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 ingresses as well as collapses have resulted in many delays. Although construction began in 2012, and was intended to be completed in 2014, completion is now estimated in 2018. The tunnel passes through permeable Tertiary sedimentary and volcano-sedimentary rocks, and Jurassic-Cretaceous limestone. A total of 45 water ingresses have occurred along the 26 km excavated. The total discharge was 1.7 m3/s at the beginning and decreased to 0.5 m3/s over time. The recharge conditions, tectonic structures, progress of the excavation and sealing by grouting affect the quantity, locations, and temporal changes of the water ingresses. The major faults and formation contacts are the most problematic sites to be considered.
With the many delays in the tunnel, driving plan and TBM were changed while the construction continued. The tunnel shows that, particularly hydrogeological conditions should be taken into consideration to the choice of TBM and driving course. However, the events that taken place are an important experience for future projects.