Signature Hole Blast Vibration Control

‘Signature Hole’ Blast Analysis, using Accurate Delay Timing Electronic Detonators *** An Innovative Technique to tackle Blast Induced Ground Vibration in Mines ***

By: Partha Das Sharma (E.mail: [email protected])

-------------------------------------------------------------------------Author: Partha Das Sharma (E.mail: [email protected])

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Control of Blast Induced Ground Vibration by ‘Signature Hole’ Blast Analysis Technique, using Accurate Delay Timing Electronic Detonators: By: Partha Das Sharma (E.mail: [email protected], [email protected]) 1. Introduction - There are many variables and site constants involved that collectively result in the formation of a complex vibration waveform. Providing a well designed blast plan and the application of proper field controls during all steps of the drilling and blasting operation help to minimize the adverse impacts of ground vibrations. The blast design should consider the proper blast-hole diameter and pattern that reflect the efficient utilization and distribution of the explosive energy loaded into the blast hole. Blast design also provides the appropriate amount of time, i.e., delay between adjacent holes in a blast to provide the explosive the optimum level of energy confinement. The parameters having the greatest effect on the composition of the ground vibration waveform are: (i) Geology between the blast site and the monitoring location and (ii) Accurate timing between blast holes in a detonation sequence. Thus, it has been observed, incorporating accurate delay timings with the help of electronic delay detonators substantial reduction in blast induced vibration and airblast occurrences take place. 2. Structural response to blast-induced ground vibration - Structural response to blastinduced ground vibration is a phenomenon that has been analyzed for many years. It is becoming increasingly important, from an environmental viewpoint, to minimize vibrations induced in urban dwellings by blasting. Research developed by the USBM, universities, and others over the last more than two decades in the blasting industry, has concluded that a residential structure’s level of response to blast induced ground vibration is dependent on both the peak particle velocity and the frequency of the waveform. The frequency is the number of oscillations that the ground particles vibrate per second as a blast vibration wave passes by the structure’s location. Researchers have shown that, above ground structures resonate whenever they are excited by a vibration containing adequate energy matching the fundamental frequency of the structure. The value of this frequency is mainly dependent upon the mass, height and stiffness of the structure. The maximum response of a house to blast induced ground vibration occurs whenever the frequency of the ground vibration matches the natural resonant frequency of the house. Likewise, if there is little or no energy at the resonant frequency of the structure, the structural response to the vibration will be negligible. When a structure is given an initial disturbance, it will vibrate at one or more of its natural frequencies, which are controlled by its mass and stiffness distribution. The highest frequencies of the system are always the multitude of the fundamental frequency. These characteristics in a structure are the controlling factor in response to a dynamic load such as ground vibration induced by a blast. There are two methods that can be used to calculate the dynamic properties of a structure. One of these methods is by theory (Computer Modal Analysis) and the other is by experiment (Frequency Response Function). ‘Computer Modal Analysis’ is done by entering into a computer the physical dimensions and the geometric and physical material properties of a structure. By adding vibration induced from traffic, blasting, construction or natural phenomena the model's response can be calculated. Whereas, ‘Frequency Response Function (FRF)’ is an experimental technique used to calculate the dynamic properties of a structure. This technique is


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widely used in different industries to solve many types of dynamic problems, such as structural failure, noise and vibration. Generally, in order to calculate the FRF of a system, the system needs to be excited with some kind of a signal. This signal is called an input signal. The input signal would be a ground vibration at the foundation of the building and the structural response to the ground vibration is the output signal. To calculate the FRF of a structure, ground vibration is generated and measured simultaneously with structural response. To do this, vibration sensors are placed on the structure and the ground. Ground vibration can be generated by detonating a small amount of explosive buried in the ground near the structure. 3. ‘Signature-Hole’ Blast Analysis for Vibration Control by using Accurate Delay Timing Electronic Detonator System – A method of controlling blast vibrations other than by modifying the scaled distance came into use some time ago. The crucial point of the methodology is the use of a pilot-blast signal which takes account of the seismic properties of all complex geology between the blast and the target locations. Therefore, it does not require any geological model or assumption. The analysis illustrates how the delay interval between blast-holes can be chosen to control and minimize the vibration energy within the structural response band of most houses. Research studies had indicated that blast vibration could be simulated by detonating a “Signature Hole” with the vibration monitored at critical locations, and then using a computer to superpose the waveforms with varying delays (Fig – 1). By choosing delay times (∆t) that create ‘destructive interference’ at frequencies that are favored by the local geology, the “ringing” vibration that excites structural elements in structures, houses and annoys neighbors could be reduced. In this method, accurate delay times are crucial to effective vibration control, scatter in the firing times limited the method severely. Electronic detonators have scatter less than a millisecond. In light of all these, researchers have started finding both limitations and new potential of this new technique of controlling blast vibration. Briefly speaking, in this system of analysis, a single hole test blast (signature hole) is detonated at the blast site. Blast sequence is simply defined as a series of single hole detonations that are separated by a given amount of time. It is the relationship between this time and the geology of the site that has the most effect on the amplitude and frequency composition of the ground vibration wave. This relationship between timing and geology has led to the development of several sophisticated computer programs to predict and modify blast induced ground vibrations. These programs process a single hole blast ground vibration signature at a given production blast location, and through thousands of mathematical iterations predict and simulate the synthetic waveform, its amplitude and frequency composition for any given delay timing between adjacent holes in a row and between consecutive rows in a blast. The “Fourier Frequency Spectrum Analysis” of this blast indicates about ‘dominant frequency characteristic’ at the recording sites. The computer analysis determine the application of delay timing between holes, between the rows and between the decks which would produce the most favorable blast induced vibrations for structures and urban dwellings. In other words, “Signature Hole Analysis” is a modeling technique is to help control adverse effects of blast induced vibrations. The process involves controlling the frequency content by adjusting delay times within a blast containing several explosive charges. The risk to adjacent structures is thereby mitigated. Thus, with the growing adoption rate of electronic initiation systems as a tool to control nuisance of vibrations, the modeling techniques are becoming more popular. The introduction of a high accuracy electronic detonator into the commercial explosives market has had many positive effects in the area of predicting and controlling blast induced ground vibrations. It has been observed that without the implementation of electronic detonators the above software techniques are very ineffective. It has also been reported that, the timing


