SBT ‐ Excavation concept for managing extreme rock mass deformation

Abstract Tunnelling through a fault zone under high overburden presents significant challenges for all parties involved. The use of ductile shotcrete lining in combination with yielding elements has proven to be an effective and meanwhile is state of the art in deep rock tunnelling. This support concept was selected from the outset of the project for the excavation of the Grassberg‐Schlagl fault system—a major fault zone within Construction Lot SBT1.1 of the Semmering Base Tunnel (SBT). Based on extensive investigations conducted during the design phase, various support types were defined, with the most robust design including the installation of an additional rigid 2nd shotcrete lining. Using this approach, approximately 75% of the fault zone was successfully excavated. The transition to a roughly 600‐meter‐long section without re‐ consolidation posed significant challenges and required continuous adaptation of the tunnelling and support concept. Through the construction of a pilot tunnel and using innovative HS‐EPS yielding elements, this particularly demanding zone was ultimately traversed successfully. This article presents the development from the initial design approaches to the ongoing modifications of the support concept during execution, also accounting for the difficulties experienced during construction.