Explosives, Theory Of Breakage And Blasting Operations

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Explosive is a compound or mixture which is capable of undergoing extremely rapid decomposition.

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Explosives, Theory Of Breakage And Blasting Operations

  1. 1. Explosives, Theory of Breakage and Blasting Operations Author: Partha Das Sharma, B.Tech(Hons.) in Mining Engineering, E.mail: sharmapd1@gmail.com, Website: http://miningandblasting.wordpress.com/
  2. 2. Introduction General types of Explosives • Commercial explosives • Military explosives 2
  3. 3. Explosive Ingredients and their Function Ingredient Chemical formula Function Ethylene glycol dinitrate C2H4(NO3)2 Explosive base – lowers freezing point Nitrocellulose (guncotton) C6H7(NO3)2O2 Explosive base – gelatinizing agent Nitroglycerin C3H5(NO3)3 Explosive base Nitrostarch Explosive base Trinitrotoluene (TNT) C7H5N3O6 Explosive base Metallic powder Al Fuel sensitizer : used in high density slurries Black powder NaNO3 + C+ S Explosive base Pentaerythritol tetranitrate C3H8N4O12 Explosive base (PETN) Lead azide Pb(N3)2 Explosive used in blasting caps Mercury fulminate Hg(ONC)2 Explosive used in blasting caps Ammonium nitrate NH4NO3 Explosive base : oxygen carrier Liquid oxygen O2 Oxygen carrier Sodium nitrate NaNO3 Oxygen carrier – lowers freezing point Potassium nitrate KNO3 Oxygen carrier Ground coal - Charcoal C Combustible, or fuel Paraffin CnH2n+2 Combustible, or fuel Sulfur S Combustible, or fuel Fuel oil (CH3)2(CH2) Combustible, or fuel Wood pulp (C6H16O3)n Combustible, absorbent Lampblack C Combustible Kieselguhr SiO2 Absorbent – prevents caking Chalk -Calcium carbonate CaCO3 Antacid Zinc oxide ZnO Antacid 3 Sodium chloride NaCI Flame depressant (permissible explosives)
  4. 4. Chemical explosives • is a compound or mixture which is capable of undergoing extremely rapid decomposition. • An explosion can be broken down into four phases • Release of gas • Intense heat • Extreme pressure, and • The explosion 4
  5. 5. Chemical explosives When the explosive is detonated, • gas is released, • temperature of the gas increases, • pressure also increases (Charles’ law). • move and break the rock. 5
  6. 6. How to compare explosives • Strength • Detonating velocity • Fume class • Detonation pressure • Water resistance • Energy • Density • Sensitivity • Physical characteristics • Sensitiveness • Storage • Flammability • Freezing 6
  7. 7. How to compare explosives • Strength : % of active material • Velocity of Detonating (VOD): is the velocity at which the detonation wave moves through the explosive (ft/s or m/s) • Fume class : the amount of toxic fumes which determine its safety to be used in particular situation such as underground operations. 7
  8. 8. How to compare explosives • Detonation pressure : is the pressure behind the detonation front. • Energy • Sensitivity : the minimum energy/pressure needed for detonation. • Sensitiveness: measure of explosion wave spreading from one stick to another. • Flammability : easiness to ignite by flame or heat 8
  9. 9. How to compare explosives • Water resistance : is the ability to resist contamination or a reduction in strength when exposed to water. Sometimes determined by the length of time it can be submerged in water and still perform as designed. • Density : is the explosive wt per given volume. Aid in blast design. 9
  10. 10. How to compare explosives • Physical characteristics: commercial explosives can take three basic forms: granular, gelatin, slurry and emulsion. The choice of form depends on the usage required. 10
  11. 11. How to compare explosives • Storage: how explosive can be stored without affecting its safety, reliability, and performance. Early nitroglycerin (NG) dynamites were extremely poor for storing due to separation of NG from the other components and creates an extremely hazardous condition. • Freezing : important for safety and performance especially in cold climate. Anitfreezing additives may be used. 11
  12. 12. Drills and Drilling • The drilling system consists of the drill: the drill steel, or rod; and the bit. The bit penetrates the rock by the force it imposes on the rock. Bits are designed for percussion, rotary drilling, or both. • Hand held drills • External –percussion drills • Down-the-hole drills • Rotary drills 12
