Block Caving(up to 160,000 tpd)

Block caving (Figure 1) is historically one ofthe lowest cost bulk underground miningmethods. Because of this, a detailed rockmechanics assessment of the ore bodymust be completed to determine itssuitability .

The principal considerations in determiningwhether or not an ore body is suitable forblock caving are: local geology, rockstrength, hydrology, pre mine rock stress,ore body geometry and the characteristicsof the capping rock.

Depending upon the project scope andtime constraints, a qualified mineengineer should be involved in order togather and interpret the data used inmaking the final decision regarding thecavability of an ore body. Mine planningefforts for block caving projects shouldinvolve engineers with practicalexperience in the parameters that areunder consideration for each particularproject.

Extensive experience in evaluating cutoffgrades and ore reserves for potential blockcaving deposits along with level placement,type of caving most applicable, (i.e., gravitydraw, slusher extraction or loader extraction)equipment, manpower, and cost estimationis required.

ISSUE NO. 105 November 2010

Introduction

One of the most important decisions facinga mine planner is the selection of asuitable mining method. Often the decisionis made without a thorough knowledge ofthe ground conditions.

Very good ground conditions are requiredfor open staging or minimum supportmethods, such as shrinkage stoping. Onthe other hand, poor quality ground is anecessity for block caving. The applicationof sublevel caving usually requirescompetent ore and incompetent easilycaved host rock. The consequences ofimplementing block caving in competent,high strength rock with little or no fracturing isextremely coarse fragmentation, excessivedrawpoint wear, high secondary blastingcosts and, in general, unacceptableproduction costs. Conversely, theconsequences of attempting sublevel cavingof weak, highly fractured ground is poorbrow control, ore loss, excess dilution,unnecessary blasting and, again,unacceptable production costs.

The choice of an underground miningmethod must be tailored to the groundconditions if the mining operation is to besuccessful. The emphasis should be putupon objectively assessing suitablemining methods and choosing themethod most compatible with groundconditions.

Copyright 2010 by Pincock, Allen and Holt, a division of Runge Inc. All Rights Reserved.

C A L E N D A R

Underground Mining Methods

Mines and Money London 2010November 30 December 1, 2010Business Design Centre, IslingtonLondon, Englandwww.mining-journal.com

Northwest Mining AssociationConventionDecember 5 10, 2010Spokane Convention CenterSpokane, Washingtonwww.nwma.org

Mineral Exploration Roundup2011January 24 27, 2011Westin Bayshore Resort & MarinaVancouver, BC, Canadawww.amebc.ca

Mining Indaba 2011February 7 10, 2011Cape Town InternationalConvention CentreCape Town, South Africawww.miningindaba.com

2011 SME Annual Meeting andExhibit / CMA 113th NationalWestern Mining ConferenceFebruary 27 March 2, 2011Colorado Convention ComplexDenver, Coloradowww.smenet.org

PDAC 2011 InternationalConvention, Trade Show andInvestors ExchangeMarch 6 9, 2011Metro Toronto Convention CentreToronto, ON, Canadawww.pdac.ca

2Cut and Fill Stoping(up to 5,000 tpd)

Cut and fill stoping (Figure 4) is anextremely versatile mining method thatcan be applied to both flat dipping andsteeply dipping deposits under almostall types of ground conditions. Itpermits the mining of erraticallyshaped ore bodies with a minimum ofdilution, and a high degree ofselectivity. Where ground conditionsare reasonably good, an overhandextraction system is employed inwhich slices of ore are removed froma stope by starting at the bottom andadvancing upward. Backfill is placed inthe stope upon completion of eachslice and serves as a working floor forextracting the succeeding slice. Whereground conditions are extremely poorin the ore body, an underhandextraction system is used in whichwork advances from the top of theorebody to bottom. In this case cementis added to the backfill material tostabilize it and provide a safe roof towork under.

