In Japan, as elsewhere, shield tunnelling began with the hand digging shields. The shield served simply to protect workers excavating with shovels. This soon led to the shields being equipped with a backhoe, or a roadheader type loading machine to help with excavation and transfer muck into the shield. The hand digging and later semi mechanised shields are referred to as ‘traditional methods’ in Japan.
Before the birth of shield tunnelling in Japan, there was a lot of work all over the country, but very little in urban environments. At around the same time Hitachi Zosen began manufacturing shield machines, shield tunnelling was adopted with alacrity. This was especially so in urban areas for metro construction. Although today there is no practical plan to construct a new metro, or extensions to existing metros in Japan (so tunnelling is driven by other sectors).
Limitations
A technical weakness and limitation of the technology compared to closed shields is the increased threat of ground settlement or collapse. Also, if operator skill is poor, the contractor cannot keep to the tunnel alignment. Otherwise, dust scattering can compromise the working environment.
Both of these weaknesses can be offset with correct and complete ground conditioning by the contractor, in which case there will be no such limitations. Where ground conditioning cannot keep the integrity of the ground or hold back water, this is of course an inappropriate method.
In Japan, compressed air is very rarely used, due to health and safety concerns, so ground conditioning is essential to make the ground steady across the entire cross section, for the entire length of the tunnel.
Flexibility
Although closed machines are not just limited to a circular cutting shape, there is less limitation on cutting section options when an open machine is used. This is because it can be equipped with a boom-style excavator that can move freely around the shield section.
This also extends to overcutting. While a closed shield is limited in overcutting range as it can only push copy or reamer cutters out from the cutterhead, an open machine can overcut freely via its loading machine or by hand mining.
So the open, semi mechanised shields are selected when special circumstances dictate. A closed shield may also struggle on projects where the TBM will encounter obstacles that will need to be removed; make a connection into an existing structure; make an extremely sharp curve or for the construction of a rectangular or horseshoe shaped cross section tunnel.
Although these machines are rarely adopted in Japan, there are many recent tunnel projects in severe conditions such as those mentioned above that would be appropriate for this technology.
Orderbook observations
Hitachi Zosen have delivered 354 units of these machines so far. But since the 1980’s, closed shields have rapidly gained popularity. The major reasons are that closed TBMs have high safety and higher efficiency compared to open shields. Though, because the latter has freedom to choose the excavation cross-section, is small in cost, and has the high adaptability to various conditions, it is adopted occasionally even today.
No units have been sold since 2006, with 16 of the last 20 years seeing no orders. The record year was 1973 with 47 orders. In general, open type is a circular semi mechanised machine (equipped with labour-saving excavation machinery). Also, we often deliver the TBMs with a full turning excavator for high-speed construction.
Shinjuku station
Shinjuku Station is one of the major stations in Tokyo, Japan. Traffic, commerce and companies are concentrated around the station and the vicinity is plagued by severe traffic jam and lack of a transfer function.
Client the Ministry of Land, Infratructure, Transport and Tourism, decided to sponsor maintenance of a connection point for public transportation, replace a viaduct and construct an underground walkway binding the new subway Fukutoshin-Line near Shinjuku Station’s south exit.
Of this walkway, 114.5 of 233m of which was underground, a semi-mechanised open TBM was adopted by contractor Konoike-Aisawa JV to bore the Tuffaceous clay with associated ground improvements.
The walkway was opened in May 2010, and linked JR Shinjuku Station Southeast entrance plaza with Tokyo Metro’s Fukutoshin Line Shinjuku-Sanchome station, and brings bright and comfortable space with its 6m width.
The tunnel passed through a small space. To ensure an adequate earth covering of 6-7m, considering the economic value of the cross-section, a rectangular shape for the tunnel was adopted. It measured 7,820mm by 4,720mm (shield od), and was an arc-shaped complex cross-section.
Furthermore, the excavation length was extremely short. The owner calculated that if they employed a closed TBM, the construction cost per meter would be overly expensive, so an open type TBM, which is inexpensive in comparison, was chosen.
Machine features
This shield body has a rectangular shape, and each side curved. This machine digs a tunnel face with two loading machines installed to the right and left, makes a space, pushes into the space, and builds the tunnel.
Its driving stroke is 500mm per advance. To prevent collapse and ground subsidence, the excavation section is being subjected to ground improvement. But to protect workers from the collapse of small landslides caused by drilling, the top of the shield body is equipped with seven movable hoods. After excavating the top, the movable hoods are pressed against the tunnel face immediately to form the roof, to ensure the safety of drilling at the face.
In addition, working platforms extended and retracted by hydraulic cylinders have been installed at the front of the body to be used during ground improvement and removal of some 60 existing piles.
In order to prevent the collapse of the ground while the shield machine stops, eight face-support hydraulic jacks are arranged to push a plate against the tunnel face.
Furthermore, the shield machine is equipped with movable sleds at the bottom of the front body for the attitude control (to prevent a nose-down). Movable sleds are also effective in correcting rolling of the machine caused by the rectangular cross section because the sleds are placed on the right and left.
Equipped with these facilities and equipment, this machine had a length of 6,500mm, and weighed 200t.
Since the gap between ground and viaduct was only 4.5m, the TBM had to be divided and lowered into the shaft by a small size overhead crane. There was a limit to size and weight of the divided TBM. The shield was divided into 12 blocks weighing up to 13t.
Features of the loading machine
The loading machine is a dedicated backhoe for semi-mechanized TBMs. It has a high capacity to break through the tunnel face. Two machines are equipped at the right and left side of TBM’s face to cover the whole flat cross section.
The backhoes have four operations, – boom up/down; boom left/right; arm protrude/draw and bucket scratch/dump. Each machine is operated independently of the other making simultaneous work possible.
Operations are controlled by joystick and the speed can be controlled freely, which allows the efficient excavating of the large section.
Features of the segment erector
Two segment erector devices were equipped, capable of complex functions, such as swinging of the arm and rolling of the clamp, are installed symmetrically in order to erect a rectangular ring consisting of eight segments.
Construction of the tunnel was completed successfully in a high-risk environment.
Conclusions
In Japan, the adoption of these machines is limited to construction under special conditions – very short tunnel length, existing obstacles, and complex line-shape and/or cross-section.
However, they are also appropriate in many situations because a visible excavating face lowers the risk of the TBM being stuck by obstacles and ground improvement in tunnel face. So, Hitach Zosen believe that the system’s value will continue in the future as a traditional machine type. It is basic, not ‘old’.
This author believes that although contractors and clients do not consider the possibility of excavating a long tunnel by open shield, an intermediate machine, between open and closed, could show the technology’s raison d’etre.
Although it is only used in special conditions at the moment, it would be good to see the flexibility of the open type leading to its being developed and changed constructively, in which case it will be adopted far and wide
