Sub-Level Caving: Where Is It Headed? 1

Sub-Level Caving: Where Is It Headed?This paper describes the challenges of mining a sill pillar under various types of paste back- fill quality. From 1994to 2002, Louvicourt mine produced approximately 12,600,000 t grading 3.5% Cu, 1.5% Zn, 27 g/mt Ag, and 0.85g/mt Au. The mining method is transverse blasthole stoping, mining primary and secondary stopes. The productionof this 4,300 t/d operation was accomplished using two mining horizons; from the 655 m level to the 415 m level andfrom the 860 m level to the 680 m level. The main sill pillar is between the 680 m level and the 655 m level. Thesecond sill pillar is between the 885 m level and the 860 m level representing a quarter of the main sill pillar in size.Since 2002, production has gradually decreased and mining activities are expected to end in mid-2005. The sill pillarproduction will represent almost 25% of the total mine production. The overall recovery of the sill pillars stopes willbe discussed in this paper. To recover the sill pillars, more than 1,500 m of development was planned in or underpaste backfill. The first sill pillar stopes under paste backfill were mined out in 2002 with excellent results

INTRODUCTIONMuch of the theory on which SLC is based was developed in Scandinavia many years ago. It was mostly based on“bin” theory and ellipsoids of movement. SLC has since fallen into disfavour.Recently a number of Australian mines have adopted SLC as a primary method and have achieved some very goodresults which has called into question the original bin theory assumptions.It is clear that the process of choke blasting (compaction of waste, very variable fragmentation through the ring)significantly conflicts with the assumptions of regular loose flow of material in a bin. The good results can only beexplained by using different models of behaviour, which have been borrowed from block caving. If these models arecorrect then they point to a number of design and operational changes that can be applied to improve theeffectiveness of SLC.This paper describes a “new” model for SLC and how this model can be used in terms of design and operation.

SUB-LEVEL CAVINGSub-level caving is considered a low cost method by the ton moved (often referred to as a “factory method”) buthigher cost due to the perceived high dilution (unfavourable costs to final finished metal). but there are existingoperations that have remained faithful to slc (even after trying alternatives) and there are new operations coming onstream using the method.It is important to recognise both the attributes and the drawbacks of the method. It might have limited application butit is a valid method worthy of consideration in the right context: usually strong, competent and massive deposits. Theknowledge base for SLC is comparatively small as very few mines use the method. Most of the "theory" comes outof Scandinavian iron ore mines and is many years old and mostly based on model studies and classical bin theory.Data on draw behaviour (what is happening in the broken mass) is difficult to come by. The other major problemwith iron ore operations is that dilution can carry very high grades so it is difficult to assess the "true" dilution(tonnes drawn from outside the current ring). The clear advantage of SLC is that it is a very predictable "factory"type method with high production potential, reasonable costs, "top-down" (low up-front capital, low stress) and verylittle ore is at risk at any one time (a few thousand tonnes in an individual ring).SLC suffers from apparent significant contradictions: a slice of broken material being drawn relatively clean whilstsurrounded by broken waste, figure 1.

Sub-Level Caving: Where Is It Headed? 2

Figure 1 - Illustration of SLC methodIt seems to defy logic. For this reason it is very important that a practical, common sense model of how, or why, itworks is presented.When a ring is blasted (typically 2,000 t) the first part of the draw is reasonably clean; then waste above and in frontof the ring starts to come into the draw point and a mixture of ore and waste is drawn; the proportion of wasteincreases until cut-off is reached. When the draw point is closed there will be ore left behind. This mixes with theprevious ore/waste in the cave; the waste therefore increases in grade as the SLC matures. The objective is to keepthe waste out for as long as possible but to try and make the most of the zones of higher grade dilution.

HOW (WHY!) DOES SLC WORK?The “classical” theory is based on ellipsoids of motion and isolated draw from a single draw point developed fromflow of loose material in bins (ref.1,2,3). There are concerns with this model as it tends to ignore differences infragmentation, material types, the significant weight of the cave which compacts the cave material, and the action ofthe blast which also consolidates the waste in front of the blasted ring. All of these factors will significantly affectthe way in which choked material moves. Conventional draw analysis suggests that dilution enters the draw at verylow draws (below 40%) but recent experience suggests that it can enter at draws well above 50%.Recent operations at an Australian gold mine appeared to be breaking some of the classic rules and included someunusual operational changes but they were achieving much better results than suggested by classic theory. When weanalysed what we “thought” was happening we concluded that the critical considerations could be broken into designand operation (layouts and operating practices) and physical (orebody characteristics):

