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CASE STUDY ON VERTICAL CRATER RETREAT MINING

BY: SUJAN DALBEHERADEVGAN SINGH

ABHAYA MISHRA

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VCR MININGVertical crater retreat (VCR) mining is a method originally developed by the Canadian mining

company INCO.

Today, VCR is an established mining method used by mines all over the world that have competent, steeply dipping ore and host rock.

VCR is based on the crater blasting technique in which powerful explosive charges are placed in large-diameter holes and fired.

Part of the blasted ore remains in the stope over the production cycle, serving as temporary support for the stope walls.

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COMPARISON FOR THE APPLICABILITY OF VCR METHOD TO GIVEN CONDITIONS:

CHARACTERSTICS REQUIREMENTS GIVEN CONDITIONS

Ore Strength Moderate to Strong Moderately Strong

Rock Strength Fairly Strong to Strong Moderately Strong

Dip of Deposit Fairly steep(>45) 70 degree

Depth Shallow to Deep 500-1000m

Ore Grade Moderate

Ore Uniformity Fairly UniformOther Given Conditions are:

1. Thickness:30-50 m2. Strike length:800m

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METHOD OF WORKING:• It starts with a geological section that’s been divided into mining blocks with regard to geological reserves. The blocks are then divided into stopes.

• The next step is to include the main and sublevel development that is needed to access and extract the stopes.

• Geological and survey data determines the grades and tonnages for the stopes.

• Planning the stopes also includes developing the drill layouts and including data such as; drilling positions, direction, depth and hole size.

SOURCE:google.com/images/vcrmining

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METHOD OF WORKING:

SOURCE:http://www.eolss.net/sample-chapters/c05/e6-37-06-02.pdf

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MINE DEVELOPMENT AND LAYOUT:

The sequence of development of VCR stopes is

• A haulage drift is excavated along the ore body at the drawpoint level.

• A draw point loading arrangement is created underneath the stope,

• The stope is undercut.

• An overcut access is excavated for drilling and charging.

SOURCE: google.com/images/ugvcrmininglayout

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CROWN PILLARS:• The last ten meters of the stope acts like a temporary crown pillar and is

normally broken using choke blasting. The mining sequence have been changed from down dip to up dip and introduced backfilling because of ground control problems.

• This change was made to eliminate the stability problems that occurred because of the crown pillars that were left behind when mining down dip, there are no crown pillars left behind when mining up dip

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ACCESS TO DEPOSITS• The access to the mine is made through two vertical shafts at ground level;

one for ore and one for personnel and machinery. • The two upper shafts go down and then through a sub-vertical shaft down to

1,000m.• These shafts are also used for ventilation purpose.• Ramps are also driven which connects two levels.

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MACHINERIES USED:• Major machineries used in VCR mining are

• ITH Drill machine

• Remote control LHD

• Small wagon

• Jumbo drills

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CROSS CUTS• A passageway driven between the entry and its parallel air course or air

courses for ventilation purposes. Also, a tunnel driven from one seam to another through or across the intervening measures; sometimes called "crosscut tunnel", or "breakthrough".

• For each stope, three 3.5 m wide x 3.5 m high draw point cross cuts are driven from the footwall loader drive.

• loader cross cuts are then driven through the ore body to the hangingwall.

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SILLS• The length of the sill is usually 8 to 10 m .• The most critical one, when the final over cut sill is blasted.

DRAW LEVEL • Level where ore can be loaded and removed.• A draw level is located beneath the stoping area, and gravity flow transfers the ore to the

loading level.• The draw point consists of a loader drive, two or three draw point cross-cuts . a tip and a

ventilation return airway. Each production stope in the retreat has one LHD assigned to it.

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STOPE DEVELPOMENT• The stope development for VCR mining consists of two primary levels, the top

drilling level and the draw point level.• Where the stope drilling heights exceed 50 m an intermediate level is also

established in between the drilling and the draw level.• All three levels are accessed by drives from the ramp located in the footwall.

