A risk-based approach to ground support design

The objective of installing support is to mitigate the risk of injuries and costs associated with rehabilitation of excavation damage and associated production downtime. It follows that the greater the likelihood of stress damage or rockfalls, due to the inherent rock mass conditions, the more work is required from the support system. In addition to this, the exposure of personnel in these excavations and the quantity of production serviced by the excavation will significantly influence the support requirements.

While support systems have evolved over many years to cater for these demands and experienced rock engineering practitioners generally apply appropriate engineering judgement, most design methods are based on a simple factor of safety and do not cater for a proper risk evaluation. In nature, rock mass characteristics are variable and considerable effort is required to understand the variability, particularly when the geology is complex. Stress fields are often unknown or are simply inferred from one or two stress measurements.

This paper describes a risk-based approach to support design, which takes the variability of rock mass conditions into consideration. A statistical block stability method is used to estimate the probability of occurrence of rockfalls of different sizes in a given length of tunnel. Elastic and elasto-plastic numerical methods are used to estimate the probability of the depth of failure exceeding a prescribed serviceability criteria. The results of these analyses are used to estimate the expected cost of excavation damage, based on the rehabilitation cost and associated production downtime.