Caving-induced subsidence behaviour of lift 1 at the Palabora Block Cave Mine

The Palabora copper mine is a large, 32 000 t/d block cave operation located in the eastern half of Limpopo, South Africa’s northern-most province. Palabora Mining Company is currently developing a second lift (Lift 2) in addition to their existing block cave operation (Lift 1). Upon breakthrough of the Lift 1 cave to the base of the open pit in 2004, significant instability in the north wall of the open pit occurred. Multiple numerical modelling studies have been conducted since this time to investigate the north wall failure and the associated subsidence (Brummer, Li and Moss, 2006; Sainsbury, Pierce and Mas Ivars, 2008; Vyazmensky et al, 2010; Woo et al, 2012). None of the published analyses have fully captured the three-dimensional evolution of the cave propagation, crown pillar failure/breakthrough, slope failure and surface cracking formation beyond the crest of the pit. To provide confidence in the prediction of caving-induced subsidence associated with the development of Lift 2, detailed three-dimensional numerical modelling has been conducted with a hybrid Abaqus/CAE-FLAC3D discrete finite difference (DFD) modelling approach. The approach has been able to accurately simulate the evolution of the north wall failure from cave initiation through to the existing subsidence zone of influence magnitude and geometry.