Chapter 26: Vibration

Vibrations from blasting impact nearby residents and structures in the form of ground and air vibrations. In the last several decades the development of vibration standards, measurement equipment and technology, and knowledge of vibration characteristics have significantly reduced the potential for structural and other damage from blasting. However, public response still remains high partly because relatively few persons are aware of the advances in science and technology designed to protect them.

This chapter presents information about both ground and air vibrations, the characteristics of vibrations, vibration monitoring, and vibration data analysis. With this information, the blaster can select parameters that can reduce or control blast-generated vibrations to a safe level while achieving the blast goals.

GROUND VIBRATIONS

Ground vibrations are one of the most common blasting complaints. The ability to feel ground vibrations at relatively low levels does not mean that the vibrations can cause structural damage. The vibration level and accompanying frequencies must be evaluated for proper assessment of the potential for damage.

Wave Types

When the confinement on a detonating explosive is released, the explosive energy enters the surrounding rock and travels outward in the form of body waves. Two different forms of body waves are created: (1) compression (P-waves); and (2) shear (S-waves). When these waves reach the ground surface and travel along it, they are referred to as surface waves. Surface wave formation is a result of the body wave energy arriving at the ground surface and of the body wave energy interacting with the interface of air and earth. These surface waves travel along the surface layer at varying depths, depending upon the wave frequency. The wave type most commonly measured at the surface is the Rayleigh (R-wave) wave. The R-wave travels at about 90% of the S-wave velocity and does not exist where the rock meets the air.

Fundamental Characteristics Of Ground Vibrations

The fundamental characteristics of ground vibrations are frequency, amplitude, and duration.

Frequency is the number of vibration cycles occurring in one second. It has the units of cycles per second or hertz.

Frequency is the number of vibration cycles occurring in one second. It has the units of cycles per second or hertz (Hz). The seismograph records ground motion by sensing the motion of a weight in a case with respect to the case while the case moves with the ground. The number of times the weight moves back and forth through the case over a period of time determines the frequency. This represents the frequency of ground movement as a result of the blast. Ground vibration frequencies generally range between 1 Hz and several hundred Hz, depending on several factors including: (1) distance between the blast and the point of interest, (2) geology, and (3) blast design. However, the generally experienced ground vibration frequencies are those above 6 Hz.

Particle velocity is the speed at which a ground particle moves when excited by the passing seismic wave. Peak particle velocity (PPV) is the maximum particle velocity. Sometimes referred to simply as peak velocity, particle velocity is generally measured in inches/second (in/sec), or millimeters/second (mm/sec). Ground particle velocity and amplitude are directly related by the vibration frequency.

Duration is simply the length of time that a vibration exists (see figure 26.1). Duration of a blast is dependent upon the total detonation time of the blast, and thus is dependent upon the number of delays and length of delay intervals. Due to wave interference, duration may be less than or equal to the total blast detonation time.

Ground Particle Motion

Ground particles are three-dimensional, generally elliptical, motions. The motion can be vertical, radial (toward and away from the blast) or transverse (side to side). These directional motions are referred to as components of the ground vibration. The seismograph senses and records these separate directional motions (see Figure 26.7).

Occasionally the peak vector sum (PVS) is used as a reference for vibration prediction or regulatory compliance. PVS is the highest vector sum value for the component waveforms and must be calculated from each time interval along the whole waveform (See equation 26.5).

$$PVS = (R^2 + V^2 + T^2)^{0.5}$$ <!-- VERIFIED -->

Equation 26.5

Where:

• $PVS$ = Peak vector sum (millimeters/second) (inches/second)
• $R$ = Radial particle velocity (millimeters/second) (inches/second)
• $V$ = Transverse particle velocity (millimeters/second) (inches/second)
• $T$ = Vertical particle velocity (millimeters/second) (inches/second)
• $t$ = Any point in time in the waveform

Caution

The peak vector sum (PVS) differs from the pseudo- vector sum obtained by using the component peak particle velocities.

Attenuation

Vibration intensities (decrease or decay in amplitude) with increasing distance as do ripples in a pond of water from the point of origin. The sequence of figures 26.3a through 26.3e illustrates a series of seismograph waveforms from a linear array of seismographs located at increasing distances from the same blast. Peak amplitude decreases in a fairly regular manner, which often makes it possible to predict what the vibration level might be at various distances from the blast. Records from such studies are also used to establish the site-specific vibration attenuation equation for a blasting seismograph. The information and decaying trend over distance of the readings from these figures is summarized in table 26.2. Generally these waveforms have three characteristics as a result of increased distance: (1) vibration amplitudes decrease, (2) frequencies will decrease, and (3) the duration can increase.

The two most influential factors on vibration amplitude are: (1) the weight of a charge and (2) the distance to the charge. In general, the following two relationships are true: (1) vibration amplitude increases as the charge-weight increases, and (2) vibration amplitude decreases as distance increases. Charge-weight and distance have a significant influence on the propagation of ground vibrations.

Table 26.2 – Summary of vibration readings from a linear array of seismographs.

In any one direction, vibrations generally decay in a predictable manner (directly with distance and inversely with charge-weight). Distance and charge-weight are used to define square root scaled distance calculated with equation 26.6.

$$SD_2 = \frac{R}{W^{0.5}}$$ <!-- VERIFIED -->

Equation 26.6

Where:

• $SD_2$ = Square root scaled distance (Sometimes written SD) (meters/kilogram^0.5) (feet/pound^0.5)
• $R$ = Distance from blast to a point of interest (meters) (feet)
• $W$ = Maximum charge-weight detonated within any 8-millisecond period (kilograms) (pounds)

This equation can be expressed as either equation 26.6a or 26.6b:

$$SD_2 = R \div \sqrt{W}$$ <!-- VERIFIED -->

Equation 26.6a

$$R = SD_2 \times \sqrt{W}$$ <!-- VERIFIED -->

Equation 26.6b

When combined with measured vibration amplitudes, the seismic characteristics of an area can be defined. Maximum PPV measurements are typically plotted on a log-log graph against scaled distance as shown in figure 26.9. In this type of graph, the relationship between the two parameters shows a linear straight line trend (rising log line) that may be fitted with an equation describing the trend. In this for example, six data points from one blast illustrate how vibration amplitudes attenuate over distance. If this trend is an accurate representation of site attenuation in that increasing scaled distances, the equation can be used for vibration amplitude prediction. The best fit trend is shown with a line, often called the "mean line". In this case, a best fit line could easily be hand drawn along the data points. When the data are scattered, a least squares analysis can provide confidence intervals on the data set. Details on least squares regression analysis are outside the scope of this handbook.

The best fit line to calculate the peak particle velocity from scaled distance is defined by equation 26.7.

$$PPV = A \times (SD_2)^B$$ <!-- VERIFIED -->

Equation 26.7

Where:

• $PPV$ = Peak Particle Velocity (millimeters/second) (inches/second)
• $SD_2$ = Square root scaled distance (meters/kilogram^0.5) (feet/pound^0.5)
• $A$ = Intercept of the line at a $SD_2$ value of 1
• $B$ = Slope of the line (note that the slope is negative).

"A" is most closely related to the type of industry blasting and "B" represents the attenuation rate of vibration based on the site conditions. Greater "B" values represent stronger attenuation characteristics.

Over the years, research has resulted in general equations that can be used in lieu of site-specific data for predicting ground vibration amplitudes from scaled distances. The attenuation rate for any area will be influenced by industry type, blast design, geology, topography, and rock type, soil thickness, etc. Table 26.3 lists vibration amplitude equations for different types of industries. Statistically, the equations may estimate the best fit or upper bound vibration level. Best fit equations are meant to approximate average vibration levels. Upper bound equations will generally estimate the highest potential vibration level, with at least 95% confidence. Keep in mind that the equations will only be valid for blasts that are well designed and implemented properly in the field.

Table 26.3 – Vibration amplitude equations for various blasting industries.

It follows that using the upper bound equations potentially results in severe limitations on the charge-weight per delay. For this reason it is a good idea to obtain site-specific information in order to avoid severe blast design limitations in terms of charge-weight per delay. The methodology on how to evaluate site-specific data is discussed in the Vibration Data Evaluation section of this chapter.

As a general rule, the peak particle velocity (ground vibration) from blasting in most geological settings will decrease by approximately ½ each time the distance is doubled. For example, at 60 meters (200 feet), the peak ground vibration will be about ½ what it is at 30 meters (100 feet). At 120 meters (400 feet), it will be about ¼ what it is at 60 meters (200 feet), and about ⅛ what it is at 30 meters (100 feet).

The same relationship also holds in reverse. If a peak vibration was measured at a distance of 120 meters (400 feet), it could be expected to be about 9 times higher at 30 meters (100 feet). While this relationship does not hold true in all cases, it is a good general rule.

EXAMPLE 26.9

Calculate the expected peak particle velocity from a coal mine blast that uses 400 kilograms of explosive per 8-millisecond delay at a distance of 500 meters from a residence.

Step 1

Calculate the scaled distance using equation 26.6.

$SD_2 = \frac{R}{W^{0.5}}$

$SD_2 = \frac{500}{400^{0.5}}$

$SD_2 = 25$

The square root scaled distance is 25 meters/kilogram^1/2.

Step 2

Calculate the expected peak particle velocity using best fit equation for coal mines from table 26.3.

$PPV = 905 \times (SD_2)^{-1.52}$

$PPV = 905 \times (25)^{-1.52}$

$PPV = 6.79$

The expected peak particle velocity is 6.79 millimeters/second.

EXAMPLE 26.10

Calculate the expected peak particle velocity from a quarry blast that uses 100 pounds of explosive per 8-millisecond delay at a distance of 800 feet from a residence.

Step 1

Calculate the scaled distance using equation 26.6.

$SD_2 = R / W^{0.5}$

$SD_2 = 800 / 100^{0.5}$

$SD_2 = 80$

The square root scaled distance is 80 feet/pound^0.5.

Step 2

Calculate the peak particle velocity using best fit equation for quarries from table 26.3.

$PPV = 112 \times (SD_2)^{-1.52}$

$PPV = 112 \times (80)^{-1.52}$

$PPV = 0.06$

The peak particle velocity is 0.06 inches/second.

Directional Effects

While vibration amplitudes tend to decay predictably in one direction, the vibration may propagate differently on other directions. These characteristics can be defined with a radial or circular array of blasting seismographs set at varying distances and directions. Vibrations traveling through less dense material, like a low velocity soil, will tend to decay faster and may contain low frequency energy. In a dense rock, vibration amplitude will tend to decay more quickly and may contain higher frequencies. Blast attenuation curves in low density materials tend to have steeper slopes, and thus the amplitudes also decay quickly. Blasts in hard rock tend to have steeper slopes. If a higher or larger discontinuity exists between a blast and a peak, the vibration amplitudes will be less than if there was no discontinuity. Vibrations will also tend to be stronger in the direction of blast initiation. When multiple structures are near a blast and in different directions, it may be wise to monitor structures in each direction.

Importance of Frequency

Vibrations from blasting will tend to have much lower amplitudes and higher dominant frequencies than earthquake vibrations. This is primarily due to a blast's lower initial energies and shorter propagation distances. Propagation velocities, amplitudes, and frequencies of both blasting and earthquake waves are related to the elastic properties of the rock, soil, and other materials through which they travel. For clarity, "propagation velocity" is the speed at which the energy travels and is dependent on the stiffness of the medium. For hard rock, it can be up to 6,100 meters/second (20,000 feet/second) and for packed soils as low as 61 meters/second (200 feet/second).

Vibrations from blasting propagate over relatively short distances when compared to earthquakes. A very important influence is dissipation, or "spreading attenuation", where the finite amount of vibration energy fills an increasingly larger volume of earth as it travels outward in all directions away from the blast. The consequence is generally that higher frequency components attenuate faster.

Other propagation effects are from geological material composition and structure. There are losses of energy through absorption, reflection, and dispersion (where different frequency components travel at different propagation velocities). Low frequency vibrations are common where the soils are thick (3 meters (10 feet) and greater) and may be found at sites with surface subsidence.

Close distance (to approximately 305 meters (1,000 feet)) from a blast, vibrations may be dominated by relatively high frequencies created from the time-delayed detonations of the individual boreholes. The exact distance varies depending on a number of factors. Pyrotechnic detonators and explosives technology only allow limited ability to influence dominant frequencies close to the blast. Electronic detonators can provide greater flexibility to influence vibration characteristics.

The surface frequency of ground vibrations from a quarry blast at a structure that sits on soils that are 20 meters thick and have a propagation velocity of 800 meters per second can be approximated by equation 26.8.

$$f = \frac{V_L}{4h}$$ <!-- VERIFIED -->

Equation 26.8

Where:

• $V_L$ = Propagation velocity of body waves in the upper layer
• $h$ = The layer thickness.

EXAMPLE 26.11

Estimate the natural frequency of ground vibrations from a quarry blast at a structure that sits on soils that are 20 meters thick and have a propagation velocity of 800 meters per second.

Calculate the frequency using equation 26.8.

$f = \frac{V_L}{4h}$

$f = \frac{800}{4 \times 20}$

$f = 10$

The frequency is 10 hertz.

Complex Waves and Wave Interference

A blast contains a number of charges detonating milliseconds apart. When these charges detonate at different delay times, vibrations from individual boreholes interact. This interaction results in complex waveforms as shown in figure 26.11. Complex waves contain many different frequencies where a simple harmonic wave contains only one frequency.

The specific frequency components of a complex wave are difficult to interpret. Often advanced mathematical computations with computer programs are necessary to evaluate the component frequencies. The most common analysis is a fast fourier transform (FFT). This analysis simply breaks a complex wave into its component single frequency waves and weights the relative energy in each frequency in graphical form.

Two or more waves may reinforce or counteract each other. This is called either constructive or destructive wave interference (See figures 26.12 and 26.13). When there is constructive wave interference, the waveform will tend to have a smooth oblong shape with a very dominant singular frequency. Destructive interference tends to result in a jagged looking waveform with a diverse frequency spectrum.

