Collapse of a highway Construction of a deep underground tunnel To investigate cause and effect | NHK News

On the afternoon of the 18th, the street in front of the house suddenly collapsed in the residential area of ​​Higashi Tsutsujigaoka 2-chome, Chofu City, Tokyo, and a hole with a width of 5 meters, a length of 2 meters was opened and a depth of 5 meters.

In the vicinity of the site, the East Japan Highway is building the “Tokyo Outer Ring Road” more than 40 meters below the ground surface, called deep underground, and according to the company, a large machine that digs a tunnel in the middle of last month means that it has passed.

The person in charge of the East Japan Highway and the experts held a meeting on the night of the 19th, and Atsushi Koizumi, president of Waseda University, who is an expert of the tunnel construction method study group, said: “Is there a causal relationship? I have to find out if it was there. It is too early to conclude. However, I cannot say there is no causal relationship and I don’t think it will suddenly disappear.”

In addition, Kenji Kato, general manager of the East Japan Highway Construction Division, said: “No cracks or water leaks have been confirmed in the tunnel body deep underground, and no damage to the tunnel has been confirmed. tunnel body “. explained.

In the future, it is necessary to check the state of the terrain of the place where the depression occurred, so it is decided to proceed with a boring investigation, etc. to investigate the cause, and it was revealed that construction will not restart until the cause can be investigated. ..

Additionally, since the East Japan Highway collapsed directly onto the underground tunnel where excavation work is taking place, we asked residents of 30 neighboring households to evacuate, noting that the causal relationship was unknown. ..

Now that the emergency restoration of the highway has been completed, there are no evacuated residents.

In this regard, Kenji Kato, General Manager of the Construction Division, said: “We regret the inconvenience to the residents. We would like to take steps to investigate the cause so that we can return to our original life as soon as possible.”

In addition, after clarifying that there were several reports from residents around the collapse site that there were cracks in the house for a month, “it is being summarized and answered how many specific offers were made. I could not do it, I visited the house where they offered me reviewing the site, but it wasn’t directly related to the fact that it had a causal relationship to construction, and I was only investigating because it was still under construction.

In the future, we will continue to focus on monitoring the condition of the depressed area and the surrounding roads, and if there is a change in the road, we will respond promptly.

The “Tokyo outer ring road” = “outer ring road” is one of the ring roads of the metropolitan area and is a high-level trunk line with 6 lanes and a total length of 85 km that connects Tokyo, Saitama and Chiba on a ring road in order to alleviate congestion in the city center. It is a path.

Of these, the section between the Kanetsu Expressway and the Tomei Expressway, which is approximately 16 kilometers north and south of Tokyo, is under construction at a depth of 40 meters or more from the ground surface called “deep underground.” .

Construction will be on the south side in February 2017
▽ Departing from Tomei Junction on Tomei Expressway in Setagaya Ward, Tokyo, heading north, we proceed underground into Komae City and Chofu City.

The construction method consists of digging a hole with the largest large machine in Japan with a diameter of about 16 meters, which is called a shield machine, to excavate the ground, scrape the ground, and lay the tunnel wall.

This time, the construction of the “Tokyo Outer Ring Road” is currently underway in the surrounding area, including below the residential area where the road collapsed.

From Nerima Ward to Setagaya Ward in Tokyo, an underground highway is being built at a depth of 40 meters or more from the earth’s surface called “Deep Depth”, north of Tomei Junction on Tomei Expressway in Setagaya Ward. It was in the middle of last month that we started heading towards, passing through the town of Komae, and reaching the basement of a residential area where the roads in the town of Chofu had collapsed.

“Deep underground” means underground at a depth of 40 meters or more from the ground surface *, and is required by law so that the subsoil can be used efficiently.

Since it is a depth that is not normally used, the law stipulates that basically no land acquisition or owner consent is required for development.

The Ministry of Lands, Infrastructure, Transport and Tourism uses “deep underground”
▽ An efficient route can be expected to shorten the construction period, etc.
▽ It is safer against earthquakes than shallow subsoil and is expected to have the effect of reducing noise and vibration.

In addition to Tokyo’s outer ring road, the Chuo Shinkansen Linear is expected to have a tunnel in the “deep underground” section of about 55 km in total in Tokyo, Kanagawa and Aichi prefectures.

* If there is a pile supporting the building, aim for the basement which is 10 meters or deeper than that.

Regarding this collapse accident, Professor Noriyuki Daiyoshi from Kanto Gakuin University, who specializes in geotechnical engineering, said: “I have never heard of a case where an excavation to a depth of 40 meters or more leads to a collapse. ” Since deep excavation does not affect the surface of the land, safety is ensured and there is a history of excavation directly below the residential area. We investigated if this depression is related to construction and if it is related. If there is, it is a new case that has never been seen before, and it is necessary to verify it properly. “

He added: “I do not know the details without verification, but considering that the accident site was excavated about a month ago and collapsed after a while, it was on the surface of the ground after excavation. In a place close to shallow, Changes such as the movement of groundwater, soil, and sand may have occurred for some reason, and cavities may have been created over time. In this case, countermeasures can be considered by performing a new form of underground monitoring. It needs to be done. “

In the subsoil, which is not more than 40 meters from the surface of the earth, what is called deep underground, and shallower than that, collapse accidents due to the construction of tunnels have so far occurred.

Of these, in November 2016, the highway collapsed on a large scale in front of JR Hakata Station in Fukuoka City, stretching approximately 30 meters in length and width and approximately 15 meters in depth.

At the time, extension work was being carried out on the Municipal Metro’s Nanakuma Line 20 meters below the collapse site, and there are reports that the collapse occurred because the bedrock force was weaker than expected and the Tunnel strength was insufficient.

Furthermore, in June, an accident occurred in which two roads directly above collapsed one after another due to the construction of the Sotetsu / Tokyu direct line tunnel that was being carried out in Yokohama city.

Regarding this accident, a later investigation revealed that the “shield machine” that was excavating the tunnel about 19 meters underground absorbed excess soil and sand, and as a result, an underground hole was created that caused the collapse. I am.