ABSTRACT
Increased demand to future-proof tunnel projects with respect to traffic has led to the proposal of some very large tunnel spans in recent road tunnel projects in Sydney. For example, four lane tunnels are currently under construction in Sydney with mined spans of approximately 20 m and Y-junction caverns exceeding 30 m spans, all with a requirement for 100-year design life. As these spans are unprecedented in Australian civil tunnels, a direct comparison with local experience is not possible and simple extrapolation of precedent designs, not necessarily adequate. This paper intends to present and discuss how recent designs that focus on first principles and the basic objectives of rock reinforcement have overcome the challenges of these designs, satisfied codes and standards requirements but at the same time provided savings with respect to ground support. The key to the design involved understanding the key failure mechanisms 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 considered acceptable for design.
