chapter 2. blasting effects and their control

Chapter 2. Blasting Effects and Their Control

INTRODUCTION

In recent years, there has been a trend in the direction of larger drilling equipment and larger diameter blastholes. Although this change has improved the effi- ciencies and reduced the costs in many operations, it has increased the potential for damage to underground openings. In addition, in many instances one now finds more sophisticated delicate instruments, automated control facilities, and a large variety of structures in proximity to blasting activity. The combined effect of larger-scale blasting activity and its proximity to various features of interest is such that there is an increased need for a more refined analysis of blasting effects and their control.

BLASTING EFFECTS ON ROCK SURFACES

The Breakage Mechanism In order to develop techniques for controlled blast-

ing, one must first understand the features of the mechanisms by which blasting causes rock breakage to occur. These features have not been easy to demonstrate, mostly due to the difficulty in making tests and observations at the high stress levels and short time durations involved.

When an explosive charge is detonated, the material surrounding the charge is subjected to a nearly instantaneous, very high pressure [on the order of 1.4 to 13.8 GPa (0.2 to 2.0 X lo6 psi), depending on the explosive]. If the charge is coupled to "average" rock, this pressure will pulverize the surrounding rock for a distance on the order of 1 to 3 charge radii in hard rock, and to a greater distance in softer rock (this is also dependent on the type of explosive). As the pressure wave passes into the rock, high tangential stresses cause radial cracks to appear, and the nearly discontinuous radial stress zones generated by the shock front may cause tangential cracks to

the explosive gases. The first three items have received much attention in the laboratory and the literature. The complex effects of gas venting are difficult to test in the laboratory because of the difficulty in reproducing the many weak planes and discontinuities typical of most field conditions, which play such a prominent role in determining the behavior of the rock mass subjected to blasting. Unfortunately, gas venting effects can be pro- jected to significant distances under certain field conditions, and are sometimes difficult to control. It is not unusual for gas venting to be the overriding factor in determining the final geometric shape and physical condition of the finished excavation.

Sources of Damage For the purposes of this discussion, damage includes

not only the breaking and rupturing of rock beyond the desired limits of excavation but also an unwanted loosening, dislocation, and disturbance of the rock mass the integrity of which one wishes to preserve (such as mine pillars, underground openings, etc.). The sources of damage include, of course, all those physical features of the rock breakage mechanism. Each of these effects must be limited to the desired zone of breakage and excavation if the integrity of the remaining rock mass is to remain undiminished. The primary zone of rock breakage usually can be controlled in the normal process of field experimentation to determine proper charge sizes and location for primary excavation. However, it frequently happens that there is damage from sources which are more difficult to account for in the design process, which are often overlooked. These are ( 1 ) the overbreak due to poor drilling control, (2) dislocation of rock (mass shifting) due to venting of explosive gases, and (3 ) loosening or dislocation due to the influence of seismic waves (ground vibrations).

appear. he extent of these-cracks depends on the en- -

ergy available in the explosive, how quickly the energy CONTROL OF ROCK BREAKAGE

is transmitted to the rock, and the strength properties of Importance the rock. The discontinuous shock front is quickly dis- In studying the rock mass and blasting design con- sipated, but the expanding gases generate a longer-acting siderations, it is important to keep in mind the geometric pressure. A compressive pulse travels to the nearest face relationships among charge size, shape, and position, and or internal rock boundary where it is reflected in tension. the physical features of the rock mass to be preserved. The tensile strengths of most rocks are roughly I/lo to Mo The features of principal interest are the external shape of their compressive strengths, so the rock may now fail and position of the rock mass relative to blasting, and in tension whereas it may have been able to support the the position and attitude of any weak planes in the rock diminished compressive phase without failure. The ten- mass. sile deflection typically produces a failure described as ~h~ sequence of ~ l ~ ~ t i ~ ~ and Excavation Events tensile slabbing or scabbing. Unfortunately, there are too many times when the

Laboratory experiments and field experience have task of preserving delicate rock is considered hopeless, pretty well established that several mechanisms are in- and because of this attitude, no further effort is ex- volved. ~ h e s e include (1) the classical case of tensile pended towards caution or control. In such cases there parallel slabbing when the pressure pulse is reflected at is often a failure to recognize the importance of the se- a free surface; (2) failure under quasi-static compressive quence of the procedures. Attention t~ this can greatly loading (the shape is normally irregular due to discon- reduce unwanted effects at minimum cost. tinuities in the rock); (3) radial cracking under the action of tangential stresses at the periphery of the ex- Perimeter Control panding pressure pulse; (4) peripheral cracking at the The requirements for perimeter control are highly discontinuous shock front which is quickly dissipated; dependent on the special needs of each particular proj- and (5) additional mass shifting due to the venting of ect. The desirable degree of control is a highly variable

1590

BLASTING

item. Nevertheless, such dramatic improvements can be made over the simple expedient of terminating pattern blasting at the perimeter that it is rare today to find a major project in which some form of pattern modification is not applied at the perimeter to improve results. Before deciding on the degree of caution to exercise, one must evaluate the cost and time of the work compared to the needs. Will concrete replace the overexcavated rock beyond a prescribed perimeter? How carefully must one preserve the integrity of the remaining rock? Can it be allowed to ravel or fail? Is the geometric shape of the perimeter of importance? The answers to such questions will help to determine the approach to the work.

If a high degree of control is needed, the two most common methods for controlling perimeter breakage are the presplitting or preshearing method, and the smooth blasting or smooth wall blasting method. A variation of the latter is usually called cushion blasting, a term which is even older in its usage.

In any method which is designed to produce precise control of the perimeter, it is extremely important to re- quire careful drilling. The final results cannot possibly be better than the drilling. Poor drilling probably accounts for more overbreak in underground excavations than does poor blasting. Extra time and effort are usually needed at the beginning of a project, since drilling is an art, and there is usually a noticeable learning curve as the work gets under way.

Presplitting Presplitting or preshearing is a method of generating

a crack in the rock along the desired limit of breakage in advance of the pattern blasting. In this method, holes are drilled just beyond the desired perimeter [usually 76 to 152 mm (3 to 6 in.) for shallow holes, 305 mm (12 in.) for deep holes]. Small-diameter explosive charges are loaded into these holes and detonated simultaneously ahead of primary blasting, generating a crack or shear along the perimeter. Size and spacing of the holes and charges are dependent on the rock characteristics and the need for smoothness and soundness of the final surface.

If an explosive charge is in full contact with the walls of the drill hole, it is said to be fully coupled. When such a charge is detonated, a very high pressure shock wave strikes the walls, usually crushing the rock for a distance of 1 to 3 charge radii. The expanding gases try to ex- pand the hole and cause radial cracks to be transmitted into the rock as the perimeter is placed in tension.

In the presplitting method, the shattering is eliminated or greatly reduced by decoupling the charge, i.e., an annular ring of air surrounds the cartridge. Ideally, the charge does not touch the rock. Although the dam- aging effect of the shock wave is thus largely eliminated, the expanding gases continue to work on the rock. And if two adjacent holes are detonated simultaneously, there is a preferential growth of the radial crack connecting the two holes, in preference to other directions. For an illustration, refer to Fig. 1. Assume that the decoupled charges A and B detonate simultaneously. Stresses are developed at particle locations x and y. At location y, the stresses a (radiating from charge A ) oppose the stresses from B, such that a crack does not develop. At location x, the stresses from A enhance those from B. The compressive stresses are not sufficient to cause the

ENHANCED

COMPRESSION

8

Fig. 1. Presplitting stress distribution. Crack propagation enhanced in rock web between holes.

rock to fail. However, the rock is much weaker in tension, and a tensile crack between the two charges is preferred over any other direction. In addition, this crack is given further preference after it begins to form, since less energy is required to extend a crack than to develop a new one.

