Determination of the Crown Pillar Thickness Between Open Pit and Underground for Coal Mining

Open pit mines cannot increase continually its depth of exploitation due to costs, safety, environmental and social expectations. In fact, many open pit mines have either already made the decision or are at the planning stage to change their mining activity from open pit to underground to remain competitive. In OP-UG transition, determining the adequate thickness of a crown pillar is one of the most essential challenges to address. Limited research has been carried out in this area, with the majority applied to the metalliferous ore deposits. This paper presents a new approach to the determination of the crown pillar thickness between Open Pit and Underground mines for coal mine deposit based on the prevention of water inrush through failure zones induced by underground operation (longwall mining). An overview of the studies conducted to-date in determining the crown pillar thickness is followed by a series of numerical modelling studies using finite difference software FLAC with selected variables including rock mass properties and depth of coal seam deposition. At the conclusion of the paper, brief guidelines for determination of crown pillar thickness between Open Pit and Underground mines are presented.