Seismic Risk Management Practices at Vale's Sudbury Operations

ABSTRACT: Vale Base Metals has been operating many underground mines in the Sudbury Basin for over a century and all mines have experienced a significant increase in the number of major seismic events over the last decade. Various mitigation strategies have been developed at Vale's North Atlantic Operations in Ontario to manage and control the risks associated with seismicity and rockbursts. The mines have seen a significant increase in the number of large seismic events, but the overall trend of rockbursts is decreasing. This paper will focus on Seismic Risk Management Strategies developed over the last decade, including both strategic and tactical control measures. Examples will be given in the paper to demonstrate the effectiveness of control measures over the last decade. Recent developments in Seismic Hazard Assessment for burst prone ground conditions are also presented in this paper. Although the strategies presented in this paper can be applied to other deep hard rock underground mine operations that face similar challenges with burst-prone ground conditions, they should be applied with caution and tailored for site specific conditions. 1. INTRODUCTION Vale Base Metals has operated several underground mines in Sudbury for over a century, and currently five mines are active (Coleman, Creighton, Copper Cliff, Garson, and Totten Mines). Numerous mining methods have been employed including cut and fill, post pillar cut and fill, and sub-level caving, however, currently the primary mining method employed is open stoping. The primary rock mechanics challenge at these operations is seismicity and rock bursting. Figure 1 shows a recent rockburst at one of the mines which was associated with a large seismic event. Seismicity has been observed to be primarily linked with: • Late stage of extraction: sill and diminishing pillars • Brittle and high strength rocks (UCShost 200 MPa) • Mining depths between 1.5 and 2.5 km below surface • Lack of confinement due to higher extraction ratio • Presence of seismically active geological structures A Seismic Risk Management Plan is an important element at any seismically active mine and is also a legal requirement in some mining jurisdictions. The following main items are key requirements: • Responsibilities • Microseismic Monitoring & Data Analysis • Seismic Hazard Assessments • Hazard Mitigation Plans/Strategies • Training This paper will focus on two key requirements described above: Hazard Mitigation Strategies and Seismic Hazard Assessments.