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Blasting Vibrations

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Blasting is, by far, the most-studied source of ground vibration, as the potential for vibrations from blasting to damage structures was recognized far earlier than that of construction heavy equipment.  There is a large scientific and technical literature of blasting vibration effects on homes and other structures worldwide.[1] I have discussed various of the critical conclusions from that literature in many other places in the CVDG, where they can be reasonably applied in construction settings. I have also pointed out the many differences between blasting vibrations and construction heavy equipment vibrations.

Although the CVDG is focused on construction heavy equipment vibrations, some additional background on blasting vibrations may be helpful to those who face nearby blasting, as it is one of the most common sources of apparent vibration damage reported to Vibrationdamage.com. In this chapter, I'll give an introduction to blasting generally and its vibration effects on structures.

Mining Blasting Vs. Construction Blasting

Mining blasting is used to extract valuable resources (coal, hard rock minerals) from the ground, while the much less common construction blasting is mostly used simply to remove rock to make way for improvements to property (e.g. highways, buildings, developments). Most mining blasting which has potential for damage involves use in surface mines, as the amounts of explosive used in surface mining are usually much greater than those used in underground mining. The explosion is often closer to structures in surface mines, with additional hazards (e.g. "flyrock") due to the less confined nature of the explosion, as well. The procedures and materials used in both mining and construction blasting are broadly similar, although there are differences in the amount of explosive employed and the degree of confinement of the explosions. Mining and construction blasting differ somewhat in the vibrations they produce, as discussed below.

About Explosives

Most explosives, at the molecular level, carry both a chemical oxidizer (e.g. oxygen, peroxide or nitrate) - to chemists, a material or structural group which can accept electrons or hydrogen atoms - and a reducer, the counterpart of an oxidizer (a different material or structural group which can easily transfer hydrogen atoms or electrons to oxidizers), usually in the same organic chemical molecular structure. Because oxidizers and reducers tend to react spontaneously with production of great quantities of energy, such structures with both built in are a little like loaded guns - primed to react at any time. There are many compounds, regularly used by organic chemists, which are potentially explosive; lots of of them are trivially easy to make from common household chemicals, even though chemists overwhelmingly use them safely. Often the goal is to avoid making these compounds unintentionally or to avoid handling them in dangerous ways.[2],[3]

When explosives are detonated, the organic structure is converted in a tiny fraction of a second via chemical reaction to gaseous products like water, carbon dioxide and nitrogen oxides. Because the process is what chemists refer to as exothermic (i.e. generating heat and occurring spontaneously, once started), the reaction produces much heat and a shock wave from the rapidly expanding gases created from the explosive decomposition. The shock wave is transmitted to the mine rock, breaking the rock as it passes. It is this shock wave, which, further away, we detect and speak of as "vibration".

A Very Short History of Blasting

Gunpowder (a finely ground mixture of sulfur, carbon and potassium nitrate) was the first explosive used for blasting rock. It wasn't very powerful, when compared with later explosives, and was supplanted by nitroglycerine, a far more powerful, and dangerous, liquid explosive. "Nitro", as it is sometimes called, is very shock-sensitive, as most people know. Alfred Nobel, whose discoveries ultimately funded the Nobel prizes, developed dynamite, nitroglycerine absorbed in a filler like diatomaceous earth, as a safer substitute for nitroglycerine. Dynamite was far less sensitive to shock than nitroglycerine and could be readily formed into easy-to-use sticks of the sort many people have seen, or even used. Dynamite isn't completely insensitive to shock (e.g. in high temperatures), but it is sufficiently less sensitive that it is usually detonated with a blasting cap. Blasting caps have a small amount of the highly shock-sensitive mercury fulminate (from the Latin root, fulminat_, "struck by lightning"), or similar compound, which starts the explosion in the dynamite.

