Blast design parameters and their impact on rock fragmentation

By

CSIR – Central Institute of Mining and Fuel Research,

Dhanbad, India 826 015

Pradeep K Singh, Chief Scientist &

Professor, Academy of Scientific & Innovative Research

Fragmentation control through effective blast design and its effect on productivity appears self-evident. In actual practice it is difficult to achieve.

Reasons: In-adequate knowledge of actual explosive energy release in

blasthole.

The effect of varying initiation practices in blast design and its effect on explosive energy release.

Reliable and statistically significant analysis of fragments.

Absence of controlled blasts in production scale to generate reproducible results.

Site specificity of blast designs.

Backdrop

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Blasting operation at a mine plays a pivotal role in overall economics of any open-cast mine. Blasting subsystem affects all other associated subsystems, i.e. loading, transporting, crushing and milling operations.

Backdrop

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Criteria for a good blast can be varied depending upon results desired (i.e. good heave, loose muck, muck profile angle, ease of digging, uniform fragmentation, or normal fragment size distribution or any combination of these performance parameters)

None of the above parameters are currently

linked to study their effect on Productivity.

Backdrop

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Blasting Performance

Throw

Back-break & Wall Control

Blast Vibration

Blasting Noise

Blast Fumes

Degree of Fragmentation

Digging and Hauling Efficiency

Blasting performance

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Link Overall Blasting Performance with Productivity

Objective

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Blast output

and productivity

Blast

design compliance

and execution

Explosive performance

Conditions at the

blasting site

Drilling pattern and

blast design

Rock mass

characterization

Planning

Execution

Results

Blast Optimisation Pyramid

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BlastingTime Scale:

<~1 ms

Drilling and Cutting~10 ms

Crushing and Grinding~100 ms

Fracture and fragmentation behaviourin rock due to explosive

and other high strain-rate loads

Rock Fragmentation

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Rock Fragmentation

Essential need in practically all mining and excavation operations. Careful tailoring of explosives properties with rock properties and blast design to achieve desired fragmentation and rock movement. Fragmentation specific to each mining method (e.g. Coarse fragments but large movement in coal mining vs. fine fragments but very limited movement in gold mining). Control and prediction of blast-induced fractures to limit damage.

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Nigahi Project, Northern Coalfields Limited (NCL)

Sonepur Bazari Project, Eastern Coalfields Limited (ECL)

Experimental Sites

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Nigahi Project Stands out as a hilly plateau with elevation of about 400- 450 m above the mean sea level.

There are three coal seams namely Turra (thickness: 13-17 m), Purewa (Bottom, Top and sometimes combined thickness: 11-12 m & 7-9 m respectively) seams.

The block has 491.8 Mt of coal reserves. The mine is currently producing 14 million tonne of coal per annum.

Sonepur Bazari Project Located in the Eastern part of Raniganj Coalfields. Four coal seams viz. R-IV, R-V, R-VI and R-VII.

The mine is producing about 4.5 Mt of coal and removal of overburden is about 12 million cubic meters.

The total coal reserve of the mine is 188.26 Mt.

Physico-Mechanical Properties of Rocks

Name of the project

Rock type/Location

Compressive

strength (MPa)

Tensile strength

(MPa)

Density

(kg/m3)

Poisson’s ratio

Young’s modulus

(GPa)

Sonepur Bazari

Sandstone (dragline bench)

37.29 3.46 2320 0.23 7.05

Sandstone (shovel bench)

36.52 3.41 2300 0.23 7.02

Nigahi

Sandstone (dragline bench)

31.73 3.53 2054 0.21 3.41

Sandstone (shovel bench)

29.56 3.23 2010 0.20 3.25

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Blast Details and Analyses

The blast design parameters data collected from 91 blasts from three experimental sites are analyzed to find out its impact on rock fragmentation level. The main important parameters which decide the fragmentation level of particular blast includes:

- burden to hole diameter ratio, - spacing to burden ratio, - stemming column length, - stiffness ratio, - explosives amount and its type, - initiation mode and charge/powder factor. The near field blast vibration signatures were also recorded to diagnose the impact of delay timing on rock fragmentation.

