Good Vibrations

How to deal effectively with airblast/vibration complaints

Today’s quarry operators face considerably greater responsibilities and liabilities than ever before. With reference to figure 1, an operator continually faces the challenge of achieving:

• acceptable productivity
• no flyrock beyond the quarry property
• safe highwall conditions and slopes
• no damage to underground mines, pipelines or utilities
• no damage to surface structures such as residential homes, industrial structures, bridges, water towers, roads, transformer stations, power lines etc.

Furthermore, the operator faces increasing pressure and awareness in dealing with:

• environmental compliance in terms of vibration, airblast,dust, fumes, flyrock, water quality in the community and reclamation
• health, labour and safety issues
• the use of sophisticated electronic detonator initiation systems and blast-design software
• codes of practice and employee competency
• local and international competition.

While most of the above can be dealt with by implementing strict policies, training, effective management and good planning, the biggest problem from a potential litigation standpoint is the possibility of damage claims resulting from ground vibration and airblast in the surrounding residential community. These problems are not likely to go away; in fact they are expected to get worse, particularly in the vicinity of new housing estates where homeowners have no association with the quarry whatsoever.

The old grandfather clause that ‘we were here first’ can no longer be applied nor defended in today’s changing political climate. Although some quarries have programmes in place to deal with the increasing number of community complaints, many others continue to operate under the grandfather clause or have not adequately planned for such occurrences, do not want to be bothered about them, or simply do not know how to deal effectively with complaints. Thus, the basis of this paper is to provide a few guidelines on how to minimize potential future litigation and how to deal with complaints.

SOURCE OF COMPLAINTS

Contrary to popular belief, for many quarry operators worldwide the major source of complaints stems from airblast and not from ground vibrations or resonant frequencies. The average breakdown of complaints is shown as a percentage in table 1.

These figures apply only to legitimate complaints which have been traced directly back to the blast designs, accompanying seismic/airblast results and influential atmospheric factors. They do not apply to false damage claims, attributed to blasting, that actually arose from other underlying agendas or motives.

Tables 2 and 3 list the controllable and non-controllable blast-design parameters and initiation systems in terms of their average influence on ground vibration and airblast respectively. It should be noted that airblast is influenced by more of the listed factors than ground vibration. Unless everything is aggressively and reasonably done to reduce the airblast to manageable levels of 120dB or less, the number of complaints may remain unaffected, regardless of how low the vibration amplitudes have been reduced. Airblast levels approaching 130dB will be very annoying and levels approaching 140dB or over will be intolerable in the community.

Airblast is always considerably easier to control than ground vibration. Obviously no one can control atmospheric conditions, but there is always the option of postponing the shot or waiting for more favorable conditions.

Once airblast levels are under control, attempts should be made to reduce the vibration amplitude. The last stage, if complaints persists, is to look at the potential of resonant frequencies in the radiated blast energy which could match that of the structure, causing amplifications. For a structure to go into resonance there must be the correct combination of vibration amplitude, resonant frequency energy and shot duration. In most cases quarry shots are so small that they do not have the duration to develop a seismic wave train of sufficient duration to cause a structure to go into resonance and to amplify it sufficiently. But for poorly constructed buildings and/or top-heavy water towers, for example, this damage potential could exist. Certainly, some surface coal mines that fire very long shots have the potential to cause this sort of damage.

VIBRATION AMPLITUDE, ATTENUATION AND RESONANT FREQUENCIES

With reference to figure 2, vibration amplitude attenuates exponentially and frequency attenuates linearly with distance. Both of these are affected by the type and transmission characteristics of the ground, including reflection and refraction at boundaries. The seismic disturbance travels in the elastic part of the rock as a volume change; there is no mass transport of material. As long as the intensity of the seismic disturbance remains below the elastic limit of the rock or breaking strain, no permanent damage occurs in the medium. Obviously, on or very close to the shot, permanent damage will occur, but further away the ground does not get damaged. In special cases of water-saturated soils, however, liquefaction could occur causing permanent damage.