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designs by above technique with electronic detonators produces blast with a distinctively shorted duration. This coupled with the higher dominant frequency content of the vibrations reduces the amplitude of structural response. Instantel has recently developed and launched a ‘Signature Hole Analysis’ software tool, which allows users to simulate a large number of charge delay times very quickly. The software, in fact, is a modeling technique used to help control blast induced vibrations.

Fig – 1 4. Advantages of the technique, ‘Signature Hole Blast Analysis’, for Vibration Control This technique provides optimum electronic timing while maintaining high level of production with efficiency by raising quantity of explosives per delay (kg/delay) and provide overall structural safety of blast surroundings. Moreover, blast with shorted duration results in mitigating effects of blast induced vibration. Therefore, as post-blast vibrations are reduced by raising frequencies, much larger blasts can be undertaken with better operational performance, without compromising stringent safety standards of environment. Thus, Signature Hole Analysis software


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tool available can be used to help optimize and improve overall operational efficiency. It has also been observed that this vibration control method is feasible for underground mining ring blasts as well. Thus, the occurrence of electronic detonators and development of computer sciences brought a new expectation in controlling the vibration level and, hence, carry out bigger blast operations both in surface and underground operations. Today electronic detonation has transformed production efficiency in mines by allowing larger blast blasts to handle, improving blasting cycle efficiency with enhanced safety and lesser ore dilution. 5. Conclusion - Since Digital Detonator system control vibrations efficiently, there are much less concern now about the impact of blasts affecting communities surrounding the mines. References: 1. Dowding, Charles H.,1985,”Blast Vibration Monitoring and Control”, Northwestern University , Evanston,IL. 2. Nicholls., H.R., Jhonson., C. F. and Duvall., W.I., - ‘Blasting Vibrations and their effects on Structures’, USBM Bull. 656. (1971). 3. Sharma, P.D.; - ‘Electronic detonators – An efficient blast initiation system’, Mining Engineers’ Journal, India, October 2008. 4. Watson., John. T; – ‘Developments with Electronic Detonators’, Proc., of Int. Conf. On Expl. & Blasting Tech, ISEE (2002). 5. Siskind ,D. E., Stagg, M. S. ,Kopp, J.W., Dowding, C.H.,1980, - ‘Structure Response and Damage Produced by Ground Vibration from Surface Mine Blasting’; Bureau of Mines RI 8507, OSM Dept. of Interior Washington, DC. 6. Medearis, K., 1976, “ Development of Rational Damage Criteria for Low Rise Structures Subjected to Blasting Vibrations”, Report to the National Crushed Stone Association, Washington, DC. 7. Doglus,A. Anderson.; - ‘Signature hole blast vibration control – Twenty years hence and beyond’, The journal of Explosives Engineers, September / October 2008 (pp. 8 to 12). 8. Christopberson, Pappilon, - ‘Vibration reduction through production – signature hole blasting’, The journal of Explosives Engineers, September / October 2008 (pp. 16 to 20). 9. Instantel: http://www.instantel.com/newsletters/jfr/jfr_q2_2008.pdf 10. Electronic Detonator for Commercial Blasting: http://www.docstoc.com/docs/5339119/Electronic-Detonator-for-commercial-blasting 11. http://knol.google.com/k/partha-das-sharma/programmable-digital-detonatorsystem/ oml631csgjs7/28


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Author’s Bio-data: Partha Das Sharma is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur, India (1979) and was associated with number of mining and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives etc., before joining the present organization, Solar Group of Explosives Industries at Nagpur (India), few years ago. Author 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 metalliferous underground mines, Controlled blasting techniques, Development of Non-primary explosive detonators (NPED), Signature hole blast analysis with Electronic detonator etc. 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], ***


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Signature Hole Blast Vibration Control

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