ABSTRACT
Portals are a crucial part of a tunnel project. This paper discusses the problems and solutions designed for the portals of a 2250m long twin-tube motorway tunnel in the Carpathian Flysch. Each tube will accommodate a two-lane road, and will be built using an EPB TBM with a diameter of 15.87 meters. Consequently, the shaft excavation will reach over 30 meters below the original ground level. The portal was designed with a double-row anchored and partially braced pile wall. To ensure the stability of the head wall during break-in and break-out operations with no pressure in the excavation chamber, the design included eight rows of soft piles surrounded by a secant pile wall. The ground conditions of the Carpathian Flysch in the area are very difficult, posing exceptional challenges for the portals. Compounding this issue is the presence of an active landslide nearby, requiring the portal to be carefully integrated with the landslide stabilization system.
