In the current mining industry climate, early and detailed planning for mine closure is becoming increasingly important. Geotechnical inputs relating to stability of open pit excavations were often limited or cursory in past mine closures. Although broad guidelines based on general observations do exist in some countries, detailed deterministic approaches and design criteria specifically applicable to open pit excavations during mine closure have not been well established. Large open pits may be most sensitive to the economic impacts of geotechnical requirements/constraints for closure, particularly if unduly conservative approaches are employed. It is therefore necessary for these requirements and constraints to be carefully assessed for each pit, and for the most cost-effective measures to be identified. It is important that slope instabilities are well-documented during operations, and that good communication between various technical departments is maintained for closure planning and coordination.
This paper presents examples of several scenarios in which pit slope stability assessment may be required at mine closure. These are focused specifically on the long-term stability of the open pit excavations, not the stability of any adjacent structures. The paper does not attempt to propose a set of guidelines but discusses techniques that may be used for assessment under various scenarios. Topics include estimation of rock mass degradation over time, assessment of long-term instability around pits, evaluation of slope buttressing options, and determination of exclusion zones around pit crests (and in-pit floors that must remain open to access).
