In this blog post, we will examine the principles behind how a decline in the groundwater level causes land subsidence, focusing on the case of Mexico City, and discuss the implications for the Jamsil area in Seoul.

Issue: Ground Abnormalities in Jamsil, Seoul, and Suspicions Regarding Groundwater

Recently, concerns among citizens have grown following sudden instances of the ground sinking in the Jamsil area of Seoul. Interest in ground stability has also increased after ground subsidence was observed on a section of the road near Seokchon Lake, and tilting was noted in surrounding buildings.
The phenomenon of the ground sinking near the surface is called ground subsidence. This occurs when water or gas present in the pores—the empty spaces within the soil or rock that make up the ground—escape, causing the particles to pack more tightly together and reducing the overall volume. In the case of the Jamsil area, although the exact cause has not yet been definitively determined, the possibility that groundwater flow changed or that some groundwater was discharged during the construction of Lotte World Tower 2 and Line 9 of the Seoul Subway has consistently been raised as a potential cause.
Furthermore, as changes in the water level of Seokchon Lake have been continuously observed, research has been conducted on the possibility of a hydrological connection between the groundwater and the lake. However, to date, it has been suggested that a combination of complex geological conditions, changes in groundwater levels, and large-scale excavation work may have contributed to the issue, making it difficult to attribute the phenomenon to a single specific cause. From this perspective, examining Mexico City—a representative case—is highly meaningful for understanding the relationship between declining groundwater levels and ground subsidence.

Mexico City: Groundwater Extraction and Decades of Subsidence

Ground subsidence in Mexico City is cited as a prime example caused by excessive groundwater extraction. In the city center, several historic buildings, including the cathedral, have tilted, and this subsidence has persisted for a long time.
Land subsidence in Mexico City has been ongoing for over 150 years, and even today, subsidence of up to several tens of centimeters per year is reported in certain areas. In particular, subsidence of 20–50 cm per year has been observed recently in some areas, and in the city center, cumulative subsidence exceeding approximately 10 meters has been confirmed over the past century. This large-scale subsidence is known to be closely linked to groundwater extraction that has continued since the mid-19th century.
Since the discovery of an artesian aquifer in 1846, extensive groundwater extraction has taken place in Mexico City over a long period. As the city grew and the population increased rapidly, deeper groundwater was extracted to secure more water for domestic and industrial use, resulting in a continuous decline in the groundwater level.
The phenomenon of ground subsidence was first identified through survey results conducted in the late 19th century, and research into its causes began in earnest in the 20th century. Initially, subsidence of about 2 cm per year was observed, but as urbanization progressed rapidly, the rate of subsidence increased significantly. Eventually, in the late 1940s, it was established that excessive groundwater extraction was the primary cause of land subsidence, and this causal relationship has since been repeatedly confirmed through numerous studies.

The Process by Which a Declining Groundwater Level Leads to Land Subsidence

Under natural conditions, the groundwater level fluctuates seasonally depending on factors such as recharge from rainfall and discharge into rivers and streams. These fluctuations alter the volume of water filling the pores, and the pore water pressure—the pressure exerted by the water within the pores—changes accordingly.
However, when humans extract large volumes of groundwater through wells or when groundwater is continuously lost during large-scale excavation projects, the groundwater level can drop sharply beyond its natural range of variation. As the volume of water in the pores decreases, pore water pressure also decreases; as a result, the soil and rock particles that make up the ground must bear a greater load directly than before.
As effective stress increases due to the decrease in pore water pressure, the particles become more tightly packed, and the voids gradually shrink. This process is called consolidation. As consolidation progresses, the volume of the voids decreases, the overall volume of the ground shrinks, and eventually, the elevation of the ground surface drops, resulting in ground subsidence.

Stages of Consolidation: Primary and Secondary Consolidation

Consolidation is generally divided into primary and secondary consolidation. Primary consolidation is directly linked to the process by which water is actually expelled from the voids, so it proceeds relatively quickly. Therefore, when large volumes of groundwater are pumped out or the groundwater level drops sharply, the resulting primary consolidation settlement can occur rapidly.
On the other hand, subsidence does not completely stop even after primary consolidation is complete. Secondary consolidation occurs as the rearrangement of fine clay particles and the gradual deformation of the clay structure continue even after the pores have been drained. The amount of settlement caused by secondary compression is generally smaller than that caused by primary compression, but it can persist for a very long time and, in the long term, may account for a significant proportion of the total settlement.
Therefore, even if measures such as reducing groundwater extraction or relocating wells are implemented, secondary compression will continue in soil where primary compression has already progressed significantly, and ground settlement may persist for years or even decades.

