Analysis of documented case studies of mine scale rock mass disasters indicates that geotechnical failure of mining structures doesn’t happen without warning. Structural damage and progressive failure is manifested by the presence of geotechnical warning signs (indicators and precursors) and can be instigated by triggers. An indicator is defined as a sign, a state or a contributing factor that points out or suggests that the rock mass may be prone to damage or failure. Usually indicators suggest that the properties of the effected rocks are different from the surrounding rock mass.
The following indicators were identified: geological (e.g. disturbance in the form of folds and dykes, layers of weak soil or rocks, ground discoloration), geotechnical (e.g. structural features such as faults, shear zones or slickensided planes, change in the mechanical properties of the rock mass, water seepage), or operational (e.g. large open spans of underground excavations, unstable shapes of underground excavations, old excavations near mining activities, a high extraction ratio, blast damage, an accumulation of water in nearby excavations, a large body of water or tailings above underground excavations, and steep slopes in open pits).
A geotechnical precursor (a telltale sign) is a state or behaviour that suggests that the geotechnical structure of the rock mass has been damaged prior to possible failure. Precursors, including results from geotechnical instrumentation, warn of the development of excess ground deformations or high stresses.
A geotechnical trigger is any stimulus that impacts rock mass behaviour. A trigger can be internal or external. External triggers come from the environment, like water inflow due to heavy rainfall or earthquakes and tectonic plate movement. Internal triggers come from our mining activities, e.g. blasting, undercutting slopes or reducing the size of underground pillars.
Based on the observed succession, it is possible to distinguish four phases of geotechnical warning: an awareness phase (during which initial precursory behaviour can be noticed), an alert phase (during which precursory behaviour is localized), an alarm or evacuation phase, (during which precursory behaviour is observed at the brunt of impending collapse), and scram (when there is very short warning for the evacuation of workers). Forecasting as part of risk management comprises of three stages: anticipation, avoidance and adjustment.
In terms of a risk management control system, indicators can be considered as hazards, precursors as incidents, and local damage and regional failure as losses.