Damage Assessment of the T7 Railway Tunnel Associated with a Large Landslide: A Case from Türkiye

This study presents the mechanism of a large landslide that has affected the single-track T7 railway tunnel, constructed in 1933 along the Diyarbakır–Fevzipaşa Railway line in Türkiye and predominantly used for freight transportation. Since its construction, the tunnel has suffered from persistent structural and operational problems, requiring repeated temporary remedial measures over nearly a century. The severity of the damage increased markedly following the 6 February 2023 earthquakes, ultimately necessitating a comprehensive reassessment of tunnel stability and long-term serviceability. To identify the causes of the observed damage and develop permanent engineering measures, detailed engineering geological and geotechnical investigations were performed. The investigations included the evaluation of historical documentation, systematic field observations, geotechnical drillings, in-situ and laboratory testing, and monitoring. The results of investigations showed that the tunnel is located within a large landslide mass approximately 220 m wide and 630 m long, characterized by multiple shear and fracture surfaces. The interaction between the landslide and the tunnel was further quantified using Light Detection and Ranging (LiDAR) measurements obtained from the tunnel interior. The results indicate cumulative tunnel displacements reaching up to 250 cm since construction, corresponding to an average long-term deformation rate of approximately 2.7 cm/year. Based on the landslide kinematics and stability assessments, it was concluded that the most effective long-term engineering solution was the relocation of the tunnel 130 m further into the mountain, beyond the landslide-affected zone. The new tunnel alignment was designed and constructed accordingly, and the tunnel was successfully completed at the end of May 2025 without encountering geotechnical or structural difficulties. The findings demonstrate that the long-standing problems of the T7 Tunnel were primarily caused by sustained landslide–tunnel interaction and have now been permanently resolved.