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Introduction:Method of Mining in Underground Metalliferous mines mainly depend on type ofDeposit:

q Regular or Irregular;

q Extent and Depth of Deposit, i.e., Massive or Pocket etc.;

q Dip and Thickness of Deposit;

q Nature of Hanging wall and Foot wall etc.

q Required production level to be achieved and

q Accordingly type of mechanization adopted in relation to drilling of blast holes forq Accordingly type of mechanization adopted in relation to drilling of blast holes forstoping operation, for development operation as well as for handling and transportof mineral within and out of stope etc.

Sublevel stoping is one of the most important methods of choice for achievinghigh production rate in Underground Metalliferous mines.

It is an overhand, vertical stoping, utilizing long hole drilling and blasting carriedout from sublevels to break the ore.

Although the stopes are unsupported, pillars are usually left between stopes andoccasionally within stopes.

The ore flows through the stope by gravity.

Sublevel stopping with Long Blast Holes:

q Demand for increased rate of production coupled with improvement indrilling technique has lead to the use of long hole blasting for ore extractionfrom Underground Metalliferous mines.

q Considerable advances have been made by introduction of long-holeand precise drilling in stopes.

q Today, most of the sublevel stopes all over the world are drilled with longblast holes.

q This has called for radical changes in stope design as compare toq This has called for radical changes in stope design as compare toconventional Sublevel stopes with increased sublevel intervals, betterpositioning of the sublevel in the ore body, improved method of orehandling from the stopes to the haulage level etc., for higher production.

q Further, by adopting improved & precise method of blasting withintroduction of new generation Emulsion explosives and ElectronicDetonators, for such Sublevel stopes the Techno-Economic level alsomaintained properly.

q It may however be noted that, large diameter holes (sometimes upto100 mm or more) reduce drilling cost per tonne of ore broken.

The Pattern of Long - Hole Drilling classed under two major categories:

Parallel hole drilling.

Ring hole drilling.

Parallel Holes are preferred in many western mines for:

Better fragmentation,

Easier setting up of drill rigs,

Greater effective drill footage and

Increased tonnage of broken rock per metre hole drilled as compared toRing drilling.

In many of the African Copper mines by adopting parallel holes as muchas 30 percent cost reduction on explosives and drilling have beenreported; which to a certain extent offset the extra cost incurred forslashing the sublevel from wall to wall.

Disadvantage with Parallel Hole Drilling:

However, the greatest disadvantage with parallel hole drilling is itsslowness due to extra time required for slashing the sublevel and lessersafety for drillers drilling on slashed out benches.

Because of these disadvantages in some of the high productionunderground metalliferous mines Ring drilling is much more common thanParallel hole drilling.

Ring Hole Drilling:

•Rings may be drilled from a central sublevel in relatively narrow stopes(about 15 to 20 m wide) to form two or more parallel sublevels in widerstopes.

• It is a general practice to drill several rings of holes before blasting tomake the drilling more continuous process and the blasting can berestricted to week-ends only, when enough time can be allowed forclearance of post blast fumes and dust.

Ring Hole Drilling:

qThe drills holes are made relatively smaller (40 to 50 mm dia.),bored with percussion rock drills mounted on a column and drifterwith extension drill steel to a maximum length of 25 to 30 m.

qThe drills holes are made relatively smaller (40 to 50 mm dia.),bored with percussion rock drills mounted on a column and drifterwith extension drill steel to a maximum length of 25 to 30 m.

q Hole deviation is a serious problem, however, deflection ofseveral metres are not uncommon.

q The effect in blasting can be disastrous as ring drilling requiresaccuracy in hole placement to obtain proper fragmentation.

q The problem of hole deviation, now a days, have been overcomegreatly because of introduction of sophisticated and accurate drillingmachines; by which the deviation is bare minimum.

Blasting Practice with Long Hole Drilling:

qWith long hole drilling, blasting practice has advanced considerably withincreasing use of stronger new generation emulsion explosives along withuse of precise and accurate millisecond NONEL / Electronic delaydetonators for obtaining better fragmentation during primary blasting.

q Judicious charging of holes in a ring using decks with suitable delaytiming ensures better fragmentation, restricted ground vibration andreasonably better economy.

q In some of the mines where ring holes system is used, it is a commonq In some of the mines where ring holes system is used, it is a commonpractice to charge every fifth hole to within one foot (approx. 0.5m) of thecollar, every second and fourth holes to within twenty feet (approx. 6 to 7 m)and the third to within ten feet (approx. 3 to 3.5 m) of the collar.

q Lesser burden on rings with wider spacing of the toes of individual holesin ring accompanied by delaying the holes from centre outwards helps ingood fragmentation.

qWhere ever possible proper distribution of charge is made by using deckcharges with in-hole electronic delay system; thereby fragmentation isimproved and blast induced ground vibration level can also be reduced.

Charging Long Hole with Explosives and Delays:

In most of the Indian condition holes are readily drilled to take advantage ofcartridges 40 to 50 mm diameter.