It is possible to use the concept of destructive interference to influence vibration amplitude and frequency. What is important to understand is that any delay time will constructively interfere with some frequencies and destructively interfere with other frequencies. The best method for determining what timing may be most effective is to analyze the vibrations from a signature borehole.

Signature Borehole Method

The signature borehole method is best used to modify frequency and should "not" be used to predict absolute vibration levels.

With this technique, a single borehole (preferably single-charged) blast is detonated within the project area and recorded with blasting seismographs at locations of concern. The recorded waveforms are free from influence by any extraneous factors other than the nature of the geology through which they traveled. The earth is a linear system, which means that when subjected to a consistent input, a consistent output will be produced. Since a production blast is simply a series of time delayed signature boreholes, we can model the signature waveforms for the influence of various timing scenarios. The end result is hopefully an acceptable delay sequence that will achieve the three desired results, whether that is: (1) reduced amplitudes, (2) higher frequencies, or (3) both, while maintaining desired fragmentation requirements.

This method is generally based on two assumptions. These include both: (1) the amount of explosive used per charge is relatively constant and is similar to that used in the signature borehole; and (2) the geology remains similar to that encountered with the signature borehole, and that the boreholes for production blast will detonate as designed.

Surface waves with frequencies of 4 hertz to 8 hertz are common at mine and quarry areas with thick soil layers, fill material, and glacial or streambed deposits. They can have relatively high amplitudes compared to vibrations propagating through solid rock at comparable distances and charge-weights. These relatively higher amplitudes of particle displacement tend to result from both constructive wave reinforcement due to charges being detonated too close together in time compared to their relatively long wave periods, and from geologically related amplitude magnification.

At low frequency sites, delay intervals of 8, 9 and 17 milliseconds are generally too short to separate vibrations effectively for frequency conditioning. For a 4 hertz wave (with a period of 250 milliseconds), 8 milliseconds is far too short to create destructive wave interference and delays closer to 60 milliseconds are needed to affect a frequency change. Constructive interference appears in the propagation paths where sites generate unusually high vibrations because the effective charge sizes are being underestimated. One way to diagnose this is to compare single-charge blast (signature blast) with production blasts. If measured vibrations are the same, then the effective charges (per delay) are also the same. Where wave reinforcement between charges must be avoided, delay between charges should be at least ¼ of the period of the dominant wave frequency.

The idea of delays is to create phase shift sufficient for destructive interference between vibrations. They do not completely separate the wavelets in time. Vibration wavelets for each delayed charge are about 100 milliseconds to 200 milliseconds long, greatly exceeding typical delays of 9 milliseconds to 42 milliseconds.

Factors Influencing Ground Vibrations

This section focuses on blast design factors the blaster-in-charge can use to reduce ground vibrations and factors that are not in the blaster's control. The single most important factor the blaster-in-charge can control is the maximum charge-weight per delay (maximum quantity of explosive that detonates within any 8-millisecond time interval). Five secondary considerations are: (1) the timing between charges (including the accuracy of the detonator initiation times), (2) geology, (3) the spatial (geometric) distribution of the charges, (4) charge coupling, and (5) charge confinement.

Charge-Weight and Timing

The charge size and timing between charges in a pattern have a strong influence on rock fragmentation and movement. Good fragmentation is related to the amount of explosive per unit volume of rock (powder factor) and proper timing, which allows each charge to break and move the rock before the next charge detonates. Similarly ground vibrations can be influenced by charge size and timing.

Vibration particle velocities are directly related to charge-weight per delay. If the delay time between adjacent charges is adequate, the vibration energy generated by each detonation will leave the blast as a separate seismic pulse. If the time intervals between charges are small or the same time, the seismic energy generated by multiple charges may act as one large charge.

Historically, limiting the maximum charge-weight that detonates within any given 8-millisecond time interval has reduced vibration amplitude. This is still a good general rule to follow for initial blast design. The practice dates to the early 1960s, following a U.S. Bureau of Mines report of tests with delay intervals of 9, 17, and 34 milliseconds. The tests resulted in the effective separation of vibration pulses from multiple charges at hard rock mines. Due to the limitations of the technology at the time, no tests were conducted with delay times shorter than 9 milliseconds.

Caution

Based on this research, many regulations and guidelines stipulate that all charges within any given 8 millisecond period must be considered as a single charge.

Caution

While the 8-millisecond criteria is still in common use, shorter time intervals may be appropriate because of improvements in detonator accuracy.

All initiation systems have scatter in the firing times of the detonators. However, the scatter in firing times for electronic detonators is in microseconds versus milliseconds for pyrotechnic detonators. Pyrotechnic detonators do not always detonate at their nominal firing times (time listed). This may result in elevated vibration levels if two or more detonators of a different delay time detonate near the same time. On the other hand, if two or more pyrotechnic detonators of the same delay are used, the timing scatter may have the same effect although more delays are being used. Blast design factors become increasingly significant in the near field, in particular when decked boreholes are used to control vibrations and the boreholes are in close proximity.

With the development of electronic detonators, the detonator accuracy has improved significantly. New production blasts can be designed for optimum timing by conducting signature blasts, monitoring the vibrations and determining the ideal delay intervals for vibration control. In vibration sensitive environments, use of the more accurate detonators may be essential. Keep in mind that vibration control must be balanced with good blast design practices and fragmentation requirements. The delay times determined for desired vibration modification may not be compatible with these items.

For initial blast design, "square root" scaled distance is often used to determine the appropriate charge-weight per delay for a given particle velocity. If site-specific data are not available, one of the historical equations from table 26.3 may be selected to determine charge-weight per delay. Once site-specific data are obtained the equations can be solved for the target vibration level.

Caution

Upper bound equations (See table 26.3) are often used for initial blast designs and will severely restrict charge weight.

For example, in the United States the upper bound of vibration equation at coal mines from table 26.3 is

$PPV = 3,330 \times (SD_2)^{-1.52}$ (Metric)      $PPV = 438 \times (SD_2)^{-1.52}$ (U.S.)

When this equation is solved for SD₂ for the target vibration level of 25 millimeters/second (1.0 inch/second), the scaled distance design value is 25 meters/kilogram^1/2 (55 feet/pound^1/2). Then maximum charge-weight can be calculated by solving equation 26.6a.

EXAMPLE 26.12

Calculate the maximum charge-weight for a blast 168 meters from the nearest point of interest using equation 26.6a and the above scaled distance solution for a vibration PPV of 25 millimeters/second.

$W = \left(\frac{R}{SD_2}\right)^2$

$W = \left(\frac{168}{25}\right)^2$

$W = 45$

The maximum charge-weight is 45 kilograms. This provides a high level of confidence that the vibration level will not exceed 25 millimeters/second.

EXAMPLE 26.13

Calculate the maximum charge-weight for a blast 550 feet from the nearest point of interest using equation 26.6a and the above scaled distance solution for a vibration PPV of 1.0 inch/second.

$W = \left(\frac{R}{SD_2}\right)^2$

$W = \left(\frac{550}{55}\right)^2$

$W = 100$

The maximum charge-weight is 100 pounds. This provides a high level of confidence that the vibration level will not exceed 1 inch/second.

Site Geology

As discussed above, blast design timing may influence ground vibration frequencies, however vibration frequency is affected primarily by the geology surrounding the blast site. For denser rock or soil, the seismic propagation velocities and associated frequencies will be high. In soft rock or where thick surface soils exist (alluvial valley floors or glacial tills), the seismic propagation velocities and frequencies are lower. In particular, an overburden of uniform thickness tends to propagate low frequency waves that are related to the overburden thickness and wave propagation velocities in the overburden.

As previously discussed, different wave types travel through the earth (i.e. body and surface waves). These wave types travel at different propagation velocities and have different physical characteristics. The site geology determines which of these wave types dominate and in what form. Another property of rock mass that affects the wave propagation and intensity is its uniformity.

Uniformity is a measure of the amount of fracturing or jointing, and its layered continuity. Waves may travel more efficiently through certain flat-lying, competent, continuous sedimentary layers, such as coastal marine limestones or similar rock types.

Vibrations will not efficiently cross dissimilar rock units (different densities and sonic velocities) because of acoustic impedance. Acoustic impedance is the energy's resistance to moving from one medium to another. The strongest acoustic impedance exists between the rock or soil and the atmosphere where the energy is not readily transmitted into the air and is reflected back into the ground.

Rock properties such as strength or moduli (density and sonic velocity) can affect vibrations in a similar way to confinement. If the rock is difficult to break, a greater percentage of the energy may be expended in seismic waves. It is fairly common for these characteristics to change with depth. For example, lower than normal vibration intensities may be generated in the weathered, weak rock near the surface of the borehole, than in normal competent rock at the bottom of the borehole.

In open, porous, and lightly cemented gravels, there is poor contact between the gravel particles, and seismic energy is not efficiently transmitted. Below the water table the voids in the gravel are filled with water, which improves the contact between particles, and thus results in more efficient transmission of seismic waves. Subsequently, vibration intensity may increase when blasting below a water table if the material is in a loose condition. A variation of this condition is found in lightly weathered rock, or rock which is highly jointed or highly fractured. However, in competent bedrock, the presence of a water table does not change the seismic transmission characteristics to any great extent.

Confinement

Charge confinement affects the vibration intensity. If a charge is deeply buried or totally confined without a nearby free face, the rock cannot be displaced (although it is damaged and the explosives send more of the energy is transferred as ground vibration. Excessive confinement can occur with excessive burden or subdrilling. Lack of confinement has the opposite effect. For shallow cover or in front of a charge, the energy is easily vented to the atmosphere and subsequently develops less ground vibration.

Attempts to reduce vibrations by reducing the powder or energy factor may increase vibrations. This is especially true if the effective burden and confinement are increased relative to the available explosive energy. Reducing the powder factor may produce as much or more vibration as before, along with poor fragmentation. These results often require remedial work and additional blasting. It is usually better to use an adequate powder factor to achieve the necessary fragmentation in a well-balanced blast design, and to control the vibrations by the charge-weight per delay.

A good example of a type of blasting with a very low powder factor is presplitting. Proof for presplit perpendicular loads tend to produce the highest amount of vibration, although charge decoupling in the borehole tends to mitigate this effect. Decking charges in the borehole may also inadvertently increase vibrations if the pattern size is not modified to maintain the appropriate powder factor.

Coupling

When small-diameter charges are placed in large-diameter boreholes, the charges are decoupled and less energy is transferred. The degree of explosives coupling affects how much energy is transferred to the rock, which influences the vibration's intensity as well as the fragmentation. Bulk products are most efficient in transferring energy to the rock because the entire borehole is filled and the explosives are in intimate contact with (coupled to) the walls of the borehole. Packaged products may not always fill the borehole and contain air gaps when loaded that diminish the energy transferred to the rock. Given equal confinement and explosives type, lower ground vibrations may occur when explosives are not coupled to the rock in blastholes.

Topography

Prominent changes in geology and topography influence the seismic waves in different directions from a source. An example is where a quarry or surface mine is located on a rock bluff at the edge of a deep alluvial valley. Vibrations behind the bluff will travel through rock and will be characterized by higher frequencies than those traveling in the half circle in front of the bluff with the deep soils.

Spatial Distribution

The geometric arrangement (spatial distribution) of the explosives charges for some types of blasting is important, for example, as when the maximum charge-weight per delay consists of a single large charge in one borehole, many small charges in many smaller boreholes, or similar variations in geometric or spatial distribution of the charges.

The spatial distribution of the explosives affects the frequency and amplitude of the ground vibration. A reduction in vibrations is often found when there are many small charges per delay, widely distributed across the blast site. There is a practical limit to the number of small charges that can reinforce each other. The more there are, the less effective is their reinforcement. For example, a group of 100 charges of 0.45 kilograms (1 pound) each separated by large distances will not generate the same intensity of vibration as a single charge of 45 kilograms (100 pounds). At times, it has been possible to make beneficial use of such scattering factors. In some quarry operations, very close to residences, it has been possible to make large physical separations between the charges, which detonate simultaneously, interspersed with delays of other delay periods.

These spatial separations also allow the blaster-in-charge to detonate two or more physically separated blasts simultaneously, without resulting in constructive reinforcement. This makes it possible to obtain more rock per blast, thus reducing the number of blasts, for improved public relations.

Detonator Timing Scatter

All pyrotechnic detonators contain inaccuracies in their firing times because of the delay elements and this results in firing time inaccuracies or timing scatter. For conventional detonators, the timing scatter increases with the total time it takes to initiate the detonator (i.e. nominal firing time). For example a 9-millisecond detonator may detonate at 8 milliseconds or 10 milliseconds while a 500-millisecond detonator may detonate at 475 milliseconds or 525 milliseconds. When charges are detonated in close proximity and near the same nominal time, the vibration may be reinforced. This is particularly true for boreholes with multiple charges.

Timing scatter in large delay patterns with many boreholes that are spatially separated may reduce ground vibrations. For example, a long-period delay detonator may be used to detonate numerous boreholes with very little probability that two boreholes will detonate at precisely the same instant. The drawback with using the inherent scatter in pyrotechnic delays to avoid simultaneous firing is that the scatter is not predictable. Therefore, there are no guarantees that the scatter will result in the desired results and is not recommended for vibration control. To eliminate timing scatter in critical vibration control areas, electronic detonators should be used. Electronic detonators have precise detonation times within 1 millisecond.

Time of Energy Release

Time of energy release is the amount of time that it takes an individual charge to detonate, and the manner and direction in which the detonation wave passes through the explosive. Examples include cases of tunnels filled with explosives (coyote blasts), very deep individual boreholes, and some instantaneous trenching blasts of hundreds or even thousands of feet in length. Detonation of large quantities of explosives in a very short period of time can lead to high vibrations, though this can be estimated using the scaled distance relations discussed earlier.