The problem in the distant field occurs when a certain combination of vibration amplitude, resonant frequency energy and shot duration act on a structure causing it to go into resonance. For one- or two-storey homes (with or without a basement) the natural critical resonant frequencies lie between 4Hz and 12Hz, but could vary from 4Hz to 20Hz depending on the type of structure, ground conditions and the geology between the shot and point of concern.

Once the critical resonant frequency of a structure is known, the aim is to select the best delay intervals in the blast design to shift the frequencies into either a lower or higher frequency spectrum away from the natural resonant frequency of the structure. In most cases, for practical blast designs, the shift is usually into the higher spectrum. This is done with single-hole signature analysis using linear superpositioning, destructive interference and phasing principles through a timing simulation. Precise electronic detonators are highly recommended, since the analysis technique assumes that the detonator firing times have a very small insignificant scatter. The analysis details will not be presented here since the technique is well know in the industry, but it is very important to understand that in order to shift the frequency, that particular frequency or frequency band must be in the original signature. If it is not there it is impossible to move the energy, although many have tried to no avail.

For structures such as water towers, bridges, smoke stacks, retaining walls, dams, or any other structures of concern, it is recommended that the resonant frequency is measured and planned for in the blast designs well in advance of the blast areas, new quarries and/or planned expansion areas; particularly with the use of precise electronic detonators.

In order to determine the natural resonant frequency of any free-standing structure with six degrees of freedom, at least two seismographs will be required. One of the seismographs will need to be placed on the ground near the structure of concern, and the other seismograph will need to be placed on the structure itself, usually on the top floor.

For the analysis to be valid, both seismographs must record the same seismic event. The seismogram obtained from the unit on the ground is referred to as the ‘ground waveform’ and the seismogram obtained from the unit on the structure is referred to as the ‘response waveform’.

Without going into the mathematics involved, the analysis basically performs a Fourier analysis using the power spectrum on each waveform, takes the ratios of the two Fourier analyses and prints out a final graph in the frequency domain. The predominant spike on the graph will generally be the natural resonant frequency of the structure (fig. 3). In some cases a structure may respond to more than one dominant frequency and the final graph will show more than one peak.

GROUND VIBRATION LEVELS REQUIRED TO CAUSE DAMAGE TO RESIDENTIAL STRUCTURES

There are two issues which need to be addressed regarding residential owners located in the vicinity of blasting operations. The first is physical property damage which may result from the blast disturbances in the form of ground vibrations that propagate through the ground and airblast (ie overpressure) which travels through the air. In order for these two disturbances to cause real damage, they would have to contain certain specific traits in terms of their amplitudes, durations and frequency relative to the structure of concern. The second issue is related to homeowner complaints in terms of their perception of blasting activities, regardless of whether any real damage is caused or not. The latter issue is always more of a problem to define or control through a simple set of guidelines and/or regulations governing all blasting activities.

Although each country has its own damage criteria, many follow or use the United States Bureau of Mines (USBM) established damage criteria. The USBM published the results of an extensive series of tests regarding ground vibration levels which could cause damage to residential one- or two-storey homes in a Report of Investigation, RI 8507 (1981). In this study direct ground vibration and airblast measurements were made in 76 homes for 219 production blasts. A typical home, using common construction materials such as concrete, blocks, brick, drywall, plaster and a wood frame, was also built near a blast site and monitored as the blasts got progressively closer. The test home was monitored with extensive sensors and instrumentation systems to measure ground vibrations, airblast, structural response, displacements, accelerations and the breaking strains of individual component members of the home in drywall, plaster, ceilings, walls, structure corners, doors, windows etc. Environmental effects were also taken into account. Results were submitted for peer review by other industry experts, including the Office of Surface Mining, and were finally published in 1981. This study, which was undoubtedly one of the best, most scientific and comprehensive studies ever performed by an independent government organization, correlated blast-induced ground vibrations to a damage potential for residential dwellings.

