This blog post examines the causes and effective countermeasures for sinkholes and road collapses, focusing on how to prevent them amid increasing urban underground space development.
The utilization of underground space is increasing to replace limited surface land in urban areas, driven by public purposes such as urban redevelopment, traffic volume control, and securing available land. This phenomenon is essential for sustainable urban development and efficient space utilization, and it is also receiving significant attention as a future direction for urban development. According to Korean government statistics, Seoul has developed and utilizes 129 underground roadways, 88 underground walkways, and a total of 578.9 km of subway lines. As demand for parking space grows, deeper and wider underground spaces are being developed to avoid existing underground facilities.
This utilization of underground space contributes to improving the quality of urban life by enabling the expansion of various public and convenience facilities beyond the limitations of surface space. For example, it provides benefits such as reducing surface traffic congestion and creating a better pedestrian environment by reconfiguring complex traffic networks underground. Furthermore, underground space development plays a crucial role in securing green spaces, which are scarce in urban centers. By relocating existing above-ground facilities like roads and parking lots underground, green spaces such as parks can be created above ground, contributing to a more pleasant urban environment.
However, as underground space development increases, concerns are also growing about the potential for sinkholes to appear throughout urban areas. Indeed, reports of five sinkholes discovered in the Jamsil area of Seoul’s Songpa District between June and August have rapidly spread fear about sinkholes. So, what exactly are these sinkholes frequently occurring in urban areas, and what causes them?
A sinkhole refers to a hole formed when underground cavities in limestone areas collapse, also known as a doline. When bedrock consists of water-soluble materials like limestone, salt, or carbonate, the flow of groundwater containing dissolved carbon dioxide dissolves the bedrock, creating underground cavities, or empty spaces. When groundwater filling these cavities drains away due to prolonged drought or excessive pumping, lowering the water table, the caves can no longer support the weight of the overlying strata. This causes the surface layer to collapse, forming a sinkhole. They typically take a funnel or cylindrical shape and are commonly found in areas with highly soluble rocks like limestone. In South Korea, limestone regions are concentrated in parts of Gangwon Province. Seoul and its metropolitan areas are mostly built on geologically stable granite and gneiss bedrock, making the occurrence of such natural sinkholes in Seoul highly unlikely.
So, what is the true nature of the sinkholes frequently occurring in Seoul’s downtown area recently? This confusion stems from mistaking road subsidence for sinkholes. As previously noted, sinkholes refer to ground collapse in limestone regions. Therefore, phenomena occurring in the Seoul metropolitan area, which is predominantly granite, are not strictly sinkholes; the accurate term is road collapse. Consequently, the incidents in the Jamsil area of Songpa-gu, Seoul, should be called road collapses.
Road subsidence occurs due to artificial construction or aging sewer pipes, differing in cause from sinkholes that form naturally in limestone areas. Primarily, the damage to aging underground utilities or changes in groundwater flow due to civil engineering works and underground structures cause soil erosion. This creates voids beneath the alluvial deposits in urban areas. Initially, the void is small, but over time, the upper part of the void gradually collapses, enlarging the void and causing subsidence at the ground surface. When a buried water main breaks, water leaks out, washing away surrounding soil and creating a void. Conversely, a broken sewer pipe causes surrounding soil to flow into the pipe, forming a void. Furthermore, the expansion rate of the void increases with higher silt content and greater fluctuations in groundwater flow velocity and level. Consequently, road subsidence occurs frequently during the rainy summer season, with most incidents happening during or immediately after rainfall. According to government reports, the primary cause of road subsidence and sinkholes is leakage from damaged, aging water and sewer pipes. The subsidence typically involves small-scale road sinkholes, approximately 30-50cm in diameter and 1m deep.
The road subsidence problem is serious not only because it involves damage to ground structures but also because it directly impacts urban safety. Particularly in urban areas, road subsidence can lead to consequences beyond simple ground sinking, potentially causing traffic accidents or large-scale property damage. For this reason, thorough management and continuous inspection of underground utilities are essential.
Urban road subsidence can be prevented by detecting underground voids within the city beforehand and filling them before surface subsidence occurs. One method for detecting underground voids is Ground Penetrating Radar (GPR) survey, a physical exploration technique. This non-destructive method sends electromagnetic waves into the ground to detect voids and is useful for locating underground utilities and voids. Since road subsidence often results from groundwater flow caused by the failure of aging underground utilities, periodic surveys of areas with old water and sewer pipes or high-risk zones are crucial. Replacing aging underground utilities at regular intervals eliminates the cause of voids forming within the ground, thereby preventing urban road subsidence.
As the need to develop deeper and wider underground spaces increases, the scale of civil engineering projects is also growing. Cavities created by indiscriminate civil engineering works are frequently causing road subsidence in urban areas. In this situation, to ensure citizen safety and sustainable urban development, meticulous construction management of civil engineering projects is essential to prevent disasters like road subsidence. Furthermore, professional research is needed to correct the meaning of road collapses, commonly called ‘sinkholes’ which have caused fear, to prevent them, and to enable swift restoration.
While the development of underground space has become an essential task for modern cities, accurately understanding and preparing for the potential risks that accompany it is paramount. Future urban development must strengthen not only technical safety but also social responsibility, ensuring safe and efficient space utilization even within the complex urban environment.
