OVERBURDEN SIDE CASTING BY BLASTING *** OPERATING LARGE OPENCAST COAL MINES IN A COST EFFECTIVE WAY. Author: Partha Das Sharma, (B.Tech-Hons.), E.mail:
[email protected]
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OVERBURDEN SIDE CASTING BY BLASTING 2
OVERBURDEN SIDE CASTING BY BLASTING Advantages of Overburden Blast Casting • Saving in operating and capital cost of excavating equipments for removal of overburden. • Saving in maintenance cost of excavating equipments for removal of overburden. • Time for removal of casting overburden by excavating machines is reduced and thereby productivity is increased. •The saving in operating, power, capital and maintenance cost of excavating machinery. 3
OVERBURDEN SIDE CASTING BY BLASTING Advantages of Overburden Blast Casting • Because of reduction in requirement of spares for maintenance etc., other hidden cost related to inventory management of spares of excavating machine also reduced. • Smaller size of excavating equipment needed with lesser manpower, as they have to handle comparatively lesser volume of overburden. • If there is mismatch of capacities of excavating equipment, by overburden blast casting, Coal production can be improved. 4
OVERBURDEN SIDE CASTING BY BLASTING
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OVERBURDEN SIDE CASTING BY BLASTING Blast design parameters: • NATURE OF OVERBURDEN, STRATA CONDITION AND GEOLOGY • NATURE AND CHARACTERISTICS OF EXPLOSIVES • 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 6
OVERBURDEN SIDE CASTING BY BLASTING 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. 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. 7
OVERBURDEN SIDE CASTING BY BLASTING
NATURE AND CHARACTERISTICS OF EXPLOSIVES: 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. 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.
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OVERBURDEN SIDE CASTING BY BLASTING NATURE AND CHARACTERISTICS OF EXPLOSIVES: 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 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. 9
OVERBURDEN SIDE CASTING BY BLASTING NATURE AND CHARACTERISTICS OF EXPLOSIVES: 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 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. 10
OVERBURDEN SIDE CASTING BY BLASTING
OTHER RELATED ASPECTS WITH REGARD TO OVERBURDEN CASTING : • Relationship between Bench height and Bench
width • Characteristics of Overburden material • Ground vibration due to overburden blast casting 11
OVERBURDEN SIDE CASTING BY BLASTING 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 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. 12
OVERBURDEN SIDE CASTING BY BLASTING Relationship between percentage of cast & Bench height and width (H/W)
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OVERBURDEN SIDE CASTING BY BLASTING
Characteristics of Overburden material: Overburden blast casting is suitable in hard and medium hard rock, when heavy blasting is required for fragmentation. 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. 14
OVERBURDEN SIDE CASTING BY BLASTING 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 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. 15
OVERBURDEN SIDE CASTING BY BLASTING 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. 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. 16
OVERBURDEN SIDE CASTING BY BLASTING POST BLAST PROFILE OF BLASTED MUCK
q A 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. 17
OVERBURDEN SIDE CASTING BY BLASTING 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 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. 18
OVERBURDEN SIDE CASTING BY BLASTING 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. 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. 19
OVERBURDEN SIDE CASTING BY BLASTING Typical Blast Initiation Sequence and Delay Patterns
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OVERBURDEN SIDE CASTING BY BLASTING Typical Blast Initiation Sequence and Delay Patterns
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OVERBURDEN SIDE CASTING BY BLASTING Modern Technology & Equipments used: Some of the modern equipments available which can be employed purposefully are: q Modern surveying equipment and its computer software provided a wealth of valuable information on a blast and post-blast scenario. These include overburden – coal ratio, optimum front hole location, custom loading information for front row hole burden, pre-blast mapping, postblast mapping & determination of cast / throw, volumetric calculation for swell factor and cast percentage for basis of evaluation of economic advantages. q High Speed Photography recording is very efficient and extremely useful instrument for analyzing of blasts. Any hindrance in free movement of projectiles or air collision of fragments can be detected and accordingly delay sequence timing can be adjusted. Now a days, high speed photography to the tune of more than 500 frames per second is in vogue, which is very useful for the purpose. 22
OVERBURDEN SIDE CASTING BY BLASTING Modern Technology & Equipments used: q Velocity of Detonation Recorder is another useful instrument which can be used to determine explosive performance in the hole. q Seismograph or Ground vibration recorder is essential in documenting the un-wanted side effects of blasting such as Ground vibration and Air blast. 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, more experimental blasts required to carryout, in order to establish the technique in a particular mines for effective cost benefits. 23
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