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Principles of rock_blasting


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Principles of rock_blasting

  1. 1. Surface Drilling p6-8 23/9/02 7:50 am Page 6 TALKING TECHNICALLY Principles of Rock Blasting Combination of Factors Blasting by design results from a large number of factors, all of which need to be brought under control in order to achieve the right result. These include the choice of drillrig and tools, the layout of the holes, the explosive, and the skill of the operators. Compression Reflection Gas Pressure Geology is the governing factor, and experience is a major ingredi- Figs 1-3 Rock breaking sequence in a normal blast. ent. Atlas Copco produces drillrigs and systems to suit all rock types, and has the experience to recom- calculated, the rock mass will yield and The ratio between spacing and burden be thrown forward. will have great impact on the blasting mend the correct approach to all result, and 1.25 can be considered as ground conditions in order to achieve the optimum result. The Benching an average ratio. The optimum burden depends upon a number of parame- following outline of the principles Bench blasting is normally carried out ters, such as rock type, required frag- involved in rock blasting is a logical by blasting a large number of parallel mentation, type of explosives, hole start point in the quest for the holes in each round. Considering deviation, and hole inclination. perfect round. the blasting mechanics, with a com- Nevertheless, as large drill holes can pression-reflection-gas pressure stage accommodate more explosives, there is in consecutive order for each charge, it a distinct relationship between burden Blasting is of vital importance to have a proper and hole diameter (see figure 6). delay between each row, and even As the bottom part of the blast is To understand the principles of rock between individual holes in each row. the constricted and critical part for blasting, it is necessary to start A proper delay will reduce rock throw, successful blasting, it is used as a basis with the rock fragmentation process improve fragmentation, and limit for deciding all other parameters. The that follows the detonation of the ground vibrations. The blast should be bottom charge, normally 1.5 x explosives in a drill hole. planned so that the rock from the first burden, from where the initiation The explosion is a very rapid row of holes has moved about one should start, requires well packed combustion, in which the energy con- third of the burden, when the next explosives of higher blasting power tained in the explosives is released in row is blasted (see figures 4 and 5). than is needed in the column charge the form of heat and gas pressure. The The horizontal distance between (see figure 7). transformation acts on the rock in three the hole and the free face is the Stemming of the top part of the consecutive stages (see figures 1-3). burden, and the parallel distance hole is used to ensure that the energy Compression: a pressure wave between holes in a row is the spacing. of explosives is properly utilised. It will propagates through the rock at a velocity of 2,500–6,000 m/sec, depending on rock type and type of explosives. This pressure wave creates microfractures which promote rock fracturing. Reflection: during the next stage, the pressure wave bounces back from the free surface, which is normally the bench wall or natural fissures in the rock. The compression wave is now transformed into tension and shear waves, increasing the fracturing process. Gas pressure: large volumes of gas are released, entering and expanding the cracks under high pressure. Where the distance between the blasthole Fig 4 Delay detonation of a typical bench blast. and the free face has been correctly 6 SURFACE DRILLING
  2. 2. Surface Drilling p6-8 23/9/02 7:50 am Page 7 TALKING TECHNICALLY base type detonation velocity features m/s nitro-glycerine dynamite 5500-4500 highly adaptable cartridged gelatin excellent in smaller holes ammonium- ANFO 2500 low cost, high safety, easy to nitrate pour or blow no water resistance, contains 5-6% fuel oil Firing pattern This firing pattern provides separate delay water slurry 4000-3000 watergel basically ANFO made water time for practically all blastholes and gives resistant gel good fragmentation as well as good 5000 emulsion stable oil/water emulsion – breakage in the bottom part of the round. heavy ANFO packaged or pumpable Fig 5 Firing sequence in delay blasting. range depends on storage time also reduce and control the fly rock Table 1 Features of common types of explosives. ejected from the blast. This tends to travel long distances, and is the main cause of on-site fatalities and damage The propagation velocity varies with Practical hole diameters for bench to equipment. Dry sand or gravel different kinds of rock, and is reduced drilling range from 30 to 400 mm. having a particle size of 4 to 9 mm by cracks and fault zones. Hard, homo- Generally, the cost of large diameter constitutes the ideal stemming