Sedat Esen
Esen Mining Consulting (EMC), Sydney, Australia
Abstract
Flyrock is considered to be one of the main environmental effects in Drill and Blast operations at quarries affecting the safety and sustainability (Licence-to-Operate) of the sites. Damage to life, mining equipment and buildings can be severe if flyrock becomes a problem. Flyrock awareness should be built into the Drill and Blast process, right from the beginning of the design stage to the firing stage. With good blasting practice and well-supervised charging of blastholes, the chances of excessive flyrock are negligible. This paper explores the causes and consequences of flyrock, particularly in bench blasting, and emphasizes the importance of proper blast design, QA/QC, stemming practices, and burden control in mitigating its occurrence.
A practical approach to determining safe blast exclusion zones is presented through the application of the Richards and Moore (2004) flyrock model, calibrated using site-specific data. The model’s effectiveness is demonstrated in a detailed case study, incorporating field observations, video analysis, and sensitivity analysis to recommend exclusion distances for both personnel and equipment. The study concludes that personnel should be excluded from within 300 meters of the blast site to account for geological and operational variability, while a minimum distance of 150 meters is advised for equipment and infrastructure.
The paper also provides practical recommendations for flyrock management at quarries, including pre-blast planning, site-specific modelling by external specialists, use of appropriate stemming materials, and implementation of pit-specific exclusion zone protocols. These findings contribute to safer and more efficient blasting practices and support the broader goals of mine-to-mill optimization in the aggregates industry.
1. Introduction
Drill and Blast operations are widely recognized as the initial stage of the comminution process and play a critical role in influencing downstream activities such as loading, hauling, crushing, and grinding. Fragmentation and loader productivity are the main KPIs whilst minimizing the environmental effects (ground vibration, airblast, flyrock, dust and fume) of blasting has always been a challenge to the industry affecting the Licence-to-Operate of the quarries.
For several decades, the quarry industry has placed increasing focus on flyrock, a key safety and operational concern in blasting.
Although fragmentation and energy efficiency dominate most mine-to-mill discussions, uncontrolled flyrock poses risks to personnel, equipment, and operational continuity. Literature addressing flyrock control has emerged alongside broader fragmentation studies, emphasizing the need for improved blast design and control methods to manage its effects while maintaining optimal fragmentation and loader productivity outcomes.
This paper presents the tools and methodologies used in a drill and blast study aimed at improving fragmentation and controlling flyrock. Through various case studies, the application of these techniques is demonstrated, reinforcing the critical link between safe blasting practices, optimal fragmentation, and downstream efficiency.
2. Flyrock Control and Determining The Blast Exclusion Zone
2.1 Flyrock
Flyrock can be defined as the rock fragments which were projected beyond the clearance zone. The clearance zone is the zone around a blast beyond which there should be no risk to personnel from flying rock fragments, and beyond which the blaster must evacuate all personnel prior to firing the blast (Stiehr, 2011).
Flyrock is one of the most blast-related incidents seen at mine sites. Some Australian examples are listed below:
• During a quarry blast, flyrock was projected more than 500 metres onto the Pacific Highway. A rock of approximately 100mm diameter was also projected onto a nearby property where it caused damage to a shed and parked vehicle;
• A rock was thrown 1300 metres from a blast consisting of 89 mm diameter blastholes;
• Flyrock resulting from a trim blast at a gold mine caused significant damage to the four drills and one excavator which parked less than 150m from the blast;
• At a gold mine, one of the blastholes caused flyrock hitting and breaking the window on a drill rig which was located 181m from the blast;
• A quarry blast had thrown material a maximum of 170 meters and striking the main office which was 150 meters from the blast and caused damage to buildings;
• A shotfirer was struck on the right side of his face by flyrock after a toe was blasted at a quarry. He was videoing the shot 75 metres from the blast area whilst sheltering behind a steel hopper with another person.
2.2 Root Causes of Flyrock
Flyrock can be generated from a bench blast (either free-faced or buffered) b)oversize blasting. This paper deals with the flyrock generated by the bench blasts only.
The causes of flyrock in a bench blast are:
• Design faults: Inappropriate face burden and stemming length, inappropriate stemming material selection (e.g.drill cuttings), poor choices of powder factor and initiation sequence.
• Deviations in implementation: “as-drilled” face burden and final stemming length being less than design; blasthole deviation (not measured and/or incorrect loading for holes with significant deviations); explosive run-away into cavity (failing to detect this issue).
• Unforeseen geological conditions: Cavities, weak seams, fault zone and broken zone in the stem zone or in the face burden area, etc.
