Appraisal of Explosive Performance by Measurement of Velocity of Detonation (Vod) in Mines - Discussion

APPRAISAL OF EXPLOSIVE PERFORMANCE BY MEASUREMENT OF VELOCITY OF DETONATION (VOD) IN MINES - DISCUSSION Partha Das Sharma, B.Tech(Hons.) in Mining Engineering; E.mail: [email protected]; Website: http://miningandblasting.wordpress.com/ *** Abstract The rate at which the detonation wave travels through an explosive column is called the velocity of detonation (VOD). The velocity of detonation is one of the most important properties of explosives. It is essential that the explosive in the field condition detonates at its optimum rate and induces sufficient detonation pressure leading to good fragmentation. By conducting continuous VOD measurements in full scale blast environments, real performance of an explosive used for blasting, can be determined. VOD of a blasting agent varies depending on mainly the rock properties (confinement), presence or absence of sensitizer (chemical formulation), blasthole diameter, and type of primer. The method of continuous VOD measurement is an indispensable and invaluable tool in explosive and primer selection. VOD monitoring can be carried out to confirm whether detonation, deflagrations or failures have taken place, to study the influence of primer size on explosive performance, and to investigate the effectiveness of decking.

1. Introduction - Blast optimization and its study have become more important in mining scenario and attract more attention than before due to overall increasing economical pressures. Besides environmental hazards, inadequate and poor blast designs result decreased efficiencies in various mining functions such as loading, hauling, crushing and grinding cycles and lead to increase in the unit cost of overall operation. If site-specific blast design is implemented in mines, the objective of decrease in overall operation cost with provision of safety and elimination of environmental hazards can be ensured. With new PC based blast monitoring instrumentation systems (Mining / Blasting software) and recently introduced new analytical methods of analysis, the mining engineers can easily eliminate most of the guess work involved in designing efficient blasts and in evaluating explosive performance in any mining and geological condition. 2. Importance of measurement of VOD - Evaluation of a blast design is carried out with the assumption that the explosives have performed as per the specifications, which may not be true in all cases. A reduction in the VOD will produce a reduction in the detonation pressure as well as in the availability of the shock energy of the explosive. It is important that the explosive detonates at its optimum rate and induces sufficient detonation pressure leading to good fragmentation. The VOD of an explosive can, therefore, be used as one of the indicators of its performance. In other words, VOD

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APPRAISAL OF EXPLOSIVE PERFORMANCE BY MEASUREMENT OF VELOCITY OF DETONATION (VOD) IN MINES - DISCUSSION

of an explosive can be used to evaluate the performance of the explosive used in mines, under specific rock and test conditions. Since VOD is a direct measurement of the source function, it provides valuable information with respect to shock, stress waves, kinetics, ground vibration, airblast, fragmentation and undesirable noxious fumes. If VOD is not monitored and an explosive product is assumed to perform as specified, the interpretation of other measurements (if any) could be erroneous when product malfunction occurs. Thus, it is important to correlate the product’s VOD characteristics to the rock environment, blast design, and other measurements. Blasting performance is directly related to the characteristics and efficiency of the explosives used. FACTORS WHICH CAN AFFECT VOD OF EXPLOSIVES The factors that can affect the VOD of an explosive include (Chiappetta, 1998): • Confinement • Formulation characteristics • Density • Sensitising agent (s) - (chemical, gas, solid, metallic) • Temperature and temperature cycling • Primer size and type • Sleep time in blasthole • Nearness to critical diameter • Nearness to critical density • Borehole loading techniques • Drop impact in borehole • Blast design (confinement, relief, delay timing, initiation direction) • Dynamic desensitising effects • Explosive column length • Blast environment • Manufacturing • Transportation • Storage-shelf life • Mixing in bulk loading system 3. VOD Measuring System- A number of VOD systems are commercially available today. These systems can be broken down into two broad types: a) Point to point VOD systems, and b) Continuous VOD systems. a. Point to point VOD measuring system - Point to point VOD systems are essentially start and stop devices which are based on an electronic timer. The first sensor cable (i.e. channels) starts a clock and the following channels stop the clock in cumulative time relative to the start signal or relative to each other subsequent channel. It is important to accurately measure and record the distances between the probe ends. VOD measurements with a point-to-point system are acceptable for some measurements, but will be limited in providing information for critical experimental measurements when trying to detect the degree of malfunctioning explosives and/or transient VOD's within the column. b. Continuous VOD Recorder -

