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Rock Fragmentation by Blasting – Sanchidrián (ed)© 2010 Taylor & Francis Group, London, ISBN 978-0-415-48296-7

Intra-hole and inter-hole effects in typical blast designs and their implications on explosives energy release and detonator delay time—A critical review

B. MohantyLassonde Institute and Department of Civil Engineering, University of Toronto, Toronto, Canada

ABSTRACT: Recent advances in the theoretical treatment of the detonation process under both ideal and non-ideal conditions are noteworthy, but they are still based on somewhat hypothetical situations. The actual variables that are essential parts of normal blasting practice have not yet been taken into account in such treatments. These include the various initiation practices employed to detonate a column of explosive, from single-point initiation to multi-point initiation in a boreholes, and the effect on detonation characteristics of both detonators and explosives under multi-deck and multi-hole blasting conditions. This review describes the most common methods of initiation of explosive column in a borehole and their implication in modifying the in-hole detonation parameters such as energy and the partitioning between shock and gas energy fractions. The phenomenon of sympathetic pressures between explosive decks and on detonator firing times, and on the sensitivity of the receptor explosive is also examined. These issues are highlighted with results from carefully controlled laboratory experiments and similar studies in quarry and underground blasting operations. On the basis of the data presented, it is concluded that despite recent progress, realistic description of the behaviour of the explosive system in a blast, and accurate prediction blast results continue to present a formidable challenge.

1 INTRODUCTION

Considerable progress has been achieved in recent years in the whole field of blasting science and technology. This applies to explosives formula-tion (e.g. emulsion and blends), initiation systems (e.g. electronic detonators), and blast simulation through advanced numerical codes. The commer-cial explosives in the market today are not only generally cheaper and safer than before, but also more flexible in their applications, and are also geared for easy transport and delivery into the boreholes in very large volumes, which the current mining practice demands. In parallel with these developments, the initiators (e.g. electric and shock tube detonators) have become increasingly precise, partly driven by the advent of highly accurate and programmable electronic detonators. Significant progress has also been made in our understand-ing of the non-ideal detonation process, which is characteristic of most commercial explosives (Braithwaite et al. 2006, Sanchidrián & López 2006, Nyberg et al. 2003).

A parallel development has involved blast simulation exercises by combining realistic detonat-ion properties with corresponding properties of the target rock to make prediction of blast results, such as throw, degree of fragmentation, fracture-plane

control blasts, and cast blasts. These efforts have been aided considerably by the availability of a wide range of readily available diagnostic equipment in the market for studying detonation behaviour of the explosive, mapping of surface and underground geometry, and quantitative assessment of frag-mentation. However, a system approach by which all the components of the blasting system could be combined to yield an accurate mine-to-mill cost estimate for a given tonnage of broken rock has met only with very limited success. This is partly due to the complexity of the system, and especially to our inability to describe the target rock to the same degree of confidence as the explosives sys-tem. Also, in our modeling effort, in order to limit the degree of complexity, it is normally assumed that the effect of multi-hole blasts is simply the sum of the contributions of individual blast holes. This not only ignores the great variety of ways that an explosive column in the borehole are initiated in actual practice, but also the effect they may have in real-time on explosive energy release. The same applies to the interaction between holes and their effect on detonation behaviour of the explosive columns and the firing time of the detonators. These sympathetic phenomena and their effect on explosive system performance are the subject of this study.

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2 BLAST DIAGNOSTICS

The blast vibration record obtained close to a blast serves as a powerful tool for the researchers in investigating blasting performance of an explo-sive system (e.g. combination of explosive, detona-tors, initiation method, blast geometry, and target rock). Figure 1 shows a vibration record (particle velocity) obtained very close to an underground multi-hole blast. This record is typical in the sense, it exhibits the normal scatter (indicated by the arrows and their direction in the figure) of the individual hole firings, and more importantly, the greatly variable amplitudes corresponding to the individual blast holes, despite similar explosive charge weights in each hole. In fact, each compo-nent of the explosive system in isolation is much more precise in terms of, for example, detonation behaviour of the explosive and the firing time of the detonator than would be inferred from exami-nation of this record. Some of these deviations may stem from the varying geometry of the blast in real time, such as hole deviation leading to change in effective burden and confinement. However, as will be shown in the following sections, both the scatter in explosive energy release and delay time can stem from the initiation practice employed and the sympathetic phenomena prevalent for both intra- and inter-hole conditions.

