Accounting for rock mass heterogeneity and buckling mechanisms in the study of excavation performance in foliated ground at Westwood mine

Operations at Westwood mine in Quebec, Canada, were temporary halted in May 2015 after three large-magnitude seismic events occurred over two days. The mechanisms leading to these events, which caused severe damage to several accesses, were not well understood at first. This was partly due to the complex geology at the site, where massive, unaltered, strong, brittle, and seismically active rock can alternate with highly altered, weak, foliated, and buckling-prone rock at the meter scale. Other ground behaviours such as the significant discrepancy in blasthole performance between secondary and primary stopes and the propagation of damage from stopes to haulage drives in some locations were also not well understood. In 2017, further geotechnical characterization of the rock mass was carried out and numerical back-analyses of several locations were completed using the continuum code FLAC3D. The objectives of the back analyses were to better understand the mechanisms controlling rock mass performance and to obtain a calibrated model for predictive stoping simulations. This paper presents the key aspects of the modelling, which include: (1) An anisotropic rock mass strength model with properties derived from field and lab strength testing, and (2) a scheme to account implicitly for the deconfinement that accompanies buckling around excavations.