While dynamite is still used all over the world, many other explosives have become available for use in special applications. These newer explosives, e.g. pentaerythritol tetranitrate (PETN), are both much more powerful and much less sensitive to shock than dynamite. They are not widely used as primary explosives in surface mining and construction blasting. Perhaps the most common surface mine explosive used today is ammonium nitrate (the oxidizer) mixed with fuel oil (the reducer).  "ANFO", as it is usually known, is cheap, powerful and insensitive to detonation other than by a "booster" or primer (e.g. dynamite or PETN), which is itself ignited by a blasting cap. ANFO is widely used in quarries and surface mines. It is still very dangerous, when misused, so, like all explosives, its safe use requires trained, qualified, knowledgeable blasting specialists.

It can matter, from a vibration standpoint, which explosive is used. For example, it has been reported that underground mining explosions using ANFO produce less vibration than equivalent explosions with dynamite or slurry.[12] Vibration is only one factor used in making a choice of which explosive to use, but it should be one which is considered well.

Blasting Procedures

Dynamite, ANFO and other explosives can be purchased pre-packed in sticks, tubular bags or other shapes for use in legitimate blasting applications. A mining drill machine is used to create, typically, 15-30 feet deep holes in a specific pattern designed for maximum blasting efficiency. Each blast hole is packed with the calculated explosive charge weight, followed by gravel or rock "ballast" placed over it to hold the explosive, and explosion, in place. The blast holes are connected with detonation cord of lengths so as to provide a specific delay between the detonation of the charge in each hole. Often, the delay is of the 8-9 thousands of a second, or "milliseconds", duration range. Typically, the explosives are detonated electrically, either by battery or, for the old plunger-driven detonators, a small dynamo. The combination of all these multiple, very closely timed, relatively small detonations is usually referred to as a "shot" by those in the blasting field.

As discussed elsewhere in the CVDG, the initial detonation is designed to break the rock, while a second detonation a few milliseconds (thousandths of a second) later heaves the rock away from the rock face, where it can be loaded for further processing. Such multiple, timed detonations are most efficient at moving the rock, while minimizing the undesirable side effects of ground vibration, noise, and rock fragments thrown large distances through the air ("flyrock").

A very active surface mine (e.g. a large coal mine) may blast once per day. A typical quarry will blast once or twice a week. Rock processing equipment (e.g. rock crushers) and transport equipment (rock haulers) may generate their own long-lasting vibrations, although these will usually be of lower velocity (PPV - Peak Particle Velocity) than the intermittent, short duration ones (usually, well under 1 second) caused by blasting.

Regulation of Blasting

In theory at least, surface mine blasting is highly regulated in the U.S. by the U.S. Bureau of Mines (USBM), Office of Surface Mining (OSM). You can see the relevant regulations in the OSM Blasting Performance Standards, 30 Code of Federal Regulations, Sec. 816.61 (first page shown at right). These regulations specify many aspects of how blasting can be done, including defining acceptable vibration velocities for the blasting as a function of vibration frequency.

Because blasting vibration has so long been recognized as a potential source of vibration damage to homes, the USBM has studied blasting vibration in many of its Reports of Investigation (the USBM RI reports). It also provides many training tools for blasters, most notably the OSM Blasting Guidance Manual. Perhaps its most important role has been in setting the widely-cited (sometimes improperly so, for heavy equipment-caused construction vibration) OSM blasting vibration standard. This standard, shown at left, is discussed in some considerable detail in the CVDG's Vibration Standards chapter, along with other vibration standards much more appropriate for construction vibration application.

As with most other vibration standards, the OSM standard PPV limits are frequency-dependent, with lower frequencies having a lower allowable vibration maximum velocity. The lower frequencies overlap structure resonance frequencies, thereby increasing the potential for damage through additive buildup of vibrations in the structure (See Resonance and Fatigue for more on this point). This frequency dependence is often a critical point when trying to understand statements to the effect that "standards were met". When such statements say that the limit is the 2.0 in/sec high frequency limit set in the OSM standard, but the actual vibration frequencies seen in the blasts are much lower in frequency, as is typically the case, then the standard says that it is the limit appropriate for the observed frequencies which must be met.

