Higashi-Ojima Trunk Line and Minami-Ojima Trunk Line Construction - Cutting and Removing Residual Piles Directly Beneath an Underground Station and Utility Tunnel Using the DO-Jet Method!
Introduction
Due to the progress of urbanization, rainwater has become less likely to soak into the ground, leading to an increase in the volume of rainwater flowing into the sewer system and causing flood damage. In light of this situation, the Tokyo Metropolitan Government Bureau of Sewerage is promoting flood control measures to realize a safe and secure life.
In the vicinity of Oshima, Koto Ward, and Komatsugawa, Edogawa Ward, as shown in Figure 1, flood control measures are being advanced through the construction of the Komatsugawa No. 2 Pumping Station, the 1.4 km long Minami-Ojima Trunk Line, and the 3.9 km long Higashi-Ojima Trunk Line.
This article introduces a case where the Higashi-Ojima Trunk Line and Minami-Ojima Trunk Line were constructed using the shield tunneling method, and ground improvement and underground obstacles were removed from the shield machine.
Construction Overview
The construction project introduced here involves developing a total of 2.1 km of sections, including 0.7 km of the Higashi-Ojima Trunk Line and the entire 1.4 km length of the Minami-Ojima Trunk Line shown in Figure 2, using the earth pressure balanced shield tunneling method as part of the flood control project in the Oshima, Koto Ward and Komatsugawa, Edogawa Ward area. The pipes were laid by excavating at a great depth of 25m to 40m, while crossing directly beneath the Toei Shinjuku Line and a utility tunnel.
In this project, it was anticipated that multiple piles left in the ground would become obstacles at the crossing points of the Toei Shinjuku Line and the utility tunnel. However, since there were structures directly above these obstacles, making ground improvement and obstacle removal from the ground surface difficult, the DO-Jet method, which allows for ground improvement and underground obstacle removal from the shield machine, was adopted.
The construction overview is shown below.
Construction Location: Oshima 5, 8, 9-chome, Koto Ward, Tokyo; Komatsugawa 1-chome, Edogawa Ward
Construction Period: October 2010 to September 2022
Construction Details: Pipe conduit work, special earth pressure balanced shield tunneling method, construction length 2069.95m
Higashi-Ojima Trunk Line: Finished inner diameter 6,000mm, length 705.25m
Minami-Ojima Trunk Line: Finished inner diameter 4,500mm, length 1364.75m
What is the DO-Jet Method?
The DO-Jet method is a construction technique that mounts an ultra-high-pressure jet system on a shield machine or similar equipment to perform underground obstacle detection, ultra-high-pressure ground improvement, and cutting/removal from the machine in a non-excavation and non-contact manner.
The obstacle removal and cutting work utilizes ultra-high-pressure injection nozzles mounted on the shield machine to perform 'forward detection,' 'ultra-high-pressure ground improvement,' and 'obstacle cutting' in that order, cutting the obstacles into sizes that can be discharged from the mud discharge port. Subsequently, the cut obstacles are taken into the chamber along with the excavated soil as the machine advances and are discharged.
The 'forward detection' performed first involves rotating the cutter head just before the obstacle while injecting ultra-high-pressure jet water and analyzing the reflected sound to identify the location of the obstacle. Based on the detection results, a cutting plan is formulated, and the scope of 'ultra-high-pressure ground improvement' and the position of the cutting lines are determined.
Next, 'ultra-high-pressure ground improvement' is performed for the purpose of stabilizing the surrounding ground during cutting work and after removal, and for protecting existing structures. An improved body is created by injecting ground improvement material (a mixture of cement milk and sodium silicate solution) from the ultra-high-pressure injection nozzle using an ultra-high-pressure jet.
After that, 'obstacle cutting' is performed by injecting cutting material (a mixture of garnet-containing abrasive and sodium silicate solution) from the cutting nozzle using an ultra-high-pressure jet while moving the mobile cutting nozzle or rotating the cutter head at low speed.
Figure 3 shows the work flow for cutting and removing obstacles using the DO-Jet method.
Removal of Obstacles Using the DO-Jet Method
In this project, underground obstacles consisting of temporary residual piles (PIP piles, etc.) that hindered excavation directly beneath the Toei Shinjuku Line and the utility tunnel were cut and removed using the DO-Jet method (see Figure 4).
Until now, the DO-Jet method had only been used to cut and remove single steel obstacles such as H-beams and steel sheet piles; this project was the first to perform cutting and removal work on PIP piles, where steel materials are covered with mortar.
As shown in Figure 4, during construction in the area where obstacles were expected, a 'wall-like obstacle' was encountered first.
Figure 5 shows the results of the shield forward detection using the DO-Jet method's forward detection system for this 'wall-like obstacle'.
From the forward detection results, it was confirmed that obstacles existed in the range shown in the figure, but it was not possible to make a clear judgment regarding the specifications (material, number of obstacle piles) of the obstacles that were continuous in a wall shape.
Therefore, based on the design drawings of previous construction, it was assumed that φ600mm PIP piles were arranged in a wall shape, and a plan was formulated and implemented to cut them in a grid pattern so that the cut pieces would be approximately 420mm, as shown in Figure 6.
On-site, after performing obstacle cutting as shown in Figure 6, shield excavation was started, but because the cutter torque value exceeded the upper management limit, it was judged that continuing excavation in this state would be difficult, and countermeasures were examined and discussed.
As a result of the examination, since the target obstacles were PIP piles with no previous record of cutting and removal, it was judged that the cutting resistance of the H-beams constrained by mortar had increased, causing the cutter torque to rise. Therefore, the plan was revised to shred the cut pieces at a 210mm pitch in the radial direction, as shown in Figure 7.
In addition, the core material of the obstructing φ600mm PIP piles was H-400, and it was found from the preliminary survey results that cutting in a single pass would be difficult, so a plan was made to perform a second cutting after the shield had advanced 300mm after the first cut.
As a result, the cutting and removal of the wall-like obstacles were successfully completed through the two-stage cutting and excavation using the DO-Jet method.
Photo 1 shows some of the obstacle cuttings recovered during the cutting and removal of the wall-like obstacles.
As mentioned above, we have demonstrated a track record of cutting and removing 'wall-like obstructions.' In this project, we also applied the DO-Jet method to cut and remove 15 Φ450mm PIP piles and 7 Φ600mm PIP piles while excavating, allowing us to pass directly underneath the subway station and utility tunnel.
Although we could not cover everything in this article, we overcame various constraints—including not only the cutting and removal of obstructions but also construction on sharp curves and the separation of the parent and child machines to allow a single shield machine to construct the Higashi-Ojima and Minami-Ojima trunk lines, which have different inner diameters—and were able to reach the destination with the shield machine in January 2022.
Past Achievements
In this project, we adopted the 'DO-Jet method' due to surface constraints and achieved results such as ground improvement and the cutting and removal of obstructions from within the shield machine. This was highly evaluated for significantly contributing to the development of shield technology, and we were awarded the 'JSCE Technology Award' for fiscal year 2022.
Conclusion
This project is still ongoing, with construction such as the laying of the Higashi-Ojima trunk line continuing.
Moving forward, we will continue to use ingenuity even in construction under severe constraints to protect the safety of the citizens of Tokyo and support a secure and comfortable life.