In the design of patterns for presplitting, the explosives concentration is a function of the ratio of the hole diameter and charge diameter and the surface area of the presplit plane (excavation boundary). With typical 15.87- to 19.05-mm (%- to %-in.) charges in a 63.5- to 76.2-mm (2%- to 3-in.) hole, the charge concentration is of the order of 3.4 to 5.3 Pa (0.07 to 0.11 lb per sq ft) of perimeter surface area.

The most difficult portion of a perimeter to preserve in the desired condition is the shoulder formed at the perimeter in the collar zone of the presplit holes. Quite often, extra holes are drilled to shallow depths in this zone to assist in the formation of rectangular corners in the rock. These holes may be loaded or left unloaded, according to the circumstances. When not loaded, they are usually referred to as guide holes. These work best when drilled within about five diameters of loaded holes. Beyond that distance, the crack propagation may not be noticeably influenced by the existence of the guide holes. When shoulder rock is being broken or shifted by presplit blasting, it may be necessary to increase the depth of stemming.

Existing stress fields in the rock have an important influence on crack propagation during presplitting. If these in-situ stresses are oriented away from the presplit plane, cracks may be favored in that direction, and the presplitting results will be unsatisfactory. Under these conditions, it may be necessary to place the holes closer together or to use a different technique for the perimeter blasting, such as smooth blasting or smooth wall blasting (this will be discussed later).

It is desirable to have the presplit holes detonate simultaneously. For this reason, it is common practice to connect the holes with detonating cord to insure simul- taneous detonation. If each hole is initiated with a separate blasting cap, a certain amount of timing scatter can be expected, depending on the cap design. However, satisfactory results usually are obtained with this method even though the timing scatter is a departure from theoretical conditions. Similarly, satisfactory results are often obtained when detonation is restricted to only 3 or 4 holes per delay due to vibration controls.

UIVDERGROUND MINING METHODS HANDBOOK

There has been much discussion over the benefits of presplitting as a method to bring about vibration isolation, i.e., to prevent vibrations from passing effectively beyond the presplit plane. Such assumed benefits may not in fact exist, and the concept should be regarded with conservatism. In most cases, a semi-infinite burden exists, and/or horizontal in-situ stresses are present. In such cases, the presplit crack immediately closes and does not present a significant barrier to vibrations, being similar to a typical joint plane in the rock. Typical stresses found in underground workings will cause such cracks or joints to close tightly. This may not occur where a limited burden exists, where a different type of problem would then exist. Sometimes a large presplit shot will cause the permanent displacement of a large mass of rock, leaving a large open crack and a disturbed rock mass. Further drilling and blasting may be restricted because of this disturbance. The open crack will serve as an effective vibration isolator only for the upper part of the small zone adjacent to the crack on the oppo- site side from future blasting. Charges placed below the level of the bottom of the crack will generate vibrations which pass undiminished below the crack through the rock mass in a normal fashion.

Pf it is desired that presplitting be used to develop a vibration barrier in a rock mass, it is possible to blast with heavier charges than normal or use more than one row of charges to produce a fractured zone. Again, it should be remembered that seismic waves will be dif- fracted around this zone so that its influence is limited.

In connection with vibration problems, it should not be overlooked that the presplit blast itself is capable of causing an unacceptable vibration. Because of the excessive confinement, the presplit blast will usually generate the largest amount of vibration for a given amount of explosive.

In many instances, the balance between costs and benefits would suggest that some form of modified presplitting be put into use. Depending on the requirements of the particular project, the diameter and spacing of holes could be increased until the perimeter condition reached its limit of acceptability. Table 1 lists sugges- tions for designing such presplitting shots as a first-order cut.

Smooth Blasting Smooth blasting is similar in concept to presplitting,

except that the charges are detonated after the primary blasting. In the ideal case, the primary blasting has been done and the rock excavated to the last row of holes. Then a separate blast is detonated for the last row. A common alternative is to detonate the perimeter charges as the last delay in a larger blast. If the latter technique

Table 1. Recommendations for Presplit Blasting

Hole Charge Diameter Spacing Concentration,

in. rnm ft m Ib per ft (kg/m)

21/2-3 64-76 2-3'15 0.6-1.1 0.18-0.25 (0.02-0.035) 4 102 3-4 0.9-1.2 0.25-0.50 (0.035-0.50) 6 152 4-6 1.2-1.8 0.35-0.75 (0.05-0.10) 8 203 6-8 1.8-2.4 0.75-1.50 (0.10-0.20)

is used, there should be some adjustment in powder factor and timing to give the greatest amount of free move- ment to the perimeter blasting. In smooth blasting, it is customary to reduce the spacing between holes to a p proximately 80% of the burden. Holes are fired simultaneously, or in groups if a vibration problem exists. Charges can be reduced slightly below those used for presplitting if a free face exists.

Both presplitting and smooth blasting usually produce good results in massive rock. Smooth blasting usually is capable of reducing venting damage in highly jointed or fractured rock.

Smooth blasting has a benefit if strong in-situ stress fields are causing presplit cracks to travel in the wrong directions. The primary blasting removes the burden and relieves most of the in-situ stress, so that the smooth blasting no longer has the same unfavorable conditions imposed.

If smooth blasting is taken to mean a completely separate blast fired after the primary round, it is a costly procedure for tunnel practice, and is not often used. In hard massive rock, no special technique is usually needed. In loose jointed rock, the stand-up time of an exposed roof is a problem, so it is not desirable to drill and blast a separate round for the perimeter. A compro- mise is to fire sections of the perimeter simultaneously on the last delay intervals.

Modifications to Perimeter Blasting Techniques It would be misleading to propose that presplitting or

smooth blasting methods must follow certain prescribed patterns. There are as many variations as the rock conditions and the imagination of the explosives engineer will allow. The plans should be tailored to the conditions and the purpose of the blasting.

The perimeter charges may be modified by diameter, length, position, density, strength, etc. Conditions can vary even within a single hole. One quick approach to modified perimeter blasting is to use a very low-density bulk blasting agent, consisting of a mixture of ANFO (ammonium nitrate-fuel oil) and expanded plastics. If higher density is satisfactory or needed, requirements may be met with low-density slurries or untamped cartridges. Going to the other extreme, one may find that conventional presplit cartridges are too large where extreme caution is required. In such cases, holes can be drilled closer together, leaving a narrow web to be broken with detonating cord.

Fracture Control in Blasting Another interesting modification to conventional

blasting is that where notches or slots are scribed in the walls of the drill holes to enhance the growth of cracks in the preferred plane. The scribing or slotting can be done with mechanical tools or by means of high-pressure water jets. These techniques offer the following advan- tages: (1) a perimeter fracture plane that is more sharply defined, (2) less shattering effect on final surfaces, (3) lower vibration levels, (4) greater spacing between holes, and (5) better extraction of a cut zone if used in a tunnel round (Oriard, 1981 ) .

Research conducted at the University of Maryland suggests that holes may be spaced up to 25 to 50 diam and that charges may be reduced to about 1/40 or less of the normal concentration (Barker, Fourney, and Dally, 1977). Plewman has demonstrated a fracture extension

BLASTING

of 50 diam (Plewman, 1968). The writer is familiar fracture proportion (Oriard, 1980; Tart, Oriard, and with a dimension-stone auarrv in which even greater plum^. 1980). fracture lengths have bee; achieved. As of this writing, the technique has been tried at two rock sites and on a project requiring partial demolition of an old concrete structure which required extremely delicate work.