In addition to the U.S. Federal regulations, individual states have their own regulations affecting blasting. Typically, they follow fairly closely the OSM regulations, but may differ in some specifics.[8] Of course, no standard is "right", if it is applied improperly to situations for which the standard was not designed, if it is misused, or if it is applied to vibration data which are, themselves, suspect for one or more reasons. The only way to know for certain if a blasting operation, for any purpose, is meeting a vibration standard at the location of a given home or structure is to measure the ground vibration at the structure for a set of truly representative blasts.

Blasting Vibrations vs. Construction Vibrations

Blasting vibrations differ in just about every significant way from construction vibrations. Vibrations caused by blasting last less than the duration of the vibrations induced in a structure, while construction vibrations from heavy equipment use last much longer than the structure vibrations. The longer durations of construction vibrations allow maximization of resonant enhancement of vibrations in homes, relative to blasting vibrations.[7],[11] Construction vibrations are also typically repeated FAR more times in a given time period than blasting vibrations. There are many examples of construction vibration plots throughout the CVDG which show this fact well.

The combination of these differences has a major effect on determination of damage potential from the two, very different, activities. The net effect of the distinctions is that typical construction vibrations emanating from heavy equipment use provide from tens to hundreds of times as much total vibration exposure as blasting (see Resonance and Fatigue for a brief description of our quantitative studies of this matter) in the same period of time. The frequency distribution of blasting vibrations also is often somewhat higher than those of construction vibrations from heavy equipment. The lower frequencies, as well as the longer durations, in construction vibrations increase the degree of resonant enhancement over that possible with blasting vibrations.[4],[7]

Documenting Blasting Vibrations

The longer duration of construction-related vibration events means that many people are likely to have witnessed them. Blasting vibrations occur infrequently, typically less than once per day; their associated vibrations last for only a second or two at most. Thus, fewer people are likely to note them in any detail. Although blasters are required to keep records of their blasting, you may not be able to get copies or they may be provided incompletely. For these reasons and others, it's a good idea to keep your own log of blasting events. The log should indicate the date and approximate time of the blast, whether the blast was seen, heard, felt or all the above, who observed it and any factual comments you feel are relevant. Having such a log may give you the leverage to force production of the blaster records. It will certainly put you in a better position if you have damage related to the blasting. The CVDG chapter, Recording Damage, has lots of tips on how to record both damage and its possible causes.

Attenuation of Blasting Vibration with Distance

As with other types of vibration, blasting vibration velocities (PPV's) decrease with increasing distance ("attenuate"). This is the reason that people are often told that they are "too far" from the blast for it to have caused damage, whether or not that is actually true for the blasts in question. Vibration amplitudes (intensities or velocities) emanating from a mining blast are usually described with some form of the following exponential equation:

A = kWbDn               

where A is a measure of the peak vibration amplitude (or velocity), W is the explosive charge weight per delay, D is the distance, and k, b, and n are constants determined by a given site or blasting procedure.[1] Typical values found in the literature for b range from 0.4 to 1.0 and for n from -1 to -2. Because the exponent, b, which describes the dependence of the vibration amplitude on the charge weight per delay of explosive, is often near 0.5 (i.e. the square root of the charge weight), one can, in principle, remove the dependence on charge weight per delay by dividing the distance by the square root of the charge weight per delay. The result is a scaled distance, which is widely used in blasting contexts to help estimate damage risk. In fact, the OSM regulations allow a blaster to avoid doing any vibration monitoring at all during blasting, so long as he meets certain scaled distance criteria.[5]

When plotted with logarithmic scales on both axes, as shown above in a diagram of vibrations from a construction blasting example, the equation describing the vibration intensity as a function of scaled distance (typically symbolized as SD or Ds), PPV = K(SD)-b, produces a line, whose slope, -b, is generally related to the type of soil or rock through which the vibration moves. Very soft soils or soft rock produce values of b up to about 1.8; harder soils or rock produce values of b as low as about 1.0, although b may be even lower in certain locales, especially in the U.S. state of Florida.[13] The higher the value of b, the more the vibration dissipates with increasing distance.

