Controlled blasting is becoming the order of the day in modern blasting technology, as the mining industry looks for benefits like vibration containment near sensitive structures.By: Tony Rorke, technical director, BME When blasting in an opencast mining environment, for instance, pit wall failure can be triggered by high vibration levels from blasting – with potentially catastrophic consequences. There are also cases where blasting operations on civils sites may need to be conducted within just a few metres of buildings or a major pipeline. This is where electronic delay detonators really play an important role, as the timing flexibility supports the detonation of small multiple charges in each blasthole to keep vibration levels down. BME has developed its AXXIS electronic delay detonators, which have a very high accuracy, and are an alternative to the traditional shocktube detonator systems. The flexible AXXIS Digital Initiation System allows any firing time up to 10 seconds at millisecond intervals to be programmed and BME is thus able to design complex firing sequences in a blast for achieving predicable and repeatable blast results. Because of the detonator accuracy, vibration from the blast can be predicted accurately and reliably. BME’s new blast design software, BlastMap III, allows complex timing designs and analysis of the results for each blast. An example of how the system ensures a controlled blast was seen recently for a new wing of a hospital in Nelspruit; the blast took place within 50 m of the existing building where patients could not be evacuated. The blast therefore had to be well controlled to ensure that vibration levels were low and that there was no fly rock towards the hospital. In large open-pits, a wall failure can mean the difference between continued operations and permanent closure of the mine site. The use of accurate electronic delay detonators helps to reduce the risk of this happening. Using the AXXIS system, it is possible to select the firing time of each blast hole in a blast to create quiet vibration zones in certain directions. By engineering the correct timing, these quiet zones can be directed towards critical structures like mine pit walls while higher vibration zones can be directed away to less sensitive areas. To maximise the full productivity benefits of using electronic delay detonators, it is vital that the blast timing design is right. There is no real value in using such detonators while continuing with a fixed timing design that does not change to address variations in rock type or blast geometry.
BlastMap III software and AXXIS provide a powerful system for getting the best blast results and making mining as efficient as possible. In addition to vibration control, the system’s benefits include finer fragmentation, dilution control in ore blasts, heave control (to achieve desired muck pile shapes) and specialised blasting scenarios.Blast preparation is also significantly faster. Logging and programming of detonators can take place as soon as the detonators are in the hole; operations can be undertaken while holes are being charged, during the stemming period or even before the detonators have been connected on the surface lines. Previously, an entire network had to be connected before programming and logging could take place. The size of electronically detonated blasts undertaken by BME has been steadily growing, reaching over 3 000 detonations in a single blast. This opens the door to valuable cost savings and higher productivity, as larger blasts mean that they can be conducted less frequently; this saves on the need to ‘walk out’ and ‘walk in’ draglines, drills and other large equipment at opencast sites. Another major plus factor is safety. Using the AXXIS system, we can initiate an event remotely up to 3 km from the blast. The system is safe because it relies on encrypted communication protocols between the blasting boxes and detonators, and employs passive communication between the intelligent connector and logger to establish connector ID and firing time. The encrypted two-way communication process allows the blaster to confirm that all the detonators are functional before pressing the fire buttons, removing any concerns about unexpected misfires. AXXIS was designed around five principles: safety, ease of operation, robustness, reliability and accuracy. It has proved its worth in these respects, even in South Africa’s harsh mining conditions. Electronic delay detonators are the future in blast timing and will become the main method for initiating blast holes in the near future. Although they may be slightly more expensive than shock tube equivalents, the overall cost is minor compared to the huge benefits that can be obtained by using electronic delay detonators.