Research on the Movement Law of Rock Strata in the Mining Transition from Open-Pit to Underground of the Sijiaying Iron Mine

The transition from open-pit to underground mining entails significant risks associated with rock mass deformation and surface subsidence, which pose critical challenges in mining engineering practice. To investigate and control the deformation behavior of overlying strata under mining-induced disturbances, a three-dimensional numerical model is developed for the goaf area at the Sijiaying Iron Mine. Deformation indicators, combined with calculations of rock movement angles and collapse angles, are utilized to elucidate the deformation characteristics and controlling mechanisms of the mine surface. The results indicate the following: (1) slope deformation in the open-pit mine exhibits notable spatial heterogeneity, characterized by a “large displacement–small deformation” phenomenon, with peak values of total displacement and total deformation reaching 92.86 mm and 3.28 mm/m, respectively; (2) the critical ranges of surface movement angle and collapse angle are determined, enabling quantitative delineation of the influence zones of underground mining on surface deformation; and (3) the dip angle of the ore body is the primary controlling factor influencing the surface subsidence. Specifically, gently dipping ore bodies predominantly exhibit vertical subsidence (associated with larger movement angles), whereas steeply dipping ore bodies display pronounced directional sliding (correlated with smaller movement angles).