Consulting seismologists often are called upon to visit the neighbors of quarries against whom complaints have been lodged. They listen to what the neighbors say and try to gain an understanding of why they complain.

"I don't have to live with this. I know they can blast less than they are doing now. They need to shoot smaller shots so they don't shake me so much." These are typical responses from neighbors to such visits.

The seismologist gets a different story at the quarry. "We can't get enough rock for the crusher. The loaders and trucks don't work more than half the time. The blaster spends more time on the job than I do and we still don't produce enough. We're down to five-hole shots with three decks per hole. I'm not making a dime and regulators say I'm getting too many blasting complaints. What am I supposed to do?"

At public meetings, complaints are directed at plant noise, dust, truck traffic, and blasting. Quarries can deal with plant noise by restricting hours and constructing berms to keep the noise on-site. Dust can be controlled with water trucks. Truck traffic is more of a problem, but public roads can be bonded if need be. There are defenses for those problems.

Blasting is different. Ground and air-blast vibrations cannot be confined to a site. They are invasive to neighbors. They come mysteriously, without warning, and they trespass on your neighbors' property and in their homes. On top of that, anything to do with explosives is sensationalized by the media.

Rules and regulations limit vibrations from blasting to protect neighboring property. Users of explosives are among the best self-regulated industries in this country, often consulting with seismologists when they exceed 25% of their allowed vibration limit. Many states limit quarries to 2.0 in. per second (in./sec) peak particle velocity (PPV) at the nearest non-owned structure. The industry complies with these regulations, but still gets complaints.

Ground vibration-an erroneous concept It sounds absurd, but no quarry has ever received a complaint about ground vibration. Who has ever received a call from an irate neighbor claiming to have been outside mowing the lawn or washing the car when the blast went off and threw him to the ground? Never. But how often has the phone rang with a homeowner claiming that the last blast "threw me out of the parlor chair?"

When a complaint does come, common practice has been to set up a seismograph in the yard to monitor the blast. One of two scenarios inevitably follow. The resident stands outside by the seismograph and proclaims after the shot, "That was nothing like the last one." Or, the resident goes inside to wait for the blast, reappearing immediately afterward wanting to know the seismograph reading. When told that the PPV was 0.1 in./sec, or even 0.5 in./sec-well below the levels necessary to damage a home-the resident erupts, "Well your machine is wrong." In some ways that person is correct.

The real culprit is not ground vibration PPV, it is structural response. Your neighbors could care less about how fast a particle on the surface of the ground in their yard is moving. All blast-vibration complaints are due to house vibration, not ground vibration. If neighbors' houses don't shake, they won't complain. Air vibration, or airblast, can shake neighbors' homes just as effectively as can ground vibration.

Three structural-response factors What can be done about it? It is not practical to measure how much each house vibrates as a result of blasting. Fortunately, the former U.S. Bureau of Mines and Vibra-Tech Engineers devised a way to calculate how much neighbors' houses could shake as a result of blasting.

This method is based on the single-degree-of-freedom vibration model. A single-degree-of-freedom house vibrates in the same direction as the ground and has some fundamental frequency at which it tends to vibrate. This method has been proven by field testing.

Three factors of ground vibrations determine how much neighbors' houses vibrate:

* ground vibration amplitude (PPV);

* ground vibration duration, which is not the same as blast duration; and

* ground vibration frequency.

To understand how these factors determine how a house vibrates, think of a more familiar vibration model-a swing. A swing is a single-degree-of-freedom vibration model that behaves remarkably similar to a vibrating house, except upside-down.

Structural response is directly and linearly proportional to ground vibration amplitude. Cutting PPV in half cuts structural response in half in a one-to-one relationship. Think of the amplitude of the ground vibration as how hard you push the swing. Push it harder and it goes higher.

Duration is important because the longer ground vibrations continue to shake the house, the greater the amplitude of the structural response. You can easily make a swing go higher without pushing it harder by simply pushing it again. The more times you push the swing, the higher it will go.

In addition, studies have shown that human tolerance to vibration decreases the longer the vibration continues. Seismographs report PPV and frequency, but duration often is ignored.

Frequency is the most important of the three factors of ground vibration. When a house is exposed to ground vibrations near its fundamental frequency, the house magnifies the vibrations. Ground vibrations at the fundamental frequency of a house are like pushing a swing whenever it comes back to you. If you push the swing at any other time, you disrupt its rhythm. Push it at the wrong time and it may just about stop swinging.

Similarly, ground vibrations very far from the fundamental frequency of the house cause it to vibrate very little. If the frequency of the ground vibrations is more than 40% greater than the fundamental frequency of the house, the house will vibrate less than the ground. Ground vibrations below the fundamental frequency of the house will still cause the house to vibrate at least as much as the ground, like pushing the swing every other time it rocks back. Ground vibrations at twice the fundamental frequency of a house produce 10 times less structural response than vibrations at the natural frequency.

Ken Medearis, for a 1976 report to the National Crushed Stone Association, measured how residential structures responded to blast vibrations. He found that the height of a residential structure primarily determined its fundamental frequency, but all residential structures have a fundamental frequency between 4 and 16 hertz.

