Editor's note: Last time, we mainly introduced the differences between the shield method and the pipe jacking method in underground pipelines. This issue will introduce new trends in shield tunneling in Japan, such as long-distance construction, sharp curve construction, and high-speed construction technologies that have been frequently applied in recent years, as well as technical trends in measures against underground obstacles.

Changes in Shield Machine Styles

Since Japan first adopted the manual-excavation open-type shield in 1920, the shield method has a history of about 100 years in Japan. From the 1970s, slurry-type and earth-pressure-type closed shield machines gradually increased, and by the mid-1980s they had become the mainstream shield machines for non-open-cut methods in cities. To solve various problems, some special shield machines have also been gradually developed.

Long-distance construction

In Japan, distances generally exceeding 1.5 km are called long-distance construction, but in recent years, ultra-long-distance construction of over 5 km has become more common. Therefore, it is very important to adopt targeted measures to improve the durability of shield machines.

1) Countermeasures for cutter wear and mechanical replacement technology

To extend the service life of cutters, methods such as using wear-resistant materials, enlarging cutter heads, arranging cutters in a stepped pattern, and developing special leading cutters that allow a larger amount of wear are adopted during construction. In addition, in the past, operators had to go to the working face to replace cutters, but now mechanical replacement technology has been developed, which can complete cutter replacement inside the machine at any time and an unlimited number of times, shortening time and improving safety.

For example, Obayashi Corporation and Hitachi Zosen Corporation jointly developed a mechanical cutter replacement technology called the "THESEUS" method, which completes cutter replacement from inside the shield machine through a movable manlock and a sliding cutter replacement device, eliminating the need for a cutter replacement shaft and realizing mechanical atmospheric-pressure cutter replacement for small shields.

Overall schematic of the THESEUS cutter replacement method

2) Countermeasures for wear of cutter bearing parts

The cutter bearing part is the heart of the shield machine, supporting the rotating cutter face. If this part is damaged, the shield machine cannot continue excavation, and it cannot be repaired within the current shaft. It is necessary to build a new shaft in front of the cutter head to replace the entire bearing set, or to excavate back from the终点 side. To prevent damage to the cutter bearing part during long-distance excavation, methods such as using materials resistant to sliding heat generation, cooling devices, increasing the number of seal layers, and automatic grease lubrication are adopted.

3) Improving the durability of tail seals

The purpose of installing tail seals is to prevent soil and groundwater from infiltrating into the shield machine through the tail gap. To maintain the elasticity of the tail seal, brush-like materials made of fine steel wire are often used. Since earth and water pressure and backfill pressure always act on this part, its sealing performance is particularly important. In long-distance construction, it is required to be wear-resistant and durable, so wear-resistant and corrosion-resistant materials, increasing the number of seal layers, increasing the number of steel wires in the brush, and automatically filling the brush with tail seal grease are adopted during construction.

Recently, materials such as delayed-hardening tail seal grease have been developed to prevent the tail brush from hardening due to the infiltration of simultaneous grouting material, losing elasticity, and causing damage to segments. For example, the new tail seal filling material "Seal Knock CR" (Cement Retarding) jointly developed by Obayashi Corporation and JX Energy can maintain a softness (consistency) that allows the grease pump to transport it for about 24 hours even if backfill grouting material is mixed in, which is twice that of conventional tail seals. It can be discharged from the tail before solidifying, preventing damage to segments.

In some cases, emergency water-stopping devices are installed between the tail brushes to cope with unexpected situations that may require replacement.

Sharp curve construction

Since the birth of articulation devices and over-excavation devices in the late 1970s, sharp curve shield construction technology has developed rapidly, and the minimum curve radius has now reached 8 m.

Although articulation devices and over-excavation devices themselves have not changed much since their development, various auxiliary methods have been developed to safely and reliably negotiate even smaller curves.

Technologies developed for sharp curve construction

For example, to prevent collapse of the extensively over-excavated part on the inner side of the curve, methods such as injecting high-viscosity filling materials and filling bag bodies with simultaneous grouting material to fix segments to the ground as early as possible and ensure reaction force for the shield machine have been developed. In addition, to make it easier for the shield machine to turn, methods such as a two-stage arrangement of articulation devices and offsetting the cutter head toward the inner side of the curve have also been developed.

High-speed construction

In the past, excavation was often accelerated by improving the thrust and shear torque performance of shield jacks, or by using one-pass segments to improve assembly efficiency. In recent years, simultaneous excavation and segment assembly technology has been developed. Construction systems using simultaneous excavation and assembly technology are roughly divided into the long jack method and the double jack method. In either case, the shield machine is longer than ordinary shield machines, so attention must be paid to sharp curve construction and the size of the starting shaft.

Slurry shield machine for the East-West Connecting Gas Pipeline Construction Project (Futtsu Section)

In the East-West Connecting Gas Pipeline Construction Project (Futtsu Section) from Shintomi in Futtsu City, Chiba Prefecture, to Tokyo Bay, Kajima Corporation adopted simultaneous excavation and assembly technology, with a total construction length of 9,030 m, an outer diameter of 3,440 mm, and a finished inner diameter of 3,000 mm.

East-West Connecting Gas Pipeline Construction Project (Futtsu Section)

Next, during high-speed construction, it is necessary to transport segments into the pit and carry out soil and other materials according to the excavation progress of the shield machine. For this purpose, some sites have installed large elevators dedicated to materials such as automatic segment transport in the pit, as well as tire-type transport equipment that can transport large quantities of supplies, machines, and materials at once.

Tire-type segment trolley

For example, in construction using earth-pressure balance shield machines, systems such as continuous conveyors and vertical conveyors have been used to replace traditional steel car transport of muck, making high-speed construction of large-section tunnels possible.

Further reading

Development of new shield construction technologies in large-section tunnels - Measures against underground obstacles

When constructing tunnels in cities, obstacles such as retaining wall steel materials and piles left from past projects are encountered, and it is often difficult to remove them directly from the ground surface. Generally, in construction, it is necessary to first treat the ground ahead of the working face to ensure ground stability and waterproofness, and then operators go to the working face to remove the obstacles. However, this method is dangerous and delays the construction schedule.

To solve this, a shield method that directly cuts obstacles such as steel materials with cutters has been developed. By setting the previously horizontal spokes at an angle so that their center protrudes, they contact the steel material little by little for cutting, effectively preventing the cutter from being unable to rotate. In addition, in this case, the excavation speed should be controlled at about 1 mm/min.

The shield machine for the Oizumi-minami (southbound) section of the Tokyo Outer Ring Road, whose cutter head can tilt 5° for cutting underground obstacles

In the next installment, the editor will continue to introduce the latest technology of the pipe jacking method. If you are interested, please stay tuned!