Soil Properties and Historical Background: Lake Texcoco and Montmorillonite

The extent of ground subsidence is closely related to the properties of the soil particles. Generally, soils with high clay content and a large proportion of expansive clay minerals experience greater consolidation, and once consolidation begins, it is difficult for the soil to return to its original state.
In particular, clay layers rich in montmorillonite expand significantly when they absorb water and contract when the water drains away. Because this process involves substantial changes in volume, such soils are particularly vulnerable to subsidence. Furthermore, repeated expansion and contraction can cause cracks and deformation in above-ground structures such as buildings and roads, making montmorillonite a clay mineral of great engineering significance.
Mexico City was built on a basin that was once formed around Lake Texcoco. The lake bed was extensively covered by a soft clay layer formed by the long-term accumulation of volcanic ejecta and fine sediments, and this clay layer was rich in montmorillonite. Subsequently, as the lake was drained and reclaimed, the city continued to expand; as a result, Mexico City came to be situated on a soft clay layer that retains a high moisture content and is prone to consolidation.
Historically, the area around Lake Texcoco suffered from repeated flooding and inundation. In particular, during the 17th century, a massive flood occurred that submerged parts of the city for an extended period. Over the following centuries, drainage systems were continuously expanded and land reclamation projects were carried out, causing the lake’s area to gradually shrink until it has largely disappeared today. However, the soft clay layer that once formed the lake bed remains intact and continues to be a significant characteristic of the city’s ground conditions.

Implications from the Mexico City Case and Their Application to the Jamsil Issue

The case of Mexico City is a prime example demonstrating that when excessive groundwater extraction combines with soft clay subsoil, severe ground subsidence can occur and persist for decades or more. In particular, in clay layers containing montmorillonite, a drop in the groundwater table can trigger not only primary consolidation but also prolonged secondary consolidation, causing subsidence to persist for a very long time.
In the Jamsil area as well, concerns were raised regarding potential changes in groundwater flow or partial groundwater outflow during the construction of Lotte World Tower 2 and Subway Line 9; interest in this issue grew as changes in the water level of Seokchon Lake were observed at the time. However, based on the results of investigations to date, it has not been conclusively determined that any specific construction project was the direct cause; rather, it has been suggested that a combination of factors—including geological structure, changes in groundwater, and excavation work—may have contributed. Therefore, if a decline in the groundwater table did indeed occur, the Mexico City case could serve as an important reference for understanding the mechanism of ground subsidence.
However, the ground conditions in Jamsil are not identical to those in Mexico City. The Jamsil area is characterized by a wide distribution of alluvial deposits from the Han River; while some sections are dominated by sand and gravel layers, others feature a relatively complex stratigraphic composition, including areas where clay and silt layers coexist. Consequently, even if the groundwater level were to drop, it is difficult to assume that large-scale consolidation, similar to that in Mexico City, would necessarily occur in the same manner. Nevertheless, considering that the area was historically influenced by rivers and floodplains, the possibility of an increased risk of subsidence in certain sections—depending on the ground characteristics—cannot be ruled out.
Another important difference lies in the form of subsidence. While Mexico City experiences gradual ground subsidence over a wide area and a long period, the ground subsidence occurring in Seoul often manifests as sinkholes or localized subsidence due to a complex interplay of various factors, such as the formation of localized voids, aging underground infrastructure, excavation work, and changes in groundwater levels. Therefore, rather than simply applying Mexico City’s response methods, it is important to develop tailored countermeasures and long-term management plans that comprehensively consider Korea’s geological environment, urban structure, ground characteristics, and the causes of subsidence.
In summary, a decrease in the groundwater level lowers the pore water pressure in the ground, increasing effective stress, which can result in long-term ground subsidence through primary and secondary consolidation. Mexico City is the most representative example of this process and is still considered one of the cities in the world experiencing the most severe ground subsidence today. In various regions of Korea, including Jamsil, efforts are needed to continuously monitor and scientifically analyze changes in groundwater levels and ground characteristics; above all, it is crucial to accurately identify the causes of ground subsidence and develop prevention and management measures tailored to local conditions.

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