Thedesignofpillarsystemsas practisedinshallowhard-rock ... · system ofbarrier pillars isused inthismineat depthsgreaterthan100m.Thesecomprise20m widebarrier pillars spaced togivedepth/span

Synopsis

This paper deals withprocedures usedfordesigning pillar systemsin hard-rock tabularmines operating at shal-low depths, typicallYless than 1000 m. Acomprehensive review ifcurrent practice identi-

fiesfour types ifpillars,namelY non-yield, yield,crush and barrier pillars,and the main prinaples

for designing thesepillars are outlined. It isshown that the currentdesign methods for pillarsystems in South Africaare mainlY empirical,each mine developing itspillar-design methodsbased largelY on prac-tical experience. Thisresults in varying pillarlqyouts, even betweenneighbouring mines,despite similarities ingeology, and depth ifmining. It is suggestedthat some current lqy-outs mqyfall short ifbeing the best as regardssqfety, the maximumsqfe extraction percent-ages,orjlexibility inaccommodatingfuturemining scenarios indeeper or otherwisedjfferent geologicalconditions.

* Department qf MiningEngineering, Universityqf the Witwatersrand.

@ The South AfricanInstitute qf Mining andMetallurgy, 1994. SAISSN oo38-223X/3.oo

+ 0.00. Paper receivedjul. 1994; revisedpaperreceived Oct. 1994.

The design of pillar systems aspractised in shallow hard-rocktabular mines in South Africaby M.U. Ozbay, J.A. Ryder, and A.J. Jager*

Introduction

Pillars have been used as stope support since theearly days of mining, and they remain the mainsupport component in most present-day shallowtabular mines. The compelling need for usingpillars is dictated by the prevailing consider-ations of rock mechanics at shallow depths,namely the large tensile stresses in the hanging-wall, and geological weaknesses in the hanging-wall rockmass.

The hangingwall of any mining excavation issubjected to vertical 'deadweight' tensile stresses.In the case of tabular excavations, according toelastic theory, the extent of the tensile zonebecomes larger with increasing ratio of miningspan to the mining depth (UH) and smallerwithincreasing ratio of the horizontal to verticalcomponents (k) of the virgin stress tensor. Thevariation in the extent of the tensile zone over a200 m span stope, as a function of increasingdepth, is shown in Figure 1, where the k ratio isassumed to be high near surface but falls offrealistically with depth. It can be seen that largeportions of the hangingwall (50 m or more) canbe subjected to vertical tensile stresses. Incontrast, at great depth (>1000 m), not only isthe tensile zone smaller but horizontal'clamping' forces generated by face fracturingtend to render the hangingwall virtually self-supporting.

At shallow depths, there are usually jointsand bedding planes which weaken the hanging-wall rockmass. For example, in the south-westregion of the Bushveld Igneous Complex (BIC),the hangingwall of the Merensky Reef typicallycontains two major weak parting planes. At 2 to4 m there is a weak pyroxenite-norite transition,and at 10 to 15 m there is a well-defined partingat the Bastard Merensky Reef contact. In manycases, these parting planes are segmented byvertical joint systems which are mostlydeveloped sympathetically to the faults anddykes.

The Journal of The South African Institute of Mining and Metallurgy

-_...-.-------

Surtaca

"'50~""'"

".,

,.., ,

"'""", 100m "'"

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

.,...'

.."~

".."',,...150m ..

'.".

'.."""

200m

750m

",--"'".." "..,.. "..

.." """'"-",'" BOOm "..

Depth below surtace

Figure 1- Tensile zone above 200 m stopes at depths of

100 m, 200 m, and 800 m. Virgin stress components a.

(vertical) and ah (horizontal) are taken as function of

depth H: a. = O,03H; ah = 30 + O,O1H

The hangingwall in shallow hard-rockmining situations can then be characterized as arockmass containing well-defined discontinuitiessubjected to deadweight tension. When unsup-ported, the hangingwall becomes susceptible to'backbreaks' if critical spans are exceeded. Invery shallow situations, hangingwall collapsescan extend up to the surface, and the full weightof the rock on the overburden may be involvedin a potential collapse. These conditions requirea support system which is not only robust butwhich is stiff and can react rapidly underrelatively little convergence. Depending on theextent of the tensile zone and position of criticalweak parting planes, the support resistancesrequired are in the order of 300 to 1200 kN/m2,for 10 to 40 m of hangingwall requiring support.As indicated in Figure 2, in-panel supportrequirements in most cases cannot be met byusing only conventional support systems, suchas props, sticks, or even grout packs. Collapses

JANUARY/FEBRUARY 1995 7 ....

~~----- --1 -.-- ---

Pillar systems as practised in shallow hard-rock tabular mines in South Africa

N~ 400EE

i 300Q)

uc:

'",~ 200VI

~1::&. 100a.:JCl)

Sandwich pack

End grain

Mat packPipe stick

- Yield stick

././

/i ",,"'.: ,,".,

"'"I.';""": ..,;;..

! ;;.i/ /

/: .~.

Stick spacing 3 m 2

Pack spacing 9 m 2

00 400

Closure (mm)

Figure 2---$upport resistance of various pack and stick support as a function of closures

~ 8

- --------

have been experienced in practice, where sticksupports on 1 x 1 m spacing could not preventcollapses once the integrity of the hangingwallwas lost. Only backfill or pillars of unmined orecan provide the required support resistance, andalmost all shallow hard-rock mines currently usepillars which are better known, and generallycost less than other methods.

Types and design of pillars as practised inshallow mines

Based on the reviewof current shallow-miningpractice,four types of pillarscan be identified.These are non-yield, crush, yield, and barrierpillars, and their typical operating characteristicsare shown in Figure 3. The typical depths atwhich these various pillar types may beexploited are indicated in Figure 4. Furtherdetails of these types of pillars and their designare discussed in the following sections.

Non-yield pillars

At very shallow depths, the tensile zone in thehangingwall can extend up to surface, and underthese conditions the design of pillars is similar tothe design of bord-and-pillar systems in coalmining. The main consideration is to ensure thatthe strength of pillars at all times exceeds, by asuitable factor of safety, the maximum averagepillar stress (APS)imposedby the coverload ofsuperincumbent strata. These pillars, which areintended to remain essentially intact and elasticduring the life of the mine, will be callednon-yield pillars, and have stress/strain charac-teristics shown by the line indicated in Figure 3.The width-to-height (w/h) ratios of these pillarsare on occasions as low as 0,7 at very shallowdepths, but are more usually in the range 2 to 5and even higher.

