Evaluation of the impact of blast damage on tunnel surrounding rock induced by C-type cumulative tube cone angle

Over-excavation and under-excavation are common issues in tunnel construction using the drilling and blasting method. In some cases, traditional blasting techniques may fail to achieve the desired results. This study systematically investigates the blasting-induced damage effects of C-type cumulative tubes with varying cone angles in different grades of tunnel surrounding rock, aiming to mitigate over-excavation and under-excavation issues while enhancing construction efficiency. First, the working principles of cumulative charges and their mechanisms in rock penetration and fragmentation were systematically analyzed based on the fundamental theory of cumulative blasting. Subsequently, numerical models of single-hole cumulative blasting were developed for both III-level and IV-level surrounding rock conditions to examine the influence of cumulative cone angles on rock crack propagation. Finally, field validation tests were conducted at a tunnel construction site in Chamdo, Tibet. Results showed that a 65° cone angle provided the best performance in III-level rock, while a 55° angle worked best in IV-level rock. Compared to traditional methods, the cumulative charge method increased contour hole spacing by 12 ~ 18% and reduced construction costs by 23.5%. It also cut over-excavation by 58.3% and improved half-hole preservation by 41.7%, ensuring better excavation profile integrity and effectively addressing excavation issues.