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Drilling & Blasting

Best practice requires ‘planning for compliance’ with statutory vibration limits rather than ‘monitoring for compliance’

Blasting operations are arguably one of the most contentious areas of surface mining and quarrying. When blasting is permitted at a site the local mineral planning authority (MPA) will set vibration limits for various locations around the site. In earlier years, the limits were set to prevent property damage, but in today’s environment the limits are more commonly set to reduce disturbance.

Nuisance complaints can sour public relations between the quarry and its neighbours and also lead to MPA involvement. Such complaints and a swell in public opinion may lead to tighter restrictions on the quarry and an ever-growing band of lawyers is always ready to advise today’s litigious society. Such opposition can render quarry extensions or future mineral extractions untenable by imposing very restrictive vibration limits.

The statutory vibration limits set out by a mineral planning authority tend to follow the formula below, outlining that the blasts are to be designed so that:

‘X% of all blasts are to be below Ymm/s as recorded at the nearest occupied premises’.
   
Where:
X = an imposed upper confidence limit (typically 95% or 98%)
Y = a maximum vibration value (typically 6mm/s but may be lower)

In order to demonstrate compliance with the condition, the monitoring and recording of blasting events is required. Well-maintained monitoring records must be held to preserve operational licenses (and also are a vital defence for potential claims). However, this is where pitfalls can arise for the unwary, and seemingly compliant operators can easily find they are facing conflict.

Scaled-distance regression

Scaled-distance regression is important to an operator planning to comply with the statutory limits set by the mineral planning authority. Scaled-distance is a weighted distance, used to normalize distance and charge weight.

It includes the distance of the observation position from the blast and also the maximum amount of explosive initiated at any moment in time during a blast (maximum instantaneous charge weight, MIC).

A monitoring location at a considerable distance from a blast using a large amount of explosive may have the same scaled-distance as a monitoring location that is close to a blast using a small amount of explosive. Being a statistical technique, regression analysis requires numerous data points spread over a range of distances in order to give reliable predictions. The predictions it gives will always be subject to a degree of uncertainty, depending on the number of data points and their scatter.

Recording data at many locations at differing distances from an initial blast allows an initial scaled-distance regression model to be produced. The regression model can be used to make predictions for vibrations produced by future blasts to ensure that future blasts will comply with the permitted vibration level. If the predicted vibration level is higher than the permitted level, the MIC of the next blast can be reduced or other measures taken to ensure vibration compliance during the next blast (single-hole signature blasts can also be used initially to provide better regression models. The waveform from such blasts can be used to calculate the inter-hole timings in order to optimize vibration reduction and maximize fragmentation).

As more blasts are conducted, the monitored vibration results can be put into the scaled-distance regression model. Updating the regression model in this way can lead to further reductions in vibrations produced and improved fragmentation at each blast. The more data there are, the more accurately the regression model will predict the likely outcome of a blast.  

Monitoring to comply

Many operators, well aware of their obligations, dutifully carry out regular vibration monitoring in accordance with their statutory requirements and will quite readily cite how important the procedure is. The following is a typical scenario: prior to a blast, the ‘nearest occupied premises’, as detailed in the MPA guidelines, is visited and a seismograph positioned to capture the day’s event(s). Post blast, the seismograph is collected, downloaded and the observed maximum peak particle velocity (PPV) (either the resultant or the maximum single plane) readings are checked for compliance. Having proven compliance (always), the results are then duly noted and archived.

Monitoring in this way, the operator will typically monitor at one location which is the same for all or most blasts. In addition, the operator will pay little or no attention to the measurable parameters that determine the PPV values, including scaled distance, burden, detonator type, blast type, rock type, orientation of the blast, bench etc.

This procedure can be called ‘monitoring for compliance’. Superficially, it may appear that nothing is wrong with this strategy and that may well be the case until the day a problem arises. For operators who normally experience a relatively harmonious existence, the sudden arrival of blasting complaints together with the threat of litigation, for whatever reason, will appear alarming. The full implication may only become apparent following a review of past blasting events when a scaled-distance regression model is constructed, revealing the true magnitude of the problem and its full ramifications.

Figure 1 illustrates an extreme example of a scaled-distance regression model for a site that has been ‘monitored for compliance’. Visual inspection of the regression curve immediately highlights the problem. As practically all the data has been recorded at the same location, the range of the results (in terms of both scaled distance and PPV) is so small that the data points present themselves as a ‘ball’ or ‘clump’.

When information such as this is interrogated, it can be seen that the slope of the best fit line is positive. This means that the predicted vibration level increases with distance. This is completely illogical. Owing to the lack of variation in scale distance data, predicting the vibration levels for future events is impossible and, therefore, planning to comply with the mineral planning authorities’ vibration limits is impossible. If the quarry faces move closer to the property, it will not be possible to design a blast to comply with the requirements.

Should the permitted vibration limits be exceeded in these circumstances, the operator would be breaking the conditions of its operating licence and, based on the monitored data, the position of the operator in court would be indefensible. The monitored vibration data proves that there was no effort made to plan to comply with the operating limits. Compliance was through chance rather than design.

Planning to comply

In order to avoid complaints and the possibility of facing the scenario described above, blasts should be planned using all vibration data and a properly designed and comprehensive monitoring strategy should be adopted.

The benefits may go well beyond meeting compliance obligations by lowering the operating costs, as increasing efficiencies and improved productivity may be possible.

Figure 2 shows an example of a scaled-distance regression model for a site where blasts have been designed so as to fulfil the legal obligation by ‘planning to comply’. Here, the scaled-distance regression curve describes a true linear relationship conforming to the logical conclusion that as the monitoring distance from the blast increases, or the amount of explosive is decreased, the lower the monitored vibrations will be.

In contrast to the ‘monitoring to comply’ method, while the ‘nearest occupied premises’ is still being diligently monitored for compliance, the use of additional seismographs positioned at varying distances from the blast site gives a range of scaled distances and, therefore, a larger spread of results (this also allows location-specific results to be analysed). These data produce a scaled-distance model from which charge weights and the related, realistic vibration predictions can be made with genuine confidence. The charge weights and predicted vibration levels can also be accounted for and verified should the situation arise when a recorded vibration level does exceed the permitted value.

Although the ‘monitoring to comply’ strategy does meet some of the requirements of the mineral planning authority and will generally comply with producing vibration limits below the permitted level, it does not show best practice. When the vibration information is presented in the form of a graph it is evident that operators do not ‘plan to comply’. Compliance when ‘monitoring to comply’ is by chance.

As such, operators should ‘plan to comply’ by implementing a dedicated recording and analysis system where they:

  • Monitor at multiple locations and distances from the blast.
  • Build a competent regression model that can be used with confidence.
  • Utilize the regression model to predict vibration levels for the next blast and alter the blast designs accordingly, if required, to ensure compliance with the permitted vibration limits. 

For further information contact Blast Log Ltd on email: [email protected]

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