1 Introduction

The Westwood mine is located approximately 80 km west of the town of Val-d'Or in Quebec, Canada. Production in the 104-mining block in Zone 2 of the Westwood mine was halted by three largemagnitude seismic events over two days in May 2015 (Fig. 1). These seismic events are technically interesting because mining activities leading up to these events were reviewed and no specific blast (production or development) could be identified as a trigger. Further, these pillar bursting events occurred in the Central Infrastructure Corridor (CIC) well away from ore zone production, where adverse seismicity in mining operations would more typically be anticipated. This paper provides an overview of the seismic events and uses numerical models to demonstrate how geological conditions and mine geometry both contributed to what was considered, by a review panel of experts, to be an unforeseen series of events.

2 Numerical back analysis

The main objective of the numerical analyses performed in this study was to improve the understanding of overall rock mass behaviour in Zone 2 at the Westwood mine and to better define the factors that contributed to the May 2015 seismicity. To achieve this, global mine-scale models and local drift-scale models were developed. Model calibration utilized a combination of the available seismic data, underground observations, the condition of boreholes, and inspection of core drill after the events. The global model was developed using FLAC3D (V. 5.0 by Itasca Consulting Group 2012) and included all development and stoping geometries in the relevant mining blocks. Drift-scale modelling utilized Phase2 (V.8.0 by RocScience 2015). Fig. 2 provides a summary of material properties and boundary conditions.

The first iteration of numerical simulations focused on the global mine model in order to narrow the range of possible stress tensors and material property combinations. The first iteration of global-scale modelling produced non-unique results. Multiple combinations of boundary conditions and material properties could reproduce the observed ground response. However, these preliminary numerical solutions required either excessively high k-ratios, or un-realistically low material strengths to induce sufficient rock mass yield to drive the pillar failure.

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