The introduction of the 1988 and 1999 Quarry Regulations have controlled the production of blast specifications and have acknowledged blasting as a branch of engineering which is more exact than was thought possible in the past. Although site trials of vibration from blasting in the form of test blasts are common, sufficient allowance is not always made for the uncertainties involved in subsequent prediction.

It is maintained that limits cannot be set for overpressure due to the variable results of blasting and changeable weather conditions. This is especially true over longer distances where the effect can be +/-14-15 dB. Similar arguments are also advanced in relation to ground vibration and flyrock.

Whilst the use of adequate margins of "safety" in the blast design in order to avoid exceeding any set limits would seem to be an appropriate solution, it is argued that the cost of such safety margins would be excessive because the uncertainties are so large. It is suggested that a better approach would be to limit the number of occasions when ground vibration or overpressure exceeds a "consent" level, e.g. 95% should be below a certain PPV level. Even better, would be to agree that EVERY blast should be DESIGNED to meet that criteria, i.e. for a 95% limit, it should be designed such that there is only a 5% probability of exceeding the limit. This design should be backed up by regular monitoring and updating of the site factors used to design subsequent blasts. This can be achieved very easily by use of a blast database.

The important thing here is that planning conditions should relate where practicable to performance, ie they should set limiting levels for ground vibration and overpressure, rather than set out good blasting practice which is the remit of the Health and Safety Executive. The operator should be left to decide the best way of achieving the performance, which is both safe and economic.