Overcoming Karst Caves during Tunnel Construction of Divac?a-Koper railway line

ABSTRACT: As predicted, during the construction of the Lokev and Beka tunnels on the second track of the Divača-Koper railway line in Slovenia, collisions between the tunnel route and large-scale karst phenomena frequently occurred, posing significant obstacles to the continuation of the works. A total of 90 such occurrences were recorded in both tunnels, with some standing out due to their size and the complexity of their remediation. Two of the larger karst phenomena identified during the construction of the Lokev tunnel were named 2TDK-054 and 2TDK-076. In both cases, they consisted of a system of karst caves located within or directly beneath the tunnel route. The article describes the additional geological and geotechnical investigations necessary for the proper placement of the bridging structures to overcome karst phenomena in the underground space. Details of the temporary and permanent re-mediation measures, as well as the execution of the reinforced concrete structures under the given challenging and spatially constrained conditions, are also presented. Special emphasis in the article is placed on the technological construction procedures that will ensure the durability and safety of the bridging structure, which will not be accessible for maintenance during its operational phase.