Numerical simulation study on safety thickness of boundary pillar and mining-induced stability during open-pit to underground transition

To investigate the safe thickness and mining-induced stability of boundary pillars during the transition from open-pit to underground mining, a specific open-pit to underground mining project was selected as the research background. A three-dimensional fine-scale model of the mining area was constructed using coupled modeling technology with MIDAS-GTS/NX and FLAC3D. The deformation and displacement, stress distribution, and evolution of plastic zones of the open-pit slope, underground stope, and isolation pillars were studied under the full lifecycle conditions of the mining area. The stability of the open-pit slope, underground stope, and boundary pillars was clarified. The results showed that backfilling and remediation of the open-pit slope can effectively prevent slope disasters. The untreated open-pit slope exhibited deformation and displacement, with a maximum displacement of 1.0 cm to 1.2 cm. The maximum compressive stress in the open-pit area was 1.5 MPa to 3 MPa, and the maximum tensile stress was 0.24 MPa to 0.34 MPa, indicating a low likelihood of tensile failure in the surrounding rock. No plastic zone distribution was observed on the slope of No. 2 open-pit, while localized plastic zones were present in the backfilled waste rock of No. 1 open-pit. The maximum deformation and displacement of the surrounding rock in the underground stope were 7 mm to 9 mm, occurring at the stope roof. The maximum compressive stress in the underground stope area was 2 MPa to 4.5 MPa, and the likelihood of compressive failure in the surrounding rock was low. Localized plastic zones were observed in the stope fill, but no breakthrough occurred between adjacent stopes. The deformation and displacement of the isolation pillars were minimal, below the determination criteria. An isolation layer of 45 m between the open-pit and underground areas effectively isolated the mining-induced disturbances from underground operations, preventing the impact of underground mining activities on the upper open-pit area. The slopes of No. 1 and No. 2 open-pits are currently stable. Underground mining has not yet had a significant impact on the open-pit area, and the underground mined-out areas are in a stable state.