JANUARY/FEBRUARY 1995

400

,,,,,,,,,,,,,,f!! :,fg,'a.,'C ,0;'s:.C:0

z

300

Residual strength

roa.::;;

-;;;200::J

to

w:h= 5

Y

100

Slenderw:h =2

c

Strain or deformation

Figure 3- Typicalstres5-$train behaviour of hard rock

pillars of different width-to-height ratios. Typicaloperating points are shown for NY (non-yield and barrier),

C (crush), and Y (yield) pillars

Typical%extraction

~ Surface

80====8=} Non-yield

60 D D D D Dpillars- ~gg ;,,(Transition)

90= = = = 8 = Yieldpillars94 = = = = 8 = Crushpillars- 1

ggg ;,,(TranSition)

95+ } Nopiliars

8 = barrier pillars

Figure 4- Typical pillar-mining systems at differentdepths (B =barrier pillar)

The most important parameter in designingnon-yield pillars is the pillar strength, crs.Currently, the strength of pillars is usuallyestimated using a modified version of theempirical formula proposed by Salamon andMunro t for coal pillars, which is in the form of:

crs=K ha w~, [1]

where crsis the pillar strength (MPa), hand ware the height and width of a pillar (m), and forSouth African coal fields, K = 7,2 MPa (strengthof 1 m3 coal), IX= -0,66 and ~ = 0,46. A modifiedversion of this formula, known as the squat-pillar formula, is used to reflect the very rapidincrease in strength of support pillars havingw/h ratios greater than approximately 5.

The Joumal of The South African Institute of Mining and Metallurgy

- -----------


In the literature describing hard-rock miningpractice using pillars, the method of selecting avalue for K is not consistent. The values used forK vary from K equal to the Ultimate eompressiveStrength (UeS) to K equal to one-third of theDes. The application of the Hoek-and-Brownrockmass-strength criterion2 to the reef ues isalso used as indicated in the formula:

0"1=0"3 + (mO"C0"3+ 0"/S)0.5 [2]

which predicts the unconfined rockmass strengthfor 'good quality' rock that comprising a pillar tobe

0"1=0,32O"c'

fors=O,l and 0"3=0.

where

0"1is the rockmass strength,O"cis the laboratory DeS,0"3is the confining stress, andm and s are the material constants as

described by Hoek and Brown2.

For hard-rock pillar design, the values of theexponents a and ~are most commonlytaken as-0,75 and 0,5, respectively. These particularexponents appear to be from a study by Hedleyand Grant3, and popularized by Wagner andSalarnon4 as quoted in Kersten 5.Hedley andGrant's work is based on a back-analysis ofpillar workings at the Quirke Mine, Elliot Lake,where the orebody is stratified conglomerates,and hanging- and footwalls consist of layeredquartzite, argillite, and limestone formations. Itis important to note that the exponents wereactually derived by considering a relativelynarrow range of w/h ratios, typically between0,7 to 1,5. Current experience in South Africa isalso beginning to suggest that for certain high-friction rocks (such as pillars made up of UG2reef or chromitite seams), the w/h ratiostrengthening may be much greater thanindicated, with an equivalent value of ~ rangingup to 1,0 or more.

Pillar stresses are normally calculated usingthe Tributary Area Theory (TAT), whichaccounts for the full cover load, Le.

qA=-

1-e

whereA is the AveragePillarStress (APS),q is the verticalcomponentof the virginstress, ande is the areal extractionratio.


-.- ..---.

The TAT method assumes that the lateralextent of mining is several times greater than themining depth, so that the pillars carry full coverload. It is operationally convenient as it leads tofIXedpillar -design dimensions for any givenseam material and depth. The TAT method tendsto be conservative, considering the fact that thepillars near permanent abutments or lines ofregional pillars carry lower stresses than the TATmethod predicts, regardless of the extent ofmining. Also, in practice, potholes or fault lossesare often left as unintentional pillars, resulting inlower extraction ratios, thus lower APS values,than initially planned.

Numerical models, e.g. two- and three-dimen-sional boundary element computer programs, areoften used for determining APS for irregularlayouts. However, these models do not currentlyprovide simple procedures for determining APSand may require onerous computer -run times, orintricate additional calculations for accurate APSdetermination. Additional features to thesemodels are required for obtaining accurate andquick determinations of APS.

The standard value of 1,6 for the Factor ofSafety (FS) used in South African coal-mine rockengineering is also used in hard-rock mining.Ryder and Ozbay6 suggested that, in hard-rockmines, the FS needs to be individually selected,and values for FS < 1,3 are not advisable unlessregional stability levels are well established.Hedley and Granp proposed values ofFS = 1,5based on their back-analysis, as shown inFigure5.

Kersten5. 7 developed an empirical method forpillar design based on back-analysis, andutilizing a spread-sheet program for carrying outthe calculations. In his method, four differentpillar-states are defined, namely: intact, slightslabbing, intense slabbing, and failed. Afterclassifying pillars, the APS values acting onthem (prior to failure) are estimated using TATmethods and then a value for K can be

[3]150

;f"i tOO

".'"'C.~'" 50w

200

Estimated pillar strength (Mpa)

Figure 5-Relationship between pillar factor of safety andpillar performance as obtained from Elliot Lake QuirkeMine3

JANUARY/FEBRUARY 1995 9 ....

------------.----- -----------


Table I

A summary of non-yield pillar layouts used in hard-rock mines

Spacingdip strike

3232322232(2)(2)(2)(2)(2)30(3)(6)

(10)2634

(32)(32)(14)

10(8)(8)(8)(6)

30303016(2)3436343636(2)20(6)

(10)(2)(2)(2)(2)(2)(2)(8)(8)(8)(6)

Spacingsmke dip

200400200400

180

Bracketed values of pillar spacing refer to skin-to-skin measurements. otherwise values are centre-to-centre measurementsUnit for dimensions is metresAverage values are used throughout

determined from back -analysis. In redesigning ahard-rock mine at an average depth of 130 m,Kersten found K values averaging approximatelyone-third of the reef DeS, by using Wagner'sperimeter rule for estimating the strength of rect-angular pillars, assuming IX = -0,75 and ~ = 0,5.

Kersten's method is simple and easy to use;however, there are certain simplifying assump-tions, and there may be difficulties estimatingthe actual state of pillars from the degree of pillarspalling without quantifying the extent offracturing into the pillar.

Table I gives a summary of the non-yieldpillar layouts currently in use; they areindividually discussed later in this paper.

Crush pillars

As depth increases, extraction ratios associatedwith coal-type bord-and-pillar layouts becomeincreasingly unfavourable. However, theincreasing mining depth results in relativelysmaller tensile zones, and this in turn permitsthe safe use of reduced levels of support resis-tance: the support no longer needs to carry thefull weight of overburden, but rather only theweight of superincumbent strata reaching up tothe furthest active weak parting in the hanging-wall. Thus small w/h ratio pillars (wlh < 3),which can provide the required reduced supportresistance in their post-peak residual strength

~ 10 JANUARY /FEBRUARY 1995

~---~-

--~--- -------.

state, can be used. For example, to support thedeadweight of as much as 35 m of immediatehangingwall strata, only 1 MPa of supportresistance is required, and this is easily providedby the residual strength of a crushed relativelyslender pillar. These types of pillars will be calledcrush pillars; they can be defined as pillarsintended to 'crush' while they are still part of theface, but which have sufficient residual strengthto provide the required support resistances to theimmediate hangingwall, both at the face and inthe back areas. These pillars can yield over alarge deformation range at their residualstrength level (e.g. point e in Figure 3).

