Critical issues in selecting conventional and mechanized tunnelling methods, lessons learned from the past

The population is increasing rapidly in urban areas. Parallel to this, the need for a better life will necessitate tunnels for rapid mobility; tunnels to solve energy, water, and sanitation problems, etc. This is reflected in the current worldwide tunnelling market, which is growing fast. In light of these, this paper is prepared to summarize the basic requirements of conventional and mechanized tunnelling methods in a comparative way and to give several examples from the past to make it clear to understand the problems encountered while applying these two tunnelling methods. The incorrect choice between the two methods may lead to tunnel construction lasting several years more than expected and the cost may be 3 or 4 times higher than originally estimated, as happened in T26 high-speed tunnel in Turkey and Yacambu-Quibor Water Tunnel in Venezuela. First, geotechnical, geological, and environmental factors limiting the selection of the tunnelling methods will be discussed given examples of T26 high-speed tunnel, Uluabat, and Kargı power tunnels from Turkey. The paper will discuss the shortest tunnel ever driven 120 m in length with a 6.2m diameter EPBTBM within the complex Marmaray Project in Istanbul, emphasizing the big cities’ environmental restrictions.