3 INITIATION PRACTICE AND ITS EFFECT ON ENERGY

Although energy calculations are based on single-point initiation and steady detonation of the explo-sive column in the borehole, the former is rarely employed in actual practice. A common practice is to employ two boosters located at two different locations in the borehole, which may be timed the same or with different delays. The explosive col-umn may also be traced with detonating cords of specific strengths and connected to these boost-ers. Figure 2 denotes the four primary modes of initiation of an explosive column, although there are many additional variations along these lines. Mode 1 denotes a single initiation point (either by a detonator or a booster) with a non-energetic downline (i.e. electric wire or shock tube); Mode 2 denotes an energetic downline (i.e. a detonating cord) connected to a booster; Mode 3 combines an energetic or a non-energetic downline with a booster and an energetic upline; whereas, Mode 4 denotes an energetic downline only. Except for Mode 4, which is specifically employed to reduce the severity of the explosion process and designed for wall-control blasts, the rest are used inter-changeably with scant regard to their effect on the explosive energy release. Their possible effect on the latter is considered of no consequence,

Figure 1. Vibration record (particle velocity) from an underground production blast (vertical lines denote designed firing times; arrows indicate deviations in firing times for selected blast holes).

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although the PETN core loads in the detonating cords can vary from 1 g/m to over 10 g/m.

The most effective means of studying the effect of the various initiation modes on explosive energy release is through the Underwater Test. This is a well established and highly reproducible diagnos-tic test. With proper controls and instrumenta-tion, the test can yield accurate information on the total energy release as well as energy partition-ing between shock and gas phases (Bjarnholt & Holmberg 1976, Roth 1983, Takahashi et al. 1998, Sanchidrián 1998, Mohanty 1999, Hagfors 2009). The latter are expressed in terms of shock energy and bubble energy respectively; the latter being considered analogous to the explosion gas energy in blasting. Although these test results cannot be directly translated yet to what would be expected in the actual borehole, or how the energy partition-ing will affect the final blast results, the test rep-resents a very powerful tool in studying explosive performance on a comparative basis under a range of relevant variables, including initiation modes (Mohanty & Joyce 1994).

The role of detonating cord along the explosive column and coupled to the booster at the bottom of the borehole is illustrated schematically in Figure 3. It is obvious that the stronger the detonating cord, the more likely that the explosive column will be initiated sideways instead of by the booster at the toe of the hole (Bhushan et al. 1986). The reverse will be true for a very week cord. The degree of reaction will also depend on the time elapsed between the initiation of the cord and booster, which will normally be initiated by a delay deto-nator. In this case, two reactions would be taking place either in sequence (Mode 2), or in parallel (as in Mode 3). The reaction in the explosive column due to the upline detonating cord will be ahead

of that due to the booster, as the assumed VOD for the explosive is 5 km/s against that of the cord (i.e. 6 km/s or more). Even on a conservative esti-mate of the velocity of shock wave in the explosive matrix, there would be more than adequate time for the sideways reaction to propagate fully across the diameter of the explosive column.

The effect of varying initiation modes on both shock and bubble energy release in a small diam-eter (40 mm) microballoon sensitized emulsion explosive under steel confinement is shown in Table 1. This represents Mode 2 type of initiation with an energetic downline connected to a detona-tor in this case. Furthermore, the initiation time of detonator is controlled with respect to that of the detonating cord (1 g/m PETN core load). The loss of energy compared to detonator initiation alone in both shock and bubble is clearly evident.

Figure 2. Four common methods of initiating an explo-sive column.

Table 1. Underwater test results for a 40 mm × 400 mm emulsion explosive under steel confinement for Mode 2 initiation for varying delay times between initiation of cord and firing of detonator; density: 1.20 g/cm3; VOD: 5260 m/s.

