One method for defining an acceptable level of risks in tunneling

Abstract

This article presents one method for defining an acceptable level of risks in tunneling. The analyses are based on a simple analytical method for definition of tunnel stability using appropriate software in order to define the stability of excavations with and without protection. These analyses helped to find an approach on how to link the Acceptable Level of Risks with values of the Safety Factor (SF) and the Probability of Failure (PF). The explained methodology is related mainly to tunnels constructed in soft rocks or fault zones, but with some adaptations, it can be applied to other tunneling problems and other engineering structures. The case history used to test the methodology is a diversion tunnel at Hydro System "Sveta Petka" near Skopje. Based on these analyses, one proposal to define an acceptable (tolerable) level of risks using the criteria of the probability of failure and potential economic costs is presented.