Blasting Fundamentals

1. Blasting Fundamentals ©Dr. B. C. Paul 2000 Note – The slides that follow present the author’s summary of knowledge commonly held by people well studied in the field. While information found here-in can be found in a variety of published sources this material does not by design or intention follow any one source of information.

2. The Role of Fragmentation in Mining • Rock masses must generally be reduced into small discrete pieces to enable materials handling and processing • Reduction is usually accomplished mechanically by ripping or scraping if softness permits • By blasting using explosives to accomplish size reduction

3. Explosive Types • Low Explosives - progress of the reaction front through the explosive chemical mixture is slower than the speed of sound • Example - Black Powder • High Explosives - reaction front is faster than speed of sound • Example - TNT and Dynamite

4. Blasting Agents • Popular explosive formulation today • Made of combinations of chemicals that are themselves not explosive • Advantage in transportation • Taking high explosives over public highways is a major permitting issue • May provide for storage of non-explosives (explosives require special powder magazines)

5. Blasting Agents Continued • Most common example is ANFO • Combination of prilled Ammonium Nitrate (a fertilizer) about 96% • Diesel Fuel about 4% • 1930’s put ANFO mix in cans and used as cheap dynamite • Late 1940s early 50s rediscovered as a bulk explosive - Texas City, Texas

6. Formulation of an Explosive • Combination of a Fuel and an Oxidizer • Fuel that is prone to fast reaction and usually produces large number of fumes and a high energy yield • Oxidizer provides for fuel to burn without kinetic limitations of atmospheric exchange • Can blast under water

7. Elements of an Explosive Detonation • Initiation energy • usually in form of heat • or shock (example - Nitroglycerine) • Oxidizer • most high explosives build it in • Confinement and Pressure • heat, confinement, and pressure are needed to boost reaction kinetics and make fuel burn with a reaction front faster than sound • Can you see what happened at Texas City

8. Initiation of a Blasting Agent • Meant to be safe - go off only when very specific design conditions are met • Cap Sensitivity • Deals with amount of shock energy to initiate • Charge in a blasting cap is not enough shock energy • generally heat alone will not provide energy threshold • Drilling into charge or getting caught in a fire generally not enough

9. Classic Blasting Initiation System • Series of electric blasting caps • Burst of electricity through the circuit initiates blasting caps • Each blasting cap contains a bridge wire similar to the element of a light bulb • The electric burst causes the bridge wire to become very hot • Below the bridge wire is a column of black powder - ignited by hot bridge wire

10. Classic Initiation • Black Powder Column functions as a delay. Variable powder column lengths allow one pulse of electricity to cause caps to go off at different pre-designed times • At base of powder column is a heat sensitive high explosive charge called a base charge • When the black powder column burns down the cap explodes

11. Initiation from a Cap • Cap is about the size of an overgrown 22 shell - could blow fingers off your hand - not blow up rock • Cap is inserted in a small hole (called a cap well) in a booster • Booster is a chunk of high explosive such as TNT or PTN • is shock sensitive but limited heat sensitivity

12. Cap Initiation Continued • Booster with a Cap inserted is called a primer • The primer is cap sensitive • When handled - can play Frisbee and Baseball with it and it won’t go off • When the cap goes off about 5 or 10 lbs of high explosive detonates • Enough energy to shatter a person or a small amount of rock

13. Blowing Things Up • The primer sits in a column of blasting agent • Shock from 5 lbs of TNT is enough to initiate the blasting agent • Main explosive charge goes off shattering the rock mass (and any stupid sucker standing in the blast area)

14. Strengths of the Modern Approach • Very limited amounts of highly explosive material used • Staged initiation ensures that main explosive charge is almost impossible to set off without design and intent • Maximizes performance of large amounts of powerful explosives

15. Initiation Alternatives • Problem of electric blasting caps is electrical surge initiates • Lighting strike can cause full detonation with people in area • large loops of wire act as antennas and can induce current from radio waves • (Remember radio silence requirements around building demolition sites) • Detonating chord - cord filled with high explosive set off by single cap

16. Detonating Chord • Cap is not eliminated but one person hooks up at the edge of the shot area after the shot is wired - greatly reduced vulnerability • Produces a surface high explosive blast - deafening sound - disturbs neighbors • Can bury the chord but still noisy and could break connection • High speed reaction through chord eliminates most opportunities for controlled delays

17. Non- El • Hollow plastic tube with an explosive coating inside • Can build delay elements into circuit • Similar to det chord but slower reaction • Limited explosive reduces noise • Still initiate with a single cap