Applying Rock Engineering Systems Approach for Prediction of Overbreak Produced in Tunnels Driven in Hard Rock

Drilling and blasting is the predominant excavation method of the tunnels driven in hard rock. Overbreak is an undesirable outcome of blasting operation that has a profound effect on the safety and cost of tunnels. Determination of overbreak and the corresponding risk is of vital importance. The main aim of this research has therefore been to assess the risk associated with overbreak as well as its prediction. To do this, a rock engineering system (RES) based model has been proposed. The research data in this study was drawn from four tunnels located in Bakhtiari dam project, Iran. For 54 blasting operations carried out in the investigated tunnels, the values of 14 causative factors in overbreak were carefully measured. The result of this study shows that the developed RES based model with a high value of coefficient of determination (R² = 0.91) is a reliable technique to predict overbreak. Furthermore, the evidence from this study suggests that the Tunneling Rock Quality Index, perimeter specific charge and burden to spacing ratio play a large part in the values of overbreak. From the risk assessment conducted for the investigated tunnels, no blast operation was found to be highly problematic. Also, the overall risk level posed by overbreak was in medium range where the overbreak might be slightly problematic, and need to be monitored. The newly suggested RES model is also evaluated using 8 blasting operations, showing high level of performance.