem to allow forthe implementation of effective control strategies aimedat reducing the rock-burst and rock-fall hazard.

It would therefore be in the interest of the miningindustry to take the lead in adopting a more positive,long-term control strategy aimed, firstly, at reducingthe occurrence of large rock-burst and rock-fall acci-dents and, secondly, at lowering the average accident-fatality rates on the mines worst affected.

Research should continue since the solution of someof the remaining questions should lead not only to asafer underground environment for men and machinesin the longer term, but also to significant cost savingsthrough more effective practices of strata control.

Acknowledgements

The author acknowledges the Management of AngloAmerican Corporation's Gold and Uranium Divisionand the Mining Operations Research Laboratory of theChamber of Mines for their support of the rock-burstresearch effort in general and for permission to publishthis paper. Members of the Rock Burst Research Project,who were responsible for many of the findings reportedhere, are also thanked for their contributions.

References1. GOVERNMENTMINING ENGINEER. Annual Report, 1976.

Pretoria, Government Printer, 1977.2. W AGNER, H. and TAINTON, C. J. An investigation into

factors affecting ground conditions in strike gullies. Johan-nesburg, Association of Mine Managers of South Africa,Papers awl Discussions, 1975/1976.

3. DE BRUYN, P. S., et al. Monitoring report, April, 1978, vo!.2, no. 4. Johannesburg, Chamber of Mines of South Africa,Human Resources Laboratory.

4. LAWRENCE, D. Final report on the influence of energyrelease rate and geology on safety, rock burst losses andproduction on a deep-level gold mine. Rock Burst ProjectReport, no. RP. 36, 1977.

5. DIE RING, J. A. C. Report on the influence of energyrelease rate on fall of ground losses and production inpanels using different support types. Rock Burst ProjectReport, no. RP. 54, 1979.

6. SALAMON, M. D. G. Rock mechanics of undergroundexcavations. Proceedings of the Third Congress of the Inter-national Society of Rock MechaniC8, vo!. I, part B. NationalAcademy of Sciences, 1974.

7. W AGNER, H. Strata control in South African gold mines.Proceedings of the 10th Canadian Rock Mechanics Sym-posium, vo!. I, 1975.

8. COOK, N. G. W., et al. Rock mechanics applicd to rock.bursts. J. S. Afr. Inst. Min. Metall., May 1966.

9. JOUGHIN, N. C. The measurement and analysis of earthmotion resulting from underground rock failure. Johannes-burg, Chamber of Mines of South Africa, Research Report,no. 73/66, 1966.

10. W AGNER, H. Mining case histories. Proceedings of theSymposium on Exploration for Rock Engineering, 1976.

11. RYDER, J. A., LING, T., and WAGNER, H. Slippage along afault plane as a rock burst mechanism - static analysis.Johannesburg, Chamber of Mines of South Africa, ResearchReport, no. 31/78, Jun. 1978.

12. CROUCH, S. L. Computer simuJation of mining in faultedground. J. S. Afr. Inst. Min. Metall., vo!. 79, no. 6. Jan.1979. pp. 159-173.

13. HEUNIS, R. A report on the interim conclusions of therock burst project (1973-1977). Rock Burst Project Report,no. RP. 37, 1977.


14. VAN DEN HEEVER, R. L. A seismic investigation of minetremors in the Klerksdorp mining complex. Johannesburg,Association of Mine Managers of South Africa, Papers andDiscussions, 1978.

15. HEUNIS, R. Rock bursts: the search for an early warningsystem. Optima, vol. 26, no. 1. 1976.

16. LAWRENCE, D. Report on the analysis of the relationshipbetween seismicity and the time of day and day of theweek. Rock Burst Project Report, no. RP. 13, 1975.

17. O'CONNOR, D. M. The relationship between seismicity andblasting on Western Deep Levels. Rock Burst ProjectReport, no. RP. 48, 1978.

18. HEUNIS, R. Report on the introduction of stabilizingpillars at "'estern Deep Levels. Rock Burst Project Report,no. RP. 66, 1979.

19. RYDER, J. A., and W AGNER, H. 2D analysis of backfill asa means of reducing energy release rates at depth. J ohannes-burg, Chamber of Mines of South Africa, Research Report,no. 47/78, Sep. 1978.

20. DIERING, J. A. C., Report on the extension of programSPUD to model a 60 X 60 grid incorporating non-linear fillelements and a comparison of fill and strike pillars. RockBurst Project Report, no. RP. 50, 1978.

21. SALAMON,M. D. G. Two-dimensional treatment of prob-lems arising from mining tabular deposits in isotropic ortransversely isotropic elastic ground. Johannesburg, Cham.ber of Mines of South Africa, Research Report, no. 11/67,1967.

