The rock mass around an underground opening is subjected to a unique stress path that results in low radial confinement and both tangential loading and unloading conditions near the wall. As a result, the rock mass strength near underground excavations is controlled by failure mechanisms dominating at low confinement. Hence, when constructing underground works in hard rock, two general scenarios are encountered:

  1. structurally controlled gravity-driven failures; and

  2. stress-induced failure with spalling and slabbing.

The former process is predominant when both the radial and the tangential stresses are low, where as the latter is prevalent when high tangential stresses drive rock mass failure. Whereas structurally controlled failures are most frequently observed at shallow depths and slabbing failure is commonly found at great depth, mining and tunnelling experience shows that these failure processes may be encountered at essentially any depth. In this keynote the authors provide an overall framework for assessing the stability of underground openings in hard rocks, regardless whether the excavations are required for mining, nuclear waste or civil engineering applications. For the prediction of stress-induced slabbing, a bi-linear failure envelope cut-off is introduced. The resulting failure envelope, combined with numerical modelling, is used to determine the depth of failure near excavations and in pillars, and to examine the effect of rock mass bulking of the failed rock on the displacement demand for support selection. An assessment of rock mass relaxation on structurally controlled failure processes is made with respect to support demand and support capacity. This keynote also includes a brief review of violent failure processes, i.e., rockbursting. Where possible, examples from mining and civil engineering projects are provided to illustrate the design challenges of underground excavations in hard rocks. Guidelines for support design are provided.

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