According to incomplete statistics, land subsidence has occurred in 96 cities and regions in China, and about 80% of the land subsidence is distributed in the eastern region. Land subsidence occurred from Haikou in the south to Harbin in the northeast. The cumulative land subsidence is between 460 and 2780 mm, and the land subsidence speed is10 ~ 56 mm/a. According to long-term monitoring and research, land subsidence is mainly caused by unreasonable exploitation of groundwater, while land subsidence caused by other factors such as crustal activity, surface dynamic (static) load, engineering construction and natural action only accounts for 5% ~ 20% of the total land subsidence. At present, most cities in the Yangtze River Delta, North China Plain, Guanzhong Plain, Huaibei Plain and Songnen Plain are experiencing and developing land subsidence in a large area. Especially in the Yangtze River Delta and North China Plain, the occurrence and development speed of land subsidence has attracted worldwide attention and caused huge economic losses.
(1) Yangtze River Delta
The Yangtze River Delta is one of the most typical areas of land subsidence in China. Among them, Shanghai is the earliest, most influential and most harmful city in China, and land subsidence disasters have also occurred in Suzhou-Xichang in Jiangsu, Hangjiahu in Zhejiang and Ningshao. At the end of 1990s, the accumulated subsidence of Suzhou, Wuxi, Changzhou, Hangjiahu and Shanghai exceeded 200mm, accounting for 1/3 of the area, and the area was nearly 1 1,000km2, and there was a trend of regional connection. The maximum cumulative settlement in the settlement center areas represented by Shanghai, Jiangsu, Suzhou, Wuxi and Jiaxing, Zhejiang reached 2.63m, 2.80m and 0.82m respectively.
After 1990, a unique geological disaster-ground fissure appeared in Suzhou, Wuxi and Changzhou. At present, more than 20 ground fissure disasters have been found, and a ground fissure zone with a length of several kilometers and a width of several tens of meters has been formed in a large area, all of which are related to the uneven ground settlement caused by excessive exploitation of groundwater.
The land subsidence in the Yangtze River Delta is mainly caused by the development and utilization of groundwater. Before 1970s, the textile industry in urban areas was developed, but there was a shortage of energy. Therefore, a large amount of groundwater is intensively exploited for air conditioning and cooling in textile mills, resulting in serious land subsidence in urban areas. Since 1980s, with the rise of township enterprises around the cities of reform and opening-up, not only a large amount of groundwater has been exploited and utilized, but also sewage has been continuously discharged into surface rivers, resulting in the general decline of surface water quality in the delta water network area where water resources are extremely abundant, and the whole area has become a water-deficient area, which has aggravated the water shortage situation of rural residents. It has promoted the rapid increase of groundwater exploitation and produced a regional groundwater level drop funnel, so the resulting land subsidence has now become a regional geological disaster centered on the city.
The Yangtze River Delta is the earliest area for land subsidence investigation, monitoring and research in China. Since 196 1, in order to investigate the land subsidence in Shanghai, a regional groundwater monitoring network has been established systematically, and existing ground leveling points have been built or used to monitor the land subsidence in urban areas, and bedrock markers and layered markers have been gradually established to monitor the deformation characteristics of different soil layers. The groundwater monitoring network in Suzhou-Wuxi-Changzhou and Hangjiahu Plain was founded in the early 1980s. With the development of various hydraulic environment investigation and evaluation, it has been continuously supplemented. At the same time, in Jiaxing, Changzhou, Suzhou and other urban areas, leveling is used for regular or irregular land subsidence monitoring. By collecting leveling data from water conservancy, urban construction, transportation and other departments at different times and in different regions according to their respective purposes, and conducting on-the-spot reconnaissance, land subsidence investigation is further carried out.
Since 1999, the land subsidence monitoring network project in the Yangtze River Delta (Jiangnan) deployed by China Geological Survey has been adjusted and further constructed, which basically constitutes the pattern of land subsidence monitoring network in this area.
At present, the Yangtze River Delta groundwater monitoring network has covered the whole region, and a three-dimensional monitoring system consisting of ground precise leveling monitoring network, bedrock markers and underground layered markers with different depths has taken shape in some important cities and areas with serious land subsidence. With the introduction of new technologies and methods, GPS, automatic monitoring and information technology have been applied to land subsidence monitoring in this area.
