Davor Garašić

Problems with Caves During Tunneling in Dinaric Karst (Croatia)

During the construction of almost all the tunnels in the Croatian Dinaric karst in the last fifty years there have been found thousands of caves that represented the major problems during the construction works. Geological features (fissures, folding, faults, etc.) are presented here, together with the hydrogeological conditions (rapid changes in groundwater levels). Special engineering geological exploration and survey of each cave, together with the stabilization of the tunnel ceiling, and groundwater protection actions according to basic engineering geological parameters are also presented here. In karst tunneling in Croatia over 150 caves longer than 500 m have been investigated. Several caves are over 300 m deep, and 10 are longer than 1,000 m. Different solutions of crossing the caves were chosen depending on the size and purpose of the tunnels (road, rail, and pedestrian tunnel or hydrotechnical tunnels). The article show interesting examples of stabilization ceilings in big cave chambers, construction of bridges inside tunnels, deviations of tunnels, filling caves, grouting, etc. A complex type of karstification has been found in the cavern at the contact between the Palaeozoic clastic impermeable formations and the Mesozoic complex of dolomitic limestones in the Vrata Tunnel and at the contact with flysch at the Ucka Tunnel. It is however quite rare to find karstification which is advancing at a similar rate in all directions. The need to have the roadway and/or tunnel above a water spring is the biggest possible engineering-geological, hydrogeological and civil engineering challenge. Significant examples are those above the Jadro water spring (Mravinci tunnel), in flysch materials or above the Zvir water spring in Rijeka (Katarina Tunnel), and in fractured Mesozoic carbonates.