Seoul manages to locate sinkholes just in time

After the Seoul Metropolitan Government started using ground penetrating radar (GPR) to detect sinkholes developing under roads, the number of sinkhole accidents dropped from 42 in the summer of 2016 to 12 this summer.

As a GPR device scanned an intersection in Dongdaemun District, eastern Seoul, on Friday, Choi Yeon-woo, who runs a road management team stared at the device’s screen.

“When the straight lines on the screen change into waves, the machine may have detected a sinkhole developing directly below it,” Choi said. “According to data collected by the device, it seems like a hole may be located some 40 centimeters (15.7 inches) below ground here.”

Choi marked the area with white spray, into which his teammates drilled a hole 5 centimeters wide and 30 centimeters deep. Through the hole, they lowered an endoscope.

Photos taken inside the hole showed a grey area for some 20 centimeters down, and then a black area.

“The black area is the sinkhole,” Choi said.

Once the sinkhole was confirmed, the team made a report to headquarters to send a repair crew.

“Usually we find that broken pipes or incomplete construction efforts lead to sinkholes,” Choi said. “Once we fill the holes, our job is done.”

“The GPR device has been a tremendous help in keeping Seoul residents safe from sinkhole accidents,” said Bae Kwang-hwan, head of the Road Management Division of the city government. “From 2014, it found some 1,780 sinkholes developing underground before they led to accidents.”

From June to July last year, a total of 42 sinkhole accidents were reported. This summer, the number dropped to 12. Sinkholes accidents tend to spike during the rainy season.

The city government said the job wasn’t always this easy.

In 2014, the city relied on a Japanese company which used a GPR device.

The GPR uses electromagnetic waves to detect empty spaces under the ground. Seoul didn’t have experts who could analyze the amplitude of the waves to determine how deep the supposed sinkhole was. The government said it picked up the skills by “looking over the shoulder” of the Japanese company.

Choi recalled there were some moments when a sinkhole was discovered just in time.

In December 2014, a 3-meter (9.8-foot) wide sinkhole was detected by a GPR device some 20 centimeters below ground on Donhwamun-ro in central Seoul. Any developing sinkhole that is wider than 2 meters requires immediate reconstruction, Choi said.

“The only reason why the road hadn’t sunk in at the time was because the asphalt above the hole was some 20 centimeters thick,” Choi said. “If it was 10 centimeters thick, the road would have collapsed. Thankfully, we found the hole in the winter, so we could fill it up before the rainy season hit in the summer.”

In addition to the GPR device, the city government also started asking taxi drivers to help detect possible sinkholes.

Since April 2014, taxi drivers have been able to press a button on their card swiping device when they see a damaged road. This sends the coordinates to the city government’s road management team, which inspects the area.

The city government said it received a total of 28,302 reports from taxis from May 2014 to last October - an average of 943 reports per month.

BY HONG JI-YU [chung.juhee@joongang.co.kr]