Of course the phrase "acceptable levels" can be quite a controversial term, in that what is deemed acceptable to one person may not be deemed acceptable to another. In this context, acceptable levels is used in reference to widely recognised standards, guidelines or established practice.

A number of guidelines and standards have been published which outline the likely vibration levels at which damage might commence. They are based on different surveys in different types of structure, so the results are quite different. While the USBM data is widely known, the British Standard is now taken as appropriate for UK residential properties.

The threshold of perception of vibration is very much lower than the onset of even cosmetic damage (plaster cracking). Humans may well start to notice vibration levels at around 0.2-0.5mm/s PPV. This compares to a level at 10Hz of 12mm/s for cosmetic damage, eg plaster cracks, and 18mm/s for structural damage to residential properties based on the USBM curves¹⁴(shown in Figure 3). It is important to remember that the values are frequency dependent (vibration levels resulting in damage rise at higher frequencies), that they refer to US type residential properties, and that exceeding the vibration level does NOT mean that damage will definitely occur.

BS7385:Part 2³⁷ provides damage curves suitable when considering UK domestic structures (Figure 4). During an extensive research project carried out by Leeds University Dept. of Mining & Mineral Engineering^15,20, new plaster cracking appeared with vibration levels at 24.1mm/s. An existing crack grew at 8.7mm/s, although the low temperatures were thought to influence this. Several blasts giving PPVs of up to 60mm/s resulted in no damage whatsoever.

The Association of Noise Consultants in the UK have produced their own assessment of the levels of vibration likely to cause damage⁴⁶ (Figure 5). The source of this information is the graph in BS7385 Part 2 for the onset of Cosmetic damage in a residential building (i.e. the red line in Figure 4.) To obtain the minor and major damage curves, they have simply multiplied the levels for cosmetic damage by 2 and 4 respectively. Standards exist from other parts of the European Union which all differ slightly from those shown so far, such as the German DIN 4150 part 3 and the Spanish UNE22-381-93.

A recent review of the literature²⁷ indicates that the criteria originally given in USBM RI8507¹³ is still appropriate guidance for the prevention of threshold cracking in homes.

Because the levels of vibration required for damage are significantly higher than the human perception level, many MPAs are setting limits which are below the thresholds of structural and cosmetic damage in order to limit nuisance. In 1992, for the 64% of MPAs specifying PPV limits³, the majority of the limits were between 2.5 and 12.5mm/s, 75% of these MPAs nonetheless received complaints. As 75% of the limits were set at 6mm/s or less and as the average actual levels were presumably below the limits, the critical level for complaints is likely to be less than 6mm/s.

It is clear that compliance with the limits above will mean that structural damage is very unlikely and, although cosmetic damage is possible, the focus will be on the issue of disturbance. It has been reported that complaints are likely to be received when vibrations reach 1.5-3mm/s^16,17 and that people are more concerned about a few high level blasts than a greater number of small ones¹⁸.

It has also been reported⁹ that complaints will start to occur with levels from 1.6-3mm/s during the day and 0.25-0.5mm/s at night and that the majority of complainants are newcomers or the newly retired. Elsewhere, 6mm/s is found by residents to be "unpleasant" and many complaints have been received with levels less than 5mm/s ¹⁹.

The USBM¹³ concluded that 5-10% of people would normally find 12-19mm/s less than acceptable. They suggest that lower levels would be necessary to achieve a similar degree of acceptability where people are at home and subject to the rattling of windows etc, leading to fright and fear of damage and injury, eg at 12mm/s up to 30% of residents might complain.

BS6472²¹ was an attempt to set vibration limits which were likely to be considered a nuisance in different situations. It was not originally intended to cover blasting, but was extended with a dedicated appendix. The interpretation of BS6472 suggests that a PPV of 8.4mm/s (obtained using Curve 60 in Figure 6) would be "satisfactory" in normal situations with less than 3 blasts per day. This value is actually a 90% value (i.e. not more than 10% should exceed it) and the standard suggests the absolute limit should not be more than 50% higher than the satisfactory level (giving a limit of 12.6mm/s!). If there are more than 3 blasts a day taking place, then the satisfactory levels are reduced, but the levels given are inconsistent with experience about reactions to actual blasting and the Association of Noise Consultants⁴⁶ report that the standard is currently the subject of a major review. Clearly there is no consensus over desirable limits for disturbance due to vibration from blasting, although the trend in limits set in planning conditions has been downward over the last two decades.

