Abstract: The primary objective of tunnel smooth blasting is to create a neat excavation contour while minimizing disturbance to the surrounding rock. This study explores the asymmetric distribution of borehole-wall loading caused by bulk explosives in horizontal boreholes and the mechanisms governing directional rock breakage and damage control on the contour side. Corresponding theoretical calculations and parameter optimization methods are established accordingly. When bulk explosives are loaded into horizontal perimeter holes, gravity causes a charge configuration characterized as “coupled at the bottom and decoupled at the top”. To represent this configuration, the concept of an arcshaped decoupled charge (ADC) is proposed. A geometric parameter system is defined alongside a revised decoupling coefficient. The borehole-wall loading is divided into coupled and decoupled segments for separate modeling. For the coupled segment, the borehole-wall pressure is calculated using an impedance-matching formulation. For the decoupled segment, a method to calculate peak-pressure distribution is developed based on the isentropic expansion of detonation products and wavefront propagation, which reveals a left-right symmetric and nonlinear attenuation of borehole-wall pressure on the uncharged side. A fluid-structure interaction numerical framework in LS-DYNA is utilized to validate the performance of concentric decoupled charging, conventional eccentric decoupled charging, and ADC. The theoretical pressure distributions agree well with the numerical results. Under the same explosive mass, ADC delivers stronger directional rock breakage and enhances energy-utilization efficiency. By increasing the charge coefficient while maintaining a comparable fragmentation effect, the required charge mass can be substantially reduced (by approximately 49.88%－69.44% under typical conditions), thereby achieving the smooth-blasting requirement of “strong breakage on the excavation side and limited damage on the contour side”. Multihole interaction analyses reveal that the inclination angle β between the perimeter-hole line and the horizontal plane substantially affects crack coalescence and damage thickness on the contour side. Thus, the charge coefficient should be adjusted in conjunction with the coupling position. Finally, field applications in a high-altitude hard-rock tunnel demonstrate the engineering feasibility of the ADC technique and its potential for producing high-quality smooth-blasting contours.

Key words: tunnel, smooth blasting, bulk explosives, arc-shaped decoupled charging, eccentric charging, borehole-wall pressure, damage distribution