The aim of this paper is to examine and compare the results of engineering geological exploration works in the design phase and during the construction of tunnel Stražina. Tunel Stražina is located near the city of Omiš on Zagreb-Split-Dubrovnik highway in Croatia. The interactive design method was applied in the tunnel design. This methodology integrates the empirical, rational and observational approaches in the geotechnical design of the tunnel. It allows for the thorough and comprehensive overview and problem solving during the construction. In addition, it minimizes the errors that may occur due to the limitations of each individual approach. Predicted underground excavation support solutions were done based on site investigation activities and other bases using the empirical and rational approaches. Geomechanical and Q classification were performed as a part of the empirical approach. Engineering-geological tunnel mapping was done during the excavation activities as a part of the geotechnical monitoring. Verification of the tunnel support system during the construction phase was done using the actual data obtained from the engineering-geological mapping and the monitoring. Thus, the actual rock mass quality along the tunnel was determined and the tunnel underground excavation support measures were optimized. Due to the complex conditions present in the rock mass, the discrepancy between estimated and actual rock mass conditions can hardly be avoided, and likewise, a certain amount of discrepancy between the predicted and actual support systems is also to be expected. The interaction of site investigation, design and construction, in addition to introducing the risk analysis, is essential for a more rational, safer and more economical construction. Development of methods and procedures for the preparation and control for each of the phases is important for a successful operation of each participant performing a completion of a particular phase, as well as for the project investor.

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