The Influence of Burden on Blast Vibration

Fragblast 1385-514x/01 /0501 -108$ 16.002001, Vol. 5, No. 1-2, pp. 108-129 ® Swets & Zeitlinger

The Influence of Burden on

Blast Vibration

D.P. BLAIR1'2 and L.W. ARMSTRONG1

ABSTRACT

There is a common belief within the blasting community that increasing the burden willincrease the blast vibration. In order to test this belief in a direct sense, it would be desirable tofire single blastholes with various burdens and monitor the vibrations at many locations. Areview of past literature indicates that such direct tests are rare and only scant data is available.Nevertheless, a detailed analysis of this and associated past work (on small-scale blocks andchoke blasts) shows no convincing evidence of an influence of burden on blast vibration. On theother hand, by considering the role of reflected waves in a simple analytical model, reasoning isgiven to show that the vibration might well be insensitive to burden.

In view of the scant data available, it was decided to conduct trials of a direct nature, inwhich 13 single blastholes were fired to a free face. The burdens chosen were 2.6 m, 5.2 m and10.4 m, and the vibration was measured at typically 10 locations over the range 5 m to 50 mfrom each hole. The results clearly show that the vibration is independent of such burdens.Furthermore, a side-by-side comparison of a choke blast with a free-face blast showed that thevibration from the holes in the choke blast was not higher than the vibration from the holes inthe free-face blast.

The present work also shows that vibration, although insensitive to burden, is not insensitiveto the condition (i.e., the degree of damage) of the surrounding rock mass. In this regard,blastholes in undamaged ground produce a significantly higher vibration than blastholes indamaged ground. This might well be the reason why pre splits and drop-cuts are observed toproduce relatively high vibrations, i.e., it is not because such blasts typically involve largeburdens, but rather that they are usually initiated in relatively undamaged ground.

Keywords: Blast vibration, burden, damage, PPV.

1 INTRODUCTION

In many mining operations, it is desirable to blast leaving broken material(from the previous blast) lying in front of the rock face. Such choke blasting

^ r i c a Australia Pty. Ltd., George Booth Drive, Kurd Kurri, NSW Australia.Corresponding author: Tel.: 61 2 4939 5200; Fax: 61 2 4939 5299; E-mail: [email protected]

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THE INFLUENCE OF BURDEN ON BLAST VIBRATION 109

has three main advantages. Firstly, this extra burden restricts horizontal facemovement and so minimally disturbs ore-waste boundaries in grade controlblasting. Secondly, it improves mine scheduling (and hence productivity)since broken material does not have to be removed prior to each blast. Thirdly,the broken material against the face helps to reduce face bursts which, in turn,helps to reduce excessive airblast.

However, because any choke blast is fired with a burden significantly largerthan that of an equivalent free-face blast, there is a common belief that chokeblasting will produce excessive vibration (see, for example [1-3]. It is thusworthwhile reviewing available data regarding the influence of burden (eitherbroken or solid material) on blast vibration. In this investigation the burden isassumed to be solid unless stated otherwise (as in choke blasts).

Liu and Ludwig [4] measured the blast vibration for a series of chargeweights, distances and burdens. Their tabulated raw data is plotted in Figure 1as a function of the traditional scaled distance d/y/W, where d is the distance(in m) from the monitor to the blasthole, and W the explosive charge weight(in kg). The burden (in m) is shown beside each VPPV value.

(mm

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Fig. 1. The raw experiment data of Liu and Ludwig [4] plotted as a function of scaled distance.The burden (in m) is shown for each data point.

110 D.P. BLAIR AND L.W. ARMSTRONG

There is obviously a significant amount of scatter in the data and this alonesuggests that caution must be exercised in attempting to extract a correlationbetween any variables. In this regard, the raw data show no con-vincingevidence of an influence of burden. Nevertheless, because of the commonbelief, Liu and Ludwig [4] assumed a dependence of burden on vibrationwithin their 4-parameter model that was used to fit a scant data set of 9observed values. Thus their resulting claim that vibration depends uponburden must be viewed with great caution.

