In the early 1960s, China's land subsidence cities were mainly Shanghai and Tianjin. By the early 1980s, it had spread to Beijing, Xi 'an, Changzhou, Wuxi, Suzhou, Jiaxing, Hangzhou, Ningbo, Taiyuan and some small and medium-sized cities in Hebei Plain. At present, land subsidence has occurred in 55 cities in 17 provinces (cities, districts) with a total area of over 6.5× 104km2.
(A) the Yangtze River Delta plain land subsidence
The Yangtze River Delta Plain is one of the most typical areas of land subsidence in China. Shanghai is the earliest, most influential and most harmful city in China. Since 1980s, land subsidence disasters have occurred in Jiangsu, Suzhou, Wuxi and Changzhou, Yangzhou, Taizhou, Nantong, Zhejiang, Hangjiahu and Shaoxing, Ningbo. At the end of 1990s, the cumulative subsidence of Suzhou-Wuxi-Changzhou, Hangjiahu and Shanghai reached 1/3, with an area of nearly 1 10,000 km2, and there was a trend of interconnection in the regions. The maximum settlement of the settlement centers represented by Shanghai urban area, Suzhou-Wuxi-Changzhou area in Jiangsu province and Jiaxing city in Zhejiang province reached 2.63m, 2.80m and 0.82m respectively.
In the early 1960s, Shanghai began to establish a land subsidence monitoring network and took various measures to prevent land subsidence, which effectively controlled land subsidence. Since 1990s, due to the large-scale urban construction, the load of high-rise buildings and the increasing exploitation of groundwater around the urban area, the ground in the central urban area is in a new stage of accelerated subsidence.
In Suzhou, Wuxi and Changzhou, although the exploitation of groundwater has been banned in recent years, the groundwater level in most areas has started to rise, but the settlement rate still reaches 20 ~ 40 mm/a, and some towns even reach 80 ~ 120 mm/a, and after 1990, ground fissures occur in Suzhou, Wuxi and Changzhou due to uneven ground settlement. At present, more than 20 ground fissures have been found, and a ground fissure zone several kilometers long and tens of meters wide has been formed in a large area. The differential settlement of ground fissure zone continues to develop and is still in a high incidence period.
The land subsidence in Hangjiahu Plain has developed to the regional level, forming multiple subsidence centers. By 2000, the cumulative settlement of Jiaxing settlement center reached 827.6mm,1.996,5438+0 ~ 2000, with an average settlement rate of 23mm/a, and in 2000, the settlement rate was 1.4mm/a, which indicated that its settlement rate was slowing down. In recent years, due to the serious over-exploitation of groundwater, the land subsidence in coastal cities of Zhejiang Province has developed rapidly. In 2002, the cumulative settlement of the settlement center in Ningbo was 484.6mm, the settlement area exceeded 175km2, and the settlement rate was 3 ~12 mm/a. Many settlement zones were formed in the western plain of Wenling City. The accumulated land subsidence in the center exceeds 1300mm, and the area with accumulated land subsidence exceeding 300mm reaches 36.45km2, making it the most serious land subsidence disaster area in Zhejiang Province.
The land subsidence in the Yangtze River Delta is mainly caused by the development and utilization of groundwater. Before 1970, the textile industry in urban areas was developed but energy was in short supply. Therefore, a large amount of groundwater is intensively exploited for air conditioning and cooling in textile mills, which leads to a sharp drop in groundwater level and serious land subsidence in urban areas. Since the 1990s, with the rise of township enterprises around the city, the water environment in the Yangtze River Delta has been deteriorating, resulting in a general decline in the surface water quality in the Yangtze River Delta water network area, which is extremely rich in water resources, making the whole area a water-deficient area, aggravating the water shortage of residents in rural areas, prompting a sharp increase in groundwater exploitation and forming a regional water level drop funnel. The resulting land subsidence has now become a regional geological environment problem centered on the city.
(2) Land subsidence in North China Plain
The areas with serious land subsidence in North China Plain are Beijing, Tianjin, Hebei and Shandong.