When development reaches the ore body, the height of the roof of the cross cut is increased to 3.5 m.

• Two different layouts are used for VCR, in the first one a single VCR drilling chamber with a width of 8.0 m and two 2.4 x 3.5 m cross cuts.

• In the second one two smaller VCR drilling chambers are excavated to about 6.0 m width which results in improved stability for the chamber

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DRILLING PATTERN

SOURCE:http://www.slideshare.net/VCR-drilling-blasting

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BLAST HOLE ARRANGEMENT:

SOURCE:http://www.ct.ufrgs.br/laprom/Underground%20Mining%20Methods.pdf

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BLASTING• Energex cartridge slurry explosive is used, it very resistant to water and has a density of

1.3g/cc.• In order to ensure filling of the hole and to avoid decoupling, the cartridges are slit so that

they can easier expand in the hole.• The explosive power is concentrated to a length of six times the hole diameter to form a point

charge. The technique used for blasting the stopes is known as the cratering technique and each blast breaks about 2.5 m to 3m of the stope back.

• The holes are plugged using 110 mm diameter x 450 mm length wooden split plugs.• For stemming dry and uniform sand is used . • Each VCR blast gives an average of 2.5 m break and gives about 2 Kt of ore. In most stopes

fragmentation of ore is very good giving an average of 0.5 m x 0.4 m x 0.3 m. • After each blast the length of the holes are measured to identify where the ground level in the

stope is and to decide which holes should be blasted next..

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BLASTING PATTERN……

SOURCE:slideshare.com/undergroundblasting

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SOURCE:slideshare.net/craterblastingtechnique

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SPECIFIC PROBLEMS HAVE BEEN ENCOUNTERED DURING STOPE BLASTING• Hole closure because of poor ground conditions in some areas.• sand baking because of too much stemming and wetness.• Blow outs because of insufficient stemming or poor placement of the charge.• coning of the hole which makes it difficult to plug the bottom of the hole.

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SUPPORT:• Roof bolt: 2.4m long 12-14 ton capacity, 500mm minimum thread length,

150mmx150mmx4mm thick, dog-eared face plate, installed on square grid.

• Maximum allowable bolt protrusion is 50 -100mm, for both roof bolt and threaded cable bolts, and 500mm for plain cable bolts.

• All shallow dip holes should be flashed out of chippings before grouting roof bolt.

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VENTILATION

• The ventilation system in VCR MINING METHOD is based on a technique that creates a pressure difference between the mine and the surface by sucking air out of the mine and letting fresh air in.

• The reason for the high temperatures is that air is heated by both autocompression, which is a function of surface temperature and depth, and by thermal radiation from the host rock.

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ANALYSIS OF GIVEN CONDITIONThe ore in a stope block is drilled with ITH drill rigs positioned in the overcut.

Holes are drilled downward until they break through into the undercut.

Vertical holes are preferred wherever possible. Hole diameters vary from 140 to 165 mm, although holes

205 mm in diameter have been tried in a few mines.

For a 165mm dia hole , hole pattern of 4 by 4m is typical.

Holes are charged from the overcut using powerful charges contained in a short section of blast hole.

The ore is mucked from stopes through the undercut using remote-controlled LHDs or recovered by a

drawpoint system underneath the stope as in sublevel stoping.

The powerful VCR charges involve higher risks for damaging the surrounding rock than sublevel open stoping.

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BLASTING ANALYSIS….• Spherical charges should be placed to obtain the maximum cratering effect. Gravity enlarges the crater

dimensions

Assuming hole diameter=165 mm and Blast hole pattern of 4 by 4m

• Let the stope of operation has length(along strike)=40m

• Width of stop is assumed to be 40m (30-50m)

• As we can control no. of blasts in a shift rather than depth of blast which depends upon drilling machine.

• So assume drill depth to be 100m.

