Stability assessments of a triple-tunnel portal with numerical analysis (south of Turkey)

Asli Can,Yasin Baskose,Candan Gokceoglu

doi:10.1680/jgere.21.00028

Abstract

The Bahce–Nurdag tunnel, which is the longest railway tunnel in Turkey with approximately 10 km was planned as two tubes. During the excavation of the second tube starting from the Nurdag portal, the tunnel-boring machine (TBM) jammed at 1267 m of the excavation. The second tube was abandoned due to TBM jamming, and it was decided to build a third tube. For this construction, additional portal and tunnel excavations were needed. These excavations resulted in additional stresses. Moreover, the project was constructed in a highly seismically active region. For this purpose, the tunnel portal section was investigated numerically by finite-element analysis. Considering the results obtained from the three-dimensional (3D) numerical analysis, the existing portal was enlarged and excavation of the third tube was started. Currently, the portal construction was successfully completed, and the third tube was excavated for approximately 2200 m with the TBM. It was concluded that the suggested methodology with the excavation and support systems was found satisfactory when the analysis results were compared with in situ conditions. This study suggests that multiple tunnels and large portals to be constructed in highly seismically active regions be simulated by 3D numerical analysis for successful construction.

Highlights

Tunnelling in dense urban areas frequently results in over-crossing or bypassing the existing tunnels (Liang et al, 2016)
Based on the geotechnical parameters determined for the exit portal of the Bahce-Nurdag Tunnel, the displacement values expected to occur on the slopes and, the effects of the tunnels on each other during the opening of the third tunnel (TBM) are presented below by using MIDAS GTS NX 3D software
As a result of all the evaluations and analyses, it was understood that the recovery of the abandoned T1 tunnel was theoretically possible, but from a safety and economic point of view, it was not the right choice

Summary

Introduction

Tunnelling in dense urban areas frequently results in over-crossing or bypassing the existing tunnels (Liang et al, 2016). Based on the geotechnical parameters determined for the exit portal of the Bahce-Nurdag Tunnel, the displacement values expected to occur on the slopes and, the effects of the tunnels on each other during the opening of the third tunnel (TBM) are presented below by using MIDAS GTS NX 3D software. The thickness of the shotcrete was taken as 15 cm; the rock bolt type is selected as 1 28 The stability analyses both in the static and dynamic case of the portal face and the side slopes were performed using the GLE (General Limit Equilibrium) / Morgenstern-Price method, which gives critical results, because of considerations of the forces in the calculation of equilibrium conditions. This shows that the designed support system has successfully fulfilled its task

Conclusion

Findings

Stages Initial Phase