Type of initiation(Mode 2)

Bubbleenergy*

Shockenergy*

MJ/kg MJ/kg

Detonator only 1.19 0.53

“ +1 g/m cord (0 ms delay) 0.81 0.32

“ +1 g/m cord (25 ms delay) 0.76 0.29

“ +1 g/m cord (500 ms delay) 0.34 0.13

*Results are average of 5 tests minimum.Notes: Detonating cord taped along side of cartridge; measured shock energy at 2 m from charge centre with-out incorporating any ‘shock energy loss factor’.

Figure 3. Effect of initiation mode on detonation reac-tion in a cartridged explosive (VOD: 5000 m/s, diameter: 50 mm, length: 5 m, reaction time frame: radial (from detonating cord) ∼25 μs, axial (from booster) ∼1 ms); light gray indicates unreacted explosive, and dark gray indicates fully reacted explosive).

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Furthermore, as the time delay between cord and detonator is increased, the loss in both shock and bubble, and therefore the total energy, becomes progressively larger. There is a 75% loss in shock energy and 70% loss in bubble for the highest delay employed (i.e. 500 ms); even for zero delay, the loss is still 40% for shock energy and 30% for bub-ble energy under this Mode 2 type of initiation. The relatively larger loss in shock energy and the general deterioration of the overall energy yield in this case is in perfect accord with that illustrated in Figure 3.

Similar results on the critical role of initiation mode on energy release are shown in Figure 4 for crushed ANFO under steel confinement for 50 mm and 100 mm diameter charges. In this case, the effect of initiation mode on energy partitioning between shock and bubble energy is most noticeable. In the smaller 50 mm diameter charges, the high-strength detonating cord (10 g/m or 50 grains/ft) by itself (i.e. Mode 4 type) and placed inside the explosive charge greatly reduces the shock energy in compar-ison with booster-only initiation (i.e. Mode 1) or booster plus cord initiation (Mode 2). In contrast, all three modes yield the same bubble energy. As expected, at the larger 100 mm diameter, both bub-ble and shock energy yields increase significantly for all three modes of initiation, with the most dramatic increase seen for the shock energy release under Mode 4, although still lower than the other two modes.

For a relatively sensitive explosive such as crushed ANFO, the results show that a 10 g/m cord by itself will initiate the explosive column across the bore-hole diameter, but will not have sufficient run-up distance to attain full VOD, and hence the greatly reduced shock energy. However, as the borehole diameter increases, the gap in the respective shock

energies will be narrower. At even larger diameters and higher strength detonating cords, there should be no difference among the three modes, although the nature of the shock front in the surrounding rock mass will be dramatically different for the two modes. That would of course alter the result-ing fracture pattern and its intensity. For relatively insensitive explosives, it is expected that the per-formance in terms of energy release will be further degraded, as the explosive will never get to full reaction across the borehole diameter. In the worst case scenario, the explosive especially in long holes will deflagrate along its upper section before the detonation front due to booster action at toe can reach this region.

It is extremely difficult to use large diameter steel confined tests in the underwater set-up. However, the smaller diameter tests described in this study do point out the critical role played by the initiation mode. For booster sensitive explosive products, the borehole diameter plays a crucial role, as it deter-mines the extent of reaction along the radial direc-tion, and linked to the strength of the detonating cord employed. The net result from such initia-tion practice could be full detonation across the borehole diameter, or partial to total failure of the explosive column its upper reaches.

4 ROLE OF SYMPATHETIC PRESSURE ON THE EXPLOSIVES SYSTEM

The two phenomena of interest in this context are the possible interaction between adjacent blast holes, as well as between two decks in the same hole. This would apply to both explosives and det-onators. Extensive experimental studies have been carried out in the recent past by several research-ers to study this aspect of the explosives system (Mohanty & Deshaies 1989, Wieland 1990, Sumiya et al. 2001, Mohanty 2007). It has been shown that one of the major causes of blast malfunction is due to pressure desensitization of the receptor explo-sive due to shock or explosion gas pressure from an adjacent hole. It has been further shown that there is a clear difference between a gassed emul-sion explosive and micro-ballooned one in the way these explosives respond to incident shock pres-sures (Nie 1997).