A safe ground vibration damage criteria was recommended by the USBM in the form of a graph, which is illustrated in figure 4. This is a plot of ground vibration amplitude versus frequency. The ground vibration amplitude (particle velocity) is measured in in/s or mm/s and represents how fast a point on the ground or a structure moves around its original rest position. The frequency is measured in Hertz (Hz) and represents the number of oscillations that the point makes around its original rest or start position over a period of one second. Both the combined particle velocity and its associated frequency are important in determining if this combination has a high or low probability of causing damage to a residential dwelling.

Basically, the USBM graph shows that if a measured ground vibration point and its associated measured frequency falls below the safe vibration limit line (shown in the solid and partly dashed lines), the probability of physical damage is extremely low or non-existent. The safe vibration limit line represents the threshold or beginning of cosmetic damage, such as very small, barely visible hairline cracks in plaster and/or drywall. If a vibration point falls above the safe vibration limit line (ie above the threshold line of cosmetic damage), the probability of real damage will increase. It does not, however, necessarily mean that physical damage is guaranteed to occur, but the higher the vibration point is above the safe limit line, the higher the probability of damage.

Another way to look at this is in terms of a structure’s elasticity, brittleness and breaking strain. If the ground vibration amplitude and frequency cause any structural component member of a home to exceed the breaking strain just once, permanent damage will occur. All parts of a home are designed to flex to a certain degree without damage, similar to the springs on a car. As long as the flex, twisting and bending is within the object’s elastic limit, no damage will occur regardless of the number of times it is distorted. But, pull the spring beyond its elastic, brittleness or breaking strain just once and permanent deformation damage will occur.

Also, all homes go through natural ‘settling’ periods throughout their life due to a host of factors related to construction, thermal strains, natural aging etc. Other than major structural or construction problems, minor defects, hairline cracks, separations, corner/wall/ceiling detachments and differential settling are to be expected, regardless of whether the home is located near blasting operations or not.

GROUND VIBRATION PERCEPTION AND ANNOYANCE

The psychological response to blasting is an entirely different matter. Human perception to ground vibrations and noise usually occurs well below the damage threshold line recommended by the USBM. In addition, noise is often interpreted by the lay person as ground vibration. Blast-induced ground vibrations and airblast (or noise) are two entirely different disturbances caused by blasting. Studies in over 20 countries have confirmed that the majority of complaints are related directly to airblast.

Although real structural damage will generally not occur in residences until ground vibrations are well over 0.5–2.0in/s (12.7–50.8mm/s), and even then only under specific frequency levels and conditions, humans will respond to ground vibrations and airblast at considerably lower levels. With reference to figure 5, human perception is very complex because it depends on each person's tolerance level, the ambient noise level, displacement, velocity, acceleration, frequency, condition of the structure and rattling effects from loose construction etc. Extensive studies by Chae in 1978, Nitro Nobel, USBM, Blasting Analysis International Inc., White Industrial Seismology Inc., Vibronics Inc., and others, have determined that the human response to ground vibrations and airblast is actually many orders of magnitude less than the level that would be required to even come close to producing actual damage. For example, the following effects were documented on humans from blasting activities (table 4).

It is evident from this data that ground vibration levels received at a structure in the order of 0.03–0.10in/s (0.76–2.54mm/s) are quite perceptible, but the probability of damage is usually non-existent. Likewise, levels in the range of 0.10–0.3in/s (2.54–7.62mm/s) can be disturbing and levels over 0.5in/s (12.7mm/s) can be very unpleasant, although permanent damage rarely occurs.

DO’S AND DON’TS

The following guidelines are recommended to help diffuse problems in the early stages of development, and to provide protection against potential litigation and large damage claims.

Quarry representative to deal with complaints

Good communication early on has always been the key to diffusing more serious problems later. Poor or no communication (whether intentional or not) with the complainer is often the sole root cause leading to major disputes and unnecessary litigation which could have been eliminated early on.

Each quarry or quarry area should assign a good public relations representative to deal with complaints in the surrounding communities. A good PR representative does not necessarily mean the quarry’s best technical engineer. The representative will often have no technical background whatsoever, but should be someone who can develop a good rapport with people in the community, gain local communities’ trust and work with the quarry’s technical people. Any communication with people in the community must always be conducted exclusively through the PR representative acting as the official liaison officer. Conversely, all complaints received by the quarry should immediately be directed to the PR representative.