The low levels which give rise to complaints are often contrasted with the levels which occur in dwellings as the result of the residents activities.²² People cause and accept higher levels of vibration by slamming doors and hammering, than are caused by blasting, although the waveforms and durations are different, blasting vibrations last longer. Slamming a door produces vibration of about 17mm/s²³ and footfalls about 0.5mm/s. It is suggested that footfalls below 1mm/s are imperceptible and barely perceptible up to 5mm/s.²⁴

However, just taking the amplitude of the vibration into consideration means that factors such as the frequency and duration are ignored. Ground vibrations from blasting usually last longer and have lower frequencies than the normal vibrations which occur in a house, and of course, do not occur as often. These factors result in blast induced vibrations being considered far more disturbing than other vibrations with possibly higher PPV values.

The apparent inconsistency of people sometimes complaining of vibration levels below the threshold of perception may be due to the fact that most measurements are made in the ground and not on the floors of buildings. Resonance in buildings can amplify the vibration, particularly in timber floors and rafters. The actual response of the building, and hence people's perception, may be significantly different depending upon the type of building. There is also the possibility that people may be complaining of vibration induced by air overpressure, as it certainly causes vibration in buildings.^20,25 Overpressure may even be the cause of some complaints attributed to ground vibration.

Few studies have been made of the response of people to such vibration in real life situations. Most of the work on people's response to vibration, including BS6472, has been based upon laboratory studies. In these studies there is no element of concern for a person's property and cannot take account of some peoples' fears which may not be logical or based on fact, but are nonetheless disturbing. The effects of blasting have been described as "fearsome" especially by elderly people.¹⁹

The effect of vibration on people is highly subjective, as one person may tolerate high levels that would be unacceptable to someone else. It is therefore difficult to offer advice on suitable levels of ground vibration and overpressure because of the uncertainties in the understanding of the response to them. Levels will have to take account of local conditions and the nature of the workings. Ground vibration limits are likely to be in the range 5-10mm/s and overpressure 120-130dB if cosmetic damage and the worst levels of disturbance are to be avoided. Current permissions mostly require ground vibration limits in the range 4-12mm/s although lower levels may be necessary for specific objectives.

There is much discussion as to whether the limits should be based upon:

• a maximum level alone;


• a statistical level, below which a specified percentage of the blasts must fall;


• the two together, eg not greater than 12mm/s and 95% below 6mm/s.

It is extremely difficult for a blasting engineer to design a blast to ensure it stays under an absolute limit because of the natural variability in vibration levels. The distribution of vibration levels against Scaled Distance is statistically normal and because a prediction is being made it is not possible to be 100% certain that a level will not be exceeded. Therefore, a confidence level of 99.9% (close to 3 standard deviations) is often taken.

Experience in the opencast coal industry has shown that occasionally, vibration levels obtained from blasts designed using the 95% confidence level at 6mm/s are often very close to the 99.9% confidence level at 12mm/s. More usually, the two results will be different, and in the example in Figure 7 the 95% limit at 6mm/s gives the higher Scaled Distance which results in a lower MIC and is therefore the more restrictive of the two limits. This type of graph is explained in much greater detail later on.


It may be that permissible levels of vibration and peak overpressure from blasting should vary with the time of day in a similar way to noise limits. One suggestion that has been made²⁶ is that higher levels could be tolerated between 09.00-15.00 hrs, middling levels from 06.00-09.00 hrs and 15.00-20.00 hrs on weekdays and lower levels at any other time. The frequency of occurrence will affect acceptable levels.

To avoid damaging caves and delicate stalactites in particular it has been suggested that workings do not approach within 10-20m, although others have suggested that a stand-off of 80m be left, so as yet there is no concensus.

Limits for planning conditions have variously been set at 120dB, 133dB and less than 133 dB. USBM limits²⁸ are 134 dB 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 reported that of 277 measurements of overpressure from various opencast coal sites none exceeded 124dB⁸. Peak overpressures are lower for a given scaled charge in loosening opencast coal overburden than when fragmenting hard rock which leads to more venting.²⁹

It is suggested⁹ that techniques for reducing vibration will also reduce overpressure and that modern techniques of blasting produce lower overpressures anyway. However, others found that over a 5 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 overpressure.³⁰

Limits have been set for unreasonable nuisance from the very short-term audible noise at 94dB(A), and 98dB(A). 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 vibrations.³² As this process is not fully understood and there is such variation in structures, no limits have yet been set for acoustic response.

No acceptable levels of flyrock have been suggested, other than to avoid it altogether outside 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 say³¹ 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, 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. Guidance on "safe" distances is needed along the lines of guidance given for hazardous industrial installations.

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.