(2) North China Plain
The North China Plain includes the plain areas of Beijing, Tianjin, Hebei, Shandong and Henan provinces (cities), with an area of 654.38+400,000 square kilometers. Land subsidence occurred in Beijing, Tianjin, Hebei and Shandong. The causes of land subsidence in North China Plain can be divided into natural factors and human factors. Natural factors include tectonic activity, self-compacting consolidation of soft soil layer and sea level rise. Human factors include groundwater, underground hot water, over-exploitation of oil and gas resources and large-scale engineering construction. According to the seismic data, the rate of land subsidence caused by tectonic activities in this area is only1~ 2mm/a ... So as far as this area is concerned, human factors, especially the over-exploitation of deep groundwater, are the main reasons for land subsidence.
The occurrence and development process of land subsidence in North China Plain is basically synchronous with or slightly lagging behind the process of groundwater exploitation. The amount of land subsidence is positively correlated with the decline of groundwater level. Its distribution range is basically consistent with the groundwater level drop funnel.
Land subsidence occurred in Beijing as early as 1935 due to the exploitation of groundwater. At that time, its range was only from Xidan to Dongdan, 1935 ~ 1952, and the maximum local land subsidence was only 58 mm. Since the 1950s, with the continuous increase of groundwater exploitation in Beijing, a regional groundwater level drop funnel centered on the eastern suburb industrial zone has gradually formed. By May of 1983, the land subsidence area in the eastern suburb of Beijing had reached 600km2, of which the area with accumulated land subsidence greater than 100mm had reached190 km2; ; The area where the settlement exceeds 200mm is about 42km2. 1966 to 1983, the central area of Laiguangying in the north sinks about 277mm；; After the accumulated land subsidence in the central area of southern Dajiaoting was about 532 mm.. 1987, the land subsidence area in Beijing increased rapidly, extending to more than 1800 km2, in which the area with subsidence greater than 200mm reached 350km2.
The development and utilization of groundwater resources in Tianjin began at 1923. According to the historical datum, with the development of groundwater, land subsidence also occurs correspondingly. Due to the small exploitation at that time, the annual settlement was only a few millimeters. After the founding of New China, with the development of industry and agriculture, the amount of groundwater exploitation increased year by year, and the land subsidence became more and more serious. The sedimentation rate of 1950 ~ 1957 is 7 ~ 12.0 mm/a, and the sedimentation rate of 1958 ~ 1966 is 30 ~ 46 mm/a, and the sedimentation center is gradually formed. 1967 ~ 1985, the settlement rate reached 80 ~ 100 mm/a, and the settlement developed rapidly. After 1986, it entered the stage of settlement control, and the settlement in most areas slowed down obviously, and the settlement rate in urban areas dropped to10 ~15 mm/a.
At present, the vast plain area south of Baodi Chengguan has experienced different degrees of land subsidence, covering an area of 8800km2. Among them, the area where the cumulative settlement exceeds 1000mm is 4080km2. The land subsidence in the south and coastal areas of this area is particularly obvious, and it is connected with the subsidence area of Hebei Province. Within this range, urban areas, Tanggu District, Hangu District, Dagang District, Haihe River downstream industrial zone and other settlement centers have been formed (Table 8. 1).
Table 8. 1 Present situation of land subsidence in Tianjin
The deep groundwater level drop funnels in Hebei Plain are formed in the urban and agricultural concentrated mining areas in the Middle East Plain, mainly including Jizhaoheng Funnel, Cangzhou Funnel, Ninghe Tanghai Funnel, Langfang Funnel, Qingxian Funnel and Bazhou Funnel. The area of the falling funnel of deep groundwater level is 439 15km2. With the further exploitation of deep groundwater, the range of groundwater level decline is expanding, and the range of each funnel is also expanding, forming a huge composite funnel covering the whole central and eastern plain, Tianjin and parts of Jidong plain, with an area of 70,000 km2 (below the 0m line of groundwater level).
With the increase of groundwater exploitation, the decline of groundwater level and the formation of groundwater level drop funnel, the geotechnical balance of strata is destroyed, and nine major land subsidence areas, namely Cangzhou, Baoding, Hengshui, Renqiu, Nangong, Bazhou, Dacheng, Quzhou and Tanghai, have gradually formed in Hebei Plain.
By the end of 1998, the land subsidence area of Hebei Plain was 48,550 km2, with an area of over 300mm 1872 km2, an area of over 500mm 6430km2 and an area of over 1000mm 755km2 (Table 8.2).
The affected area of land subsidence in Dezhou City, Shandong Province has reached 2037.5km2, with cumulative subsidence 150 ~ 387mm, subsidence center subsidence of 300~387mm and average annual subsidence of 25 ~ 32.25mm Since 1989, the cumulative subsidence in Jining City has reached 208.9mm, and the area with subsidence exceeding 60mm is nearly 90km2.
