Blasting
Acceptable Levels Page 3.
Acceptable Levels of Overpressure
BS6472-2: 2008 21 quotes research by the USBM which indicates that a poorly mounted window that is pre-stressed can crack at 150dB, with most windows cracking at around 170dB or higher432.
Limits for planning conditions have been set at various levels, including 120dB and 133dB. USBM limits28 are 134dB for nuisance. At this level 5% of residents would be expected to complain that they were startled and frightened; even 120dB will lead to rattling windows, feelings of annoyance and fright. It is suggested that, because of the variability in the levels from apparently similar blasts, a 95% confidence limit should be used rather than an absolute one.
It is suggested 9 that techniques for reducing vibration will also reduce overpressure and that modern techniques of blasting produce lower overpressures anyway435. However, others found that over a five year period the proportion of ground vibration readings over 2.5mm/s reduced from 50% to 1.5% but that complaints had increased. It was concluded that this was due to overpressure because the complaints occurred in weather conditions likely to enhance overpressure30 437.
Acceptable Levels of Noise
Limits have been set for unreasonable nuisance from the very short-term audible noise at 94dB(A), and 98dB(A).32 To avoid potential problems, it is clearly preferable to blast at the noisier times of the day. There is some suggestion that livestock may be affected by noise/vibration. Direct noise from the blast is rarely a problem if the use of exposed detonating cord is avoided. However there is sometimes a problem with rattling and vibrations that take place as the building responds acoustically to the ground or air overpressure vibrations. As this process is not fully understood and there is such variation in structures, no limits have yet been set for acoustic response.
Acceptable Levels of Flyrock
No acceptable levels of flyrock have been suggested, other than to avoid it altogether outside the site boundary, and to minimise the amount of rock travelling more than a few metres from the muckpile within the site. If the assessment of the danger zone suggests it may extend beyond the site boundary, then the Approved Code of Practice for Shotfiring Operations requires a controlled space to be created, where people are either excluded or protected, and not endangered by the blast.
It is said within the minerals industry that it is not possible to be sure of avoiding flyrock or of predicting when significant instances are likely to happen. Nonetheless, the Health and Safety Executive say 31 that 83% of flyrock incidents might have been avoided if the whole operation had been supervised by a person with knowledge of explosives, blast design, controlled drilling and careful observation of quarry faces (using one of a number of different types of profiling equipment), and accepted blasting practices had been followed.
Use of the good practice described below should reduce both the severity and frequency of significant flyrock mishaps. They will not however be entirely avoided and buffer zones between the active face and sensitive areas will remain necessary.
Acceptable levels of fumes
Although no guidelines are given for control of fumes in open pit environments, general limits are given by Health and Safety Executive and updated annually.33 In April 2003, two Chemical Hazard Alert Notices for Nitrogen Monoxide (CHAN28) and Nitrogen Dioxide (CHAN29). This was based on a review of evidence by the Health and Safety Commission's Working Group on the Assessment of Toxic Chemicals (WATCH), who decided the current Occupational Exposure Standards may not be adequate to protect occupational health. The implications of these changes are more likely to be felt by the employees rather than local residents and so no further detail will be given here.
Table 2. Exposure limits for NO, NO2 and CO
|
Long Term Exposure Limit* |
Short Term Exposure Limit** |
Nitrogen Monoxide |
Was 3ppm, Now 1ppm |
Was 5ppm, Now no limit given |
Nitrogen Dioxide |
Was 3 ppm, Now 1ppm |
Was 5 ppm, Now no limit given |
Carbon Monoxide |
30 ppm |
1200 ppm |
| * 8 hour time weighted average reference period | ||
| ** 15 minute reference period | ||
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