Figure 2 shows the VPPV measured by Blair and Birney [5] for themonitoring of single blastholes, with burdens of either 6 m or 3 m, fired 900 mbelow the surface vibration detectors. It is quite obvious that this data forunderground blasting also shows no convincing evidence of a dependence ofvibration on burden.

Figure 3 shows the data measured by Bergmann et al. [6] for blasting in smallblocks of granite. Pressure gauges (rather than geophones or accelerometers)were used to measure the induced vibration, and so the results are given interms of the gauge pressure (in MPa). The solid curve is the traditional least

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squares power curve fit to the data. Again, in this case there is no convincingevidence of an effect of burden on blast vibration (gauge pressure, in thiscase). Unfortunately, these authors, too, assumed a burden dependence andincorporated this effect into a rather questionable 9-parameter model.

Incidentally, it was this model that also predicted the much-touted claim thatan optimum velocity of detonation (VoD) for an explosive is 1.3 times the rocksonic velocity, and so this added claim must also be viewed with great caution.As it stands, the most common sense interpretation of Figure 3 is that all the datamay be fit by the usual power curve (the solid line), irrespective of burden.

Heilig et al. [7] monitored a series of blasts in a quarry in which the face waseither free or confined by a muckpile. Figure 4 shows their data plotted over thesame range in scaled distances. Their original data for the confined blasts covereda scaled distance that was approximately twice that of the unconfined blasts, andit is statistically unwise to compare data sets over vastly different regimes. Thelarge amount of scatter in the data is quite evident, and raises the questionregarding the significance to be placed on the fact that the regression line for theconfined blasts lies above that for the unconfined blasts as shown in Heilig et al.[7] The data is shown in Figure 4 for similar ranges of scaled distance.


Davies and Goldsmith [8] outline a series of Hypotheses testing that can beused in multiple linear regression. When their statistical method is applied tothe data of Figure 4, the conclusion is that all the data is best fit by a singleline. Inother words there is no significant difference in the separate regression lines forthe free-face and confined blasts. Incidentally, the same conclusion results evenif all the data for the confined blasts are used in the statistical analysis.

An alternative method of analysing the data is to plot the 95% confidencebounds on the mean vibration for any scaled distance using the statisticalmethod outlined in Draper and Smith [9]. These confidence bounds are shownin Figure 4, and it is quite clear that, for all scaled distances, the 95% con-fidence region for the mean vibration produced by the confined blasts signi-ficantly overlaps that for the unconfined blasts. This finding, alone, suggeststhat the data of Heilig et al. [7] show no convincing evidence that confinedblasts produce vibrations different to those from unconfined blasts.

Despite a detailed literature review, we have been unable to locate theoriginal reasoning behind the claim that increased burden results in increasedvibration. The experimental evidence reviewed above certainly does not seem

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to support it. Perhaps this claim is based, in part, on the perceived role ofconfined explosive gases, and if so, would require clear experimental evidencefor support. Perhaps the claim is also based upon a perception.that theexplosive in an over-burdened blasthole, since it cannot move materialforward, will back-react to produce a large vibration in the opposite direction.

This perception warrants further investigation. At first glance, it mightappear akin to Newton's Third Law of Motion-to every action there is anequal and opposite reaction. However, the blasthole does not 'know' the extentof the burden until a wave has travelled out to the face and then returned. Thusthere is a time delay between the action of the blasthole and its reaction off theface. In this regard (and neglecting any role of explosive gases) the influenceof burden on blast vibrations must be analysed from a viewpoint of travellingwaves. The situation is illustrated by the two-wave system shown in Figure 5;the monitoring line will be referenced in the modelling section.