Detailed comparison between small diameter gassed and micro-ballooned explosive products has also been carried out by Mohanty & Deshaies (1992). Table 2 shows the sympathetic distances in water for a standard 220 g Pentolite primer for these products, with and without a detonator. All the detonators in each test were initiated at the same instant; the donor Pentolite always with a ‘0’ period LP detonator, whereas, the receptor

Figure 4. Comparison of Bubble and Shock energy release under steel confined for crushed ANFO in two diameters and three modes of initiation: booster only—Mode 1; booster + det. cord (10 g/m or 50 gr/ft)—Mode 2, and det. cord (10 g/m or 50 gr/ft)—Mode 4.

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detonators were LP #10, with a nominal firing time of 4 s. For all tests, the receptor explosive fired at its designated time, whereas, inside the sympathetic zone, these products detonated instantaneously with the donor, plus the time it took for the shock to arrive at the target (e.g. for a separation distance of 2 m, the transit time would be 1.3 ms).

These results show that the sympathetic dis-tance for detonation is larger than either of the explosive components. This implies that the explosive matrix, depending on the type of void it incorporates, greatly amplifies the ‘transmitted’ pressure within the explosive matrix, air bubble being a much better amplifier of shock than the glass micro-balloons. Thus, the detonator requires incident pressures in the range of 75 MPa to ini-tiate sympathetically, but only 14 MPa on the outside of the cartridge, when it is embedded in the gassed slurry matrix. For the micro-ballooned emulsion, the corresponding incident pressure for sympathetic initiation is 40 MPa, still only about half that required to initiate the detonator on its own in water. It should be noted here that the cal-culated pressures shown in Table 2 have been veri-fied through actual measurements up to a distance of 50 cm in water from the Pentolite donor. This shock amplification effect is further confirmed by examining the nature of the transmitted shock in the same micro-ballooned emulsion explosive matrix, which is shown in Figure 5. In this case, the shock inside the cartridge was measured at various distance form the Pentolite donor. The shape of the transmitted pulse is very different from that of the standard incident shock in water because of the interaction with (and possible breakage of ) the micro-balloons. The internal shock pressure rises monotonically as the distance is reduced, but the shape undergoes a dramatic change. However, when the donor-receptor distance is reduced to 0.8 m from 1.0 m, there is an extremely rapid

rise in the shock amplitude, which now measures 90.5 MPa compared to only 15 MPa at 1 m dis-tance. This value of course exceeds the threshold pressure of 75 MPa for initiation of the detonator. This would result in sympathetic detonation of the receptor explosive cartridge.

Although the magnitude of the incident pressure from an adjacent borehole in actual blasting in rock would be different than in water, the same

Table 2. Distance for sympathetic detonation in water for a typical pyrotechnic detonator, a gassed slurry, and emulsion explosive with micro-balloons, with a 220 g Pentolite primer serving as the donor. Both are plastic film wrapped car-tridge (50 mm × 400 mm) explosive, with a density of 1.20 g/cm3.

Explosive product (Receptor)

Sympatheticdistancecm

Calculated incident shock pressureMPa

LP Detonator (#10) 46 75

Water-gel slurry cartridge (gassed, without detonator) 13 310

Water-gel slurry cartridge (gassed, with same detonator) 200 14

Emulsion cartridge (micro-balloon, without detonator) 6 742

Emulsion cartridge (micro-balloon, with same detonator) 80 40

Notes: Detonator placed centrally in the cartridge, and activated at same instant as the Pentolite donor.Cartridge oriented ‘broadside’ to Pentolite donor.Results are average of 5 tests minimum for each configuration.

15.2 MPa

Pmax: 90.5 MPa

Distance

0.8 m

7.3 MPa

1 m

2 m

4 m 3.2 MPa

Time (ms)

Pre

ssu

re (

7.5

MP

a/d

iv)

Figure 5. Shock pressure inside a micro-ballooned emulsion explosive cartridge (50 mm × 400 mm) from detonation of a 220 g Pentolite donor at various distances in water (shock pressures measured with Tourmaline pressure transducers).

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phenomenon would prevail, with void-sensitized products, especially gassed ones, being more sus-ceptible to sympathetic initiation or deflagration than others.

