Abstract
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.
Highlights
Problems encountered in the excavation of road and railway tunnels worldwide arise in those excavated under shallow overburden thicknesses of weak soils
A significant problem that occurs in shallow tunnels excavated through weak ground in residential areas is the negative effect of excavation under the buildings along the tunnel route
Evaluation of preliminary tunnel support systems Zhang et al [41] proposed a theoretical model for the stability analysis of a reinforced tunnel face in cohesive-frictional soils with the limit equilibrium method and strength reduction technique
Summary
Problems encountered in the excavation of road and railway tunnels worldwide arise in those excavated under shallow overburden thicknesses of weak soils. Aygar and Gokceoglu [26] have emphasized that providing the stability of the tunnel face and ceiling is the most crucial factor of support systems for large-diameter urban railway tunnels excavated through weak units under a shallow overburden thickness.
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