Investigation on the transition depth of open pit to underground mining in Sijiaying iron mine

When the expansion of an open pit limit is restricted by the permanent facilities located near the boundary of the pit, the traditional economic evaluation method is insufficient to determine the ultimate transition depth of mining from open pit to underground. A new method needs to be constructed to determine the transition depth from open pit mining to underground. In the context of mining in the transition from open pit to the underground at Sijiaying Iron Mine located in North China, this study investigates the ultimate depth of the open pit mining under the conditions of the New Luan River located on the footwall and a Provincial Highway on the hanging wall near the open pit slope. Formulas for calculating the limiting depth of open pit mining when immovable permanent facilities exist on the hanging wall, footwall, and both sides of orebodies are established, respectively. Using these established formulas verifies the economic feasibility of the new method. The calculated transition depth of mining from the open pit to the underground for the case mine is 388 m. The result shows that the presented formulas achieve significance and usability in ascertaining transition depth from the open pit to the underground under the constraints of various permanent immovable facilities on the hanging wall and footwall of the ore body.