2.3 Determining the Blast Exclusion Zone – A Case Study
There are two main flyrock models (Richards and Moore 2004, McKenzie 2009) which have been widely used in the industry. In this paper, a case study was presented using Richards and Moore’s model. According to Richards and Moore (2004), there are three common sources of flyrock (Figure 10):
• The face of the blast, in which flyrock is generated through a ‘face burst’;
• The bench top, through a phenomenon known as ‘cratering’; and
• The stemming zone, where flyrock is generated through stemming ejection or ‘rifling’.
Face burst occurs when front row burdens are insufficient to contain the explosive energy. This mechanism can produce flyrock in front of the blast area. Stemming ejection (or rifling) occurs when stemming material is of poor quality or where the hole is not fully stemmed (e.g. hang ups). This mechanism can produce flyrock behind the blast area, depending on the angle of the blast hole.
Note that cratering calculations are removed from original model as cratering calculations are not representative of the blasts at this quarry. Cratering is not valid as stemming length is well above 20 times hole diameter and should not cause cratering effect.
The equations shown in Figure 10 provide a tool which can be used to predict the maximum flyrock distance likely to result from a blast, given the specific parameters of that blast. Based on this prediction, a safety factor is applied to give a minimum blast clearance distance. The safety factor applied for buildings and equipment is 2.0. For humans the safety factor is 4.0.
The site constant, K, as shown in Figure 10 accounts for the blasting response of the rock mass at a specific site. K takes a value between 13 and 27 depending on the observed blast outcomes at that site. The model can be “tuned” to an individual site’s blasting conditions based on a history of measured blast outcomes and maximum rock movement.
A few survey points were marked on the ground around the blast to determine the horizontal distance of the flyrock. Videos of the blast and other blasts were analysed. Maximum horizontal flyrock distance is determined as approximately 45m and the maximum vertical distance is approximately 25m.
Base case blast had 89mm hole diameter, 10.6m hole length, 2.2m stemming length and 4m face burden. The model constant is calibrated (K=21) to match the observed collar projection and maximum flyrock range of 45m. Base case stemming lengths of 2.2m at 89mm pattern indicate a Scaled Depth of Burial (see Figure 11 for explanation of this terminology) of 1.41 using pumped emulsion with density of 1.20g/cm3 which shouldn’t cause significant flyrock distances provided that stemming collar is not in a broken ground and stemming material is appropriate (free flowing ensuring bridging does not occur). Face holes are designed with minimum of 3.2m face burden.
The calibrated model determines the blast exclusion zone for personnel as 193m. It assumes that a) minimum of 2.2m stemming is applied; b) bridging does not occur and appropriate crushed aggregate fills the stemming column c) minimum face burden is 4m.
Figure 10. Flyrock model (Richards and Moore, 2004)
A sensitivity analysis (see Appendix 1) was carried out by varying the K constant, stemming length and face burden to account for the natural variation that may exist in geology, face burden and loading. It is shown that medium (10%) variation in geology (K constant), face burden and loading (stemming length) increases the personnel exclusion zone distance to 277m. Therefore, it is recommended to adopt 300m as the exclusion zone distance for personnel. This would ensure that natural variation in geology, face burden and loading is accounted for the selection of the distance.
Based on the safety factor of 2 for the equipment, the nominal exclusion zone distance is calculated as 138m considering the variation (medium: 10%) in the loading, face burden and stemming. Therefore, it is suggested to use a minimum of 150m of clearance distance for the equipment and infrastructure around the quarry.
Figure 11. Scaled Depth of Burial (Chiappetta, 2008)
2.4 Some Suggestions for Managing the Flyrock Issues at Quarries
All employees should be removed to a safe location away from the blast area during blasting. Blast exclusion zone calculations should be carried by an external consultant.
All entrances to the blast area should be securely guarded to prevent inadvertent entry of employees or visitors. Pit plans showing the blast exclusions zones for equipment and personnel as well as guard positions should be prepared for each blast. Good communication is a key to a safe blasting operation.
Proper blast design and an effective blasting plan will reduce the chances for flyrock. Most flyrock incidents occur because a) the burden has not been checked b)appropriate stemming material and/or stemming length were not used. Laser profiling and boretracking are useful tools for checking the face burdens and adopt the correct explosive loading process (Appendix 2). Risk assessment must be completed for each blast to manage the environmental effects of blasting including flyrock.
Crushed aggregate with size of 1/10th of the hole diameter should be used. Stemming length calculations should be based on the scaled depth of burial given by Chiappetta et al. (1983) or Stiehr (2011).
In this paper, the author recommends a flyrock modelling and blast exclusion zone calculations to be carried out by an external blast consultant. In the absence of such reports, I recommend using below guidelines at quarries until a report is prepared and made available for use:
• For non-blast personnel 800m in front of the shot and 400m to the side and rear of the shot.
• Blast personnel should be positioned greater than 400 metres from the shot and not positioned in the direct line of fire and within retreat distance of a protective structure (i.e. fixed plant or blasting bell).
