Objective With continuously increasing coal mining depth, the risks of hazards such as roof collapse and personnel entrapment caused by high in situ stress and mining-induced pressure have significantly intensified. Drilling emerges as a key means of rapidly establishing rescue passageways. However, the drilling process tends to face multiple challenges, including complex geological structures, unstable collapsed bodies, and low drilling efficiency.

Methods  This study aims to reveal the rock-breaking mechanisms during the drilling of collapsed bodies, optimize drilling parameters to ensure reliable borehole formation, and achieve efficient drilling of collapsed bodies. To this end, this study developed a large-scale experimental platform that integrated impact-rotary horizontal drilling and real-time measurement while drilling. This platform allowed for real-time acquisition and control of nine drilling parameters, including drilling speed, propulsive force, propulsive pressure, rotational pressure, and percussive pressure, meeting the requirements of full-process dynamic monitoring. Based on this platform, model experiments for simulating typical fabrics and structures of collapsed bodies were designed using M20, C30, and C50 materials, followed by the comparison of the variation patterns of drilling parameters under varying strengths and states.

Results and Conclusions  The strength and occurrence states of the drilled rock masses can be effectively characterized using parameters including the curve morphology of the drilling stroke, the abrupt change response of drilling speed, the transition interval and periodic fluctuations of propulsive force, and the fluctuating amplitude of rotational pressure. In combination with borehole morphology and fracturing patterns, this study proposed that the strategy characterized by low impact pressure, low propulsive pressure, and high rotational speed, which assists in reducing disturbance and improving drilling stability in roadway penetration sections. The experiments revealed the rock-breaking mechanism of graded responses based on rock block size, cascaded transformation of fracturing modes, and progressive and cyclic drilling within the collapsed bodies. Specifically, small loose fragments were prone to be disturbed and discharged. Medium-sized rock blocks tended to form temporary support structures together with loose fragments and were then fractured and involved in the cutting discharge circulation. In contrast, large, isolated rocks either entered the discharge circulation after partial fracturing or remained in place after forming penetration pathways due to their high integrity. This reflects the interactions between the drill system and rocks, as well as the dynamic structural evolution, during drilling. The results of this study provide theoretical support for rock identification, parameter optimization, and safety control during the drilling of underground collapsed bodies, thus contributing to scientific decision-making and precise operations during emergency rescue.