One of the rock sites is a construction site in relatively massive and competent limestone. In experimental blasting for fracture control at this site, vee-shaped notches were scribed mechanically in vertical holes [diam of 63.5 mm (2% in.)]. It was found possible to generate preshear fractures in this limestone with approximately ?h to ?4 normal loading per unit of surface area of perimeter wall when the holes were scribed. The rock did not fracture at loadings or spacings equivalent to those used successfully in laboratory experiments. How- ever, the method was considered successful at this site and proved that it could be used under typical field conditions for large-scale blasting.

The second site was a research chamber in highly foliated rock appended to the Peachtree Center subway station in Atlanta, Georgia. As expected, it was far easier to develop fractures parallel to foliation than it was to develop them perpendicular to foliation. As before, fractures developed at reduced loadings. However, in this highly anisotropic rock, approximately equal results were obtained at the same spacing with the use of heavier charges detonated conventionally without scribing (Oriard, 1979).

A third case involved the use of explosives for partial demolition of the old concrete lock walls at Lock and Dam No. 1, Minneapolis, MN. Field tests were very successful in demonstrating that explosives charges in notched holes could be reduced to about ?4 of those in holes that were not notched. Good stemming was required to accomplish this reduction.

Mechanical scribing tools were developed at each of these sites. The writer has not yet had the opportunity to test high-pressure water jet scribing, but feels that it has the potential for producing slightly better results than mechanical scribing.

It is probable that field results will not consistently equal those obtained in the laboratory. There may be need for more field data before we can scale up the laboratory data successfully. In this writer's opinion, the question of the fracture toughness of a given material is complicated by another factor which we might call the beam strength. A relatively small block of material is more easily broken in the laboratory than the same material in a semi-infinite mass of rock in the field.

For the enhancement of fracture growth in a presplitting operation, the ideal explosive would be a slow explosive which produces a large volume of gas, where the gas expansion can be maintained for a relatively long period of time. Confining the gases is very important. One must use good stemming so that the gases are p r e vented from escaping before the fracture growth is wm- pleted. This was dramatically demonstrated at the limestone site previously mentioned. After a given shot failed to develop a presplit fracture, the stemming was changed and the shot was then repeated successfully because the stemming was not ejected the second time. Similarly, the test blasting in concrete demonstrated the importance of good stemming as an adjunct to good

Grade Control In underground bench type blasting, usually there

are two aspects of grade wntrol that are of primary interest. One of these is the depth of the rock breakage; the other is the smoothness of the surface breakage.

In homogeneous isotropic rock, each charge in a blasting round tends to form a crude crater, breaking upward and outward from the bottom of the charge. The size and shape of this crude crater are a function of the charge concentration and the character of the rock. Because of this tendency to form a crater, it is necessary to drill the blastholes deeper than the desired grade level; otherwise high spots in the rock surface will protrude above the grade level. This additional drilling is usually called subgrade drilling. As a first-order rule of thumb, the depth of subgrade drilling is approximately '/3 the dimension of the hole burden (distance to free face). However, the range of subgrade drilling can extend from less than zero to as much as half the burden. If a soft layer is encountered at grade level with hard rock above, the breakage may actually extend below the level of the charges. If a soft layer is found a few feet above grade level with hard rock at grade, there will be a tendency for explosive energy to be dissipated in the soft layer, and deeper drilling will be needed to break to grade.

The smoothness of the grade surface is also determined strongly by the character of the rock. When blasting in horizontally bedded rock with prominent horizontal parting planes, one can achieve smooth planar bottom surfaces. However, when blasting in massive unbedded rock, or rock with weak planes at some attitude other than horizontal, the smoothness of the grade surface is strongly dependent on the spacing between the holes. Smaller charges at closer spacings will develop a smoother surface. For planning purposes, the predicted surface can be modeled as contiguous craters whose bottoms are ?L3 times burden below grade.

VIBRATIONS (ELASTIC WAVES)

After the primary shock front or pressure pulse has passed beyond the zone in which shattering or fractur- ing of the rock occurs, it passes through the rock in the form of vibrations or elastic waves. As this energy passes through the rock, it takes on different forms which travel at different velocities and cause different types of deformation to occur in the rock. The fastest traveling wave originally was given the name primary or P-wave. This is a compressional wave, sometimes called a radial wave or longitudinal wave, because the rock is deformed in the radial direction from the energy source. Follow- ing the P-wave is a slower traveling wave which was originally called a secondary wave or S-wave. This is a shear wave, sometimes called a transverse wave. Al- though this wave travels in the same direction as the P-wave, the deformation of the rock is at right angles (or transverse) to the direction of the wave travel. The P-wave and S-wave move through the main mass of the rock and have the general name body waves.

As the body waves arrive at the ground surface, new forms of waves are generated. One group of these waves is known as surface waves because they travel along the ground surface. Their motion is quite different from that

1594 UNDERGROLIND MINING METHODS HANDBOOK

of the body waves, being characterized by larger amplitudes, lower frequencies, and a lower propagation velocity. Structures which rest on the ground surface are usually located far enough from the blasting for the surface waves to develop, and they receive the strongest part of the motion from these waves. In underground measurements, the body waves tend to be the most significant.

If one makes the usual assumption that there is an elastic half space that is homogeneous and isotropic, elastic wave theory describes the wave motions that can be anticipated. In practice, it is simpler and more re- liable to determine particle motions by means of field measurements rather than through theoretical calcula- tions. However, it is important to remember that the different forms of energy are propagated at different velocities. The compressional or dilational wave is propagated with the velocity

c,= [E(1 - -p) /p( l - -2p)( l + p)1112 = [(A + 2G)/pI1l2

where

and

E is the modulus of elasticity; p is mass density; and p is Poisson's ratio. The constants A and G are known as Lame's constants. G is also known as the shear modulus.

Compressional wave transmission velocities for most rock types fall in the range of 1524 m/s (5000 fps) to around 6096 m/s (20,000 fps), correspondingly less for weathered or decomposed rock. Most soils fall in the range of about 152 m/s (500 fps) to about 1220 m/s (4000 fps).

The shear wave travels at the velocity

The ratio of compressional and shear velocities is

Poisson's ratio for most rock materials is very nearly 0.25. Thus, the velocity ratio Cp /C , is often very nearly 0 = 1.73.

The Rayleigh wave is named after Lord Rayleigh who was the first to examine the case of this seismic wave traveling along the boundary of a free surface. This wave is characterized by particle motion that is polarized in a vertical plane parallel to the direction of the wave propagation, and the particle motion is ellipti- cal retrograde. When Poisson's ratio is equal to 0.25, the velocity of the Rayleigh wave is 0.92 times the velocity of the shear wave.

Not only do these different wave forms travel at different velocities, but they have the additional character- istic of attenuating at different rates. In the case of spherical symmetry in a nondispersive medium, such as the outward-advancing body wave, elastic theory shows that the amplitude is inversely proportional to the distance. In contrast, surface waves have an amplitude which is inversely proportional to the square root of the distance.

Therefore, when the point of observation is close to the energy source, there will be a complex combination of several different wave forms. However, as one moves farther from the source, the wave forms become separated, arriving at different times and producing different types of particle motion. The more distant the point of observation is from the source, the more prominent will the surface waves be compared to the body waves.

