Project Background

A major gold mining operation with multiple open pits plans to continue excavation of an existing pit where the orebody lies within a sedimentary rock sequence that includes schists and shales. The pit is 700 m deep and will be excavated up to 950 m deep, making it one of the largest open pits ever mined. Slope failures are likely in such a large pit, demanding accurate geotechnical insight to maximize economic return through optimized slope design that also maintains operational safety.

Objectives

• Optimize the pit slope angles to reduce waste removal, increasing the value of the project.
• Ensure acceptable Factors of Safety (FoS) for pit slope failures.
• Identify potential pit slope failure mechanisms to develop targeted depressurization and slope deformation monitoring. 

ITASCA’s Role

ITASCA delivered a comprehensive geomechanical assessment using industry-leading numerical modeling tools. Mine-scale FLAC3D models were constructed and run for each scenario using the IMASS model, a Hoek-Brown-based model that includes strain-softening. More than 60 faults were explicitly defined as interfaces along which slip and/or separation can occur. The anisotropy within the bedded rock units was represented using ubiquitous joints, which provide an orientation of weakness in each FLAC3D model zone.

3DEC models were also constructed and run for parts of the pit to include explicit representation of selected joint sets. Pore pressures were defined, and the effects of active depressurization were included. Strength reduction analyses were performed to estimate the Factor of Safety (FoS).

Featured ITASCA Software

FLAC3D

3DEC

Plan view showing domains and explicit structures in FLAC3D model.

Results & Impact

ITASCA’s analysis equipped the client with insights to guide operations to a safer, more profitable design for one of the largest open pits ever developed.

• Informed Decision-making: Assessment of whether each design and pore pressure scenario achieves the Design Acceptability Criteria (DAC) based on the resulting FoS values.
• Targeted Risk Mitigation and Improved Safety: 3DEC analyses revealed potential instabilities and failure mechanisms to inform redesign, implementation of depressurization measures, and focused slope deformation monitoring.
• Design Optimization: Calibration and back-analysis improved understanding of historical deformations and slope failures, and forward-analysis provided insight for future slope steepening in areas with high FoS or flattening in areas with low FoS.

Isometric view showing domains and explicit faults and joints in 3DEC model.

Isometric view showing total cumulative displacement contours in 3DEC model.

References

• Wines, D. R. (2016). A comparison of slope stability analyses in two and three dimensions. J. S. Afr. Inst. Min. Metall. 116(5), 399–406.