Hazardous geological processes on Hanoi territory (Vietnam)

Geological structure and history of geological evolution, tectonics, as well as physical and geographicalposition and climate, are controlling factors of exogenous processes that take place in the territory of Hanoi. The most importantof the engineering geological factors are tectonic faults, presence of thick series of unstable soils and sinking of ground waters leveland settlement of earth surface. Hanoi, the capital of Vietnam, is a city with an area of more than 3.3 thousand km2 and 6.2 million people,located in the delta area of the Red River (Bac Bo Plain). It is flat and filled with alluvial soils brought by the Red River. It consistsof Neocene and quaternary sediments. The quaternary sediments are divided into two sediments (Pleistocene and Holocene). The Pleistocenesediments are formed by gravel, sand and clay and divided into three formations: 1. Le Chi formation: it consists of gravel interstratifiedwith lenses of clayey silt. The thickness ranges from 25.0 to 30.0 m.; 2. Hanoi formation: it consists of gravel intermixed withsandy silty clay. The thickness ranges from 20.0 to 24.5 m.; 3. Vinh Phuc formation: it consists of marine sediments of sand, silt mixedwith pebbles in the lower part, and layers of silty clay in the upper part. The thickness is 10.0 to 45.0 m. The Holocene sediments aredivided into two formations: 1. Hai Hung formation: it consists of clayey silt and sandy layers. The upper part of this sedimentary formationconsists of a layer of marine clay with organic substances. The thickness is from 0.6 to 20.0 m.; 2. Thai Binh formation: it consistsof clayey silt mixed with pebbly sand layers, of alluvial-proluvial genesis. The alluvial swamp sediments consist of clay and peat.The thickness is from 0.9 to 35.0 m. Most of the ground water under the Hanoi plain is found in the quaternary sediments, in two mainsaquifers (Holocene and Pleistocene). The Holocene aquifer is sufficient for small-scale water supply. The Pleistocene aquifer consists ofalluvial sand, gravel, pebble and cobble. The thickness of aquifer varies from several meters to tens of meters. The aquifer has a significantpotential for the supply of groundwater. At present, the total water withdrawal is estimated about 750,000 m3/day. Land subsidencewas developed in the central and southeast parts of Hanoi where groundwater is extensively exploited. The city of Hanoi is underlain bysediments, including organic and inorganic clays, silt, peat, that are compressible. The demand for groundwater supply of Hanoi hasincreased ceaselessly, especially in the recent 15 years. Combining these two factors, land subsidence is an obvious geotechnical problemthat Hanoi is facing.

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