Both theory and observation suggest that the particle motion transmitted to a free surface is more prominent than for the same wave within the body of the solid. For a wave arriving at normal incidence, the particle amplitude may be doubled. This is of interest when considering underground structures.

Kinetics of Wave Motion The displacement or amplitude of the ground wave is

the distance from a particle at rest to its peak or trough as the wave passes. Typical displacements for blasting vibrations fall in the range from 0.025 to 2.5 mm (0.001 to 0.1 in.). The term amplitude is used also to refer to the trace amplitude on the seismogram (recording of the motion).

The frequency of a vibration is the number of cycles that pass a given point in unit time, usually expressed as cycles per second or hertz. Frequencies of interest for blasting vibrations usually fall in the range from 1 to 500 Hz, and most are in the range from 5 to 100 Hz.

The period of a vibration is the length of time required for one complete cycle to pass a given point, usually expressed in seconds. Period is the inverse of frequency.

Particle velocity is the time rate of change of particle displacement. I t is the velocity of the motion of a particle during the passage of the seismic wave. Parti- cle velocity is not to be confused with propagation velocity.

Propagation velocity is the velocity with which a wave travels through a given medium. The propagation velocity varies widely according to the elastic properties of the medium. Typical P-wave velocities in rock range from about 1500 m/s (5000 fps) to about 6000 m/s (20,000 fps). The term velocity will denote particle velocity unless otherwise specified.

Acceleration is the time rate of change of particle velocity. It refers to the acceleration of a particle as the seismic wave passes this particle. For simple harmonic motion, the following relationships apply: x is displacement at time t; A is maximum value of x which is equal to the amplitude (zero to peak); f is frequency; v is particle velocity; a is acceleration; and w is angular frequency.

Some useful formulae are:

w = 2-f x = A sin wt

Xmax. = A v = dx/dt = w cos wt

= w sin wt + 7r/2) v,,,, = 2 ~ f A

a = d2x/dt2 = -w2A sin wt = w Z A sin (wt + m)

a,,,, = 47r2f2A.

Damage Criteria Parameters Over the years there have been many attempts to

BLASTING 1595

select suitable criteria for limiting vibrations or for rep- resenting the ability of some entity (structure, rock slope, etc.) to withstand vibrations. The two parameters which have been used most often to express the intensity of a vibration are acceleration and particle velocity. Dis- placement has received somewhat less emphasis. The purpose of these various researches generally has been to find a single value of some vibration parameter which can be used to express damage potential. For a specific type of vibration and an identified structure or response system, the problem can often be solved theoretically through the use of response spectra.

In attempting to find a simplified approach to blasting vibrations, a number of investigators (Duvall, et al., 1963; Devine, et al., 1966; Oriard, 1970; Nichols, et al., 1971; Hendron and Oriard, 1972; and others) have found it practical to use values of particle velocity as criteria in preference to other single-valued parameters because particle velocity appears to have the best cor- relation in the frequency range encompassed by most blasting vibrations. Nevertheless, it is this writer's con- clusion that for most structures a single-valued velocity criterion is less conservative at low frequencies and more conservative at high frequencies. One reason for this is the larger response that occurs in most structures at low frequencies. Even without an enhanced response, distor- tion (strain) of the structure plays an important role in determining the extent of damage. Thus, large displace ments and low frequencies tend to be more harmful than small displacements and high frequencies, even if the assumed criterion parameter remains constant.

Therefore one should be aware of the time history of the motion, as well as any single value used to express intensity. Ideally, one should look also at the response time of the structure and compare that to the input signal. There can be an important advantage towards greater liberalism if the response time of the structure is large compared to the rise time or frequency of the input signal. For example, it is not ordinarily suitable to establish a criterion for shock waves on the basis of peak pressure. For such short-duration transients, the damage potential is more related to impulse, which takes the time history into account. The time histories of both shock wave and structure are important. The total duration of the motion is also significant in instances where the motion reaches or exceeds a certain threshold level.

In the case of underground openings in rock, the span of the opening becomes an item of considerable importance, not only because the span has an important bearing on the static stability of the opening but also because it has an important bearing on the amount of seismic energy that is reflected at the surface of the opening, hence the dynamic stability. This reflected energy is a function of the span of the opening, the wave length of the incoming seismic signal, and the angle of incidence of the signal.

In spite of the technical difficulties in specifying a single value of a given parameter, it turns out in practice that one can select a threshold value that is restricted to certain types of vibrations and certain types of structures (or whatever might be the entity of concern). One can be as conservative as desired in selecting that value. Therefore, particle velocity can serve as a useful parameter for describing the damage potential of b!asting vibra-

tions, even though the degree of conservatism may change with frequency. In other words, there may be a different limiting value of particle velocity for 100- to 200-Hz vibration from tunnel blasting than there would be for a 2- to 20-Hz vibration from a bench blast. Rec- ognizing this, one can refine the analyses and still avoid the complications of developing response spectra. How- ever, with advances in instrumentation technology, it is becoming ever easier to determine the spectral content of vibrations, and it is expected that there will be an increased use of criteria which take this into account.

Data Scaling In order to compare blasts of different sizes at differ-

ent distances, it is customary to scale the distance factor by some function of the explosive weight. Two methods are currently in popular use in the US. One of these scales distance by the ?4 power of the charge weight per delay, the other by the I/? power. Ambraseys and Hendron (1968) have suggested the use of cube root scaling. Research sponsored by the US Bureau of Mines has led to the recommendation for square root scaling (Devine, et al., 1966). This writer uses cube root scaling for blast waves in water, sometimes for seismic body waves in rock, but usually uses square root scaling for surface ground motion data, and emphasizes the need to consider modifications due to spatial and time distri- butions of energy (Oriard, 1972). Cube root scaling demands certain theoretical limitations that are often not met under conditions found for many types of construction blasting.

Empirical prediction curves based on cube root scaling are shown in Fig. 2. In this figure, R is range (in feet), W is charge weight per delay (in pounds), and V is peak particle velocity (in inches per second).

Similar curves for square root scaling are shown in Fig. 3. The trend lines on the graph can be expressed mathematically in the form:

V = H (DI W"2)-B k , , k p , k3. . . where V is peak particle velocity; H is velocity at unity scaled distance, D is distance, W is charge weight per delay, B is slope of trend line, and k are factors defining firing time variations, travel time variations, coupling, confinement, geology, isolation, spatial distribution, etc.

Scaling by the ?4 power is considered to be the preferred method by some who argue that it can be shown to be dimensionally correct by the Buckingham Pi theorem. Yet this is only true for a spherical charge or for a cylindrical charge whose height changes in a specified manner with a change in radius. Although these conditions are sometimes approached in the field, it often happens that they are not. In many blasting cases, the hole depth is a fixed constant, and charge weight is changed by increasing the hole diameter, thus being proportional to the change in radius squared. One needs to recognize that theoretical conditions are seldom met perfectly, and the data is empirical. Neither of these scaling methods accounts perfectly for all the conditions that may be encountered. Neither accounts for the many possible variations and complexities in blast design and geometry. One charge may approach similarity to the classical case of the concentrated spherical charge; another may be a very long small-diameter cylinder; and another may be many small charges in many holes deto-

1596 UNDERGROUND MINING METHODS HANDBOOK

Fig. 2. Peak particle velocity vs. scaled range according to cube root scaling (Ambraseys and Hendron, 1968).

nating simultaneously throughout a large mass of rock. Since correction factors or judgment must be applied for many such factors, the degree of theoretical correctness of scaling laws does not solve the problem for the explosives engineer. Additional input is needed.

