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The T3 railway tunnel—under construction within the scope of the Bursa-Yenişehir high-speed train project—is located in northwestern Turkey and has a length of 1250 m. The tunnel is being constructed entirely in silty clay/clayey silt, sand, and clay units, and it is designed in accordance with the principles of weak ground tunneling described by the new Austrian tunneling method (NATM). When the tunnel excavations began in 2013, during and after portal excavations, a failure occurred at the entrance of the tunnel; thus, a revision of the support systems became mandatory. While the excavation works proceeded after the revision phase, a collapse occurred again at the tunnel face where the overburden thickness was low. This study presents the mechanisms of the collapse that took place at the portal location and in the middle of the tunnel. The proposed tunnel support systems and their numerical analyses are also discussed, because the case is interesting for the tunnel community and will inform future tunnel construction work. For this reason, the relationship between portal excavation and tunnel excavation stability is described. Consequently, tunnels excavated through weak ground conditions are considered, and the importance of considering the face stability of tunnels in tunneling studies is underlined.

Due to the increasing population and resulting transportation needs, the number of subway and high-speed railway projects has also increased. The geometric constraints of such projects have caused many tunnels to be built in weak ground. Thus, weak ground tunnelling has attracted the attention of tunnel engineers and researchers. The main purposes of this study are to analyse the T4 tunnel excavated in weak ground and to compare the results obtained from the analytical solutions and 3D numerical analyses. This study specifically considers the T4 tunnel support system used in the Ankara İzmir High Speed Railway Project (Afyonkarahisar-Banaz Section). The T4 tunnel route encounters weak ground composed of layers of extremely weak mudstone, clayey sand, weakly cemented sandstone, and silty–clay matrix with pebbles. The tunnel overburden ranges from 10 to 35 m, which is shallow. After the excavation work of the T4 tunnel, severe deformation and critical stability problems in the shallow part (where the overburden is approximately 10 m) were encountered inside the tunnel, leading to a halt in construction. This was followed by revisions to the tunnel support system, leading to successful completion of the tunnel excavation. Numerical simulations of the low overburden section are performed using the commercially available FLAC3D program that uses the finite difference method. The characteristics of insufficient/ineffective support systems and adequate support systems for shallow tunnels excavated through weak ground are discussed in this study. Additionally, problems that pertain to the tunnel itself and its support system are discussed. The results of the 3D numerical analyses and analytical solutions are compared, and the advantages of 3D numerical analyses are discussed. The importance and necessity of tunnel face stability and roof stability for tunnel stability in weak ground is illustrated. Consequently, solutions based on analytical and numerical analyses are presented, and the analysis methodology and solutions proposed in the study can help guide weak ground tunnelling design and evaluation.

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