Legal license boundary restrictions and economic considerations for extraction of vein type iron orebody in Erzincan/Bizmisen area of Turkey limit the overall production by open pit mining. Based on overall Stripping Ratio (OSR) assessments, ore production is decided to proceed by open pit mining to a certain depth and continue with a cut and fill type underground mining operation. A crown pillar is to be designed in the transition region from surface to underground operations. Crown pillar dimensioning effects the overall ore recovery in such small-scale ore bodies. Proper dimensioning has to ensure the stability of open pit mine slopes and underground mine structural units as underground mining progresses to upper levels. Dimensional optimization of the crown pillar is performed by empirical, deterministic and numerical modeling approaches. First, scaled span method was employed to estimate the safe crown pillar thickness providing the required service life. Later, rigid analysis method was used to check shear type failure from the abutments. Finally, 2D finite element modeling analysis was used to ascertain the stable crown pillar thickness. Input parameters of numerical models were produced from geotechnical site investigation work and laboratory experiments. Disturbance effect of surface mine production blasting on the slopes of rock mass above the underground workings was taken into account in deciding on the rock mass properties.
Reserve is the economically valuable ore mineralization that is host within the earth crust. Especially in low grade, low profit ores, mining cost outlines the feasibility conditions for reserve extraction. Surface mining is mostly preferred due to low mining cost, ease and speed of production. In spite of its advantages, surface mining may not be feasibly applicable for the complete orebody extraction mainly due to two reasons.
The first reason is the feasible open pit layout. Economic depth of excavation is defined by comparing the Maximum Allowable Stripping Ratio (MASR) (see Eq. (1).) and Overall Stripping Ratio (OSR) (see Eq. (2).).
where a is surface mining cost per ton of ore, b is overburden stripping cost per m3, c is underground mining cost per ton of ore, V is volume of overburden, and M is weight of orebody. V is calculated by subtracting the volume of a cone representing the surface mine from the orebody it includes. Economic open pit layout may not contain whole of the orebody. In this case, rest of the orebody is exploited by U/G (underground) mining.