BLAST FRAGMENTATION IN MINES

BLAST FRAGMENTATION IN MINESPresented by MGAYA FRANCIS, 2011-04-02327 GY424: ROCK EXCAVATION AND SUPPORT Course Instructor: Dr. Kinabo

CONTENTSMechanism of Rock breakage by Blasting

Factors of Blast Design

Effects of Controllable Blast parameters on Fragmentation

Effects of Discontinuities on Rock fragmentation by Blasting

Extent of Blast damage zone

ReferencesROCK BREAKAGE BY BLASTINGBlasting technology is the process of fracturing material by the use of a calculated amount of explosive so that a predetermined volume of material is broken.When an explosive charge is detonated, chemical reaction occur which,very rapidly changes the solid or liquid explosive mass into a hot gases.This reaction starts at the point of initiation where detonator is connected with explosives and forms a convex like shock wave (Compressive wave) on its leading edge that acts on the borehole wall and propagates through the explosive column.Ahead of the reaction zone are undetonated explosive products and behind the reaction zone are expanding hot gasses.

Cont..Rock fragmentation by blasting is achieved by dynamic loading introduced into the rock mass. The explosive loading of rock can be separated into two phases, the shock wave and gas pressure phase. Rapid the detonation process, the quicker the energy release from explosives mass, in the form of a shockwave followed by gas pressure, is applied to the borehole wall. In other words, faster the detonation velocity of the explosive, quicker is the energy applied to the borehole wall, and for a shorter time period.Conversely, with a slower detonation velocity, the energy is applied more slowly, and for a longer time period. The degree of coupling between the explosive and the borehole wall will have an effect on how efficiently the shockwave is transmitted into the rock.

Cont..Pumped or poured explosives will result in better transmission of energy than cartridge products with an annular space between the cartridge and the borehole wall.The pressure that builds up in the borehole depends not only upon explosive composition, but also the physical characteristics of the rock.Strong competent rock will result in higher pressures than weak, compressible rock.When the shock wave reaches the borehole wall the fragmentation process begins.When the shockwave first encounters the borehole wall, the compressive strength of the rock is exceeded by the shockwave and the zone immediately surrounding the borehole is crushed.

Cont..As the shockwave radiates outward at declining velocity, its intensity drops below the compressive strength of the rock and compressive crushing stops.The radius of this crushed zone varies with the compressive strength of the rock and the intensity of the shock wave, but seldom exceeds twice the diameter of the borehole.However, beyond this crushed zone, the intensity is still above the tensile strength of the rock and it causes the surrounding rock mass to expand and fail in tension, resulting in radial cracking.The hot gas following the shockwave expands into the radial cracks and extends them further. This is the zone where most of the fragmentation process takes place.

Important points learned through past experienceWithin the range of conventional blasting, the physical characteristics of the rock are more important than the characteristics of the explosives used and can have a greater impact on the success or failure of a blast.Final-size fragmentation is usually obtained before any appreciable rock movement or throw occurs.Rock can absorb only so much energy and only at a certain maximum rate before it will fail.The final displacement of the bulk of the rock is more a function of the duration of the gas pressure than its intensity.FACTORS OF BLAST DESIGNBlast designing is not a science, but knowledge, experience, studying and analyzing past practices in relation to rock strata & geology etc., makes blaster to achieve perfection.Thus, for a blaster, valuable tool is the file of blast reports that he builds as he gains experience.Not only do these provide evidence of the quality of his work, but they also provide a wealth of information upon which he can draw as future blasting situations develop.The two main factors in Blast design are, Controllable and Uncontrollable factors.

Controllable factors These are the factors of blast design that are manipulated by the engineer.For the purposes of blast design, the controllable parameters are classified in the following groups:

Geometric: Hole diameter, Hole depth, Sub-drill depth, Burden, Spacing, Bench height, Stemming height etc. Physicochemical or pertaining to explosives: Types of explosives, strength, energy, priming systems, etc. Time: Delay timing and initiation sequence.

