Cave mining-thestateoftheart - SAIMM · Cavemining-thestateoftheart orebodies bylow-cost methods, theroleqf cavemining willhave tobedUined Inthe past, caving was generallY consideredfor

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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 ....

Cave mining-the state of the art

orebodies by low-costmethods, the role qfcave mining will haveto be dUined In thepast, caving wasgenerallY consideredforrockmasses that caveandfragment readilY.The ability to definecavability andfragmen-tation, the availabilityqf large, robust

load-haul-dump units,a better understandingqf draw-control

requirements, improveddrilling equipment forsecondary blasting, andreliable cost data haveshown that competentorebodies with coarse

fragmentation can beexploited by cavemining at a much lowercost than by drill-and-blast methods.

~ 280

It is common to find that old established

mines, which have developed standards during

the course of successfully mining the easy

tonnage in the upper levels of the orebody, have a

resistance to change and do not adjust to the

ground-control problems that occur as mining

proceeds to greater depths, or as the rock types

change. Mines that have experienced continuous

problems are more amenable to adopting new

techniques to cope with a changing mining

situation. Detailed knowledge of local and

regional structural geology, the use of an accepted

rockmass classification to characterize the

rockmass, and knowledge of the regional and

induced stress environment are prerequisites forgood mine planning. It is encouraging to note that

these aspects are receiving more and more

attention.The Laubscher rockmass classification

system that provides both rockmass ratings androckmass strength is necessary for designpurposes. The rockmass ratings (RMR) definethe geological environment, and the adjusted ormining rockmass ratings (MRMR)consider theeffect of the mining operation on the rockmass.The ratings, details of the mining environment,and the way in which this affects the rockmassand geological interpretation are used to definethe cavability, subsidence angles, failure zones,fragmentation, undercut-face shape, cave-frontorientation, undercutting sequence, overallmining sequence, and support design.

Factors Affecting Caving Operations

Twenty five parameters that should beconsidered before the implementation of anycave mining operation are set out in Table H. Theparameters in capital letters are a function of theparameters that follow in the same box. Many ofthe parameters are uniquely defined by theorebody and the mining system, and are notdiscussed further. The parameters consideredlater are common to all cave-mining systems andneed to be addressed if any form of cave miningis contemplated.

OCTOBER 1994

Cavability

Monitoring of a large number of cavingoperations has shown that two types of cavingcan occur. These have been defined as stress andsubsidence caving.

Stress caving occurs in virgin cave blockswhen the stresses in the cave back exceed thestrength of the rockmass. Caving can stop whena stable arch develops in the cave back. Theundercut must be increased in size, or boundaryweakening must be undertaken to induce furthercaving.

Subsidence caving occurs when adjacentmining has removed the lateral restraint on theblock being caved. This can result in rapidpropagation of the cave, with limited bulking.Figure 1 illustrates the effect of removing thelateral restraint from block 16 at Shabani. Block16 had a hydraulic radius of 28 with an MRMRof 64 and a stable, arched back. The adjacentblock, no. 7, was caved and resulted in areduction in the MRMR in block 16 to 56, atwhich stage caving occurred. For a range ofMRMR, Figure 1 illustrates worldwide cavingand stable situations.

The stresses in the cave back can bemodified to an extent by the shape of the cavefront. Numerical modelling can be a useful tool,helping the engineer to determine the stresspattern associated with several possible miningsequences. An undercut face, concave towar~sthe caved area, provides better control of majorstructures. In orebodies with a range of MRMR,the onset of continuous caving will be based onthe lower rating zones if these are continuous inplan and section. This is illustrated in Figure 2B,where the class 5 and 4B zones are shown to becontinuous. In Figure 2A, the pods of class 2rock are sufficiently large to influence caving,and the cavability should be based on the ratingof these pods.

