Study on Parameter Optimization of Presplitting-smooth Combined Blasting for Soft Rock Roadway Excavation

To address the technical challenge of blast-induced overbreak at tunnel crowns during soft rock roadway excavation, this study proposes an optimized presplit-smooth blasting synergistic method. Theoretical analysis grounded in wave impedance principles demonstrates that the wave impedance disparity between pre-split cracks and surrounding rock creates a high-reflectivity interface, fundamentally altering stress wave propagation dynamics. When the stress wave reaches this interface, most of its energy is reflected, with only a small portion transmitted, resulting in substantial attenuation of wave intensity. This phenomenon transforms the initially transient, intense impact energy into a millisecond-scale, gradually released load, effectively mitigating energy superposition from subsequent stress waves and preventing cumulative damage within the rock mass, thereby significantly improving the dynamic stability of the geological structure. Numerical simulation results demonstrate that the synergistic method outperforms conventional smooth blasting by effectively confining the damage zone within the presplit fracture boundary, resulting in a remarkable reduction in crown damage depth to 0.3 m. Furthermore, this approach substantially decreases peak particle velocity (PPV) while transforming the loading profile from a transient sharp-rise/slow-decay' impact to a more controlled millisecond-level delayed-release' pattern featuring gradual pressure buildup and deferred peak intensity. Field implementation of the optimized synergistic scheme in the kaolinized diorite porphyry roadway at the-350 m level of the Gushan Iron Mine demonstrated exceptional contour control, maintaining intact surrounding rock in weak interlayer zones without spalling or collapse. The study verifies that the presplit-smooth blasting synergistic method, leveraging stress barrier effects and meticulous parameter optimization, achieves a substantial reduction in blast-induced damage and vibration while effectively mitigating overbreak in soft rock roadway excavation.