22. VVISEMAN,N. Final report on the in situ performance ofpassive supports. Rock Burst Project Report, no. RP. 24,1976.

23. BRINK, A.v.Z. Rock burst prediction research in SouthAfrican gold mines. Proceedings of the Second Conference onAcoustic Emission/Microseismic Activity in Geologic Struc-tures and Materials, 1978.

24. BRINK, A.v.Z. Report on rock burst prediction researchat Western Deep Levels, Limited. Rock Burst Project Report,no. 64, 1980.

Colloquium: Materiaalseleksie/Materials selectionThe first colloquium of the Materials Engineering

Specialist Division of the South African Institute ofMining and Metallurgy was held in Pretoria on 28thNovember, 1979. The subject was the Selection ofMaterials - Part I: Choice and Specification of Struc-tural and Alloy Steel/Materiaalseleksie - Deel I: Keuseen Spesifisering van Strukturele en Legeringstaal.

Die colloquium is geopen deur die voorsitter van diespesialisteafdeling, dr. J. P. Hugo. Hy rig 'n spesialewoord van verwelkoming aan die sowat sewentig personeteenwoordig by die eerste colloquium van die spesialiste-afdeling. Hy deel die colloquium-gangers mee dat daarbeplan word om twee tot drie colloquia per jaar te houen dat daar ook besoeke aan nywerhede in die voor-uitsig gestel word.

Professor G. T. van Rooyen van die Universiteit vanPretoria lei die onderwerp in met 'n insiggewende lesingoor die beginsels van materiaalspesifisering. Hy wysd,aarop dat die verbruikers en vervaardigers die tweesleutelgroepe by materiaalspesifikasie vorm en dat diemetallurg tussen die twee groepe staan. Die volgendeaspekte is van besondere belang by materiaal-spesifikasies :

(i) Die inhoudelike aspek van die spesifikasieVoldoening aan die spesifikasies ten opsigte vanchemiese analise is nie altyd 'n waarborg dat diemateriaal aan die verlangde meganiese vereistes salvoldoen nie. Dit kan selfs gebeur dat 'n strengerspesifikasie 'n swakker materiaal vir 'n spesifieketoepassing lewer omdat sekere legeringselementewat buite spesifikasie le, dalk 'n voordeliger invloedkon gehad het.

(ii) KwaliteitsversekeringDit is uiters belangrik dat voldoende gehalte-versekering toegepas word om te verseker dat daarwel aan 'n bepaalde spesifikasie voldoen word.

(iii) Die koste-aspekDirekte koste wat voortspruit uit die vervaardigingvan 'n defektiewe komponent is belangrik maar diegevolgskade wat kan ontstaan uit 'n artikel watsou faal, moet ook in ag geneem word.

(iv) Die ontwerp van 'n onderdeelMet 'n goeie ontwerp kan swak materiaal bv.gebruik word, maar aan die ander kant moet 'u

150 APRIL 1980 .JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND MfTALLURGY

(Dr R. J. Dippenaar:Rapporteur)

""--

strawwe spesifikasie nie neergeIe word om vir 'nswak ontwerp te kompenseer nie.

Ten slotte wys professor Van Rooyen daarop dat dieoordeelkundige gebruik van spesifikasies van die aller-grootste belang is. Daar moet deeglik kennis geneemword van die vereistes wat die toepassing stel en diegebruiker moet goed onderIe wees in die kennis van dietoepaslike ma teriaaleienska ppe.

Dr Rocco de ViIliers, of Iscor, read a paper on therationalization of structural steel specifications in SouthAfrica. His paper was aimed at the man in industry whohas to select a steel for a specific purpose. This includesthe designer, the works engineer, and the fabricator.He emphasized that the supply of steel has to be cateredfor by the steel merchant and service centre to an ever-increasing extent. Serious difficulties arise in theirinventory planning, which could be greatly relieved ifagreement could be reached on a basic range of specifica-tions that can meet the bulk of the needs in South Africa.Specialized requirements could then be treated as suchby designer as well as manufacturer. Dr de ViIliersproceeded to outline the various types of wrought-carbonand Iow-alloy steels required with respect to a rationaliza-tion in their specifications.

The paper that was read by Mr J. R. Campbell, ofBritish Engine Insurance Company, dealt with theproblems facing the practising engineer who has to assessthe acceptability of known defects in components inservice. He discussed specific examples of known defectsin structures with reference to code philosophy, andcontemplated a fracture-mechanics approach to specifi-cations.

Mr J. G. Price, of TUV Rheinland (SA), discussedmanagement problems associated with the specificationof a sophisticated stainless steel that had been used fora torque thickener at a gold mine. In order to initiate aproper quality assurance programme, it had beenfound necessary to adopt a computerized controlmechanism on all aspects of materials supply, identifica-tion, and control.

After a lively discussion, the meeting adjourned for asocial function in the clubhouse of the university.

I.

the development ofrock-burst control strategies ... · the development ofrock-burst control...

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