• No mobile plant is to be within 300 metres of the initiation point without signed site manager’s approval.
• Where a blast is to occur within 100 metres of fixed plant an appropriate blasting specialist should be engaged to design and control the loading and firing process.
3. Conclusions
Effective control strategies for flyrock are presented in this paper. Some of the key conclusions are as follows:
• Safe blasting requires that rock throw be controlled to prevent danger from flyrock. Flyrock incidents have occurred in the past and investigations of these incidents commonly conclude that the flyrock was due to over charging and/or under confinement of the explosive charge.
• A practical flyrock model was presented in order to determine the safe blast exclusion zone for the mining equipment and personnel. Some key guidelines were suggested to minimise the occurrence of the flyrock.
• An external specialist should be engaged to carry out the blast exclusion zone calculations.
• Risk assessment should be conducted for each blast to consider the flyrock risk. Face profiling and boretracking must be considered as tools to minimize the risk. A good Drill&Blast design software will certainly help mitigate the risk by including the QA/QC data. Exclusion zone map should be prepared prior to the blast showing the exclusion zones for both quarry personnel and mining equipment.
Appendix 1. Sensitivity Analysis – Exclusion Zone Distance
Appendix 2. Face profiling and boretracking
References
Adel, G, Kojovic, T, Thornton, D. 2006. Mine-to-Mill Optimization of Aggregate Production. JKMRC Semi-annual Report No:4. 86 pages.
Benzer, H, 2005. Modeling and simulation of a fully air swept ball mill in a raw material grinding circuit. Powder Technology 150:145– 154.
Chavez, R, Leclercq, F, McClure, R, 2007. Applying up-to-date Blasting Technology and Mine to Mill Concept in Quarries. International Society of Explosives Engineers –
Annual Conference on Explosives and Blasting Technique. 12 pages.
Chiappetta, R., Bauer, A., Dailey, P. and Burchell, S., 1983. “The Use of High-Speed
Motion Picture Photography in Blast Evaluation and Design”, Proceedings of the Ninth
Annual Conference on Explosives and Blasting Technique. Dallas, TX. International Society of Explosives Engineers, pp 258-309.
Cox, N, Cotton P, 1995. Improvements in quarry blasting cost effectiveness.
International Society of Explosives Engineers – Annual Conference on Explosives and Blasting Technique. pp 78-92.
Cunningham, C V B, 1983. The Kuz-Ram model for prediction of fragmentation from blasting. Proceedings of the first international symposium on rock fragmentation by blasting, Lulea, Sweden, 439-453.
Cunningham, C V B, 1987. Fragmentation estimations and the Kuz-Ram model – Four years on. Proceedings of the second international symposium on rock fragmentation by blasting, Keystone, Colorado, 475-487.
Cunningham, C.V.B. 2005. The Kuz-Ram fragmentation model—20 years on. In R.
Holmberg (ed.), Proc. 3rd EFEE World Conf. on Explosives and Blasting, Brighton, UK, 13–16 September, pp. 201–210. Reading, UK: European Federation of Explosives Engineers.
Dance, A., Valery Jnr., W., Jankovic, A., La Rosa, D. and Esen, S. 2006.
Higher Productivity Through Cooperative Effort: A Method Of Revealing And Correcting Hidden Operating Inefficiencies. SAG2006 – HPGR, Geometallurgy, Testing.
International Conference on Autogenous and Semiautogenous Grinding Technology, Volume 4, 375 – 390, Vancouver, Canada. Djordjevic, N, 1999. Two-component of blast fragmentation.
Proceedings of 6th international symposium of rock fragmentation by blasting – FRAGBLAST 6, Johannesburg, South Africa.
South African Institute of Mining and Metallurgy, 213-219. Elliott, R, Ethier, R, Levaque J, 1999. Lafarge Exshaw finer fragmentation study.
International Society of Explosives Engineers – Annual Conference on Explosives and Blasting Technique. pp 333-353.
Eloranta, J. 1995. Selection of powder factor in large diameter blastholes, EXPLO 95 Conference, AusIMM, Brisbane, September, PP 25-28.
Esen, S., LaRosa, D., Dance, A., Valery, W. and Jankovic, A. 2007. Integration and optimisation of Blasting and Comminution Processes. EXPLO 2007. Australia. pp 95-103.
Esen S. 2013. Fragmentation Modelling and the Effects of ROM Fragmentation on Comminution Circuits. 23rd International Mining Congress & Exhibition of Turkey. pp 252-260.
Fujimoto, S, 1993. Reducing specific power usage in cement plants, World Cem. 7 : 25– 35.
Kanchibotla S.S., Valery W. and Morrell, S. 1998. Modelling fines in blast fragmentation
