Monitoring and back numerical analyses in the Konjsko tunnel

The Konjsko tunnel is an integral part of the Zagreb-Split-Dubrovnik highway, section Prgomet-Dugopolje. It consists of two tunnel tubes having 1326.0 and 1133.8 meters in length. The greatest overburden amounts to 150 m. This paper presents geotechnical measurement results and back numerical analyses. Back numerical analyses in combination with geotechnical measurements enable safer and more rational approach to designing and performing underground constructions. They contribute to the development of cognition on rock massifs and to determining its physically-mechanical parameters, joining it with rock classifying results. They can help to verify or to modify characteristics of the primary support system, of the foreseen progress length, estimated time of non-supported ranges, as well as time and schedule of performing all works regarding the excavation stability. 2 ENGINEERING AND GEOLOGICAL CHARACTERISTICS OF THE TUNNEL AREA 2.1 Investigation works For the purposes of the Konjsko tunnel design, geological, engineering-geological, hydro geological and geotechnical investigations have been performed in the wider area of the south and north tube route (Grabovac et al., 2004). The engineeringgeological and geotechnical investigation works have been performed in order to define the composition and physical and mechanical properties of deposits, thickness of quaternary deposits, as well as elevations at which flysch deposits appear on the western part of both tubes. Positions of limestone sediments and characteristic discontinuities have also been recorded. Selected samples of marl and limestones were laboratory tested. The above mentioned investigations and obtained data provided a design engineers’ base for geotechnical categorization of rock masses (according to RMR and Q systems), for designing of the tunnel support system and for selection of the tunnel excavation technology. Investigation drilling was performed at eight points in total. Three drillings were performed at the west tunnel tube centreline and one at east portals. Drilling further could not be performed due to lack of available access. Among the geophysical investigations, seizmic down-hole measurements were performed in three drill holes, geo-electrical