Optimizing Fragmentation
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QM February 2005 www.qmj.co.uk 33 T he ratio of bench height (H) to effective firing burden (Be) represents the stiffness ratio in a surface mine bench blast. Changing the bench height, effective burden and/or the mode of firing may change this ratio in field-scale blasts.This is because, for a given burden, there exists a maximum bench height to produce a full crater.The breakage angle for a given burden increases with the increase in bench height up to a certain point.This phenomenon is attributable to the decrease in the burden rock stiffness, which makes the bench more flexible for improved fragmentation results. Lundberg et al. 1 reported a host of problems due to increased burden rock stiffness. The mechanism of rock fragmentation also considers the role of flexion in rock breakage during and after the radial fracturing and spalling phases; the pressure applied by the explosion gases in front of the explosive column acts on the burden rock mass, which behaves like a beam embedded in the bottom of the blasthole and in the stemming region.This beam is subject to breakage by deformation, fracturing and flexion, as depicted in figure 1. The potential problems in relation to the changes in stiffness ratio are summarized in the table 1. Sarathy 4 proposed the concept of optimum bench heights for varying burdens with respect to the drill hole diameter, as given in table 2. It is interesting to note that, ➤ Optimizing Fragmentation A study of the impact of stiffness ratio on the fragmentation of sandstone strata in an opencast coal mine By Dr Piyush Rai*,Abhai Kumar Ranjan** and B. Singh*** *Reader in Mining Engineering (coal mining) **Post-graduate scholar, Department of Mining Engineering, Institute of Technology, Banaras Hindu University,Varanasi, India ***Superintendent of Mines, Northern Coalfields Ltd., Singrauli, India Fig. 1. Mechanism of breakage by flexion, after Ouchterlony 2 Stiffness Ratio Values Fragmentation Air Blast Results Fly Rock Results Ground Vibration Results 1 Poor Severe Severe Severe 2 Fair Fair Fair Fair 3 Good Good Good Good 4 Excellent Excellent Excellent Excellent Table 1.Potential problems related to stiffness ratio, after Konya 3 Table 2.Optimum bench heights for different drill hole diameters, after Sarathy 4 Drill Hole Diameter, mm Average Burden, m Bench Height, m Ideal Desirable Sub-optimal 100 3–4 9–12 6–8 3–4 150 4–5 12–15 8–10 4–5 250 7–9 21–27 14–18 7–5 311 8–10 24–30 16–20 8–10
Transcript of Optimizing Fragmentation
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The ratio of benchheight (H) to effectivefiring burden (Be)represents the stiffness
ratio in a surface mine bench blast.Changing the bench height,effective burden and/or the modeof firing may change this ratio infield-scale blasts.This is because,for a given burden, there exists amaximum bench height toproduce a full crater.The breakageangle for a given burden increaseswith the increase in bench heightup to a certain point.Thisphenomenon is attributable to thedecrease in the burden rockstiffness, which makes the benchmore flexible for improvedfragmentation results. Lundberg etal.1 reported a host of problemsdue to increased burden rockstiffness.
The mechanism of rockfragmentation also considers therole of flexion in rock breakageduring and after the radialfracturing and spalling phases; thepressure applied by the explosiongases in front of the explosivecolumn acts on the burden rockmass, which behaves like a beamembedded in the bottom of theblasthole and in the stemmingregion.This beam is subject tobreakage by deformation, fracturingand flexion, as depicted in figure 1.
The potential problems inrelation to the changes in stiffnessratio are summarized in thetable 1.
Sarathy4 proposed the conceptof optimum bench heights forvarying burdens with respect tothe drill hole diameter, as given intable 2.
It is interesting to note that, �
OptimizingFragmentationA study of the impact of stiffness ratio on the fragmentation of sandstonestrata in an opencast coal mine
By Dr Piyush Rai*,Abhai Kumar Ranjan** and B. Singh***
*Reader in MiningEngineering (coalmining)**Post-graduatescholar, Department ofMining Engineering,Institute of Technology,Banaras HinduUniversity, Varanasi,India***Superintendent ofMines, NorthernCoalfields Ltd.,Singrauli, India
Fig. 1. Mechanism ofbreakage by flexion,after Ouchterlony2
StiffnessRatioValues
FragmentationAir BlastResults
FlyRock Results
GroundVibrationResults
1 Poor Severe Severe Severe
2 Fair Fair Fair Fair
3 Good Good Good Good
4 Excellent Excellent Excellent Excellent
Table 1. Potential problems related to stiffness ratio,after Konya3
Table 2. Optimum bench heights for different drill holediameters, after Sarathy4
Drill HoleDiameter,
mm
AverageBurden,m
Bench Height, m
Ideal Desirable Sub-optimal
100 3–4 9–12 6–8 3–4
150 4–5 12–15 8–10 4–5
250 7–9 21–27 14–18 7–5
311 8–10 24–30 16–20 8–10
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even under similar bench-heightconditions, blasts can be firedusing different firing patterns thatchange the effective burden5,which changes the stiffness ratioand the stiffness of the bench.Therole of effective burden whenfiring the blasts thus becomes
consequential. Studies pertainingto the stiffness ratio can be madeaccordingly.
