This lecture focuses on brittle rock and rock mass failure modes to illustrate how highly stressed rock behaviour near deep excavations differs from rock far from an excavation, i.e., it fails predominantly by spalling rather than by shear. As a consequence, the often adopted shear failure models do not represent spalling type failure processes and thus are often inadequate for the design of tunnels and their support systems. It is also discussed how swelling rock problems in tunnelling might be related to brittle failure processes and how a change to more appropriate modelling approaches might assist in understanding and controlling swelling. Recent developments and implications of practical importance are highlighted, particularly with respect to the selection of strength parameters for support design of underground excavations in highly stressed rock. Several case examples are used to highlight how a better understanding of the spalling behaviour may help to prevent costly delays and rehabilitation work.
Some of the lessons learned with brittle failing rock in deep underground construction and Alpine tunnelling was previously presented in keynote lectures: at GeoEng 2000 (Kaiser et al. 2000), summarizing a decade of collaborative research work on brittle rock failure; at two Rockburst and Seismicity in Mines Symposia (Kaiser et al. 2005 and 2009), introducing new means of complex data interpretation in seismically active mines; at GEAT'99 and '05 (Kaiser & Tannant 1999 and Kaiser 2006), focusing on experiences from Alpine tunnelling in Switzerland; and at the Asian Rock Mechanics Symposium (Kaiser 2006), highlighting the impact of brittle failure on constructability.
When building in highly stressed rock, instability is imminent and careful designs with a clear understanding of the rock behaviour is of utmost importance for risk management and for ease of construction (Kaiser 2006).