Tunneling Excavation Induced Settlements in Granular Soils:Case of Keçiören Subway (Ankara)

Abstract

The need for underground transportation is increasing to reduce traffic within metropolitan cities. During excavation, ground loss within the soft soils and associated ground settlements might occur. Tunneling induced ground settlement has to be within tolerable limits. Otherwise, damage to ground level and infrastructures are experienced. In this study, reasons of ground settlement occurred during the excavation of twin subway tunnels at Keçiören (Ankara) are evaluated. In 2006, the first phase excavation caused ground settlement with no considerable damage. The second phase excavation started in 2012, both within the saturated sandy and gravelly soils of Çubuk Creek’s alluvium. At the time, several collapses and associated ground settlements were reported. Due, an investigation program started including resistivity surveying, borehole drilling and in-situ testing. Since ground settlements were known, back calculation was performed to obtain the settlement trough, inflection point and the change of contraction increment. Moreover, the stress conditions and external stresses exerted by existing structures are reviewed. Collapses with volume up to 80 m3 were observed nearby the buildings and free field during the second phase excavation which were determined through 2D resistivity data and boreholes. It was concluded that, the change of settlement at ground level and on the tunnel segments are related to contraction increment based on the numerical modelling of the twin tunnels. Possible reasons for collapse can be listed as TBM cutting head and shield defect, change in ground structure due to underground water level changes, and the impact of the previously excavated tunnel in 2006.

Monitoring the Ground Deformation Caused by Ankara M4 Subway Tunnel with SAR Images

“…In the first case, the groundwater depth is 4.5 m and the soils are silty sand, good grade sand, bad grade sand, silty gravel and fine gravel. The tunnel is 13.5 m deep and 5.90 m in diameter from the surface and was dug with a TBM (Sojoudi, H., Ulamis, K., & Kilic, R., 2021). During the excavation of the 2nd line in 2006, no serious damage was observed in the surrounding structures.…”