The blasthole (HI, say) on the left is located a distance b (burden) from thefree face (shown as the solid line). The blasthole (H2, say) on the right has aninfinite burden, with a fictitious line (shown dashed) placed at b just toemphasise the only difference between both holes in ideal, identical geology.There is just one wave system (which may include various types such as p-waves, and s-waves) radiating from H2, and we may call it the direct wave.However, there are two wave systems associated with HI: the direct wavesystem and the reflected wave system. It is very important to note that the onlydifference between HI and H2 is the reflected wave system, since the directwave system is identical for both blastholes.

reflectedwave system

directwave system

directwave system

monitoringline I

Fig. 5. The two-wave system for the influence of burden (b) on blast vibration.


Thus the simple model of Figure 5 immediately suggests that if thereflected wave can be eliminated by some means, then blast vibration iscompletely independent of burden. In any real situation there are at least fourreasons to suspect that the reflected wave might not be significant. Firstly, themost dominant reflection would only occur for plane waves normally incidenton a planar free face. However, due to the three-dimensional geometry and thefinite velocity of detonation (VoD) of the explosive, an essentially conicalwave hits the face at non-normal incidence. In fact, a three-dimensionalDynamic Finite Element Model (DFEM) of this situation has verified the weaknature of the reflected wave. Secondly, the direct wave travelling towards thefree face encounters ground that is usually more damaged than ground at anequal distance behind the blasthole. Thirdly, this wave is then incident on aragged (non-planar) face that will promote incoherent back-scattering of theenergy rather than simple reflection. Fourthly, this back-scattered wave nowtravels back to the blasthole and beyond through ground even more damagedby the direct wave.

The fact that previous experimental data shows little evidence for a burdeninfluence on blast vibration might well be due to one or more of thesemechanisms. Although the role of wave reflection from a smooth surface canbe reasonably modelled, it is more difficult to model other influences such asback-scattering from a ragged face and wave propagation through variouslydamaged material. Furthermore, there may also be some role played by theconfined explosive gases. The combined influence of all these mechanisms isbest determined experimentally.

Thus there are three main aims of the present work. Firstly, to analyticallyevaluate the influence of burden on blast vibration using a simple model thatignores any role of explosive gases. Secondly, and most importantly, tomeasure directly, the influence of burden on blast vibration by firing a series ofsingle blastholes at various distances to free faces. Thirdly, to compare thevibrations from a free-face blast with a choked blast.

2 A SIMPLE MODEL FOR THE INFLUENCE OF BURDENON BLAST VIBRATION

One of us (DPB) is currently completing a series of models to yield fastanalytical solutions for the vibration produced from an explosive sourcelocated near free faces. The source is either spherical [10] or cylindrical with a


finite VoD [11], and fired near one or two faces. The wave mode conversionupon reflection at each boundary is accomplished by a modified method ofimages. Material attenuation (viscoelasticity) is also modelled approximately,and non-linearity is built-in at the elemental level for the cylindrical blastholemodel. Only the spherical source model is considered here, and only forvibrations detected along the monitoring line shown in Figure 5. Since thesource produces only p-waves, then only p-waves will be reflected for alllocations along this monitoring line (for locations off this line, the incidentp-waves could also produce reflected s-waves via mode conversion).

Figure 6 shows an example of the two pressure-time functions, PI and P2,used in the present model, and either function is applied to the wall of thespherical cavity. The function PI (heavy line) is given by /*exp(-otf),m which tis time and a is a constant (=10,000 s"1 here, for the time, t, in s). The functionP2 (light line) is given by an 8th order band-pass Butterworth response overthe range 400 Hz to 800 Hz. The function PI has been expanded in time by afactor of 20 (its actual time duration is approximately 0.1 ms rather than 2.0ms) in order to show its detail.

The spherical cavity response to PI has been derived by Jiang et al. [12, 13]in a scale-independent form, and their equations have been re-cast for the

Heavy line- t*exp(-oct) function (PI)

Light line - Butterworth function (P2)

10TIME (ms)

Fig. 6. The two pressure-time functions used in the spherical source model. The time scale forPI has been expanded by a factor of 20.


