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Design Software for Electronic Detonators

Mike HigginsPrincipal

Soft-Blast Pty LtdBrisbane, Queensland, Australia

Kai RiihiojaSenior Research Programmer

Julius Kruttschnitt Mineral Research CentreBrisbane, Queensland, Australia

Abstract

A major difference between electronic and pyrotechnic detonators is that, in almost all but the simplestof blast layouts, computer software is required to plan a blast using electronic detonators. While it maybe possible to program delay times without software, using only the manufacturer’s control hardware,the full potential of electronic detonators can only be realized through software.

Until recently, the only software available has been provided by the suppliers of the electronicdetonators. These usually have just enough features to create a hole layout and to assign times to thedetonators in the holes. The software is also able to communicate only with the manufacturer’sdetonators. This means that the client may be compelled to change several parts of the blasting process when a different delay system is adopted, from design through to implementation and reporting, eventhough these may be sub-optimal at the client’s mine.

While many authors have demonstrated improvements in blasting outcomes using electronic detonators, very few have discussed how the delay times were calculated. Where the calculations are discussed, it is also obvious that the delay times must be monitored and re-evaluated as conditions in the mine change.For example, the principles of superposition and scaled distance, burden relief and energy cooperationbetween holes should be checked constantly for different locations within the mine, changes in rockproperties, or even for differences between design and actual hole positions.

The most common methods in blast design software to calculate delay times are inter-hole interval,relief contour and direct editing. Although they are simple in concept, they can result in fundamentalerrors if they are not applied correctly. The software for electronic blast designs should be capable ofnot just a timing calculation, but also of showing, to some degree, the impact and interaction of thecalculated times.

Introduction

Much has been written to date on the advantages of using programmable electronic detonators overpyrotechnics. In general, these can be summarised as:

� Optimum delay time – the detonators can be set to detonate at the time chosen by the user, instead of the restricted times of pyrotechnic detonators;

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2004G Volume 1 - Design Sofware for Electronic Delays

� Precision and accuracy – the detonators will detonate at the set time, with a much higher degree of accuracy i.e. scatter is negligible by comparison;

� Reduced inventory – only one type of detonator is stored in the magazine and inserted in the blast hole, with zero likelihood of error.

Conversely, the disadvantages are often not discussed as extensively, but are well known, such as higher unit cost, increased labour time to install and check, and a dependence on instruments and the skills touse them. A further disadvantage, that is often overlooked (with some notable exceptions), is thedependence on software to determine the delay time for each detonator.

Controlling a Blast

Blasting in rock is a process of controlling the interaction of three distinct elements: rock, explosivesand time. Unfortunately, we don’t often have the luxury of selecting the best rock mass to blast, but we can measure the rock properties. To compensate for the difficulties in the rock mass, we try to select an appropriate explosive, and to distribute it through the rock mass in blast holes of specific dimensions(diameter and length), orientation (dip, bearing) and arrangement (burden, spacing, pattern), and insufficient quantity to break the rock. The final ingredient, time, is then applied in such a way as toensure that each blast hole is able to break the rock assigned to it, typically by providing enough timebetween holes so that the rock is removed progressively by holes detonating in a specific sequence.

Pyrotechnic detonators have a significant degree of scatter, so the interval between detonators must be of sufficient length to ensure that the potential for overlap is minimised or even removed. In many cases,where long downhole delays are used or where the sequence is critical, this usually results in each holeacting independently of the other holes.

The example below (Figure 1) shows the effect of scatter on an otherwise simple blast. The blast is asimple pre-split – the top row uses pyrotechnic detonators, the second uses electronics. The holes areplaced at 3 m (10 ft) spacing. The top row has 250 ms delays in the hole, defined with 1% scatter(2.5 ms, equal to one standard deviation), with no delay between the holes, and connected by detonating cord with a burn rate of 6400 m/s (21,000 ft/s), so that the actual delay time between holes should beabout 0.47 ms. The detonators in the second row are programmed at an interval of 0.5 ms between holes, with a scatter of 0.01 % and the first detonator set for 257.5 ms. A Monte Carlo simulation is then run on both blasts simultaneously.

As can be seen from the simulation, the scatter in the pyrotechnic detonators is more than enough tothrow the holes completely out of sequence, leading to almost zero co-operation between holes andhigher levels of damage and vibration. The scatter is five times the inter-hole interval, so that the onlyway that a sequence can be imposed on the top row is to place a delay of greater length than the scatter between each hole. This would extend the duration of the blast by 351 ms (for a 9 ms delay), whichitself could have a detrimental effect on blast vibration.