All rockmasses will cave. The manner oftheir caving and the resultant fragmentation sizedistribution need to be predicted if cave miningis to be implemented successfully. The rate ofcaving can be slowed down by control of thedraw since the cave can propagate only if thereis space into which the rock can move. The rateof caving can be increased by a more rapidadvance of the undercut, but problems can ariseif this allows an air gap to form over a largearea. In this situation, the intersection of majorstructures, heavy blasting, and the influx ofwater can result in damaging airblasts. Rapid,uncontrolled caving can result in an early influxof waste dilution.

The rate of undercutting (RV) should becontrolled so that rate of caving (RC) is fasterthan the rate of damage (RD);

RC>RU>RD.

The Joumal of The South African Institute of Mining and Metaliurgy


Table 11

Parameters to be considered before the implementation of cave mining

PRIMARY FRAGMENTATION

Rockmass strength (MRMR)Geological structuresJoint/fracture spacingJoint condition ratingsStress or subsidence cavingInduced stress

LAYOUT

FragmentationDrawpoint spacing and size

Method of draw

UNDERCUTTING SEQUENCE(pre/advance/post)

Regional stressesRockmass strengthRockburst potentialRate of advanceOre requirements

DEVELOPMENT

LayoutSequenceProductionDrilling and blasting

PRACTICAL EXCAVATION SIZE

Rockmass strengthIn situ stressInduced stressCaving stressesSecondary blasting

DRAWPOINT INTERACTION

Drawpoint spacingFragmentationTime frame of working drawpoints

SECONDARY BLASTING/BREAKING

Secondary fragmentationDraw methodDrawpoint sizeSize of equipment and grizzly spacing

SUPPORT REPAIR

Tonnage drawnPoint and column loadingBrow wearSecondary blasting

The Joumal of The South African Institute of Mining and Metallurgy OCTOBER 1994 281 ....

70

~60::E~::E

50

40

30

20

10

A I

0 10 20 50

ClASS lA,B ClASS 2A,B ClASS 3A,B

~~1««1

Cavemining-the state of the art

100

90

80

0I0

30I

100 150

40 60I

200

7010 20 50I

250 feet50

Figure 1-A stability diagram for various mines worldwide

HYDRAULIC RADIUS =Area/perimeter

CROSS SECTION BLOCK 7/2

STABLE: Only local support required.

TRANSITIONAL: Supportable in upper band, with archingin middle band and intermittent caving/arching in lowerband depending on outside influences such as blasting,water.

CA VING: Progressive caving of cave back or walls intopreviously caved areas.

Cavability, function of:

Rockmass strengthGeological structureIn situ stressWaterInduced stress ]

MRMR

0 Stable~ Transitional. Caving

80metres

CROSS SECTION BLOCK WEST/2

~

Figure 2-Geomechanics classification data

OCTOBER 1994282

.'., ..'; .

I

lOOm

ClASS 4A,B ClASS 5A,B

I:;;~"~',":;;':"I ~



INPUT DATA =

CAVINGOPERATION =

Good geotechnicaIinformation,as wellasmonitoring of the rate of caving and rockmassdamage, is needed to fine-tune this relationship.

Fragmentation

In caving operations, fragmentation has abearing on the following:

>- Drawpoint spacing>- Dilution entry into the draw column>- Draw control>- Drawpoint productivity>- Secondary blasting/breaking costs>- Secondary blasting damage.

The input data needed for the calculation ofthe primary fragmentation and the factors thatdetermine the secondary fragmentation as afunction of the caving operation are shown inFigure 3.

Caving results in primary fragmentation,which can be defined as the size distribution ofthe particles that separate from the cave backand enter the draw column. The primaryfragmentation from stress caving is generallyfiner than that from subsidence caving owing tothe rapid propagation of caving in the latter case,with disintegration of the rockmass, primarilyalong favourably oriented joint sets, and littleshearing of intact rock. The orientation of thecave front or back with respect to the joint setsand direction of principal stress can have asignificant effect on the primary fragmentation.