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Partha Das Sharma, B.Tech(Hons.) in Mining Engineering; E.mail: [email protected]; Website: http://miningandblasting.wordpress.com/


i. Resistance wire continuous VOD system- The continuous resistance wire method was developed in the early 1960s by the United States Bureau of Mines (USBM). Operation is based on the basic Ohm's law, (E = RI), where E = Voltage, R = Resistance and I = Current. When the current is held constant against a shortened (i.e. detonated) wire of known resistance per unit length, a voltage drop can be measured instantaneously at any point in time. The voltage drop is equivalent to the length of resistance wire consumed in the detonation. Resistance wire probes actually consist of two wires which must be physically shorted out by the detonation through ionisation. Some resistance wire probes consist of just two insulated wires twisted together and other probes consist of one coated wire placed inside of a small metal tube which acts as the second wire. Providing that the wires are adequately shorted during the detonation, the resistance wire method does provide a truly continuous VOD along the explosive column due to the high sampling rates ranging from 1.25 MHz to over 10 MHz. If the wires are not adequately shorted in a continuous and reliable fashion, erroneous results, excessive electronic noise and severe drop outs are the norm. In such cases the results are usually undeciphered or no readings are obtained. ii. SLIFER continuous VOD system- The SLIFER (Shorted Location Indication by Frequency of Electrical Resonance) system was originally developed by Sandia National Laboratories to measure the propagation of shock waves from nuclear explosions. It consists of a shorted length of coaxial cable placed in an explosive. The cable forms part of an oscillator circuit, the frequency of which is governed by the length of the cable. As the explosive detonates and crushes the cable, the effective length of the cable decreases and the frequency of oscillation increases. By monitoring this frequency as a function of time, the rate of cable length change can be determined, leading directly to the measurement of VOD.An on board electronics package, enables the measured frequency to be converted to a voltage in real time. This information is then linearised and VOD derived from the slope of the resulting trace; a plot of cable length with time. iii. TDR continuous VOD system (VODR-I)- Field VOD measurements are performed with a continuous VOD recorder (VODR-1) developed by EG&G and BAI. The VODR-1 is a commercial version of the CORRTEX system, which was originally developed by Los Alamos National Laboratory (USA) to measure shock wave propagation in nuclear tests to determine nuclear explosive yield [Refer: ‘Explosive-array performance measurement using TDR’, Creator/Author McKown, T.O. (Los Alamos National Lab., NM (United States)] ; Eilers, D.D. [Raytheon Services Nevada, Las Vegas, NV (United States), http://www.osti.gov/bridge/product.biblio.jsp?osti_id=10139067 ]. In the measurement of explosive performance, a coaxial cable is placed in an explosive filled borehole with the far end of the cable taped on the primer. Operation is similar to that of RADAR where a pulse of radio waves is sent out and an echo or reflected pulse is returned to give ranging information. The VODR-1 uses standard coaxial cables to carry a fast rise time electrical pulse back and forth every 5200 μs. Displacement versus time data can be obtained for the entire length of explosive column and the stemming region to determine explosive VOD, cap timing, shock wave transmission, etc. One of the unique features of the TDR systems is that the electrical pulse is always reflected from the cable end, regardless of whether the coaxial cable is shorted or not. In comparison to the resistance wire system and/or coaxial cables used in the SLIFER system, the TDR does not require the sensing cables to be shorted in order to acquire data.

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Application of Measured VODs - The velocity of detonation (VOD) of an explosive can be used to indicate a number of important characteristics regarding the explosive's performance, under specific field and test conditions. When correctly interpreted, the results can be used to • • • • • • • •

Evaluate the consistency of detonation Confirm whether detonation, deflagrations or failures have taken place Study the influence of primer size on explosive performance. Compare laboratory and field VODs Rate the performance of explosives The VOD of the explosive may be used to calculate the detonation pressure, Explosive Performance Term and to match impedance of rock and explosive. The actual firing time of delays can be noted from VOD signals of multiple holes. This information can be used to decide maximum charge per delay and to control blast vibration. Knowing VOD, it would be easier to recommend distribution of charge column in the hole according to hard/soft bands. Explosives with high velocities of detonation are considered to have a higher shock energy component and would be most suitable for blasting hard competent rock.

Because VOD is a direct measurement of the source function, it can provide valuable information with respect to shock, stress waves, kinetics, ground vibration, airblast, fragmentation and undesirable noxious fumes.

