Assessment of the Shallow Cukurcayir 1 Tunnel (NE Turkiye) Constructed in a Residential Area and in Weak Ground Using 3D Numerical Analysis

Tunnels excavated in weak ground can develop problems due to squeezing phenomena. This situation is much more critical in large-diameter and double-tube tunnels. The effects of the tubes on each other are also important. The NATM (New Austrian Tunneling Method) is employed in the design of the support systems for these types of tunnels, which is based on the flexible outer arch principle and allows for deformations or minimization of deformations by increasing the lining thickness and using a rigid pavement. The risk of deformations occurring in a flexible outer arch is determined by the occurrence of deformations in shallow tunnels, which can affect the surface; a ground reaction curve can quickly develop and lead to a collapse. It is also known that the stability of the tunnel whose support and excavation is completed first can be impacted by subsequent excavations for the other tunnel. The purpose of the present study was to describe the geological and geotechnical conditions of a shallow tunnel excavated in weak ground and to assess its stability using three-dimensional numerical analysis. For this purpose, the Cukurcayir 1 tunnel—a double-tube tunnel—was employed as a case study. The 3D numerical analyses were performed using the Flac3d V2.10-222 software. The results are discussed in detail and recommendations for the necessary support system for tunnel roof and face stability are given.