As was mentioned earlier in the section on elastic waves, both particle motions and attenuation rates are different for the different types of waves. When the point of observation is close to the energy source, there will be a complex combination of several different wave forms. As we move farther from the source, the wave forms become separated, arriving at different times and producing different particle motions. The more distant the point of observation is from the source, the more prominent will be the surface waves compared to the body waves. Thus, the various scaling techniques for prediction purposes are merely an effort to express the average attenuation rate of a given type of blasting in a given type of geological setting.

For particle velocities below about 0.05 m/s (2.0 ips), there is relatively little significance to differences in the prediction methods, since the predicted values them- selves are not ordinarily significant. For predictions very close to the source of energy, the methods show a more significant departure from each other. Unfortunately, this is the area in which blast-design parameters have an important effect on the vibration results.

For a prediction of body waves generated for single charges or greatly concentrated charges, cube root data would sometimes be the best choice. For a prediction of surface waves generated by a charge distributed in a

SCALED DISTANCE (5) Fig. 3. Peak particle velocity vs. scaled distance accord-

ing to square root scaling (Oriard, 1972).

number of holes, square root data would be a better choice. Both methods might give trend lines that are too conservative for complicated geometrical arrangements, e.g., if the charge per delay were to be distributed over a linear distance that is appreciable compared to the distance from the point of observation, or if there is timing scatter in the detonation of individual holes. Another example would be large coyote blasts in which a tunnel is filled with explosives and detonated. Under a number of such conditions, the trend line will often drop to a flatter slope at lower values of scaled distance. Failure to realize this might have the effect of prohibiting work that would be quite feasible and safe.

It can be an error in judgment to become overly engrossed with the best fit concept when there are questions about the basic data. For example, it is very common to find errors in the values assigned to charge weight per delay, and these values have a primary input to the data plot. In many of the complex firing patterns in use today, there can only be an estimate of the charge weight that detonates at any given instant of time because of the variations in actual firing times from the theoretical or nominal.

It is important for the person using some prediction method to realize the differences in conservatism under different field conditions. Hendron and Oriard (1972) have compared the effects on quantity-distance predictions using several different methods. Consider the long- range accumulation of data by several investigators as a basis for predicting a certain particle velocity. Fig. 4 shows a graph of charge per delay vs. range to generate a particle velocity of 0.05 m/s (2.0 ips) or threshold of damage predicted by Langefors (1963) for residential structures, and a conservative prediction used by Devine in the absence of field data. In this general case, Hen-

BLASTING

1000 I I 1 4 3

- -

- 4 I00 - - - - -

- E - w 0 -

- a - w - -

It is especially difficult to compare differences in the

Table 2. Typlcal Values of Allowable Charge Welght per Delay for Vlbratlon Llmlt of

2.0 In. per Sec*

Distance to Allowable Charge Equipment, ftt per Delay, Ibt

10 0.25 15 0.5 20 1 .o 25 1.5 50 6 75 14

w c)

2 10, -

/ - - - - - - - - - - -

- -

I I I t , I I I

dron is the most conservative at small distances and the most liberal at great distances. Langefors is the most liberal at small distances and the most conservative at great distances. The Oriard values fall between the other two. Devine's line falls outside the range of the others and represents an extreme beyond which Devine does not expect any data. Table 2 shows the Oriard prediction line presented as a table of allowable charge weight per delay vs. distance for 0.05 m/s (2.0 ips sec).

It is common for typical blasting to be monitored with a single seismograph. Examining such data, one finds a great deal of scatter in the data, and it is usually very difficult to decide what power function would best suit the data. An even more important fact to keep in mind is that the data fit often changes dramatically as a few more data points are gathered. It would not be rare to find that a horizontal line or even a line with reverse slope would best fit a given number of data points, and that a completely new line would be required after a few

100 25 1 50 56 200 100 250 156

' Oriard t Metric equivalents: A x 0.3048 = m; Ib x 0.453 592 4 = kg.

more points are added. As more data is added, it becomes increasingly evident that we are not usually inter-

10 100 1000 lo,oo~ ested in finding the best fit, but in finding the envelope RANGE 111.)

or upper bound of the data. Predictions or limits based on averages can be disastrous. It is not good enough to

Fig. 4. Relation between charge per delay and range for say that the average blast won't cause damage, if we residential vibration limits. don't want any blast to cause damage.

methods for scaling distance in the middle range. For example, take the series of points shown in Table 3. These are points which were selected to fit exactly on a line drawn for scaling to the Yi power, as in Fig. 5. Using the same data, but scaling the distance by the 1/4 power, 9'3 power, and 2/3 power of the charge weight per delay, one can plot three additional lines for the re- spective scaling factors. Fig. 5 reveals that scaling to a lower fractional power has the effect of producing a line which is concave downward, whereas scaling to a higher fractional power has the effect of producing a line which is concave upward. The curvature increases with departure from the best fit line. Also, within the data for any single line, the curvature increases as the line is extended upward or downward from middle-range values.

In these hypothetical examples, the data were selected to fit perfectly formed lines. Of course, actual field data would show a great deal of scatter. Typical field data in middle-range values would show a convino ing fit to any of these approaches. Therefore, within that

Table 3. Partlcle Veloclty Vs. Scaled Dlstance for Different Fractional Powers of Dlstance Scallng (Hypothetical Fleld Data)

Particle Charge per Velocity, ips' Distance, ft' Delay, Ibs' Dl w '14 DIW '13 Dl w '12 D/W2/~

2.0 500 625.0 100 58.5 1 .O 1000 1040.6 176 98.7 0.9 1 100 1 138.5 189 105.3 0.8 1200 1 142.6 206 114.8 0.7 1300 1140.2 224 124.4 0.6 1400 1085.1 244 136.2 0.5 1500 997.2 267 150.1 0.4 1600 855.6 296 168.5 0.3 1700 673.6 334 193.9 0.2 1800 459.2 389 233.3 0.1 1900 216.9 495 316.2

'Metric equivalents: ips 0.025 4 = mls; A x 0.3048 = m; Ib x 0.453 592 4 = kg.


SCALED DISTANCF (Fae l / P o u n d l a l

Fig. 5. Comparison of data for different scaling powers.

range, there is very little significance to the best fit concept.

The important thing is to recognize that scaling is just a means of comparing blasting data for different distances and different charge quantities. Judgment must come in recognizing what happens when we wish to ma- nipulate the data for various purposes. Suppose, for example, that a small-scale test is arranged with small charge weight and small distance in order to determine the response characteristics and assume that this test produces the limiting scaled distance. Suppose further that the final blast will be a larger charge at greater distance, but will be designed for the same scaled distance so as to generate the limiting vibration. What is the correct charge weight? A simple arithmetic manipulation will show that scaling to the ?h power gives the more conservative prediction in this instance; scaling to the % power is less conservative. Take the reverse case for comparison. Suppose that the limiting-scaled distance has been determined by a large blast at great distance, and that the final blast will take place at small distance and small charge weight. What is the limiting charge weight? In this case, scaling to the !h power gives a less conservative prediction and scaling to the % power is more conservative.

This exercise should reveal to the reader how important it is to recognize where a given test value falls in the expected range of data scatter. If the test value were unusually low, extreme conservatism should be used for predicting future events. If the test value were high, a more liberal prediction could be made safely. And, of course, it is more important to know the expected range of data than it is to use a specific scaling law.

Factors Which Affect the Vibration There are a great many variables which have an in-

fluence on vibrations generated by blasting. We will offer a few comments on those variables which most commonly have the predominant effects. These are (1) distance and geometry, (2) charge weight per in-

stant of time, (3) geological environment, and (4) confinement.

