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.

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

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

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)

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

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

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

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

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

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

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

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

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)

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

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

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

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