Damage Characteristics of Surrounding Rock for Large Cross-section Tunnel Bench Blasting Excavation: A Case Study

Drilling and blasting methods of excavating the rock mass will cause a certain degree of damage to the surrounding rock. The damage of surrounding rock caused by tunnel blasting excavation is significant for tunnel support design and long-term tunnel stability study.

This study is based on the excavation project of the Longnan tunnel of the Ganzhou-Shenzhen high-speed railway with a bench blasting method of grade III surrounding rocks. The cross-hole acoustic wave method was used to test the acoustic wave velocity of surrounding rock in different parts of the same section of the tunnel after excavation. The distribution characteristics of the damage variable of surrounding rock were analyzed, and the damage depth of the surrounding rock in different parts of the tunnel was determined, revealing the relationship between the degree of damage and the damage depth of the surrounding rock. LS-DYNA numerical simulation software was used to simulate the damage evolution and distribution characteristics of the tunnel surrounding rock under the same working condition with eight cycles of blasting excavation, which was consistent with the acoustic test results.

The results of acoustic testing and numerical simulation showed the maximum damage of the surrounding rock at the foot of an upper step arch of the bench method tunnel but the minimum damage depth; the maximum damage depth of surrounding rock of the tunnel was located at the bottom of the inversion arch.

Based on the damage distribution characteristics of the tunnel surrounding rock, the initial supporting bolt length for the Longnan Tunnel Grade III rock was determined to be 3.5 m ~ 4 m based on engineering analogies and relevant specifications.