The increasing incidence of road cave-ins and sinkholes in urban centers with active underground development has raised concerns. This article explores the causes of road cave-ins, exploration techniques and management measures to prevent them, and emphasizes the importance of utilizing underground space safely.
As a substitute for limited surface space in urban centers, underground space is increasingly being utilized for public purposes such as urban renewal, traffic control, and land availability. This phenomenon is essential for the sustainable development and efficient utilization of space in cities, and it is also attracting a lot of attention as the direction of future urban development. According to Korean government statistics, Seoul has 129 underpasses, 88 underground sidewalks, and a total length of 578.9 kilometers of subways, and as the demand for parking space grows, deeper and wider underground spaces are being developed by avoiding other existing underground facilities.
The utilization of these underground spaces is contributing to improving the quality of urban life by enabling various public facilities and amenities in urban centers to be expanded beyond the limits of surface space. For example, by reconfiguring complex transportation networks underground, the benefits include reducing congestion on the surface and creating a better pedestrian environment. The development of underground spaces also plays an important role in freeing up scarce green space in urban centers. By relocating existing above-ground facilities such as roads and parking lots underground, park-like green spaces can be created above ground, contributing to a more pleasant urban environment.
However, as the development of underground spaces continues to increase, there is also a growing concern that sinkholes may occur in urban centers. In fact, sinkhole fears are spreading rapidly in Seoul, where five sinkholes were discovered between June and August in the Jamsil neighborhood of Songpa-gu, Seoul. What are sinkholes, and what causes them to occur so frequently in urban centers?
A sinkhole, also known as a doline, is a hole created by subsidence in a limestone formation. When the bedrock is composed of materials that can dissolve in water, such as limestone, salt, and carbonates, a flow of groundwater with dissolved carbonic acid gas dissolves the bedrock and creates a cavity, or void, in the ground. When the groundwater level drops because the groundwater that was filling the cavity drains away due to prolonged drought or excessive pumping, the cave cannot support the weight of the earth and the upper strata collapse, forming a sinkhole. They are usually funnel-shaped or cylindrical in shape and are common in areas with highly soluble rocks such as limestone. In Korea, limestone areas are centered in parts of Gangwon-do Province, while Seoul and the downtown area are mostly geologically stable granite and gneiss, so it is highly unlikely that such natural sinkholes will occur in Seoul.
So, what is the real reason for the recent spate of sinkholes in the Seoul city center? It’s actually a confusion between a pothole and a sinkhole. As we have seen, a sinkhole is a collapse of the ground in limestone areas, so the phenomenon in the Seoul metropolitan area, which is mostly granite, is not technically a sinkhole, but a road depression. Therefore, the sinkhole in Jamsil, Songpa-gu, Seoul should be called a road pothole.
Road potholes are caused by man-made construction or old sewer lines, so they have a different cause than sinkholes that form naturally in limestone areas. As soil is lost due to the breakage of old underground buried pipes or changes in groundwater flow caused by civil engineering works and underground structures, empty space is created under the alluvial layer in urban areas, and although the cavity is small at first, as time passes, the top of the cavity gradually collapses, causing the cavity to grow and create a depression on the surface. In the case of a broken water supply pipe buried in the ground, the water leaks out and causes the surrounding soil to spill out, but in the case of a broken sewer pipe, the surrounding soil spills into the sewer pipe and causes the cavity. The more fine-grained the soil, the greater the flow rate of groundwater and the greater the change in water level, the faster the cavity expands. Therefore, road depressions are frequent during the summer months when rainfall is high, and most of them occur during and after rain. According to government data, the cause of road subsidence and potholes is water leakage due to damage to old water and sewer pipes, and pavement depressions are small-scale road depressions with a diameter of 30 to 50 centimeters and a depth of 1 meter.
The problem of road potholes is not just about the damage to the ground structure, but also about the safety of urban centers. Especially in urban centers, potholes can cause more than just ground settlement; they can lead to traffic accidents and large-scale property damage. For this reason, it is essential that underground utilities are well managed and constantly inspected.
Potholes in urban centers can be prevented by detecting underground cavities within the city center and filling them before they cause surface depressions. One way to detect underground cavities is to use ground penetrating radar (GPR) surveys, a physical exploration technique. This is a non-destructive survey that sends electromagnetic waves into the ground to probe for cavities and is useful for detecting underground burials and cavities. Given that road potholes are caused by groundwater flow due to the breakdown of old underground pipes, potholes in urban centers can be prevented by periodically surveying for old water and sewer pipe burials or at-risk areas, and replacing old pipes at regular intervals to eliminate the source of cavities in the ground.
With the increasing need to develop deeper and wider underground spaces, the scale of civil engineering works is getting larger and larger, and the cavities caused by indiscriminate civil engineering works are causing frequent urban road cave-ins. In this situation, to ensure the safety of citizens and sustainable development of cities, it is necessary to thoroughly manage the construction of civil works to prevent disasters such as road cave-ins from occurring. In addition, it is necessary to study the meaning of road cave-ins, which are often referred to as “sinkholes” and have created a sense of fear, so that they can be corrected, prevented, and quickly restored.
The development of underground spaces is becoming an integral part of modern cities, but it is vital that we understand and prepare for the risks that come with it. In addition to technical safety, future urban developments will need to be socially responsible to ensure safe and efficient use of space in the complex environment of urban centers.