The w/h ratios for crush pillars are typically1,7 to 2,5. Many mines exploiting MerenskyReef use crush pillars with strike dimensions of2, 3, 4, or 6 m and dip dimensions of 2 or 3 m,separated by 0,5 to 3 m wide ventilation holings.These pillars are normally aligned on strike onthe down-dip side of strike gullies, often with a1 m siding.


--- --- --- --~--- --- --. -----


Table 11

A summary of crush pillar layouts used in hard-rock mining

Spacingdip strike

363636253535353536

(32)

(3)(4)(4)(3)(2)(2)(2)(2)(2)(2) 140

Spacing

strike dip

170

Bracketed values of pillar spacing refer to skin-to-skin measurements, otherwise values are centre-to-centre measurementsUnit for dimensions is metresAverage values are used throughout

The design of crush-pillar layouts is mostcommonly carried out by initially using pillardimensions which have been successful insimilar geological situations elsewhere8.Depending on the performance of the newlayouts, the pillar dimensions and spacing areadjusted until the pillars provide the requiredbehaviour. Alternatively, the initial w/h ratio fordesign can be specified to be 2, and if pillars donot crush initially, the w/h ratio is decreaseduntil stable crushing is achieved. The w/h rationormally does not exceed 2,5, and should not beless than about 1,5 in narrow stope widths. Insome cases, the failure mode of pillars guides thew/h ratio selection. Structural weaknesses, suchas thin bands of soft material or a relativelyweak foundation rock, can provide the 'crushing'mechanism8.

Table II gives a summary of the crush-pillarlayouts currently in use; they are individuallydiscussed later.

Yield pillars

The correct depth at which the transition fromnon-yield to crush-pillar systems should takeplace, is not immediately apparent and requirescareful consideration. Ozbay and Roberts9defined a transition zone; where the pillarstresses are too high for non-yield pillars ofreasonable size, and yet are too low to causeimmediate fracturing at the face so that crushpillars can safely be formed in their residualstrength state. This is a difficult situation whereover-sizing of crush pillars could result in pillarbursting. In some cases, for example coal minesemploying longwalls, small w/h ratio pillars canbe designed to be intact initially but then areallowed to be loaded beyond their peak strengthin a stable manner. Ideally, such pillars could bedesigned with a factor of safety (FS) of 1 so that

the pillars can exert their maximum support to


the hangingwalllO. These pillars will be calledyield pillars; they are defined as pillars which areintended to have an FS > 1 or even FS equal to 1when first formed, but then to yield in a stablemanner at stress levels near to peak strength(point Y in Figure 3). The w/h ratios are as lowas 3 but often approach 5.

Yield pillars are initially intact but, as miningprogresses, the stresses acting on them increaseto the levels equivalent to their strength.Provided that the pillar's post-peak stiffness isless than the stiffness of the loading strata, thesepillars can yield in a stable manner. Such stablepost-peak behaviour is assured if the post-peakmodulus is not negative, Le. a horizontal orrising post-yield stress-strain behaviour.

At present, the design of hard-rock yieldpillars is also largely empirical. Little is known ofthe fundamental behaviour of yield pillars, andinconsistencies are often evident between theoryand practice. Theoretical studies indicate that thelevels of regional or local stiffness do not favourstability for slender hard-rock pillars, yet pre-liminary observations indicate that stable loadshedding of such pillars can take place at w/hratios ofless than 2,511,12.Also, it is thoughtthat pillarswith w/h ratios 2: 5 cannot fail in anunstable manner, yet bursting of 5 x 5 m pillarsin the back areas was reportedl3. These casesneed to be rationally analysed using numericaland laboratory modelling, together with thecollection of further in situ data.

Table IIIgives a summary of the yield-pillarlayouts used in various hard-rock mines, whichare discussed in further detail later.

JANUARY IFEBRUARY 1 995 11 <Ill

-~-~---~ ~- ~ --~ ~---~

Spacingstrike dip

400

400400400400400400

(250)

120 120

(150) (150)


Table 11/

A summary of yield pillar layouts used in hard-rock mining

IN-PANEL PILLARS

Spacingdip strike

323232323232323636(2)

(40)(35)(15)(32)

30(2)(2)(2)(2)(2)(2)(2)(2)

(25)(3)

(40)(2)(2)

Bracketed values of pillar spacing refer to skin-ta-skin measurements, otherwise values are centre-ta-centre measurementsUnit for dimensions is metresAverage values are used throughout

Barrier pillars

Pillar layouts in shallow mines involving smallin-panel pillars often include large w/h ratiopillars, known as bam'er pillars to provideregional stability to the entire mine. Theimportant consideration in the design of regionalpillars is their strength; they are designed toremain essentially elastic and intact over theentire life of the mine. According to Salamon14choice of squat pillars having w/h ratios> 10,should ensure indestructibility of the barrierpillars, though the possibility of foundationfailures (e.g. punching or footwall heave) needsto be checked in highly-stressed situations, orwhere hanging- or footwalls are relatively weak.

Barrier pillars are usually rib or rectangularin shape, and oriented on dip or strike. Whereappropriate, unpayable areas (e.g. potholes, faultlosses) may be incorporated into regional pillarlayouts. However, it is very important that thesepillars are not oriented parallel to geologicalweaknesses or joint sets in the rockmass, whichcould otherwise both weaken the regional pillars,and promote the possibility of large falls ofground in the stopes. It appears that there isuncertainty and diversity of opinion as to theneed for, and design of barrier pillars. A widerange of designs, sizes, and spacings wereencountered in the industry survey.

The main purposes of leaving barrier pillarsare:

~ to compartmentalize the mining intodistinct regions, so that, if a collapseshould occur in one region, it will beprevented from spreading intoneighbouring stopes

~ 12 JANUARY/FEBRUARY 1995

--~- "- -- ------

~ to reduce excessive closures and surfacesubsidences which would otherwise occurin panels supported only by crush pillars

~ to assist in the control of the tensile zoneand the prevention of surface subsidence

~ to increase effective strata stiffnesssubstantially so as to reduce the possibilityof large-scale instabilities (pillar runs) instopes supported by non-yield pillars.

The recently observed increase in seismicitylevels on the deeper mines of the BIC, indicatesthat barrier pillars may also be required to restrictvolumetric stope closures and correspondingseismic energy release potentials. The design ofpillars for this purpose may differ from barrierpillars used at present, and will need detailedinvestigation.

