On the morning of the 25th of last month, at a construction site in Yangpyeong-dong 1-ga, Yeongdeungpo District, Seoul, excavation work was in full swing to build a 20-story office building with 4 basement floors on a site of 10,000㎡.
An orange machine placed on the sidewalk next to the site caught attention. Its name is "Ground Penetrating Radar (GPR)." It is a device that sends radio waves underground and analyzes the reflected signals to detect the subsurface structure. On this day, Yeongdeungpo District deployed GPR equipment to conduct underground inspections in preparation for sinkholes.
Lee Tae-ho (58), an executive director of Wide Shin EN&C, which received the underground cavity detection service from Yeongdeungpo District, began to push the GPR equipment forward. The monitor displayed what was filling up to 5 meters underground, layer by layer.
The executive said, "If you push it forward like this... You can see the monitor, right? It automatically shows the underground situation, and it is also recorded." He added, "The top is asphalt, right? Below that is aggregate, and the areas where the signals are distorted are piles of stones."
◇If an 'X' or inverted 'U' shape appears on the GPR monitor, it suggests a hollow space underground
According to the Ministry of Land, Infrastructure and Transport, 864 sinkholes occurred nationwide over the past five years from 2020 to last year. This means that a road collapsed somewhere in the country once every two days.
The necessary equipment to prevent such accidents is GPR. It is divided into 'handheld' type, which is operated manually, and 'vehicle-mounted' type, which operates while mounted on a vehicle.
The handheld type provides 2D cross-sectional images. The vehicle-mounted type scans a wide area and displays a 3D image. The vehicle-mounted type can detect up to 20 kilometers in a day, mainly used on main roads, while the handheld type is often used to detect underground in narrow and complex sidewalks.
The price of one unit of equipment ranges from 30 million to 150 million won for the handheld type and from 200 million to 1.5 billion won for the vehicle-mounted type. The executive noted, "The vehicle-mounted type is visually better, but there are sections where accuracy declines, so we also use the handheld type."
On this day, Yeongdeungpo District first deployed handheld GPR equipment to detect underground cavities near the excavation site. This machine, approximately 1 meter in width and length, analyzed what condition the underground was in, within a width of 50 to 100 cm.
If an 'X' or inverted 'U' shape appears on the monitor attached to the GPR equipment, it indicates the possibility of empty space underground. A representative of Wide Shin EN&C said, "If abnormal signals are detected on the monitor, we use a remote program to verify and determine whether it is indeed a cavity."
Yeongdeungpo District looked underground once using the handheld device before deploying the vehicle-mounted equipment to search for sinkholes. It took about 3 minutes to inspect a 500-meter section. No cavities were discovered during that day's inspection.
◇Seoul City plans to introduce new technology to measure up to 20m underground
Yeongdeungpo District inspects the roads in the district with GPR once every five years. Last year, it surveyed 142 km, discovering 17 underground cavities, thus preventing sinkholes. This year, they plan to conduct inspections over a distance of 66 km with a budget of 144 million won.
However, many local governments are unable to conduct advance sinkhole detection due to reasons such as insufficient budgets. From 2020 to last year, only five regions—Seoul City, Busan City, Gwangju Metropolitan City, Ulsan City, and Gyeongsangbuk-do—used equipment to conduct underground site investigations.
There is also a significant difference in the lengths of sinkhole detection sections. Last year, Seoul City surveyed 5,100 km, Busan City 1,014 km, while Gwangju City only surveyed 38 km and Ulsan City 6 km. Furthermore, the only places with their own GPR equipment are Seoul City and Busan City.
GPR equipment also has technical limitations. When it rains and the ground is wet, radio waves are reflected by moisture, leading to inaccurate detection results. Moreover, recent large sinkholes have occurred deeper than 2 meters, which GPR can detect. Although GPR can identify structures up to 5 meters underground, its accuracy decreases in the 2 to 5 meter range.
Accordingly, Seoul City plans to install a new technology called "ground subsidence observation network" to measure subsurface changes up to approximately 20 meters deep. It is scheduled to be installed this month at the construction site of section 1, phase 4 of subway line 9, where a sinkhole occurred in Myeongil-dong.
Other technologies are also being used abroad. Florida and Israel have adopted Interferometric Synthetic Aperture Radar (InSAR) technology that utilizes satellites. This method involves capturing the same spot multiple times to detect minute changes in the surface, and it has been attempted for detecting large-scale subsurface deformations.
Ryu Dong-woo, a senior researcher at the Korea Institute of Geoscience and Mineral Resources, noted, "It is virtually impossible to survey all areas of a city extensively," stressing the importance of pre-selecting and mapping vulnerable underground geological environments susceptible to excavation or leaks for management.