Abstract: After the conversion of an open-pit mine to a underground mine，in order to determine the reasonable safe thickness of the isolation pillars，various theories such as the K.B Rubeneyi method， the thickness-span ratio method，and the load transfer intersection line method were used to calculate the thickness of the boundary isolation pillars to be 12.0 ~14.33 m.Combined with similar engineering cases from both domestic and international sources，the reasonable thickness of the isolation pillars for this mine was finally determined to be 15 m.A three-dimensional numerical model was established for verification， and the impact of underground mining on the stability of the open-pit slope and the isolation pillars was an ⁃ alyzed.The simulation results show that after underground mining，the maximum surface displacement is generally at the millimeter level，the plastic distribution is mainly dominated by a stable plastic zone，the maximum minimum principal stress is much smaller than the tensile strength of the rock mass itself，and the stress concentration phenomenon is not obvious.The open-pit slope can maintain good stability，and no obvious failure characteristics are observed.The cross-sectional analysis of the top and bottom of the isolation pillar shows that the 15 m thick isolation pillar forms an effective stress buffering and deformation coordination structure，which reduces the stress disturbance of underground mining on the open-pit bot ⁃ tom，has certain safety，and can ensure the overall stability of the slope and production system of the open-pit to underground mining.Based on the comprehensive analysis of theory，engineering analogy，and numerical simulation results，it is recommended that the isolation pillar thickness for the open-pit to un ⁃ derground mining conversion of this mine be 15 m.The research and application of the reasonable thick ⁃ ness of the isolation pillar for the open-pit to underground mining conversion of this mine can provide tech ⁃ nical support for this mine's open-pit to underground mining.

Key words: open-pit to underground mining, isolating pillar, numerical simulation, stability analysis