Table 8.2 Statistical Table of Settlement Area of Main Land Subsidence Areas in Hebei Plain
Due to the long-term over-exploitation of groundwater, the North China Plain has become one of the regions with the most serious over-exploitation of groundwater in the world, and it is also one of the regions with the largest area of groundwater level drop funnel, the largest area of land subsidence and the most complicated types, among which the Beijing-Tianjin-Hebei-Shandong region is the most prominent. Large-scale land subsidence poses a serious threat to the safety of local people's lives and property, and has become an important factor restricting the sustainable development of local economy.
Land subsidence directly leads to the loss of ground elevation resources in the coastal low plain area of North China Plain, which in turn leads to the subsidence of railway subgrade and the aggravation of storm surge disaster. Due to the influence of flood discharge, the ground accumulated water for a long time, and the factory building was flooded, resulting in serious economic losses. Due to uneven ground settlement, buildings are damaged and large-scale municipal infrastructure is destroyed; Due to land subsidence, many geological disasters such as ground subsidence and ground fissures have been caused, which directly threaten the safety of people's lives and property. Moreover, with the development of local economy, the greater the disaster loss, which restricts the sustainable development of social economy. At the same time, the process of world industrialization for more than 100 years has led to global temperature rise, sea level change superimposed on land subsidence, which has always threatened the survival of human beings in estuaries and coastal areas, including low-altitude areas in North China Plain.
The investigation and monitoring of land subsidence in North China Plain are relatively low. Except Tianjin, there is no special land subsidence monitoring network and monitoring system, and there is no comprehensive and systematic land subsidence research results. The data of regional land subsidence mainly comes from the Beijing-Tianjin-Tangshan land deformation regional network laid by the State Seismological Bureau, but its measurement density is low and its measurement frequency is low, which is far from controlling the land subsidence range in North China Plain. Moreover, the investigation and monitoring of land subsidence in North China Plain are limited by conditions, divided by administration, and lack of unified technical standards and planning. The layout of investigation and monitoring is not reasonable in time and space, and the obtained data are not comparable, which is far from meeting the needs of national disaster prevention and mitigation. Therefore, it is of great social and economic significance to investigate and monitor the land subsidence in North China Plain.
(3) Guanzhong plain area
Over-exploitation of groundwater in Guanzhong Plain has caused a large area of land subsidence, especially in Xi 'an area. The confined aquifer in the suburb of Xi 'an is fine sand, and the gravel layer and clay layer are interbedded with unequal thickness. The thickness of aquifer decreases from west to east and from north to south, and the thickness of clay layer increases gradually. This structure is beneficial to the discharge of bound water in cohesive soil layer when the confined water level is greatly reduced due to a large amount of exploitation of confined water.
At the initial stage of Xi 'an groundwater exploitation, the buried depth of confined water is only 25 ~ 35m. In the 1970s and 1980s, the water level dropped sharply due to the massive exploitation of confined water. By the early 1990s, the number of confined water wells in Xi 'an City had increased to more than 530, with an annual output of over 654.38+400 million m3. The confined water level drops by 60 ~100m, and the area of the falling funnel is 200km2. A large area of confined water level in the southeast suburb falls below the aquifer roof, and the buried depth of water level drops to 90 ~130 m. The sharp drop of confined water level means that the pore water pressure decreases, and the water in the cohesive soil layer is released to the aquifer, which in turn leads to the release of pressure in the cohesive soil layer and land subsidence.
The ground subsidence in Xi 'an was found in 1959. From 1972 to 1983, the maximum accumulated land subsidence in the urban area of Xi is 777mm, and there are five subsidence centers, with an average annual subsidence of 30 ~ 70 mm Up to 1988, the maximum accumulated subsidence has reached 1.34m, settlement/kloc. In the 1990s, the scope of land subsidence expanded, with the cumulative subsidence exceeding 200mm being about 150km2, the cumulative subsidence exceeding 600mm in the southeast suburb and 42km2 exceeding 1000mm. The number of settlement centers has increased to seven, with the cumulative settlement exceeding 2,000mm and the maximum cumulative settlement reaching 2,600mm, which is faster than before. The Xi 'an Bell Tower, built in the Ming Dynasty, sank by 395 mm, and the Big Wild Goose Pagoda, which has a history of over 300 years, also sank by 1 198 mm. ..
The land subsidence in Xi 'an has the following characteristics: ① the subsidence is closely related to the exploitation of confined water; ② The inhomogeneity and difference of land subsidence.
