Over Burden Side Casting

OVERBURDEN SIDE CASTING BY BLASTING

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OPERATING LARGE

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OPERATING LARGE

OPENCAST COAL MINES

IN A COST

EFFECTIVE WAY.Author: Partha Das Sharma, (B.Tech-Hons.),

E.mail: [email protected]

OVERBURDEN

SIDE CASTING

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BY

BLASTING


Advantages of Overburden Blast Casting

• Saving in operating and capital cost of excavatingequipments for removal of overburden.

• Saving in maintenance cost of excavatingequipments for removal of overburden.

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equipments for removal of overburden.

• Time for removal of casting overburden byexcavating machines is reduced and therebyproductivity is increased.

•The saving in operating, power, capital and maintenance cost of excavating machinery.


Advantages of Overburden Blast Casting

•••• Because of reduction in requirement of sparesfor maintenance etc., other hidden cost related toinventory management of spares of excavatingmachine also reduced.

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machine also reduced.

•••• Smaller size of excavating equipment neededwith lesser manpower, as they have to handlecomparatively lesser volume of overburden.

•••• If there is mismatch of capacities of excavatingequipment, by overburden blast casting, Coalproduction can be improved.


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Blast design parameters:

• NATURE OF OVERBURDEN, STRATA CONDITION AND GEOLOGY

• NATURE AND CHARACTERISTICS OF EXPLOSIVES

• PARAMETER DESIGN AND BLAST GEOMETRY

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• PARAMETER DESIGN AND BLAST GEOMETRY

a) Fixing of Spacing and Burden

b) Stemming

c) Blast hole Inclination

d) Blast hole Diameter

e) Initiation sequences and selection of Delay Intervals


NATURE AND CHARACTERISTICS OF EXPLOSIVES:

It has been seen in developed countries like USA, Canada, Australia etc., high energy explosives can give better casting results.

However, due to cheapness ANFO is being used in dry holes for casting purposes.

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for casting purposes.

In India, due to non-availability of better quality of prilled Ammonium Nitrate most of the overburden blast casting experiments in open pit workings was done with high density aluminized cartridge Slurry explosives or with Site mixed Bulk Emulsion explosives.

In fact, in overburden blast casting the energy of explosives going down in boreholes is most important factor.



A study of blast casting with various types of explosives using high-speed photography showed that by maintaining constant hole diameter, burden could be increased when going from ANFO to higher energy explosives.

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At the same time for a given hole diameter and constant burden, throw velocities could also be increased when going from ANFO to higher energy explosives.

Side Casting requires increased burden ejection velocity.

This can be accomplished only by increasing Energy of explosives per unit of mass of overburden thrown.



The way of increasing energy is to keep higher specific charge, i.e., at higher Kg of explosives per Cum of rock.

As in cast blasting the ejection velocity and lateral displacement of burden rock is most important, and are related to the weight rather than the volume, thus energy factors of the explosives used must be used in designing pattern for blast

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explosives used must be used in designing pattern for blast casting.

Therefore, it is the amount energy rather than the quantity of explosives that controls the displacement of rock.

It may be mentioned here that, breakage mechanism of rock by explosives is done by two ways, i.e., by stress wave generated from explosive detonation (Shock) force and by the borehole pressure created by the detonation gas pressure.



Majority of shattering in rock, rock breakage, fragmentation etc. are effected by stress wave generated by explosive and detonation gas pressure is mainly responsible for heaving and throw.

As better-fragmented rock (acquires more surface area) is displaced more effectively at longer distance than poorly fragmented rock, the explosive characteristic should be such that it provides sufficient shock wave for achieving better fragmentation and

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sufficient shock wave for achieving better fragmentation and considerable amount of gas energy for maximum throw of blasted overburden.

It has been observed that, Emulsion explosives address both shock and gas energy well and effective in overburden blast castings.

For effective overburden blast casting, many authors have suggested a higher specific charge with higher explosives energy, by a factor of 1.4 to 1.6 than the conventional blast.


OTHER RELATED ASPECTS WITH REGARD TO OTHER RELATED ASPECTS WITH REGARD TO OTHER RELATED ASPECTS WITH REGARD TO OTHER RELATED ASPECTS WITH REGARD TO

OVERBURDEN CASTINGOVERBURDEN CASTINGOVERBURDEN CASTINGOVERBURDEN CASTING ::::

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• Relationship between Bench height and Bench width

• Characteristics of Overburden material

• Ground vibration due to overburden blast casting


Relationship between Bench height and Bench width:

Higher the height of Bench more percentage of material thrown into spoil dump.

Percentage of cast also depends upon width, i.e., distance of high wall from the free face.

More energy needed to throw overburden, if the distance of

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More energy needed to throw overburden, if the distance of high wall is more from the free face, i.e., if width of bench is more.

Thus, there exists relationship between the percentage of blasted material thrown and H/W ratio (Bench height and bench width ratio).