Sub-Level Caving: Where Is It Headed? 3

Operations• differential fragmentation: finer fragmented material flows more readily than coarse material; dilution should be

coarser than the ore; (fine material can easily flow "through" coarse material)• compaction: compacted material does not flow as readily as freshly blasted material; the blast must compact the

"waste" in front of the fresh ore so that it does not flow as readily as the freshly blasted ore• temporary arching: coarser material at the top of the ring impedes flow of waste from above; coarser material

draws over a much wider "arch" than fine material; coarse material can temporarily "hang-up" whilst the finermaterial below the arch is drawn (but very coarse or unblasted material will allow waste to flow around the oreand reduce recovery – a delicate balancing act) draw coverage: the more the ore is undercut by development themore likely it is that the ore will flow into the draw point

• interactive draw: draw points drawn together along a flat face result in a much wider zone of moving material;the increase in recovery can be significant; the material in the ring can be drawn to much flatter angles (allowingflatter ring angles)

• ground support: the support is primarily for the brow; the brow has to stay stable for that short period of draw andthen charging of the next ring; support intensity will be much more than required for normal tunnel stability; thebrow has to accept the blast damage and has an extra degree of freedom

•• blasting:"" have to break the rock to the right fragmentation, even but not too fine and without causing too muchdamage but sufficient to fluff the ore and compact the waste; powder factors are generally more than 30% greaterthan for un-choked blasting

Some of the changes in layout and operations are illustrated in figure 2.

Figure 2(a) - Effect of interactive draw

Sub-Level Caving: Where Is It Headed? 4

Figure 2(b) - Effect of draw point spacingThe primary objective for a SLC operation should be delayed dilution entry. Classical theory and practice hasdilution entering at 20% to 40% extraction of the ring tonnage; more recent operations have reportedly achieved 80%and better; but we have also seen operations where the dilution was overrunning the ore at less than 10% draw. Thelater the dilution enters the muckpile the greater the proportion of "clean" ore and the higher the grade factor for thesame extraction. (And to repeat: dilution in this instance is material outside the current ring - if it carries grade somuch the better).

Orebody ConditionsWhat the orebody will allow you to do is a function of "mining difficulty": geometry, rock mass conditions, majorstructure, stresses, grade distribution etc.:•• strong rock: allows small pillars; small drive cross-cut intervals increases coverage•• competent rock: dependent on jointing and formation of wedges; brows are mostly disturbed by blasting and then

by gravity; competent rock masses allow wide backs; competent rock implies few joints and strong joint surfaces(little infilling, irregular and rough surfaces); jointing may cause wedge failures in one direction but not in theother; therefore a design decision might be to align development in a favourable direction

•• few major structures: prevent massive wedges and/or hole cut-offs•• steep dip: keeps the low grade dilution source further away from the current draw points so most of the dilution

comes in over the top of a mixture of ore and waste from caving at much higher levels than the current extractionlevel

•• massive: sufficiently large footprint for high production rates; most dilution comes from the boundary betweenore and waste - the more massive the deposit the smaller the proportion of material from the boundary; the moremassive the deposit the higher the development yield from waste development

•• fragmentation: must consider the fragmentation from blasting and the fragmentation of the material that caves;joint frequency, joint condition and joint direction will all affect fragmentation; a competent rock mass is usually

Sub-Level Caving: Where Is It Headed? 5

the most suitable as larger, widely spaced holes will still result in good fragmentation but the caving will be verycoarse; this results in blasted material finer than the cave and an "even" fragmentation for the ore with a minimumof both oversize and fines

•• no "muddy" material: want a minimum of very weak or rapidly weathering material to avoid problems of muckrushes, over-compaction and hang-ups

•• caving: caving is seldom an issue with SLC as it usually starts out of the bottom of an open pit; lack of cavingfrom the hanging wall is usually an advantage as it delays the introduction of dilution; but, choke conditions mustbe maintained so at dips below 50o(say) the occurrence of voids can be an issue; caving is a function of rock massconditions and the hydraulic radius

"Ideal" is not found in mining. Compromises are inevitable. Currently most massive deposits are either block cavedor if they will not cave readily they are open stoped with cemented fill. It would appear that the concern is withdilution and recovery; the response being that recovery will be higher and dilution lower with a filling method. Thisis partially correct but recoveries are usually well below 100% and dilution often runs at over 15%. A mature SLCcan achieve recoveries in excess of 100% and grade factors well over 80% (proportion of pure waste less than 20%).This assumes a certain amount of “cheating”: it assumes that there is mineralised waste around the ore (not unusual);and that the dilution grade steadily increases (but this is the expected attribute of SLC which must be maximised).

