Introduction
Overbreak is a term commonly encountered in rock blasting operations. It occurs when the extension of rock breakage goes beyond the designed boundaries of excavation. This phenomenon is a critical concern in mining, tunnelling, and construction, as it can lead to increased costs, safety hazards, and structural instability (Singh & Narendrula, 2020). It is therefore important to understand the causes, types, and mitigation strategies to optimize blasting operations and ensuring project success.
Causes of overbreak
Geological factors
Variability of geological conditions is one of the main causes of overbreak in rock blasting. Discontinuities such as fractures, faults, and bedding planes create zones of weakness that can propagate unintended rock breakage (Brady & Brown, 2013). Additionally, the presence of highly fractured or weathered rock increases susceptibility to overbreak.
Blast design issues
Poor blast design significantly contributes to overbreak. Common issues include:
- Improper burden and spacing: Insufficient burden or excessive spacing can cause uneven energy distribution, leading to uncontrolled rock breakage (Chiappetta, 2015).
- Incorrect charge placement: Misaligned or excessively deep charges increase the likelihood of energy escaping into undesired zones.
- Explosive overcharge: Overcharging results in excessive energy release, causing fractures beyond the planned excavation limits.
Operational factors
Operational inefficiencies, such as poor drilling accuracy and suboptimal initiation sequences, also play a role. Misaligned boreholes or deviation from the blast plan often result in uneven energy distribution, contributing to overbreak (Smith & Anderson, 2019).
Environmental factors
Environmental considerations, including groundwater presence and high in-situ stresses, exacerbate overbreak risks. Water weakens rock strength, while stress concentrations amplify fracture propagation (Hoek & Brown, 1997).
Types of overbreak
Blast-induced overbreak
This type occurs directly due to excessive energy from blasting. Factors include high charge densities, improper delay timing, and misalignment of holes (Chiappetta, 2015).
Geologically induced overbreak
Here, the pre-existing weaknesses in the rock mass, such as joints or faults, dictate the extent of overbreak. This is more prevalent in highly fractured or anisotropic rock masses (Singh & Narendrula, 2020).
Stress-induced overbreak
High in-situ stresses cause rock spalling or slabbing around excavation boundaries. This type is common in deep underground operations, where stress redistribution occurs post-blasting (Hoek & Brown, 1997).
Mitigation strategies
Geological mapping and modeling
Thorough geological surveys and 3D modeling enable precise identification of weak zones. This information helps adjust blast designs to minimize overbreak (Brady & Brown, 2013).
Optimized Blast Design
- Controlled blasting techniques
Techniques like pre-splitting, smooth blasting, and cushion blasting reduce overbreak by creating controlled fracture patterns (Chiappetta, 2015).
- Proper charge distribution
Using the right amount and type of explosive ensures controlled energy release. Deck charging and electronic detonators improve precision (Smith & Anderson, 2019).
Improved drilling accuracy
Advanced drilling equipment and GPS technology ensure borehole alignment with design specifications, reducing deviation and overbreak risks.
Stress management
Stress relief measures, such as sequential excavation or stress redistribution techniques, mitigate stress-induced overbreak (Hoek & Brown, 1997).
Real-time monitoring
Implementing vibration and overbreak monitoring systems allows immediate adjustments to blasting parameters, reducing overbreak occurrences (Singh & Narendrula, 2020).
Conclusion
Overbreak in rock blasting is a multifaceted issue influenced by geological, design, operational, and environmental factors. By understanding its causes and types, engineers can adopt effective mitigation strategies to minimize its impact. These practices not only enhance project efficiency but also improve safety and reduce costs, underscoring the importance of meticulous planning and execution in blasting operations.
Reference
Brady, B. H. G., & Brown, E. T. (2013). Rock Mechanics for Underground Mining (3rd ed.). Springer Science & Business Media.
Chiappetta, R. F. (2015). Controlled blasting practices for surface and underground projects. Journal of Explosives Engineering, 32(2), 45-52.
Hoek, E., & Brown, E. T. (1997). Practical estimates of rock mass strength. International Journal of Rock Mechanics and Mining Sciences, 34(8), 1165-1186.
Singh, R., & Narendrula, R. (2020). Overbreak control in tunneling projects: Lessons learned. Tunneling and Underground Space Technology, 98, 103280.
Smith, J., & Anderson, L. (2019). Advances in precision drilling and blasting techniques. Mining Engineering Journal, 71(5), 30-38.
