ABSTRACT
This paper presents the design and construction of a tunnel excavation through a geological fault zone of the OeBB Koralmtunnel lot KAT3. Drawing on the experience gained from advancing the parallel southern tunnel tube and aiming to minimize geotechnical risks for the subsequent excavation of the northern tube’s TBM drive, a comprehensive redesign of the excavation concept and methods was undertaken. Five alternative solutions were evaluated through a risk analysis to optimize necessary measures, reducing potential risks and costs to an acceptable level. The proposed measures included ground improvement solutions through grouting from the TBM or the neighboring tube, the construction of a pre-injection gallery in the northern tube, and pre-excavation of the fault zone using NATM (New Austrian Tunneling Method) with either a top-heading or full-face approach, followed by backfilling. Based on this evaluation, the decision was made to excavate the fault zone using a conventional top-heading drive from an adjacent cross-passage. Subsequently this section was successfully excavated with a TBM, without any construction delays. The paper discusses the geological and geotechnical conditions, the pre-excavation measures implemented, and the observed system behavior during both the conventional excavation and the TBM advance. It emphasizes the importance of a consequent application of the observational method and its derived measures within a geotechnical safety management plan for controlling residual risks.