Secondary fragmentation is the reductionin size of the original particle that enters thedraw column as it moves through the drawcolumn. The processes to which particles aresubjected determine the fragmentation sizedistribution in the particles that report to thedrawpoints. A strong, well-jointed material canresult in a stable particle shape at a low drawheight. Figure 4 shows the decrease in fragmen-tation for different draw heights and less-jointed(coarse) to well-jointed (fine) rockmasses. Arange in rockmass ratings will result in a widerange in fragmentation size distribution ascompared with that produced by rock with asingle rating, since the fine material produced bythe former tends to cushion the larger blocks andprevents further attrition of these blocks. This isillustrated in Figure 2B, in which class 5 andclass 4 material is shown to cushion the largerprimary fragments from class 3. A slow rate ofdraw allows a higher probability of time-dependent failure as the caving stresses work onparticles in the draw column.

Fragmentation is the major factordetermining drawpoint productivity. Experiencehas shown that 2 m3 is the largest size of blockthat can be moved by a 6 yd LHD and still allowan acceptable rate of production to bemaintained. In Figure 5 the productivity of alayout using 3,5 yd, 6 yd, and 8 yd LHDs and agrizzly are related to the percentage of fragmentslarger than 2 m3. The usage of secondaryexplosives is based on the amount of oversizethat cannot be handled by a 6 yd LHD.

Orientation-CaveFront/Joints

PRODUCTION = Secondary BlastingDamage and Costs

Operational Costs and Overall Productivity

Figure ~nput data for the calculation of fragmentation



350mm14" Gr

500mm20' Gr 3yd IJID 6yd lJID

100

""

90

.S 80(f)(f)

'"70

'""

80~ 50~ 40

"... 30

"'"

2010

0,1 0.01

. 15% Drilled and Blasted= 8% of Operating Cosls

0.1

SIZE m3

*105m DrawHeight

Primary Fragmentation- - - - Secondary Fragmentation

1.0 2.0

Figure 4-Size distribution of cave fragmentation

1400

1300

1200

1100

1000

900--':a BOO<Z>[; 7000..

'"600

§ 5000

E-400

300

200

100

0

""'"

""....

0,1 1 5 10 2030 5070Percentage + 2m3 - Secondary Fragmentation

10.0 100.0

350"">.<0325

300"'"'"275 .:=.

250 ~225 S

'"200 c3I

175 ~i(J

150 i:C

125 §100 'e

<=>75 i:'

50 i0

25 ~

Figure 5-LHD productivity on a round trip of 100 m with 60 per cent utilization

~ 284

A computer simulation program has beendeveloped for the calculation of primary andsecondary fragmentation.

Drawzone Spacing

Drawpoint spacings for grizzly and slusherlayouts reflect the spacing of the drawzonesbecause of the close spacing of the drawpoints.However, in the case ofLHD layouts with anominal drawpoint spacing of 15 m, thedrawzone spacing can vary across the majorapex (pillar) from 18 to 24 m, depending on thelength of the drawbell (Figure 6). This situationoccurs when the length of the drawpoint crosscutis increased to ensure that the LHD is straightbefore it loads. In this case, the major consid-eration, optimum ore recovery, is prejudiced byincorrect usage of equipment or a desire toachieve idealized loading conditions.

OCTOBER 1994

Sand-model tests have shown that there is arelationship between the spacing of drawpointsand the interaction of drawzones. Widely spaceddrawpoints develop isolated drawzones, withdiameters defined by the fragmentation. Whendrawpoints are spaced at 1,5 times the diameterof the isolated drawzone (IDZ) , interactionoccurs. The interaction improves as thedrawpoint spacing is decreased, as shown inFigure 7. The flow lines and the stresses thatdevelop around a drawzone are shown in Figure8. The sand-model results have been confirmedby observation of the fine material extractedduring cave mining and by the behaviour ofmaterial in bins. The question is whether theinteractive theory based on isolated drawzonescan be wholly applied to coarse material, wherearch spans of 20 m have been observed. Thecollapse of large arches will affect the large areaoverlying the drawpoint, as shown in Figure 9.The formation and failure of arches lead to widedrawzones in coarse material, so that thedrawzone spacing can be increased to thespacings shown in Figure 9. The frictionalproperties of the caved material must also berecognized. Low-friction material could flowgreater distances when under high overburdenload, and this could mean a wider drawzonespacing.

