Satellite data shed light on massive sinkhole in Tokyo neighborhood
Underground construction work may be linked to road cave-in
Nikkei staff writers
TOKYO -- The ground level at places around a road cave-in in Tokyo’s Chofu had sunk or risen by between two and three centimeters shortly after a tunnel-boring machine passed underground there, according to a Nikkei analysis of satellite data.
The heavy machinery was drilling an underground tunnel in the city Chofu in Tokyo as part of the Tokyo Outer Ring Road construction project. A section of road in a residential area that it had passed under collapsed in October 2020.
The ongoing construction work was occurring 40 meters or more below ground, a level that is deemed “deep underground” in Japan. Deep underground space is usually not used by landowners directly above.
Nikkei’s analysis also showed that the ground surface had shifted in places other than those directly above the tunnel. A causal link between the rise and fall of the ground surface and the tunnel work is unclear pending a full investigation.
The Tokyo Outer Ring Road is a major thoroughfare that links areas about 15 km from central Tokyo -- from Nerima Ward’s Oizumi to Ichikawa in Chiba Prefecture through Saitama Prefecture.
Businesses that serve the public interest, such as road and railway operators, can use deep underground space without the consent of landowners above. They must get approval from the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) or the prefectural government.
In 2001, a special law that aimed to more effectively use underground space came into effect. The law covers the country’s three metropolitan areas -- the Tokyo metropolitan area, the Kinki metropolitan area, which includes Osaka, and the Chubu metropolitan area, home to Nagoya.
The Chofu cave-in came despite previous safety assurances from MLIT and two expressway operators -- East Nippon Expressway Co. and Central Nippon Express Co.
The ministry and the two expressway operators had said they thought that “any phenomenon undermining safety on the ground surface” would not happen, as sufficient safety measures had been taken.
When the cave-in happened in October 2020, construction work was underway to link the Kanetsu Expressway and the Tomei Expressway. The site of the cave-in is located between Chuo Junction and Tomei Junction.
One month before the collapse was discovered on Oct. 18, 2020, a tunnel-boring machine 16 meters in diameter -- equivalent to a five-story building -- passed underground at a depth of 47 meters.
Nikkei analyzed the movements of the ground surface there and in surrounding areas using a technology called “interferometric synthetic aperture radar,” or “InSAR.” The technology makes it possible to observe changes on the ground surface using satellites and radio waves. Nikkei obtained data from Italian satellite date analysis company Tre Altamira and Japan’s Space Shift and examined how the ground surface changed in an area 530 meters east to west, and 870 meters north to south
As of April 8, 2020, before the tunnel work began, there were only two spots in the area where the ground surface had risen by one centimeter or more.
But as of Sept. 20, less than a week after the machine passed below the cave-in site on Sept. 14, rapid changes could be seen. There were many spots where the ground surface had risen or sunk by one centimeter or more.
As of Oct. 12, shortly before the cave-in occurred, there were even spots where the ground surface had sunk by up to around three centimeters.
The Nikkei analysis also showed that the ground surface declined appreciably on the east side of the cave-in site, and it rose on the west side.
There were also widespread changes in locations other than those above the tunnel. Many changes of one centimeter or more were observed on the opposite side of a nearby river.
From mid-September 2020, rapid changes took place in spots where there had been almost no ground surface movements between April and August that year.
Some spots where the ground sank were found to have sunk deeper over time, and some where the surface rose gradually returned to their previous level.
It was on Oct. 18, Chofu residents reported a sinkhole five meters wide, three meters long and five meters deep. It was located right above the tunnel construction site.
1st underground cavern
On Nov. 4, East Nippon Expressway announced it had discovered an underground cavern 30 meters long, four meters wide and three meters deep, five meters below the surface and right above the tunnel site.
2nd underground cavern
On Nov. 22, the company announced it had found another space at a different location. The second cavity was 27 meters long, three meters wide and four meters deep, and was four meters below the surface. Roads and houses are believed to be above the cavern.
Some residents that lived in the area but not on the tunnel site complained of unusual occurrences on their properties.
In one area where the surface sank, a man complained that the gap in concrete walls had widened and a four-centimeter wide crack had appeared.
“Manholes in the area seem to keep rising us,” he added.
The cave-in site is located in a quiet residential area. Many residents heard noises and felt vibrations when the tunnel work was underway, including those who do not live on the direct path of the tunnel.
A woman in her 30s said that “thuds” began sounding throughout her house in mid-September. “I thought someone was playing music at full volume,” she said. “I didn’t realize it was because of the [tunnel] construction work at first.”
A housing complex separate from the tunnel’s path put up a sign urging residents to be careful not to be too loud in their everyday activities after complaints about noise and vibrations came in.
MLIT, East Nippon Expressway and Central Nippon Express set up a panel of experts in response to the cave-in.
The panel is headed by Atsushi Koizumi, a professor emeritus at Waseda University.
After looking at Nikkei’s analysis, some panel members and senior officials at MLIT acknowledged that the ground surface sank by about two centimeters near the sinkhole location.
The expert panel is said to be acquiring data, including through a boring survey, without using InSAR satellite data. The technology is not thought to be able to track widespread movements across the surface, nor capture detailed changes over time.
The panel released some of its findings on Dec. 18.
The use of deep underground space enables planners to design routes that shorten project times and cut costs.
The use of deep underground space has so far been approved for four projects in Japan, including the Tokyo-Nagoya section of the Linear Chuo Shinkansen project, which will use magnetically levitated bullet trains.
If Japan is to promote the further use of underground space, authorities need to thoroughly investigate the Chofu cave-in and take measures to prevent any future incidents.