Ryder and Ozbay6 give the followingsummary of the criteria used in selecting thespacing of barrier pillars: a conservative rule ofthumb sometimes used is to keep the span, L,less than about one-quarter of the depth, H, Le.H/L > 4.

A number of theoretical results can be citedto support this rule.

Firstly, the height of the tensile zone is lowerat H/L = 4 than it is at H/L = 2, thus reducing

the burden placed on the in-stope yield or crushpillars for tensile-zone control.

Secondly, the hangingwall stiffness falls offfairly rapidly for H/L < approximately 2, and thiscould prejudice the stability of certain in-stopenon-yield pillar layouts.


- -- - - -


Thirdly, in-stope closures and surfacesubsidences in a yielding-pillar layout increasein direct proportion to span, L, but are often atacceptable levels if H/L is approximately 4. Inshallow non-yield pillar layouts, much lower H/Lratios may be acceptable since the in-stopepillars themselves provide substantial tensile-zone control. However, for adequate compart-mentalization L should rarely exceed 250 m.Moreover, at low H/L ratios, adequate factors ofsafety, to prevent regional instability, becomedifficult or impossible to achieve, and this placesan increased premium on the choice of anadequately high factor of safety for the strengthof non-yield in-stope pillars.

The final choice of an operational regionalpillar span, L, can be guided by numericalmodelling, but this may well require modificationand experimentation in practice, particularly inthe light of local geological conditionsencountered during mining.

Pillar-design methods and current practice

Impala Platinum Mines initially used 3 x 3 mnon-yield pillars, spaced 30 m on strike and 32 mon dip, at shallow depths. This system workeduntil the mining depth reached 160 m, at whichdepth a large collapse occurred. The pillar sizeswere then increased to 5 x 5 m. These5 x 5 m non-yield pillars are still being used inshallow operations on both Merensky and UG2Reefs at depths of about 400 to 500 m. Pillarswith w/h ratios.. 5 are currently believed toprovide a positive post-peak slope, and can besafely loaded beyond their elastic limit. A typicalexample is the pillar layout used at ImpalaPlatinum Mines, where 5 x 5 m pillars (in achequer-board pattern of 2 pillars per 32 x 30 m)have been adequately supporting thehangingwall at depths of 500 m or greater, andare free from any sign of fracturing until they areabout 30 m in the back area. The mode ofyielding of these pillars is generally stablealthough occasional 'bumps' are said to occur inthe back areas.

Numerous instrumented field experiments andpillar-design studies have been carried out atImpala over the past 20 years13.15-22. These

studies suggest that 5 x 5 m pillar properties varywidely, depending on geological factors such asthe local density of jointing. Occasionally, pillarscan fail through to the core at comparatively lowstress levels. However, in most cases it wouldseem that 5 x 5m pillars are significantly strongerthan once thought-under high stress the edgesspall but the central core tends to remain intact.For example, Spencer21estimated an APS equal to219 MPa for an elastic unfalled 5 x 5 m pillar in aUG2 stope at a depth of 588 m. This value wasconsistent with in situ stress measurements,


whereas the strength estimate using formulae,accepted at that time, was only 85 MPa. An

interesting observation was that the pillar was

free of any fracturing only when it was first cut,

and started fracturing as the face advanced away

from the pillar. Petroscope observations showedthat, within ten months, pillar -sidewall fracturing

extended 0,4 m deep into the pillar. The remain-ing inner part of the pillar was unfractured. It is

not impossible, therefore, that because of theirstrength, many of the 5 x 5 m pillars at Impala are

not functioning as yield pillars at all, but are

instead operating as conventional non-yield

pillars. Large parts of the deeper areas of Impala

are now being mined on more slender yield pillars(minimum w/ h ratio of about 3). These appear to

be functioning satisfactorily at present, although

seismicity levels are occasionally a cause for

concern.

At Impala, experience shows that open spans

of more than 120 m cannot be achieved2°. Asystem of barrier pillars is used in this mine at

depths greater than 100 m. These comprise 20 m

wide barrier pillars spaced to give depth/span

ratios approximately 217. At relatively deeperworkings, the barrier pillars on the Merensky

and UG2 workings are superimposed, and arespaced at about 400 m.

RPM Rustenburg Section started mining with

total extraction of Merensky Reef in 1927.Initially the support consisted of stonewalls and

timber props, and this proved satisfactory. The

timber props showed no signs of deformation,implying low elastic convergence and a self-

supporting hangingwall. As mining advanced

deeper than 200 m, trials were carried out toreplace stonewalls by stonepacks (stone-filled

skeleton packs). Backbreaks were experienced

with this system when the strike span reached

60 m. Lines of crush pillars spaced 45 m on dip

were then introduced. The dip spacing of crushpillars was subsequently reduced to 36 m as

backbreaks still occurred at the 45 m spacing. In1974, longwall mining with crush pillars was

introduced at the Frank Shaft area at a depth of

500 m. The layout in longwall mining is typically

2 to 2,5 m wide strike pillars with a dip spacing

of 36 m. The pillars in the 19 longwall stope at

Frank Shaft measure 6 m long, 2 m wide and

1 m high, separated by 2 m wide ventilationholings. The mechanism of stable crushing isexplainedB as the eventual crushing of large

crystals in the yielding pyroxenite matrix. Thepillar ends up as an intensely fractured but still

well-knit mass of crushed rock. This pillar system

has generally been successful in providing therequired hangingwall support, although some

pillars are said to be 'punching' or even 'bursting'

150 m behind the face on the Townlandslongwall, due to abnormal amounts of strong

norites or anorthosites in pillars affected by

JANUARY IFEBRUARY 1995 13 ...

---- -- - -------~-

---.----


rolling-reef conditions. At present, RPM usesthree different support systems, depending on theground conditions

>- grout packs (along gullies) and timberprops (in stope)

>- crush pillars (along gullies) and timberprops (in stope)

>- crush pillars (along gullies), grout packsand timber props (in stope).

No barrier pillars are being used at the RPMmines.

RPM Union Section mines deeper and widerMerensky Reef than that mined at the RPMRustenburg Section. Initially, mining was carriedout using 0,6 x 0,6 m mat packs at skin-to-skindistances of 3 m on dip and 2,4 m on strike.Spans could not be kept open beyond 30 to 40 mwith this support system and crush pillars alongstrike gullies were introduced in 1977. Thesepillars measured 1,5 m wide and 3 m long with1 m ventilation holings, and were spaced 35 mcentre-to-centre along dip23.Backbreakspersisted and it was thought that the pillars weretoo slender.

