An Introduction to Subway Construction Technology Construction work for a subway station is done by digging from the road above the ground using the Cut and Cover Method, and the section between the tunnels is built using the Shield Method. The Shield Method uses the box-shaped station tunnel, which is constructed by the Cut and Cover Method, as a base, and the circular shield is used to bore the tunnel through to the next station. Recently employed shield methods are the Slurry Shield Method and the Earth Pressure Balanced Shield Method.

The Process of the Cut and Cover Method Preparation for construction· Investigation of the underground utilities The location of gas and water main etc. under the road is thoroughly inspected. · Transplanting trees and shrubs The roadside trees above the site are moved temporarily. · Relocation of the underground utility lines Any gas or water main lines that are an obstacle in the pile driving are relocated to another area under the road. · Relocation of the roadside facilities Guardrails, guidance signs etc. are relocated to another area of the road. · Reduction of sidewalk space In order to provide the space needed for construction work, part of the sidewalk is cut and used for car traffic. Building an Earth Supporting-Wall· Column-type diaphragm After the holes are dug to the necessary depth by earth augar, the strong mortar is poured in the holes and set H shaped steels within the mortar in the holes. · Wall-type diaphragm After digging a deep ditch by the excavation machine, the structural steel is inserted in it and concrete is poured into the ditch to form the underground wall which will be used as a station structure. · Setting H shape Steel After holes are dug to the necessary depth by earth auger, strong mortar is poured into the bottom (penetration depth) and the H shape steel is inserted.

Placing temporary road deck· Placing temporary road deck The pavement is dismantled and after digging 1.2 m in depth, a support girder (H shaped steel, strong enough to support cars) is put into place. Then a concrete or steel temporary road deck is laid. Excavation work·  . Excavation The digging is done manually as far down as the underground utilities, and then the rest of the digging is carried out by machine. · The placement of waling and strutting Along with the excavation, in order to prevent landslide or deformation of the ground waling (H shaped steel butten) and strutting (H shaped steel support) is used. Suspended Guard of Underground Utilities· Suspended Guard of Underground Utilities Existing underground utilities, such as gas, water, etc., are suspended with steel beams and wire to protect their location and maintain existing condition.

Construction of Structure After excavation, the structure is made from the bottom up as follows.  · Placing reinforcement bar  · Forming work  · Casting in concrete In some cases the flying arch method is used where the top floor or middle floor is constructed and excavation is carried out through the hole left open. First the support walls are put in place and then the bottom floor.

Backfilling Work· Backfilling Work Once the structure is completed the road is reconstructed to its previous condition. The surface above the top floor is backfilled with liquefaction processed soil. Liquefaction processed soil: a mixture of slurry (made from the soil taken at the beginning of construction and mixed with water) and cement or other hardening material. Removal of Temporary Road Deck · Removal of Temporary Road Deck Once the backfilling is complete, the support girder and temporary road deck are removed. The road foundation is laid and temporarily paved. Pile Removal · Pile Removal The steel piles that are no longer needed are removed and replenished with mortar. Restoration of the Underground Utilities ·  Restoration of the Underground Utilities Any gas and water main lines that were relocated are now restored to their original place. Reconstruction of the Road ·  Reconstruction of the Road Any guardrails, guidance signs etc. that were relocated to another area of the road are restored. · The part of the sidewalk that was cut and used for car traffic during the construction work is now restored. ·  The temporary pavement is demolished and the road completely reconstructed.

Shield Method Categories   Closed   Slurry Shield Method The face is stabilized by sending the slurry. Earth Pressure Balanced Shield Method The face is stabilized by equalizing the pressure of earth excavation with the machine.    Open   Used only when the boring face can be independent

Slurry Shield Method

Earth Pressure Balanced Shield Method                                                                                           

Open Shield     Partly mechanized shield of the large double track cross section.     The face is stabilized by increasing the air pressure.

Partly mechanized shield of the small horseshoe-shaped cross section     Partly mechanized shield of the small horseshoe-shaped cross section.     Used for the passage to the entrance & exit.                                                     

The Principle of the Slurry Shield Method The Principle of the Commonly Used Slurry Shield Method

Types of Shield Tunnels The shield tunnel method was developed by Frenchman, M. I. Brunel, in the 19th century, and first used in the section of the London underground that passes under the Thames River. This method was first used in Japan in 1944 when the Ministry of Transport constructed the first circular tunnel, the Kanmon tunnel. The first time it was employed for a subway was when building a semi-circular tunnel near Kokkaigijikomae station on the Marunouchi line, which opened in 1959. The contemporary type of circular shield has been used ever since, starting near Toyocho station on the Tozai line, which opened for service in 1967. Recently, while constructing the Namboku line, more than 70 % of construction used some type of the various shield methods.

Single Track Shield Double Track Shield

Triple Track Shield Eyeglass-Type Station Roof Shield Removable Triple-Faced Shield Triple-Faced Shield (island platform (opposite platforms)

Chronology of Shield Stations Station constructed using the Single Track Pneumatic Shield Kiba station, Tozai line, 1964 Station constructed using the Eyeglass-Type with Comb Girder Shield Method Shin-ochanomizu station, Chiyoda line, 1966 (same method used for Kokkaigijidomae station, Chiyoda line and Nagatacho station, Namboku line) Station constructed using the Eyeglass-Type Roof Shield Method Nagatacho station, Yurakucho line, 1971 (same method used for Nagatacho station, Hanzomon line and Mitsukoshimae station, Hanzomon line) Station constructed using the Removable Triple-Faced Slurry Shield Method Shirokanedai station, Namboku line, 1993 Station constructed using the Triple-Faced Shield Method Kiyosumi-shirakawa station, Hanzomon line, 1999

Removable Triple-Faced Slurry Shield The section between Meguro and Shirokane ventilation room on the Namboku line, including station-tunnel-station were constructed successively using the Removable Triple-Faced Slurry Shield Method. By attaching side shields, conversion from a double track shield machine into the Triple-Faced Shield machine makes a smooth transition from tunnel construction to construction of the station platform area without stopping. Up until now, the tunnel section was constructed and then the station area constructed separately using cut-and-cover method, but with the Removable Triple-Faced Slurry Shield Machine, one machine can be used for both the tunnel and the station, which is much more economical. This machine, along with the Nested Parent-Child Shield Machine, were awarded the 1998 Technology Award for "Construction of the subway tunnel with a flexible cross-section type shield machine" by the Japan Society of Civil Engineers.

Nested Parent-Child Slurry Shield The section between Shirokane-takanawa and Azabu-juban stations on the Namboku line, is a double track tunnel and storage track, altogether a triple track tunnel, was constructed continuously using the Nested Parent-Child Shield Machine. The "Parent" is the largest of its kind in the world with a diameter of 14 m, to bore the triple track tunnel, and the "child", approximately 10 m in diameter is built into the center. Up until now, one machine would be used to bore the triple track tunnel section and another machine for the double track tunnel section, but since the Nested Parent-Child Shield Machine can be used for more than one size cross-section, only one machine is necessary, making it very economical. This machine, along with the Removable Triple-Faced Slurry Shield Machine, were awarded the 1998 Technology Award for "Construction of the subway tunnel with a flexible cross-section type shield machine" by the Japan Society of Civil Engineers.