Thursday 3 June 2010

Explosives Used for Civil Engineering Works

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

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