EVALUATIONLike all mining methods, the costs per unit are easy to calculate. The difficulty is in predicting productivity per unit(primarily the production rate per draw point) and the head grade (a function of planned and unplanned dilution,which in turn is dependent on the point of dilution entry and the degree of mixing and the grade of the dilution). Toadd to this difficulty, there are very few SLC operations. And none of these operations have either fully monitored orpublished their characteristic draw behaviour (as far as we know). But this should not detract from what ispotentially a very productive and cost effective method - a method that can deliver the lowest cost-to-metal undercertain circumstances. We are sorely in need of data. But estimations and predictions must still be made:

Grade PredictionThis is the key concern and the one in which current predictive methods do not recognise the benefits of interactivedraw. The following draw model illustrated in figure 3 has been based on models developed for block caving (ref 4)as well from previous experience with SLC operations. The results from the model agree reasonably well with theachievements of more recent SLC operations but none of these operations is technically or operationally ideal. Webelieve that a well engineered and operated SLC in ideal circumstances (geometry and rock mass) can deliver betterresults than predicted by the model.

Sub-Level Caving: Where Is It Headed? 6

Figure 3 - Mixture of ore and waste with drawThe model shows the proportion of ore (below the line) and dilution (above the line) as the extraction increases(along the x axis). It can be seen that very high extractions can be achieved dependent on the shape of the curve andthe grade of the ore as well as the grade of the waste. Ideally we should also have a model that estimates the grade ofthe dilution (the mixture of pure waste, mineralised waste and the ore left behind from each recovered ring). Amethodology to estimate the dilution “bin” is illustrated in figure 4.

Figure 4 Mechanics of dilution grade estimation

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This is very simplistic as we know that the dilution grade will decrease as the ring is drawn and the waste comesfrom further away. But in the absence of real data this methodology was used on a recent project and allowedreasonable sensitivities to be run.

Production RateWe have used the draw point as the key operational indicator; the number of available draw points will determine theoverall production rate. The longer term rate per draw point (in effect a production drift or cross-cut) will depend onthe time taken for a series of activities: drilling, charging and blasting (including any brow repairs and re-drilling);mucking and any secondary breakage and any hang-up clearance. Keeping the face flat and achieving interactivedraw will also conflict with the availability of a draw point.Typical results for reasonably well run operations are 500 to 600t/dy per available draw point. A large LHD willtherefore require some four to five draw points to keep it supplied with broken muck. If interactive draw is alsoconsidered and the draw is across a group of four to five draw points then at least ten draw points may be requiredper LHD. It is critical to have as large an LHD as possible operating.The overall maximum production rate of any mining method is very difficult to estimate on paper but typicalratesof-fall through the deposit in good conditions have been in the region of 65m per year.

DISCUSSIONThere is clearly a conflict between the grandiose title of this paper and the reality of fitting it all into eight pagesincluding figures. We have not been able to do justice to the subject. But we hope that we have shown that there aresituations where SLC is an appropriate method. In a recent project it showed very superior economics over moreconventional methods achieving over 100% recovery with grade factors over 80% and operating costs of less than$12/t. And SLC can be very productive in terms of rate-of-fall through a deposit – generally double what could beachieved with filling methods.The only methods that can compete on cost-to-finished-metal are possibly block caving and partial extraction. Butvery few ore bodies are really suitable; they need to be:•• strong and competent•• have a large footprint with very steep dip•• and preferably have mineralised wasteCoupled with this is the uncertainty of outcome as there is very little documentation on draw behaviour on which tobase reasonable draw models. But we are reasonably certain of what contributes to efficient achievements. Andcurrently there is work being done to confirm and improve the current rather simplistic and rough models illustratedin this paper.The major advances are with interactive draw and the understanding of what contributes to the success of SLC andhow to ensure that this success is achieved.

REFERENCESKVAPIL, R.1982 The Mechanics and Design of Sublevel Caving Systems. Underground Mining MethodsHandbook, SME, pp.880-897LAUBSCHER, D.H. 1994 Cave Mining – The State of the Art, Journal of the SAIMM, Oct 1994, pp 279-293

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