Back-analysis of the geotechnical stability of high waste rock piles

Managing mine waste rock (WR) presents significant challenges due to its environmental impact and stability concerns. Typically, WRs are disposed of in piles using methods tailored to project specifications. The present paper is based on a large waste rock pile (WRP) built during the open-pit backfilling operation at the Canadian Malartic Mine. Backfilling is a cost-effective disposal method that minimizes the environmental impacts of mining operations. However, it often results in the formation of high WRPs that are prone to geotechnical instability during dumping and construction. Stability analyses of these structures often do not follow conventional geotechnical practices, since WR generated through blasting consists of coarse, angular grains, easily exceeding one metre in diameter, leading to considerable heterogeneity and variability in geotechnical properties within the piles. This adds significant uncertainty to conventional testing methods, resulting in potentially unreliable safety assessments. This paper examines the geotechnical stability of a high WRP using a multifaceted approach aimed at reducing these uncertainties. The method incorporates Finite Element modelling using the Hardening Soil constitutive model, with parameters calibrated after various triaxial laboratory tests, and considers the actual geometric configuration and construction sequence of the pile. The results have been validated through deformation monitoring data collected throughout all construction stages, to ensure short-term stability of the pile during truck dumping operations. Numerical modelling using 2D and 3D approaches revealed that, while shear strength was realistically captured, deformations were significantly overestimated due to limitations in laboratoryderived stiffness parameters. Back-analysis using recalibrated stiffness moduli improved alignment with field monitoring, suggesting current design guidelines may be conservative for WRPs in hard rock mines.