Type of Explosive

The choice of explosive is not normally very important in determining vibration intensity at far distances from a blast. The energy output variation for the most common commercial explosives is small, usually less than 10% to 20%. The normal scatter in vibration data usually obscures any variations in vibration associated with explosive type. For uncommon explosive products, or for close-in work, careful evaluation of explosives type is advisable. Test blasts may be needed to evaluate the expected vibration amplitudes.

Summary Of Ground Vibration Control Procedures

Factors that either increase or decrease the ground vibrations are summarized in tables 26.4a and 26.4b. The level of significance each variable has on ground vibration is given. Knowing the extent to which these factors influence vibration is the first step in controlling ground vibrations.

Table 26.4a – Factors within the blaster-in-charge's control that influence ground vibrations.

Table 26.4b – Factors outside the blaster-in-charge's control that influence vibrations.

Field procedures used to reduce ground vibrations are outlined in tables 26.5a and 26.5b. Specific procedure recommendations are given in each category.

Table 26.5a – Procedures to reduce ground vibrations regarding blast design factors.

Table 26.5b – Procedures to reduce ground vibrations regarding blast implementation factors.

AIR VIBRATIONS

When a blast is detonated, air vibrations radiate out in all directions. The frequency, amplitude and duration of the vibrations at a given site will depend on the size of the blast, blast confinement and atmospheric conditions. Earlier chapters have focused on blast size as a function of charge-weight/delay to break rock. This section will focus on the atmospheric and blast design parameters affecting air vibrations.

Air Vibration Characteristics

Air in the atmosphere is a fluid that propagates particle motions in the same manner as water. These air vibrations are pressure waves traveling through the atmosphere like concussions (P waves) in the ground. No shear (S waves) exists in the atmosphere, since fluids have no shear strength.

Air Overpressure

When generated by a blast, air vibrations are measured as time-histories of air overpressure. Air overpressure is the additional pressure generated from a blast above normal atmospheric pressure i.e. pressure by the weight of the atmosphere on the air at the surface.

Air vibrations are audible to the humans ear at frequencies above 20 hertz and are referred to as "sound" or "noise". Air vibrations with frequencies less than 20 hertz are inaudible and are sometimes called "airblast" or "air concussion." Air concussion often feels like a gust of wind by an observer. As a general rule, surface detonations like detonating cord or lightly confined blasts will cause audible air vibrations and confined blasts will cause inaudible air concussions.

Air overpressure originates from the five different potential sources as described in table 26.6.

Table 26.6 – Air overpressure sources.

The air pressure pulse is related to rock volume of the blast and total displacement of the rock volume. When a large volume of rock subsequently moves, such as in a cast blast, it creates a massive air overpressure generating an effect similar to that of a transient wind. Such "winds" are also often felt in tunnels and are generated by a combination of expanding gases and rapidly moving rock and dust.

The gas release pulse occurs when a void exists between the explosive charge and the free face. Expanding gases rapidly follow the path of least resistance and vent out of the face. This causes a sharp pressure rise in the air at the face. The same result is possible for a blast with weak rock near a mud seam.

The stemming release pulse occurs when borehole pressure overwhelms the loading capability of the stemming or the stemming is simply insufficient. The more energy and angular the stemming, the less likely energy will escape through it. If expelled or lifted before detonation is complete, the overpressure emitted from this source increases.

Ground vibrations energy transmitted into the atmosphere is called the rock pressure pulse. This type of acoustic source is minimal. At frequencies above 20 hertz, the effect is audible, but not often noticed.

Lastly, detonating cord, surface delays, and other explosive devices placed on the ground surface and exposed directly to the atmosphere, will cause audible sound. Covering these explosives with soil can eliminate this noise.

Speed of Sound

Sound waves in air travel much more slowly than ground vibrations and are affected by temperature and wind direction. At sea level, the velocity of sound in air is approximately 335 meters/second (1,100 feet/second) at a temperature of 7°C (45°F) and no wind. The speed of sound can be estimated if the temperature and wind direction and speed can be measured. This is shown in equations 26.9 (metric units) or 26.10 (U.S. units). As the temperature and wind velocity increase, the sonic velocity affects the arrival time of air overpressures in a given direction.

$$V_s = 331 + (0.606 \times T) + (0.93 \times V_W)$$ <!-- VERIFIED -->

Equation 26.9

Where:

• $V_s$ = Sound velocity in air (meters/second)
• $T$ = Air temperature (Celsius)
• $V_W$ = Wind velocity (kilometers/second), $V_W$ is positive for downwind and negative for upwind.

$$V_s = 1,052 + (1.1 \times T) + (1.5 \times V_W)$$ <!-- VERIFIED -->

Equation 26.10

Where:

• $V_s$ = Sound velocity in air (feet/second)
• $T$ = Air temperature (Fahrenheit)
• $V_W$ = Wind velocity (miles/hour), $V_W$ is positive for downwind and negative for upwind.

Air Overpressure Characteristics

Air overpressure are pressure waves that create a compression or positive pressure (push) followed by a dilation or negative pressure (pull) effect. The amplitude is often measured in pascals (Pa), millibars (mb), or pounds/inch² (psi) above the ambient pressure. The pressures are reported as time histories.

Blasting air overpressure (p) may also be reported as the sound equivalent in decibels (dB) and is calculated from measured pressures with equations 26.11, provided here for reference.

$$p_{dB} = 20 \times log\frac{P}{P_{ref}}$$ <!-- VERIFIED -->

Equation 26.11

Where:

• $p_{dB}$ = Pressure level (decibels)
• $P$ = Measured overpressure (pascals) (millibars) (pounds/inch²)
• $P_{ref}$ = Reference pressure (2 × 10⁻⁵ pascals) (2 × 10⁻⁷ millibars) (2.9 × 10⁻⁹ pounds/inch²)

Pressure Unit Conversions

For simple time histories, air overpressure frequency can be determined with the same methodology used for vibrations. Frequencies associated with air pressures will often vary depending on the type of blasting. The coal mine pressure pulse shown in figure 26.14 has a low frequency of about 2 hertz. The trench blast time history is predominantly high frequency. The latter would be audible while the coal mine blast would be inaudible unless inside the building (20 hertz).

Weighting Scales

Air overpressure from blasting (typically inaudible) and sound are different in terms of frequency although both are measured as pressure and reported in units of decibels. It is inappropriate to compare these two types of measurements because of the measuring equipment capabilities. Audible sound at noise measurements are usually made with standard sound level meters that have frequency scales commonly referred to as A, B, C and D weighting. However, These differ from the proper response to encompass the expected range of frequency from blasting of less than 20 hertz. Therefore, use of such weighting scales provides only accurate information at blasting seismographs or pressure gauges designed to measure low frequency air concussions. Noise level (frequency) measurements are typically given in dBA (A), dBB (B), dBlin, or dBC (C), corresponding to different weighting scales. Blasting induced air pressure is best reported as dB (L) or "linear". Linear response refers to flat frequency response from 2 to 250 hertz. The ISEE seismograph specifications in Appendix D also recommends flat response "linear" measurement in the 2 to 250 hertz range.

Attenuation Of Air Overpressures

In a uniform atmosphere with no wind or temperature inversions, air pressure attenuates with distance in a predictable manner from the source. Some variations in amplitude may occur with changing geologic and topographic conditions around a blast site. Three factors most influential on air overpressure are: (1) the size of a charge, (2) the distance from the charge, and (3) charge confinement. As would be expected, open air detonations create significant air overpressures and heavily confined blasts generate low air overpressures.

Figure 26.8 illustrates the attenuation of air overpressure at five different locations for the same blast. Note that the air overpressure waveforms at each location are very similar but with decreasing attenuation. Also note that the air overpressure arrival time compared to the ground vibration arrival times increases with distance corresponding to the slower travel velocity of the air overpressure wave. These air overpressures may be estimated with mathematical expressions based on prior studies or site specific measurements with blasting seismographs. Cube root scaled distance (SD₃) is used to evaluate air overpressure attenuation. On the graph in figure 26.15, the distance is scaled (normalized) by the cube root of the maximum charge-weight per delay, rather than the square root. This choice of scaling is the most common for air overpressure data, and usually provides plots best scatter in the data plot.

In any one direction, the air overpressures will generally decay in a predictable manner in direct relation to the charge-weight per delay and distance from the charge. Cube-root scaled distance (SD₃) using equation 26.12 is typically used to evaluate blasting air overpressures.

$$SD_3 = \frac{R}{W^{0.33}}$$ <!-- VERIFIED -->

Equation 26.12

Where:

• $R$ = Distance from the blast to a point (meters) (feet)
• $W$ = Charge-weight detonated within any 8 millisecond delay period (kilograms) (pounds)

This equation can be expressed as either of the following equivalent forms:

$$R = SD_3 \times W^{0.33}$$ <!-- VERIFIED -->

Equation 26.12a, m

$$R = SD_3 \times (W)^{0.33}$$ <!-- VERIFIED -->

Equation 26.12b

When the air overpressure measurements from a single blast are plotted as a function of their scaled-distances, an attenuation relationship becomes apparent. Figure 26.15 shows seven air overpressure levels plotted with their scaled distance value.

The overpressure attenuation rate for any area will be influenced by blast design, topography, and atmospheric conditions (e.g. wind speed, temperature inversions). Table 26.7 lists attenuation equations and figure 26.16 plots the equations for different types of blast design based on differing levels of confinement. Statistically, the equations may estimate the expected air overpressure level. Best fit equations are meant to estimate the expected air overpressure level. Keep in mind that the equations will only be useful for blasts that are well designed and implemented properly in the field. In this case, a best fit line can easily be hand drawn above the data points. When the data are scattered, a least squares regression analysis can provide confidence intervals on the data and line. Details on least squares regression analysis are given later in section "Vibration Data Evaluation."

The best fit line to calculate the air overpressure from scaled distance is defined by equation 26.13.

$$P = A \times (SD_3)^B$$ <!-- VERIFIED -->

Equation 26.13

Where:

• $P$ = Air overpressure (pascals or millimeters) (pounds/inch²)
• $SD_3$ = Square root scaled distance (meters/kilogram^{1/3}) (feet/pound^{1/3})
• $A$ = Intercept of the line at a $SD_3$ value of 1
• $B$ = Slope of the line (note that the slope is negative)

Air Overpressure Prediction Equations

Table 26.7 – Air overpressure prediction equations.

As noted earlier, ground vibrations typically attenuate to about 1/3 their former value for each doubling of the distance, equivalent to a decay slope of (-1.6). Air vibrations attenuate slower than ground vibrations. Most data from blasting operations fall in the range of an attenuation of 6 decibels per doubling of the distance (-1.0 slope) to 7.2 decibels per doubling of the distance (-1.2 slope). A slope of (-1.10) means that the pressure is proportional to the distance, attenuating to ½ the pressure when the distance is doubled.

When charges are detonated on the ground surface, air overpressure decay is more rapid, with slopes typically in the range of approximately (-1.28) to (-1.42). As with ground vibrations, the very high frequencies attenuate more quickly than the lower frequencies. Therefore, it is more common to have steeper slopes near the source especially for the detonation in air of charges of booster(high-velocity) explosives.

At large distances, the observed pressures are more sensitive to atmospheric conditions than those close to the source. This occasionally explains the reaction of some people in their homes who notice the blasting at times of unfavorable weather conditions. At such times, air overpressures at greater distances would not be greater than those near the source, but could be greater than those at some intermediate location. Because atmospheric conditions change frequently, the decay of air overpressures is less predictable than that for ground vibrations.

Factors Influencing Air Overpressures

The seven factors influencing air overpressures listed in table 26.8 are discussed below and are arranged roughly in order of decreasing influence. The relative influence of each factor is specific to each site and the type of blasting operation.

Table 26.8 – Factors influencing air overpressure.

Charge Size

Charge-weight affects air overpressure in a manner similar to its effect on ground vibration. The weight of the charge dictates the amount of energy released. The amount of energy received at a point is dependent on the distance from the blast as related in the scaled distance discussion above. Maximum air overpressures will be generated when explosives are detonated in the atmosphere during disposal operations.

For initial blast designs, cube root scaled distance may be used to determine the appropriate charge-weight per delay for a given target pressure. If site-specific data are not available, one of the historical equations from table 26.7 may be selected to determine charge-weight per delay. Best fit equations are used for initial blast designs. Once site-specific data are obtained the equations can be modified. For example, for an open air detonation during disposal operations, the best fit equation from table 26.7 is:

$$P = 3398 \times SD_3^{-1.0} \text{ (Metric)}$$ <!-- VERIFIED -->

$$P = 187 \times SD_3^{-1.0} \text{ (U.S.)}$$ <!-- VERIFIED -->

If the air overpressure target is 133 decibels (0.900 millibars) (0.013 pounds/inch²) for design purposes, by solving these equations for SD₃ results in a scaled distance design value of 406 meters/kilogram^{1/3} and 1,030 feet/pound^{1/3}. Then maximum charge-weight per delay to be detonated can be calculated by solving equation 26.12a.

EXAMPLE 26.14

Calculate the maximum charge-weight per delay for a blast at a distance of 1,500 meters to stay under 133 decibels using equation 26.12a and the scaled distance solution above.

$$W = \left(\frac{R}{SD_3}\right)^3$$ <!-- VERIFIED -->

$$W = \left(\frac{1500}{406}\right)^3$$

$W = 50$

The maximum charge-weight per delay to stay under 133 decibels is 50 kilograms.

EXAMPLE 26.15

Calculate the maximum charge-weight for a blast at a distance of 4,900 feet to stay under 133 decibels using equation 26.12a and the scaled distance solution on the previous page.

$$W = \left(\frac{R}{SD_3}\right)^3$$ <!-- VERIFIED -->

$$W = \left(\frac{4900}{1030}\right)^3$$

$W = 102$

The maximum charge-weight to stay under 133 decibels is 102 pounds.

Depth of Burial

For the same charge-weight, the resulting air overpressure is strongly affected by the depth of burial of the charge center. A deeply buried or heavily confined charge will cause mostly ground vibrations. Lightly confined blasts will transfer more of the available energy to the atmosphere. The depth of burial could be either the depth below the ground surface or the distance in an open face. The results may also be affected by how rapidly the rock moves.