In order to evaluate these variables, we need to look at the results of extensive field experience because an acceptable theoretical approach has not yet been developed for calculating ground motions from typical blasting.

In some instances it suffices to assume maximum conservative values for all modifying factors and accept the conservatism that this introduces into predictions or data treatment. For example, we could assume that all k factors equal 1.0, and assume a high value for H. However, there are instances when such high levels of conservatism introduce unacceptable costs into a project or make it technically impossible to perform the work. Clearly, such conditions call for refinement of the approach, and one needs to look again at all significant variables.

Distance and Geometry

At first glance, there would seem to be no ambiguity in a term such as distance. However, it happens quite often that there is indeed some question as to its mean- ing. One must often try to define it as some effective distance to multiple charges controlled by the orientation of the firing sequence, or some effective distance to a charge with a high ratio of length to diameter, or some effective distance modification because the surface area covered by the blast is large compared to the distance to the point of observation, or effective distance modification because of some geometric isolation. In other words, spatial distribution can be an important consid- eration, especially when the distance is small. Modifica- tions are introduced by deviation from the concept of a point charge.

Charge Weight Per Delay

Similarly, there can be ambiguity regarding the term charge weight per delay. It would be more meaningful to define the term as charge weight detonating within a specified interval of time and within a specified distance interval. In the case of single charges whose dimensions are small compared to the distance to the point of observation, the effects are controlled by sequential detonation. Variable time periods are available for selection from the types of initiators on the market. These include electric caps in long-period series (usually % to 1 sec), short-period series (usually 20 to 250 m-sec), inter- mediate series (usually 1/4 to $5 sec), or combinations of these. An additional alternative method for elective firing is the use of a sequential timer, a detonating device which fires a series of electrical circuits with a preselected time interval between circuits. In non- electric caps, similar selections are available with the added feature that the initiators can be used in a combination of surface and down-the-hole initiation for an unlimited series. However, it is extremely important to be aware of deviations from nominal firing times.

Geological Environment The elastic properties of the medium through which

the waves pass will have a strong influence on the character of the waves. The two fundamental characteristics of the motion which are of most interest to us are the frequency and the amplitude. As a general comment, we

BLASTING

can state that a soft medium will transmit waves with lower frequency and larger amplitude, whereas a hard medium will transmit waves with a higher frequency and smaller amplitude. Thus, motions in soft saturated soils would be quite different from those in rock. Similarly, as the rock becomes harder and more brittle, the motions would show an increase toward even higher frequencies and smaller amplitudes.

Distance and charge size also have an influence on the character of the vibrations. High frequencies are at- tenuated with distance; and the smaller the size of the explosive charge, the higher will be the generated

Confinement The concept of confinement is partly a matter of geo-

logical environment and partly a matter of blast design. The greater the physical confinement of a charge, the greater will be the vibration generated by the detonation of that charge, up to the limits which the elastic properties of the confining material will allow. A typical upper limit in construction practice comes from the detonation of a presplit blast. Ideally, such a blast has a semi-infinite (great) burden, the maximum to which the material is capable of confining the charges, and as little energy as possible is dissipated in generating more than a single fracture plane through the rock. Such a blast represents the normal upper limit of vibration for a particular geological setting in typical construction or mining practice. Under some circumstances, presplitting may generate vibrations which are about twice the typical upper bound of down-hole bench blasting.

Minimum Delay Time for Vibration Control Currently, there is a very widespread concept that

short time intervals between detonations are not effective in the control of ground vibrations from blasting. It is unfortunate that this concept has become so deeply ingrained. I t has resulted in some very costly decisions on major projects.

The following advice is presented for the considera- tion of the reader: If the question is an important one for your project, cautiously test the reaction of your geological setting to the desired blasting technique. As a general guideline, shorter time intervals can be used in hard, brittle, heterogeneous rock, especially if the charges are small and in boreholes at relativelv close s~acines.

L - One may need to use longer time intervals in soft, elastic, homogeneous materials especially if the charges are large and placed in widely spaced boreholes. A case history of a large-scale blasting operation, along with further theoretical discussion, has been presented by Oriard and Emmert (1980).

Effects on Natural and Man-Made Structures Different man-made structures, underground open-

ings, slopes, and other entities will have different strengths to resist vibration damage. Furthermore, the range is quite large. Therefore, to be definitive, it is necessary to

separate these entities into appropriate groups. One common group needing definition is that encompassing nonengineered structures such as small mine offices, residences, and the like. Although their strengths vary widely, a great deal of experience has been gained in the past 40 years in the observation of residential structures, prompted by the unfavorable reaction of homeowners to

blasting vibrations. I t is now generally agreed that a typical blasting vibration in the middle to upper frequency range with a peak particle velocity less than 0.05 m/s (2.0 ips) will not be harmful to such nonengineered structures in average condition. Most such structures will not be damaged until the particle velocity of the ground motion reaches a value near or above 0.1 m/s (4.0 ips). Major damage may occur in some at about 0.2 m/s (8.0 ips), but others will still not be harmed at this level. The writer has observed cases where 50-year old frame residences were subjected to ground motions of more than 0.25 m/s (10 ips) without incurring damage.

The reader is expected to have a primary interest in underground mining, where blasting normally generates ground vibrations in the middle to upper frequency range, say 20 to 200 Hz. However, if the vibration is characterized by a very low frequency, say 1 to 5 HZ, and the structure is thought to be unusually weak, it would be prudent to reduce the foregoing values by about half, depending on conditions.

If surface mining is involved, the reader should become familiar with the latest regulations covering that activity. At the time of this writing, it appears possible that new regulations will be forthcoming.

Explosives users often limit vibrations to lower levels than necessary because of the adverse response of hu- mans. There is a trend toward giving a greater role to this aspect of blasting effects, and certain regulations and specifications are being revised to more conservative levels for this reason.

Of course, engineered structures have greater strengths than residential structures and have a similar range of strengths. Because of their great variety, it is difficult to define them by categories. However, many engineered concrete structures have been subjected to particle velocities in the range of 0.25 to 0.38 m/s (10 to 15 ips) without incurring damage, and the writer has observed several which were subjected to particle velocities in the range of 0.5 to 0.8 m / s (20 to 30 ips) and did not incur damage. Tied-down, heavily braced steel structures may withstand as much as 5 m/s (200 ips), even at low frequencies.

For rock and soil slopes, a similar scale can be for- mulated. At the low end of the scale, slope displacements have been observed at low levels of vibration but were also observed in the absence of vibrations. Usually it appears that no primary influence is found at velocities under 0.05 to 0.1 m/s (2 to 4 ips), but the time history may be changed under special circumstances. At 0.05 to 0.1 m/s (2 to 4 ips), we may expect the occasional falling of loose stones on slopes. At 0.13 to 0.38 m/s ( 5 to 15 ips), we may expect the falling of partly loosened sections of rock underground and on above-ground slopes, sections of rock that would otherwise remain in place for long periods of time. Above 0.6 m/s (25 ips), we would expect some damage to occur in the relatively unsound rock types, but we find also that damage may not occur in sound rock at much higher vibration levels. Much depends on whether the seismic waves merely pass through a section of confined rock, or whether the waves are reflected at a free boundary at the rock surface. One can observe sections of sound rock that show no visible signs of damage even after they have been subjected to particle velocities in excess of 2.5 m/s (100 ips). Thus, it is


not merely a question of variations in inherent strength but also a question of the geometric shapes in relation to the surfaces of reflection.

