Prediction maps of land subsidence due to groundwater extraction in Hanoi (Vietnam)
The article presents research results of progressive land subsidence caused by consolidation of compressible water-bearing strata as a result of groundwater pumping in Hanoi (Vietnam). The investigations included collecting and analyzing data on the geology, hydrology, soil properties and observed cases of subsidence from 10 monitoring stations. The city of Hanoi is underlain by sediments consisting of organic and inorganic clay, silt, peat, sand, and gravel. Pumping of groundwater causes consolidation of compressible aquifer layers. To simulate the time-dependent surface settlement caused by groundwater extraction, the multifactorial correlation analysis method (MCA) and finite element method (FEM) were used. The calculated subsidence correlated satisfactorily with the data obtained from field observations. Modeling is completed for forecasting the development of land subsidence in 92 points by the finite element method. The prediction maps are compiled for the surface subsidence in the year of 2020 and 2030. The predicted results showed that in 2013, most of the area in the central part of Hanoi showed the highest values of land subsidence ranging from 30 to 104 cm. In this zone, a large groundwater depression cone has been formed in the Pleistocene complex. The bulk of the area in the zone is covered by the soft soil strata, with their thickness exceeding 5 m. The suburb areas and the outer part of the depression cone manifest less significant land subsidence, ranging from 0 to 30 cm, although weak water-saturated soils with the thickness exceeding 5 m occur there. The land subsidence is insignificant in the areas along Red River and Duong River, ranging only from 10 to 29 cm. In comparison with 2013, the map shows that in 2020 and 2030 the area subject to surface subsidence will continue to expand; although the subsidence value will increase insignificantly, only within 1-15 cm, with an average of 6 cm. The land subsidence rate will be significantly reduced. Recommendations are proposed for zoning the potential extraction of groundwater in the city territory.