Cut and fill stopes can often be highlymechanized so that employeeproductivity is good, but ore productionfrom each stope must be periodicallyinterrupted to allow for placement ofbackfill materials. A sufficient number

Blasthole (Sublevel) Stoping(up to 20,000 tpd)

This stoping method (Figure 2), alongwith its variations of sublevel stoping,vertical crater retreat (VCR) or endslicing, is especially suitable for orebodies with the followingcharacteristics:

1. The rock in the ore bodies and inthe host ground is reasonablycompetent.

2. The ore zones have relatively largehorizontal and vertical dimensions.

3. The number and size of barren orwaste zones within the ore body isminimal.

When conditions are favorable forblasthole stoping, this method willproduce reasonably low mining costsbecause it can be highly mechanizedand result in good productivity peremployee. Large diameter blastholescombined with modern, dieselpowered load hauldump (LHD)equipment can usually be used togood advantage when this stopingmethod is employed. However, closesupervision is required and aneffective preventative maintenanceprogram must be enforced.

Shrinkage Stoping(up to 2,000 tpd)

Shrinkage stoping (Figure 3) is thepreferred method for mining steeplydipping relatively narrow vein deposits thathave competent rock for the hanging walland footwall. This method does,however, require that 60 to 70 percent ofthe broken ore be left in the stope untilmining of the stope is completed, sincethe broken ore pile serves as the workplatform for the miners. Consequently, thetotal revenue from the broken ore isdelayed until sometime after each stope iscompleted. Also, ores that are susceptibleto rapid oxidation upon exposure to air aregenerally not considered for shrinkagestoping.

Drilling and blasting is difficult tomechanize in shrinkage stopesbecause of the inherently uneven floorsand generally limited working space.Drawing of the broken ore from thestopes can be mechanized usingslushers or LHD equipment loadingfrom drawpoints, or it may be doneconventionally with chute loading of traincars or mine trucks.

In general, shrinkage stoping is laborintensive and its use is limited torelatively small producers of highergrade ores.

Figure 2: Blasthole (Sublevel) Stoping

Drawpoint

Large holedrilling,parallelholes

Sublevel

Stope

Blasted ore

Loading crosscut

Transport drift

Manway

Figure 1: Block Caving

Undercut preparation

Grizzlydrift

Grizzlylevel

Mainlevel

Transport drift

Fingerraise

33

of stopes must be made available sothat mine production does not sufferbecause of the backfilling operations.

The backfill material usually consists ofdeslimed concentrator tailings, and maybe augmented by waste muck from minedevelopment or by surface sand andgravel. Cement is commonly added tothe backfill material to help stabilize it,either to provide a sound working floor orto make a safe roof for underhandstoping. Hydraulic placement of the fill isthe most common practice, with thematerial prepared in a surface plant andtransported underground throughpipelines.

Open Stoping(up to 20,000 tpd)

Open stoping is a mining method forextracting small, erratic ore depositsencountered as veins, sills or mantos inrelatively competent rock . The methoddoes not require a large investment inexpensive mining equipment and can beused to effectively follow ore shoots on ablast round to blast round basis. In flatlying ore zones, the footwall of thedeposit is used as a working floor, whilein steeply dipping deposits timberstaging is commonly used to work fromor pillars of low grade material may beadvantageously used as a working base.

Transport of the broken rock down to thehaulage level may be by gravity alone, orutilize slushers where the dip of the orezones is too flat to permit 100 percentgravity movement of the blasted ore.

Open stoping of small deposits permitshigh selectivity of the material to bemined, but daily production is generallyvery limited at mines employing it as theprincipal method of extraction. It iscommonly used as a scavengingmethod for recovering ore that mightotherwise be lost at larger mines wherethe principal production methods arebased on blasthole or shrinkagestoping systems. It is a mining systemcommonly employed at smallerprecious metal mines where groundconditions are good, and the orebodyconsists of small ore shoots. It isespecially prevalent in many lesserdeveloped nations where miningregulations are less stringent, laborcosts lower and mining equipment isrelatively high priced.