4. Discussion on in-hole VOD of explosives and blast result – Behaviour of in-the-hole-VODs results blast performances in the mines. These are discussed below: 1. VOD of a blasting agent depends heavily on formulation characteristics, rock properties (confinement), blasthole diameter and priming. 2. A VOD record can provide excellent information on the integrity, structural and geomechanical condition of the surrounding rock mass. 3. When an explosive is primed adequately, VOD at hole bottom is high and decreases towards the end of the explosive column. So, VOD of a commercial explosive is not constant. 4. Low VOD at the hole bottom indicates poor priming of blasting agent. Inadequate priming creates low order detonation resulting in poor explosive performance at the hole bottom. It leads to hard toe (difficult digging) and coarse fragmentation. 5. Since performance of primer and blasting agent affect blasting results, selection of explosives should be made properly for each blasting site. VOD measurement carried out at full scale is an excellent and indispensable tool for the selection. 6. VOD measurements conducted during full scale blasting certainly offers important information to be used in blast design optimization and the detection of low order detonation, deflagration and misfire. 7. The difference between the value of VOD measured in laboratory for an unconfined explosive and the value of VOD measured in the hole increases when the behaviour of the detonation of the product is not ideal. This difference increases for the less homogeneous products (ANFO) while the

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difference is not as great for the dynamites and watergels. It has been observed that confined VODs were 1.2 to 1.4 times greater than the corresponding unconfined VOD values. 8. Influence on stemming length on VOD: If in a blast hole explosive charge is adequately confined, the VOD of explosives are not expected to increase by increasing the stemming length. 9. Influence of Blast hole diameter on VOD: In general, the larger the diameter, the higher the velocity of detonation until the explosive's maximum velocity is reached. It has been observed,in case of a gassed heavy ANFO type emulsion explosives, about 10 per cent higher in production holes (311 mm diameter) than for the smaller (140 or 165 mm) diameter holes. 10. Influence of sleep time on VOD: The length of time an explosive remains in the blasthole is called ‘sleep time’. It should not be too long so that an explosive can remain in a blasthole without a change in the chemical composition or its physical properties. Earlier work shows that the VOD of a highly water-resistant bulk explosive (water gel) deteriorated rapidly with increasing sleep time in wet blastholes. 11. Influence of wet blastholes on VOD: All explosives deteriorate progressively in wet holes; the amount of deterioration increases with the severity and period of exposure. The performance of a highly water-resistant explosive (emulsion) when loaded into dry blastholes compared to wet holes indicated that the explosive’s ability to fragment and displace rock in a blast would be significantly reduced when explosive is charged into wet blastholes. 12. Influence of density of an explosive on VOD: The density of an explosive relates to its mass to the volume it occupies in a blasthole. The density of an explosive can affect its performance. ‘Cup density’ is the term used where site mixed system (bulk explosives) loaded, indicate the density of the explosive at normal temperature and pressure. The cup density of the SMS series varies from 0.8 to 0.9 gm/cc. For most bulk explosives, as density increases, velocity of detonation increases and sensitivity decreases. If the density is too high the explosive will not sustain the detonation reaction and the charge will misfire. 13. It has been observed that a single point priming is sufficient to reliably initiate and sustain the steady state VOD of explosives up to 10m long column without any additional booster charge. There is no obvious advantage of bottom or decked priming in respect of VOD values or the release of shock energy of the explosive. Therefore, all cap sensitive explosives can be loaded at the bottom to tackle toe problem in mines. 14. The bulk explosive performance deteriorates with contamination (with drill cutting), particularly, it is observed, when it is contaminated more than two times that what it happens during normal course of charging. References: * Anon (1987) Explosives and Rock Blasting. Atlas Powder Company. * Persson, P.A., Holmberg, R. and Lee, J. (1994) Rock Blasting and Explosives Engineering, CRC.