• So volume of blast=(40x40x100)=160000cubic meter

• Assuming specific gravity of ore=2,

• Total output per round of blast=160000x2=320000kg(320 ton)

• This is close to per shift production(1000/3=333.33tn)

• So we require 1 blasting in the stope per shift.

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PRODUCTION ANALYSIS:• Given Production Target is 1000tn/day

• Asuming 3 shifts per day, production per shift=1000/3=333.33tn

• Assuming Capacity of Remote control LHD is 3tn

• So no. of cycles=333.33/3=112(assuming 1LHD) per shift

• Assuming no. of cylces by 1LHD is 22, no. of LHD required is 112/22=6

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PRODUCTION ANALYSIS:• Assuming in one day avg. 2 cycle is completed in 3 shifts and working on drilling over 4*4 m^2

• On charging of 1m of hole, let 1.5m retreat ore is produced by a single blasting.

• Volume of ore produced in one cycle=4*4*1.5=24 m^3

• Volume of ore produced in one day = 24*2=42 m^3

• Assuming some extra production of 8m^3, total production per cycle is 50 m^3

• Assuming 50 m^3 are produced by development of drives and crosscuts.

• So in one stope=50+50=100m^3 per day

• Let in one day 5 stopes are depillared then100*5=500 m^3 per day

• Assuming copper ore which has specific gravity 2

• So total production per day=500*2=1000m^3

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VENTILATION ANALYSIS:• Mine Rules

• Air velocity should be 0.5-3 m/sec and recommended is 1.2 m/s

• 2.5 m^3/min per tn of production

• 6m^3/min per employee worked in mine

• So for per shift production 333.33 tn(1000/3) required quantity of air should be 2.5*333.33=8333.25

m^3/min=138.88 m^3/sec

• Assuming 600 men are working in a shift so required quantity of air should be 600*6=3600m^3/min

• To have efficient ventilation

• Resistance should be minimized

• Use of control devices should be reduced

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Assuming in one crosscut

• Quantity of air required=20m^3/sec

• Length of air way=300m

• Cross section of air way=2.5m*3m

• Resistance coefficient=0.01 NS^2m^-4

• Pressure required & Power of ventilation/air power required=

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BACKFILLING:• It is done for further extraction of ore in next stope.

• It should be done in such a way that it should sufficient enough such that it act as self

supporting during the extraction of other.

• Upon completion of the ore extraction, the stope is often backfilled from the top drift,

providing rock stability for upcoming blasts.

• It can be backfilled with cemented fill to provide wall support for the blasting of successive

stopes.

• This process is repeated until the ore body is mined.

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ADVANTAGES:Higher tonnage per day and lower stoping cost.

Lower development cost since it eliminates raise boring and slot-cutting.

Increased safety of operations because drilling and blasting are carried out from above and there

is no need for the miner to enter the actual stope.

Improvement in fragmentation (the method yields lowest powder factor).

Reduced labour requirements and drilling and charging time.

Reduced dilution and over break.

Elimination of up-hole drilling and up-hole loading of explosives

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CONCLUSION

• For the given production target• Hole diameter should be 165mm• 1 blasting is required per shift in one stope• No. of LHD required 6• 5 stopes can be extracted per day• Quantity of air should be 138.88 m^3 per sec• Support can be done with roof bolting and wire mesh• At last supports can be backfilled with cemented fill

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REFERENCES:• Mine environment and ventilation by G.B. Mishra

• Ground control by S. peng

• Geotechnical aspects of vertical crater mining method in a deep mine by s.c. goel

• Techniques in ug mining  by Richard E. Gertsch, Richard Lee Bullock

• http://www.slideshare.net/underground-blasting

• http://www.slideshare.net/short-delay-blasting

• http://www.eolss.net/ug metal mining.pdf

• http://www.ct.ufrgs.br/laprom/Underground%20Mining%20Methods.pdf