World's First Large-Scale Tunnel Widening by Shield Machine

Fujii:
Obayashi Corporation oversaw the shield tunneling project of the 5.5 km underground tunnel section of the main line of the Yokohama Circular Northern Route project.
In the middle of the 11.5-meter-diameter tunnel, we constructed bifurcation sections connected to the entrance and exit above the ground. This was done by widening along 200 m of the tunnel using an expanding shield and a large-diameter steel pipe roof. It was an unprecedentedly large-scale underground widening project using an enlargement shield excavator.
The construction required considerable attention to safety and security, with a residential area and infrastructure facilities such as highways and railroads above the ground.

Simultaneous construction at four bifurcation locations

Nakano:
I was not particularly conscious of the fact that the project was the world's first attempt, but since there was no precedent, I was quite concerned about how to proceed with it. I was mainly responsible for the frame construction in the excavated area, and the first major challenge was dealing with a frame with an elliptical cross section that gradually widened. It was trumpet-shaped or conical-shaped, so we had to deal with each block individually, in terms of cross-section, reinforcing steel, formwork, supports and others.

Fujii:
Although this was our first such attempt, we needed to work on four locations simultaneously, including the bifurcation sections of both upper and lower lines. Arranging the schedule to construct four sections right after the completion of the main line was a challenge. When we actually started the construction, we found out the need to use methods different from the ones initially planned and thus the design for the change. I still remember racking our brains to proceed with this project.

Nakano:
The drawing of the shape after completion was in front of us, but our job was to make it a reality. For example, the storage space in the tunnel was very limited, making the material management for all four locations mind-boggling. There were many other difficulties, but we managed to meet the deadline by improving efficiency, for example, by gradually increasing the length of progress at a time, from 4 m to 6 m and then to 8 m.

Tsuyoshi FUJII Deputy Resident Engineer, Deputy Project Director (at that time)

The Lesson from the Leading-Edge Project

Fujii:
Now we have a proven record of a large-scale tunnel widening project using an enlargement shield excavator. But frankly, we are not sure how we should showcase this as one of our strengths.

Nakano:
Indeed. I suppose there is not much difference in the technology companies have, despite their strengths and weaknesses. What we can say, however, is that workers at our company are not afraid to tackle the unknown. No matter how difficult the task, we never look for an excuse. No one says, "This can’t be done.”

Fujii:
We say engineers’ work starts after much thinking and running into troubles. When I was a site manager of a different project, I felt the difficulty of being in a decision-making position, but through the shield tunneling of the Yokohama Circular Northern Route project, I realized that the process where the members rack their brains and work frantically to achieve the task at hand is more difficult than the act of judgment of top management. After that tough project, I feel that we can overcome almost anything.

Nakano:
The project always made me tense and I felt my heartbeat speed up, but towards the end of the project, I became more relaxed.

Koji NAKANO Project Engineer (at that time)

Role of the site manager

Nakano:
I consider "contemplation" as the key in construction work. I examine all possible options, thinking them through alone, sharing opinions with others, and getting together with several people to listen to their ideas. And when urged to make a decision, one needs to decide "what not to do" rather than "what to do.” I try not to narrow down options to just one or to take things too definitively.

Fujii:
To tackle “difficult” projects, I psyche myself for the challenge. If the leader gives up, the project will stop so I demonstrate my commitment to the others around me. That kind of mentality makes me aware of various issues that might arise during the construction, enabling me to look at situations from a different angle and to come up with new solutions. If you have a strong desire, things will naturally work out.

Nevertheless, for projects like this one, with no precedents, team unity is the most crucial. Such situation demands a trusting relationship between the boss and subordinates. Another indispensable quality for a site manager is to always be there for the workers, not letting them deal with daily troubles alone.