Research and exploration of longwall fully mechanized mining in steeply dipping coal seams

To address the challenges of safe and efficient mining in steeply dipping coal seams in China and support the intelligent transformation of the coal industry, this article focuses on fully mechanized longwall mining technology for such seams. It systematically reviews the theoretical and technological evolution of longwall mining in steeply dipping coal seams and, in light of intelligent mining trends, outlines future scientific issues and technological development directions in this field. Since its formal definition in 1996, steeply dipping coal seams, with dip angles ranging from 35° to 55°, have seen their upper mining limit gradually extend to 70° due to advances in technology and equipment. These seams are widely distributed across China with substantial reserves, serving as vital resources for regional socioeconomic development while also posing key challenges for intelligent mining. The article systematically outlines the four developmental stages of longwall mining in steeply dipping coal seams: non mechanization dominance, mechanization exploration, breakthrough in fully mechanized mining, and technological system maturation. From the perspectives of thin, medium thick, and thick or ultra-thick seams, it summarizes the applicability, advantages, and limitations of various mining methods such as wire saw mining, pseudo inclined directional longwall mining with segmental dense support, fully mechanized mining, and fully mechanized caving. It notes that fully mechanized longwall mining, including caving, remains the mainstream technological direction. In terms of theoretical research, key advances are introduced, including the relationship between support and surrounding rock in steeply dipping longwall faces, roof structure stability, floor sliding mechanisms, support surrounding rock system dynamics, coal wall stability, flying gangue mechanisms, overburden structure and disaster inducing mechanisms. Technological innovations such as longwall pseudo inclined mining, nonlinear face layout, and combined horizontal section fully mechanized caving mining along with related intelligent equipment development are also reviewed. In response to the current trend toward intelligent mining, the establishment of digital twin simulation platforms based on longwall mining, the development of intelligent integrated equipment, and practical cases of intelligent mine applications are discussed. It is emphasized that intelligent fully mechanized longwall mining is the essential pathway toward safe and efficient extraction of steeply dipping coal seams. Finally, the article identifies pressing research issues such as intelligent surrounding rock control, collaborative stability of equipment systems, and reliability of complex environmental perception. Future technological development directions are proposed, offering theoretical and technical support for promoting safe, efficient, and intelligent mining in steeply dipping coal seams.