An assessment on the inner lining need for a large-span tunnel (a case from Turkey, Akyazı Tunnel, Trabzon)

Abstract

Stability of large underground structures depends on not only rock mass conditions but also excavation stages and support systems. In addition, complexity of geological conditions and interaction of excavation with in situ conditions affect the performance of design. The Akyazı tunnel (Trabzon, Turkey) is 2478 m long and was designed as 3 lanes and double tubes, and the number of lanes at the junction part increases to 5. Tunnel cross-sectional area from the Akyazı tunnel reaches 3 lanes and 150 m2 to 5 lanes and 438 m2 gradually. The width of the tunnel at the junction point reaches to 31 m. In this study, the necessity of inner lining concrete in tunnels excavated in good rock mass conditions is assessed and the results of the analyses are discussed. The inner lining concrete increases the cost of tunnel construction, as well as extending the construction period. With the procedure followed in the Akyazı tunnel, the top heading part of the tunnel was constructed without inner lining. No stability and drainage problem have not been encountered for 4 years. Currently, the tunnel construction was completed successfully, and the tunnel was opened to traffic. Consequently, the excavation stages, support systems described, and especially inner lining necessities discussed in this study may be applicable for extremely large tunnel sections.