The pillars were subsequently made squatterby changing their dimensions to 2 x 2 m with2 m holings. The extent of crushing was lesscompared to 1,5 x 3 m pillars. Wood andCoetzer24reported that the performance of thispillar system had been satisfactory and that nofurther backbreaks had occurred. Current pillardimensions are 4 x 2 m if sidings are used, or4 x 3 m if pillars are cut immediately adjacent togullies. No barrier or regional pillars have beenemployed in this mine. Bord stability remainsthe major problem as the hangingwall rock(~arzburgite) is he~vily jointed inplaces, givingnse to plug-type failure along vertical planesadjacent to pillars.

Union Section has also been mining the UG2Reeffor the last 5 years. At about 35 m belowthe old Merensky workings, square pillars withside length of 5 m are being used to give 90 percent areal extraction. At greater depths (-700 m)pillar side length is reduced to 4 m. Plug types ofbord failure occur occasionally.

RPM Amandelbult Section initially employed4 x 2 m strike pillars at 45 m dip centre-to-centresp~cing. This system could not prevent collapseswhich extended up to the pyroxenite-noritetransition, 2 m above the stope. The spacing wassubsequently reduced to 36 m and pillardimensions of 3 m long and 4 m wide were usedsuccessfully. The stoping width is relativelyhigh, being about 1,3 m, and the average miningdepth is about 400 m.

~ 14 JANUARY/FEBRUARY 1995

- - -------- --- --- -- ~-----

Western Platinum Mines previously used alayout comprising the development of dip centregullies, 150 m apart, from which strike gullieswere developed 40 m apart on dip. As miningproceeded away from the centre gullies, pillarswere established on a line 2 m below the strikegullies. The dimensions of the pillars varied from10 x 10 m to 2 x 6 m. When encountered,potholes were utilized to form pillars. Betweenthe pillars 15 cm diameter timber props wereplaced at 2 x 2 m spacing. This system providedthe required support system for hangingwallcontrol. Presently, the Western Platinum Mineemploys up-dip mining. Where possible,potholes (about 16 per cent of the reef) areincorporated as pillars. Nominally, dip-pillars aredesigned to be 20 x 5 m with a 2 m holingbetween pillars. The strike distance betweenpillars is 36 m. Without using barrier pillars,overall extraction ratios of about 89 per cent isbeing achieved. Hangingwall conditions aregood, and high rates of face advance areachieved.

Lebowa Platinum (Atok Section) mines fromnear surface to a maximum depth of about450 m. A uniform pillar design is usedcomprising 3 x 2 m pillars separated byventilation holings at a dip spacing of 30 mcentre-to-centre. The stope width is 0,8 to 0,9 m.The hangingwall-pyroxenite beam is only 0,15 mthick, and the Bastard Merensky parting is at10 m in the hangingwall. No major falls havebeen reported; typical falls of ground are lessthan 0,5 m thick. No barrier pillars are used,apart from numerous unmined pothole blocks.

Lavino Chrome Mine initially used nominally3 to 4 m wide 1,8 to 2,0 m high dip pillars at astrike distance (skin-to-skin) of 20 m, althoughthe actual layout shows considerable variationsin these dimensions. Scaling and various degreesof fracturing were observed in some of the pillarswhich had a width of less than 3 m at depths ofabout 100 m. The fracturing appeared to havebeen governed by the serpentinized bedding at

~h~mid-height of the pillars. A shallow-dippingJOIntsystem parallel to the strike occasionallycaused bord failures extending up to 1 m intothe hangingwall. Also, larger falls, extending4 to 5 m into the hangingwall, occurred betweensteep dipping joints. The current pillar layout ismore conservative. It consists of 5 x 5 m pillarson 6 m bords both on strike and dip. A 20 mwide strike-orientated barrier pillar at a depth of120 m, now separates the old workings from thecurrent mining area. This system of pillarsappears to have prevented pillar fracturing andbord failures. Currently, Lavino Chrome Mine isintroducing 20 m wide strike-orientated barrierpillars.



Acknowledgements

This research is part of a

project funded by the Safety

in Mines Research Advisory

Committee (SIMRAC) of the

Ministty of Mineral and

Energy Affairs of South Africa

under GAP027/028.

Permission to publish this

material by the management

of the mines involved, and the

guidance and help providedby the personnel of these

mines are greatly appreciated.

References

1. SALAMON,M.D.G., and

MUNRO,A.H. A study of

the strength of coal

pillars. J S. Iifr. Inst. Min.

Metall. 1967. vo!. 68, pp.

55-67.2. HoEK, E. and BROWN,E.T.

Underground

excavations in rock.

London, IMM. 1980.

3. HEDLEY,D.G.F., andGRANT,F. Stope-pillar

design for the Elliot Lake

Uranium Mines. Bul/.

Can. Inst. Min. Metal.,

1972. vo!. 65. pp. 37-44.

4. WAGNER,H., and

SALAMON,M.D.G. Pillar

design. Series of Lectures

given at Sappers Rust,

Pretoria, Aug. 1979.

5. KERSTEN,R. (1984). The

design of pillars in the

shrinkage stoping of a

South African gold mine.

J S. Aft. Inst. Min.

Metal/., vo!. 84, no. 11.pp. 365-368.

6. RYDER,I.A., and OZBAY,

M.V. A methodology fordesigning pillar layouts

for shallow mining.

Proceedings ISRM

International Symposium

on Static and I!Ynamic

Considerations in Rock

Engineering, Swaziland.

1990. pp. 273-286.

7. KERSTEN,R. Personal

communication, 1993.

8. CAHNBLEY, H. Personal


9. OZBAY,M.V., and

RoBERTS,M.K.C. Yield

pillars in stope support.

Proceedings SANGORM

Symposium on Rock

Mechanics in Africa,

Swaziland, 1988.pp. 317-326.

Winterveld Chrome Mine used 3 x 3 mpillars, with 10 m spacing both along strike anddip. Following a large collapse involving about50 pillars which took place at 70 m belowsurface, pillar sizes were increased, and a systemof strike pillars of alternating dimensions-8 x 8 m and 4 x 4 m-was introduced. This layoutwas later changed to 6 x 6 m pillars located alongstrike with ventilation holings of 2 m betweenthem. The skin-to-skin dip spacings are 28 m. Asystem of barrier pillars was subsequently addedto the layout, and comprises 10 m widecontinuous dip pillars 180 m apart. At depths ofabout 500 m, no signs of visible scaling areapparent on the in-panel pillars except foroccasional slight slabbing occurring at the up-dipcorners of the pillars or intense curved fracturingat pillar mid-height. Winterveld is introducing10 m wide dip-barrier pillars at depths greaterthan 500 m.

