ABSTRACT
Managing design uncertainties is a persistent challenge in tunnel engineering, with the Observational Method (OM) offering an attractive alternative. However, its adoption is often resisted due to perceptions of increased risk compared to a pre-defined conservative design. This study introduces a probabilistic framework to quantify structural safety when applying the OM compared to the standard approach. Using a case study of Sydney’s Ashfield Shale, the framework leverages stress-strength interference models and Bayesian updating to compare the OM to traditional methods. The study also presents a method to derive instrumentation trigger levels through a target probability of failure, reducing epistemic uncertainties during construction and linking design risk tolerance to cost. The proposed solution offers a transparent approach to link design, risk tolerance, and cost.