Blasting Effects on Concrete As in the case of rock, concrete can be damaged by

either excessive vibration or by rupture. It has been this writer's experience to see more damage from rupture than from excessive vibration. Establishing realistic limits for either of these blasting effects is made more complex by the fact that the ability of concrete to resist damage is not merely a function of the strength of the concrete fabric; and the fabric strength itself varies with time for freshly poured concrete. Most of our knowl- edge about the ability of concrete to resist vibration comes from field experience, although there has been some limited research conducted by such agencies as the Corps of Engineers and the Portland Cement Associa- tion. Such research results were encouraging in that no damage occurred to concrete specimens of varying ages of curing which were subjected to moderate levels of vibration. However, these test conditions were not severe enough to be applicable to the field conditions that are often encountered in mining and construction. Ex- perience demonstrates that most engineered concrete structures are able to withstand particle velocities in the range of 0.5 to 0.8 m/s (20 to 30 ips) at medium frequencies, say in the range of 20 to 100 Hz, and can withstand much higher particle velocities in the range of several thousand hertz.

In a recent case, this writer conducted experimental test blasting near freshly poured concrete at a construction site. The concrete was composed of a low- strength mix and formed in cubes 0.9 m (3 ft) on each side. These cubes were not subject to the type of deflection one might expect for structural sections. They were intended to represent mass or fill concrete. When they had attained a compressive strength of 2.8 MPa (400 psi), they were subjected to particle velocities of about 1.8 m/s (70 ips). When they had attained a compressive strength of about 8.3 MPa (1200 psi), they were subjected to more than 2.5 m/s (100 ips). This final blast destroyed the surrounding rock and threw the concrete into the muck pile. However, it was not damaged by the vibrations it had received (Oriard, 1980; Oriard and Coulson, 1980).

Even without field experience, there is a justification for recommending values in the range of 0.25 to 0.38 m/s (10 to 15 ips) for the case of tensile slabbing. This

is based on the assumption that the pressure in a medium generated by a traveling stress wave can be represented by the product of the acoustical impedance and the particle velocity. Assuming a tensile strength of concrete of about 4 io that in compression, one might expect tensile slabbing at about 0.25 m/s (10 ips). This figure can be increased due to the fact that the dynamic strength of concrete is higher than its static strength. The exact value is a function of the time history of the incoming stress pulse. The faster the rise time of the pulse, the greater will be the strength of the concrete in resisting it. It is this writer's opinion that it is important to recognize what order of magnitude of strain can be expected and to take this into account when selecting limits. An example of the values of particle velocity that would generally be acceptable for engineered concrete structures is illustrated in Tables 4-6. The values are those which have been recommended by this writer for use in the blasting of rock near existing concrete sections of various ages. Oriard and Coulson (1980) give additional examples of criteria developed for the Tennessee Valley Authority to provide protection to concrete.

The Distance Factor shown in the tables was a means used to reduce the allowable particle velocity with increasing distance because of the attenuation of fre- quenky with distance. Experience demonstrates that higher particle velocity can be allowed at higher frequencies. At lower frequencies, greater deflections (hence greater strains) will be induced in those structures which are capable of responding in that fashion. (There is a special concern for structural walls of freshly poured concrete). This is not entirely a question of resonance. Rigid systems at resonance are usually more durable.

The limits are varied according to the age of the concrete. In the period of 0 to 4 hr, the concrete has not started to set and it can still tolerate vibration. (Of course, structural concrete forms can be damaged by severe shaking at this time). From 4 to 24 hr, the concrete has begun to set, but has very little strength. After 7 days, the concrete has a strength that is approximately 3$ of the ultimate (28-day) strength.

Although these recommended limits are far more liberal than some which have appeared in project specifications in the past, experience has shown that they are acceptable for many engineered concrete structures, and will often save very large sums of money by allowing the construction work to proceed a t a much higher rate.

The reader should not apply these criteria indiscrimi-

Table 4. An lllustratlon of Particle Veloclty and Distance Crlterla for Blasting Near Concrete (Case History)'

Nonstructural Fill Structural Concrete Walls, Unspecified Electrical Time From Batching and Mass Concrete Structural Slabs, etc. Equipment, ips

0- 4 hr 4 ips x D F t 2 ips x DFt 2

4-24 hr 1 ips x DF 1/4 ips x DF 2

1- 3 days 1.5 ips x DF 1 ips x DF 2

3- 7 days 3 ips x DF 2 ips x DF 2 7-10 days 8 ips x DF 5 ips x DF 2 over 10 days 15 ips x DF 10 ips x DF 2

'Intended as an illustration of a case history, not a general recommendation for all cases. (No allowance for form bracing, which would provide additional strength.) tDistance factor: 0-50 ft*, multiply x 1.0; -150 ft, multiply x 0.8; 150-250 ft, multiply x 0.7; and over 250 ft, multiply x 0.6. *Metric equivalent: ips x 0.0254 = mls.

Table 5. An lllustratlon of Allowable Charge Welght per Delay tor Nonstructural Flll and Mass Concrete'

Age of Concrete (From Batching Time) Distance to

Concrete, ftt 0 to 4 hr 4 to 24 hr 1 to 3 Days 3 to 7 Days 7 to 10 Days Over 10 Days, Ibt

10 0.6 0.1 0.1 7 0.4 1.4 3 15 1.3 0.2 0.4 0.9 3 7 20 2.4 0.4 0.7 1.7 5 12 25 3.7 0.6 1 2.6 8 19 50 11 2.6 3 8 28 63 75 26 4 7 18 63 143

100 47 8 13 33 113 255 150 93 16 27 65 223 500 200 165 29 48 116 396 500 250 221 39 65 156 500 500

'Intended as an illustration of a case history, not a general recommendation for all cases. (No allowance for form bracing, which would provide additional strength.) tMetric equivalents: f l x 0.3048 = m; Ib x 0.453 592 4 = kg.

nately to all concrete structures. They are presented as an illustration of the fact that past criteria have often been unnecessarily conservative. This liberalization should not be attempted without an understanding of the time-history of the input motion and time-history of the response. This understanding is critical to the suc- cess of establishing criteria. Its importance cannot be overemphasized.

When assessing the effects of blasting on freshly poured concrete, one should also consider the strength of the forms which will hold the concrete in the early stages of its curing. Before the concrete has attained a strength of its own, and especially when it is still in a semiliquid state, the forms must be capable of resisting any shaking that might be generated.

If forms are es~eciallv strong and well-braced. it is

cause the rock supporting the concrete has been shifted or ruptured, rather than the concrete alone.

