Case study of slope failure during construction of an open pit mine in Indonesia

This paper presents back-analyses of a failed slope at an open pit lignite mine in Muara Enim, Indonesia. The slope, which was being raised by the dumping of excavated material, failed at a dump height of 24 m, well before reaching the design height of 80 m. The primary cause of failure was determined to be the presence of a previously unknown weak layer in the foundation, combined with high excess pore pressures generated by a relatively fast dumping rate. The failure resulted in significant disruption of the mining operation, in addition to environmental problems and a high cost of remediation. Finite element analyses were performed to gain a better understanding of the influence of the weak layer at the time of the failure. The case history emphasizes the need for a thorough site investigation and the risks associated with fast dumping on saturated weak clay overlain by a stiff residual soil layer with low permeability.