The confinement of a blast or "depth of burial" is usually considered to be the depth to the center of the charge (or, more accurately, the center of mass of the charges). Therefore, a long cylindrical charge will have a relatively larger portion closer to the ground surface, i.e., less ground cover on top of the charge. A spherical charge will be heavily confined at the center of mass with a more uniform ground cover over all of the charge. Therefore, surface mines, which typically use long cylindrical charges, may experience higher air overpressures at the nominal depth of burial than would those operations where the height/diameter ratios of the charges are smaller (more cover even all of the charges).

In addition to the main charge, explosives that are detonated on the surface such as detonating cord or surface delay connectors can cause a significant amount of audible air overpressure. These tend to generate sharp high frequency noise, more noticeable to the public than the inaudible air overpressure. These may generate higher air overpressures than the main buried charges.

Volume of Displaced Rock

Displacement of the rock into the air generates an air overpressure pulse. Blasts that displace large volumes of rock tend to create high air overpressures with low frequency energy. For example, cast blasting intentionally displaces a large volume of rock onto the adjacent pit. The sudden displacement acts like a large piston displacing the air. The resulting air overpressures will be a low-frequency pulse that may affect a large area, create complaints, and may damage structures.

The use of delays in the blast pattern mitigates this tendency in the same way that it does for ground vibration. With delays, the work takes place in many smaller discrete, separate volumes, rather than in a single large hit. In addition to the benefit of the separate pulses, the longer it takes for the total volume of rock to be displaced, the more subdued will be the mass air movement.

Delay Time Intervals

In addition to the charge-weight per delay, it is useful to consider the timing intervals between boreholes and the relative position of the charges along a face.

Caution A delay pattern may produce a destructive sequence timing spread (zeitspreiz) that coincides with or exceeds the velocity of sound in air, thus reinforcing the air overpressures generated by successive charges.

This effect is far less significant than charge-weight per delay, but partly offsets the benefits of using separate delays. As a routine part of blast design for large operations in highly populated areas, the blaster may wish to calculate the time lapse between borehole detonation and the velocity at which the detonation appears to travel across the face from the neighbor's perspective.

Figure 26.17 illustrates this point. In this example, boreholes are drilled on a 6.1 meter (20 foot) pattern and connected with detonating cord. The delay sequence would progress across the bench at a velocity of 254 meters/second (833 feet/second), slower than the velocity of sound. If, however, the boreholes were drilled on an 8.5 meter (27 foot) square pattern, the sequence would progress at 942 meters/second (1,125 feet/second), or near the velocity of sound. Air overpressures would be amplified across the bench from upper right to upper left. If there is only one orientation of interest, it is helpful to have the detonation progress away from that direction to extend time intervals between air overpressure pulses. If overpressures are higher than desired, and reinforcement appears to be occurring, slower sequences can be tested.

Type Of Explosive

The difference between explosive types is more pronounced if charges are detonated in air. As the depth of burial increases, the differences become less pronounced. An experienced blaster-in-charge may be able to tell the difference in the sounds of exposed charges and buried charges. A high brisance explosive will have a high velocity of detonation and generate a higher pressure in its wave front at a higher frequency. An explosive with a lower velocity of detonation will generate a correspondingly lower pressure and a wave front that is not as steep. Thus, a pulse of lower frequency and longer duration characterizes the waveform. For a charge detonated in air, or vented directly into the air, the primary wave is not a true oscillation but a shock wave with a very steep front followed by an asymmetrical rarefaction, directly related to the brisance.

Atmospheric Conditions

On a calm clear day with steady atmospheric conditions, air overpressures are very predictable. But as the weather conditions change, air overpressure propagation will change from day to day, or even hour to hour based on wind direction, temperature and barometric pressure changes. Two of the most important atmospheric factors affecting the propagation of air overpressures are: (1) temperature inversions and (2) wind. Barometric pressure and humidity have little effect and generally need not be considered.

Under normal conditions in the troposphere, the air temperature decreases slowly with altitude (i.e. the air becomes colder by 1.9°C (3.5°F) for every 305 meters (1,000 feet) of rise above the surface). The velocity of sound in air changes with temperature, and the velocity is lower at higher altitudes. This change in the velocity of sound produces a refraction that causes the "sound" waves in the ground the same and reflect upward toward the lower-velocity layers in the ground). This refraction of the sound waves limits the distance to which sounds can be heard.

Temperature Inversion A temperature inversion occurs when there is a layer of warm air above a layer of colder air.

In the event of a temperature inversion, the sound waves reflect downward toward the ground surface, and sounds are more easily heard at greater distances. If a temperature inversion exists, there may be a relatively quiet zone at some intermediate distance, and sounds increase again at greater distances.

Temperature layers in the atmosphere have an effect that is roughly similar to that of the reflecting surfaces of a lake. Sounds are reflected off the lake surface especially when the water is smooth and quiet and do not dissipate as multiway as over rough terrain. Urban settings, including paved streets and buildings with smooth walls, sometimes provide a complex pattern of reflections and shadow zones. It is comparable to walking past a noise source, going behind a building, then coming out into the open again.

Wind direction will cause air overpressure to be enhanced downwind. The wind bends the angles of the wave fronts, so far they are bent downward when the waves are propagating downwind and upward when the waves are propagating upwind (See figure 26.18). Thus, the downwind sound pressure does not move slowly due the spread level. For a 32 kilometer/hour (20 mile/hour) wind, an additional 10 to 20 decibels may be recorded downwind at a rate 38 to 76 decibels compared to a no wind condition. Mid elevations do not have a significant effect; but strong turbulent winds may send the sound as well as disrupt the continuity of the air overpressures. Sometimes there are wind layers in the atmosphere whose winds may move in opposite directions. Such layers may cause the reflection of sound waves. Wind pressure can be estimated with equations 26.14 or 26.15. 134 decibels is the equivalent of a 45 kilometer/hour (28 mile/hour) wind.

$$P = (0.14 \times V)^2 \text{ (Metric)}$$ <!-- VERIFIED -->

Equation 26.14

Where:

• $P$ = Pressure (millibars)
• $V$ = Wind speed (kilometers/hour)

$$P = (1.78 \times 10^{-5} \times V)^2 \text{ (U.S.)}$$ <!-- VERIFIED -->

Equation 26.15

Where:

• $P$ = Pressure (pounds/inch²)
• $V$ = Wind speed (miles/hour)

Wind speeds vary considerably with elevation above the ground surface, because of "drag" or the resistance caused by a surface as air flows across it. Any gas or fluid moving across a surface experiences drag and it depends on such factors as the roughness and irregularity of the surface. Drag causes wind to move more slowly at ground level and to move more rapidly and less turbulently above the surface, and the difference becomes more pronounced with stronger winds.

Topography

Topography is the shape of the landscape, including hills and valleys. Topographic relief may either enhance or reduce sound pressure levels. As an example there would be a shadow zone behind a hill or beyond the lip of an open pit, where the observer is shielded from the sound of activities deep within the pit. A location somewhat farther away may be positioned on a hillside rising above the pit and be more exposed to the air overpressure where the sound pressure levels may be higher than those in the shadow zone even though the observer is farther away. Similarly, if a sound is generated between steep walls in a valley, it will be reflected between the walls and die off more slowly than the same sound generated on an open plain. Air vibrations may be enhanced from ridge to ridge, valley to valley.

Blasters should be aware that topography of the area and shape of the excavated zone may have an effect on the air overpressure. Some field conditions may reduce air overpressure (e.g. hills, large stockpiles, or the shape of a pit). Other field conditions may increase air overpressure (e.g. reflection from a high back wall or reflection from the walls in a narrow canyon). A long row of boreholes parallel to a high back wall, detonated simultaneously, would tend to enhance the reflection of air waves. A pattern of more delays, or the countouring of rows, either perpendicular or at an angle, to the back wall would be expected to reduce that type of reflection.

Air Overpressure Control Procedures

Factors that either increase or decrease the intensity of air overpressures are summarized in tables 26.9a and 26.9b. The level of significance for each parameter is variable. Knowing the extent of the influence of these factors is the first step in controlling air overpressures. Procedures to adjust blast design and blast implementation helpful to minimize air overpressures are listed and summarized in tables 26.10a and 26.10b.

Factors Within The Blaster-In-Charge's Control

Table 26.9a – Factors within the blaster-in-charge's control that influence air overpressures.

Factors Beyond The Blaster-In-Charge's Control

Table 26.9b – Factors beyond the blaster-in-charge's control that influence air overpressures.

Procedures Regarding Blast Design Factors To Reduce Air Overpressure

Table 26.10a – Procedures regarding blast design factors to reduce air overpressures.

Procedures Regarding Blast Implementation Factors To Reduce Air Overpressure

Table 26.10b – Procedures regarding blast implementation factors to reduce air overpressure.

VIBRATION MONITORING – BLASTING SEISMOGRAPHS

Blasting seismographs are equipped to monitor ground vibration and air overpressure. Ground vibrations are resolved in three component directions and air overpressures are measured as a pressure wave. Each vibration "event" that automatically activates (triggers) the blasting seismograph is stored digitally or printed in the field.

The accuracy of any recording is dependent on how the blasting seismograph is manufactured and how it is deployed in the field. To ensure that all blasting seismographs function correctly, the International Society of Explosives Engineers (ISEE) adopted the ISEE Performance Specifications for Blasting Seismographs included as Appendix D. This standard sets the minimum performance criteria to be met for each blasting seismograph by the manufacturer. The manufacturer's specifications should include this information for the user.

The ISEE Performance Specifications for Blasting Seismographs requires a nominal upper frequency limit of 250 hertz. This frequency limit will accommodate the vast majority of monitoring situations and is based on a sample rate of 1,024 samples per second. In those areas where it may be necessary to record higher frequencies than 250 hertz, the sample rate needs to be increased to at least 4 times the desired upper frequency bound.

For blasting very close to special structures with well-defined structural properties (such as buried pipelines or building support columns), strain on these elements should be measured with dynamic strain gages (DSGs). Typical vibration criteria such as peak particle velocity are not applicable in these cases, and engineering knowledge, both for the appropriate criteria as well as the measurement technology, should be obtained from professionals experienced in such work.

The second equally important standard is the ISEE Field Practice Guidelines for Blasting Seismographs. These guidelines are included as Appendix E and outline the user's responsibility to ensure accurate defensible vibration recordings. Improper placement or programming of the unit will result in missed or errant data.

Types of Blasting Seismographs

Most blasting seismographs can be programmed to function as a waveform or peak "histogram" recorder. Waveform recorders should always be used for monitoring blast-induced vibration events. Waveform recorders measure vibrations in time histories over a finite recording period (e.g. 5, 10, or 20 seconds). Peak recorders measure vibrations continuously and report the highest value that occurs within a predetermined time interval (e.g. 5, 10, or 15 minutes). Peak recorders are primarily used for sources of continuous and semi-continuous vibrations such as highway traffic, pile driving, dynamic compactors, or other recurring events where the waveform from each event will tend to be similar. Some seismographs have a combination histogram/waveform mode where the waveform is recorded if it exceeds a certain level, while continuously recording peak information.

All blasting seismographs manufactured today record and store the data digitally. Some units have onboard printers so the results can be printed immediately in the field for review. Units without printers must have the data downloaded to a computer for printing but also have the advantage of enhanced analysis of the waveforms.

All blasting seismographs contain a geophone sensor to measure ground vibrations, a microphone to measure air overpressure, and a data collection/processing base (Figure 26.19). In some of these, the geophone is internal to the data collection/processing base. The microphone is always a structural element.

Typical Blasting Seismograph Data

Each blasting seismograph record will contain general information, summary information, graphical information, and component waveforms (time histories). If the record is printed with an onboard field printer, it is called a "strip chart" (See figure 26.20). Full page printouts are the result of the data being downloaded to a computer and printed (See figure 26.21). Each printout will contain the basic information listed in table 26.11. The most difference among records is their resolution.

Basic Information In Each Seismograph Printout

Table 26.11– Basic information in each seismograph printout.

Proper Set-up Of Blasting Seismographs

Proper set-up and programming of a blasting seismograph is the user's responsibility. The user may be the blaster, a member of the blasting crew or a third party monitoring company. The ISEE Field Practice Guidelines for Blasting Seismographs addresses the fundamentals of the set-up in the three categories: (1) general guidelines, (2) ground vibration monitoring, and (3) air overpressure monitoring.

The general guidelines focus on the system operating characteristics and maintenance. Be sure to choose the right blasting seismograph for the monitoring environment and reporting requirements.

For ground vibrations, good coupling between the vibration sensor (geophone) and the ground is important. The anticipated accelerations level determines the appropriate coupling method, which may be burial, spiking, or attachment (to rock). The sensor should be prevented from skipping sideways, as well as from rocking or "jumping" from the surface. These tendencies are dependent on the size, shape, and mass of the sensor, as well as the supporting surface. When a sensor is poorly coupled to the ground the result is often a higher reported vibration value.

In soils, burial and firm backfilling is recommended for vibration sensors. Burial is most effective when the density of the sensor matches that of the soil, often around 1,922 kilograms/meter³ (120 pounds/foot³). Poor locations for sensors would be on grass or plant roots or on the surface of loose fill soil. When such locations cannot be avoided, dig a hole to the bottom of the loose soil zone so that the sensor can be placed in the firm, underlying soil. Thus, test the sensor spikes and compact ground with the geophone to reduce distortion.

Sand bags used on top of a geophone can prevent movement of moderate vibration amplitudes, but they must be large enough to contain the ground all around the sensor. Sand bags that do not touch the ground may increase the likelihood of erroneous recordings. They merely add to the weight of the sensor and create an inverted pendulum, that increases the recorded vibration levels above the actual level.

