Geotechnical Challenges and Adaptive Excavation Strategies: The Konovo Tunnel Case Study

Summary

Abstract

The Konovo Tunnel is an approximately 500-m-long single-tube tunnel situated at the junction of the expressway on the third development axis between Velenje and Slovenj Gradec. Excavation was performed under an overburden of up to 50 m within highly heterogeneous geological conditions, comprising tectonically intensely fractured dolomite, sandy marl with embedded dolomite blocks, marlstone, claystone, and localized coal seams. Despite its relatively short alignment, the project required the management of several significant geotechnical challenges. One of the key issues involved a continuous section of marlstone and claystone occurring in a comparatively homogeneous sequence beneath two residential structures under approximately 35 m of cover. In this zone, a technologically adapted, phased excavation scheme was implemented to ensure adequate control of surface deformations and to maintain structural safety. Additional critical challenges occurred in the portal areas. The northern portal intersected a 58 m section located within a slope of very low stability. To mitigate the instability risks and achieve a safe connection to the underground alignment, a covered-trench construction method supported by piles was applied. At the southern portal, an atypical constraint required the tunnel to be excavated and broken through beneath extremely shallow overburden due to permitting limitations. The breakthrough was executed through the excavation of a southern pre-cut, enabling controlled exposure of the tunnel face. Although relatively short in length, the Konovo Tunnel encompassed a broad spectrum of geotechnical and construction challenges, all of which were successfully addressed through continuous geological and geotechnical monitoring and responsive adaptation of the construction methods.