Australian Standard 2187.2 (Appendix J) is used by state and local regulators and owners of assets for determiningvibration and airblast limits for blasting. It sets guideline limits for human comfort and damage which vary, depending on the duration of the blasting project and the occupancy of neighbouring properties (residential or commercial). Most quarries abide by the lowest human comfort limits of 5mm or 10mm per second. The damagelimits range up to 50mm per second for high frequencies (above 40 Hz), but are conservative and designed to eliminate the possibility of superficial damage in the most susceptible properties.
If a quarry is assessed on vibration levels at two properties and it can be established that the properties are regularly unoccupied(eg every day between 2:30pm and 3:30pm), a higher, frequency-based limit could be applied if blasting occurs in that time. Most construction projects have different limits that depend on the time of day or day of the week, and general industrial noise limits are regulated in this manner by the Environmental Protection Agency. However, many state and local laws do not make a distinction, applying a flat ?human comfort? level, regardless of the state of occupancy.
THE BEST LIMIT IS NOT NECESSARILY THE LOWEST LIMIT
AS2187.2 allows for negotiation of higher limits with occupiers of affected properties. As a rule, increasing the allowable vibration limit from 10mm to 25mm per second will reduce the number of blastholes and blasts to extract a given volume of rock by 60 per cent. This can be achieved through use of larger charge weights, diameter blastholes and blasts. In construction blasting, higher limits are often used in this way to reduce the:
? Amount of supplementary mechanical rock breaking.
? Overall number of blasts and blastholes to be drilled.
? Overall duration of construction noise.
Experience shows that people find noise sources annoying, but may tolerate higher levels of short, transient blast vibrations in return for a reduction in these nuisances.
Of course, there are many quarries with a large number of neighbours who will only accept the lowest limit. This can make negotiations difficult, but negotiating higher limits can have cost savings while aiming to reduce the impact on the community.
SETTING THE MONITOR CORRECTLY
Quarry managers can aid compliance by mounting their vibration and airblast monitors correctly. Incorrect mounting will increase the measured levels of vibration and reduce the measured frequencies of the waveform. To mount correctly:
? Make sure the geophone is level.
? ?Earth? the geophone, ie the geophone should be buried in the earth or firmly bonded to a pre-cast block of concrete or aluminium. Using spikes is an alternative, but difficult where the ground is too hard for the spikes to penetrate or too soft to create a good coupling. Simply placing the geophone on the ground is generally unacceptable, as in side-by-sidecomparisons, a spiked or free standing geophone may record a higher vibration level than an embedded one.
? Keep the geophone away from objects that may provide ?feedback?, eg water tanks, telephone poles, sheds and houses shake at their natural frequency when the vibration waveform passes by. If the geophone is too close, it will measure the object?s response rather than the original blast vibrations.
? Don?t mount the geophone on any other structure or man-made surface unless thatis what you are interested in measuring. AS2187.2 requires the measurement of ground vibrations, not the response of the structure.
? Microphones should be mounted in theopen, facing the blast and with a windsock (foam ball) fitted. Natural wind gusts can register as low-frequency overpressures of 115 dB or more. If blasting on a windy day, it is useful to manually trigger the monitor to record periods of background wind overpressure levels before and after the blast (especially important if the monitor is downwind of the blast). These records may explain an exceedance during a subsequent investigation.
ACTIVE COMMUNICATION WITH THE NEIGHBOURS
Humans are sensitive to vibration and airblast and can detect levels below commonly used limits for human comfort.This means that even if a quarry complies with the lowest limit of 5mm per second, some people may still complain about blast vibrations and/or airblast.
At the same time, people are tolerant of levels above the human comfort limits when they are made aware that the blast is imminent. To ensure awareness, it is important to engage neighbours personally (either by phone or in person) to advise of the blast a few minutes prior. If this is impractical or too invasive, an SMS from email broadcast services may be practical. This allows a quarry operator to send an email blast notification as an SMS message to multiple receivers in an instant.
Of course, it is important to ensure the blast is fired on time. This may mean the blast has to be ready to fire hours or, in extreme cases, days in advance. In one case, a large blast adjacent to a major rail line had to be ready two days in advance of the 24-hour firing window to ensure it was fired on time. The next available firing time window was three months away.
HANDLING DAMAGE AND NUISANCE CLAIMS
Even if a quarry has 100 per cent compliance with the strictest vibration and airblast limits,a damage claim is still possible. It is prudent to have a plan for dealing with these claims.
It is essential that damage claims are notified as soon as possible after blasting, so the evidence is as fresh as possible. To achieve this, neighbours should be encouraged to report suspected damage immediately and provided with direct contact details so a representative can attend their property as soon as they report the damage.
Treat every complaint with respect and credibility, no matter how incredible it may seem. The complainant?s perception is the effective reality, and dismissing a claim without investigation will never satisfy the complainant, even if the recorded vibration and airblast levels were within limits.
It is important to determine exactly what the complaint is about. Most complainantsdon?t know the difference between vibrationand airblast and may misidentify the sourceof their anguish. Vibration and airblast have different sources within the blast, and different mechanisms of transmission, but manifest themselves within homes and buildings in similar ways, eg they cause windows to rattle and walls or floors to move. Some people are not so much annoyed by the vibration or airblast levels as by the fact that the blast occurred at all. It is possible for neighbours further from the blast to experience a higher level of airblast due to atmospheric inversion effects.
Be aware that the neighbour may be more concerned about other aspects of the operation,using the complaint about blast noise/vibration to voice their opinion, eg they may be tired of the dust created by trucks going past the gate, but powerless to stop the trucks, they complain about the blasts.
Find out what the claimant is seeking in response or compensation. It could simply be reassurance. Of course, one must be mindful that claims may ?snowball? to other affected parties before deciding to settle.
Finally, remember that damage due to blasting vibrations is rare. The forces andstrains of blast vibrations within the Standard limits are less than those created by normal everyday environmental forces such as changes in temperature, humidity and human activity (eg slamming doors). This can be demonstrated by fitting continuous autonomous strain gauges across existing defects. An example is illustrated in Figure 1.
It can be seen that the dilation and contraction of the crack that occurs every day, without blasting is much larger than any movement that occurred when the blast was fired. Such ?direct? measures can make it much easier for neighbours and regulators to understand the true effects of blasting on structures and are an important risk control measure in construction and quarry blasting applications.
Martin Adam is the principal blasting engineer for Orica Quarry Services.