SECTION 10 SHRINKAGE METHODS Mike Turner …

1 SECTION 10 SHRINKAGE METHODSMike TurnerAustralian Mining ConsultantsMay 2000 Australian Centre for GeomechanicsUnderground Mining MethodsMike TurnerShrinkage Disadvantages .. Advantages .. Orebody Dip .. Orebody Competence .. Hangingwall and Footwall Competence .. Orebody Geometry .. Broken Ore Orebody .. OreDrive DrawpointCross-cut Method .. Chutes/Rail Drawpoint Cleaning .. Operational Guidelines .. Stope Hangingwall and Footwall .. Rolling SHRINKAGE .. SemiShrinkage .. Cut andfill SHRINKAGE .. Longhole SHRINKAGE .. Alimak Transverse SHRINKAGE .. 12 PlatesPLATE1; ! BOOKMARK NOT ;SHRINKAGESTOPE HANGINGWALL SUPPORT,HOLEMARK-UP !

2 BOOKMARK NOT ; LONGSECTION OFRAIL/ ! BOOKMARK ; ISOMETRIC VIEW OFRAIL/ 16 FIGURE3; ISOMETRIC VIEW 17 FIGURE4; ISOMETRIC VIEW 18 FIGURE5; ISOMETRIC VIEW OFRAIL/ 19 FIGURE6; LONGSECTION 20 Australian Centre for GeomechanicsUnderground Mining MethodsMike TurnerShrinkage MethodsSection 10, Page SHRINKAGE stoping the oreis generally mined in horizontal slices, starting from thebottom and advancing updip. The hangingwall and footwall are supported by brokenore left in the mined out stope. This broken ore servesas the working platform forairleg operators in the stope, and sufficientore is drawn out atthe bottom, followingblasting, to provide suitableheadroom for these stoping is applicable to the mining of steeply dipping orebodies where theorebody is competent enough to work under andthe walls are sufficiently strong to beself bulks and increases in volume by 30 to40% due to blasting.

3 Approximately 30 to40% of the blasted ore must therefore bedrawn off following blasting in order to re-establish a suitable working space above the blasted the stope reaches it supper limit the remaining 60% of the broken ore is ore is drawn off at the lower elevation from either drawpoint cross-cuts, usingboggers (LHDs), rocker-shovels or scraper (slushers), or from ore-drive chutes. Bothcross-cuts and chutes have to be closely spaced to facilitate even ore drawdown. Airpowered scrapers can be used in the stope to level off the broken mining was a common stoping method prior to mechanised mining and theintroduction of hydraulic fill. Sub-levelstoping, cut and fill mining and sub-levelcaving have generally replaced SHRINKAGE mining in Australia, Europe and Canada.

4 Themethod is still used in a handful of non-mechanised minesin South America, Africa,Asia and method is labour intensive and requiresexperience in such METHODS . The workingconditions can also be difficult and relatively hazardous due tothe fact thatthe brokenore is used as a working platform and has to be drawn down daily. Voids, unevendrawdowns and hang-ups can also cause method is also a relatively low productivity method and the majority of the oreremains in the stope for extended periods of time. This has a major impact during theinitial production phase of amine nearly 3 times as much ore has to broken thandrawn in order to achieve full production.

5 There is also a risk of over-drawing stopes,which leads to stope drilling difficulties and a requirement for timber Centre for GeomechanicsUnderground Mining MethodsMike TurnerShrinkage MethodsSection 10, Page mining still remains, however, a mining method that can be implemented andpractised with minimal capital investment inequipment and fill. The method requiresthe least capital investment and can be operated with small mining is applicable for orebodies with; competent ore (operators working underneath backs) stablehangingwall and ore that is not affected by storage in the stopes (oxidisation,cementation, combustion) width from 1m to to moderate DipThe method is suitable to very steep orebodies, greater than 70generally, anddefinitely exceeding the angle ofrepose ( greater than 45).

6 This is required due to afew problems with orebodies at lower dip angles;-Uneven and difficult daily draw-downs-Hang-ups during final drawdown-Hangingwall CompetenceThe following estimates of applicable RockMass Quality (Q) are based on personalexperience. Most SHRINKAGE operations pre-date the established rock mass mining is applicable for orebodies with good ground conditions (Q>10),generally only requiring spot bolting. Any additional mesh or extensive bolting willtend to reduce productivityand could lead to draw problems and orebody should be sufficiently competentto be safe to mine without mesh andwithout excessive bolting.

7 Mesh and bolts inthe blasted ore adversely affect the flow ofdrawdowns. This requirement alone would make it difficult to implement shrinkagemining as a method for Western Australia,without daily geotechnical Centre for GeomechanicsUnderground Mining MethodsMike TurnerShrinkage MethodsSection 10, Page and Footwall CompetenceThe rock mass quality of the hangingwall and footwall rock masses should also be atleast fair (Q>4). Poor quality walls tend to unravel during drawdown of broken hangingwall and footwall rock masses are only supported in most SHRINKAGE minesby the broken ore. This support is minor,and once the stope is finished and beingdrawn down there is no support on the walls.

8 The hangingwall and footwall rockmasses both have to be competent for SHRINKAGE stoping. Large equipment for drillinglong cable reinforcement holesis unsuitable for the stopes and internal reinforcement isusually limited to short bolts,holes drilled with airlegs and bolts manually can be left in SHRINKAGE stopes, butthey can have a major impact on the flow ofbroken ore and therefore have to bedesigned to minimise this split sets and grouted bars can be used for local wallsupport, and the plates orbolts are also designed to facilitate the attachment of safety chains for the GeometryDue to the required flow of broken ore within the stoped out void,and the difficulty ininstalling reinforcement and leaving pillars.

9 The orebodies should have reasonablyregular ore boundaries and problems can be associated withchanging stoping widthsand variable strikeorientations. Pillars are usually the major stabilising option as cable bolt reinforcing isdifficult to install in these limited working spaces (Plate 1). Ore DeteriorationThe orebody should be stable following blasting, with no oxidisation, or other physicalor chemical deterioration or alteration thatcould result in combustion or the cementingtogether of the rock sulphur contents and partially weathered orebodies would have to be tested todetermine their susceptibilityto combustion and cementing. The clay content of shearzones could also have a detrimental impact on the flow of broken of the ore leads to great difficultyin extracting the brokenore, especially theore sitting above pillars (between the chutes or drawpoint cross-cuts).

10 WidthShrinkage mining can generally be usedfor orebody widths from 1m to Centre for GeomechanicsUnderground Mining MethodsMike TurnerShrinkage MethodsSection 10, Page 6 The minimum stoping width is only limited tothat required for drilling operations. Thehangingwall and footwall of wider orebodies canalso be stabilised using internal wider than 12m canbe mined using a transverseshrinkage method, but thisrequires pillars being left between adjacent mining is only applicable in lowto moderately stressed sections. Highstresses can lead to stress fracturing inthe backs which will require more support,reduced productivity and increasedrisk of hang-ups due to mesh and bolts in the related stope closure also leads to pinching of the broken ore andsubsequent difficulties during two main SHRINKAGE options include ore drive rail cleaningusing chutes (thestandard SHRINKAGE method) and drawpointcross-cut cleaning using boggers or development strategy for the two mainoptions are covered below, includingore drive rail cleaning with finger rises, cones and chutes andbogger (LHD)