With 40 / 50 mm diameter cartridges burden of 2 to 2.5 m are blasted; with 32 mmcartridges burden is kept of the order of 1.25 to 1.8 m. The type of explosives usedranges from free flowing bulk loaded ANFO in dry hole condition to high strengthEmulsion explosives in wet hole condition.

The charge factor / explosives consumption in this type of blasting is generally keptof the order of 0.1 to 0.25 kg / tonne of blasted ore.

Short delay interval of 25 ms between holes in each row or ring starting from theShort delay interval of 25 ms between holes in each row or ring starting from theeasiest breaking section in the middle and progressing towards the walls are used.

Under normal circumstances, it may be possible to charge holes extending toabout 1.5 to 1.8 m (not more than the burden distance) of the collar without undulyovercharging, but in cases where the configuration of ore body is such that the holesare very closely spaced at the collar zone, it may be desirable to charge alternateholes only close to the collar and others upto 3 to 4.5 m from collar in order to reducethe charge factor.

The best pattern is decided by a series of actual trial blasts and it is alwaysadvisable to slightly overcharge the holes, as a failed blast could mean considerableloss of production.

Vertical Crater Retreat (VCR) – Advanced method of Sublevel stoping:

qWith the advent of large diameter (100 mm to 200mm) rotary and downthe hole percussion drills, it became practical to adopt modified version oflarge diameter parallel hole drilling and blasting for sublevel stoping.

qWith this system hole deviation is no longer a problem (< 2%) with large,parallel holes; which can now be extended to about 90 m and accordinglysublevel spacing is increased considerably.

q The major innovation in VCR method is of blasting.

q In this method, horizontal slice blasting of ore body with near sphericalcharges into the undercut is under taken.

q Spherical placement of explosives is the most efficient in terms offragmentation and explosives consumption.

q Holes are charged from collar after plugging the opposite end.

q The size of charge is generally restricted to a Length-to-Diameter ratio of6:1; which suffices in the practice to simulate a spherical charge.

q All holes in the stope are blasted together.

Vertical Crater Retreat (VCR)

q After the broken ore from the stope is drawn, the next slice of ore(generally about 4 to 5 m thickness in one slice) is charged and blasted asbefore.

q As mentioned, drilling is carried out from sublevel and is usuallycomplete before slice blasting commences; the high level of efficiency andproductivity is obtained with VCR method of sublevel stoping.

q Rock mechanics, blasting and material handling considerations are theimportant points in deciding dimension of stopes, location of boundarypillars etc.

q In addition, because of the unique reliance on long hole drilling andblasting special attention in sublevel stoping must be paid to rock-breakage/ blast design, hole diameter, length, burden, explosives selection, powderfactor etc., to obtain desired Techno-economics of the mining system.

Basting in Vertical Crater Retreat (VCR) sub-level stope:

q Blasting is done in stope worked by Vertical retreat method by drillingholes in slashed sublevel.

q The holes are drilled parallel, mostly either vertical or inclined alongdip of the ore body.

q A few holes at both the ends of the rows near the walls are drilledinclined into the walls to prevent any ore to be left out in the vicinity ofwalls.

q Blasting is done in slices, after the holes are charged from collar byplugging at the opposite end.

q Extensive surveys are carried out before charging in order to knowthe exact extent of holes, their depth, inclination, end points etc., toascertain correctly the placement of plugging at the bottom of holes.

q Delay sequence is used by using NONEL. Delays are so adjustedthat, centre holes of middle rows are blasted first. Side holes are blastedsubsequently.

Drilling and blasting for Depillaring of various pillars :

As the competent nature of rocks called for adoption of Sublevel openstoping, thus open stoping essentially results in leaving a network ofmine pillars for maintenance of overall stability of the mine.

In sublevel open stoping, as pressure abetments are quite high in thevarious pillars, these pillars encounter high level of lateral stresses andwith prolonged working these pillars undergo continuous deterioration.

One of the methods of relieving ground stresses is to mass blast thesepillars regularly.pillars regularly.

Drilling activities in such high stress zone encounter constraints such as(a) Failure of Drill drive and spalling from it, (b) Failure or collapsing ofdrill holes, (c) Development of extension fractures etc.

Thus, in such a high stress zone, explosives charging into the hole andblasting thereof are important aspect, which very carefully and efficientlyshould be organized.

Deck charging is done to minimize the maximum charge per delay,which prevent back-break, ground vibrations etc.

Discussion on modern precision Drilling and Electronic Delay Detonatorsystem for enhancement of efficiency in Mines:

q Computerized drilling and electronic detonators add precision at the minesto lower downstream costs.

q Drilling and Blasting is the most important activities in working of anymines.

q Computerized drills, Measure-While-Drilling (MWD) systems, andelectronic detonators are gaining wider acceptance in quarries, opencast andunderground mines.

q Cost centers such as drilling, blasting, excavation, hauling and crushingare seen as interrelated variables in the total cost equation.

q As the first step in this interrelated process, improved results from drillingand blasting; even if it costs more; can significantly improve costs andproductivity of downstream cost centers such as excavating, hauling, andcrushing.

q Thus, gaining a competitive edge in a mature and basic industry, modernsystem of Drilling & Blasting methods have taken a major leap forward inenhancement of Techno-Economics advantages in working of mines.

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