Numerical simulation of crown pillar behaviour in transition from open pit to underground mining

Crown pillars provide regional and local support by isolating the ground surface from underground mine workings. Topography above the underground mine may be a relatively flat ground surface or an open-pit structure. Depending on what lies above and design of the underground mine, the crown pillar behaviour will differ. In transitioning setups (open pit to underground), large open pit collapses have taken place as a result of crown pillars located at the transition zone. Hence, this paper focuses on crown pillars between open pit and underground to better understand their behaviour. Taking the Zuuntsagaan Fluorite mine as an example where open pit will transform to underground mining, a remnant ore is to be left as a crown pillar to separate the two mining sections. Through numerical simulation in FLAC3D 7.0, stress distribution and failure mechanisms acting around the crown pillar were monitored as underground mining progresses. Effect of crown pillar geometrical parameters was evaluated, thus crown pillar thickness, span and dip. Further, the open pit geometry influence was also considered on the overall behaviour of the crown pillar. It was found out that in transition from open pit(OP) to underground(UG), slope and stope walls closing in on the crown pillar induce stresses that act as loading from the pillar sides, which in turn influence the failure process.