For paved surfaces, or other smooth surfaces, bolting or various chemical substances may be used to provide anchoring. Coupling to loose or hollow concrete slabs may also result in inaccurate readings. The key to remember is what the sensor is coupled to is what it is recording. If it is coupled to the ground, then ground vibration is being recorded. If it coupled to any other object, then it is recording the response of that surface or the object to the vibration.

An air overpressure is measured with the microphone. The microphone should be placed facing the blast in the stand provided by the manufacturer. Obstructions in front of the microphone buildings, walls and trees should be avoided. Also avoid reflective surfaces behind the microphone. It is subject to an object, to place the microphone near the structure.

Blast Event Determination

Once data is obtained from a blasting seismograph, the operator must determine if the event is blast-related or not. If a blasting seismograph is deployed immediately before a blast, then only one event is likely to be recorded. However, if a unit is deployed in the field on a continuous basis, extraneous events are likely to be recorded in addition to blast events. The questions becomes, "Which events on that blaster are blast related and which are not?" The answer is that each blast results in a vibration waveform unique to that event, i.e., like a fingerprint.

When a particle begins to vibrate, the three dimensional vibration is recorded as the L, T, and V component of motion simultaneously. The seismograph monitors vibration continuously, but does not save the data in an event until the vibrations or "airblast" exceeds a pre-trigger level. Once triggered, from ~1 to 5 second "pre-trigger" data is saved along with the post-trigger data to ensure capturing of the full waveform from beginning to end. A typical blast induced waveform will show all three vibration components that last the same time, the waveform peaks will occur at different times, and the air overpressures should arrive one second later than the ground vibrations for each 335 meters (1,100 feet) of distance. If air overpressures triggers the unit in a blast, in most cases the ground vibrations will have been missed unless the blast is within 335 meters (1,100 feet) to clear that the ground vibrations are captured in the pre-trigger data.

The vibration's duration is a function of three factors: (1) blast design, (2) distance from the blast to the seismograph, and (3) geology. The number and type of delays in the blast establishes the duration of the blast. The duration of the vibration event will last at least as long as the blast duration. As explained above, the different wave types spread out as they travel away from the blasting site lengthening the duration of the vibrations. As such, a vibration event close to a blast will have about the same duration as the blast. For example, a blast at a far distance with a delay duration of 8 seconds will generate vibrations of one up to several seconds in duration.

Since ground vibrations waves typically travel from 5 to 20 times faster than air overpressures, for distant blasts the arrival of the ground vibration will occur several seconds before the arrival of the air overpressure. This fact explains the response of some observers. Sometimes a person will detect the passage of the ground vibration and will also detect the arrival of the air overpressures structure noise is generated for both (for example, the rattling of windows). They may believe they felt two blasts at that time, even though it was only a single blast.

VIBRATION DATA EVALUATION

As previously discussed, ground vibration and air overpressure can be predicted with square root and cube root scaling respectively. This section describes how the field data can be analyzed to predict accurately vibrations at a given site. These methodologies can be used to justify blast designs for vibration control to regulatory personnel or form the basis of a quality control program. The discussion will focus on ground vibration and square root scaled distance, but the same technique can be used for air overpressure and cube root scaled distance.

Normalizing The Data

Peak particle velocity and air overpressure intensities decay geometrically with distance from a blast according to an exponential function. This means that, very close in to a blast, the peak amplitude increases rapidly with a small decrease of the distance. Likewise, at greater distances it takes a fairly large change in distance to cause a significant reduction in amplitude.

In figure 26.22 the horizontal axis is distance from a blast and the vertical axis is peak particle velocity. The peak particle velocities generated at different distances is represented by the lower line for a fixed charge-weight of 65 kilograms (144 pounds). If the charge-weight is increased to 226 kilograms (496 pounds), the line is similar, although its location on the graph is different because of the greater charge-weight. The graph could be used to predict vibration intensities but would be difficult to interpret with many such lines.

In order to combine data from many blasts with different charge-weights, it is common to normalize (scale) the distance. The data are then plotted on a log-log graph to give the appearance of linearity (See figures 26.9 and 26.13). Normalizing data reduces two or more variables to a common basis in order to reduce the number of variables. For blast vibrations, distance and charge-weight are combined to give a single number, which can be plotted on a graph or used in calculations. The most common way of combining distance and explosive energy is to divide the distance by the square root (for ground vibrations) or cube root (for air overpressure) of the maximum charge-weight/delay. The number obtained is called normalized distance or scaled distance. With a statistically significant number of measurements, best fit and upper bound (95% confidence interval) regressions can be determined. Square root scaled distance (SD₂) and cube root scaled distance (SD₃) can be used to predict ground vibration and air overpressure amplitudes, respectively.

The equation for the best fit line can be determined using least squares regression analysis or manually based on a hand drawn line slope and intercept. Most blasting seismograph manufacturers have software to perform regression analysis and many spreadsheet packages also have the statistical capabilities.

The relation between vibrations (either ground or air) and scaled distance (charge-weight and distance) is non-linear, and represented by equation 26.16.

$$c = A \times SD_x^B$$ <!-- VERIFIED -->

Equation 26.16

Where:

• $c$ = Amplitude of either the ground vibration or air overpressure
• $SD_x$ = Scaled distance of either the ground vibration (SD₂) or air overpressure (SD₃)
• $A$ = Intercept of the best fit line at 1
• $B$ = Slope of the best fit line

For the readers' reference, equation 26.16 is equivalent to the logarithm form:

$$log(c) = log(A) – B \times log(SD)$$ <!-- VERIFIED -->

Equation 26.17

A data set should have a linear appearance, albeit with scatter. As an alternative, with good data set plotted on log-log paper, a best fit line can be hand drawn through the middle of the data. The best fit line is a line that minimizes the squares of the vertical distances of the data points from the line. Extending the line to a scaled distance of 1 yields the y-intercept or "A" value. The slope is the line "B" represents the rate of data decay with increasing scaled distance. Both axes are common for a given blast site and blasting methodology. All the historical equations of tables 26.3 (or figure 26.8 for Ground Vibration) and Table 26.7 (or figure 26.15 for Air Overpressure) were developed using the method of effective and distance combined into scaled distance.

An advantage of the statistical method is that a statistical factor called the correlation coefficient (also called R²) indicates how well the data conforms to the best fit line. If all of the data points fall on the best fit line the "R²" value would be one. A good statistical relationship exists if the "R²" is greater than 0.70. This does not mean that the data set is not valid if the coefficient is less than 0.70. It may mean less confidence, the greater the amount of data scatter.

To ensure that the analysis is valid, a statistically significant number of data points are needed. For blast vibrations 30 or more data points that span at least 3 orders of magnitude either SD are recommended. What may not a few vibration measurements have been made since a blast, particularly if measured with just one seismograph, the few data points constitute what is known in statistics as a "small sample". If only a small sample of data is available, it is better to start a historical data set with prior gathering bound basis for reference until adequate data are available. Regression based on small samples may give regression lines that are way off (but usually be conservative's very steep slopes overall).

Because blasting varies with formation, geology, blast design, and explosive product and packaging, data should be separated into specific groupings. If data from multiple locations are combined, data from many blasts at one location, or many different blast types (open trench, presplit, etc) within one location are combined, the regression coefficient will not be as representative. Regression homogeneous raw data types and there are variations in other blast parameters, the data will not fall on a straight line or the data will have scatter. When multiple blast types (open trench, presplitting, production, etc) at the same site are plotted together the data will become more scattered. For the same blast design and geology the data should plotted on a narrow band. It is wise to know the normal range of the band. If a particular data point is unusually low or high, the field procedures for blast loading, explosive product performance and/or blasting seismograph setup may need to be reviewed.

Ground Vibration Data

For ground vibrations, peak particle velocity is plotted against square root scaled distance using a log-log graph as shown in figure 26.23. For this data set, a least squares regression analysis yields the peak particle velocity equation that takes the form of equation 26.7, where the R² equals 0.83. Thus, scaled distance is a good predictor of ground vibrations with these equations.

$$PPV = 377 \times (SD_2)^{-1.58} \text{ (Metric)}$$ <!-- VERIFIED -->

$$PPV = 37 \times (SD_2)^{-1.58} \text{ (U.S.)}$$ <!-- VERIFIED -->

This equation represents the best fit line of the data set. The best fit equation is best suited to estimate ground vibration levels for the same type of blasting that the data set represents. The line labeled upper bound represents the upper bound of an envelope that encompasses 95% of the data set. The upper bound line is parallel to the best fit line (same slope) and bounds 97.5 % of the data points. PPV predictions made with the equation of this line will be higher than predictions made by the best fit equation. The lower bound of the 95% envelope (not shown) is rarely used for vibration analysis.

On a plotted graph, the upper bound of the data set can be obtained by drawing a line parallel to the first line (same slope) that encompasses the entire data set and determining a new "A" intercept. Through statistical analysis, the 95% interval upper bound is determined by finding standard error of estimate (in this case 1.34). The upper bound is obtained by multiplying intercept "A" by the standard error twice (377 × 1.74 × 1.74 = 1,141) to yield the following equations.

$$PPV = 1,141 \times (SD_2)^{-1.58} \text{ (Metric)}$$ <!-- VERIFIED -->

$$PPV = 112 \times (SD_2)^{-1.58} \text{ (U.S.)}$$ <!-- VERIFIED -->

Figure 26.23 shows the best fit and upper bound lines of the data set. Take care not to over extrapolate using the best fit line by going beyond the scaled distance range. With this data set, predictions should not be made at scaled distance beyond 90 meters/kilogram^{1/2} (200 feet/pound^{1/2}) unless the limitations of the data are fully understood by the blaster-in-charge.

Caution For blasts with the same scaled distance and many boreholes per delay there is more potential for scatter.

Scatter in the data is due to the three factors when (1) pyrotechnic detonators may not initiate at the nominal firing time; (2) wave forms from separate boreholes may constructively or destructively interfere, or (3) blast designs are different.

There will be some variation in seismic wave attenuation (decay) with changes in geologic conditions around a blasting site. However, distance is far more influential than changes in geology.

Caution If any recording location yields anomalous readings, the blasting seismograph must be checked, adjusted, or moved and additional data gathered before determining attenuation lines for the area.

There might also be anomalous readings at the location of a blasting seismograph because of sensor coupling, soil conditions (soft spot or grass roots), or monitoring interfaces (hollow concrete slab). These would usually tend to exaggerate the vibration intensity. Always document anomalous vibration data with explanations of why data were discarded. When anomalous data is encountered take appropriate caution. Data rejection may be appropriate when data authenticity and circumstances are verified.

PPV Estimates Using The Ground Vibration Regression

Estimates or predictions of vibration amplitude can be obtained by using either the graph with plotted best fit and upper bound confidence levels or the equations of the lines. Once the attenuation relationship is established, scaled distance values can be determined that will facilitate blast design and ensure safe vibration levels. With experience, the blaster-in-charge can learn to predict whether the vibrations from future blasting are likely to be of relatively low, medium, or high amplitude. The amplitude is based on the charge-weight-per delay and distance of the blast. To illustrate this point, figure 26.23 shows the best fit and upper bound equations for a quarry. The negative slope of the value indicates the decay rate. The steeper the slope, the faster the vibrations attenuate.

The graphical method is convenient for visualizing the relationship and estimating PPV from SD₂. Using the line equations, yields specific values based on the regression analysis. Scientific calculators are readily available and economically affordable to blasters and should be used for all vibration calculations.

If the graph must be used, the PPV for a particular blast is estimated from SD₂ based on the desired line. To illustrate the use of the graph, suppose a blast will consist of 10 boreholes, each loaded with 11.4 kilograms (25 pounds) of explosive and that each borehole will detonate separately, at 25 millisecond intervals. This gives a maximum charge-weight per delay of 11.4 kilograms (25 pounds). Further assume that the distance to a structure is 30.5 meters (100 feet). First, first, determine the scaled distance, which is 61 meters (200 feet) divided by the cube root of the maximum charge-weight per delay 11.4 kilograms (25 pounds). The result will be 9.0 meters/kilogram^{1/2} (20 feet/pound^{1/2}).

Now go to the graph and find a scaled distance of 9.0 meters/kilogram^{1/2} (20 feet/pound^{1/2}) on the horizontal axis (the x-axis), and project a vertical line unit it intersects the prediction line on the graph. Usually, the blaster-in-charge is interested in knowing the upper bound for typical down-hole bench blasting, which means that there is a 95% probability that the actual vibration intensity will not exceed this amount. From the point where the upper bound line is intersected, draw a horizontal line to the vertical axis (y-axis) and find 110 pounds (.617 inch/second) is the prediction. That is, for a scaled distance of 9.0 meters/kilogram^{1/2} (20 feet/pound^{1/2}) the peak particle velocity would be expected to remain at or below 90 millimeters/second (3.5 inches/second) about 95% of the time for typical down-hole bench blasting. If the resulting amplitude is too high, the blast must be redesigned for a smaller charge-weight/delay.

In practice it is better to use the actual equation for the upper bound line. For example:

EXAMPLE 26.16

Estimate the peak particle velocity for a scale distance of 9.0 meters/kilogram^{1/2} using the upper bound equation from figure 26.23.

$$PPV = 1,141 \times (SD_2)^{-1.58}$$ <!-- VERIFIED -->

$PPV = 1,141 \times (9.0)^{-1.58}$

$PPV = 89$

The estimated peak particle velocity is 89 millimeters/second.

EXAMPLE 26.17

Estimate the peak particle velocity for a scale distance of 20 feet/pound^{1/2} using the upper bound equation from figure 26.23.

$$PPV = 112 \times (SD_2)^{-1.58}$$ <!-- VERIFIED -->

$PPV = 112 \times (20)^{-1.58}$

$PPV = 3.5$

The estimated peak particle velocity is 3.5 inches/second.

As seen in figure 26.23, the vibration amplitude can vary considerably from the best fit line to the upper bound line and sometimes even be lower. This means that the ground response factor "A" is not always 1,141 (112). Remember that the "A" term represents all changes in vibration amplitude as caused by: (1) blast design, (2) spatial configuration, (3) initiation direction, (4) quality control, or (5) geological conditions. Values also will go higher or lower, for blasting with high or low confinement, high or low coupling and high or low rock strength.

