Future tunnelling projects in Istanbul and some considerations for using mechanized tunnelling based on previous experiences

The total length of the tunnels planned to be excavated in Istanbul in the near future is 185 km for metro tunnels and 47 km for utility tunnels. Majority of the tunnels will be excavated with tunnel boring machines and the total cost of the operation is estimated to be around 10 billion Euros. In this paper the projects concerning future tunneling activities will be summarized including geological characteristics of the related area. The geology of Istanbul is complex and some of the tunnels have been currently excavated in young Cenozoic formations and some of them in complex series of Paleozoic formations. The Paleozoic rock formations are cut frequently by andesite and diabase dykes, fracturing the main rock, decreasing the daily advance rates and increasing the tunneling cost tremendously. Sometimes in EPB (Earth Pressure Balance) TBM applications, excessive ground deformations may cause damages to the surrounding buildings like experienced in Otogar-Esenler metro tunnels which caused an extra cost of 35.6 million US Dollars of the project. Possible difficulties which may be encountered during tunnel excavations will be cited in the paper and some recommendations will be made to ease the success of the tunneling operations.