There is still a need to continue with three-dimensional model tests to establish some poorlydefined principles, such as interaction acrossmajor apexes when the spacing of groups ofinteractive drawzones is increased. Numericalmodelling could possibly provide the solutionsfor the draw behaviour of coarse-fragmentedcave material.

Draw Control

The draw-control requirements are shown inFigure 10 and Table Ill. The grade and fragmen-tation distribution for the dilution zone must beknown if sound draw control is to be practised.Figure 11 shows the value distribution forcolumns A and B. Both columns have the sameaverage grade of 1,4 per cent, but the valuedistribution is different. The high grade at thetop of column B means that a larger tonnage ofdilution can be tolerated before the shutoff gradeis reached.

In LHDlayouts, a major factor in poor drawcontrol is the drawing of fine material at theexpense of coarse material. A strict draw-controldiscipline is required in that the coarse ore mustbe drilled and blasted at the end of the shift inwhich it reported in the drawpoint.

The Joumal of The South African Institute of Mining and Metallurgy

iPlX DII.IJBlU. APD: DRAIBlU.

IT- -TI~


Drawbell

Dranone

Drawpoinl

18m 22m 24m11 1411.1 11111"11.111111"'11.111jl12mljjl Illllljl8mlljlljl~mll

I 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} \ riJ24mll I I I Iloml...J2omll I} \ [12m

W18l.JI I \ I I L..-~ I j I \ I j L ~ I I L ~ I I L ~ j

~~..l:(

n ~1 en )WdW "Wd~tj:

.. ..30m

ProduclionLevel

Figure &-Maximum and minimum drawzone spacing (isolated drawzone = 10 m area of influence = 225 m')

11

Lor

Pressure

Zone

Vedle.1 StressInere.sed by.dj.eent workingdrorpoint

- - Vedle.tR.dl.1

A. DIPs @ 2.2 X IDZ DIAM.DIL. ENTRY 15%

B. DIPs @ 1.5 X IDZ DIAM.DIL. ENTRY 60%

---

c. DIPs @ 1.1 X IDZ DIAM.DIL. ENTRY 85%

D. DPs @ 1.1 X IDZ WORKEDIN ISOLATION DIL. ENTRY 25%

IT- -TI~

Figure 7- The results of three-dimensional sand-model experimentsFigure 8-Flow lines and inferred stresses betweenadjacent working operations


Loading Width I!aximum/llinimum Spacing of DraYZones5m = 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


Rockmass Class

FF/m

Rock Size Range

Loading Width

5m =4m =3m =2m =

550-7

0.01-0.3m

4

20-1.50.1-2m

3

5-0.40.4-5m

Isolated Drawzone Diameter

2

1.5-0.21.5-9m

6.5m6m

9ma.5mam

13m12.5m12m

Figure 9-Maximum/minimum spacing of drawzones based on isolated drawzone diameter

INPUT DATA =

EFFECT OFLAYOUT =

PRODUCTIONINFLUENCE =

Figure 1o-Draw-control requirements

Ore Recovered + Dilution =Production Tonnage and Grade

~ 286

It has been established that the draw willangle towards less dense areas. This principlecan be used to move the material overlying themajor apex by the differential draw of lines ofdrawbells so that zones of varying density arecreated.