5 SYMPATHETIC PRESSURE AND DETONATOR DELAY TRAIN

The effect of sympathetic pressures on the delay firing characteristics of a detonator would also be of similar concern, as the burning of the propellant in the delay elements would be pressure-dependent. The effect of incident underwater shock on the delay time of a short period shock tube detona-tor with a nominal firing time of 500 ms is shown in Figure 6. Without a pre-shock from a Pentolite donor in water, the detonator fires near its nominal firing time. (i.e. 510 ms vs. 500 ms nominal). How-ever, as the incident shock is applied at various stage of burning of the delay element (i.e. ‘cooking time’), the firing time of the detonator increases significantly. As expected, the worst increase from nominal occurs when the incident shock is applied very early in the burning of the delay element. When the shock is applied towards the end of the cooking time, there is negligible difference between actual firing time and the nominal. In addition, this increase in firing time is observed to be a function of the amplitude of the incident shock, irrespective of the cooking time. For a cooking time of 25 ms, and a 65 MPa (9.4 kpsi) incident shock, the firing time is observed to be 610 ms compared to 550 ms for a 30 MPa (4.3 kpsi) shock, both being 19% and 8% higher than nominal firing time respectively.

The exact magnitude of increase will depend on the detailed design of the detonator housing as well as the delay train, but in relative terms all designs

will be affected by pre-shock. Under multi-hole blasting conditions, the detonators in successive holes will be ‘shocked’ a number of times during their ‘cooking phase’ with varying amplitudes, before they actually detonate. Thus, at least some of the observed scatter in firing times of detona-tors in actual blasting operation may be attributed to the sympathetic pressure phenomenon.

6 PRESSURE EFFECTS IN DECK BLASTS

It is common practice to introduce decking of explosive charges in a borehole on cost considera-tions, but more often to reduce vibration levels. But the practice of introducing deck charges in a blast and the requirement for employing inter-deck stemming are taken for granted, and their possi-ble role in any deck malfunction is seldom tested. Lee et al. (2000) have conducted a full-scale instru-mented quarry blast trial to investigate the inter-deck blasting problems. The trial was carried out in a dolomitic limestone quarry, and the explosive use was a doped emulsion (60/40) with two decks per hole. The crushed stone inter-deck stemming was 1.2 m. All the holes were instrumented with VOD probes. The results showed that the top deck either deflagrated or malfunctioned in 10 of the 15 holes blasted. These malfunctions were attributed to sympathetic pressure from the bottom explosive deck acting on the top decks.

The current study summarizes some of the findings from an extensive series of full-scale pro-duction blasts conducted to determine the role of stemming in both double-deck and triple-deck blasts. Typical blast designs in relatively hard lime-stone quarry are shown in Figures 7 and 8. Two types of explosives were used: a doped emulsion (70/30) and a TNT slurry explosive. These single-row production blasts employed 165 mm diameter blastholes, 15 m high bench, and 5 m spacing. The full density (1.45 g/cm3) slurry was in 125 mm car-tridge form, which was simply airmailed from top of the bench, whereas the gassed doped emulsion (density: 1.25 g/cm3) was pumped into the holes. All the explosive decks were probed to measure VOD in each, as well as pseudo-VOD in the stemming sections. Figure 7 shows the results from a 9-hole production blast with doped emulsion. All holes were initiated with the same down-hole delay; the delay between decks and holes were provided by appropriate surface delays, so as to keep the delay interval fixed at 42 ms.

The results from an 8-hole single-row blast with doped emulsion are shown in Figure 7. The numerals against borehole denote length (in ft), nominal firing time (within rectangle), actual firing time (within oval), and the measured VOD

Figure 6. Variation of firing times of a 500 ms pyro-technic delay detonator (shock tube) as a function of pre-compression and ‘cooking time’.

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Figure 7. Evidence of malfunctioning doped emulsion explosive (70/30) decks in a limestone quarry blast. Borehole diameter: 165 mm; bench height: 15 m; crushed stone decks (hatched figures); numerals against explosive decks denote nominal firing time (bottom rectangle) or length (in ft); oval symbol denotes observed firing times, and top number against each explosive deck denotes measured VOD (typically ∼5200 m/s). Note: a minimum length of 3.3 m of stem-ming to prevent sympathetic effect between decks.

Figure 8. Evidence of malfunctioning slurry explosive (TNT) decks in a limestone quarry blast (specifications identi-cal to those in Figure 7). Hole #1 loaded with doped emulsion; ‘no VOD’ against deck denotes defective probe, and not necessarily malfunction of explosive deck. Note also the minimum inter-deck stemming length of 2.4 m (8') to prevent sympathetic pressure problems with the next deck.

