Application of Shaped Charge Tubes with Different Materials in Smooth Blasting of Fractured Surrounding Rock

This study investigates limitations of conventional shaped-charge blasting in tunnel smooth blasting applications, including excessive charge concentration, operational challenges in charge placement, and induced over-break and surrounding rock damage from concentrated bottom charges. To address these problems, an innovative PVC semi-tubular clasp-shaped charge tube was developed. Following initial effectiveness validation, a reusable steel slit-type charge tube was subsequently designed specifically for weak and fractured surrounding rock mass. Numerical simulations comparing shaped-charge effects between PVC and steel slit-type charge tubes revealed that the steel variant exhibited a substantially greater peak stress concentration at the slit and generated longer fracture propagation lengths than its PVC counterpart. The steel slit-shaped charge tube achieved a peak pressure ratio of 5.51 between the slit and non-slit directions, significantly surpassing the PVC tube's ratio of 1.65, thereby demonstrating superior directional energy concentration and protection of the surrounding rock mass. In double-hole blasting tests, the steel slit-shaped charge tube achieved results equivalent to those of the PVC tube at a significantly lower linear charge density (113 g/m vs. 314 g/m), demonstrating enhanced directional fracturing efficiency and superior protection for the weak and fractured surrounding rock mass. Field tests in jointed tunnel rock confirmed that the PVC-shaped charge tube enabled uniform explosive distribution and controlled directional energy release, effectively increasing half-cast hole rate while minimizing damage to the retained surrounding rock mass. Comparative blasting tests with/without the steel slit-shaped charge tube confirmed its effective directional energy concentration and reusability. Results demonstrate this steel tube's superior suitability for blasting projects in weak and fractured surrounding rock masses, achieving effective disturbance control with reduced explosive quantities while showing significant potential for practical engineering applications and widespread implementation.