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DASHED LINE - ELASTIC CASE

0.5 1.5 2 2.5

BURDEN (m)

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Fig. 7. The vibration as a function of burden in materials of varying attenuation (I/Q) Themonitor is placed 50m from the spherical source, SI.

present specific case. White [14] has given the spherical cavity response to aHeaviside unit step function, and so the response to P2 is obtained by firstdifferentiating White's solution (with respect to time) and then convolving thiswith P2. These particular pressure-time functions are used since the first (PI)has a traditional history of application, and the second (P2) produces a reason-able degree of oscillatory response and so allows the possibility of constructiveand destructive interference of the direct wave with the reflected wave.

Figure 7 shows the influence of burden, b, on the peak vibration producedby a spherical source (SI) for an applied load given by PI. The detector islocated 50 m from the source and along the monitoring line (Fig. 5). In thepresent models, the peak vibration is defined as the peak particle velocity forthe only non-zero component (radial) lying in a direction along the monitoringline. The results are shown for various materials whose attenuation isdescribed by the constant-Q viscoelastic model of Kjartansson [15]. TheVPPV values have been normalised to that for the case of infinite burden.

Figure 8 shows the influence of burden, b, on the peak vibration producedby a spherical source (S2) for an applied load given by P2. In this figure theVPPV values have been normalised to that for the elastic case at infiniteburden.


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The vibration as a function of burden in materials of varying attenuation (I/Q) Theis placed 50m from the spherical source, S2.

The degree of overlap of the direct wave with the reflected wave isobviously dependent upon the width of these waves, and so it is not surprisingthat the peak vibration depends upon the original pressure-time function andthe material Q as well as the burden.

This simple model shows that the vibration can never increase by more thana factor of 2.0 due to the burden. Ironically, the model also shows that, undercertain conditions, the vibration decreases with increasing burden, which isprecisely opposite to the common belief.

3 VIBRATION FROM BLASTHOLES FIRED WITHVARIOUS BURDENS

The single blasthole trials were conducted over the period August 1997 toAugust 1998, in the Oroya South region of the Fimiston Open Pit Operationsof Kalgoorlie Consolidated Gold Mines (KCGM). All blastholes had adiameter 165 mm, were back-filled to a depth 10.2 m, charged with Energan2640 explosive having charge weights in the range 125 kg to 150 kg, andstemmed with approximately 4.5 m of crushed aggregate. Figure 9 shows aplan view of some of the blastholes and monitor locations. In all cases the


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monitors (triaxial accelerometer arrays) were bonded directly to rock in anapproximate line that was approximately normal to the bench face. Theburden trials commenced in January 1998. Some months prior, two blastholeswere initiated in relatively undamaged (virgin) rock when the operating benchface was 60 m east of the closest hole. These two holes are shown as thecrossed squares in Figure 9 and their resultant vibration will be discussed later.

The standard burden for blastholes at this KCGM site is 5.2m, and thepresent trials were conducted using burdens of 0.5, 1.0 and 2.0 times thestandard. It is for this reason that the free face blastholes appear staggered inthe figure. A total of 13 blastholes was monitored (with other holes elsewhere,not shown), with four repeats of 5.2 m and 2.6 m, and five repeats of 10.4m.The separation of blastholes along the face was 20 m since previousexperience had shown that at this separation there was minimal influence ofone hole on another.

Figure 10 shows the vector peak particle velocity (VPPV) as a function ofthe scaled distance and various burdens. Each data set is fit with a least squares


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Fig. 10. The VPPV as a function of burden for the single blasthole trials.

power curve. It is quite obvious that the burden has an insignificant effect uponthe blast vibration, in fact the curve fits for the 2.6 m and 10.4 m burdens arepractically indistinguishable. However, a formal statistical analysis wasconducted on this data by first transforming each set to log10 (VPPV) as afunction of logio (scaled distance) and then applying the multiple linearregression analysis of Davies and Goldsmith [8]. The conclusion was found tobe that all the data is best treated as a single data set rather than grouped byburden.

