Inverse Analysis of Cadia Tailings Dam Failure

On March 9, 2018, south of the town of Orange in Central West New South Wales, Australia, the tailings dam at the Northern Tailings Storage Facility (NTSF) of Cadia Valley failed. At the time of failure, buttresses were being constructed on the downstream slope of the dam to increase the factor of safety. The upper buttress, towards the crest, was completed in March 2018 while an excavation at the toe to remove accumulated tailings was left open from January 2018 until failure. The cause of the failure was determined to be a low-density or weak zone in the Forest Reef Volcanics (FRV) unit-A that was exposed in the toe excavation. The FRV unit-A is a highly weathered, relatively weak, compressible, and strain weakening volcaniclastic material that underlies parts of the NTSF dam. Using values of Atterberg limits for the FRV unit-A from the failure area, peak, fully softened, and residual shear strength envelopes were estimated using existing empirical correlations. The estimated stress-dependent strength envelopes were then used in an inverse 2D limit equilibrium analysis to evaluate the mobilized strength of the FRV unit-A. The mobilized strength envelope was then compared to the shear strength models used in the design limit equilibrium analysis for the NTSF dam. This comparison shows that correlations yielded a reasonable fully softened strength envelope for the FRV unit-A that could have been used for the design of the subject slope. This could have prevented the over-estimate of the factor of safety using the design strength models because of the potential for softening during the toe excavation. A similar analysis should be conducted for future projects to evaluate tailings dam foundation stability.