Sublevel Caving(up to 50,000 tpd)

Sublevel caving (Figure 5) can beapplied to those large ore deposits inwhich the ore itself is relatively strong,but the host rock is weak enough tocave when the ore is removed. The

geometry of the ore deposit influencesthe selection of this stoping method. Asteeply dipping deposit is moresatisfactory for its application than arelatively flat deposit, unless the latter hasconsiderable thickness. Sublevel cavingcan often be used to extract ore bodieswhose limited size or rock competencyprecludes extraction by the block cavingsystem, and is flexible enough to beapplied to irregular ore bodies of varyingwidths.

The principal disadvantages of sublevelcaving is the resulting high dilution ofthe ore caused by caving of wastematerial from the walls and therelatively high development cost to bringthe mine into production. Sincesublevel caving induces failure of thewall rock and overburden, surfacesubsidence results in locating allpermanent structures outside of thearea of influence.

Sublevel caving mines lend themselvesto mechanization and mining activitiescan be specialized simplifying the trainingof underground personnel. Miningactivities on each level are similar, i.e.,development of the levels, productiondrilling on the intermediate levels andproduction blasting with ore extraction onthe upper levels, consequently thesupervision of the activities is also

Figure 4: Cut and Fill Stoping

Exhaust airway

Hydraulic sandfill

Ramp

Ore pass

Figure 3: Shrinkage Stoping

Drawpoints of chutes

Transport drift

Cross cuts for loading

Ore left in stope

Raise

Timbered manway(also ventilation)

44

Consultants for Mining and Financial Solutions

to be successfully implemented.Because of the large amount of timberused, both methods present a definitefire hazard for the entire mine. Thecharacteristics of top slicing and squaresetting preclude mechanization of theiroperations, so their application islimited to very high grade ore bodies.

Resuing is a method of stoping inwhich the ore is broken and removedfirst followed by the blasting of the wasteor vice versa. Usually the material whichbreaks easier is blasted first. Thebroken waste is left in the stope asfilling and a plank floor laid on the fill toprevent mixing of ore and waste.Resuing is applicable where the ore isnot frozen to the stope walls and worksbest if there is a considerable differencebetween the hardness of the ore andthe wall rocks. The method is laborintensive and is rarely practicedanymore, except in very high grade,narrow vein, gold and silver deposits.

This months article was provided byPAHs Mining and Geological ServicesDepartment.

simplified and interference between theactivities is minimized.

Room and Pillar Stoping(up to 20,000 tpd)

Tabular, flat dipping ore deposits incompetent rock are usually mined byroom and pillar stoping methods (Figure6). If the ore zone is continuous over longdistances, a regular pattern of supportpillars can be laid out to yield maximumrecovery of the ore and at the same timeprovide sufficient support for the hangingwall or roof. If the ore zones are erratic,random support pillars can be left inareas of waste or low grade material.

The principal advantage of room and pillarstoping is that it is readily adaptable tomechanized mining equipment, whichresults in high productivity and a relativelylow cost per ton of material extracted. Forlarge ore bodies, a large number of workingplaces can be easily developed so that highdaily rates of production can be countedupon. Most of the mine development work isin ore so waste extraction is kept to aminimum.

The main disadvantage of room andpillar mining is that a large area of roofis continuously exposed where workactivities or movement of men andsupplies are carried out. Consequently,roof soundness is a primary concern forthe safety of personnel and groundsupport is generally a major cost,especially in rooms with high backs.Also, recirculation of ventilating air canbe difficult to minimize in room and pillarmines .

Miscellaneous StopingMethods

Three stoping systems that werecommonly used in the past, butbecause of their labor intensivecharacteristics are no longer favoredare the square set, top slicing andresuing methods.

The square set and top slicingmethods are used in extremely poorground where other extractionmethods are not practical. Bothmethods require large amounts oftimber and an experienced work force

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Figure 6: Room and Pillar Stoping

PillarVertical benching

Benching of thicker parts of ore body Pillar Front benching

Figure 5: Sublevel Caving

Ore pass

Main level

Drilled

Development ofnew sublevels

Production drilling

Mining = Blasting and loading

Caved hanging wall