Caution The vibration amplitude is only part of the vibration concern and does not consider the frequency of the vibration event.

EXAMPLE 26.18

Estimate the vibration of a blast that uses a maximum charge-weight per delay of 4,545 kilograms (10,000 pounds) at a distance of 610 meters (2,000 feet), estimate the ground vibration amplitude using the data analyzed in figure 26.23. The method to estimate the intensity is summarized in table 26.12.

Method To Estimate Ground Vibration Amplitude

Table 26.12 – Solutions for example 26.16 and 26.17.

This larger blast results in the same scaled distance values as determined for the smaller blast discussed above. But, while the same vibration intensity (PPV) is expected at the same scaled distance, the frequency characteristics of the two blasts will be different. Generally high-frequency vibrations are dominant for near field blasts and low frequency vibrations persist for far field blasts (larger blasts at a large distances). In this instance, peak particle velocity predictions will be the same but the frequency content and vibration duration will be different because the distances differ.

Estimating Charge-Weight

Once the vibration characteristics of an area are defined by field data, or a historical reference line is selected from Table 26.3, blasts can be designed to a scaled distance that will ensure target vibration levels with a high level of confidence. For example, assume a vibration limit for a project is 25 millimeters/second (1.0 inch/second). What is the appropriate scaled distance based on the upper bound equation of figure 26.23?

First, the upper-bound equations of figure 26.23 must be rewritten to solve for the scaled distance.

Upper Bound: $PPV = 1,141 \times (SD_2)^{-1.58}$ (Metric) $PPV = 112 \times (SD_2)^{-1.58}$ (U.S.)

When solved for scaled distance these become:

Upper Bound:

$$SD_2 = \left(\frac{PPV}{1,141}\right)^{-0.63} \text{ (Metric)}$$ <!-- VERIFIED -->

$$SD_2 = \left(\frac{25}{1,141}\right)^{-0.63}$$

$SD_2 = 27$

$$SD_2 = \left(\frac{PPV}{112}\right)^{-0.63} \text{ (U.S.)}$$ <!-- VERIFIED -->

$$SD_2 = \left(\frac{1.0}{112}\right)^{-0.63}$$

$SD_2 = 58$

The upper bound square root scaled distance is 27 meters/kilogram^{1/2} (58 feet/pound^{1/2})

Rounding up the scaled distance value is appropriate because ground vibration amplitude decreases with increasing scaled distance. Now the maximum charge-weight per delay can be calculated from the scaled distance equation 26.6a based on this site specific information and the distance.

EXAMPLE 26.19

Calculate the maximum charge-weight per delay at a distance of 305 meters using equation 26.6a.

$$W = \left(\frac{R}{SD_2}\right)^2$$ <!-- VERIFIED -->

$$W = \left(\frac{305}{27}\right)^2$$

$W = 127$

The maximum charge-weight per delay is 127 kilograms.

EXAMPLE 26.20

Calculate the maximum charge-weight at a distance of 1,000 feet using equation 26.6a.

$$W = \left(\frac{R}{SD_2}\right)^2$$ <!-- VERIFIED -->

$$W = \left(\frac{1000}{58}\right)^2$$

$W = 297$

The maximum charge-weight is 297 pounds.

At this point, blasts can be easily designed with the appropriate charge-weight based on any distance. For small diameter boreholes, a single charge per borehole may be appropriate. For larger diameter boreholes, decks may be needed. The upper bound equation provides conservative charge-weights per delay with a high level of confidence of remaining within vibration limits.

Extrapolating Vibrations With Only One Vibration Data Point

Extrapolating a single vibration amplitude to predict particle velocity at other locations can be performed, but it is not recommended except for cases where no better data are available. The following predictive method can be used if a vibration measurement is available, the distance from the blast to the measurement location is known and the distance from the blast to the point of prediction is known. Preferably the two locations are in the same direction from the blast. To extrapolate vibration amplitudes use equation 26.18.

$$V_2 = V_1 \times \left(\frac{R_2}{R_1}\right)^{-1.6}$$ <!-- VERIFIED -->

Equation 26.18

Where:

• $V_2$ = Estimated vibration at the prediction location (millimeters/second) (inches/second)
• $V_1$ = Measured vibration (millimeters/second) (inches/second)
• $R_2$ = Distance from the blast to the measurement location
• $R_1$ = Distance from the blast to the point of prediction

This is simply a general slope equation using an assumed slope of (-1.6) from the general equation of table 26.3. If low velocity soils exist in an area, a shallower slope may be selected based on experience or the equations of table 26.3. If hard rock persists a steeper slope may be justifiable.

EXAMPLE 26.21

Estimate the vibration amplitude using equation 26.18 at distances of 75 meters and 300 meters from a blast where there is a blasting seismograph record of 5.8 millimeters/second at 150 meters.

$$V_2 = V_1 \times \left(\frac{R_2}{R_1}\right)^{-1.6}$$ <!-- VERIFIED -->

$$V_2 = 5.8 \times \left(\frac{75}{150}\right)^{-1.6}$$

$V_2 = 17.6$

The extrapolated vibration amplitude at 75 meters is 17.6 millimeters/second.

$$V_2 = 5.8 \times \left(\frac{300}{150}\right)^{-1.6}$$

$V_2 = 1.9$

The extrapolated vibration amplitude at 300 meters is 1.9 millimeters/second.

EXAMPLE 26.22

Extrapolate the vibration amplitude using equation 26.18 at a distance of 250 feet and 2,000 feet on a blast site where there is an available blasting seismograph record of 0.23 inches/second at 500 feet.

$$V_2 = V_1 \times \left(\frac{R_2}{R_1}\right)^{-1.6}$$ <!-- VERIFIED -->

$$V_2 = 0.23 \times \left(\frac{250}{500}\right)^{-1.6}$$

$V_2 = 0.70$

The extrapolated vibration amplitude at 250 feet is 0.70 inches/second.

$$V_2 = 0.23 \times \left(\frac{2000}{500}\right)^{-1.6}$$

$V_2 = 0.025$

The extrapolated vibration amplitude at 2,000 feet is 0.025 inches/second.

The slope value determines the rate of change of the predicted value. A general slope of (-1.6) means that the predicted vibration will be approximately three times higher than the measured vibration at half the distance. Likewise, it will be approximately three times less than the measured vibration at double the distance.

Air Overpressure Data

Estimating charge-weights for air overpressure control is very similar to the method of estimating for ground vibration and the limitations are the same. Air overpressure is plotted against cube root scaled distance using a log-log graph as shown in figure 26.24. For this data set, a least squares regression analysis yields the air overpressure equation that takes the form of equation 26.13, and R² equals 0.854. Thus, scaled distance is a good predictor of air overpressure with these best fit equations.

Best Fit: $$P = 480 \times (SD_3)^{-0.895} \text{ (Metric)}$$ <!-- VERIFIED -->

$$P = 0.24 \times (SD_3)^{-0.895} \text{ (U.S.)}$$ <!-- VERIFIED -->

The best fit equation is to estimate expected air overpressure level. The line labeled upper bound represents the upper bound of an envelope that encompasses 95% of the data set. The upper bound line is parallel to the best fit line (same slope) and lies above 97.5 % of the data points. Amplitude predictions made with the equation of this line will be higher than predictions made by the best fit equation.

On a plotted graph, the upper bound of the data set can be obtained by drawing a line parallel to the first line (same slope) that encompasses the entire data set and determining a new "A" intercept. Through statistical analysis, the 95% interval (upper bound) is determined by finding standard error of estimate (in this case 1.48). The upper bound is obtained by multiplying intercept "A" by the standard error twice (480 × 1.48 × 1.48) to yield:

Upper Bound: $P = 1,489 \times (SD_3)^{-0.895}$ (Metric) $P = 0.490 \times (SD_3)^{-0.895}$ (U.S.)

The graph shows the best fit and upper bound equations of the data set. Take care to not over-extrapolate using the best fit line by going beyond the scaled distance range.

Caution With this data set, predictions should not be made at scaled distances beyond 240 meters/kilogram^{1/3} (600 feet/pound^{1/3}) unless the limitations of the data are fully understood by the blaster-in charge.

There will be strong variation in air overpressure attenuation with changes in explosive confinement and atmospheric conditions. Therefore, if the relationship was obtained based on average confinement and there is a face breach and/or stemming ejection the predictive values will be low. If venting does occur then select the appropriate historical equation in table 26.7 that addresses lack of confinement.

Anomalous Overpressure Data

If any recording location yields anomalous readings, the blasting seismograph must be checked, adjusted, or moved and additional data gathered before determining attenuation lines for the area. There might also be anomalous readings at the location of a blasting seismograph because of atmospheric focusing.

Always document anomalous overpressure data with explanations of why data were discarded. When anomalous data is encountered take appropriate caution. Data rejection may be appropriate when data authenticity and circumstances are verified.

Amplitude Estimates Using The Air Overpressure Regression

Estimates or predictions of air overpressure can be obtained by using either the graph with plotted best fit and upper bound confidence levels or the equations of the lines. Once the attenuation relationship is established, scaled distance values can be determined that will facilitate blast design and ensure safe vibration levels. The amplitude is based on the charge-weight per delay, confinement and distance of the blast. To illustrate this point, figure 26.24 shows the best fit and upper bound equations for a quarry.

Using the graphical method, the air overpressure for a particular blast is estimated from SD₃ based on the appropriate line. To illustrate the use of the graph, suppose a blast will consist of 10 boreholes, loaded with 32.8 kilograms (65 pounds) of explosives and that each borehole will detonate separately, at 25 millisecond intervals. This gives a maximum charge-weight per delay of 32.8 kilograms (65 pounds). Further assume that the distance to a structure is 30.5 meters (100 feet). First, first, determine the scaled distance, which is 61 meters (200 feet) divided by the cube root of the maximum charge-weight per delay 32.8 kilograms (65 pounds). The result will be 9.5 meters/kilogram^{1/3} (50 feet/pound^{1/3}).

Now go to the graph and find a scaled distance of 20 meters/kilogram^{1/3} (51 feet/pound^{1/3}) on the horizontal axis (the x-axis), and project a vertical line until it intersects the prediction line on the graph. Usually, the upper bound is used for predictions, which means that there is a 95% probability that the actual vibration intensity will not exceed this amount. From the point where the upper bound line is intersected, draw a horizontal line to the vertical axis (y-axis) and find 110 pascals (.617 inch/second) is the prediction. That is, for a scaled distance of 15 feet/pound^{1/3}) the peak particle velocity would be expected to remain at or below 90 millimeters/second (3.5 inches/second) about 95% of the time for typical down-hole bench blasting.

In practice it is better to use the actual equation for the upper bound line.

Upper Bound:

$$P = 1,489 \times (SD_3)^{-0.895} \text{ (Metric)}$$ <!-- VERIFIED -->

$$P = 0.490 \times (SD_3)^{-0.895} \text{ (U.S.)}$$ <!-- VERIFIED -->

$P = 0.490 \times (51)^{-0.895}$

$P = 0.015$

The maximum air overpressure to be expected is 105 pascals (0.015 pounds/inch²). Using equation 26.11 to convert to decibels, the air overpressure is 134 decibels. If this air overpressure is too high, the blast should be redesigned to reduce the charge-weight/delay.

As seen in figure 26.24, the air overpressure can vary considerably from the best fit line to the upper bound line. Remember that the "A" term represents all changes in vibration intensity as caused by: blast design, spatial configuration, initiation direction, and quality control in the field; not only atmospheric conditions. Values also will go higher or lower, for blasting with high or low confinement, high or low coupling and high or low rock strength.

Estimating Charge-Weight

Once the vibration characteristics of an area are defined by field data, or a historical reference line is selected from table 26.7, blasts can be designed to a scaled distance that will ensure target vibration levels with a high level of confidence. If this air overpressure is too high the blast should be redesigned to reduce the charge-weight/delay. For example, assume an air overpressure limit for a project is 134 decibels, what is the appropriate scaled distance based on the equation of figure 26.24?

First the upper bound equations of figure 26.24 must be rewritten to solve for scaled distance.

Upper Bound: $P = 1,489 \times (SD_3)^{-0.895}$ (Metric) $P = 0.490 \times (SD_3)^{1/3}$ (U.S.)

When solved for SD₃ these become:

$$SD_3 = \left(\frac{P}{1,489}\right)^{-1.12} \text{ (Metric)}$$ <!-- VERIFIED -->

Now calculate the upper bound cube root scaled distance for air overpressure of 134 decibels (100 pascals, 0.0145 pounds/inch²) using the upper bound equation and round up to the nearest whole number.

$$SD_3 = \left(\frac{100}{1,489}\right)^{-1.12}$$

$SD_3 = 21.15$

$$SD_3 = \left(\frac{P}{0.490}\right)^{-1.12} \text{ (U.S.)}$$ <!-- VERIFIED -->

$$SD_3 = \left(\frac{0.0145}{0.490}\right)^{-1.12}$$

$SD_3 = 53.4$

The upper-bound cube root scaled distance is 21.15 meters/kilogram^{1/3} or 53.4 feet/pound^{1/3} to remain below 134 decibels at this quarry.

Rounding up maintains simplicity and is appropriate because air overpressure decreases with increasing scaled distance. Now use scaled distance to determine an allowable charge-weight per delay based on equation 26.12a.

EXAMPLE 26.23

Calculate the allowable charge-weight per delay at a distance of 305 meters with a SD₃ of 21.15 meters/kilogram^{1/3} using equation 26.12a.

$$W = \left(\frac{R}{SD_3}\right)^3$$ <!-- VERIFIED -->

$$W = \left(\frac{305}{21.15}\right)^3$$

$W = 2,999$

The allowable charge-weight per delay is 2,999 kilograms.