Randfontein Estates Gold Mine mines mainlythe reefs in the Elsburg series. Depending on thedistance between the individual reefs, the miningis carried out along either a single reef withstoping widths of 1 to 3 m, or multiple reefs withstoping widths of 5 to 8 m. Many pillar-systemdesigns are used to cater for a large range ofgeotechnical conditions. At depths of about700 m, in the areas where the stoping widthvaries from 1 to 3 m, the typical crush-pillarlayout comprises 1,6 to 3 m wide and 9 m longstrike pillars, separated by 2 m ventilationholings with a dip spacing of 32 m. Wherepossible, the w/h ratio of the crush pillars is keptat 1,65 as greater w/h ratio pillars are said to beburst-prone. Intermediate low-friction layersprovide a stable crushing mechanism resultingfrom the enhanced vertical slabbing of the topand bottom portions of the pillar caused by thepartingsB. In the high stoping width areas, twodifferent mining methods are employed. In thebackfill sections, 7 m wide cuts are taken alongthe strike direction at 7 m intervals on dip. Themined-out area is subsequently backfilled, andthe 7 m wide strike strip pillars betweenbackfilled sections are mined. This method hasgenerally proved successful. In the bord-and-pillar sections, the primary extraction is carriedout in the same way as in the backfill sections,followed by a secondary extraction process ofcutting 7 m wide slots through the strike pillars,creating individual square pillars measuring7 x 7 m. Prior to secondary extraction, the pillarsdo not show any sign of slabbing or failure.Soon after secondary extraction, the pillars startfracturing. Occasional backbreaks and severepillar-sidewall fracturing are observed in thebord-and-pillar sections. Computer simulationsshow that the average stress acting on thesepillars is approximately 120 MPa25.Backbreakswere a common phenomenon in shallow sections

The Journal of The South Afncan Institute of Mining and Metallurgy

--- -------------

of the workings at Randfontein Estates. Theyused to occur at spans between 100 and 250 mdepending on local conditions. The incidence ofbackbreaks fell after the introduction of barrierpillars, but could only be reduced to insignificantlevels after the introduction of crush-pillarsystems. Regional pillars are 20 m wide (strike)and 60 m long (dip), spaced 170 m and 140 mon strike and dip, respectively. In the newerareas, the barrier-pillar size and spacing are30 x 30 m and 150 m, respectively.

}oelGold Mine uses 3 m square pillars locatedalong the up-dip side of the gully and separatedby 3 m holings at depths of 500 m. The dip-centre spacing of the pillars is about 40 m. Theyielding mechanism of the in-panel pillars is inthe form of stable punching of the pillars into thefootwall, which results in some footwall heave inthe back-areas, but is otherwise satisfactory. Themine has introduced 6 x 9 m regional pillars on a120 x 120 m grid.

Beatrix mines at depths of about 800 to1000 m. The mining height varies from 0,9 to6 m. In the low stoping-width areas (0,9 to 2 m),the layout comprises 5 x 5 m pillars with centresseparated 40 m on strike and 35 m on dip. In thehigh stoping-width areas (3 to 6 m), nominally8 m wide strike, rib pillars are left at a dipspacing of 15 m. The mode of yielding is stablepunching of pillars into hanging- and footwalls.No barrier pillars are employed.

Durban Roodepoort Deep (DRD) currentlymines Main Reef at depths of 800 to 1000 m.Pillars were introduced in 1987 to prevent localhangingwall collapses in high stope width areas(> 2 m). Initially pillars with w/h ratio equal to 2were used but these remained uncrushed atconsiderable spans. The pillar dimensions werechanged to w/h is equal to 1,7, and observationsindicated that the pillars were yielding in a stablemanner 20 to 30 m behind the face. At present,rib pillars located along dip with 2 m ventilationholings, are being employed. Grout packs areinstalled on both sides of the pillars, owing tohigh gully sides. The skin-to-skin open spanalong the strike direction is 25 m, and barr.ierpillars are left at a spacing of approximately250 m 11. Durban Roodepoort Deep hasintroduced barrier pillars at a spacing of250m.

JANUARY/FEBRUARY 1995 15 ...


10. SALAMON,M.D.G.

Stability, instability and

design of pillar workings.

Inter. f Rock Meek. Min.

Sa., vo!. 7. 1970.pp. 613-631.

11. NOBLE,K. Personal


12. LOUGHER,D.R. Personal


13. SPENCER,D.A., and

KorzE, T.j. Observed

crush pillar failure

between two regional

stability pillars at Impala

Platinum Mines.

International Symposium

on Static and Dynamic

Considerations in Rock

Engineenng, ISRM,

Swaziland, 1990.

14. SALAMON,M.D.G. Rock

mechanics of

underground

excavations. Proceedings

3rd Congress ISRM,

Denver, 1974. vo!. 2,pp. 951-1099.

15. DuNHAM,R.K. A rock-

mechanics investigation

into room and pillar

mining at Impala

Platinum Mines.

Proceedings Symposium

on Explorationfor Rock

Engineering.

johannesburg, 1976.pp. 263-270.

16. LEAR, C.D. An interim

assessment of strike

yield pillar experiment of

728-726 and 828-827stopes, Wildebeestfontein

North Mine, Impala

Platinum Mines. Report

No. 108/80, 1980.

Manganese Mines (Wessels, Nchwaning andGloria), in general, employ the bord-and-pillarmethod of mining, using 8 m wide bords and6 to 20 m wide square pillars in 2 to 8 m highstopes. The w/h ratios are mostly in the order of1 to 1,5. At typical depths of around 300 m, thepillars do not show any sign of fracturing orslabbing. In some cases, bord stability causessome concern, and the roof stability in thesecases is ensured by employing 2,4 m long roofbolts with 1,5 m spacing in the well-laminatedand jointed-banded ironstone hangingwall wherethis is exposed. No barrier pillars are used.

Koretsi West Asbestos Mine extracts crocido-lite asbestos from a horizon where economicconcentrations of fibre seams occur over astoping width of 1,8 to 2,0 m. Mining takes placeat a depth of approximately 160 m, and thestrata dip westward at 3 to 4 degrees. Thehanging- and footwall strata comprise well-laminated banded ironstones. Bord-and-pillarmining is practised with 2 x 2 m pillars and 6 mbords, resulting in 94 per cent extraction. Thebanded ironstone roof is very laminated, and issupported by 1,2 m rootbolts on a 1,2 x 1,2 mgrid. The pillars show no signs of failure, andthe pillar system appears to be totally adequateto provide overall stability to the mine, partic-ularly when considering that ore bodies in thearea are generally small, often measuring lessthan 500 by 500 m.

Non-yield pillars-Collapsed cases

Non-yield pillar layouts carry the risk of a pillarrun, which is a sudden collapse of pillars overlarge areas, often associated with seismicity,subsidence, and airblast. Some of the non-yieldpillar-layout collapses are summarized in TableIVand briefly reviewed.