Dilution Entry

The percentage of dilution entry is thepercentage of the ore column that has beendrawn before dilution appears in the drawpoint,and is a function of the amount of mixing thatoccurs in the draw columns. The mixing is afunction of the following:

OCTOBER 1994 The Joumal of The South Afdcan InstItute of Mining and Metallurgy

0 0

02 0.2

0.4 0.4

0.6 0.6

0.8 1.6

1.2 2.0

1.6 1.2

2.0 0.8

20% 40%[ 1[ 1

Value of A 1.9% 1.4%

%Dra. B 0.9% 1.4%

,,C:-A-,,

B'41

80 60 40 20RMR

RMRRange Curves Examples Ratings0 - 14 NO.l A 50 - 6015 - 29 NO.230 - 49 No.3

+50 No.4 B 5 - 60

20 40 60 80 90 120 140HEIGHT OF INTERACTION ZONE IllZ

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

55 21m 90m


Application

Calculation of tonnageRecording of tonnages producedControlling the draw

A

Unpay

70% 100%

1% Ora. [

I 11.0% 0.7%

1.4% 1.1%

128% 150%

11 Ore Recovery

0.5% 85%

0.7% 96%

200

Drawpoints

Average Ore Grade = 1.4%Shut off Grade = 0.7%

Figure 11-Grade analysis

~ Ore draw height~ Range in fragmentation of both ore and

waste~ Drawzone spacing~ Range in tonnages drawn from drawpoints.

B

The range in fragmentation size distributionand the minimum drawzone spacing across themajor apex will give the height of the interactionzone (HIZ). This is illustrated in Figure 12.There is a volume increase as the cavepropagates, so that a certain amount of materialis drawn before the cave reaches the dilutionzone. The volume increase or swell factors arebased on the fragmentation and are applied tocolumn height. The following are typical swellfactors: fine fragmentation 1,16, medium 1,12,coarse 1,08.

A draw-control factor is based on thevariation in tonnages from working drawpoints.This is illustrated in Figure 13. If productiondata are not available, the draw-control engineermust predict a likely draw pattern. A formulabased on the above factors has been developedto determine the dilution entry percentage:-

Dilution entry = (A -B)/A x Cx 100,

whereA =Draw-column height x swell factorB =Height of interactionC=Draw-control factor.

The graph for dilution entry was originallydrawn as a straight line, but undergroundobservations show that, where there is earlydilution, the rate of influx follows a curved linewith a long ore 'tail', as shown in Figure 14.Figure 15 shows that dilution entry is alsoaffected by the attitude of the drawzone, whichcan angle towards higher overburden loads.

,RMR OF ALL MATERIAL IN THE POTENTIAL DRAW COLUMN TO BE USED IN CALCULATION AS FINES FLOW MUCH FURTHER THAN COARSE

MINJ},{ill! SPACING OF DRA1fZONES

ACROSS THE MAlOR APEXIBm

15m -- --12m

9m

VERTICAL LINE "(!' LOCATED AT HIGHEST RATING OF MATERIAL IN DRAW COLUMN

Figure 12-Height of the interaction zone (HIZ)



1.00.90.80.70.60.50.40.30.20.10

2 ID 11

Standard Deviation x 100 of Tonnage of Working Drawpoints

DiPs

Monthly

Tonnage

Wll

2000

E/4

1800

Ell

800

W12 E/2

1000 2500w/3

600E/3 w/41500 800

Mean = 1375 Standard Deviation = 682/100 = 7

Draw Control Factor = 0.6

Figure 13- The draw-control factor

A-B x C x 100A

A = Ore Draw Column Height x Swell Factor = 168mB = Height of Interaction Zone = 90mC = Draw-control Factor = 0.6

= Dilution Entry

168 - 90 x 0.6 x 100168 = 28%

0100

C 9

0 80M 70P 600S 50

I 40T 30I 200N 10

0

200 250

DILUTION /W ASTE

0 20 25040 60 80 100

Percentage Draw

200

Figure 14-Calculation of dilution entry

~ 288 OCTOBER 1994

Layouts

A factor that needs to be resolvedis the correctshape of the major apex. It is considered that ashaped pillar will assist in the recovery of fineore. Where coarse material results in majorarching, a square-topped major apex (pillar) ispreferable in terms of arch failure and browwear, as shown in Figure 16. The main area ofbrow wear is immediately above the drawpoint.If the vertical height of pillar above the brow issmall, as shown in Figure 16A, failure of the topsection will reduce the strength of the lowersection and result in aggravated brow wear.