The division of the distance by the square root of the charge weight is usually referred to as "square root scaling" in the scientific literature, since its goal is to place a series of blasts with different amounts of explosive on the same source energy basis. Square root scaling is normally used for any vibration data resulting from blasting in drill holes with multiple charges - perhaps the most common situation in construction and mine blasting. For point source blasting in particular, cube root scaling, i.e. dividing the distance by the cube root of the charge weight per delay, sometimes provides better correlation of the observed velocities with distance.[9],[10] The dependence of blasting vibration on scaled distance is also discussed in more detail in the CVDG chapter, Vibration and Distance.

Blasting Vibration and Damage Probabilities

Because blasting vibrations have been so extensively studied, something is known of the statistical probabilities for damage to a structure as the vibration velocity increases. The diagram at the beginning of this chapter shows what these probabilities look like for three different levels of damage.[6] Damage probabilities increase very rapidly once one exceeds a threshold level, typically at or near the standard limit. Thus, any vibration which is over blasting standard limits should be seen as unacceptably likely to cause damage to structures. You can find much more discussion of this point in our chapter, Vibration and Damage.

Is That All?

This may be more information than most people want to know about blasting, particularly construction blasting, although it is only an introduction to a tiny fraction of a huge scientific literature pertaining to it. But, if your home or building is ever damaged by blasting, you may well find yourself needing to know at least the basics presented here. Other information about blasting vibration science is found throughout the CVDG. The CVDG Pro chapter, Calculating Vibration Amplitudes, has considerably more information about blasting vibration and means of modeling its decrease with distance.


[1] We reference many blasting studies, mostly related to side effects of blasting like vibration, noise and flyrock, throughout the CVDG. For the learner, some of the most important and influential in that area are:
Structure Response and Damage Produced by Ground Vibration From Surface Mine Blasting, D. E. Siskind, M. S. Stagg, J. W. Kopp, and C. H. Dowding, United States Bureau of Mines Report of Investigations 8507 (USBM RI 8507), 1980
OSMRE Blasting Guidance Manual, Michael F. Rosenthal and Gregory L. Morlock, Office of Surface Mining Reclamation and Enforcement, United States Department of the Interior, 1987
Effects of Repeated Blasting on a Wood-Frame House,
Mark S. Stagg, David E. Siskind, Michael G. Stevens, and Charles H. Dowding, United States Bureau of Mines Report of Investigations 8896 (USBM RI 8896), 1984
Blasting Vibrations and Their Effects on Structures, Harry R. Nicholls. Charles F. Johnson. and Wilbur I. Duvall, United States Bureau of Mines Bulletin 656, 1976
Our More Information chapter provides a link to the
OSM Blasting Download Page (https://www.osmre.gov/resources/blasting/ARBlast.shtm), where you can get free copies of these, and many more, blasting vibration studies.
[2] Homeowners can make toxic and/or explosive chemicals by accident when they mix, contrary to packaging cautions, certain household chemicals. Read the packaging before you attempt to mix household cleaning agents. Untrained non-chemists should NOT attempt to make or handle explosive materials. I once had a non-chemist college fraternity brother who lost a few fingers on one hand because he was making in his fraternity room a well-known contact explosive from commonly available, non-explosive materials. He made a few grams of the explosive (about one-tenth of an ounce), the detonation of which destroyed his fraternity room and put him in the hospital for several weeks. I, as an organic chemist, had repeatedly warned him of the dangers before the explosion. Of course, these days, an unusual interest in explosives might also attract the attention of the U.S. Department of Homeland Security.

[3] Wikipedia has a basic article on explosives at https://en.wikipedia.org/wiki/Explosive. It gives an overview, but no formulae or procedures, fortunately, for making explosive compounds.
 
