Overbreak in tunneling
Overbreak is a section of the damaged zone in tunneling that extends beyond the calculated profile for support pieces that fall during rock excavation. It elevates the entire cost of tunneling to 15% or even more than the original construction budget.
Influence factors for overbreak
Over-break is discovered to be the result of the complicated interplay between poor excavation sequences, unprofessional blasting procedures, and insufficient main support. The presence of unfavorable joints, their orientations, non-cohesive filling materials, and poor groundwater conditions all contribute to the incidence of quick overbreak.
Tunnel blasting, like conventional rock slope blasting, can be controlled by using controlled blasting techniques including pre-splitting, trim blasting, and line drilling, which operates best in huge rock formations. A simple crack between adjacent blast holes does not occur in geologically complicated material. A shattered shatter zone occurs, which protects the final rock face from the effects of radial cracks emerging from the production blast holes.
Control blasting techniques will not be able to generate a good smooth face when the rock has multiple joints between blast holes and those joints intersect the face at less than a 15-degree angle. In reality, the joints must intersect the face at an angle more than 30 degrees for the wall to have a fair degree of overbreak. Anything less will result in fractures intersecting the jointing planes and significant pieces of material falling out of the face during excavation. Overbreak can also be caused by a badly designed blast plan with pre-split holes that are too close together or overloaded.
A pre-split line can also be hampered by close joint spacing. When this occurs, the answer is to reduce the hole spacing and powder weight, increase the burden, and/or detonate the pre-split holes simultaneously.
In less huge rock formations, the excavator operator's competence is crucial. Some machines can generate a significant amount of thrust, burrowing into an unblasted wall and badly ruining the final shape. Soft seams and mud seams are two more geologic elements that influence the outcome of control blasting procedures. It is difficult to obtain satisfactory results if the rock face is crossed by several mud seams.
Overbreak caused by the perimeter charge factor can be reduced by optimum advancement in a blasting round. This is due to the availability of a better free face and a lesser burden on the contour holes. Greater advancement may reduce the negative impacts of the perimeter charge factor on overbreak.
Minimize the overbreak in tunneling
Overbreak in tunneling can be minimized through an optimum blast pattern because it reduces blast-induced damage to the surrounding rock mass.
Optimal progress in a blasting round can prevent overbreak caused by the perimeter charge factor. This is due to the availability of a more open face and less stress on the contour holes. Increased progress could help to mitigate the detrimental effects of the perimeter charge factor on overbreak.
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