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Impact of Hole Diameter on Velocity of Detonation

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Impact of Booster Placement on VOD of Explosives

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VOD trace when explosives were not contaminated

Impact of Cleaning of mouth of Holes on VOD of Explosives

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Deck Blasting with Different Initiation System and Resultant Fragmentation

Explosives- 415 kg

Deck-4.5 m

Explosives- 120 kg

6.3 m

20 m

450 ms 500 ms

Mean- 0.555 m (dia. of equivalent sphere) Mode- 0.412 m (dia. of equivalent sphere) Index of uniformity – 1.91

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Explosives- 415 kg

Deck-4.5 m

Explosives- 120 kg

6.3 m

20 m

450 ms 450 ms

Mean- 0.690 m (dia. Of equivalent sphere) Mode-0.412 m (dia. Of equivalent sphere) Index of uniformity – 2.17

Deck Blasting with Different Initiation System and Resultant Fragmentation

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Mean- 0.377 m (dia. Of equivalent sphere) Mode-0.191 m (dia. Of equivalent sphere) Index of uniformity – 2.19

Explosives- 415 kg

Deck-4.5 m

Explosives- 120 kg

6.3 m

20 m

450 ms

D-cord

Deck Blasting with Different Initiation System and Resultant Fragmentation

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Scattering Test Results

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Fragmented size analysis - medium hard OB bench

Loading cycle of 10 cubic meter shovel

Nigahi Project

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Sonepur Bazari Project

Fragmented size analysis - hard OB bench

Loading cycle of 10 cubic meter shovel

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Burden to Hole Diameter Ratio Vs Mean Fragment Size

Mean fragment size increases with increase in the ratio of burden to hole diameter.

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Spacing to Burden Ratio Vs Mean Fragment Size

As most of the data have little variation in spacing to burden ratio, the outcomes of the graphs are not so significant.

However, spacing to burden ratio between 1.1 and 1.3 shows good results except for a few blasts which are having low index of uniformity (n) due to presence of joints and back break of previous blast.

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Stemming Length to Burden Ratio Vs Mean Fragment Size

The data points are relatively scattered but general trend shows that mean fragment size of fragmented rock decreases with the decrease in stemming length to burden ratio

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Charge/Powder Factor Vs Mean Fragment Size

Mean fragment size decreases with increase in charge factor. A few scattered data in this graph are due to the geological discontinuities of rock mass of the blasting patch

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Stiffness (Bench Height to Burden Ratio) Vs Mean Fragment Size

It is observed that stiffness value of less than 2 gives coarser fragmentation and the best optimum value comes between 2 and 3

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Joint Plane Orientation and Spacing

Joint and bedding planes act as natural pre-splits during blasting and if possible, should be used to improve performance.

Spacing of joints within a rock mass will have significant impact on the size distribution of the blasted muck. In general, the joint spacing will also improve the fragmentation level.

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FREE FACE

Blast Design and Resulted Muck Profile at Hard OB Bench

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Blast Design and Resulted Muck Profile at Medium Hard OB Bench

FREE FACE

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Conclusions.... Optimum blasting should comprise the generation of fragment size distribution with suitable muck pile optimal for loading, which should improve the downstream operations. This study is confined to the effect of blast design parameters on the fragment size distribution of the blasted muck. The main conclusions of the study are: Mean fragment particle size increases with the increase in

the burden to hole diameter ratio. This increase was mainly due to the increase in burden as the hole diameter was kept constant.

Mean fragment size and index of uniformity (n) of the blasted

muck decreases with the increase in the spacing to burden ratio. The optimum value of spacing to burden ratio in most of the blasts ranges from 1.1 to 1.3 and it resulted into excellent fragmentation.

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Conclusions

Stemming length to burden ratio was plotted against mean fragment size and the general trend shows that mean fragment size of fragmented rock decreases with the decrease of stemming length to burden ratio.

As anticipated, the increase in the charge/powder factor will

increase the rock fragmentation level i.e. decrease the mean fragment size of the rock.

Change in burden with respect to bench height has

significant effect on rock fragmentation. Therefore, the stiffness (bench height to burden ratio) value of less than 2 gives coarser fragmentation and the best optimum value was around 3.

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Perfection is achieved, not when there is nothing more to add, but when nothing

left to take away