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* Chiappetta, R.F. 1993. Continuous velocity of detonation measurements in full scale blast environments, Proceedings of the International Congress on Mine Design, Kingston, Ontario,Canada: 759-785. Rotterdam: Balkema. * Chiappetta, R.F. 1998. Blast monitoring instrumentation and analysis techniques, with an emphasis on field applications. FRAGBLASTInternational Journal of Blasting and Fragmentation,Vol.2, No. 1: 79-122. Rotterdam: Balkema. * Esen, S., Bilgin, H.A. & BoBo, T. 2000. Effect of explosive on fragmentation. The 4th Drilling and Blasting Symposium, Ankara, Turkey: 63-72 (in Turkish). * Bergmann, O.R. (1983) Effect of explosive properties, rock type and delays on fragmentation in large model blasts, Proc. 1st Int. Symp. on Rock Fragmentation by Blasting, Lulea, August, pp. 71-78. * Cameron, A and Grouchel, P. (1990) The effects of the quality of the bulk commercial explosives on blast performance. Proc. 3rd Int. Symp. on Rock Fragmentation by Blasting, Brisbane, August 26-31, pp. 335-343. * Louw, M.J., Sarracino, R.S. and Vather, S.M. (1993) Comparison of the theoretical and measured velocities of detonation for selected explosives. Journal of South Afr. Ins. Min. Metal, Vol.93, No.6, June, pp.147-153. * Mainardi, D., Robinson, P. (1997) In hole measurement of velocities of detonation. Explosive Engineering, December, pp. 2-7 * Adhikari, G.R. and A. K. Ghose (1999) Various approaches to blast design for surface mines, Journal of Mines, Metals & Fuels, January-February, pp. 24-30. * Crosby, W. A., Bauer, A.W. and Warkentin, J.P.F. (1991) State-of-the-art explosive VOD measurement system. Proc. 7th Conf. Explosives and Blasting Technique, pp.23-34. * Chiappetta, R.F. (1991) Generating site specific blast designs with state-of-the-art blast monitoring instrumentation and PC based analytical techniques, Proc. 17th Conf. On Explosives and Blasting Technology, Las Vegas, pp. 79-101. * Ladds, C.G., Dvery, O.L., Jordan, D.J.P., Rorke, A.J. and Cunningham, C.V.B (1993) Blast monitoring at ATCOM for improved blasting efficiency, Journal of Explosives Engineering, Sept-Oct., pp. 10-12 & 43-5 * Lee, R (2001) A new way of thinking for priming bulk blasting agents in wet boreholes. Abstracts of the 10th High Tech Seminar organised by Blasting Analysis International, Inc., USA. * Moxon, N.T., Hopkins, M.L. and Danell, R.E (1992). Portable continuous velocity of detonation recorder systems. Explosive Engineering, December, pp. 34-40 * Ouchterlony, F, Nie, S, Nyberg, U and Deng, J (1997) Monitoring of large open cut rounds by VOD, PPV and gas pressure measurements, FRAGBLAST - International Journal of Blasting and Fragmentation, Vol. 1: 3-25

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* Venkatesh, H.S., Adhikari, G.R. and Theresraj, A.I. (1998). In-the-hole detonation velocity measurement - a case study, National Seminar on outlook for Fossil fuels & Non-Metallic Mining and Mineral Based Industries, Chennai, April. -------------------------------------------------------------------------------------------------------------------------Partha Das Sharma’s Bio-data:

Partha Das Sharma (P.D.Sharma) is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur, India (1979) He has very rich experience both in Mining operation and Marketing / Export / offering of Technical Services of Explosives, ANFO, Bulk explosives, Blast designing etc. Visited number of countries in Africa, South East Asia etc. He was associated with number of mining PSUs and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives, Solar Explosives before being a Consultant. He has presented number of technical papers in many of the seminars and journals on varied topics like Overburden side casting by blasting, Blast induced Ground Vibration and its control, Tunnel blasting, Drilling & blasting in metal underground mines, Controlled blasting techniques, Development of Non-primary explosive detonators (NPED), Hot hole blasting, Signature hole blast analysis with Electronic detonator, Acid Mine Drainage (AMD),Mining and Industrial dust etc. TECHNICAL PAPERS PRESENTED IN SEMINARS/JOURNALS: * Overburden Blast Casting with SMS Explosives – A case Study, Special Issue on Explosives & Blasting, Indian Mining & Engineering Journal, November 1998. * Blast Casting with SMS – A case study at Sasti Opencast mine, “Visfotak” ‘98, National Seminar on Explosives, Nagpur (India) * Control of adverse effects of Explosives Blasting in mines by using Shock tubes (Non-electric) Initiation system and its Future challenges; Advances in drilling and blasting techniques- Procc. of DRILL BLAST ’99 – National Seminar on drilling and blasting, Bhubaneswar, (India) January 2000. * Overburden side-casting by blasting – An effective way of reducing operating cost in large opencast mines; Journal of Mines Metals and Fuel, November 2004 (Sp., issue on development in surface mining technology – Calcutta, India). * Overburden side-casting by blasting – Operating Large Opencast Coal Mines in a cost effective way; Procc. st of 1 Asian Mining Congress - Asian Mining: Towards a new resurgence (Vol. I), Seminar organised by MGMI th th at KOLKATA (India) from 16 – 18 January 2006 (pp. 307 – 315). * Non-Primary explosive detonator (NPED) – An eco-friendly initiating system for commercial blasting is the need-of-the-hour for Indian mines; Journal of Mines Metals and Fuel, March 2006.