Impala Platinum. The introduction of pillarsat the Impala Mines came after a major collapseat a depth of 40 m. In spite of the shallow depthof the stopes, no surface subsidence was observed,and it was estimated that the collapse extendedonly up to 9 m above the Bastard Merensky Reef.Pillars with dimensions 3 x 3 m and located32 m on dip and 30 m on strike, were then intro-duced. This system worked until mining reacheda depth of 160 m, where a massive collapse overan area of 600 x 900 m occurred, and surfacesubsidence was observed!7. Soon thereafter,another collapse measuring 350 x 450 m at 50 mbelow surface took place in a neighbouring minewhich was also using 3 x 3 m pillars. Assumingthat the extraction ratio was

9e=l--=O 99

32 x 30 '[4]

and a rock density of 2900 kg/m3 applies for theoverburden strata, the APS levels acting on thepillars prior to the failure can be estimated tohave been 490 MPa at 160 m depth and 150 MPaat 50 m depth (using the formulaA =q/(l-e)where A is the APS). In reality, the stress levelswhich caused failure should be somewhat lessthan these values since, as stated before, theabove formula ignores the irregularities of thelayout geometry and the presence of potholes.

Lorraine Iron Ore Mine. Tincelin andSinou26reported on the nature of the suddenpillar collapses in the Lorraine Iron Ore Mine inFrance. Seven sudden collapses took place in thismine, all resulting in seismicity, with two ofthem causing fatalities. Bord-and-pillar miningwas used for the extraction of a 5 m thick flat-lying reef at a depth of 140 m. Wide (12 to 15 m)square pillars, with 5 m skin-to-skin distancewere employed, giving extraction ratios of about60 to 70 per cent. The characteristics common toall seven cases were identified as:

Table IV

A summary of collapsed cases of non-yield pillar layouts used in hard-rock mines

Dimensionsstrike dip height

3

315

7

34,5

3

3

15

7

3

12

1,1

1,1

5

51

31,9

Bracketed values of pillar spacing refer to skin-to-skin measurements, otherwise values are centre-to-centre measurements

Unit for dimensions is metresAverage values are used throughout

~ 16

-~--~~~- -~~

---~ ---~~

JANUARY/FEBRUARY 1995 The Journal of The South African Institute of Mining and Metallurgy

-~ ~ ~ -~ ~ ~-~~-- ~~- -~

-~


17. KoTZE. T.). Strata controlproblems resulting frommining at shallow depthin the hard rockmeasures of SouthAfrican BushveldComplex. Proceedings5th ISRMCongress,Melbourne, 1982.pp. D67-D76.

18. )ACK, B., HEPWORTH,N.,and GAY N.C. Strata-control investigations atImpala platinum Mine.Unpublished Report,Chamber of MinesResearch Organization,1982.

19. KoTZE, T.). The natureand magnitude of surfacesubsidence resulting frommining at relativelyshallow depths onplatinum mines.Proceedings if SANGORMSymposium on The fffect

if Underground Miningon Surface, 1986.pp. 47-51.

20. KOTZE,T.). The measuredresponse of theimmediate hangingwallof a Merensky Reefsupported by clusterpacks only. Rockengineering problemsrelated to hard rockmining at shallowintermediate depth.Proceedings ifSANGORM Symposium,Rustenburg, 1993.pp. 38-43.

21. SPENCER, D.A.UsingFLACto model the depthof fracturing intochromotite pillar- a casestudy. Rock EngineeringProblems Related to HardRock Mining at ShallowIntermediate Depth.Proceedings SymposiumSANGORM, Rustenburg,1993. pp. 20-27.

22. LoUGHER,D.R., andMELLowSHIPP. Stratacontrol problemsassociated with geologicalstructureson ImpalaPlatinum Mines. RockEngineering ProblemsRelated to Hard RockMining at ShallowIntermediate Depth.Proceedings if SANGORMSymposium, Rustenburg,1993. pp. 68-75.

23. KoRF,C.W. Stick andsupport on UnionSectionRustenburg PlatinumMines. AMM papers anddiscussions, 1978.pp. 71-82.

~ the immediate hangingwall was firm andsound

~ the w/h ratio of pillars was 3 or less~ the collapses were sudden, and associated

with seismicity, surface subsidence, andair blast.

It is important to note that these characteris-tics are similar to those of the Coalbrook disasterof 1962, where an area of 2,5 km2 supported by4400 pillars collapsed in 20 minutes. In both ofthese cases of sudden collapse, very little priorwarning was noticeable in terms of pillarslabbing or hangingwall-fracturing.

Otjihase Copper Mine. The mineexperienced a large collapse while miningoperations were taking place at depths of 200 m.A system of primary and secondary extraction ofthe 3 to 10 m thick orebody was being used.Initially, the primary extraction was carried outusing 7 m wide strike pillars to give 60 per centextraction, followed by a secondary extractionprocess of cutting 14 m wide slots through thestrike pillars, creating individual square pillarsmeasuring 7 x 7 m, and resulting in 88 per centfinal extraction. The collapse extended over astrike length of about 800 m within two days.Indications were that the failure commenced atthe corners of pillars, and extended into thehanging- and footwall. This can be interpretedas foundation failure, which might have reducedthe effective w/h ratio of the already slenderpillars (w/h equal to 1,7).

Winterveld. Korf23and Ortlepp27reported apillar collapse in the Winterveld Chrome Mine at70 m below surface. The lateral extent of thecollapse was 45 x 100 m, involving some 50pillars. The height of the collapse into thehangingwall was estimated to be 10 m. Thepillars in this case were designed to carry theload imposed on them by only the 10 m ofhangingwall rock up to the major parting. In hisdiscussion, Ortlepp emphasized that thebehaviour of pillars would not be determinedonly by the 10 m height of the hangingwall, butby the elastic stope convergence, which was, inturn, determined by the regional geometry, andthe height of the entire superincumbent strata.

Quirke Uranium Mine28. The mines stratifiedconglomerates, and uses rib pillars of 3 to 6 mwidth located along dip. The mining heightvaries from 3 to 6 m, and pillar w/h ratios from0,7 to 1,5. On-reef haulages are located at 76 mintervals on down-dip, and are protected bystrike-rib pillars of the same dimensions as thein-stope pillars. The bord widths employed varyfrom 15 to 20 m. In 1981, the Quirke Mineexperienced a large sudden collapse in a flat-dipping section of the reef, covering an area ofabout 500 m on strike and 300 m on dip. Themining depth was 450 m, and the extractionratio was, as quoted, about 70 to 85 per cent.


The pillar failures were noted to be initiallygradual and subsequently sudden, involvingbursting of stope pillars. No barrier pillars werepresent in the collapsed area.

Pomfret Asbestos Mine and LavinoChrome Mine. Both mines experienced collapsesof large areas supported on slender pillars about20 years ago. Further details of these collapsecases have not been obtained.