ObjectivesThe present study investigates theinfluence of stiffness ratio on thedegree of fragmentation in field-
scale blasts in order to establish arelationship between thefragment size and the stiffnessratio.To meet the aboveobjective, blasts were conductedin one of the opencast projects ofNorthern Coalfields Ltd (NCL) inSingrauli, India.
The explosive and rockparameters were kept identical asfar as possible in order tocategorically assess the impact ofstiffness ratio alone on thefragmentation results (underalmost equivalent blast geometryparameters). In order to keep therock parameters constant, all theblasts were conducted on thesame bench with no (orinsignificant) geological variations.
The studies were performedon a moderately strong sandstoneoverburden formation withcompressive strength rangingfrom 10–25MPa and tensilestrength from 0.8–2.0MPa.Thesandstone was fine-to-mediumgrained.The coal in this particularcoalfield is poor in grade and themine is linked to a thermal powerplant.
The explosive used in theSingrauli opencast coalfield wasan emulsion comprising hydratedammonium nitrate plus oil (burntfurnace oil, wax or paraffin), plussodium nitrite as gassing agent,plus sodium mono-oleate asemulsifier, doped on site with12–20% ammonium nitrate prillsby site mixing trucks.Thisexplosive was used throughoutthe entire project with noperceptible variations in its natureand properties, hence theexplosive parameters have alsobeen assumed to be identical fordifferent blasts studied on thesame overburden formation.
Field study andmethodologyFour field-scale blasts wereconducted and studied on thesandstone overburden bench.This bench was worked by alarge-capacity shovel anddumptruck combination. Detailsof the four study blasts, withrespect to blast geometry andother relevant parameters, aretabulated in table 3.As is evidentfrom the table, the blasts wereconducted at varying stiffnessratios.To measure thefragmentation in the blastedmuckpiles, image capturing,processing and analysis techniques
IP Free Face
High Wall
PreviouslyBlastedMuck
– 100ms= 125ms
150ms==
Table 3. Influence of stiffness ratio on fragmentation
Parameters Blast 1 Blast 2 Blast 3 Blast 4
Hole diameter(mm), φ
259 259 269 269
Mesh area (m),S xB
9 x 8 9 x 8 10 x 9 10 x 9
Bench height(m), H
12.5 16 19 22.5
Sub-grade drilling(m), J
1 1 1 2.5
Stemming (m),T 4.5 5 5 5
Decking (m), DK 1 1 2 0
No. of holes 42 44 65 52
No. of rows 11 12 15 14
Explosive quantity(kg), Qt
18,634 26,565 51,350 47,832.5
Column chargelength (m), CCL
7 10 12 17.5
Initiation system Detonating Fuse Detonating Fuse Detonating Fuse Detonating Fuse
Drilling pattern Rectangular Rectangular Rectangular Rectangular
Firing pattern Diagonal Diagonal Diagonal Diagonal
Delay (ms) 0/100/125/150 0/100/125/150 0/100/125/150 0/100/125/150
Volume broken(theoretical in m3)
37,800 50,688 111,150 105,300
Effective burden(m), Be
5.97 5.97 6.68 6.68
Stiffness ratio(H/Be)
2 2.6 2.9 3.4
Mean fragment size(m), K50
0.4557 0.4069 0.3475 0.3376
Coarse fragmentsize (m), K90
0.7267 0.6815 0.5233 0.5476
Maximum fragmentsize (m), K100
0.9625 0.8415 0.6865 0.6963
Fig. 2. Representativedrilling and firingpattern for blast no. 1
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were employed.The imagingtechnique has been used withrespect to blasting andfragmentation for the past twodecades.This technique canprecisely quantify the geometricaspects of the broken fragmentsusing images in two dimensions6.The basis of this method is tocapture scaled images of theblasted muckpile using acamera in the field, and then todigitize and measure thedelineated fragments to providea measure of the particle sizedistributions7,8.
In the present study, theblasted muckpile wasdocumented with scaledphotographs captured from thefront of the blasted bench bykeeping a suitable scale atphysically convenient locations onthe huge muckpiles. Obviously asingle photo can not evaluate alarge muckpile, hence a series ofphotographs was obtainedperiodically to cover the entireexcavation sequence of themuckpile.
A number of additionalphotographs were captured in theevent of exceptional fieldsituations, such as the occurrenceof large-size boulders or a largeamount of fines, evidence ofgeological features etc.Typically200–250 broken rock fragmentscould be easily captured in oneimage frame.Analysis of 20–25photographs would yield a
statistically representative samplefor characterizing thefragmentation in one blastedmuckpile.This is as per therecommendations of Paley9,Exadyktlos and Tsourtrelis10,Panlagio and Franklin11 and Scottet al.12.