For successful overburden blast casting H/W ratio should be kept between 0.7 and 1.0.

OVERBURDEN SIDE CASTING BY BLASTINGRelationship between percentage of cast & Bench height and width

(H/W)

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Characteristics of Overburden material:

Overburden blast casting is suitable in hard and medium hard rock, when heavy blasting is required for fragmentation.

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In general, overburden blast casting is not feasible enough in unconsolidated, loose rock like soft clay, loose sand etc., which do not require blasting at all.

Blast casting can effectively be done in dry as well as wet ground conditions, only suitable explosives are to be selected as per ground condition.


Ground vibration due to overburden blast casting :

For effective blast casting of overburden the requirement of specific charge (i.e., requirement of explosives in Kg per Volume of rock blasted in Kg/ Cum) is quite high as compared to blasting required for conventional workings.

Because of that, the ground vibration, in case of overburden blast casting is higher than that of conventional method of

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blast casting is higher than that of conventional method of workings.

But strangely, the degree of increase in ground vibration is not as per the increase in specific charge.

This may be because in successful overburden blast casting the excess energy of explosives used for better fragmentation and for moving the blasted material away from the blasting site and not in generating ground vibration.


Ground vibration due to overburden blast casting :

Since, slightly higher ground vibration is generated in case of overburden blast casting, it is suggested to adopt this technique in the mines where residential / industrial buildings and permanent structures are far away from blast site.

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Mines closer to permanent structures should carryout some experimental blasts to assess the effects on ground vibrations before carrying out this technique on a regular basis.

Techniques such as, use of pre-splitting of production blast and introduction of some delays for group of holes in a row subject to restriction of maximum charge per delay, may be tried, in order to reduce ground vibration.

OVERBURDEN SIDE CASTING BY BLASTINGPOST BLAST PROFILE OF BLASTED MUCK

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qA sizable portion of the overburden is thrown in de-coaled area.

q The maximum depression is generally observed at the high wall side from the top and the blasted muck profile smoothly goes towards de-coaled area resulting notable depression at the free face side also.


Blast pattern and Initiation sequences of Blast Castings:

Initiation of sequences and selection of delay intervals are one of the most important criteria in overburden blast casting.

Effectiveness of casting can be increased when all the holes in the rows are detonated simultaneously and

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holes in the rows are detonated simultaneously and subsequent rows are detonated with milli-second delays.

Here our prime objective is to have throw, row by row without hindrances. For that, clear free faces are to be achieved for each row and the movement of materials of each row should not have any hindrances like air collisions or flight collisions of the fragmented material.


Blast pattern and Initiation sequences of Blast Castings:

For that purpose, row-by-row initiation sequence is preferable for creation of free faces at the front of each row.

Secondly, the delay intervals between rows should be selected in such a way to prevent any hindrances in movement of fragments like air collision.

It has been observed with increasing inter- row delay intervals from front to rear row the casting of overburden is better.

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from front to rear row the casting of overburden is better.

On an average, the delay interval of about 10 milli-second per meter of burden between first and second rows, while increasing proportionately to about 20 milli-second per meter of burden between fifth and sixth rows, should be given.

Where there is restriction in respect of ground vibration, a group of holes in the row, subject to allowable maximum charge per delay can be fired with minimum delay interval between the groups of holes in the same row.


Typical Blast Initiation Sequence and Delay Patterns

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Typical Blast Initiation Sequence and Delay Patterns

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Modern Technology & Equipments used:

Some of the modern equipments available which can be employedpurposefully are:

q Modern surveying equipment and its computer software provided awealth of valuable information on a blast and post-blast scenario. Theseinclude overburden – coal ratio, optimum front hole location, customloading information for front row hole burden, pre-blast mapping, post-blast mapping & determination of cast / throw, volumetric calculation for


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blast mapping & determination of cast / throw, volumetric calculation forswell factor and cast percentage for basis of evaluation of economicadvantages.

q High Speed Photography recording is very efficient and extremely usefulinstrument for analyzing of blasts. Any hindrance in free movement ofprojectiles or air collision of fragments can be detected and accordinglydelay sequence timing can be adjusted. Now a days, high speed photographyto the tune of more than 500 frames per second is in vogue, which is veryuseful for the purpose.


Modern Technology & Equipments used:

q Velocity of Detonation Recorder is another useful instrumentwhich can be used to determine explosive performance in the hole.

q Seismograph or Ground vibration recorder is essential indocumenting the un-wanted side effects of blasting such as Groundvibration and Air blast.

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Conclusion :

q Thus, it has been observed from the above examples that the technique of overburden side casting by blasting is very much cost effective.

q The optimum blasting parameter is mine specific. Therefore, moreexperimental blasts required to carryout, in order to establish thetechnique in a particular mines for effective cost benefits.

THANK THANK THANK THANK YOUYOUYOUYOU

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Over Burden Side Casting