Geotechnical risk assessment and management for maintenance of water conveyance tunnels in South Eastern Australia

In SE Australia it is estimated that there are more than 500 km of large diameter rock tunnels constructed over a period of more than 50 years for water supply, hydro-electricity, sewage and other purposes. Owners are required to implement an inspection and maintenance program for these critical assets to ensure ongoing reliability of performance and in particular to prevent tunnel blockages from rockfalls. Major cost and time goes into the planning of a tunnel outage for inspection and repairs. Thus it is critical that the frequency between outages is determined on the basis of sound engineering and commercial principles. A critical aspect is the assessment of geotechnical risk at the planning stages for tunnel outages. A Quantitative Risk Assessment (QRA) is considered the most defensible method for prediction of risk of collapse to guide the timing for inspection and repair work.

This paper presents a straight forward QRA method adopted recently by the author on a range of tunnel projects in SE Australia during the planning stage to determine the annual probability of collapse and key geotechnical issues required to be considered during repairs. It also discusses procedures adopted to manage geotechnical risk during an outage with due regard to the range of likely treatment methods that may be required and safe access for personnel.

This risk assessment approach applied in recent years in Australia for dams and slopes can also be adopted for tunnels. This method represents a powerful geotechnical tool as it attempts to quantify the annual probability of failure in numerical terms. It also forces the practitioner to look closely at the range of failure mechanisms possible and their individual contribution to the combined probability of failure. The method may also be applied for the preliminary design for new tunnel projects.