A century of tunnel-landslide interaction: Anatomy of T7 Tunnel, Nurhak–Gölbaşı (Türkiye)

The T7 Tunnel, a single-track railway tunnel constructed in 1933 on the Diyarbakır-Fevzipaşa line in southeastern Türkiye, has experienced progressive damage due to tunnel-landslide interaction for nearly a century. Primarily used for freight transport, the tunnel underwent various temporary interventions to maintain traffic until the damage became critical following the 6 February 2023 Türkiye Earthquake. This study investigates the causes and extent of deterioration and proposes a permanent engineering solution. A comprehensive characterization was conducted using historical records, field observations, drilling data, in-situ and laboratory tests, and Light Detection and Ranging (LiDAR) measurements. The investigation revealed a 220 m wide, 630 m long landslide with multiple fracture surfaces enveloping the tunnel. LiDAR data showed cumulative tunnel displacements of up to 250 cm, corresponding to an average movement rate of 2.7 cm/year since construction. Based on landslide analysis, the optimal engineering decision was to relocate the tunnel alignment 130 m deeper into stable bedrock. The new tunnel was constructed and completed without incident on May 30, 2025. Post-construction monitoring confirms no deformation inside or outside the tunnel, indicating that the century-long engineering challenges have been permanently resolved. The methodology and engineering approaches presented provide a valuable reference for managing tunnel-landslide hazards in mountainous terrains.