Rock damage assessment in a large unstable slope from microseismic monitoring - MMG Century mine (Queensland, Australia) case study

• Microseismic monitoring data is used to fully understand the slope failure development in an open pit mine;
• Strong correlation between spatial and temporal slope style of activity was found;
• Rock mass damaged and failure mechanism was obtained by combining microseismic and geotechnical data;
• Potential use of a microseismic network as a management and safety tool are exploited.

Movements, instability and failures in open pit mines can pose important geotechnical problems, leading to major impacts on the safety of personnel and the mining operations. In particular, large slope scale rockslides represent a significant challenge, as these types of instabilities require accurate observations and monitoring. However, in many cases engineers can only rely on surface displacements for their interpretation of the failure mechanism because there is no information on the extension of the deformation into the slope. More recently, several attempts have been made to monitor the volume of rock of unstable slope in open pit and natural slopes, using the microseismic technique. Nevertheless, the link between ground deformations, failure mechanism and microseismic data was rarely addressed in the details of these studies. In this paper, a case study of the SW Wall instability at Century mine (Queensland, Australia) is discussed. Since 2009, the pit wall has been affected by several multi-batter failures, associated with continuous bedding planes. Geotechnical investigations, supported by numerical modelling, have interpreted those instabilities as potential development of deep-seated failure. Consequently, in early 2013, the slope angle at the base of the slope was reduced and a buttress was left to avoid further progression of the instability into the lower section of the wall. Slope performance while mining has been primarily managed through surface monitoring (geodetic prisms and ground-based radar). However, as there were still concerns, a microseismic monitoring program was proposed by MMG geotechnical personnel and subsequently implemented. Our work integrated the approaches, analysing both ground deformation and microseismic data in order to reach a more complete understanding of rock damage at depth and mechanisms of instability.