For the purposes of processingand analysing the capturedimages, Fragalyst Version 2.0commercial software, developedby Central Mining ResearchInstitute, Nagpur Centre, incollaboration with WaveletGroup, Pune, India, was used.Although a wide range of image-analysis software is commerciallyavailable, ie Wipfrag, Split,Tucipetc, Fragalyst has been wellproven under Indian conditions.
Results anddiscussionAs is evident from theobservations provided in table 3,two blasts with hole diameters of269mm and two blasts with holediameters of 259mm werestudied.A perusal of table 3indicates that blast nos. 1, 2, 3 and4 were conducted at stiffnessratios of 2.0, 2.6, 2.9 and 3.4,respectively.The variation in thestiffness ratio could be studied inthe field because of variations inthe height of the same bench atdifferent locations. Furthermore,all four blasts were drilled in astraight rectangular pattern butfired diagonally.A representative
drilling and firing pattern for blastno. 1 is shown in figure 2.Thenotation 0/100/125/150 in figure 2and table 3 signifies that theinitiation point (IP) is representedby digit ‘0’ and the subsequentdelay interval between thediagonal firing rows is 100ms,125ms and 150ms. Blast nos. 3and 4 were fired with greaternumber of firing rows (15 and 14respectively) in comparison toblast nos. 1 and 2 (11 and 12respectively). Barring these fewexceptions, which were almostinevitable under the fieldconditions, all the blasts havebeen assumed to be more or lessequivalent from the viewpoint ofblast geometry, rock andexplosive parameters. Hence,the comparative fragmentationstudy of the performance of theblasts, made on the basis of theresults obtained from imageanalysis of the blasted material,could adequately reveal theimpact of stiffness ratio (H/Be)alone.
Looking at table 3 for results offragment size and its distribution,it is quite evident that with theincrease in the stiffness ratio,fragmentation inside the blastedmuckpile improves.The results interms of cumulative fragment sizedistribution curves, as providedby the software, are illustrated infigures 3, 4, 5 and 6 for blastnumbers 1, 2, 3 and 4respectively. �
Figs 3, 4, 5 and 6.Fragment sizedistribution curves forblasts no. 1, 2, 3 and 4respectively
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Field photographs (figs 7, 8 and9) reveal the occurrence of collarboulders from the back rows andboulders inside the muckpile forblast nos. 1 and 2, whereas thephotographs captured on themuckpile of blast nos. 3 and 4(figs 10, 11 and 12) by and large
reveal good fragmentation insidethe muckpile with gooduniformity. Nevertheless, boulderswere observed along the backrows of these blasts(figs 11and 12).
The impact of a change in thestiffness ratio is clearly evident
from the relationship exhibited infigure 13, which reveals that withan increase in the stiffness ratiothere is a significant reduction inthe fragment size. Furthermore, italso reveals that there is a drasticreduction in the fragment sizewhen the stiffness ratio increasesfrom 2.5 to 3. Beyond a stiffnessratio of 3, the curve is observedto be almost flat.The findings arein line with earlier work reportedby Konya3.
Smith13 also reported that anincrease in bench height for agiven burden makes the benchmore flexible, giving betterfragmentation. Lundborg et al.1
mentioned that a smallerstiffness ratio implies greaterstiffness, which, in turn, offersgreater resistance to breakagein bench blasts.The poorbreakage along the collar regionmay be attributed to theincreased bench stiffness5.
Fig. 7. Presence of large collar boulder (blast 1)Fig. 8. Poor fragmentation with boulders inside the muckpile (blast1)
Fig. 9. Large boulders coming from the back rows (blast 2)Fig. 10. Good fragmentation inside the muckpile with occasionalboulders from the back rows (blast 3)
Fig. 11. Good fragmentation with good uniformity inside themuckpile (blast 4) Fig. 12. Oversizes along the back rows (blast 4)
Fig. 13. Graph showingthe relationshipbetween stiffness ratioand screen size forblast nos. 1, 2, 3 and4
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Drilling, Blasting & BreakingSarathy14 also reported that abench height-to-burden ratio of2.5 to 3 provides the idealsituation for fragmentation assmaller values than this result in a‘cratering’ effect, which generatesflyrock, airblast, vibrations andoversizes.
ConclusionBased on this case study, it can beconcluded that for moderatelyhard, medium-to-fine-grainedsandstone benches, stiffness ratioexerts considerable impact onfragmentation.
For such benches the fragmentsize first decreases and thenincreases with an increase instiffness ratio.The point ofinflexion corresponds to amagnitude of almost 3, whichmeans that for moderately high,medium-to-fine-grained sandstonebenches, optimum results interms of fragment size anddistribution are obtained atstiffness ratio of almost 3.
AcknowledgementsThe authors would like toexpress their gratitude for theassistance and co-operation ofstaff, officers and management ofNorthern Coalfields Ltd. �
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