As noted previously, there were two blastholes initiated in virgin(undamaged) ground prior to the burden trials (see Fig. 9). It is worthwhilecomparing the VPPV measured for virgin ground with that measured for theburden trials. According to the statistical analysis, all the burden data may beconsidered as a single data set, and so this set may then be compared with thedata obtained for virgin ground. The results are shown in Figure 11. In thiscase the Davies and Goldsmith [8] analysis shows that the data is best fit bytwo separate lines each having the same slope. Figure 11 also shows the 95%confidence bounds on the mean, and it is quite obvious that these regions arequite distinct except for a small degree of overlap in the very near field. Thusboth statistical analyses show that the vibration produced from blastholes fired


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in the undamaged ground is significantly larger than the vibration producedfrom blastholes in the burden trials. The significance of these findings is raisedin the Discussion section.

4 VIBRATIONS FROM A FREE-FACE BLAST ANDA CHOKE BLAST

Figure 12 shows the location of the vibration monitors and blastholes for theside-by-side comparison of a choke blast of 55 holes with a free-face blast of33 holes. Each hole had the following nominal design parameters: diameter165 mm, depth 11.3 m, charge weight 190 kg (Energan 2640) and stemminglength 3.9 m. The local geology was similar for both blasts.

Nine triaxial accelerometer arrays were bonded directly to the pit wall inorder to measure the vibration behind each blast. The initiation sequence wassimilar for both blasts (17 ms between each hole, 100 ms between each row).The choke blast was fired with approximately 15 m of broken material (from a


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Fig. 12. The location of monitors and blastholes for the two blasts.

previous blast) lying in front. The choke blast was fired first (13th Feb, 1999)and the free-face blast fired one day later.

The peak vibration for the case of a single blasthole is convenientlydescribed by the vector peak particle velocity (VPPV) which is just one pointon the entire vibration vector trace. However, such a measure gives noinformation on how the vibration varies with time. In order to distinguishbetween the choke blast and the free-face blast, a vibration measure (such asthe amplitude) is required for all times throughout these multi-hole blasts.

Traditionally, the vector sum waveform, Vs(t), is used. However, the vectorsum is not a measure of the vibration amplitude as a function of time. In thisregard Farnbach [16] shows how the vibration envelope function may beobtained as a function of time for any single waveform by using Hilberttransform techniques. This envelope function gives the instantaneous vib-ration amplitude at any particular time. For example, the envelope function ofa single component sine wave is constant and equal to its amplitude, whereasthe vector sum (which reduces to the modulus of the sine wave in this case)is not.

The work of Farnbach [16] is now extended to the analysis of triaxialmeasurements. If L{t), T(t) and V(t) are the triaxial components of the


vibration as a function of time, t, then the traditional vector sum,Vs(t) is givenby:

Vs(t) = (1)

If HL(t), HT{t) and Hv(t) are the Hilbert transforms of L(t), T(t) and V{t),respectively, then an envelope function, E(t), may be defined as:

E{t) = (2)

This particular form of the envelope function is chosen in order to ensure thatthe peak value of Vs(t) is almost identical to the peak value of E(t). In thisregard, Figure 13 shows the vector sum and the envelope function for the casein which L(t), T(t) and V(t) are given by sine waves of amplitudes 1, 2 and 3units, respectively.

In the present work, the Hilbert Transforms are calculated efficiently usingFast Fourier Transform (FFT) techniques. Tapering is used at the beginningand end of each waveform in this simple example to avoid FFT time windoweffects. The vector peak is given by (I2 + 22 + 32)1'2 = 3.742, and agrees withthe peak value of the envelope function. Neglecting the taper regions, it is

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Fig. 14. The vibration mean envelope functions for the choke and free-face blasts.

expected that such a superposition of sine waves should give a total amplitudethat is constant with time. The envelope function clearly meets this expec-tation whereas the vector sum does not.

