Analytical solutions and 3D numerical analyses of a shallow tunnel excavated in weak ground: a case from Turkey

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.

Tunnels

“…For this reason, both squeezing and swelling potential are the most important factors in tunnels excavated in weak rocks and soils. Two different tunnel support system approaches are used for tunnels excavated in weak rocks and soils [19][20][21][22]. These are called the passive method and the active method.…”

Intelligent decision method for stability assessment of shield tunnel based on multi-objective data mining

“…Nevertheless, due to the assumptions of numerical simulations, the degree of reduction of actual working conditions is low. Numerical simulations thus have a higher error, so numerical simulation technology is mostly used to verify the actual results [21,28,29].…”

Evaluation of uncertainties in travertine rock mass for the Antalya (Turkey) Metro tunnel excavation

“…In particular, when examining the geological conditions of complex areas such as karst development, various safety accidents such as collapses [1] and runoff water [2] can easily be triggered, affecting all stakeholders as well as those not directly involved in the project [3]. Several case studies reported worldwide [4][5][6][7][8] suggest that uncertainties in the geology and complexity of tunnel construction processes often result in construction delays and cost overruns [9][10][11][12]. Travertines from Plio-Quaternary are exposed in the vast area around the Antalya region.…”