Misfires in blasting, the failure of an explosive charge to detonate as planned, pose significant safety risks and operational challenges. Understanding their causes and implementing strict safety protocols for their management are crucial in industries like mining, quarrying, and construction.
Common Causes of Misfires
Several factors, often interconnected, can lead to a blasting misfire. These can be broadly categorized as issues related to the explosives themselves, the initiation system, and environmental or human factors.
Explosive and initiation system failures
- Defective products: manufacturing defects in explosives or detonators can lead to a failure to initiate or propagate the detonation. This can include issues with the chemical composition, density, or physical integrity of the explosive.
- Improper storage and handling: explosives are sensitive to environmental conditions. Exposure to moisture, extreme temperatures, or improper handling can degrade their performance and lead to misfires. The age of the explosives can also be a factor, as their chemical properties can change over time.
- Inadequate priming: the primer, which is the initial and most sensitive part of the explosive charge, must be correctly positioned and have sufficient energy to initiate the main charge. Improper priming is a common cause of misfires.
- Initiation system malfunctions: the entire system used to trigger the detonation, from the blasting machine to the detonators and connecting wires or tubes, must function flawlessly. Common issues include damaged wire tubes, poor connections, incorrect firing sequence, insufficient firing current.
Environmental and human factors
- Water ingress: water entering the borehole can desensitize the explosives or short-circuit electrical connections, leading to a misfire. This is a particular concern in wet or rainy conditions.
- Ground conditions: the geology of the blast area can influence the effectiveness of the blast. Very soft or fractured ground may not provide enough confinement for the explosive energy to build up sufficiently for full detonation.
- Human error: mistakes during the loading and connection of the blast, such as incorrect wiring, improper placement of the primer, or failure to follow the blast design, are significant contributors to misfires. Inadequate training and supervision can increase the likelihood of such errors.
- Cut-offs: this occurs when the detonation of one charge severs the initiation line of an adjacent charge before it has had a chance to detonate.
Safe Management of Misfires
The safe handling of a misfire is a critical and high-risk procedure that requires strict adherence to established protocols. The primary goal is to safely neutralize the unexploded charge without causing an accidental detonation.
Immediate actions
- Secure the area: The first and most crucial step is to immediately secure the blast area and prevent any unauthorized personnel from entering. The blast guard should remain in place.
- Waiting period: A mandatory waiting period must be observed before anyone approaches the blast area. This allows for the possibility of a delayed detonation (a hangfire). The duration of this waiting period varies depending on the type of initiation system used (e.g., 30 minutes for non-electric systems, and a shorter time for electric systems after the firing lines have been disconnected and short-circuited).
- Visual inspection: After the waiting period, a qualified and authorized person, typically the blaster in charge, will carefully inspect the blast site from a safe distance to identify any signs of a misfire, such as undetonated charges, intact detonator wires, or areas where the rock has not been properly fragmented.
Misfire remediation procedures
If a misfire is confirmed, several methods can be used to deal with the unexploded charge. The choice of method depends on the specific circumstances of the misfire.
- Re-firing: if the initiation lines are accessible and intact, the simplest and often safest method is to attempt to re-fire the charge. This involves reconnecting the firing lines and attempting to initiate the detonator again.
- Washing out: for some types of explosives, it may be possible to wash them out of the borehole using water or compressed air. This is a delicate operation that must be performed with non-sparking tools and equipment.
- Drilling a relief hole: a new hole is drilled parallel to the misfired hole, at a safe distance. This new hole is then loaded with a small charge and detonated. The shockwave and ground movement from this secondary blast are intended to detonate the misfired charge sympathetically or to break up the rock around it, allowing for safe excavation.
- Manual removal: in some rare and carefully assessed situations, it may be necessary to manually remove the stemming material (the inert material packed in the top of the borehole) to access and remove the unexploded charge. This is an extremely hazardous procedure and should only be undertaken by highly experienced personnel with specialized equipment.
Throughout the entire process of managing a misfire, meticulous documentation and communication are essential. All personnel involved must be fully aware of the situation and the procedures being followed. Following the successful remediation of a misfire, a thorough investigation should be conducted to determine the cause and implement corrective actions to prevent future occurrences.
Video credit to Ing. A. Sadiq Seidu,PE-GHIE.
