Blasting
Introduction
The purpose of mine and quarry blasting is to fracture or fragment the rock mass to enable excavation. The mechanism of fracturing is beyond the scope of this summary, but it is important to understand that not all the energy produced when the explosive is detonated goes into breaking the rock. Some of it is "lost" in the form of heat, sound (causing noise), displacement (when this is excessive it causes flyrock) and ground shaking (causing vibrations). Blasting can therefore be seen to give rise to a number of unwanted and sometimes disturbing effects, but with modern technology and techniques, these impacts can be minimised.
This section was originally written in 2004, but has been reviewed and updated using information gathered for the thematic review of ALSF projects, published in 2008. The relevant report (Reducing the Environmental Effect of Aggregate Quarrying: Dust, Noise and Vibration L0089) contains additional information which can be referenced, and a comprehensive summary of recent research into the control of environmental effects of blasting. The key findings are now included in this section.
Limestone Quarry Blast Video Clip (133K)
Vibrations
Vibrations transmitted through the ground (seismic waves) and pressure waves through the air (overpressure) shake buildings and people and may cause nuisance or, in extreme cases, damage. The effects of the two factors are difficult for even an expert to distinguish without instrumentation such as that developed as part of two ALSF projectsL0045 L0052. However, the air overpressure wave may arrive after the ground vibration by up to two seconds over a distance of 1km. The perception of both factors is likely to be stronger inside a building than outside. Figure 1 gives a good example of ground vibration entering a house at the foundation, the overpressure reaching the house at roof level, and the response of an upstairs wall in the building to both of these different vibrations. It also shows how the ground vibration can be amplified within a building.
Figure 1. The response of a plasterboard wall to ground vibration and air overpressure
Noise
Audible noise is the sound of the gasses venting from the blast area, and rocks collapsing into the muck pile. Noise is part of the pressure wave so it occurs at the same time as overpressure. It may be augmented by the rattling of windows, etc. which can be caused by the overpressure or by the structure shaking in response to the ground vibration event. Equipment is now available to determine whether noise caused by the structural response of a building is the result of ground vibration or air overpressure (the air blast)426. This needs to be identified if the effects are to be minimised
Flyrock
Flyrock is the name given to fragments of rock propelled into the air by the explosions. This is clearly potentially dangerous to people and property both inside and outside the site, and therefore should more correctly be thought of as a health and safety issue, rather than an environmental matter.
Dust
Dust can be an issue on some sites, but as the blasting takes place within the excavation it is usually confined within the workings. Dust is discussed in more detail in another section.
Fumes
Fumes can occasionally be produced by the incomplete reaction of the explosive chemical. There are a variety of causes and is recognisable by the presence of orange fumes.
Under the Quarries Regulations, 1999, a written specification must be prepared for each blasting operation to ensure, so far as is reasonably practicable, that when blasting occurs it will not give rise to danger. Other steps need to be taken to ensure that the impact of the effects listed above is minimised. To achieve this requires a comprehensive understanding of the blast design parameters and the mechanisms which control the environmental impacts.
There are a significant number of publications covering the subject of blasting which are referred to through the text and listed in the reference section. Two in particular are The Environmental Effects of Production Blasting from Surface Mineral Workings 195 and the Scottish Executive's PAN50 Annex D L0185 which forms a guide to good practice.
Continued with Blasting and Energy TOP Return to Introduction