Blasting

Potential Effects

Problems due to Ground Vibration

It is clear that the main reason for complaints of vibration is not usually structural or even cosmetic damage, but concern over possible damage and/or nuisance. The threshold of human perception (sometimes as low as 0.5mm/s) is well below the levels at which damage occurs and below the levels currently required in planning conditions. Good public relations and explanations reduce anxiety and will help to reduce complaints. However, complaints continue to be received and reflect significant disturbance to the complainant whatever the reason and so potential nuisance to residents is a major planning matter. It is increasingly acknowledged that damage criteria are no longer adequate and that nuisance criteria must be adopted.

There is usually some familiarisation with a disturbance over a period of time and people gain confidence that their houses are not being damaged. Others, who may feel that their early complaints have been ignored, may become increasingly vociferous.

In areas where caves are present, great care must be taken to avoid damage, as they may be SSSIs, of tourist interest, or used by cavers for recreation. No examples of damage have been identified or reported. A study of blast vibrations in railway tunnels 7 has shown that if the tunnel is in good repair then extremely high vibration levels are required to cause any damage. Indeed, levels of up to 263mm/s were recorded with no associated damage. Many of the techniques described in this section are now being successfully applied to maintain tight control over ground vibration levels in quarries which are adjacent to caves and tunnels, as well as other important structures.

Problems due to Overpressure

Overpressure may vibrate buildings but damage would seem to be rare. Damage in the form of broken windows is possible at 140dB 8 but still extremely unlikely432. More frequently it adds to the perception of vibration and causes complaints by making windows, ornaments, etc, rattle and startling people, which is possible at 120dB9. It is probably less of a problem than ground vibration especially where the use of surface detonating cord and secondary blasting are avoided10.

Recent work has shown the effect that wavefront reinforcement caused by inappropriate detonator delays can have on increasing overpressure levels433 434 435. The burden of the front row (distance from the first row of holes to the face) is another blast design parameter which can affect the overpressure, which rises as burden increases. The effect is more pronounced in front of the face than behind it436.

The levels of overpressure and noise can be significantly affected by meteorological conditions437 430, e.g. temperature inversions. Areas in which levels are enhanced will generally be down-wind and the increase in level about 5dB. In addition, in advance of warm fronts and behind cold fronts, the overpressure waves can be channelled towards small focal zones. Although these generally occur adjacent to steep temperature gradients it is wind shear that is the more important factor. These small focal zones exist for about 10% of the time at distances of 3-4km from the site of the blast and can be in any direction relative to the wind. The overpressure increases can be as high as 15dB.

Problems due to Flyrock

Significant improvements have been made in recent years in reducing the number of flyrock incidents. It should be considered more of a Health and Safety issue than an environmental one and is a key factor to consider in good blast design, as stipulated in the UK Quarries Regulations 1999L0292.

Problems due to fumes

Blasting operations produce toxic and non-toxic gases as a normal by-product regardless of the types of explosive materials used. The orange fumes that are sometimes visible after a blast using ANFO (a mix of ammonium nitrate and fuel oil) indicate the presence of nitrogen dioxide (NO2), but carbon monoxide and nitrogen monoxide will also be produced. It is due to the incorrect burning of the explosive caused by excess water or incorrect/incomplete mixing of the ANFO. This is more frequent than it used to be, due to the increased use of site mixed ANFO as opposed to factory produced explosives. In most circumstances, the fumes dissipate readily, but if there is a temperature inversion and little wind, then they can linger at the bottom of the site and could be a potential health hazard.

Recent work 251 has examined the fume characteristics (particularly carbon monoxide, nitrogen monoxide and nitrogen dioxide) that were produced from three separate underground "dead-end" blasts using ANFO. The explosive types were: laboratory-mixed porous prill, site-mixed porous prill and laboratory-mixed dense prill. Of particular importance was the degree of control that could be achieved regarding the production of nitrogen monoxide and especially nitrogen dioxide. The work showed the importance of using the correct prill, the right amount of diesel and ensuring that it is thoroughly mixed.

There are reports of incidents 5 where carbon monoxide (CO) from outdoor blasting operations has migrated into underground enclosed spaces, including the basement of a domestic property. It appears that a unique set of circumstances combined to contribute to the high concentrations of CO, and this would appear to be a very rare occurrence.