Experimental study on the influences of charging structure with various filling mediums on rock blasting performances

Decoupled charge blasting is widely used in tunnel contour excavation to mitigate overbreak/underbreak and the extent of excavation damaged zone, which plays a pivotal role in enhancing the safety and efficiency of tunnel construction. This study experimentally investigates the influences of filling medium (air, water, sand and clay) on blasting performances. Five single hole blast (SHB) tests with a blasthole length of 1.0 m and blasthole diameter of 45 mm are first conducted in an underground tunnel. The ground vibration signals are captured using three-component seismic vibrometers installed at different distances away from the blasthole. The blast induced crater is automatically measured using a 3D laser scanner and the post-blast rock fragmentation is analyzed using the open-source program ImageJ. Subsequently, the peak pressure, particle velocity and displacement on the borehole wall with respect to various filling mediums are theoretically deduced and analyzed. The experimental results indicated that the fully coupled blasting produces the largest near-field peak particle velocity (PPV), finest rock fragmentation and largest crater, but also the highest PPV attenuation rate. Among these decoupled charge blasts, water-decoupled blasting results in the finest fragmentation and maximum crater, whereas the corresponding PPV is at a much smaller level than coupled blasting. Sand and clay-decoupled blasts yield smaller PPV and crater but coarser fragmentation than water-decoupled blasting. Air-decoupled charge structure causes the minimum PPV and crater size, as well as the coarsest fragmentation. The theoretical analysis shows that the fully coupled blasting yields the largest borehole wall pressure (BWP), particle velocity and displacement on borehole wall than those induced by decoupled blasts, as well as the maximum attenuation rate of BWP and velocity. Additionally, water-decoupled blasting exhibits higher energy transfer efficiency so that a finer rock fragmentation and a larger crater are obtained than air, clay and sand decoupled blasts, but more attention should be paid to the blast vibration.