The Gerede is the longest and most problematic water transmission tunnel in Turkey, at 31.6 km long with an overburden of 600 m. The tunnel will transmit water from the Ulusu river to the Camlidere reservoir. High-pressure water ingress and collapses have resulted in many delays. Although construction began in 2012, and was intended to be completed in 2014, completion is now estimated to be in 2019. The tunnel passes through permeable Tertiary sedimentary and volcano-sedimentary rocks, and Jurassic–Cretaceous limestone. Water ingress has occurred on 45 occasions along the 26 km excavated. The initial total discharge was 1.7 m3 s−1 and decreased to 0.5 m3 s−1 over time. The recharge conditions, tectonic structure, progress of the excavation and sealing by grouting affect the quantity, locations and temporal changes of the inflow of water. The major faults and formation contacts are the most problematic sites to be considered.
With the many delays in the tunnel, the driving plan and tunnel boring machine (TBM) were changed while the construction continued. The tunnel shows that hydrogeological conditions, in particular, should be taken into consideration when choosing the TBM and driving course. However, the problems we encountered provided valuable experience for future projects.