Strategies for surface crown pillar design using numerical modelling – A case study

Abstract

The extraction of shallow ore deposits using underground mining methods rather than open pit mining is not uncommon nowadays, especially with the increased environmental constraints. Such scenario results in the creation of a surface crown pillar - the rock cover consisting of ore rocks that separate the uppermost stopes from the overlying surface. Although the problem of surface crown pillar stability has been researched for more than 30 years, the design guidelines are still limited. This is largely because of the wide variety and complexity of site-specific conditions. Thus, empirical and analytical methods are only used for preliminary design and more comprehensive methods such as numerical modelling must be adopted for the final design. This paper investigates strategies for the design of a surface crown pillar at a gold mining operation having a shallow, steeply dipping orebody that extends underneath an existing tailings storage facility. The orebody is in the vicinity of two regional faults. In this study, numerical modelling approaches are developed using the finite difference code FLAC3D 7.0. Critical factors such as in-situ stress regime, major geological structures, mining sequences, slanted surface topography and surcharge are considered. A model parametric study is carried out to investigate the effect of fault properties and horizontal stress ratios on surface crown pillar stability. Mining-induced surface subsidence is then analyzed to shed light on the anticipated maximum differential settlement and distortion developed in the tailings dam foundation as a result of the mining activities. Predicted differential settlement results could serve as another criterion for crown pillar sizing from the perspective of overlying infrastructure stability. The reactivation of two nearby faults due to mining activities is also explored. It appears that larger shear displacements occur on the fault that is parallel to the orebody strike compared to the cross fault.