Two TBMs, one for each line, are specially designed to suit the particular geotechnical conditions of the Project. Their cutter heads are fitted wit "drag bits" and "disk cutters" and they are capable of boring through a variety of ground conditions. The TBM are pushed forward by a ring of hydraulic thrust rams during the rotation of the cutter. Immediately after the completion of each forward move, 8 precast concrete segments are installed around the tunnel perimeter to form a closed ring, which constitutes the final lining. Subsequently, a cement-based grout is injected behind the segments in order to fill the gap between the face of the excavated ground and the external side of the segmental lining.

B. NATM METHOD

The new Austrian Tunneling Method was chosen because of its flexibility to cater for a variety of ground conditions and for the different sizes and shapes of the excavated caverns. The proper selection of the main support elements associated with this method leads to a stable excavation and, more important, to an efficient control of surface settlements. The latter is particularly important in the present case of shallow tunneling in an urban environment, since the protection of overlying structures is a very important issue. Following the NATM excavation, the waterproofing membrane is laid and thepermanent in situ concrete lining is cast.

C. CUT AND COVER METHOD

According to this method, an open pit is excavated from the ground surface to the required depth. The vertical sides of the pit are retained by strutted/ anchored Berliner walls or concrete pile walls. Subsequently, the waterproofing membrane is laid, followed by the casting of the permanent structure, which has the form of a closed concrete box. Finally the whole structure is backfield and the traffic is reinstated.