The tunnels under Istanbul will be a mix of different principles. The red part of the alignment will be immersed tunnel, the grey parts will be done mainly by tunnel boring machines (TBM) and the yellow parts will be either cut and cover technique (C&C), New Austrian Tunneling Method (NATM) or other traditional methods.
Bored and Other Tunnels
Connecting to the immersed tunnel are tunnels bored through rock using tunnel boring machines (TBMs). One tunnel in each direction is provided with one track in each. The tunnels are spaced far enough apart to avoid affecting each other significantly during construction. Short connecting tunnels are provided at frequent intervals so that escape to the parallel tunnel is possible in an emergency.
When the tunnels become very shallow, or the rock becomes very soft, the tunnels are constructed either using a soft-ground TBM or directly by digging down from the surface (cut-and-cover tunnels). In places where the tunnel shape changes, such as where trains can cross from one tunnel to the other, other methods using a number of specialized procedures may be used (such as New Austrian Tunnelling Method (NATM), drill-and-blast, and road-header). Similar procedures may be used to excavate Sirkeci Station that will be formed in a cavern deep underground. Two other stations will be constructed below ground using cut-and-cover techniques; these will be located at Yenikapı and Üsküdar. Where cut-and-cover tunnels are used, they will most probably be constructed as a single box with a central dividing wall between the two tracks.
Altogether, there will be approximately 12 kilometres of tunnels constructed by these methods. These works extend from west of Yedikule Station to the end of the immersed tunnel somewhere east of Sarayburnu Park on the European side, and from end of the immersed tunnel north of Kız Kulesi near the Üsküdar Ferry Terminal to Carrefour near Sögütlüçesme Station on the Asian side.
All tunnels and stations will be waterproofed to prevent leaks and ventilation will be provided. Similar design principles as used for the underground metro stations will be used for the commuter rail stations.
Tunnel boring machines has undergone an extensive improvement during the past decades. The pictures show examples of such a modern machine. The diameter of the shield can with today's techniques exceed 15 meter.
Tunnels drilled under the city will be interconnected every 200 meters to ensure that service personnel can come easily from one duct to the next, and to ensure safe rescuing routes and access for rescuing personnel in case of an accident in one of the bores.
Modern tunnel boring machines can be quite sophisticated in the way they operate. The picture shows a three faced machine used in Japan that can drill an oval shaped tunnel. This technique may be used where the station platforms have to be constructed.
The picture shows a typical interior from a tunnel constructed for commuter trains by tunnel boring machine technique (TBM).
Where cross over tracks or spur lines are needed different tunneling methods can be combined. Here TBM technique and NATM technique from the Copenhagen Metro.