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Blasting Vibrations
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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 a brief discussion of blasting generally and its vibration effects on structures, with an introduction to the technical language of blasting. This chapter is not intended to teach the proper and safe use of explosives. See the papers and sources referenced in this chapter and throughout the CVDG for more information on these important topics. Mining Blasting Vs. Construction Blasting Mining blasting is used to extract valuable resources (coal, building materials, 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, pipelines, 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; such blasting presents additional hazards (e.g. "flyrock" (rock chunks thrown through the air) and "airblast" (shock and sound)), due to the less confined nature of the explosion. The procedures and materials used in both mining and construction blasting are broadly similar. 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, molecular structures with both structural elements built in are a little like loaded guns - prepared to react at any time. There are many compounds, regularly used by organic chemists, which are potentially explosive; some of them are trivially easy to make from common household chemicals, even though chemists overwhelmingly use them safely. Often a chemist's goal is to avoid making these compounds unintentionally, to circumvent handling them in dangerous ways or to destroy them safely when their formation cannot be readily avoided.[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. When used in mining or construction, the shock wave is transmitted to the 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, but was, and is, dangerous if not handled with care. It was supplanted in blasting by nitroglycerine, a far more powerful, and hazardous, 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. Military explosives are mostly formulations of 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX). PETN and RDX are not widely used as primary explosives in surface mining and construction blasting. There are many other explosive compounds used in specialized military applications, as well. The most common explosive used today in mines and quarries is a combination of ammonium nitrate (AN - the oxidizer) mixed with fuel oil (FO - the reducer) in a specific ratio. "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 and very powerful and perilous, when misused, so, like all explosives, its safe use requires trained, qualified, knowledgeable blasting specialists. Explosive Power Although there are a number of different measures of explosive "power", the one most commonly seen is "detonation velocity". It is a measure of the speed at which the reacting front moves through the explosive. Unstable materials which have low detonation velocities tend to burn, rather than detonate. Those which detonate with high velocity direct more of the energy released into the explosion. Gunpowder has the lowest detonation velocity, followed by ANFO, then nitroglycerine, PETN, and RDX, among the explosives indicated above. There are many others; some, mostly for military use, have still higher detonation velocities. 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 commercial explosives can be purchased pre-packed in sticks, tubular bags or other shapes for use in legitimate blasting applications. They are also used as thickened or gelled water slurries, which are pumped into blast holes. 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, where movement of the "plunger" generates the electrical pulse which starts the explosion. 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 often designed to break the rock, while a second detonation a few milliseconds 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 dislodging 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) generate their own vibrations, although these are usually of lower ground velocity (PPV - Peak Particle Velocity) and are of longer duration than the intermittent, short duration ones (usually, well under 1 second)[11] 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 ground vibration velocities generated by the blasting as a function of vibration frequency.[15] 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 vibration 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 often 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, or based on, 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 properly 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. Perhaps most importantly, 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 in most examples. Thus, fewer people are likely to note them in any detail, even though both the sound and vibration from blast shots are noticeable. 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, if you have concerns about nearby blasting. 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 general 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 from the blast, 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 (a measure of the instantaneous energy released in the explosion), 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. OSM regulations allow a blaster to avoid doing any vibration monitoring at all during blasting, so long as his blast meets certain scaled distance criteria.[5] The table below shows those criteria.
When plotted with logarithmic scales on both axes, as shown below 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 value, -b, is generally related to the type of soil or rock through which the vibration moves. Note that the b exponent in this case in not the same one referred to above, which characterizes the dependence of the vibration velocity on charge weight; this one indicates the attenuation of the ground vibration peak particle velocity with distance. 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. In most calculations of vibration velocity using the blasting PPV equation, where the specific value of b is unknown, it is usually assumed to have a value of 1.6, although that assumption is not always realized in real-life blasting.[14] The K constant is often referred to as a "confinement factor" and typically takes values from 20 to 250, with the average K value about 150 (in U.S. units). Highly confined blasts can have K values over 600.[14] When measured blasting PPV's are compared with corresponding calculated ones for the same locations, the correlation between them is often low. We have several times correlated seismograph-measured blasting PPV's with calculated ones and found correlation coefficients (r2) as low as 0.1 (cf. 1.0 for perfect correlation). This is just a scientific way of describing the limited value of calculated PPV's in blasting. There is no substitute for actual vibration monitoring in blasting, even though OSM regulations do not require vibration monitoring in all situations.
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 a version of what these probabilities look like for three different levels of damage.[6] Other data from USBM RI 8507 indicate that low frequency blasts show a greater change in damage probabilities as the PPV increases than do high frequency blasts, where damage probability rises very rapidly from its onset point.[6] In all cases, damage probabilities increase very rapidly once one exceeds a threshold level, typically at or near the standard limit. Thus, any vibration which is over properly applied blasting standard limits should be seen as potentially having an unacceptable probability of damage to structures. There is much more discussion of this point in the CVDG chapter, Vibration and Damage. You can calculate your own blasting damage probabilities from blasting PPV's with the Vibrationdamage.com Ground Vibration and Safe Distance Calculator, free to registered users of the CVDG and CVDG Pro. 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 and technical 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.
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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 from vibrations caused by construction heavy equipment. There is a large
scientific and technical literature of
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 
