The choice of demolition method depends on the project conditions, site constraints, sensitivity of the neighbourhood and availability of equipment. Implosion is one of the demolition methods. Implosion is the preferred method for efficiently demolishing large structures.
>>> Pre-blast Considerations
If it is intended to blast a building structure, the Registered Specialist Contractor (Demolition) shall carry out a comprehensive Risk Assessment Report and an Environmental Assessment Report on the effect of implosion on the affected neighbourhood. With positive results on both the risk assessment and environmental impact assessment and agreed by the relevant approval Authority, the Registered Specialist Contractor (Demolition) may begin studying the structure of the building and develop a blasting design. The design may include pre-weakening of the structure, the strategy in placement of the explosives and time delay so that the building will collapse in a safe manner. Pre-weakening of the structure may include cutting out a portion of the shear walls and other structural elements. A test blast may be conducted to verify the strength of the structural member and to fine tune the explosive design. Protection of the adjacent properties and habitats is also an important consideration.
>>> General Concerns
General concerns and good practices in controlled demolition by blasting are discussed in the following:
- Pre-weakening of the structure shall be designed to ensure the structural stability before the implosion;
- To minimize the dispersion of building debris into adjoining land after blasting, a trench or bund wall shall be installed outside the building to contain the debris, unless a basement exists;
- A good design will provide adequate and sufficient time delay to allow only one or two floors of the building debris to fall on ground level at a time in order to limit the magnitude of the impact on the ground;
- A good design will cause the structure to fall towards the center of the building and/or within the protected area;
- The design must also identify an exclusion zone to evacuate all residents or inhabitants during the blasting. The impacts of noise and dust generated during the blasting shall be considered. Radius of the typical exclusion zone shall not be less than 2.5 times the building height;
- If there are slopes and earth retaining walls or features, a geotechnical assessment shall be conducted to ensure that the blasting will not affect the stability of these features;
- The entire site shall be under 24-hour security from the installation of explosive until final blasting. Handling and storage of explosives shall be in conformance with the Dangerous Goods Ordinance, any requirements of the Commissioner of Mines and other relevant regulations. The implosion expert shall have proven experience and track records in design and supervision of blasting similar building structures to the satisfaction of the Commissioner of Mines. The blasting expert shall have acquired the relevant training and practical experience in using the proposed explosives. The blasting expert shall obtain from the Commissioner of Mines an authorization to carry out blasting. All personnel must be evacuated from the site before and during blasting;
- The Registered Specialist Contractor (Demolition) must coordinate with the government and local community to determine the best procedures in notification, schedules for the events, traffic routing, design for the sequence of events, evacuating residents, clear out personnel from the building and assigning responsibilities during blasting. For the purpose of crowd control, blasting should be carried out in the early morning of a Sunday or public holiday;
- An emergency plan shall be prepared to handle emergency situation such as premature explosion, misfire or interruption due to bad weather including thunder and lightning;
- After the explosion, the blasting expert must check to make sure that there is no unfired explosive left on site. The entire area must remain clear and under security control until the unfired explosives have been detonated or safely dealt with by the blasting expert;
- As far as practicable, non-electrical initiation system should be used to avoid the risk of pre-mature detonation by stray currents, external electro-magnetic waves or radio frequencies. The installation shall include a redundant system to ensure successful detonation. Nitroglycerine based explosives are not permitted to be used.
- The Registered Specialist Contractor (Demolition) must provide evidence of his capability to safely perform the demolition and shall illustrate to the approving authorities that the procedures are safe;
- The mode of collapse shall be demonstrated to ascertain that: (1) no part of building beyond the protected area; (2) the impact of the structural collapse will not cause significant vibration affecting any underground tunnels and any underground utilities and any adjoining properties;
- The structural safety of the building to be imploded shall be checked and certified to be sound and safe at all stages prior to implosion.
Demolishing steel columns is a bit more difficult, as the dense material is much stronger. For buildings with a steel support structure, blasters typically use the specialized explosive material cyclotrimethylenetrinitramine, called RDX for short. RDX-based explosive compounds expand at a very high rate of speed, up to 27,000 ft/s (8,230 m/s).