Construction of a subway tunnel beneath the downtown, highway, and two rivers

The Fukuoka City Subway Nanakuma Line, which connects Hashimoto Station in the southwest of the city to Tenjin-Minami Station in the city center, was to be extended to Hakata Station, a major-scale terminal station, to improve convenience. Between Tejin-Minami and Hakata stations, Subway Nanakuma Line, Intermediate station (tentative name) was to be newly built, and we were involved in the open-cut construction of the new station’s structure, about two-thirds on the Tenjin-Minami side. We were also responsible for the shield tunneling using the structure as a launching base and connecting work between the tunnel and Tenjin-Minami, where the shield machines would reach.

In this project, twin shield tunnels each with a length of 570-meter are excavated for upper and lower lines. Above the ground are Nakasu, one of the busiest shopping and entertainment districts in western Japan, National Route 202 with approximately 30,000 vehicles passing daily, and two rivers, Naka River and Hakata River. We took care of various issues including the construction noise, vibration, ground subsidence, and groundwater runoff.

Occurring of an unexpected circumstance

The original plan was to build the structure for the Intermediate Station by open-cut method and from there launch the shield machine to excavate the first tunnel to Tenjin-Minami Station, dismantle the machine once there, and transport it to Intermediate Station, reassemble it, and excavate another tunnel toward Tenjin-Minami Station.
But right after receiving the construction order, a buried cultural property was discovered at the site, requiring an archaeological survey and halting the construction for about a year. But it was impossible to extend the construction period because the schedule for the replacement of the Haruyoshi Bridge, located above the construction site, was to begin after the tunneling work had passed through the area.

With a drastic change of plan, the shield machine was launched before the completion of the structure of the station, and road decking beams, intermediate piles, and soil-retaining support works were replaced. The shield tunnel was excavated while the station was built by open-cut construction.
For the shield launching base, the site for two lanes of the road above ground was used. By using this limited space, approximately 75 dump trucks filled with excavated soil were carried out per day. In addition, we had to bring in and out not only segments and other materials but also materials and equipment for the open-cut construction. The whole management was very difficult.

Reaching the arrival wall of the station in operation

As the tunneling under two bridges required preventive measures against ground subsidence and bridge deformation, we introduced the double-checking system of ensuring the proper face pressure and thorough soil volume control.

The process of connecting the tunnel with Tenjin-Minami Station, the climax of the shield construction, was also a big challenge which involved excavating the arrival wall of the station in operation as the shield reached the station. If water flowed into the station, the subway would become unusable.
As a preventive measure, we built a workroom made of steel on the station side of the arrival wall so that the outflow of water would be contained within the space. But outflow didn’t occur, and the two tunnels were successfully completed in July 2019 and February 2020, respectively. We completed the project in about 18 months, adhering to the construction schedule.

Embraced the spirit of shield construction with Obayashi

At Obayashi, many employees involved in shield construction had no particular desire to work in this field before joining the company. But I had a special interest in shield tunneling and talked about it at my job interview.
What I find fascinating about shield construction is that it is a large-scale project carried out steadily but unnoticed by busy passersby above the ground. With such a mindset, I joined Obayashi in 1993 and my first job was shield construction for the Tokyo Metro subway. Then I was transferred to the Shield Engineering Department, which was in charge of field support, sales support, and technical development. Thereafter, I worked at the construction site and in the engineering department, engaged in shield tunneling for the Rinkai Line, Yokohama Municipal Subway, Pittsburgh Subway, and more.

In shield tunneling, excavation is conducted with no visual confirmation of the ground. So just before the completion of the tunnel, the feeling of excitement reaches its peak and when the machine finally reaches the end, the feeling is indescribable. This may be hard to comprehend for someone who has never experienced it but the feeling never changes no matter how many times you experience it.
Whenever I ride a train that goes through the tunnel I was involved in, the memories of the construction and the joy of completing it come back to me.

Overcoming Challenges in Unity

At Obayashi, employees take the initiative to visit construction sites to cooperate with workers there. That’s our corporate culture and I like it very much.
Also, employees never operate on feel but with numbers and logic. This kind of culture nurtured our shield technology, including the Rheological Foam Shield Tunneling method, DOT Tunneling method, and URUP method.

Anther strength of Obayashi is the unity in overcoming challenges. Especially in this case where irregularities occurred, the open-cut construction team and the shield construction team worked together, with the support of the design and engineering departments, to complete a very “good construction”.
We may have to tackle more difficult challenges, but we will continue to make the best of our technical capabilities and teamwork for safe and excellent tunnelling work.