Improvements to the Observational Method in New South Wales Road Tunnel Construction

Phil Clark

November 12, 2021Sydney Symposium 2021

The past five years has seen an unprecedented boom in tunnel construction in Sydney. Road tunnels in particular, continue to push both design and construction to their limits, no less than when Sydney Harbour Tunnel was constructed 30 years ago. Integral to the safe and efficient construction of road tunnels has been the application of the Observational Method in design and construction. This paper describes some important “lessons learned” in implementing the Observational Method in New South Wales road tunnels since the construction of the Sydney Harbour Tunnel.

Underground infrastructure construction in New South Wales in the early 1990s had very few precedents. Construction of the major underground excavations relied on application of the principles of the Observational Method as described by Peck in 1969. This concentrated on validating design assumptions against detailed monitoring data. As confidence in the ability to predict the behaviour of rock masses in the Sydney Region increased, it could be argued that appreciation of the fundamentals of the Observational Method diminished to a process of collecting data for the sake of collecting data, rather than being a live tool to identify and manage geotechnical hazards. The tunnel collapses in the Cross City Tunnel (2004) and Lane Cove Tunnel (2005) led to the industry reassessing tunnel construction risk management. The Permit to Tunnel (PTT) process was born, and at its heart, it provides a means to manage geotechnical risk via a formalised process that includes reviewing of observations by both designers and constructors and agreement to continue construction, within the main principles of the Observational Method.

However, over the past 5 years, the author has observed waning deference to the Observational Method. Construction processes used on major projects, including the Observational Method, the PTT and associated processes have become opportunities for contractors and designers to modify certified designs in an ad hoc manner, often without design changes being adequately reviewed against key criteria such as safety, stability and durability. This paper aims to identify key concerns with the current implementation of the Observational Method through the PTT process. Recommendations are proposed to reset practice to ensure designs are constructed safely and owners are provided with assurances that construction delivers the intended design.

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