Explosives

[edit] Introduction

Demolition is the most high risk activity in the construction sector, often involving the use of explosives. The production of sophisticated explosives and initiation methods has made the process both a fast and economic. Explosives can be controlled to precise timing and can follow a wide range of predetermined sequences allowing complex structures to collapse under their own weight.

The Explosives Regulations came into force in 2014.

The main advantages of using explosives as a means of demolition are:

• The demolition programme can be shortened.
• The duration of any nuisance to neighbors can be much shorter than more traditional demolition methods.
• There is a high degree of safety.

The basic methods of explosive demolition are:

• Telescoping. For example for cooling towers.
• Toppling. For example for chimneys.
• Implosion. For example for high-rise buildings.
• Progressive collapse. Typically used for buildings with a large floor area.

Explosives can be used in two main ways:

• Borehole charges: Holes are drilled and explosives are installed into position. This is suitable for controlled demolition of concrete, masonry and rock blasting.
• Kicking charges: Used in pre-weakened structures to collapse or displace supporting members.

[edit] Planning permission

Where demolition of any kind of building is proposed, the council may wish to be involved in agreeing how the demolition will be carried out and the site restoration plan for afterwards. Demolition is dealt with under the Building Act 1984, which generally requires six weeks notice to be given to the local authority to demolition before demolition begins.

The Construction (Design and Management) Regulations 2015 must also be complied with when carrying out demolition works, as well as a health and safety plan to be produced by the principal contractor.

[edit] Preparation

Best practice generally involves the pre-weakening of structures before any explosives are installed. A method must be devised which maintains the stability of the structure under wind loads despite partial removal of supports. If a certain direction of collapse is preferred then specific load-bearing members will be retained or removed accordingly. This should be set out in a method statement along with the type and quantity of explosives, the safety and protection arrangements and the necessary exclusion zone.

[edit] Exclusion zone

The person with responsibility for designing the exclusion zone is the explosives engineer. An exclusion zone should be established at a distance from and surrounding the structure to be demolished. It must be ensured that everyone should be outside the exclusion zone at the time of the blast. The exclusion zone is made up of four areas, each of which should be individually assessed:

• Plan area: The structure’s plan area.
• Designed drop area: Where it is intended that the structure drops.
• Predicted debris area: Where it is predicted debris will fall outside of the drop area.
• Buffer area: Between the predicted debris area and the exclusion zone perimeter.

The structure’s height is not the sole determinant of the exclusion zone size. Debris projection from explosive charges must be considered, as must potential scale of ground vibration.

The following information should be considered when planning an exclusion zone:

• Designed collapse mechanism.
• The materials used in the structure.
• Any planned pre-weakening of the structure.
• Types of explosives that are to be used.
• Charge weights and detonation sequences.
• Test blast results and historical data from explosives use.
• The topography and conditions of the site.
• The extent of the surrounding structures and whether there may need to be closures or public evacuation (this may involve contact with local police).
• Potential for noise, vibration and dust emissions.

A risk assessment must be carried out together with contingency planning for repair and cleaning to services and buildings in the vicinity that may be affected by the demolition.

[edit] Characteristics of explosives

Common to all explosives is that they react suddenly to form large volumes of gas at high temperatures. Very high pressure results from this release of gas. The properties of different specific types of explosive can be expressed in terms of their:

• Density: Enables the energy per unit length of a borehole to be varied as required by the site conditions.
• Detonation pressure: Measure of the explosive’s fragmenting ability.
• Fume characteristics: The quantity of noxious fumes that an explosive produces.
• Gas volume: The total quantity of gases produced on detonation.
• Sensitiveness: The explosive’s ability to maintain the detonation wave.
• Sensitivity: The ease of initiation of an explosive.
• Stability: A measure of chemical stability in storage conditions when stored.
• Strength: This describes the energy content of an explosive, often measured as relative bulk strength or relative weight strength.
• Velocity of detonation: This describes the speed of the detonation wave as it passes through a column of explosives.
• Water resistance: The effect of water on the explosive.

[edit] Types of explosive

The most commonly used types of explosive for construction demolition are:

[edit] Slurry explosives:

Ammonium nitrate and other nitrates in solution. Materials are sensitised by air during the manufacturing process and have good fume characteristics and are suitable for wet conditions.

[edit] Emulsion explosives:

Entrapped air is incorporated in the emulsion in microscopic spheres of glass, plastic or resin that act as bulking agents. Fine aluminium can be added to produce different strength of explosive.

[edit] Nitroglycerine-based explosives:

• Gelatine explosives. Nitroglycerine is thickened with nitrocellulose to give a plastic consistency. Gelatine explosives have high bulk strength, water resistance and propagation.
• Nitroglycerine powder explosives. Made from ammonium nitrate and a combustible material. Compositions are reliably sensitive.
• Semi-gelatinous explosives: Part-gelatine and part-powder, semi-gelatinous explosives can be adapted for specific requirements.

[edit] Ammonium nitrate/fuel oil (ANFO):

ANFO explosives are low bulk strength and not water resistant. A high blasting efficiency can be achieved when mixed in the correct proportions.

[edit] Nitramine-based explosives:

Nitramine-based explosives are useful in operations where a very high detonation pressure is required. They have good plasticity and excellent water resistance.

[edit] Related articles on Designing Buildings Wiki:

• Achieve safety in demolition.
• Asbestos.
• Building survey.
• Conservation area.
• Demolition.
• Environmental Impact Assessment.
• Health and Safety.
• Notify HSE.
• Planning permission.
• Pre-demolition and pre-refurbishment audits.
• Risk assessment.
• Site waste management plan.
• Temporary works.

[edit] External references

• ‘Introduction to Civil Engineering Construction’ (3rd ed.), HOLMES, R., The College of Estate Management (1995)
• HSE – Establishing exclusion zones when using explosives in demolition (factsheet)
• Planning Portal - Demolition