1935 The land subsidence in Beijing only occurred in the area from Xidan to Dongdan, and the local maximum land subsidence in 1935 ~ 1952 was only 58 mm. After 1950s, with the increasing exploitation of groundwater in Beijing, a regional groundwater level drop funnel centered on the eastern suburb industrial zone gradually formed. By May of 1983, the land subsidence area in the eastern suburb of Beijing reached 600km2, the accumulated land subsidence was greater than 100mm, and the subsidence area reached 190km2. The settlement area larger than 200mm is about 42km2. From 1966 to 1983, the land subsidence center in Laiguangying, the northern suburb of Beijing, was about 277mm, and the land subsidence center in Dajiaoting, the southern suburb was about 532 mm After 1987, the affected area of Beijing's land subsidence rapidly expanded to 1800km2, and the area with subsidence greater than 200mm reached 3,500.
Tianjin is located on the coast of Bohai Sea, with a ground elevation of 3 ~ 5m. It is one of the low-altitude industrial port cities in China. Before 1959, the annual settlement of groundwater mining area is about10 ~ 20 mm; After 1959, the subsidence in most areas of Tianjin exceeded 50mm, and 1975 reached 262 mm/a. From 1959 to 1988, the cumulative subsidence in Tianjin urban area is about 1356mm, the maximum subsidence is 2628mm, and the subsidence area exceeds 7300km2.
The land subsidence in Hebei Plain was mainly formed in 1980s. With the decline of groundwater level and the formation of groundwater level drop funnel, nine main land subsidence areas such as Cangzhou, Baoding, Hengshui, Renqiu, Nangong, Bazhou, Dacheng, Quzhou and Tanghai have gradually formed in Hebei Plain. By the end of 1998, the land subsidence in Hebei plain was over 200mm, covering an area of 48550km2. The accumulated land subsidence in Cangzhou has reached 2250 mm, and with the differential fluctuation of basement fault blocks, more than 20 ground fissures have appeared.
The affected area of land subsidence in Dezhou City, Shandong Province has reached 2037.5km2, with cumulative subsidence 150 ~ 387mm and subsidence center of 300~387mm. From the late 1970s to the present, the average annual settlement is 25 ~ 32.25mm. Since 1989, the accumulated settlement in Jining City is 208.9mm, and the area with settlement exceeding 60mm is nearly 90km2, and the maximum annual settlement rate in the center reaches 48.8mm.
Because the Quaternary sediments in Beijing are relatively dense, the land subsidence and its harm are lighter than those in Tianjin and Hebei Plain. However, high-rise buildings and other major projects have high requirements for ground deformation, and their potential threats cannot be ignored.
(3) Land subsidence in Wei Fen graben basin.
Wei Fen graben is distributed obliquely along Weihe River in Shaanxi Province and the fifth great basin in Shanxi Province. China is a region with strong tectonic activity, and the basement structure changes greatly. Because of the exploitation of groundwater, not only the ground subsidence is caused, but also a number of ground fissures distributed along the structural line are formed, and three serious ground subsidence and ground fissure areas, Xi 'an, Datong and Taiyuan, are gradually formed. Among them, the accumulated land subsidence in Xi 'an area reached 2.6m, resulting in ground fissures 13, with a total length of 73km. The accumulated land subsidence in Taiyuan City is more than 3m, and the ground fissure disaster has intensified in recent years, with a length of 15km. Land subsidence and many ground fissures occurred in Datong, Yuci, Linfen and Yuncheng basins.
Xi 'an land subsidence has a history of more than 40 years since it was discovered in the early 1960s. By the end of 1996, the cumulative land subsidence in Xi 'an has reached 150km2, with a diffusion range of 200km2 and a maximum subsidence rate of 300mm/a. Seven subsidence centers have been formed, with a cumulative subsidence of over 2000mm and a maximum subsidence of 2600mm.
The range of land subsidence in Taiyuan City, Shanxi Province is about 40km long from north to south and 15km wide from east to west, forming multiple subsidence centers. At present, the subsidence area is developing to the high-tech development zone in the east. 1980, the area of Taiyuan basin with subsidence greater than 100mm is 108km2, the subsidence center is in Wu Jiabao area, and the cumulative subsidence reaches 819mm; 1990, the cumulative subsidence in Wu Jiabao is 2600mm, 10 year subsidence 178 1mm, with an average annual decline178.1mm; The cumulative settlement in 2002 was 3700 mm.
The land subsidence in Datong City, Shanxi Province appeared in the late 1970s. The cumulative land subsidence in the settlement center is generally 40 ~ 50 mm, the maximum cumulative land subsidence is 124mm, and the average settlement speed is 8 ~ 10 mm/a ... There is a good correspondence between the land subsidence and the groundwater drop funnel in Datong City in time and space.