Figure 1 - Pyrotechnic versus electronic – simulation of scatter effect

Cunningham et al (2001) has noted “that the greatest learning curve is in the area of short intervals,since these ranges have not been possible in the past”. This has been the subject of many papers where improved fragmentation and vibration control has been achieved, “but these huge benefits are only what should be expected through having eliminated out-of-sequence firing and crowding”. It is quiteprobable that many success stories, and also many failures, can be attributed to shorter delay times.Obviously, some effort must be put into determining an appropriate delay interval.

Calculating the Times

At Gastown, PA, Elkin and Bartley (2003) showed that through careful application of seed waveforms,it was possible to significantly reduce vibration levels at the residences adjacent to the mine. However, as mining progressed along the strip, the orientation of the blast to the residences changed, so that timing had to be changed from 22 ms inter-hole and 89 ms inter-row, to 33 and 76 ms respectively.

Similarly, at Waihi, New Zealand, Jackson and Louw (2003) demonstrated a significant decrease invibration levels by superposition of full blast waveforms and breaking the blasts into several smallpanels. The improvements subsequently allowed an increase in powder factor, with improvements incrusher throughput and excavator production. The next stage to consider is increasing the pattern size,but this is waiting for further improvement in fragmentation and expected changes in conditions as thepit deepens.

Rosenstock (2003) discussed the possibility of forcing explosive decks to interact, both horizontally and vertically, by fine tuning of the detonation times in response to the physical properties of the rock mass.He also demonstrated how excessive burden relief (insufficient burden) could produce oversize.Rossmanith (2003) looked at the implications of hole interaction in greater detail, advocating that “anew body of knowledge” on the dynamics of rock breakage is required to achieve the full potential ofthe new technology.



Various aspects of the technology and processes for optimising detonation times, as outlined in theabove papers, exist to some degree, but not in a single, formalised software package. The studies atGastown and Waihi show that the techniques need to be part of the design process so that engineers can monitor and adjust timing as required. The new techniques proposed by Rossmanith and Rosenstockwill require new models in blast design, but are not beyond the capabilities of some of the currentsoftware.

One of the most common methods for checking blast timing in the design process is to plot contours ofthe detonation times. This gives a graphical indication of the sequence of detonation, and is often usedto infer the progressive development of the free face in the blast and the direction of movement of themuckpile. It would seem straightforward to apply this in reverse to calculate electronic delay times –define the required contour, and apply it to the layout of blast holes to calculate the time at each hole.An alternative method is to apply an interval between holes, and then simply “walk the blast” on thecomputer screen, incrementing the delay time at each hole as they are “connected”.

Pitfalls in the Methods

The following figures show bench blasts analysed in JKSimBlast. The analyses are equally applicableto stope and tunnelling blasts, but bench blasts are used here for their simpler geometry.

For a perfect blast design, the results can be exactly the same for either calculation method. Figure 2shows a simple four row blast (7 m x 9 m, 23 ft x 29.5 ft), with the initiating hole at the top left corner(row 1, hole 1). The times have been calculated with 26 ms inter-hole and 52 ms inter-row, to give a V2 contour (in echelon). The contours are separated at 7.97 ms /m (2.43 ms/ft), at an angle of 21.25 degreesto the front row.

Figure 2 – detonation contours – “design” holes



If the actual hole locations are the same as the designed locations, then either method can be used tocalculate the detonation times. Problems arise, however, when the hole positions are shifted. InFigure 3, the fourth and fifth holes in row three are shifted 0.5 m (1.6 ft) west and north, and 0.5 m east and south respectively, and the times are calculated by the contour method, as above. If an intervalmethod had been used, there would be no change in the detonator times and the contours would bendwith the holes. But in the contour method, the times change with the position of the holes. Becauseelectronic detonators allow fine tuning of the delay times, if this design had been optimised for aminimum relief time of 21 ms between holes, then holes 4 and 6 in row 3 would be on the verge offailure due to over-confinement at detonation.

