Bridge standards - Are they appropriate for tunnel design?

The use of Limit State concepts in ground engineering designs has always been a subject of controversy or scepticism. In favour, its fundamental objective is to identify limiting conditions of failure (Ultimate Limit State) and serviceability (Serviceability Limit State) that need to be addressed as part of the design. On the other hand, particularly in rock tunnelling, there are designers who consider it inappropriate or even meaningless to attempt to apply limit state design concepts as encapsulated by some of the existing codes that apply a number of factors to both load and strength. The major issues in tunnelling, including collapses, are mostly associated with unexpected ground conditions or fundamental design flaws that are not dealt with by applying codified factors to loads and/or strength. Nonetheless there is increased bureaucratic pressure on ground engineering and tunnelling professionals to carry out design works for tunnelling in accordance with Limit State concepts that are devised for other types of structures such as AS 5100 Bridge design standard. This paper discusses some of the difficulties in applying AS 5100 to tunnel design in Australia with a focus on rock reinforcement design using rock bolts. The objective is to highlight the discrepancies between design concepts with focus on support behaviour and the requirements of AS 5100.