VietNamNet Bridge – After a family in Yen Son Commune, Quoc Oai District, Hanoi, drilled a well, subsidence occurred in the neighborhood, unnerving locals. Geological experts say the phenomenon may have been caused by underground caverns (Karst) and the weak soil or sand layers above them, which are easily washed away.

The broken wall of a neighbor's house.

On April 3 the family of Mr. Nguyen Dac Cuong in Yen Son Commune drilled a well. When workers drilled to a depth of 52m, some subsidence occurred in the neighborhood. The walls of nearby homes cracked. Long cracks also appeared on adjacent road and in yards. This is the first time the phenomenon has been reported in the area.

Mr. Nguyen Van Binh, from the Department of Geology of the Hanoi-based University of Natural Resources and Environment, says the subsidence is related to the underground cave systems and the presence of weak soil or fine sand layers, which are easily washed away.

In his analysis, the drilling of wells in the areas with weak soil and fine sand layers above a fractured limestone layer and underground Karst caves accidentally creates a path, washing away the fine materials into cracks and underground caves, creating voids in the soil. As the size of the voids grows large enough, the initial equilibrium of the soil is broken and land subsidence occurs.

However, Binh says that this phenomenon also depends on many other factors, such as the size and the depth of Karst caves as well as the composition and thickness of the layers above.

"For the case in Quoc Oai District, if care is taken to drill wells with the proper measures to protect the well walls, the subsidence phenomenon can be avoided," Binh adds.

Some experts agree with Binh, saying that there are underground Karst caves in this area and workers might be drilling into them, causing an imbalance to the upper layer, and leading to the subsidence. "The neighborhood was affected by a phenomenon called shift deformation,” one expert says.

Experts suggest putting an end to groundwater exploitation in this area, and propose that local authorities supply residents with tap water.

The local government is preparing to remove households from the subsided area and has banned drilling wells in the area. The local authorities have also asked the Hanoi City People's Committee to help clarify the phenomenon.

S. Tung