Investigation on overbreak and underbreak of pre-stressed tunnels under the impact of decoupled charge blasting

Abstract

Decoupled charge blasting has shown its superior performance in achieving smooth tunnel profiles in underground excavation. In this study, through establishing an underground tunnel blasting similarity modeling (UTBSM) test system that incorporates a static stress loading device (SSL), a highspeed digital image correlation device (DIC) and a 3D laser scanning device (3DLS), field blast tests with concrete models involving set-aside circular and rectangular openings are first conducted to examine the influence of filling medium on the depth and area of overbreak and underbreak. Subsequently, a 3D finite element model is developed using LS-DYNA and validated against one of the test results, and the influences of decoupled coefficient, decoupled charge mode, filling medium, and in-situ stress condition are comprehensively investigated. Finally, an energy analysis encompassing internal energy and kinetic energy is performed. The experimental and numerical results show that water-decoupled blasting results in the largest overbreak and smallest underbreak due to the highest internal energy and kinetic energy, and the expanded polystyrene foam (EPS) effectively reduces the blasting energy exerted on concrete so that causes the minimum overbreak depth and volume. Both the overbreak and damage in burden decrease with increasing decoupled coefficient. Radial decoupled charge mode produces a higher energy transfer efficiency than axial charge mode, whereas eccentric charge mode outperforms in controlling overbreak and improving the excavation efficiency. Additionally, the increase of in-situ stress leads to a reduction in overbreak depth and damage volume in the burden, indicating that in-situ stress plays a favorable role in overbreak control during contour blasting. However, the restraining effect of in-situ stress on rock fracture warrants sufficient attention as it may result in blasting failure.