The harm of ground subsidence in Xi 'an is mainly enough to aggravate the activity of ground fissures, leading to a great increase in the vertical activity of ground fissures. Due to the uneven development of subsidence in various urban areas, there have been 1 1 obvious cracks with a total length of 76.68km, which develop at a speed of 5 ~ 3~4mm in vertical direction and 3~4mm in horizontal direction every year. The vertical activity rate of ground fissures in the southeast suburb is 30 ~ 50 mm/year.
Ground fissures cause uneven settlement of the foundation of nearby buildings, which leads to cracking of buildings, dislocation of underground pipelines and destruction of urban roads. According to incomplete statistics, 1996, 170 buildings, 57 factories and workshops, nearly 2,000 houses were destroyed, 74 roads were damaged, and water supply and gas supply pipelines were broken more than 40 times. In addition, dozens of deep wells were scrapped due to the rise of well pipes, which endangered the safety of many cultural relics. At the end of 2004, the Big Wild Goose Pagoda tilted 60 to the northwest. According to statistics, the direct economic losses caused by ground fissures totaled 654.38 billion yuan.
(4) Huaibei plain area
Fuyang City in Huaibei Plain is a water-deficient city. Before 1980s, urban drinking water was the surface water of spring. But after 1980s, with the development of industry and the growth of population, Quanhe became a sewage ditch, and the urban industrial and domestic water had to be changed to groundwater. According to the investigation, the groundwater in Fuyang City, especially the middle and deep groundwater with a depth of about 250m, has good water quality and abundant water. This makes the middle and deep groundwater become the object of exploitation in Fuyang waterworks, self-provided wells and industrial enterprises, and the water intake is increasing year by year. At present, there are more than 200 deep wells with a depth of more than 200 m in the built-up area of Fuyang City, with a density of more than 5 holes /km2. Although the allowable exploitation amount of groundwater in the middle and deep layers is 68,000m3/day, the actual exploitation amount has reached 1 .4000m3/day, exceeding the exploitation amount1times. As a result, the groundwater level in the middle and deep layers continues to decline, with an average annual decline of 1.62m, forming a groundwater level drop funnel of1.200km2..
Due to the above situation, the maximum land subsidence in Fuyang City has reached 1.4m, ranking fifth in China, and it continues to sink at a rate of 40 ~ 50 mm per year. This has caused a series of environmental geological problems, such as uneven settlement of highway bridges and large buildings in flood season, fracture of drainage pipes and scrapping of deep wells. Fuyang sluice, which is located in the settlement center and used to regulate the flow of Heying River, also cracked in many places, causing the pier dislocation and affecting the flood control ability.
(5) Songnen Plain
In addition to the large-scale exploitation of groundwater in Songnen Plain, oil and gas development mainly in Daqing area also caused land subsidence.
Daqing Oil&Gas Development Zone in Songnen Plain is located in Xiaoxing 'anling-Inner Mongolia geosyncline fold area, Xiaoxing 'anling-Songnen massif, and the central and western fault depression (fault) zone, that is, the central and western Songnen Basin (plain), with a single landform type. The general terrain is high in the north and low in the south. Generally, the ground elevation is above 130m and the natural slope is about 0. 14‰. Controlled by geological structure, oil-bearing strata with a thickness of about 6000 meters have been deposited since Jurassic. There are Jurassic, Cretaceous, Tertiary and Quaternary. Cretaceous is mud bank rock and sandstone deposited in inland lake basin, with a thickness of nearly 3000m, which is the main reservoir structure and exploitation horizon of oil and underground hot water, and the exploitation depth is generally between1000 ~ 3000 m. Neogene sandstone, gravel and Quaternary alluvium are the main water supply horizons in Daqing area, and the exploitable groundwater is 230 million m3/a, but the current exploitation amount is 390 million m3/a. Among them, the annual groundwater exploitation for oil recovery technology is 300 million m3, accounting for almost 80% of the groundwater exploitation. Groundwater has formed a huge descending funnel, the water level in the center of the funnel has dropped to 50m, and the area of the funnel has reached 5560km2, which almost covers Daqing City and spreads to neighboring counties (cities) adjacent to Daqing. Every year, many wells are drained and scrapped because of the decrease in groundwater exploitation. According to incomplete statistics, due to years of oil production and unreasonable exploitation of groundwater, the groundwater level in Daqing city and its surrounding areas has dropped 16 ~ 19m. The sharp drop of groundwater level is the main cause of ground deformation, but the specific settlement data is still blank because the land subsidence monitoring in oil and gas development zones has not been carried out at present.
8. Problems in1.2
Because the development process, degree and harm of land subsidence are different in different places, the monitoring and prevention measures adopted in different places are also different, and most areas where land subsidence has occurred have not taken monitoring and prevention measures. The common problems are the lack of unified regional planning and information communication, the outdated technology of main instruments and equipment, which can not adapt to the development trend of regional land subsidence and the continuous update of monitoring technology at home and abroad.