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(in km/s). The average firing time is slightly lower than the nominal time, but well within the expected scatter at the beginning of the blast, but the delay times becomes significantly larger as one proceeds from hole #1 to hole #9. The VOD in the doped emulsion averaged ∼5250 m/s. It can be seen that a minimum stemming length required would be 3.3 m in a 165 mm diameter borehole. Explosive deck malfunction results, as evidenced by low VOD, when the stemming length is reduced below this value (see hole nos. 1 and 5, where the VOD in the middle deck has been reduced to 1500 m/s and 1530 m/s respectively). This can be contrasted with holes #6, #8, and #9, where with a minimum stem-ming length of 3.1 m, all the decks fired at their specified velocity. On the other hand, a low VOD or deflagration in the middle deck helps restore detonability of the upper deck (both upper decks in holes #1 and #5 and detonated at ∼5300 m/s).

Figure 8 shows the corresponding situation for a full density TNT slurry blast (except hole #1, which was loaded with doped emulsion, and hence the higher VOD) again with three decks per hole. The average VOD for the slurry explosive in this case is found to be ∼4400 m/s, in agreement with specification. However, the delay time scatter in this case is much higher than with the blast in shown in Figure 7. Also, the progressive increase in firing time with propagation of the blast that was observed with the emulsion explosive blast (Figure 7) is not observed here. This could be due to the difference in transmitted pressure in the explosive matrix between a void- sensitized prod-uct and a non-gassed product such as TNT slurry. However, the major difference lies in the minimum stemming length required to prevent malfunction of the upper explosive decks—for the emulsion explosive this would be 3.3 m, whereas for the non-gassed TNT slurry, a minimum stemming length of 2.4 m would be required for the same 162 mm diameter borehole. This difference is in conformity with the earlier discussion on the increased suscep-tibility of gassed product to pressure amplification within the matrix and its effect on detonation char-acteristics of both detonator and the explosive.

7 CONCLUSIONS

Controlled experiments in both laboratory and full-scale production blasts have shown the critical role played by voids incorporated in the explosive matrix. This can lead to either deflagration or sym-pathetic initiation of explosive columns, the latter being related to shock pressure amplification inside gassed explosives leading to sympathetic initiation of the detonator. Blasting malfunctions, such as altered energy release, out of sequence firing, and

instantaneous detonations are more common than has been assumed until now. Its cause can be directly traced to the effect of transmitted pressure in both intra-hole and inter-hole situations. The widely accepted practice of employing deck blast design to reduce blasting vibrations, but without taking into account the minimum inter-deck stemming length required to prevent deck malfunctions requires re-examination. This study has shown that there is a significant difference in terms of the required stem-ming length between a gassed explosive product and a non-gassed one; for a gassed doped emulsion the required length would be 20 borehole diameters; for non-gassed slurry it would be 16 borehole diam-eters. These figures would be prohibitive, especially for gassed products, in assuring adequate amount of explosive energy throughout the rock mass. Improved means of stemming to reduce this length would be a natural target for future research.

The explosive energy release, even for non-decked blasts, has been shown to be critically affected by the mode of initiation, especially by the common practice of using various combinations of detonat-ing cord and booster in the same hole. All advanced blast modeling efforts would have to take this into account to make any progress in blast predictions on a reasonable basis. Since the bulk of the deto-nators would continue to be based on the current pyrotechnic approach, the technology of embed-ding delay elements as well as the overall design of these detonators should be reviewed in terms of increased shock resistance and improved firing time accuracy in the presence of sympathetic pressures. The corresponding information on pressure effects on the performance of electronic detonators, if available, should be more widely distributed. These efforts should proceed hand in hand with controlled experiments in a production blasting environment, especially on the nature of sympathetic shocks in multi-hole or multi-deck blasts.

ACKNOWLEDGEMENTS

The author gratefully acknowledges the help and support provided over the years by numerous quar-ries and mines, and the former ICI Explosives for some of the work reported here. This was also partly supported by grants from the Natural Sciences and Engineering Research Council of Canada.

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