The Role of Pillars in Raise Caving

Abstract

Mining advances to greater depth, where the control of rock pressure is central for a successful operation. To handle the increasing rock pressure at LKAB’s Kiruna iron ore mine, LKAB and Montanuniversität Leoben develop a novel mining method called “raise caving”. Raise caving is based on an active stress management approach. De-stressing slots, which are separated by substantial pillars, are developed first and provide stress shadows for stope development and subsequent large-scale mineral extraction. At an advanced stage of extraction, these pillars are extracted, too. The pillars are decisive for the success of the raise caving method. They are responsible for the control of stresses, the stability of the hanging wall, and the control of seismic energy release. Hence, studies on pillars have a high importance. This contribution investigates the effect of pillar behavior on the overall extraction system by means of numerical simulations. Two different types for pillar behavior are examined, namely infinitely strong pillars and pillars which yield or crush in the process of extraction. In the second case, pillar stress strain curves are created and replicated numerically. The investigated behavior of pillars is based on available studies on the pillar stress strain behavior. Results show the importance of intact (not overloaded) pillars in the initial de-stressing phase in raise caving. These pillars are essential for controlling the stress levels at the position of the raise bore holes for the initial slot development. Pillar overloading results not only in an increase of the spatial extent of the stress shadow but also in a growth of stresses in the abutment areas and neighboring not yet overloaded pillars. It has been found that this instance can endanger the mining activities in the de-stressing phase. Furthermore, it has been found that the layout of the slot-pillar system together with the extraction sequence is decisive for ensuring that pillars are not overloaded in the de-stressing phase.