EXAMPLE 26.24

Calculate the allowable charge-weight per delay at a distance of 1,000 feet with a SD₃ of 53.4 feet/pound^{1/3} using equation 26.12a.

$$W = \left(\frac{R}{SD_3}\right)^3$$ <!-- VERIFIED -->

$$W = \left(\frac{1000}{53.4}\right)^3$$

$W = 6,567$

The allowable charge-weight per delay is 6,567 pounds.

Therefore, any charge-weight under 2,999 kilograms (about 6,567 pounds) would result in an air overpressure of less than 134 decibels with a high level of confidence at 305 meters (1,000 feet), barring any unforeseen venting of gases at this quarry site.

Caution If the calculation indicates a limitation on the charge-weight that cannot accommodate the project needs, the blaster should reevaluate the proposed design. It may be necessary to resolve uncertainties with test blasts using the proposed explosive products and blast designs.

VIBRATION DAMAGE PREVENTION

As discussed previously, all blasts have residual energy that is manifested as ground vibration and air overpressure. At this residual energy arrives at nearby man-made structures such as homes, commercial buildings, pipelines, water towers, etc., these structures will respond to the vibration energy. If the response of the structure is significant, unintended damage outside the blast site is possible. It is incumbent upon all blasters-in-charge to design blasts in order to prevent damage to structures outside the permit or project work area.

Vibration monitoring at structures measure the energy transmitted into building foundations and superstructures. Each structure will respond differently as ground vibrations enter a structure through the foundation and air vibrations enter a structure through the superstructure. Buried structures such as pipelines will not respond to air vibration. Most safe vibration level criteria have a high probability of non-damage based on the type of structure. The most restrictive vibration limits are set to ensure the prevention of damage to residential structures.

Structure Response

As ground and air vibrations reach a structure, each will cause it to shake or respond. This structure response is dependent on the vibration characteristics (frequency and amplitude) and structure type. The response characteristics are primarily related to structure mass and the stiffness. A structure may not only respond as a whole but may also have components with their own response characteristics. However, whether or not a structure responds to a blast vibration depends on the character of the vibration. That a structure may respond is not in and of itself an indication that damage may occur. All materials have the ability to resist deformation from dynamic forces. It is only when those forces cause strains that exceed the strength of the material that damage occurs.

Structures below the ground surface, such as pipelines, are well connected or coupled to the ground and vibrate with the ground. Structures above the ground, like a residence, receive ground vibrations through the foundation and air vibrations through the walls and roof. Both cause the superstructure of the building to vibrate. The response can be visualized with a simple pole model. For ground vibrations, structure response at the top of the pole depends on the frequencies and amplification of the vibration at the bottom of the pole. If shaken at the pole's natural frequency, the top will move significantly more compared with the bottom. Motion at the top is amplified from the bottom motion. For air vibrations, the energy enters the top of the pole. The low frequency event (concussions) is normally only a one or two cycle event.

Residential structure response to blast vibration has been researched extensively by the U.S. Bureau of Mines (USBM) (i.e. USBM RI 8507 and others). The two general types of responses within a structure caused by external vibrations are (1) whole-structure response, or the racking motions of the entire structure or building (See figure 26.25, responses to lower frequencies of 4 hertz to 12 hertz, and (2) mid-wall response or motions within individual panels or components of the building, normally out of plane with walls (See figure 26.26, responses to mid range frequencies of 12 hertz to 20 hertz).

Ground vibrations impact the basement or foundation walls below ground. Since the structure is free standing, the above ground portion can move in the two horizontal directions while the foundation is fixed. Differential motions between the upper and lower corners may cause cracking. Mid-wall responses are typically responsible for window rattling, picture tilting, etc.

The USBM observed structure amplification in the corners of one to two story homes typically between 1 to 4 times the ground vibration when the ground vibration frequency was in the range of 4 hertz to 12 hertz. Mid-wall amplification factors were typically 1 to 6 times the ground vibration, the higher amplifications occur at two story structures. Structure response below ground level is the same as or less than the incoming vibrations.

Like ground vibrations, air vibrations can cause both whole structure and mid-wall responses. If an air overpressure of sufficient amplitude has mostly low-frequency energy (< 12 hertz), the whole structure will respond similarly to ground vibrations. If an overpressure of sufficient amplitude has mostly mid-range frequencies (12 hertz to 20 hertz) the wave will cause the mid-walls to respond in a mid response. Two-story structures are more sensitive to air vibrations than are one-story structures.

When walls shake loose objects on the wall or sitting on shelves tend and generate noise within the structure. This noise is very perceptible to the occupants and may create concern that the structure is being damaged by the blasting.

Importance Of Frequency To Structure Response

Frequency is a very important component of ground vibration because it affects how structures respond. When the vibration frequency closely matches a natural or fundamental frequency of a structure or structural component the structure or component will tend to respond more vigorously or the structure resonates. In other words the incoming ground vibrations are amplified in the upper portion of the structure. Alternatively, if the ground vibration frequency is well above the natural frequency very little seismic energy transits into the structure, and there will be little if any response. It is also the case that if the vibration frequency is well below the whole structure natural frequency, that the structure will tend to move with the ground with very little differential motion.

High-frequency energy generates low structure response. When the length of a single high-frequency wave is short as compared to the dimension of a structure the structure will not significantly respond. On the other hand, if a low-frequency wave is long compared to the dimensions of structures it may significantly respond. Accordingly, low frequencies tend to efficiently couple energy into structures and promote higher-amplitude, long-duration response.

Deflections and bending strains of individual panels or components are greater when structures are subjected to vibrations at these natural frequencies. Whole-structure motions are similar to the type of distortion a cardboard box experiences when pushed on one side, whereby changing a rectangular side into a parallelogram. These distortions generate shear strains in walls and ceilings and have more potential for cracking than in the bending mode. Even with very low amplitudes, ground vibrations with low frequencies and long duration can generate responses in structures. If complaints are being received from ground vibration at a significant distance, this possibility should be investigated, and if necessary to prevent damage, the blaster-in-charge should adjust the design accordingly.

Damage Potential From Ground Vibrations

Structures can withstand some level of dynamic vibration because of the elastic nature of their materials. In fact, structures are subjected to dynamic vibrations from a wide variety of environmental activities both from the inside and outside. If the vibration levels are high enough, and the structure response generates strains that exceed the critical tensile strains of the building materials, threshold cosmetic damage may occur.

The term damage is often used very loosely in reference to cracking in structure materials. However, not all materials have the same strength properties. Damage classifications used today are listed and described in table 26.13.

Damage Classifications (USBM RI 8507)

Table 26.13 – Classification of Damage (USBM RI 8507).

Old plaster or sheetrock is the weakest material found in structures. Threshold damage from blasting includes the lengthening of existing hairline cracks in old plaster and sheetrock. However these types of cracks develop in all houses independent of any blasting, caused by aging, environmental (weather) change, and normal wear and tear. Safe blast vibration limits are most often based on protecting these most susceptible of materials. If the vibration intensity exceeds standard criteria, there is merely an increasing probability of threshold damage, not a significant probability of threshold damage.

Summary Of Ground Vibration Criteria

Table 26.14 provides a summary of vibration levels recommended to prevent cracking or damage in residential structures. These levels represent the range of intensities commonly found in regulations, specifications and individual recommendations for damage prevention. All criteria have a basis in amplitude and frequency, both of which play a significant role in structure response and cracking potential. If vibration levels are not included in contract specifications or regulations, the blaster-in-charge must select the appropriate vibration levels that will ensure prevention of damage to nearby structures. The levels selected should be based on the blaster's evaluation of nearby structures and/or the preblast surveys. Then the appropriate vibration level must be placed in the blast plan for field implementation.

The most common recommendations are very conservative and designed to prevent threshold damage (extensions of existing hairline cracks) to residential structures. The lowest vibration levels shown in table 26.14 are for below minor or major damage levels. The blaster should remember that low-frequency vibrations have the greater potential to cause damage than high-frequency vibrations. However, for blasters that need to work close to homes, it is beneficial to know that most homes are capable of withstanding higher vibration intensities at higher frequencies.

Summary Of Ground Vibration Criteria

Table 26.14 – Range of common residential criteria and effects. (The above criteria apply only to residences, not to any other facilities or materials.)

Vibration Effects On Concrete

Safe vibration levels for concrete are dependent on the location of the concrete in the structure. If blasting within the concrete, it can be treated as a man-made rock. The perimeter of the blasting and excavations can be handled by controlled blasting techniques, not by vibration limitations. Blasting in old concrete requires a particle velocity of 9525 millimeters/second (375 inches/second) and overpressures can be 15,240 mm/s/second (600 inches/second) with no break up at contact.

When blasting outside the boundaries of the concrete, vibration limits are based on the age of the concrete and the location of the concrete within the building. A common general specification limiting vibrations in green concrete was prepared for the Tennessee Valley Authority (TVA) in 1976, and is shown in table 26.15. In this table the recommended maximum PPV is given by the formula in the right hand side of the table.

TVA Blasting Vibration Limits For Mass Concrete

Table 26.15 – TVA Blasting vibration limits for mass concrete.

The TVA specifications also include a description of three field conditions and the limitations that accompany them as listed in table 26.16.

Field Conditions Causing TVA Specification Limits For Mass concrete

Table 26.16 – Field conditions causing TVA specification limits for mass concrete.

In Field Conditions 2 and 3 of table 26.16, the blasting takes place in rock layers whose boundaries fall below the concrete, where layer separation could disrupt the concrete. In those situations, the blasting can proceed without supervisory approval if the slope angle from the bottom of the concrete to the bottom of the explosives charge is at least 2 horizontal to 1 vertical. Blasting may also take place directly against the concrete, but only with supervisory approval.

Caution Supervisors may apply the base motion and must be treated more cautiously. For the types of concrete superstructures expected in TVA construction, and the types of blasting that might be used, it was considered appropriate to restrict the ground vibrations to about half the level used for mass concrete.

These TVA specifications are regarded as being conservative and more restrictive than necessary in many cases. However, they can be followed easily in the field without concern about damage. The limitations may be exceeded with supervisory control when considered necessary. When blasting near concrete, each case should be evaluated on its own conditions.

Vibration Effects on Buried Pipelines and Utilities

Blasting near buried pipelines or utilities often occurs when trenching at adjacent projects. In addition, there are many different types of materials used for utility lines or conduits, from welded steel to high plastic and ceramic. There has been no documented case of elastic vibration damage to buried utilities from blasting.

The prevention of over break or ground fracturing between the blast site and the location of existing pipelines should be the primary concern. If possible, test blasting could be employed to evaluate ground movement potential before full-scale blasting begins. If there is a vibration limit for the project, the levels should be monitored during the test blasting.

USBM RI 9523, Surface Mine Blasting Near Pressurized Transmission Pipelines is the most comprehensive summary of dynamic protection for buried pipelines. During the study no vibration damage to the pipelines occurred at amplitudes up to 600 millimeters/second (24 inches/second). The final boreholes were drilled between the parallel lines. When heaved out of the ground during the blast, two of the five lines were severely bent, but did not break. The recommended safe vibration level is 127 millimeters/second (5.0 inches/second).

Vibration Effects on Water Wells

Blasting near water wells generally occurs in rural settings where public water supplies are not available. Most wells will exist far outside of the blast site. These "vertical pipelines" are well coupled to the ground and respond with the ground. Most wells are lined with a steel or plastic casing down to bed rock and from there to the bottom left as an open borehole. Vibration damage to the well casing or borehole is not possible below 50.8 millimeters/second (2.0 inches/second) although a temporary increase in turbidity may occur in the water. In these cases, the water clears shortly after the event. See 3 Vibration from Blasting (Siskind, 2000) for a summary of the research reports.

Vibration Effects on Historic Structures

Historic structures need special mention because they can span a far greater range of vibration sensitivities than residences. However, neither their age nor general condition forms a complete criterion. Historic structures should be evaluated with an understanding of how they might respond to ground motion. These types of evaluations are beyond the scope of this book and may require the advice of a structural engineer.

Vibration Effects on Underground Mines and Tunnels

The integrity of mines and tunnels must be maintained when blasting nearby. Instability and rock falls have the potential to injure the workforce or public inside the man-made features and may adversely affect the ventilation network that ensures fresh air circulation. Often the regulatory agency responsible for workforce safety must grant approval prior to blasting.

Numerous studies as outlined in 3 Vibration from Blasting (Siskind, 2000) discuss the impacts of vibrations on underground mines and tunnels. Vibrations on the roof of underground mines tend to be high frequency and relatively low amplitude. Roof collapse and pillar failure is not likely below 305 millimeters/second (12 inches/second) and small pieces of rock falling are not likely at less than 50 to 127 millimeters/second (2 to 5 inches/second). If blasting near abandoned underground facilities where worker safety is not an issue, vibration limits may be established to prevent roof collapse for subsidence prevention if necessary to protect structures on the surface.

Damage Potential - Air Vibrations

The acoustic energy from a blast will normally arrive at some time after the ground vibration. The time of arrival is strongly dependent on the distance from the blast and can be expected to arrive about 1 second later than the ground vibration for each 335 meters (1,100 feet) of distance from the blast. In cases where the blast is close to a structure, the ground and air vibrations will arrive close together. At the distances, for example 600 meters (2,000 feet), the air vibrations will arrive about 2 seconds after the ground vibration. This may account for the "double blast" that some residents report as the house responds twice to the same event. Atmospheric conditions may also have an effect on the distance to which the public can perceive the air vibrations from blasting.

Air vibrations enter a house through the superstructure and cause racking and mid-wall response. These responses can generate vibrations comparable to ground vibrations. A notable effect will be window response and knock-knock rattling as the air vibrations move strongly impact mid-walls.

Threshold damage from air overpressure is unusual, but at elevated levels nearing 150 decibels, cracks in large glass windows may occur. Most criteria for air overpressure are largely founded upon perception and not damage potential. For construction and quarry blasting where the acoustic frequencies are high, an air overpressure of 140 decibels was often used in limiting criteria. A level of 133 decibels is now very common for all types of blasting. This level corresponds to a structure response level comparable to approximately 12.7 millimeters/second (0.5 inches/second) of ground vibration. Table 26.17 contains typical air overpressure criteria for damage prevention.

