Possible causes of unexpected cracks in a large-span railway tunnel after ring closure

The T4 tunnel constructed within the scope of the Ankara - İzmir High-Speed Train Project (Western Part of Turkey), is 2350 m long. While the excavation works in the tunnel were continuing, cracks occurred in the sections where the ring was closed, and invert concrete was completed. The aim of this study is to explain the possible causes of these unexpected cracks. T4 Tunnel is located in the northern horst of the Gediz Graben. This horst-graben system is controlled by normal active faults. For this reason, the causes of cracks cannot be explained only by the quality of the rock mass and the support inadequacy. Although the supports were increased after the occurrences of the cracks, the deformations continued. As an additional measure, consolidation injections were performed, and deformations were stopped in the section where cracks occurred. To understand the possible causes of cracking, besides the deformation measurements in the tunnel, the deformations in the rock mass and support system were measured by 3 sets of extensometers. According to the results obtained from the extra measurements, it has been determined that the highest stresses occur in the shoulder region close to the active normal fault and the deformations reach up to 15 m from the tunnel wall. In other words, it was understood that the plastic zone formed around the tunnel reached up to 15 m and was not symmetrical. Considering this finding, a consolidation injection application was performed, and the tunnel was successfully completed.