More thought must be given to the design ofLHD layouts in order to provide the maximumamount of manoeuvring space for the minimumsize of drift opening so that larger machines canbe used within the optimum drawzone spacings.Another aspect that needs attention is the designof LHDs to reduce the length and increase thewidth. Whilst the use of large machines might bean attraction, it is recommended that caution beexercised and that a decision on machine size bebased on the correct assessment of the requireddrawzone spacing in terms of fragmentation. Theloss of revenue that can result from high dilutionand ore loss far exceeds the lower operatingcosts associated with larger machines.

Eight different horizontal LHD layouts andtwo inclined drawpoint LHD layouts are in use atvarious operating mines. An example of aninclined LHDlayout is shown in Figure 17.

Undercutting

Undercutting is one of the most importantaspects of cave mining since, not only is acomplete undercut necessary to induce caving,but the undercut method can reduce thedamaging effect of induced stresses.

The normal undercutting sequence is todevelop the drawbell and then to break theundercut into the drawbell, as shown in Figure 18.In environments of high stress, the pillars andbrows are damaged by the advancing abutmentstresses. The Henderson Mine technique ofdeveloping the drawbell with long holes from theundercut level reduces the time interval andextent of damage associated with postundercutting. In order to preserve stability,Henderson Mine has also found it necessary todelay the development of the drawbell drift untilthe bell has to be blasted (Figure 19).

The damage caused to pillars around driftsand drawbells by abutment stresses issignificant, being the major factor in brow wearand excavation collapse. Rockbursts are alsolocated in these areas. The solution is tocomplete the undercut before the development ofthe drawpoints and drawbells. The 'advancedundercut' technique is shown in Figure 20.



A. Average Loading Above Drawpoints

t t t t ~ t

f

.n...~ ::::

FOOTWALL V un

DRA WPOINTS

B. Higher Loading To Side Of DrawpointsSIMULATED HILL

FEATURE ffiGHEROVERBURDEN LOADING

~ ~MODEL

CORNER

EFFECT

Figure 15-lnclined drawpoint layout showing the effect of different overburden loading (three-dimensional sand-model experiments)

MORE STABLE

ARCHES

FAIR FINES

DRAW

STABLE ARCHES

HIGHER

Figure 16-Shapes of major apex/pillars


POOR FINES

DRAW

BETTERARCH FAILURE

OCTOBER 1994 289 ...


Inclination - 40'

Spacing. DIP

121518

STRIKE1012

12/15

VERTICAL10

12.515

Figure 17-An example of the layout of an inclined drawpoint

A. Production Level Layout Detail

I~-EI~

,I II II I--~--r r-~---__~__L L_~---

E!

B. Cross Section

Figure 18-LHD layout at El Teniente

~ 290 OCTOBER 1994

C. Isometric View ofBlockcaving with LHD

The Joumal of The South Afncan Insmute of Mining and Metallurgy

-----

C. Medium/Low

~Stress Weak Ground .!;.:!

Production Drift c;f;'j

Drawpoint andDrawbell Drift ~Fully~ Temporary --- Full y---

Supported Supported Supported


In the past it was considered that the heightof the undercut had a significant influence oncaving and, possibly, the flow of ore. Theasbestos mines in Zimbabwe had unoercuts of30 m with no resultant improvement in caving orfragmentation. The long time involved incompleting the undercut often led to ground-control problems. Good results are obtained withundercuts of minimum height provided thatcomplete undercutting is achieved. Wheregravity is needed for the flow of blasted undercutore, the undercut height needs to be only halfthe width of the major apex. This results in anangle of repose of 45 degrees and allows the oreto flow freely.

Support Requirements

In areas of high stress, weak rock will deformplastically and strong rock will exhibit brittle,often violent, failure. If there is a large differencebetween the RMR and MRMR, yielding supportsystems are required. This is explained inFigure 21.