There is really no acceptable basis for making a calculation to control this type of damage because of the many variations that can exist in the rock and in the blast designs. Someone in the field must examine the rock conditions and estimate the manner in which the rock could be broken by the blasts being considered, then form a judgment as to whether or not it is safe to proceed. The writer has developed criteria to prevent rupture damage under various rock and soil conditions (Oriard, 1973 and 1980). In many cases the field conditions justify the use of more liberal criteria than those cited here. A judgment must be made on the basis of the vibration characteristics, the structural response characteristics, and the possibilities of block motion or -

possible to allow even higher levels of vibration than ground rupture. those shown in the tables. The writer is involved in Mechanical and Electrical other projects where substantially higher particle velocities were allowed, but only under special circumstances. There is such a wide range in response characteris-

rn addition to tensile slabbing or structural deflec- tics of mechanical and electrical equipment that it is tion, concrete may also be damaged through dislocation, Very difficult, and probably misleading, to give specific i.e.. a ~ermanent displacement. This mav come about recommendations for vibration limits. Even within a

, . through vibration, rupture, or venting of explosive gases. narrow category, such as electrical switches or relays, More commonly, this type of damage comes about be- the sensitivity to vibration will often vary by at least one

Table 6. An lllustratlon of Allowable Charge Welght per Delay tor Structural Concrete Walls, Structural Slabs, etc.*

Age of Concrete (From Batching Time) Distance to

Concrete, ftt 0 to 4 hr 4 to 24 hr 1 to 3 Days 3 to 7 Days 7 to 10 Days Over 10 Days, Ibt

10 0.25 - 0.1 0.25 0.7 1.9 15 0.5 - 0.2 0.5 1.7 4 20 1 .O - 0.4 1 .O 3.1 7 25 1.5 0.1 0.6 1.5 4.8 11 50 6 0.4 2.6 6 15 37 75 11 0.8 4 11 35

100 20 1.5 8 20 62 84

150 150 39 3 16 39 123 295 200 70 5 29 70 21 9 500 250 93 7 39 93 293 500

'Intended as an illustration of a case history, not a general recommendation for all cases. (No allowance for form bracing, which would provide additional strength.) tMetric equivalents: ft x 0.3048 = m; Ib x 0.453 592 4 = kg.


order of magnitude. Some switches can be tripped at particle velocities of as low as 0.025 m/s (1 ips) or less. Others might not be affected at 0.5 m/s (20 ips). Just as with structures, many electrical and mechanical devices will have sensitivities that are frequency-dependent, i.e., their sensitivities are greater in certain frequency ranges.

Similar to electrical switches, mechanical devices and controls have a wide range of sensitivities. Some are so durable that the structure containing them may collapse before damage is incurred. Others are highly sensitive.

Probably the first step in the judgment process is to determine what will be the consequence if a switch is tripped or if some other device is activated, affected, or broken by vibration. If the results are inconsequential, the question is academic, and there may be no need to determine a limit. If the results induced by the vibration cannot be tolerated, two alternatives may be considered. One is, of course, to keep vibration levels below the critical level. Another approach is often more attractive, but often overlooked, i.e., either to isolate the device from vibration or to lock out its action, e.g., tying down a relay or bypassing electrically an automatic switch.

Except for such items as sensitive switches, most electronic and electrical equipment will not be physically damaged by particle velocities of the order of 0.05 m/s (2.0 ips), and most mechanical equipment is far less sensitive. The frequently quoted limit of 0.05 m/s (2.0 ips) is, therefore, appropriate to limit physical damage to most of this equipment, but often inappropriate to prevent activation of automatic devices.

AIR WAVES

Air Waves Above Ground It is expected that the reader will be interested pri-

marily in underground work. For this reason, only limited comment will be made about above-ground effects. Occasionally it may happen that air waves from shaft or tunnel blasting could be of concern to above-ground structures or people, especially if the work is taking place in an urban setting, or in proximity to mine offices or processing facilities.

An overpressure of the order of 7 kPa (1.0 psi) can be expected to cause widespread window damage. It may also cause minor architectural damage to unusually weak structures. Overpressures of the order of 0.7 kPa (0.1 psi) are not expected to cause damage except under unusual circumstances. An example of design criteria for window glass subjected to sonic boom is shown in Fig. 6 (Pittsburgh Plate Glass Industries, 1969). A factor of safety of 2.5 is included in the recommendations. The reader is advised to review applicable regulations, laws, and ordinances pertinent to his location and type of operation. For example, at the time of this writing, the Office of Surface Mining requires compliance to an airblast overpressure limit of 128 dB (0.0073 psi, 0.05 kPa) for residences.

There are several factors which are important in the control of air waves. Atmospheric conditions are very important in the above-ground transmission of air overpressures. In an area of concern, it is desirable to avoid blasting during times when refraction, reflection, and fo-

Fig. 6. Design criteria for window glass subjected to sonic boom.

cusing of air waves could build up air overpressures to potentially harmful or annoying. values. If air waves are a problem, isotherms, temperature inversions, wind shears, and unfavorable wind directions should be avoided. If human response is a matter of interest, it is desirable to avoid nighttime and other quiet hours, and to avoid weekend and holiday blasting. The choice of products can influence the results also. Down-hole electric detonators will generate lower overpressures than surface connectors and detonating fuse. The use of stemming can be used to reduce overpressures. In many instances, there is a feasibility to using some sort of barrier or isolation device, such as a shaft cover.

Air Waves Underground The behavior of blast waves in air and in under-

ground chambers and openings is far more complicated than the behavior of ground vibrations. Through complex reflection and focusing, air overpressures may be maintained at high values in an underground environment. To illustrate overpressures developed in a typical room-and-pillar mining operation, Olson and Fletcher (1971) recorded air waves from three mine production blasts initiated with conventional ?h -sec delay detonators. For that particular location and blasting system, the overpressures could be expressed by the equation:

P = 4.9 X 10"Dl w'3)-2.15 where P is overpressure in pounds per square inch, D is distance from the blast in feet, and W is zero-delay charge weight in pounds.

This writer has also found it useful to follow the concept of overpressures developed in vented chambers according to the volume of the chamber. Measurements at the NORAD facilities (Smart, 1971) showed good agreement with tests in partially closed chambers (Weibull, 1968). See Fig. 7.

BLASTING 1603

CHARGE -VOLUME RATIO. KG / ~3

Fig. 7. NORAD overpressure measurements in comparison with vented chamber tests.

REFERENCES AND BIBLIOGRAPHY

Ambraseys, N.R., and Hendron, A.J., Jr., 1968, "Dynamic Behavior of Rock Masses," Rock Mechanics in Engineer- ing Practice, K.G. Stagg and O.C. Zienkiewicz, eds., John Wiley and Sons, pp. 203-227.

Barber, D.B., Fourney, W.L., and Dally, J.W., 1979, "Frac- ture Control in Tunnel Blasting," University of Maryland. Aug.

Barber, D.B., Fourney, W.L., and Dally, J.W., 1978, "Blast- ing Parallel Hole Cuts with Fracture Plane Control," Tunnels and Tunneling, May.

Devine, J.F., et al., 1966, "Effect of Charge Weight on Vibration Levels from Quarry Blasting," RI 6774, US Bureau of Mines.

Duvall, W.I., et al., 1963, "Vibrations from Instantaneous and Millisecond-Delayed Quarry Blasts," RI 6151, US Bureau of Mines.

Hendron, A.J., and Oriard, L.L., 1972, "Specifications for Controlled Blasting in Civil Engineering Projects," Pro- ceedings, First North American Rapid Excavation and Tunneling Conference, Vol. 2, AIME, New York.

Langefors, U., and Kihlstrom, B., 1963, The Modern Tech- nique o f Rock Blasting, Almqvist and Wiksell, Stockholm, Sweden.

Nicholls, H.R., Johnson, C.F., and Duvall, W.I., 1971, "Blasting Vibrations and Their Effects on Structures," Bulletin 656, US Bureau of Mines.

Olson, J.J., and Fletcher, L.R., 1971, "Airblast Overpressure Levels from Confined Underground Production Blasts," RI 7574, US Bureau of Mines.

Oriard, L.L., 1972, "Blasting Operations in the Urban En- vironment," Bulletin, Association of Engineering Geolo- gists, Vol. 9, No. 1, Oct.

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chapter 2. blasting effects and their control

Documents

surrounding rock

rock surfaces

softer rock

hard rock

average rock

rock breakage mechanism

causes rock breakage

remaining rock mass