Figure 3 – detonation contours – “actual” holes

Figures 4 and 5 show a 4D energy distribution analysis. This analysis calculates the amount of energy at every point in the rock mass, based on the properties and distribution of the explosives in the blast. A3D analysis is static – in other words, it assumes that the energy from all holes is available to the entire rock mass – but a 4D analysis includes a co-operation time, where every point is assigned the detonation time of the nearest deck, and is also weighted by any other deck detonating within the co-operation time.The calculated energy level is affected by the inverse square of the distance. In both figures, the co-operation time is set at 26 ms.

Figure 4 shows the 4D energy distribution for the designed blast, and Figure 5 shows the altered blast.Note the decrease in available energy between holes 3-4 and 3-5. This area could be a potential source of oversize (or the areas in front of 3-4 and behind 3-5, if the concept of insufficient burden (Rosenstock 2003) holds), if the blast parameters were fine tuned close to the minimum values for the rock mass. Isit possible that, where some blasts with electronic detonators have been reported as failures in the past,that those blasts unknowingly exceeded the criteria for the rock mass?



Figure 4 – 4D energy distribution – “design” holes

Figure 5 – 4D energy distribution – “actual” holes

Implementing a Blast

Cunningham (2001) has made the point that the task of programming detonators can lead to errors. The hardware for programming and firing electronic detonators allows the user to modify the delay times, as well as upload the times directly from a computer or via some medium of transfer. This leads to several



questions: where and when is the decision made on the firing times; how is that information transferredto the blast; and if changes are required, how will they be made, and how will they be recorded?

A simple approach is to draw a plan of the blast, showing the holes and detonation times, and thenassign the times to each detonator as it is inserted in the hole. This will be very tedious and subject totranscription errors. The data could be uploaded to the hardware, thus avoiding errors in input, but thiswill now introduce the possibility of selecting the wrong time for a blast hole. If the ID number of each delay, and the hole it is in, is known beforehand, then the detonator times could be assigned to eachdetonator in the design software, but this assumes that the data is accurate and can be easily transferred to and from the office. For most blasts in most mines, this would require good coordination between the office and the blast (which may be next to impossible for underground operations) or may ultimatelyrequire that the office come to the blast. For this to occur, the skill requirements of all personnelinvolved in the process would need careful evaluation in order to determine the most appropriateprocess.

In order to maximise the opportunity provided by electronic detonators, there must be a concerted effort in the precision and accuracy of planning, implementation, measurement and record keeping, both indesign and actual data. A particular effort should be made to record the programmed times for eachdetonator in the blast, and the actual location of each hole in the blast. This should be an early goal inintroducing these systems to a mine.

Software Requirements

The suggested minimum capabilities for a useful software package for electronic blast design andanalysis are:

� Import actual hole locations and detonator times and ID numbers;

� Calculate detonator times by contour and inter-hole, plus manual editing;

� Export a simple ASCII file of the properties of each detonator – ID, delay time, location in blast (hole number or co-ordinates), depth in hole

� Analysis of the effect of hole location and explosive mass or energy versus detonation time (burden relief);

� Analysis of predicted vibration waveform or frequency reinforcement;

� If the analyses are not included in the software, it should be possible to export all blast data in an acceptable format so that the analysis can be performed in other software

And finally, to facilitate the transfer of data from the software to the hardware, the manufacturers ofelectronic detonators should provide a program for conversion from a simple ASCII file format to theformat used by their equipment, and vice versa. Alternatively, the manufacturers could agree on a single standard format for import and export of electronic blast data.



References

Cunningham, C.V.B., “Pre-Set Delay Electronic Detonators: Merits Opposite Programmable Systems”,EXPLO 2001, Hunter Valley, NSW, October 28 – 31, 2001

Elkin, J. and Bartley, D., “Digital Detonators keep PA Coal Mine Operating”, Proceedings of theTwenty-Ninth Annual Conference on Explosives and Blasting Technique, Nashville, TN, February 2-5,2003

Jackson, B. and Louw, T., “The Evaluation of Electronic Detonators at Martha Mine, New Zealand,2002”, Proceedings of the Twenty-Ninth Annual Conference on Explosives and Blasting Technique,Nashville, TN, February 2-5, 2003

Rosenstock, W., “Cast Blasting in the Light of the Electronic Option”, Proceedings of the Twenty-NinthAnnual Conference on Explosives and Blasting Technique, Nashville, TN, February 2-5, 2003

Rossmanith, H.P., “The Mechanics and Physics of Electronic Blasting”, Proceedings of the Twenty-Ninth Annual Conference on Explosives and Blasting Technique, Nashville, TN, February 2-5, 2003



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