In the late 1970s, the USBM also studied the natural frequency of residential structures. Its findings yielded a range of 4 to 12 hertz for the natural frequency of residential structures, supporting those of Medearis. The USBM also discovered that while houses vibrate as a single-degree-of-freedom between 4 and 12 hertz, the natural frequency of the house's midwalls tends to occur between 12 and 20 hertz.

Midwall vibrations cause house rattle, making the vibration more noticeable. Engineers know that to control the response of a structure that has more than one natural frequency, you must control the two lowest natural frequencies. For residential structures, this means minimizing vibrations between 4 and 20 hertz.

All three factors of ground vibration play a role in neighbors' perception of blast vibrations. Reducing ground vibration amplitude reduces house vibration amplitude proportionally. Reduce ground vibration duration and structural response decreases while the neighbors' tolerance increases. Finally, doubling the frequency can achieve 10 times the reduction in structural response.

A blast vibration-analysis program such as Vibra-Tech's RSVP considers all three factors of structural response, relates the PPV to the frequency and the time effects to determine structural vibration, and relates that to the induced strain in a house to determine the damage potential.

Damage is actually caused by strain, the stretching of building components. When the ground around a structure begins to vibrate, the structure also begins to vibrate, although not immediately. Inertia causes the top of the house to lag behind the foundation somewhat. This lag causes the house to stretch, or strain. If the strain is too great, cracks form.

Reducing structural response is the only certain way to reduce the effect of blast vibrations on neighbors and their property. RSVP analysis can simultaneously show regulatory compliance, calculate structural response (what neighbors feel in their homes), and determine the risk of property damage.

Most blasters and quarry operators are already well aware of the importance of ground vibration amplitude, frequency, and duration. Many also are well aware of the traditional techniques used to control each. When complaints exist, operators often require their blaster to use additional load decks to reduce PPV. Other common steps include using high explosives and other high-detonation-velocity products instead of ANFO to increase the frequency; shortening delays to increase frequency and decrease duration; and when necessary, using smaller shots to decrease PPV and vibration duration. It turns out, none of these options can solve structural response problems. Some actually make it worse and cause more complaints.

The only certain effect of smaller shots is to require more frequent blasts. Smaller, more frequent blasts not only intrude on neighbors more often, but they increase the costs to produce stone.

Blame Mother Nature, not the blaster The key to being a good neighbor doesn't lie in hurting your bottom line, but rather in understanding what causes the high amplitude, long duration and low-frequency vibrations that generate complaints. Vibra-Tech discovered through its IsoSeismic Survey technique that the biggest cause of blast-vibration complaints is not in the quarry, it's the geology where the houses are built. The IsoSeismic Survey reveals the dynamic characteristics of the geology surrounding an operation-to map the way the ground wants to vibrate.

When an explosive charge is detonated in a borehole, the ground experiences a shock that lasts for only a few tens of milliseconds. A shock like this cannot produce a certain frequency any more than you can change the frequency of a swing by the way you push it. The resulting ground vibration is determined entirely by the geology, not the explosive, and it represents the natural mode of vibration.

Since most of the geology vibrating is outside the quarry, that same geology is primarily responsible for the characteristics of the vibration event. If the surrounding geology is very similar, it tends to vibrate the same way, sending higher-amplitude and longer-duration vibrations to greater distances (Figure 1)#.

If the geology changes abruptly, the amplitude of the vibrations may die out very quickly, but low frequency may dominate the remaining vibration (Figure 2). The very short charge detonation can yield several seconds of vibration (Figure 3), and the PPV can be higher at some locations farther away than at others nearby (Figure 4).

Blast vibrations always will be more or less perceptible to certain neighbors because of where they live, not because of blast design (Figure 5). The more complex the local geology, the more likely an operation is to experience pockets of complainants. These trends have been observed at all types of quarries and mines in every part of the country.

Good neighbors can be profitable Once the influence of geology is understood, blasts can be configured in such a way that the delays disrupt the ground's natural rhythm, pushing it at all the wrong times to make it vibrate. If the ground can't vibrate very well, neither can the houses built on it. The result is that the ground vibrates at a lower amplitude, for a shorter duration and with less low-frequency energy to shake the neighbors.

Computer modeling, used in conjunction with the large volume of blast-vibration data collected by the IsoSeismic system, can determine how blasts should best be configured to minimize the vibration perceived by neighbors and still meet production goals. Smaller blasts with more load decks and shorter delay intervals rarely are successful at doing this.

A quarry may never be able to eliminate complaints (unless it can change the geology around the operation), but there are ways to decrease blasting complaints without increasing costs and reducing production.

For those who still think only peak particle velocity (PPV) is important, are any of these situations familiar?

* PPV is low and complaints still come in.

* A neighbor complains about a blast one day but says the next day's blast was better, even though PPV increased.

* One neighbor complains, even though closer residents don't.

* Two neighbors complain, but one only complains about stripping shots while the other only complains about production shots.

* Reducing pounds-per-delay by adding decks elicits more complaints.

* Neighbors don't complain about a big shot but many of them call about a small trimming shot.

All of these situations are common and are the result of ground-vibration factors other than amplitude producing structural response.