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* Open pit blasting with in-hole delays and / or pre-splitting of production blast – Measures to control adverse impact of complex vibration arising due to presence of underground workings in the vicinity or in otherwise sensitive areas; Mining Engineers’ Journal, August 2006. * Tunnel blasting – emulsion explosives and proper blast design are the pre- requisite for better efficiency; Journal of Mines Metals and Fuel, September 2005. * Improved Blasting technique is the key to achieve Techno-Economics of high production Underground Metalliferous mines; Indian Mining & Engineering Journal, December 2006. * Enhancement of drilling & blasting efficiency in O/C & U/G mines – Use of modern precision drilling, electronic delay detonator system and other sophisticated equipments with new generation emulsion explosives are the need-of-the-hour; Mining Engineers’ Journal, February 2007. * Improved Blasting with precision drilling patterns in Underground Metalliferous mines; Procc. ‘Golden th th Jubilee Seminar’ on Present status of Mining and future Prospects, organized by MEAI (6 to 8 April 2007) at Hyderabad, India. * Reduction of Ore dilution/Ore loss in underground metalliferous mines, lies on mitigation of blast induced vibration to a great extent; Mining Engineers’ Journal, August 2007. * Controlled Blasting Techniques – Means to mitigate adverse impact of blasting in Open pits, Quarry, Tunnel, UG metal mines and construction workings; Mining Engineers’ Journal, January 2008. * Controlled Blasting Techniques – Means to mitigate adverse impact of blasting; Asian mining: Solutions for nd development and expansion (Vol. II), Procc. of 2 Asian Mining Congress, organized by MGMI at Kolkata th th (India) dt. 17 – 19 January 2008 (pp. 287 – 295). * ‘Electronic detonators – An efficient blast initiation system, Mining Engineers’ Journal, India, October 2008. * ‘Electronic detonators – Results in substantial techno-economic benefits for large mining operations’, Mining Engineers’ Journal, India, February 2009. * Innovative “Signature-Hole Blast Analysis” Technique to predict and control ground vibration in mines; Asian mining – Resurgence of mining in Asia: Prospect and challenges, Vol. II (pp. 211 – 223), Proceedings of rd nd th 3 Asian Mining Congress (22 – 25 January 2010, at Kolkata, India), Organised by MGMI, Kolkata. * Charging and blasting in hot strata condition in opencast coal mines: identifying crucial aspects for effective safety management; Journal of Mines, Metals & Fuels; India; January – February 2010; (pp. 21). * Techniques of controlled blasting for mines, tunnels and construction workings – to mitigate various blast induced adverse effects; Journal of Mines, Metals & Fuels; June 2010 (pp. 152-161). * Factors in designing of blasts, flyrock, industrial explosives used and safe operation of bulk explosives in opencast mines; Journal of Mines, Metals & Fuels; September 2010 (pp. 255 - 261). * Acid Drainage in Mines, African Mining Brief Online Jan - Feb 2011, (http://www.ambriefonline.com/janfeb11%20guest.html), Acid Mine Drainage (AMD) Author’s Published Books: 1. "Acid mine drainage (AMD) and It's control", Lambert Academic Publishing, Germany, (ISBN 978-3-83835522-1). 2. “Mining and Blasting Techniques”, LAP Lambert Academic Publishing, Germany, (ISBN 978-3-8383-7439-0). 3. “Mining Operations”, LAP Lambert Academic Publishing, Germany, (ISBN: 978-3-8383-8172-5).

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4. “Keeping World Environment Safer and Greener”, LAP Lambert Academic Publishing, Germany. ISBN: 978-38383-8149-7. 5. “Man And Environment”, LAP Lambert Academic Publishing, Germany. ISBN: 978-3-8383-8338-5. 6. “ENVIRONMENT AND POLLUTION”, LAP Lambert Academic Publishing, Germany. ISBN: 978-3-8383-8651-5 Currently, author has following useful blogs on Web: • http://miningandblasting.wordpress.com/ http://saferenvironment.wordpress.com • • http://www.environmentengineering.blogspot.com • www.coalandfuel.blogspot.com Author can be contacted at E-mail: [email protected], [email protected], ------------------------------------------------------------------------------------------------------------------Disclaimer: Views expressed in the article are solely of the author’s own and do not necessarily belong to any of the Company. ***

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Appraisal of Explosive Performance by Measurement of Velocity of Detonation (Vod) in Mines - Discussion

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