Figure 19-1sometric view of a panel-cave operation

Blasted Hor. Holes - --

A. Very High Stress

Production DriftOnly

~-

=- m--- ~=- ~-

.~~

~ly Supporte~

NarrowUndercut

ProductionLevel

B. High Stress

Production Driftand Drawpoint

'--= m ~---

~Fully

Supported

NarrowUndercut

ProductionLevel

Figure ~ifferent sequences of advanced undercutting

The Joumal of The South African Institute of Mining and Metallurgy OCTOBER 1994

NarrowUndercut

291 ....


00

2010

20

M 30

R40

M

R 50

60

70

80

90

R M R30 70 8050 6040

SYSTEMS

Failure

. RIGID SUPPORT SYSTEMS. ..

i'. .

100

Figure 21-Support requirements for caving operations

Acknowledgements

This paper presents an updateof the technology of cave

mining. It is not possible to

quote references since the

bulk of the data supporting

the contents of this paper

have not been published and

the basic concepts are known

to mining engineers.

However, it is appropriate to

acknowledge the contributions

from discussions with the

following people in canada,

Chile, South Africa, and

Zimbabwe: R. Alvarez, P.).

Bartlett, N.).W. Bell, T. Carew,

A.R. Guest, C. Page,

D. Stacey, and A. Susaeta.

Thanks are due toP.). Bartlett for assisting in the

final preparation of the paper.

The simulation program for

the calculation of primary and

secondary fragmentation

referred to in the text was

writtenbyG.5. Esterhuizen at Pretoria

University.

~ 292

Table IV

Support techniques

90 100

t

High

stress

= 1 m + (0,33 W x F)

=1 m

= Rigid rebar

0,5 mm x 100 mm aperture

Cables = 1 m + 1,5 W

Mesh-reinforced shotcrete

Rigid steel archesMassive concrete

Large washers (triangles)Tendon straps

25 mm rope-cable slings

Pre-stressed cables have little application inunderground situations unless it is to stabilizefractured rock in a low-stress environment. Theneed for effective lateral constraint of the rockand of lining surfaces such as concrete cannot betoo highly emphasized. Support techniques areillustrated in Table IV.

Conclusions

~ Cavability can be assessed providedaccurate geotechnical data are availableand the geological variations arerecognized. The mining rockmass rating(MRMR) system provides the necessarydata for the empirical definition of theundercut dimension in terms of thehydraulic radius.

~ Numerical modelling can assist theengineer in understanding and definingthe stress environment.

~ Fragmentation is a major factor in anassessment of the feasibility of cavemining in large competent orebodies.Programs are being developed for thedetermination of fragmentation. and eventhe less sophisticated programs providegood design data. The economic viabilityof caving in competent orebodies isdetermined by LHD productivity and thecost of breaking large fragments.

Low stress

Birdcage cables from undercut levelInclined pipes

GroutingExtra bolts and cablesPlates, straps, and arches

W is the span of the tunnel and F is based onMRMR = 0-20: F = 1,4 MRMR = 21-30: F = 1,30MRMR=41-50: F= 1,1 MRMR=51-60: F= 1,05

OCTOBER 1994

MRMR = 31-40: F = 1,2MRMR > 61 : F = 1,0



~ DrawpoinUdrawzone spacings for coarsermaterial need to be examined in terms ofrecovery and improved miningenvironments. Spacings must not beincreased to lower operating costs at theexpense of ore recovery.

~ The interactive theory of draw and thediameter of the isolated drawzone can beused in the design of drawzone spacing.

~ Complications occur when the drawzonespacing is designed on the primaryfragmentation and the secondary fragmen-tation is significantly different. .

The Journal of The South African Institute of Mining and Metallurgy OCTOBER 1994 293 ....

Documents

Cave mining-thestateoftheart - SAIMM · Cavemining-thestateoftheart orebodies bylow-cost methods, theroleqf cavemining willhave tobedUined Inthe past, caving was generallY consideredfor