(1) Yangtze River Delta
1) In the past, the land subsidence monitoring network was laid according to the situation that land subsidence occurred first in some areas such as cities, and a unified land subsidence monitoring network was not established. With the expansion of land subsidence in the whole area, the monitoring work failed to keep up in time, and its limitations became increasingly prominent.
2) The land subsidence monitoring network is limited by administrative jurisdiction, lacking regional unified planning, and the monitoring is extremely unbalanced in different places. Especially in the north of the Yangtze River and the south bank of Hangzhou Bay (except Ningbo), the monitoring work is blank and the land subsidence is unknown, which is likely to embark on the old road of remediation after severe disasters and should be paid attention to by relevant departments.
3) The current monitoring methods and standards, including network density and frequency, are not unified. If the general accuracy in Shanghai is 1:50000, and the local accuracy reaches 1: 1:000, carry out the first and second level leveling. The frequency is three times a month, 1 time, and also four times a year, twice, 1 time. Jiangsu Province has only set up two leveling lines for land subsidence monitoring in Changzhou City, and the monitoring frequency is 1 time per month. The existing leveling network dedicated to monitoring land subsidence in Hangjiahu Lake is distributed along the main roads. In recent years, the control range can reach 3500km2, with monitoring 1 time per year. However, due to the influence of funds, the monitoring frequency cannot be guaranteed. In addition, Shanghai suburbs mainly collect surveying and mapping departments, while Suzhou, Wuxi and Changzhou mainly collect third-and fourth-class leveling data from urban construction and water conservancy departments in different periods to investigate land subsidence. Therefore, the ground leveling data belong to different departments, the sources are complex, the distribution is uneven, the consistency of data reference system can not be guaranteed, there are few overlapping points, the reliability is poor, and the measurement time is different, so it is difficult to systematically, comprehensively, timely and reliably grasp the distribution and development law of regional land subsidence.
4) Bedrock marker and layered marker are relatively sound in Shanghai, with one in Hangjiahu area, basically completed in Suzhou, Wuxi and Changzhou, and almost none in northern Jiangsu. Therefore, the research on the vertical distribution and causes of land subsidence is weak, and it is difficult to put forward targeted suggestions to control land subsidence.
5) Although the groundwater monitoring network has been established in the region, the monitoring wells are unevenly distributed and have different accuracy. In recent years, monitoring points have been destroyed many times, and individual aquifers lack control and monitoring facilities in quite a few areas, including the surrounding areas of the work area.
6) At present, the technical means used in land subsidence monitoring are generally backward, and the problems of low efficiency and long construction period still exist, so it is difficult to objectively reflect the needs of the expanding monitoring network in time. Although some new technologies and methods have been introduced, they are not mature and perfect, and there are no executable technical standards or regulations in the implementation process.
7) Ground fissure caused by differential ground subsidence is a new geological disaster in this area, but the density of existing monitoring network is obviously insufficient. It is still necessary to encrypt the layout to accurately record its development and change process and improve the quality of data monitoring and analysis.
(2) North China Plain
The problems in the North China Plain are shown in Table 8.3.
As the provinces (cities) in North China Plain are limited by administrative divisions, they work in their respective regions and submit relevant land subsidence isoline maps. After synthesizing relevant maps, the contour map of land subsidence in North China Plain is obtained. As can be seen from the figure, due to different monitoring standards and methods, the submitted land subsidence isolines have different years, and the settlement in many places is only inferred. It is obvious that different land subsidence amounts are obtained in the same place, which cannot fully reflect the reality. Therefore, at present, the land subsidence in all parts of North China Plain.
(3) Guanzhong Plain, Huaibei Plain and Songnen Plain.
These three plains are all river alluvial plains, and the land subsidence caused by groundwater overexploitation and oil and gas exploitation has seriously affected the ecological environment and sustainable economic development, but so far no systematic special investigation and monitoring of land subsidence has been carried out in these areas.
Table 8.3 Problems in Monitoring Facilities of Land Subsidence in North China Plain
In view of the above problems, the future land subsidence monitoring network needs to establish a land subsidence monitoring network with reasonable spatial layout and advanced and feasible technology on the basis of unified planning and design, unified technical standards and unified data platform. On the basis of traditional survey, advanced GPS, InSAR and LIDAR technologies are applied to monitor and correct each other, and accurate land subsidence is obtained, which provides reliable data basis for the sustainable development of the whole social economy and urban construction planning.