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Uncontrollable factorsThese are design factors that are out of the engineers control, and should be taken into account during Blast design.These include: GeologyMaterials strength and properties ( by strength we mean the compressive strength)Structural discontinuities ( strike, dip, jointing system, faults etc.) Weather conditionsWater etc.

EFFECTS OF CONTROLLABLE BLAST PARAMETERS ON FRAGMENTATIONHeight of Bench ( H )Is the vertical height from the toe to the crest.The bench height limits the size of the charge diameter and the burden.The optimum ratio ( H / B ) is larger than 3.If H / B = 1.5 - 4, the fragments will be large, with overbreak / backbreak around holes and toe problems.If the bench height is very large, there can be problems of blasthole deviation, which will not only affect rock fragmentation but will also increase risk of generating strong vibrations, flyrock, and overbreak. Because the drilling pattern and subsequently the explosives consumption will not remain constant in the different levels of the blasthole.

Cont..Burden ( B ) and Spacing ( S )The burden is the minimum distance from the axis of a blasthole to the free face, and spacing is the distance between blastholes in the same row.These parameters depend basically upon the drilling diameter, the height of the bench and the desired degree of fragmentation and displacement.Burden values all fall in the range of 20-40 D. Excessive burden resists penetration by explosion gases to effectively fracture and displace the rock and part of the energy may become seismic intensifying blast vibrations.Small burden lets the gases escape and expand with high speed towards the free face, pushing the fragmented rock and projecting it uncontrollably, provoking an increase in overpressure of the air, noise and flyrock.

Cont..Spacing is calculated as a function of burden, delay timing between blastholes and initiation sequence.Very small spacing causes excessive crushing between charges and superficial crater breakage, large blocks in front of the blastholes and toe problems.Excessive spacing between blastholes causes inadequate fracturing between charges, along with toe problems and an irregular face.

Cont..Blasthole diameter ( D )Drillhole diameter plays an important role in the distribution of explosives in a blast. Intuitively, it has a major impact on fragmentation.For small diameter holes, due to a better distribution of energy in blasting, smaller diameter boreholes result in a lower powder factor.In the case of jointed rock, the use of small diameter boreholes is imperative, otherwise fragmentation could be unacceptable if the joints and discontinuities are widely separated and form blocks in situ.In these cases it is recommended that the spacing between blastholes be smaller than the mean separation distance between discontinuities, which necessitates smaller holes.For large diameter holes, a higher shock energy can be delivered to the rock mass, aiding fragmentation.Cont..Blasthole InclinationThe benefits of inclined drilling are better fragmentation, displacement and swelling of the muckpile, less subdrilling and better use of the explosive energy, lower vibration levels and less risk of toe appearance.

Cont..Stemming MaterialIf stemming is insufficient, then there will be a premature escape of the gases into the atmosphere which will produce airblast and dangerous flyrock.On the other hand, if the stemming is excessive, there will be a large quantity of boulders coming from the top part of the bench, poor swelling of the muckpile and an elevated vibration level.

Cont..Stemming Height ( T )The optimum lengths of stemming increase as the quality and competence of the rock decrease, varying between 20D and 60D, where D is the diameter of the borehole.Whenever possible, a stemming length of more than 25D should be maintained in order to avoid problems of airblast, flyrock, cutoffs, and overbreak.

Cont..Subdrilling ( J )If the subdrilling is small, then the rock will not be completely sheared off at floor level, which will result in toe appearance and a considerable increase in loading costs.However, if subdrilling is excessive, there will be excessive fragmentation in the top part of the underlying bench, causing drilling problems of the same and affecting slope stability in the end zones of the open pit.

