Explosives Used for Civil Engineering Works

• Rock blasting

– Explosives are used to breakup rock and hard soils for tunnelling, quarrying and for excavation works where mechanical plant cannot perform the task.

• Demolition

– Used for wide range of structures

• Chimneys, cooling towers, complex structures.

– BS 5607

Types of Explosives

• An explosive charge on detonation reacts to form a large volume of gas at high temperature. The release of the gas is almost instantaneous and produces high pressure shock waves which continue until the reaction is complete.

– Slow explosives

– High explosives

– Initiating explosives

Slow explosives

• Black powder

– Mixture of saltpetre, charcoal and sulphur

– Ignited by exposure to an open blame, relatively low gas pressure produced

– Main effect is a burst in action causing displacement and fragmentation

– Used in quarrying and in safety fuse court

– Velocity of detonation about 600 m per second

High explosives

• Nitroglycerine

• Ammonium nitrate + fuel oils AN/FO

• Slurries

Nitroglycerine

• The action of nitric and sulphuric acids on glycerine

• The mixture is highly sensitive to impact and temperature and can be detonated by the action of a gentle blow.

• Nitroglycerine explosive compounds are often referred to as dynamite

Ammonium Nitrate

• Nitroglycerine-based explosives are expensive

• Mixture of ammonium nitrate and oil (AN/FO) provides good alternative

– Can be poured directly into a drill hole

• AN/FO is not water resistant and therefore needs gelatinising

Slurries

• Consist of aluminium particles, or TNT (trinitrotoluene) plus stabilising compound, suspended in water

– Good in wet

– More expensive than AN/FO

Initiating explosives

• Required because high explosives are not sensitive to detonation by spark or flame

• Extremely sensitive and easily exploded.

– Produces sufficient shock and temperature rise to induce a reaction in the high explosive.

– Used to form detonators

– Can be ignited by safety fuse cord or electrically.

Safety fuse cord

• Black powder core sheathed in plastic.

• Designed to burn at a rate of approximately 100m/s and a short length is led to the detonating cap embedded in the high explosive.

– Can be ignited by flame but usually linked to igniter cord which has burning rates between 3.5 m per second and 50 m per second.

Explosion in soil and rock

• An explosion converts a chemical substance into a gas which then produces enormous pressure.

• The process the rapid 2000 to 6000 m per second shattering the rock adjacent to the explosive and exposing the surrounding areas to stress.

Explosion in soil and rock

• Upon the ignition of a spherical charge placed in rock a shock wave is propagated, causing crushing and possibly liquefaction of the rock.

• Radial cracks are also formed which fade out with increasing distance from the wave front, as the energy is dispersed to a greater volume of the medium.

Explosives and demolition

• Telescoping

– Cooling towers

• Toppling

– Chimneys and bunkers

• Implosion

– High rise buildings

• Progressive collapse

– Long buildings