Abstract

Excavation and implementation of tunnel support system of T341 tunnel of Ankara-Sivas High Speed Train Project were completed recently. Throughout the tunnel excavation, serious stability problems occurred both in the entrance portal section and tunnel. The purposes of this study are to explain the failure mechanism and to propose a suitable support system for the section between Km: 341 + 040 − 341 + 114. Tunnel works were interrupted after the collapse, which occurred in a section of approximately 74 m between Km: 341 + 040 − 341 + 114. As a result of the collapse in the tunnel, local settlements emerged on the surface and it was observed that the effects of the collapse on the surface, as well. To surpass the collapsed section, extra reinforcements were performed by grouting taking place both at the surface and inside the tunnel. Afterwards, tunnel excavation was completed in relatively short stages by applying double umbrella system and rigid support system. Consequently, the causes of the failure, excavation and support system design and results of the numerical analyses are discussed.

Introduction

The T341 tunnel of Ankara – Sivas High-Speed Railway Project of Turkey is located between Km: 340 + 860 (entrance portal) − Km: 342 + 080 (exit portal), was completed successfully. The length of the tunnel is 1220 m. Figure 1 shows the location of the tunnel, and some Google Earth views of the tunnel portal locations are given in figure 2. In March 2020, after the tunnel excavation and supports were completed, inner lining concrete was started. However, sudden deformations occurred between Km: 341 + 040 − 341 + 114, and a sudden failure occurred in this part of the tunnel (figure 3). After this stage, the tunnel works had to be suspended. The main purpose of the present study is to explain the failure mechanism and to propose a new design system. After the ring is closed and the deformations are completed, a failure in a tunnel is not expected in general. Therefore, investigation of this unusual situation is an interesting case for tunneling. For the purpose of the study, the previous documents including geological and geotechnical investigations, projects, construction stages and deformation measurements were examined, and the field observations were performed.

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