EFFECTS OF DISCONTINUITIES ON ROCK FRAGMENTATION BY BLASTINGDuring designing of a blast, the rock mass is considered to be homogeneous. In reality rock contains features like joints and other discontinuities, which have a pronounced effect on the blasting results.The quality of the rock mass in which an excavation is being driven has a strong influence on the level of dynamic forces to which the rock can be subjected and sustain damage.Generally all actual rock masses have different types of discontinuities spread throughout the rock mass. Blasting in a homogenous isotropic medium naturally does not result in the samefragmentation pattern as when the medium is permeated with discontinuities.In most rock materials, fissures occur, thus reducing the explosive induced stresses due to shock wave reflections.

Orientation of Discontinuities

Blasting results are affected by the orientation of the rock mass structures.Three cases which have to be considered are i) shooting with the dip, ii) shooting against the dip and iii) shooting along the strike.While shooting with the dip back break increases, toe problem decreases resulting in a smooth floor and throw of the blast increases resulting in scattered and low muck pile.When shooing against the dip one finds less back break, more toe problems resulting in uneven floor and throw of the blast decreases resulting in higher muck pile profile.Finally, when shooting along the strike one finds that the floor can be highly sawtoothed due to the different rock types intersecting the floor. For the same reasons the backbreak is irregular.

Orientation of Discontinuities

With Dipb) Against Dipc) Along StrikeAperture of DiscontinuitiesAs joint surface separation increases, it becomes difficult to get a smooth excavation profile from blasting, as open joints hinder the crack propagation between the perimeter holes.On the other hand the chance of overbreak reduces if joints are tight and cemented.

Frequency of DiscontinuitiesIf discontinuities are present then the effective area of influence of a hole reduces.Because the gaps of the joints will not only hinder the propagation of radial cracks but will also provide easy passage for the gases to escape, thus reducing the borehole pressure.The fragmentation and heave of blasted material will be reduced as a consequence.

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The Blast induced damage zone can be defined as zone beyond the designed perimeter of a rock tunnel or excavation where rock mass has been damaged by blasting.The quality of the rock mass in this zone can be significantly reduced, thus leading to reduction in strength and stiffness with ultimate consequence on the stability of the excavation.Overbreak results in the need to remove additional volumes of material at additional cost, which both the contractors and owners strive to avoid.The remaining damaged rock can pose long term stability problems. Therefore it is important to deal with both scenarios through damage control guidelines.

EXTENT OF BLAST DAMAGE ZONEBlast damage

Cont..The extent of rock damage can be approximately correlated with the peak particle velocity, which is proportional to strain as a measure of the damage potential of the wave motion.From extensive studies made of structural damage to buildings and constructions due to detonation in a drill hole of a single charge, we know that reliable predictions of damage can be made if we know the peak particle velocity.The peak particle velocity can be predicted using the empirical equation: V = K W RWhere: V is the peak particle velocity in mm/sec, W is the charge weight in kg, and R is distance in m.The constants K, , depend on the structural and elastic properties of the rock mass and vary with each particular blasting site. Typical values for hard rock masses are : K = 0.7 m/s. = 0.7 and = 1.4.

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Some common explosives and recommended burdens used for tunneling in Sweden.

ExampleFor a hole diameter of 48mm and 17mm Gurit pipe charges, a burden of 0.8 is normal. From figure a, we find that this charge will result in a damage zone of about 0.3m. It is apparent from this example that a reduction of the damage zone can be obtained by a reduction of the charge concentration per meter of borehole. This obviously results in increased costs for drill and blast operation, but these are balanced by the advantage of a safer roof and decreased costs for grouting and maintenance.

REFERENCESErickson, K.B. Investigating The Extent of Damage From a Single Blasthole. 2014.Holmberg, R., J, Lee , P.A, Persson. Rock Blasting and Explosives. 1994.M. Z. Abu Bakar, S. M. Tariq, M.B. Hayat, M. K. Zahoor, M. U. Khan. Influence Of Geological Discontinuities Upon Fragmentation by Blasting. 2013.Saiang, D. BlastInduced Damaged Zone Studies. 2011.Sharma, P.D. Mining and Blasting Weblog.University of Arizona Lecture Notes. 2006.