material. geneous rocks, with high propagation drilling and blasting is cheaper per Inclined holes give less back break, velocity, are best fragmented by an cubic metre than using small holes. safer benches and less boulders, when explosive having high velocity of deto- However, rock fragmentation is compared to vertical holes. nation (VOD). improved by higher specific drilling. An extensive range of different The explosive is initiated with deto- Types of Explosive types and grades of explosives is avail- nators which can be electric or non- able to suit various blasting applica- electric. Electric systems have the The geology frequently has more tions. A breakdown is presented in advantage that the complete circuit effect on the fragmentation than does table 1. can easily be checked with an Ohm- the explosive used in the blast. The In dry conditions, ANFO has meter to ensure that all connections properties that influence the result of become the most used blasting agent, and detonators are correct before the blast are compressive strength, due to its availability and economy. blasting. To eliminate the risk for spon- tensile strength, density, propagation The blast hole diameter, together taneous ignition from lightning, non- velocity, hardness and structure. In with the type of explosive used, will electric systems, including detonating general, rock has a tensile strength determine burden and hole depth. cord, are used. which is 8 to 10 times lower than the compressive strength. The tensile Boulders and flyrock strength has to be exceeded during come from this zone the blast, otherwise the rock will not break. High rock density requires more explosives to achieve the displacement. Burden as a function of Back break Drill Hole Diameter Stemming (length ~ burden) Practical Values Burden Column charge only light charge needed for good fragmentation Subdrilling Bottom charge = 0,3 burden Hole Diameter, mm requires well packed Spacing Equal to 1,25 Burden high blasting power Fig 6 Burden as a function of drill hole diameter. Fig 7 Charging for optimum fragmentation. SURFACE DRILLING 7
  3. 3. Surface Drilling p6-8 23/9/02 7:50 am Page 8 TALKING TECHNICALLY Hole Deviation In case α is greater than ~15˚ the hole deflects perpendicularily Collaring A main factor influencing fragmenta- misalignment to foliation (bedding). tion and the overall blasting result, is Collaring offset that the drill hole follows its designed α α path along its entire length. As straight holes are important, hole deviation should be avoided as far as possible. To make the practical outcome cor- respond to the pre-calculated blasting results, a first requirement is that the Planned blast holes are actually drilled as as- hole sumed in the theoretical pattern. This means that the holes must be collared in the exact spot, and drilled in the cor- rect direction and to the proper depth. Figure 8 illustrates various causes of Fig 10 Influence of bedding and foliation hole deviation. Precision in collaring on drilling. In-hole and hole alignment can be achieved deviation with proper surveying and mark-ups of the drill pattern grid, coupled with drill double drill steel support for improved angle indicator mounted on the feed, visibility and rod guidance. The ROC Incorrect and hole depth instrument. It is also D-series are furnished with an interme- depth Due to collar error essential to have a good view of the diate drill steel support on the feed. collaring procedure from the operator’s The most severe cause, which is Fig 8 Various causes of hole deviation. cabin. Atlas Copco equips their modern more difficult to overcome, is the ROC range of crawler drills with in-hole deviation during drilling, usually because of geological conditions. Figures 9 and 10 illustrate the influence of bedding and foliation. The drill hole tends to deviate to a direction perpendicular to the joint- ing. The longer the holes, the more accentuated is the deflection. It is often claimed that the deviation is proportion- al to the depth to the power of two. Experience shows that the approach angle of the drill bit towards the bed- ding is crucial. There seems to be a tendency for the bit to follow parallel to the bedding where the angle of approach is smaller than 15 degrees. Drilling through homogeneous rock, such as isotropic granite with sparse jointing, causes little, or no, in- hole deviation. There are various ways and means to reduce this problem: A stiff drill string, and small clear- ance between the hole and the drill string components, give straighter holes. For top hammer drilling, Atlas Copco provides TAC tubes to be added behind the bit. The usage of TAC tubes will improve the flushing and reduce the risk of getting stuck. A combination of reduced feed force and increased rotation speed gives less deviation. DTH drilling, COPROD drilling, and rotary drilling all give less deviation than top hammer drilling. Less hole depth, and consequently low benches, gives better control of deviation. Fig 9 Hole deviation in a presplit rock wall. by Hans Fernberg 8 SURFACE DRILLING