Underground cave activity can cause 'death' sinkholes

According to experts from the Institute of Geology, the phenomenon of land subsidence in some residential areas occurs because the cover layer on carbonate rock is eroded into the underlying karst cave.

The landslide in Nam Son 2 area, Cam Pha, Quang Ninh from July 30 to the morning of August 1 caused the asphalt road and part of the sidewalk to sink 2.5 meters deep and 5 meters in diameter.

Dr. Tran Quoc Cuong, Institute of Geology, Vietnam Academy of Science and Technology, said that there are two types of sinkholes (also known as death holes). First, due to the development of underground karst caves and the existence of weak soil layers in coastal areas. Second, there are stimulating factors, including the role of fluctuations (raising and lowering) of underground water levels related to tides and rain, factors that increase the load of structures and filled soil layers.

In Vietnam, areas with similar geology have experienced land subsidence due to underground underground space such as: Quy Hop, Nghe An, Thuy Nguyen (Hai Phong); Bang Lung, Cho Don, ( Bac Kan ). In Quy Hop, Nghe An, in just two months of 2021, there were 11 subsidence points. Many holes are 2-7 m wide and 2-2.5 m deep.

The sinking process is summarized in the figure below with the example of a sinkhole located right above the underground space of the carbide (drainage zone). The development process from a to d according to the figure, in which 1 (carbonate rock) and 2 (critical layer of soil and rock covering the surface of underground carbonate rock).

Research by the Institute of Geology shows that in coastal areas, the overlying layer of carbonate rock (limestone) is eroded and eroded underground into the underlying karst cave. Due to stimulating factors, materials of the critical soil layer are eroded and drawn into the underground space through cracks and fracture zones of carbonate rock at the cave ceiling. The loss of critical layers can also occur at karst funnels.

Geological and tectonic structures are also considered to be indirect causes, creating favorable conditions for the development of subsidence. The development of the filled layer, especially in the sea encroachment area, also contributes to promoting the process of subsidence. In addition, the stimulating factor worth considering is the fluctuation of groundwater levels related to tides and rainfall. According to Mr. Cuong, more detailed assessments are needed soon to serve urban management work, minimizing the impact of landslides.

A hole about 2.5 m deep and 5 m in diameter appeared in Cam Pha City. Photo: Bao Long

According to experts from the Institute of Geology, people can identify signs related to landslides through unusual depressions and sinkholes on roads, signs of wall cracks, water loss, or sudden increase in water consumption according to the meter (water supply system broken or ruptured due to sinkholes).

For people's construction projects in areas prone to sinkholes, to minimize property damage, pile foundations should be used for the project, Mr. Cuong recommended.

Nhu Quynh

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