Figure 14 shows the mean vibration envelope (averaged over all 9 monitors)for each blast. However, a clearer comparison of the vibration from each blastmay be made by averaging each envelope function at each detector over asliding time window of specified width; these time-averaged envelopes maythen be averaged over all detectors to form a vibration smoothed envelopefunction. Figure 15 shows the vibration smoothed envelope function for bothblasts, the window width for smoothing the envelope function for eachmonitor was arbitrarily chosen to be 0.1 s.

Figures 14 and 15 show that there is no evidence to suggest that the chokeblast has produced vibrations larger than those of the free-face blast,especially over the first 0.5 s or so. In this regard, it should be appreciated thatthe first row of overburdened holes in the choke blast has completely initiatedafter 0.2 s.

However, there were some slight differences in the actual design of eachblast (such as charge weights and number of blastholes) and a definitivecomment regarding relative vibration levels can only be made after thesevariables are taken into account. Therefore, each blast was modelled using the


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Fig. 16. Monte Carlo solution for the vibration smoothed envelope functions for the choke andfree-face blasts.

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Monte Carlo technique outlined in Blair [17]. Figure 16 shows the MonteCarlo results for both blasts.

It should be appreciated that the Monte Carlo model does not account forthe fact that holes in the choke blast were overburdened, in other words, allblastholes in both blasts are assumed to behave in a similar manner withregard to vibration.

Thus the blast design, itself, is such that there should be little difference invibration between the choke blast and the free-face blast for the first 0.4 s,approximately. The fact that only a small difference was observed is directevidence that the overburdened holes in the choke blast did not producevibration larger than that of corresponding holes in the free-face blast. Themeasured peak vibration for the choke blast (at approximately 0.52 s Fig. 15)is larger than that due to the free-face blast solely because of the design (asshown by Fig. 16).

5 DISCUSSION

In the introduction four possible mechanisms were suggested to explain whyvibration waves reflected off a free face might have insignificant amplitude. Itwas also shown that an absence of reflected waves would imply that vibrationwas independent of burden provided any role of explosive gases wasneglected. In this regard it is interesting to note that we found no evidence of areflected wave in any of our experiments. Thus it is reasonable to assume thatone or more of the four mechanisms was in operation.

Nevertheless, analytical models were used to investigate the case wherewaves were reflected with maximum amplitude (i.e., reflected waves due towaves incident normally on the free surface). Such models will yield an upperbound to the influence of burden on blast vibration. The Dynamic FiniteElement Model (DFEM), on the other hand, showed that it is more likely thatany reflection will not be significant anyway, due to the three-dimensionalnature of the problem. The results shown in Figures 7 and 8 for the analyticalmodel neglected the influence of explosive gases on burden (confinement)because it was too difficult to model this mechanism. However, the work ofBrent et al. [18] shows that this mechanism is probably insignificant, anyway.In this regard, they conducted gas pressure measurements in some of thesingle blastholes and demonstrated that the extent of gas penetration beyondthe immediate confines of the blasthole was also independent of burden in the


present trials. Furthermore, the observed data of Figure 10 directly impliesthat any influence of explosive gases on vibration (although difficult toquantify) must, too, be independent of confinement for the present burdens of2.6 m, 5.2 m and 10.4m. The fact that explosive gases play such aninsignificant role in this aspect obviously improves the relevance of theanalytical model and DFEM for the prediction of vibration as a function ofburden. Obviously, in the extreme case when the burden approaches zero, gas-venting and reduced vibration will occur. However, this is not relevant to thepresent trials that have only considered burden variations within a reasonablerange.

