Design of large span tunnels and caverns: back to basicsKeynote Address

Increased demand to future-proof tunnel projects with respect to traffic has led to the proposal of some very large spans in recent road tunnel projects in Australia. For example, four lane tunnels are currently under construction in Sydney with mined spans of approximately 20 m and Y-junction caverns of unprecedented spans for road tunnels in Australia, all with a requirement for 100-year design life. As these spans are unprecedented in Australian civil tunnels, a direct comparison with local past experience is not possible and simple extrapolation of precedent designs, although potentially solving the problem, often result in uneconomical solutions that do not necessarily target the actual failure mechanisms involved in the excavation of such large spans. International experience could certainly be used but adequate design justification would still have to be provided. Although there is certainly room for cutting edge innovation, robust solutions can also be achieved by simply going back to basics. As a result, this paper intends to present and discuss how designs that focus on first principles and the basic objectives of rock reinforcement may allow for a better understanding of the design requirements and how to satisfy codes and standards but also provide savings with respect to ground support. The key to the design involves understanding the failure mechanism that needs to be addressed, its relationship with the different actions of rock bolting, i.e. suspension/anchorage and/or rock reinforcement and what could be acceptable.