In order to finish urban shallow tunnels (driven by NATM) projects as scheduled shafts should be sunk at certain intervals to form new excavation faces. Entering the tunnel from nearby shafts in addition to the tunnel axis to create new excavation faces is time consuming and costly. For this reason, to form new excavation faces directly from the shafts sunk in tunnel axis seems more economical. However, the greatest problem in such a study appears to be the stability. The stress accumulations at tunnel-shaft intersection areas contain hazard from the aspect of stability. Although it is not likely to face serious stability problem in strong rock, this situation is potentially prone to reaching to dangerous levels especially in weak rocks. In this study the stability at the shaft sunk in the tunnel axis and the intersection points of Goztepe station as part of the 2nd stage of Izmir Metro, was analyzed by numerical modelling. The accuracy of the numerical modelling results was later verified comparing with the results of in-situ surveys and measurements. During the study, shaft-tunnel intersection areas have been consolidated by umbrella arch method. The main reason for this strengthening was the obvious difference in the sizes of the shaft and the tunnel i.e., shaft crossectional areas was smaller than that of tunnel. Therefore, stability of tunnel-shaft intersection area was designed accounting that the whole load of the shaft was too be carried by the tunnel support and the ground itself.
Formation of new excavation faces along the route of conventional inner city tunnels shortens the completion time scheduled for tunnel construction. This may be achieved by either sinking shafts aligned with the axis of the main tunnel at certain intervals or by intersecting the main tunnel via subsidiary tunnels under suitable topographic conditions. The former way is considered to be both time consuming and costly. Therefore, the most cost efficient way appears to be sinking shafts along the tunnel axis to the cutting face provided that ground conditions are satisfactory and adequate reinforcement system is employed.
Certain reinforcement systems cannot be literally distinguished. Thus, pipe-arch reinforcement process should be separated from other systems. In this process, the main tunnel is systematically excavated under the protection of previously constructed pipe jacking. The pipe arch process is one of the safest and most efficient ways of constructing large diameter tunnels over short distances. It is likely to encounter various applications of Pipe Arch method in literature (Hoek, 2003; Kim et al. 2004, Miura, 2003; Gibbs et al. 2002). The pipes can be exercised on the ground by two different methods, being pre-drilling and case- drilling. Both methods process advantages and disadvantages (Volkmann, 2004).
In this study, deformations were investigated at the intersection of shaft no.8, one of the eight service shafts sunk on the axis of main tunnel route, and 5.2 km. long main tunnel constructed as the second stage of Izmir Metro Project.