[4] Structure Response and Damage Produced by Ground Vibration From Surface Mine Blasting, D. E. Siskind, M. S. Stagg, J. W. Kopp, and C. H. Dowding, United States Bureau of Mines Report of Investigations 8507 (USBM RI 8507), 1980, p. 6, et seq.  
[5] OSM Blasting Performance Standards, 30 Code of Federal Regulations, Sec. 816.67 Use of explosives: Control of adverse effects
 
[6] The diagram shown was created from data in Structure Response and Damage Produced by Ground Vibration From Surface Mine Blasting, D. E. Siskind, M. S. Stagg, J. W. Kopp, and C. H. Dowding, United States Bureau of Mines Report of Investigations 8507 (USBM RI 8507), 1980, pp. 58-60. These probabilities represent the risk of damage to a group of homes of similar construction from a single blast producing a given PPV, not the risk of damage to a single home from multiple blasts.  
[7] "The relatively short duration (impulse-type loading) of blasting vibrations does not lead to any important resonant effects in building components. However, with periodic excitation due to pile-driving, vibrators and traffic considerable resonance effect is sometimes possible." Swiss Standard for Vibrational Damage to Buildings, J. Studer and A. Susstrunk, Proceedings, X. Int. Conf. ISSMFE, Stockholm, Vol 3., p. 308 (1981)
[8] A now-dated compilation of cities, states, provinces, and countries having blasting vibration regulations is:  Blast Vibration and Seismograph Section Report, International Society of Explosives Engineers, 2003G Volume 1, Appendix 3, p. 11; a formal paper of the findings appeared as: A Survey of Blasting Vibration Regulations, L.C. Schneider, Fragblast, International Journal for Blasting and Fragmentation, Volume 5, 2001 - Issue 3, pp. 133-156  
[9] Cube root scaling was tested for these data, but did not provide any better correlation of the data than square root scaling.  
[10] For comparison with heavy equipment reference values, which range from about 0.1 to 1.5 in/sec at the FTA reference distance of 25 feet for normal and approved use of equipment, I have used one of the sets of data which comprise part of the plot for construction blasting, involving a measurement of PPV of 0.46 in/sec at 325 ft. from a rock blast using 13 pounds of explosive charge per delay. Calculating with the Vibrationdamage.com Ground Vibration and Safe Distance Calculator using that information, one arrives at a predicted PPV, for the observed attenuation exponent of 1.7, at the reference distance of 25 feet, for this blast of 36 in/sec. Another blast in this same location, using 24.74 lbs. of explosive per delay gives a similar velocity at the 25 ft. reference distance of 32 in/sec. This calculation is provided only for comparison purposes with construction vibrations and may not be quantitatively accurate in all blasting circumstances, but it gives an idea of the relative energies involved. Blasting is almost always carried out at distances far greater than 25 feet from structures. It is also highly regulated by municipal, state and Federal governments to avoid damage as much as possible, unlike construction heavy equipment vibration.

Note that unapproved use of construction heavy equipment may generate substantially higher vibration velocities. For example, a calculation of the vibration velocity from pounding with an excavator bucket to demolish asphalt, based the velocity necessary to bring about the observed breaking of monolithic concrete in several different locations 180 feet away, gives a minimum velocity of 10 in/sec, at the reference distance of 25 ft., for such pounding. It is noteworthy that the contractor involved in this work had already banned such pounding by its employees, because it was "hard on the equipment".
 
[11] "The 1-sec time was an overall compromise that was long enough to include all the significant energy in a typical mine blast vibration measured near the source." Structure Response and Damage Produced by Ground Vibration From Surface Mine Blasting, D. E. Siskind, M. S. Stagg, J. W. Kopp, and C. H. Dowding, United States Bureau of Mines Report of Investigations 8507 (USBM RI 8507), 1980, p. 32
 
[12] Vibrations From Underground Blasting, James J. Snodgrass and David E. Siskind, United States Bureau of Mines Report of Investigations 7937, 1974, p. 30  
[13] Blasting vibrations in South Florida show vibration attenuation (b) exponents in the range 0.9 to 0.7: Attenuation of Blasting Vibrations In South Florida, M. Johnson, J. Pepper, G. Mclellan, Proceedings Of the Twenty-sixth Annual Conference on Explosives and Blasting Technique, February 13-16, 2000, International Society of Explosives Engineers, 2000; Lee County Blasting Study, F. J. Lucca and A. Williams, June 2005 (Report)  
 

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