Photo/Illutration A manhole rises up about 1.2 meters high in the middle of a road. In the background, workers remove another protruding manhole in Wajima, Ishikawa Prefecture, on Jan. 31. (Saori Kuroda)

Underground manhole shafts are rising from the road by over a meter in some areas impacted by the Noto Peninsula earthquake, obstructing roads, hampering recovery efforts and vexing citizens.

The phenomenon of rising or “floating” manholes is believed to be caused by ground liquefaction due to the earthquake.

According to the water and sewerage bureau of Wajima, Ishikawa Prefecture, manholes are cylindrical structures that are connected to the sewage pipes several meters below the ground.

Although the central government has been working on countermeasures nationwide, Wajima and Suzu cities had not yet taken action to prevent their manholes from floating.

Both cities were hit by major tremors and manholes have been popping up all over the place. Cones have been placed to warn people about the obstructions and some roads have been closed to traffic.

“If the raised manholes are not removed, vehicles carrying disaster refuse will not be able to enter the area,” said an official of Wajima's public works department.

In late January, on a city street in central Wajima, manholes were found to have risen above ground at three locations within about 40 meters of one another. Some were protruding from the street by about 1.2 meters.

A man who owns a parking lot facing the street said, “I am having trouble getting my car in.”

On Jan, 31, the city cut the protruding sections of the manholes with a machine, covered the holes with steel plates and leveled them with soil. Now cars can drive through.

The city is removing these protruding manholes one by one, but their exact number has not yet been determined. This is because, in some places, manholes rose long after the earthquake.

However, removing these protrusions is merely an emergency measure for road restoration.

The connecting sewage pipes that were pulled up by the floating manholes are also damaged, so additional work is required to restore the water supply.

WHY DID THE QUAKE CAUSE MANHOLES TO RISE?

Toshihiro Noda, a professor in geotechnical engineering at Nagoya University’s Graduate School, said that this phenomenon was caused by “liquefaction of the ground.”

When earth rich in groundwater loosens and liquefies due to earthquake shaking, objects with a density greater than that of the ground will sink, while lighter objects will float, Noda said.

Manholes with hollow interiors will float because they are less dense than the ground is.

Noda believes that manholes began to surface a short time after the earthquake because of aftershocks. It is believed that the additional tremors accelerated the loosening of the ground, he said.

In Urayasu, Chiba Prefecture, which suffered significant liquefaction damage in the 2011 Great East Japan Earthquake, the aftershocks also increased the damage.

The central government considers manhole lifting to be a “risk of transportation dysfunction.”

As part of the disaster prevention measures approved by the Cabinet in 2018 in response to the Hokkaido earthquake, the land ministry urgently implemented measures to prevent manholes from floating up in approximately 200 kilometers of sewage pipes in areas where liquefaction damage had occurred in the past.

The ministry had completed most installations of valves and weights by fiscal 2020.

However, there are still many areas in Japan where these measures have not yet been implemented.

According to the ministry, only 47,000 km of the 86,000 km of major sewage pipes in Japan have been earthquake-proofed, including measures to prevent manholes from rising.

Neither Wajima nor Suzu had taken these measures before the New Years’ Day earthquake struck.

“There are certain restrictions on construction work because it is necessary to proceed while ensuring that sewage pipes remain in working condition," a ministry official said. "Disaster prevention and mitigation measures vary widely, and each municipality has its own priorities.”

The official noted, “It will be difficult to complete the measures all at once throughout the country.”

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