Within the scope of the Ankara-Istanbul High Speed Train Project, the T29 tunnel was opened to traffic after completing excavation and support implementation. The tunnel was excavated in weak to very weak graphitic schists causing severe face stability problems and deformations on the tunnel supports during excavation stage. In the progress of the excavation stage, a serious face failure occurred at Km: 225 + 784, and the effects of this face failure were observed on the surface. Since the cover thickness is approximately 35 m in this section of the tunnel, it was decided to pass this zone with a cut and cover structure. The portal excavation was carried out from the failure zone, and it was planned to provide stability with the closure of the surface by constructing a cut-and-cover structure immediately afterwards. However, slope failures occurred during the portal excavations. This section was reworked, and it was decided to support this area with a pile system. Consequently, in this study, the reasons for the face failure and slope failures are investigated, and the construction stages are explained and discussed.
T29 tunnel, which is located between Bilecik and Bozüyük within the scope of Ankara-Istanbul High Speed Train Project, is located between Km: 225 + 600 − 225 + 120. The total length of the tunnel is 520 m and the maximum overburden thickness is 40 m. Location map and the satellite image are given in Figures 1 and 2, respectively. During the tunnel excavation, on November 01, 2019, at Km: 225 + 784, the tunnel excavation was interrupted as the collapse in the tunnel face spread to inside of the tunnel. This collapse caused an effect of about 6 x 4 meters on the surface and on the tunnel face (Figures 3 and 4).
The impact of the collapse in the tunnels is observed at the surface, especially in shallow tunnels. Special solutions are required, especially in cohesionless or low cohesion soils (Aygar and Gokceoglu, 2020a). In addition, the most important factor in this cohesionless soil is the stability of the tunnel face and tunnel ceiling (Aygar and Gokceoglu, 2020b, c and d). In order to pass the collapsed section, the main support systems are ground improvement and umbrella system from the surface or in the tunnel (Aygar and Gokceoğlu, 2020e). In addition, tunnel and portal relationship is always an important situation in tunnelling. Because a collapse in the portal significantly affects the tunnel section and causes unexpected damages and time losses. For this reason, tunnel face and ceiling stability should be provided together with tunnel excavation, especially in shallow tunnels.