As noted earlier in regard to Figure 11, the statistical evidence suggests thatthe vibration data for blasting in damaged ground is significantly lower thanthat for blasting in undamaged ground. Furthermore, the slope of both datasets was found to be similar, i.e., the difference is due to the offset alone. Thefact that the slopes are similar strongly suggests that the attenuation ofvibration as a function of distance is the same for both data sets. This, in turn,also suggests that the observed difference in attenuation (i.e., the offset)occurs directly at the source. In other words, the source, itself, produces lessvibration if it is located in damaged ground. This inference is not surprising,since source radiation is expected to be strongly dependent upon its couplingto the surrounding rock mass. On the other hand, for the frequencies of presentinterest, the attenuation of peak vibration with distance (and hence the slope)is primarily dependent on geometric spreading rather than rock masscondition.

Thus, although vibration appears to be independent of burden over theranges of present interest (or at least insensitive to it), the vibration is notinsensitive to the local rock condition surrounding the blasthole. This finding,alone, suggests that even if an apparent dependence of vibration on burdenwas to be found at a particular site, then it would be quite difficult to isolate theburden influence, itself, unambiguously. For example, an increase in vibrationwith burden might simply reflect the fact that the holes further from the faceare fired in ground less damaged than that slightly closer to the face. In thiscase it would not be a burden effect per se, but rather an influence due toground condition. Alternatively expressed, the blast vibration is not dependentupon the volume of ground that the charged hole has to excavate, but isdependent upon the condition of the rock mass close to the hole.

Blast vibrations due to presplits and drop-cuts are often claimed to be highrelative to the charge weights used. The large burden typical of such situations


is the popular reason generally given for the high vibration in these cases.However, the present work strongly suggests that the high vibration is not dueto the large burden, but rather to the fact that all such blastholes are-typicallyinitiated in undamaged ground.

According to Figure 14, the vibration from some of the overburdened holesin the front row of the choke blast appears to be somewhat lower than thevibration from corresponding holes in the free-face blast, even though all suchholes had similar scaled distances in relation to the monitors. In this regard itis interesting to note that broken material lying in front of the face (i.e., as inthe choke blast) provides some degree of confinement to vibration wavesincident on the solid face-broken rock boundary. In other words this boundaryis neither completely free to move (as in a free-face blast) nor completely fixed(as in the case of a rigid boundary).

It is well known in dynamics that a partially restricted boundary is morelikely to absorb incident waves (i.e., reduce any reflected waves) than is acompletely free or fixed boundary. Based upon this reasoning and if a. reflectedwave happens to occur to any significant degree, it might well be expected thatchoke blasts would reduce blast vibrations rather than increase them. Thisoutcome is certainly not inconsistent with the present experimental data. Forexample, if the outgoing wave system (the direct wave shown in Fig. 5) fromeach blasthole is optimally incident on the face, then the choke blast has abetter chance of absorbing such waves than does the free-face blast. In thisaspect, choke blasting might well provide a better insurance against highvibrations reaching the wall of an open pit.

6 CONCLUSIONS

A literature survey of previous results showed that there was no convincingevidence to support the popular claim that vibration increases with increasingburden. On the other hand, by simply considering the role of reflected waves,reasoning was given to show that the vibration might well be insensitive toburden in many situations.

Although the influence of explosive gases on vibration would be difficult tomodel, the evidence of Brent et al. [18] suggests that such influences might beinsignificant anyway. This justifies neglecting any role of explosive gases inthe present models used to predict the influence of burden on blast vibration.These models, which also assumed optimal reflection of vibration waves,


showed that the vibration can never increase by more than a factor of 2.0 dueto burden. The models also predicted that vibration can either increase ordecrease with increasing burden depending upon the source type and the rockattenuation (i.e., rock Q). In particular, for a source of very small timeduration, the vibration invariably decreases with increasing burden (see Fig. 7)which is precisely opposite to the common belief.

