ABSTRACT
In the recent years, the shielded TBMs are implemented for tunneling in the shaft boxes of the subway stations to provide a direct connection between the tunnel routes throughout the shaft. This ensures no machine stoppage within boring process that decreases the operational costs and then a short distance between surface and the underground platforms is created. However, this adaption can result in some adverse effects such as break-ups of the overburden or blow-outs of the support medium. In this paper, challenges regarding a shallow tunneling using a shield machine through a rectangular shaft box with water bearing soil are described. A case study was considered for evaluation of the instabilities during tunnel excavation, and the relevant calculations regarding the face support pressure are given. Finally, two proposed approaches are presented in order to apply in the similar situations to avoid surface instabilities and some discussions on the recommended solutions are given.
