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Equipment:

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For major steelwork demolition.

Specialised explosives.

Quick and clean.

Work conducted on intact structure.

The principle of high energy cutting devices, or shaped charges as they are commonly known, has become an increasingly important demolition tool.

As early as 1880 scientists noted that if an explosive charge was hollowed, the hollow was mirrored in the target after initiation of the charge. Over the years the hollow has been modified to take on specific shapes such as wedges and cones.

By inserting a thin layer of material - usually a metal like mild steel, lead, copper, silver or sometimes even gold - to line the hollow, the jet formed by the explosive becomes a thin stream of the liner material.

As the detonation wave sweeps over the liner it collapses, forming the jet and slug (see diagram 3 below) which will cut through the structure on which the charge is placed.

The most advanced forms of shaped charges are used in space programmes in applications such as rocket separation. These charges, however, have a velocity of detonation of 9 500 metres a second, much higher than the 7 500 metres a second common to the more general applications.

Shaped charges have been widely used within the demolition industry, enabling steel framed structures to be demolished in much the same way as reinforced concrete structures.

The principle of shaped charges has made it possible to demolish, with maximum safety and precision, complex structures which would have been dangerous to demolish by other means.

Cross-section of charge prior to detonation, showing metalic sheath and explosive core charge.

Immediately after detonation, before sheath vaporizes.

The sheath has vaporized, and the cutting jet of vaporized metal begins to take shape and form a slug.

The jet slug is half formed. At this stage some cutting would occur, but it would be very inefficient.

The jet slug is fully formed and is at the optimum time for cutting.