However, the experimental waveforms also showed that the reflected wavewas too insignificant to detect. This observation, alone, is consistent with thefact that the vibration was found to be independent of burden as shown by thedata in Figure 10 for the single blastholes, and by the data of Figures 14 and 15for production blasts.

The present work has also shown that it is not the burden that determinesthe vibration but rather the condition of the rock mass surrounding theblasthole.

ACKNOWLEDGEMENTS

The considerable effort of KCGM (especially Ian Brunton) and Roche Miningpersonnel in all the vibration trials is gratefully acknowledged, particularlywith regard to site availability and drilling of test holes. Orica colleagues, GilSmith, Dave Kay and Sahul Rafiudeen, also gave invaluable assistance duringvarious stages of the monitoring, and Dave Kennedy performed the three-dimensional DFEM analysis for a single blasthole firing to a free face.

REFERENCES

1. Ashby, J.P.: Production blasting and the development of open pit slopes. Proc. Sixth Conf.on Explosives and Blasting Technique, Florida, 1980, pp. 291-311.

2. Dowding, C.H.: Blast Vibration Monitoring and Control. Prentice-Hall, 1985, 297.3. Floyd, J.L.: The development and implementation of efficient wall control blast designs.

Explosives Eng. 15 (1998), pp. 12-18.4. Liu, Q., Ludwig, G.: A blast damage study in blasthole open stope mining. Proc. Fifth Int.

Symp. on Rock Fragmentation by Blasting, Vienna, 1996, pp. 451-459.5. Blair, D.P. and Birney, B.: Vibration Signatures Due to Single Blastholes Fired in the

Charlotte Deeps. ICI Confidential Internal Report, 1994, 10.6. Bergmann, O.R., Riggle, J.W. and Wu, F.C.: Model rock blasting effect of explosives

properties and other variables on blasting results. Int. J. Rock Mech. Min. Sci. & Geomech.Abstr. 10 (1973), pp. 585-612.

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7. Heilig, J., Zoitsas, A. and Cox, N.: Free face blasting: Is it the best for quarrying? Proc. 41stAnnual Conf. Institute of Quarrying, Australia, 1997.

8. Davies, O.L. and Goldsmith, P.L.: Statistical Methods in Research and Production.Longman, London, 1972. --

9. Draper, N.R. and Smith, H.: Applied Regression Analysis, 2nd ed. John Wiley and Sons,1981, 709.

10. Blake, F.G.: Spherical wave propagation in solid media../. Acoust. Soc. Am. 24(2) (1952),pp. 211-215.

11. Blair, D.P. and Minchintonm A.: On the damage zone surrounding a single blasthole. FifthInt. Symp. Rock Fragmentation by Blasting, Montreal. Canada, 1996, pp. 121-130.

12. Jiang, J., Baird, G.R. and Blair, D.P.: Dynamic response of a half-space to a buried sphericalsource. Geophys. J. Int. 119 (1994), pp. 753-765.

13. Jiang, J., Blair, D.P. and Baird, GR.: Dynamic response of an elastic and viscoelastic full-space to a spherical source. Int. J. Numer. Anal. Methods Geomech. 19 (1995), pp. 181-193.

14. White, J.E.: Underground sound. Application of Seismic Waves. Elsevier, 1983, 253.15. Kjartansson, E.: Constant Q wave propagation and attenuation. J. Geophys. Res. 84 (1979),

pp. 4737^748.16. Farnbach, J.S.: The complex envelope in seismic signal analysis. Bull. Seism. Soc. Am. 65

(1975), pp. 951-962.17. Blair, D.P.: Statistical Models for ground vibration and airblast. Int. J. Blasting and

Fragmentation. 3 (1999), pp. 335-364.18. Brent, G.F., Smith, G.E. and Lye, G.: Studies on the Effect of Burden on Blast Damage and

the Implementation of New Blasting Practices at KCGM's Fimiston Mine. EXPLO 2001,Aus. I.M.M., NSW, Australia, [in press].

The Influence of Burden on Blast Vibration

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