Safety of underground developments questioned after sinkholes appear

Accidents have shaken confidence in the safety of underground development. The government and developers need to take thorough measures to dispel distrust.

Since last autumn, roads have sunk and underground cavities have been discovered one after another in a residential area in the city of Chofu, Tokyo. A tunnel for the Tokyo-Gaikan Expressway is being constructed about 47 meters below the area.

An expert panel set up by East Nippon Expressway Co., the main operator of the project, identified the construction as the cause. The construction work has already been suspended, and about 1,000 households in the area will receive compensation. The amount will be determined by factors such as damage to homes.

First of all, reconstruction work in the affected area must be done quickly, and an environment in which residents can live with peace of mind must be restored. Information on the matter must be disclosed steadily, and an explanation of the causes and countermeasures must be given to the residents.

According to the investigation by the expert panel, when chemicals were applied to soften the soil, the excavator mistakenly removed too much soil. Sinkholes were caused because the ground contained a “particular type of soil” comprising small stones and sand, the panel said.

As measures to prevent a recurrence, the committee urged the company to conduct boring surveys of the ground as necessary, even during construction, and improve ground monitoring.

In response, some experts have said soil in Japan is complex and this situation is not a special case. If that is the case, shouldn’t such measures have been taken before the accidents occurred?

Residents had been complaining of vibration and other abnormalities for a month before the sinkholes appeared. The project operators bear a heavy responsibility for not taking appropriate measures at that time and failing to prevent the accidents.

The construction of the Tokyo-Gaikan Expressway was being carried out deep underground, at depths of more than 40 meters below ground level. It is the second project to be covered by the law on special measures concerning public use of areas deep underground, which came into effect in 2001.

Under the law, public works projects conducted deep underground in the three major metropolitan areas centering on Tokyo, Osaka and Nagoya do not require the consent of local residents or the acquisition of land once they are approved by the national or prefectural governments. This has the advantage of reducing the construction period and costs.

In the past, there has been little demand for such projects, and it was thought that the impact of deep underground construction on the surface of the earth was unlikely. Now that this assumption has been shattered, it is necessary for the government and project operators to fundamentally review and correct their safety measures.

Fatal accidents have occurred in underground construction work using excavators. The government must investigate accidents and strengthen measures. Legislative revisions are also worthy of consideration to provide more detailed explanations to residents.

Construction at great depths is also planned for sections totaling 50 kilometers between Shinagawa and Nagoya stations for the Linear Chuo Shinkansen line. It must not be forgotten that the safety of the surface is a prerequisite for underground development.

— The original Japanese article appeared in The Yomiuri Shimbun on April 6, 2021.