Discussion

In summary, it would appear that pillar systemsin use in South African hard-rock mines are, forthe most part, performing satisfactorily. It isapparent, nevertheless, that current pillar-designmethods are largely empirically based, and thatoften very different layouts are being used inneighbouring mines or in mines with similargeometries. It is entirely possible therefore, thatsome current layouts fall far short of beingoptimal in respect of (i) real safety, (ii) maxi-mum safe extraction percentages, or (Hi) flexi-bility in accommodating future mining scenariosin deeper or otherwise different geologicalconditions.

In broad terms, the merits and demerits ofthe pillar systems currently in use can besummarized as follows.

~ Non-yield pillars in shallow operations,when properly implemented perform well;they maintain safety, and limit mining-induced disturbances to a minimum atrelatively minor costs. However, they are,at present, difficult to design rationally,and therefore carry with them the risk ofconceivably major mining disasters (pillarruns, heavy backbreaks) , or alternatively,that of wasteful extraction percentages.

~ Similar remarks apply, with even greaterforce, to yield-pillar layouts; whichnevertheless at intermediate depths,provide the potential for giving increasedextraction percentages while maintainingsafe conditions.

~ Crush-pillar systems seem, in practice, tobe much easier to design, and provide highextraction percentages with adequatebackbreak prevention and regional supportin intermediate depth conditions. Theirmain possible drawback would seem to bethat they allow greater mining disturbances(stope closures, surface subsidence, andface-stress concentrations) to take placewhen compared with non-yielding oryield-pillar layouts.

~ Barrier pillars provide a final very robustline of defence against potential majormining disasters such as large pillar runsor regional backbreaks. However, they arenot universally used, and procedures fortheir rational design are poorly defined atpresent.

JANUARY IFEBRUARY 1 995 17 ....

,- - - -- -.-- -p--

-- ---,--------


24. WOOD. LW.. andCoETZER.P.M. Follow-upon stickand pillarsupport at Union SectionRustenburg PlatinumMines. AMM. 1986.pp. 95-109.

25. KEEN. M. Personalcommunication. 1994.

26. TINCELIN.E.. and SINOU.P.Collapse of areas workedby the small pillarmethod. PracticalConclusions and anAttempt to FormulateLaws for the PhenomenaObserved. Proceedings3rd InternationalCor!ference on StrataControl 1960.pp. 572-590.

27. ORTLEPP.W.D.Contributed remarks tothe paper by Korf. C.W.on stick and support onUnion SectionRustenburg PlatinumMines. 1978.AMMpapers and discussions,1978. pp. 71-82.

28. HEDLEY.D.G.F..RoXBURGH. I.W.. and

MUPPALANENIS.N. A casestory of rockbursts atElliot Lake. Stability inUnderground Mining If.New York. University ofKentucky. AIME. 1984.pp. 210-234.

~ 18

The pillar layouts discussed in this reportmay not readily provide solutions to some of theproblems which will probably be experienced asmining depths increase. The experience gainedfrom deep gold mining will undoubtedly be calledupon for designing deeper mine layouts in theBIC,and elsewhere. However, the high k ratios(typically k > 1), and highly jointed and serpen-tinized nature of the BICrockmass, may bringabout different mining conditions from thoseencountered in typical deep gold mines, whichwill require further fundamental and appliedresearch.

Strategies against seismicity related pro-blems need to be identified as soon as the seis-mic monitoring programmes, currently beingcarried out, indicate increased seismicity levelsat relatively deeper parts of the Blc. Whether touse barrier (stabilizing) pillars or backfill, or acombination of the two, and when and how thetransition from pillar to non-pillar mining can beadopted, needs to be investigated. As an alter-native to the commonly used deep-level miningmethods, consideration should also be given topillar mining at great depths as a logical exten-sion of the current methods. For example, a75 per cent extraction using pillars with w/hratios of 7 or more, or by using crush pillars atdepth may be acceptable as rock engineeringstrategies, provided that these methods are wellunderstood, practical from a mining point ofview, and economical.

Conclusions

>- The methods and design criteria for pillarmining have evolved from practicalexperience. These empirical methods donot always accommodate the differing rockproperties, variability in rockmassconditions, and stress states encounteredin the BICand other base metal provinces.

>- The most commonly used hard-rock pillar-strength formula

crs= K h-D,7Swo.s

is similar to Salamon and Munro'sformula 1,but the parameters appear to bebased on Elliot Lake Mine experience,which has different rock types, and wherepillar w/h ratios range only from 0,7 to1,5. Based on very limited currentevidence, it is possible that, at least forsome BICrock types, the relative strengthof squat pillars versus slender pillars maybe considerably understated by thisformula.

JANUARY /FEBRUARY 1995

~_..---------------

[5]

>- Generally in the BIC,35 m bord widths arestable under relatively good hangingwallconditions. However, bord stability isdifficult to achieve where the hangingwallrockmass is weak, laminated or severelyjointed and, under these conditions,severely reduced bord widths seem toachieve adequate stability.

>- Non-yield pillar layouts carry the risk of apillar run, which is a sudden collapse ofmany pillars over a large area with exten-sive hangingwall failure, and is oftenassociated with seismicity, subsidence,and airblast. The pillar-run cases in hard-rock mining, have similarities with regardto failure modes, and it is interesting tonote that most sudden collapse cases tookplace at relatively shallow depths, Le.150 m or less. Again, methods andprocedures for adequately determiningpillar strengths and stresses are needed toensure that pillar runs do not occur innon-yield pillar layouts.

>- Based on current theory, low w/h ratiosfor hard rock pillars (will < 5) are likely tofail in an unstable manner, yet stablefailure of these pillars has been observedin some cases. Conversely, pillars havingw/h ;;, 5 should fail in a stable manner, yetcases of bursting have been noted. Furthertheoretical, laboratory, and in situ studieswould lead to a better understanding ofthe mechanisms governing stable orunstable pillar-failure modes.

>- A few direct attempts have been made toassess the numerical value of the residualstrength of pillars. A rule-of-thumb usedin shallow mining, based partly on in situobservations, is that a mature 2:1 yieldpillar has a residual strength of at least5 to 10 per cent of its peak strength. It ismost likely, for example, that verydifferent figures might apply for differentrock types, and for varying will ratiosinvolved. Laboratory testing, numericalmodelling, and in situ monitoring studiesare needed to develop and generalize thecurrent empirical design procedures.

>- Although barrier pillars do not exhibitstability problems, the variations in theirsize and spacing in different mines (e.g.RPM does not use barrier pillars) indicatea need for improved understanding of therole of barrier pillars in affecting thestability of tabular pillar workings.Increasing levels of seismicity bring aboutthe question of whether stabilizing pillarsshould be used for energy-release ratecontrol in future deep-mining conditions intheBlc..


---- -----

Documents

Thedesignofpillarsystemsas practisedinshallowhard-rock ... · system ofbarrier pillars isused inthismineat depthsgreaterthan100m.Thesecomprise20m widebarrier pillars spaced togivedepth/span