  13. 13. Theory of Breakage Purpose of blasting • One solid piece → smaller pieces (fragmentation) → to be moved or excavated (movement). • Underground blasting, for example, requires greater fragmentation than surface blasting because of the size of the equipment that can be used and the difficulty of access. • Get the desired results with a minimum cost 13
  14. 14. Theory of Breakage Involves two basic processes: • Radial cracking • Flexural rupture • Rock is stronger in compression than in tension. Therefore, the easiest way to break rock is to subject it to a tensile stress greater than its ultimate strength in tension. • Rocks are heterogeneous (contain different types of rocks). They differ in their density. 14
  15. 15. Theory of Breakage Free face Compression Borehole waves Radial cracking 15
  16. 16. Theory of Breakage • The distance from the borehole to the free face is the burden. • The denser the rock the faster the waves • Proper fragmentation when enough to travel to the face and back overcoming the tensile strength of the rock. • Along the face the outermost edge is stretched in tension which causes cracks. 16
  17. 17. Flexural Rupture • The second process in breaking rock by bending the rock to the point where the outside edge, the side in tension, breaks. • Caused by the rapid expansion of gases in borehole. • Analogous to the bending and breaking of a beam. • Movement or displacement are required in addition to cracking. 17
  18. 18. Flexural Rupture • After detonation the redial cracks expands and the gas starts to the movement by putting a CS against the borehole wall causing its bending. • The deeper the hole, the greater the burden and borehole spacing. 18
  19. 19. Blast Design • Is the safe and economic way to do blasting • Factors affecting blasting design • Geological factors (out of blaster’s control) • Controllable factors • Borehole dia. • Burden • Spacing • Stemming • Design of the delay firing system. 19
  20. 20. Burden & spacing Burden is the distance from the blast hole to the nearest perpendicular free face. Spacing Burden 20 Free face
  21. 21. Burden & spacing determination Andersen Formula B= (dL)0.5 • B : burden, ft d : borehole dia, in • L : borehole Length, ft • Langefors’ Formula V= (db/33) [Ps/cf(E/V)]0.5 • V : burden, m db : dia of drill bit, mm • P : degree of packing = 1-1.6 kg/dm3 • s : wt strength of explosives (1.3 for gelatin) • c : rock constant, generally 0.45 • f : 1 degree of fraction, for straight hole = 1 • E/V = ratio of spacing to burden 21
  22. 22. Spacing determination Spacing is the distance between blast holes fired in the same row • It is necessary to complete burden calculations before determining the spacing. S= (BL)0.5 • B : burden, ft • L : borehole Length, ft 22
  23. 23. Controlled Blasting To control overbreak and to aid the stability of the remaining rock formation. • There are following methods: • Line drilling (unloaded), • Cushion blasting • Smooth-wall blasting • Presplitting 23
  24. 24. Controlled Blasting – Line drilling • Provides a plane of weakness to which the rock can break. • Helps to reflect shock waves, • Reduces the shattering effect of the rock outside the perimeter. • Do not exceed 3 in in dia and are spaced one to four diameters apart (due to cost). • Are not loaded • Requires more drilling more than the other controlled blasting methods. • Is not very effective in non-homogeneous formations. 24
  25. 25. Controlled Blasting – Line drilling Free Unloaded face line drill holes 25
  26. 26. Cushion Blasting • Requires a single row of holes ( 2 to 3.5 in) in dia. • Permits a reduction in the No. of holes required by line-drilling • Unlike line-drilling holes, the cushion holes are loaded with light charges. • Holes are fully stemmed between charges, allowing no air gap, and are fired after the production shot has been excavated. • The stemming acts as a cushion to protect the finished wall from the shock waves. The larger the borehole, the greater the cushion. • Not suitable for underground - tough stemming requirements. • Drawbacks: (1) requires removal of excavated material before firing (costly due to production delay – no excavation for entire area at once). (2) Sometimes the production shot can break back to the cushion holes, creating redrilling problems and causing loading changes. 26
  27. 27. Smooth-wall Blasting • Similar to cushion blasting 27
  28. 28. Pre-splitting • Creates a plane of shear in solid rows along the desired excavation before the production blast. • All holes are loaded like cushion blasting • Reduces overbreak • Reduces the vibration 28

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