Typical Air Overpressure Damage Criteria

Table 26.17 – Typical air overpressure criteria.

The air overpressure of 134 decibels recommended as a limit by the U.S. Bureau of Mines is slightly less than one-half of the linear overpressure of 140 decibels that served previously as a long-term common standard. Neither of these has been shown to cause window breakage or threshold damage.

Air vibration damage from blasting is unusual. Hence it is also unusual that an unfavorable atmospheric condition would play a role in actual damage from normal blasting operations. Air overpressures at great distances will not be greater than those near the source, even though they might be greater than those at intermediate distances. However, the blaster may get an increase in complaints when unfavorable atmospheric conditions exist. At such times, persons located at a great distance from the blasting may perceive air vibration that they would not ordinarily notice under favorable atmospheric conditions.

Recommended Guidelines for Blast Vibration Monitoring

The following set of guidelines can be used for blast vibration monitoring. If structures other than those discussed below exist near the blasting, the blaster-in-charge should contact an experienced person in evaluating vibration effects and setting appropriate limits to protect the structures. These guidelines provide for a high probability of damage prevention and are based upon scientific visual observations of cosmetic cracking as documented in U.S. Bureau of Mines RI's 8507, 8896 and 8485. Any deviation from these recommendations should be accompanied by a documented technical justification.

Caution Whenever the term damage is used in this guideline, it shall be construed to apply to threshold cosmetic cracking.

Basic Guidelines For Vibration Monitoring

These guidelines apply to any dwelling, public building, school, church, or commercial or institutional building owned a certificate of occupancy (where applicable) or its equivalent. They do not apply to buildings owned, leased, or controlled by the blasting company or on property for which the owner has provided a written waiver to the blasting company.

The application of these guidelines provides for a high level of non-damage probability concerning damage of any sort including small, hairline cosmetic cracks in weak wall coverings, even in those structures that are old and distressed. Equipment structures and utilities, including those constructed of masonry and concrete, can withstand higher levels of excitation (vibration).

Monitoring of ground vibration and air overpressure must be conducted with blasting seismographs that meet the guidelines established by the ISEE Performance Specifications for Blasting Seismographs (See Appendix D). Furthermore the blasting seismographs must be deployed in the field according to the ISEE Field Practice Guidelines for Blasting Seismographs (See Appendix E).

Guideline for Ground Vibration Levels

At all blasting operations, the maximum single axis ground vibration (peak particle velocity) shall not exceed the limitation specified in figure 26.27 at any building.

When blasting is not monitored with a blasting seismograph, the blasting shall comply with the following scaled distance factors at the nearest building (See table 26.18). The appropriate scaled distance factor is dependent on the distance from the blast to the nearest building. The maximum allowable charge-weight of explosives (W) in kilograms (pounds) detonated within any eight-millisecond interval is calculated from the distance (R), in meters (feet) from the blast to the nearest building not owned, leased, or controlled by the blasting operation.

Scaled Distance Equations

Table 26.18 – Scaled distance equations. <!-- VERIFIED -->

The use of the table 26.18 scaled distance equations provides a high level of confidence for the prevention of damage. However, seismograph monitoring provides the best data for gauging the effect of vibrations. Thus monitoring with a blasting seismograph supersedes the necessity of designing blasts with the above ultra-conservative scaled distance factors.

Higher ground vibration limits for buildings, man-made structures, or utilities other than those described previously may be independently established based on technical justification by engineers and personnel familiar with the blasting related projects.

Guideline for Air Overpressure Levels

Air overpressure shall not exceed the maximum limit of 133 decibel (0.9 millibars or 0.013 pounds/inch²) at the location of any building. The limit of 133 decibels is primarily based on perception and has no potential to cause any damage to buildings.

Higher air overpressure limits for buildings, man-made structures, or utilities other than those described above may be independently established based on technical justification by engineers and personnel familiar with blasting related projects.

Other Industry Standards for Damage Prevention

Other authority organizations (see table 26.19) have established guidelines for vibration limits. Generally these limits are supported by the technical literature and conform to the U.S. Bureau of Mines recommendations. The recommended levels are generally for prevention of damage but in some instances are set to minimize annoyance. Furthermore, each country may have its own standards and those standards should be referred to when blasting work is done. Following is a partial list of some organizations that have developed guidelines or standards for vibration control.

Industry Standards For Damage Prevention

Table 26.19 – Industry standards for damage prevention.

HUMAN RESPONSE

During a blast vibration event, people inside a building will respond differently than people outside a building. Ground vibrations arrive first and air vibrations arrive second. Each will cause the structure to vibrate. People inside the structure are immersed in the vibration event and often cannot tell the difference between the two effects. The windows may rattle and there may be other structure responses that enhance their perception of the event. They can perceive structure vibrations that are well below levels that could possibly cause threshold damage. On the other hand, a person outside a structure will not notice any of the structure response. Therefore, their perception of the event will generally be much less.

Human response to blasting is subjective, as two people will react differently to the same vibration event depending on where they are in the structure, their frame of mind, and their personalities. Unfavorable reactions to vibration will often result in complaints. When a resident feels a blast, they may become concerned about damage to their home.

The factors which contribute to human response to air vibrations are the same as those from ground vibration. In both cases, observers are more likely to respond unfavorably inside a building than outside, and the response is increased if the observer is inside his own home rather than some other building. These sounds are more unwelcome if they suddenly, are not expected, or have a short rise to peak pressure, be distinct from those of sonic booms.

Beginning in the 1930s, physiological tests conducted on human volunteers attempted to determine their sensitivities to vibrations. Although people are sensitive to sounds and vibrations, it is difficult to quantify their perception. Inside a structure, people will feel the building shake and hear the objects around them rattle such as windows and knick-knacks on walls. When noise is perceived, some people will say that they felt very strong vibrations, even if the vibrations was too low to be felt outside. The reactions of people are best understood when observed in their own homes during times of real-life events. These reactions may not be the same as those of volunteers under laboratory test conditions.

The threshold of perception for ground vibration is about 0.51 millimeters/second (0.02 inches/second) for most people at typical blasting frequencies. This is 1/100 of the limit of 50 millimeters/second (2 inches/second) commonly used for construction blasting. These occasional or transient vibrations have minimal physiological impacts. Keep in mind that air vibrations also cause a structure to vibrate. Like a ground vibration, these are physiological hazards (risk associated with air overpressures from commercial blasting operations).

From a sociological perspective, people in different living environments will perceive blasting as either positive or negative. Sociological perspectives are based on population density, education level, community needs, employment needs, etc. of the area. If the project is not perceived as beneficial to a community, like a quarry, the project will be unwelcome. On the other hand, a bridge demolition project that is highly desired to make way for a new bridge will be favorably viewed. Long term projects may communities are often viewed unfavorably as opposed to short term projects. While the physical effects of vibrations do not change over time, the social attitudes may change greatly over time. Even for desired blasting projects, once the novelty has worn off, complaints and damage claims may occur.

Blasting will continue to be accepted as long as observers perceive the work as beneficial. That fact is illustrated in a sonic boom study for the supersonic transport (SST) system. People who believed that the sonic booms were necessary and unavoidable generated two and one-half times less complaints than the people who thought otherwise. In some mining communities, tolerances are higher because of the perception of the value of the work to the community in general, although specific individuals may object not only to the physical perception of vibration and sounds, but also to dust, traffic and other aspects of the operations that do not contribute to their own personal interests.

When the blast vibrations follow a warning signal, the surprise associated with the event may be mitigated. A warning signal is most often used for workforce safety, but has the side benefit of notifying nearby residents of impending blasts. This shows that there is value in public relations programs that stress workforce and public safety.

Summary

Vibrations from blasting cause man-made structures to vibrate. The blaster-in-charge is responsible for evaluating blast area conditions and designing blasts that generate vibration intensities below levels necessary to protect these nearby structures. Available explosive products and initiation systems give the blaster-in-charge numerous options for blast design that will ensure proper fragmentation and vibration control. The vibration intensities generated are predictable to the extent that the explosive products perform as expected in the field and the blasts are loaded in the field as designed.

Vibration data from multiple locations for a single blast will plot along a simple line. For numerous blasts at the same site, the data points will tend to exhibit some scatter because of minor geology, topographic, and blast design changes. The scatter of data should fall within a narrow band for a single blasting methodology. If more than one type of blast design is used at a site because of contract specifications, mining methodologies, geologic conditions, etc., the band of data scatter will also increase. From the data, vibration equations can be developed. The upper bound of the dataset is used to estimate future vibration intensities and facilitate future blast design. If a blasting project is small, historical vibration equations are available to estimate vibration intensities.

Vibration monitoring with blasting seismographs is recommended for all blasting. The vibration data allows the blaster to evaluate the performance of each blast in terms of both blast design and rock vow implementation. A blaster with a good understanding of blast design parameters that increase or decrease the vibration intensity should be able to recognize those vibrations that fall outside the range of the typical data. If a particular blast causes vibration intensity close to or outside of the upper bound of the vibration equations being used at the site, the blaster should investigate to understand the cause of the anomaly.

Furthermore vibration monitoring is essential for liability protection. The blaster's goal is to restrict blast vibration levels at man-made structures outside the project, work, or permit area to ensure a high confidence level of damage prevention. Each type of structure will tolerate different vibration levels. Engineered and buried structures will tolerate higher vibration levels. Residential structures tend to be the most sensitive. The blaster should outline the vibration controls and vibration level justifications as part of the blast plan for the project. Often, vibration levels are established by local regulations or contract specifications. Those are typically set for protecting residential structures and should also be discussed in the blast plan. To ensure maximum liability protection, blasters must select the appropriate monitoring equipment and monitoring locations, use personnel trained in the use of blasting seismographs and keep good records of the blasting activities as discussed in chapter 27.

REFERENCES

Landon, R. L., and E. Hosalum. 2009. Report of ground vibration and airblast attenuation and character at a Pennsy Supply, Silver Spring Quarry. Pennsylvania Department of Environmental Protection.

Nicholls, H. R., C. F. Johnson, W. I. Duvall. 1971. Blasting vibrations and their effects on structures. Bulletin 656, United States Department of Interior, Bureau of Mines.

Oriard, L. L. 2005. Explosives Engineering: Construction Vibrations and Geotechnology. International Society of Explosives Engineers, Cleveland, OH.

Oriard, L. L. 1999. The Effects of Vibrations and Environmental Forces: A Guide for the Investigations of Structures. International Society of Explosives Engineers, Cleveland, OH.

Siskind, D. E., V. J. Stachura, M. S. Stagg, J.W. Kopp. 1980. Structure response and damage produced by airblast from surface mining. Report of Investigations 8485, United States Department of Interior, Bureau of Mines.

Siskind, D. E., M.S. Stagg, J.W. Kopp, C.H. Dowding. 1980. Structure response and damage produced by ground vibration from surface mining. Report of Investigations 8507, United States Department of Interior, Bureau of Mines.

Siskind, D. E. 2000. Vibrations from Blasting. International Society of Explosives Engineers, Cleveland, OH.

ADDITIONAL RESOURCES

Aimone-Martin, C. T., M. A. McKenna, D.E. Siskind, C.H. Dowding. 2005. Comparative study of structure response to coal mine blasting. Final report to the Office of Surface Mining, ID CTO 5-21203.

Crum, S. V. 1997. House response from blast-induced low frequency ground vibrations and inspections for interior related cracking. Final report to the Office of Surface Mining, ID 143880-P096-214RC.

D. B. Stephens and Associates. Comparative Study of Domestic Water Wells in Coal Mine Blasting Summary Report. Final Report to the Office of Surface Mining, ID CTO 5-21064 2002.

Dowding, C. H. 1996. Construction Vibrations. Prentice Hall, Englewood Cliffs, NJ.

Dowding, C. H. 2008. Micrometer Crack Response to Vibration and Weather. International Society of Explosives Engineers, Cleveland, OH.

Society of Mining and Metallurgy and Exploration. 1992, SME Mining Engineering Handbook, 2nd Edition, Hartman, H. L., Senior, Editor, chapter 9.2.2, Monitoring and control of blast effects, Society for Mining, Metallurgy and Exploration, Inc.

International Society of Explosives Engineers (ISEE). 1998. ISEE Blasters' Handbook™, 17th Edition. ISEE, Cleveland, OH.

Lusk, B., J. Silva, K. Eltschlager, J. Hoffman. 2010. Acoustic responses of structures to blasting analyzed against comfort levels of residents near surface coal operations. Final report to the Office of Surface Mining. ID EPD06.

Perkins, B., Jr., and W.F. Jackson, 1964, Handbook for Predicting Air Vibration Focusing, Ballistic Research Laboratories, Aberdeen, MD Report No. 1240, 309pp.

Pittsburgh Plate Glass Company. 1965. Technical Service Report No. 101, Pittsburgh Plate Glass Company.

Rosenthal, M. F. and Morlock, G. L. 1995. OSM Blasting Guidance Manual, Office of Surface Mining Reclamation and Enforcement. 1985.

Stagg, M.S., D.E. Siskind, M.G. Stevens, and C.H. Dowding. 1984. Effects of repeated blasting on a wood frame house, Report of Investigation 8896, United States Department of Interior, Bureau of Mines.

Tart R.G., L. L. Oriard, and J. H. Plump. 1980. Blast Damage Criteria for a Massive Concrete Structure. Preprint 80-175.

Mississippi Development Commission Vibrations, ASCE National Meeting, Portland, Oregon, 1980.

Wiss, J.F., and P. Linehan, 1978 Control of Vibration and Blast Noise from Surface Coal Mining, U.S. Bureau of Mines Contract, J0255022, 2,111 pp.

Wyoming Blaster's Training Modules, US Department of the Interior, Office of Surface Mining Reclamation and Enforcement, 2008.