Overbreak in tunneling refers to the phenomenon where rock excavation goes beyond the calculated profile, resulting in additional damage to the surrounding rock mass and increased costs to the construction project. There are several factors that can contribute to overbreak, including poor excavation sequences, unprofessional blasting procedures, and insufficient primary support.
One major factor that can cause overbreak is the presence of unfavorable joints in the rock, which can lead to fractures intersecting the jointing planes and causing large pieces of material to fall out of the face during excavation. This can be mitigated using controlled blasting techniques, such as pre-splitting, trim blasting, and line drilling, which are particularly effective in large rock formations. However, these techniques may not be effective when the rock has multiple joints between blast holes, and those joints intersect the face at less than a 15-degree angle. Another factor that can cause overbreak is a badly designed blast plan with pre-split holes that are too close together or overloaded. To minimize overbreak in this case, the hole spacing and powder weight can be reduced, the burden can be increased, and/or the pre-split holes can be detonated simultaneously. The competence of the excavator operator is also a crucial factor in minimizing overbreak in less huge rock formations. Soft seams and mud seams can also influence the outcome of control blasting procedures, and it can be difficult to obtain satisfactory results if the rock face is crossed by several mud seams.
Overall, the most effective way to minimize overbreak in tunneling is through an optimum blast pattern and optimal progress in a blasting round, which can prevent overbreak caused by the perimeter charge factor and reduce blast-induced damage to the surrounding rock mass.
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