Tunneling problems in weak rock conditions under shallow overburden and comparison between full-face excavation and sequence excavation methods

The T8 tunnel was constructed within the scope of the Ankara – Sivas High-Speed Railway project. The tunnel route is formed completely by very weak and weak lithological units. The overburden decreases to 10 m, and the highest overburden thickness reaches up to 60 m. The tunnel is large with a 150 m2 excavation area and 14 m diameter. During the excavations in the section with an overburden thickness of 12 m, the supports failed, and the tunnel was completely collapsed. The purposes of the present study are to discuss the causes of the failure and engineering measures and to compare NATM and ADECO-RS methods with three-dimensional (3D) numerical analyses. Considering the mechanism of failure, extra engineering measures were proposed and the T8 tunnel was completed successfully. For the purpose of the study, a series of 3D numerical analyses were performed, and the results are discussed. The results obtained from the 3D numerical analyses showed that the ADECO-RS method yielded higher performance than the NATM method, for very weak and weak ground conditions (Category C).