Blast Performance Monitoring
The use of modern technology to measure blast performance
by Rob Farnfield
The last decade has seen many developments in the field of blast performance monitoring. The Technical Services Department of Exchem Explosives has access to a wide range of such technology, either developed in association with parent company EPC Groupe or bought in from specialist suppliers.
This article describes this technology and gives examples of its uses.
Quarries in the UK already undertake a significant amount of surveying for each blast carried out. The data collected includes 3-D face profiles, hole surveys and in-hole surveys. Few operators realize the potential of applying the same technologies and equipment as an aid to measuring blast performance.
It is a relatively simple task to carry out a 3-D survey of the blasted rock and the standard profiling software can then be used to produce rock pile profiles. These profiles can be used to compare the performance of explosives or the impact of the burden value, initiation pattern, geology etc.
Figure 1 shows two rock pile profiles generated using this technique. The profiles shown are for two blasts using the same explosive but different burdens and, therefore, blast ratios.
If the operation has a good survey control system it is also possible to profile the face after the blast has been mucked out to give additional information, such as:
- actual blasted volume/tonnage
- degree of back-break
- bulkage of the rock pile (can give an idea of digability).
There are currently a number of systems available to measure fragmentation and these are almost exclusively based on photographs or videos of the broken rock pile. Exchem Explosives employ the highly respected WipFrag software to measure the fragmentation in the post-blast rock pile. The associated WipJoint software can also be used to assess the natural in-situ fragmentation visible in the quarry face.
Figure 2 shows a typical image suitable for fragmentation assessment. The image includes two balls of known size to allow for both scaling and correction of the distortion induced by the angle of the rock pile.
In reality, a series of images is processed to give an overall fragmentation assessment for the blast.
The greatest difficulty with any fragmentation assessment system is to develop a ‘fair’ sampling strategy. Factors to be considered include when and where to take pictures as well as how many images to analyse. Difficulties often arise relating to the safety of obtaining such images. In general, the more pictures over the excavation period the better the estimate.
Other options for fragmentation assessment include using the same technology to automatically process images taken of the load in a dumptruck as it approaches the primary crusher.
It is becoming common for operators to video each blast and such video footage can yield valuable information relating to blast performance.
There are also a number of software packages available to process digital video to produce information on the velocity of the blasted faces and data on the firing times of surface detonators.
Figure 3 shows part of the process of analysis of a digital video recording of a blast. The software illustrated is tracing the location of markers to give face velocities.
The results of such analysis can be used to assess the effect of burden and explosive type on face velocity and can help in the optimization of inter-row delay periods.
Technology is also available to monitor face velocities using radar techniques.
It is in the field of in-hole monitoring that the greatest advances have been made in recent years. These developments mean that suppliers can now quote performance parameters for explosives based on actual, realistic, field measurements rather than theoretical values.
Velocity of detonation (VOD)
VOD is related to the shattering ability of the explosive and also the detonation pressure. Exchem use equipment from MREL in Canada that measures the length of cable by means of changing resistance. Such equipment is capable of taking measurements at a rate of several million times a second, giving excellent resolution.
Figure 4 shows a typical VOD recording from a blasthole with a packaged explosive base charge and an ANFO column charge.
Data captured with the VOD-monitoring technology outlined above can also be processed to obtain information on initiation times. It is possible to monitor the initiation time of each in-hole detonator and surface connector in a blast, albeit with a good deal of effort and cost. Exchem’s biggest such exercise involved recording the entire detonator firing times in a triple-row 39-hole blast.
Figure 5 shows a VOD recording from a double-decked hole fired with highly accurate electronic detonators.
Relatively cheap in-hole sensors are now available to allow the actual monitoring of explosive detonation pressures. The biggest technological challenge here is the very high data sampling rates required to capture reliable data under field conditions. Dual-channel, battery-powered, digital oscilloscopes are now available to accomplish this task.
Similar technology can also be used to detect the pressure waves travelling from one blasthole to another or through the stemming material. For this application lower data sampling rates are required, and Exchem employ the MREL Microtrap VOD unit for this task. This unit has been upgraded with an additional four voltage channels, a combination which allows for simultaneous monitoring of in-hole pressure (four locations), initiation times and velocity of detonation.
Figure 6 shows a pressure recording made 0.5m into the stemming area from the detonation of a blasthole.
The recording of such data is currently being used to investigate the factors controlling the amplitude of the shock wave as it arrives at other blastholes.
Equipment and techniques have been outlined that allow for the monitoring of blast performance under quarry conditions. Exchem use this technology to test new products, for research and development, and as part of their quality-control system. There is also great potential for the use of this technology by quarry operators or drilling and blasting contractors.
Exchem Explosives, Rough Close Works, PO Box 4, Alfreton, Derbys DE55 7AB; tel: (01773) 832253; fax: (01773) 520723.