Any and all complaints must be responded to immediately, even if the initial contact with the complainer is only a phone call on the day of the incident, followed by a scheduled face-to-face meeting as soon as possible thereafter. This procedure should always be followed no matter how trivial the complaint may appear from a technical perspective and regardless of whether the home is a very modest dwelling or an expensive and elaborate multi-million pound residence.

The PR representative must remember that a homeowner has absolutely no interest whatsoever in hearing technical jargon related to blast designs, seismic waveforms, Fourier analysis, delay time, electronic detonators and/or compliance with the law. Most homeowners only want to be assured that the blasting is not causing damage to their homes, that the incident will be investigated, and that the quarry is taking reasonable ‘genuine’ measures to reduce the blasting impact on their property.

Unless the homeowner hears these words, any further communications will generally be meaningless and disputes will tend to escalate from there on.

During the initial contact with the homeowner, the PR representative should also attempt to determine the real basis of the complaint. In many instances, blasting is often initially referred to as the problem, but the underlying root of the complaint may be related to non-blasting activities such as dust, rock fragments on public roads causing cracked windscreens, or quarry noise during the night shift. At other times it might be a ploy to sell the property to the quarry, in an attempt to get more money for the sale, and/or be related to the fear of property devaluation as extraction encroaches closer to the property. Knowing this type of information at the start can avoid spending considerable amounts of time and effort on trying to resolve complaints for the wrong reasons.

A good PR programme may include but is not limited to:

• constant communication updates and visits with the homeowner and local regulatory bodies
• full damage inspection of the homeowner’s property by an independent qualified inspector
• meetings with people and regulators in the community to educate them on blasting (in simple terms) and to keep them updated
• quarry sponsorship of community events
• quarry tours and the witnessing of a carefully planned blast, followed by a hospitality event at the quarry
• making the homeowners part of the programme by asking them to call the PR person after each shot to report on how it felt.

The PR representative should never admit to a problem, assume damages or settle even very small claims until a full, qualified damage inspection of the property has been completed. If blasting is concluded to be a contributing factor to the claimed damages, compensation should be paid immediately and without any fanfare. On the other hand, if blasting was not the cause, even if the claim is for only a minor amount, the quarry must stand firm and be consistent in its policies. Inconsistencies of policies in the community will generate mistrust and lead to greater problems.

Need for good record-keeping

Good record-keeping is becoming increasingly important and even more essential regarding ground vibration/airblast measurements for:

• compliance with any prevailing regulations
• protection against litigation
• use in blast designs and diagnostics.

Consideration should also be given to standardizing these records into appropriate databases, since this will force everyone, including seismic subcontractors, to comply in documenting essential information, and will expedite the transfer of information electronically and allow for convenient statistical analysis when necessary. It should always be anticipated that the blasting reports and seismic/airblast records could end up in court, and that some discretion should be used in preparing the records before they are filed away.

A proprietary agreement with strict penalties for the disclosure of proprietary information should be in place with all subcontractors involved in seismic/airblast measurements or any outsider who has a need to access the data. Here again, seismic/airblast results should only be communicated to homeowners via the PR representative and not by the seismic/airblast contractor.

Seismic/airblast monitoring

More seismic/airblast monitoring in and around the surrounding quarry communities is expected and recommended, particularly where new housing estates are developed. Seismographs should be strategically placed to cover the immediate complaint area, population densities and other critical areas of concern. The best arrangement is usually with a seismic array line of at least three seismographs from the shot area towards the area of concern. Large quarries with multiple areas of concern will require multiple seismic arrays.

Seismic arrays are also conducive to statistical predictive analyses using linear regression. Linear-regression analysis is recommended for each seismic array, since it is anticipated that complaints could surface where no previous measurements have been taken. Linear-regression analysis will allow the operator to confidently interpolate or extrapolate amplitude predictions at a location of ‘no measurement’, providing that at least 25 to 30 data pair points were previously available for analysis.

