This paper discusses the design and construction of the initial tunnel section of the OeBB Koralmtunnel lot KAT3 near the western portal. Based on the experience obtained during the advance of the exploration tunnel nearly nine years earlier very challenging tunneling conditions were expected due to multiple fault zones combined with shallow overburden. During the advance of the exploration tunnel large surface settlements developed rapidly, leading to a situation where the overall stability of the structure was considered as compromised. A thorough analysis of the displacement data from the tunnel and ground surface together with the geological documentation provided a comprehensive understanding of the critical situation and allowed implementing mitigation measures and the tunnel excavation to be continued without major problems. The findings and experience from the exploration tunnel provided the basis for the design for the following tunnel advances. The paper focuses on the lessons learned from the exploration tunnel and their influence on the design. The applied monitoring program is discussed in detail, emphasizing the analysis of the system behavior in the light of the encountered geological conditions.

1 Introduction

The 32.9 km long Koralm tunnel connects the cities Graz and Klagenfurt and is the main part of the new Baltic Adriatic high capacity railway. The tunnel alignment crosses Neogene sediments on both sides of the Koralm massif, which is composed of various gneiss subtypes, schists and (to a lesser degree) silicate marbles. The overburden varies from 5 m at the portal area to approximately 1200 m in the central section of the advance. Due to the length and general strategic importance of the project, an extensive geological and geotechnical exploration program was conducted – including the construction of two exploratory tunnels on both sides of the massif.

On the Carinthian side (western portal), the exploratory tunnel was constructed as the top heading of the final southern tube, and passed through various faulted Neogene sediments before advancing through the main Lavanttal fault zone into the gneisses of the central massif.

The exploratory tunnel Mitterpichling advanced westwards from the intermediate access towards the western portal and over a 300 m section encountered severely faulted rock mass conditions with low overburden. While passing beneath a major regional road both the tunnel and surface deformations increased considerably leading to a critical situation, which due to the rapid on-site analyses of the system behavior and subsequent quick introduction of countermeasures was stabilized without incidence (Moritz et al. 2006). Nevertheless, with measured surface settlements up to 220 mm and visible cracks on the surface the zone was deemed highly critical for the completion of the final tunnel drives.

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