Seismographs should have the ability to monitor 24h a day, since it is important to monitor all other non-blasting disturbances in the community. It is not uncommon to find that ambient levels in the community can be as high or higher than those produced by blasting. This can occur and a result of thunder and lightning, normal traffic, farm equipment, construction activities etc. This type of information has often been very effective in eliminating and/or reducing large damage claims.

Seismograph measurements should always be obtained directly at the complaint property unless recent nearby measurements are available. There is nothing more credible than a direct reading actually recorded at the property, even if it is only one measurement, although obviously the more the better.

Local and other regulator bodies

Mines and quarries need to be proactive and have a ‘firm hand’ in communications with local and other regulatory bodies. In many cases, the regulatory authorities do not understand mining, explosives applications and structural damage, and it will be in everyone’s interest for quarries to assist them with training, provide information and suggest reasonable damage criteria which all parties can live and operate with in the near future.

The lack of quarry involvement or positive lobbying in these areas can often lead to outrageous limits being imposed by the regulators under political pressure. This has been experienced in many parts of the world. For example, in a major coal mine in Australia, regulators set an arbitrary limit of 5mm/s for nearby residential homeowners, while in Sparta, Tennessee, a limit of 2mm/s was regulated for all blasting. The coal mine in Australia eventually had to abandon casting for a less-efficient mining method; and the inhabitants of Sparta basically made a firm statement that they did not want any blasting in their area. The crucial point here is that it is always easier to influence a proposed regulation by being proactive, then by trying to change or undo a regulation later.

Pre-blast surveys

The need for pre-blast surveys in some areas of the community is expected to increase, while in new neighbourhoods or developments, such surveys are essential. A pre-blast survey for a structure basically involves documenting all predominant and minor structural conditions, defects and workmanship, to establish the ‘now’ condition, as a base for comparing any future damage claims. There is an obvious cost to this, but in the long term a carefully conducted pre-blast survey of residential and/or industrial structures can save a huge amount of time, resources and money, and will help to diffuse exaggerated future damage claims. In many parts of the US, pre-blast survey inspections must be performed, by law, on all structures located up to 0.5 miles (0.8km) from the quarry boundary line. This is an example of a situation where the US mining industry did not get involved in lobbying early enough and is now stuck with this requirement and cost forever!

Pre-blast surveys are recommended for certain dwellings and structures, particularly those in expensive neighbourhoods or industrial structures housing sensitive equipment, machinery and/or processes, but quarries do need to use some discretion in deciding which structures or areas in the community warrant such inspections.

The most important thing in conducting pre-blast surveys is accurate, detailed and consistent documentation by a qualified, reputable firm. On completion of a pre-blast survey, it is also important to get the property owner to sign the report.

Importance of seismogram verification by qualified personnel

Seismograms should be verified and interpreted periodically by a qualified independent engineer or specialist who understands blasting applications, seismic/airblast theory and structural damage. A labourer or technician who sets up a seismograph is not necessarily qualified to verify or interpret the results. The significance of this requirement almost always shows up during serious disputes, expensive damage claims, compliance infringements and definitely in all court cases.

Likewise, statements from non-qualified people (ie building contractors, painters, utility workers etc) suggesting to the homeowner that damage was caused by blasting, should always be challenged and their company contacted immediately to inquire about their qualifications in making such a statement.

CONCLUSIONS

Quarries cannot ignore the subject of vibration/airblast. Sooner or later complaints and/or damage claims will surface. In order to protect against litigation, reduce the monetary claim and/or offer reasonable mitigating solutions, quarries need to communicate more effectively with their neighbours, set up PR community programmes, perform more seismic/airblast monitoring and pre-blast surveys, identify critical points of concern in advance, determine the resonant frequencies, and plan accordingly to provide the best possible blast designs.

This article is based on a paper presented at the Institute of Quarrying Southern Africa's annual conference in March 2001. The author, R. Frank Chiappetta, is president and explosives applications engineer with Blasting Analysis International Inc. His co-author, Andre van Vreden, is blasting consultant with Ancor Rock Fragmentation.