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Subsidence occurs worldwide

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Subsidence in China

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Subsidence in China

 

Introduction

Land subsidence in China occurs in different regions. It is primarily caused by excessive groundwater withdrawal. Other reasons for the subsidence include the oil, warm groundwater withdrawal and the neotectonic movement. The common characteristics of land subsidence in China are slow, accumulative, irreversible, and other unique properties. The range of subsidence still keeps extending and the accumulative subsidence increasing though some measures taken. Adjustment of the aquifer exploitation practice is a subsidiary way to control land subsidence, but it cannot solve this problem completely. In a specfic way of groundwater changing, the contribution of a certain soil layer to the total subsidence depends on its compressibility and thickness. Besides the elasticity, both cohesive soil layers (aquitards) and sand layers (aquifers) are observed to be plastic and creep when the groundwater level fluctuates in a specific way, which often leads to subsidence delay.

Basic situation

In China, land subsidence mainly occurs in 17 provinces (cities) located in the
eastern and central regions including Shanghai, Tianjin, Jiangsu, and Hebei
provinces. Total subsidence area is more than 7×104 km2 and distributes
in the following regions (as shown in the right): (1) Yangtse Delta, such as
Suzhou, Wuxi, Changzhou, and Jiaxing; (2) Huang-huai-hai [Huanghe River
(Yellow River)-Huaihe River-Haihe River] Plain (sometimes called Northern
Plain of China), such as Tianjin, Cangzhou, Hengshui, and Anyang; (3)
Songnen (Songhuajiang River-Nenjiang River) Plain and Xialiaohe (lower
reaches of Liaohe River) Plain, such as Daqin, Haerbin Shenyang and
Yingkou; (4) River valley and intermontane basin, such as Xi’an, Taiyuan and
Linfen in Fenwe valley (Fenhe River and Weihe River Valley) and Datong in
a intermontane basin; and (5) Southeastern coast plain, such as Ningbo,
Taizhou, Haikou, plan Taibei. There are not only delta, coastal plan alluvial
and diluvial plains, but also inland basins in existing subsidence regions, of
which the most serious subsidence develops in Huang-huai-hai plain and
Yangtse delta.

Land subsidence occurred first in the big coastal cities, such as Shanghai
and Tianjin, and it gradually developed to inland after the 1980s. Two big
land subsidence areas have been formed. The centers of these two areas
are Yangtse Delta and Huang-huai-hai Plain, respectively. They represent
two different types of land subsidence with difierent geological backgrounds
in East China. Their maximal accumulative settlement has exceeded 3 m. The highest rate of subsidence is over 100 mm/a. The ground in some areas was lower than the average sea level in the coastal region (Cui et al. 2002). At present, the land subsidence in most regions still continues and the subsidence area still expands. Even in Shanghai and Tianjin where the land subsidence has been partly controlled, the land subsidence is still going on though its rate has been slowed down (approximately 10-15 mm/year).

More than 1,000 earth fissures (fissures in soils or rocks that can extend up to the ground surface) have been reported in Huang-huai-hai Plain, Yangtse Delta, Fenwei Valley, and some inland basins due to differential subsidence (Xue et al. 2003). They scatter in 450 places where the buildings above the fissures are damaged, which induces huge economic loss.

Figure 21.The distribution of land subsidence in China(Xue, 2005).

Yangtse delta

Yangtse delta is one of the most typical regions of land subsidence in China. Shanghai is the first city in which land subsidence was reported and its effect and damage were the greatest. Land subsidence in Shanghai was first reported in 1921. The average accumulative subsidence at the center of city was approximately 1.93 m from 1921 to the end of 2001, with the maximum being 2.63 m (Sun 2002). From the 1980s, the land subsidence area has extended to all of Shanghai with the shifting of the groundwater pumping center from the urban area to the suburbs.

Figure 22.The aquifer structure of Shanghai is illustrated in the left picture. There is a phreatic aquifer and five confined aquifers from top to bottom. These aquifers consist of silt, ?ne to medium sand, and coarse sand. Between these aquifers are six aquitards, which consist of clay and silty clay.

The land subsidence in Shanghai experienced the following stages : developing (1921–1948), obviously developing (1949–1956), violently developing (1957–1961), slowing down (1962–1965), rebounding (1966–1971), slightly developing (1972–1989) and slowly developing (1990–2001). The corresponding subsidence rates are approximately 24, 40, 100, 60, -3, 3.5, and 16 mm/a, respectively (SGEAEB 2002; Chai et al. 2004).

Besides Shanghai, land subsidence has also taken place in other cities of Yangtse delta since the 1960s, such as Suzhou, Wuxi, Changzhou, Nantong, Taizhou, Jiaxing, and Tongxiang. By the end of the last century, the area where the accumulative subsidence exceeded 200 mm was approximately 10,000 km2, which was equivalent to 1/3 of the whole region. The subsidence development followed a trend. The accumulative subsidence was over 1 m in Suzhou, Wuxi and Changzhou (SXC), and the maximum was more than 2 m (Sun 2002). In history, these three cities were known for their beauty, prosperity, and free of pollution. However, when the surface water was polluted with the development of town industry in the recent 30 years, people turned to exploit groundwater and caused groundwater level (GWL) to decline rapidly. At first, the separate drawdown cones were developed in the three cities, then with the cones extending, all of them joined together and formed a coalesced drawdown cone. At the same time, land subsidence took place, which was coincident with the drawdown cones in the time and space. These observations indicated that the main cause of land subsidence is excessive groundwater withdrawal in the SXC area. Now, both the drawdown cones and the land subsidence areas are separate in SXC, Shanghai, Jiaxing and Huzhou. However, land subsidence would develop rapidly and the drawdown cones and land subsidence areas of the whole delta would join together soon if the effective control measures were not taken. Though groundwater exploitation has been banned in SXC and nearby areas in the recent years, the rate of subsidence is still 20–40 mm/a and the subsidence area still increases. What is more, 14 earth fissures have been found in these areas since 1990. These fissures resulted from the differential subsidence and their length is up to several kilometers (Sun 2002). The fissures also have obvious direction, most of which extend along NE or NNW.

China North Plain

Tianjin is located in the subsidence region of China North Plain and the thicknesses of the Quaternary deposit range from several hundred meters to 1,000 m in the plain. There are under-consolidated soils in the deposit. The land subsidence of Tianjin is different from that of Shanghai from geological perspective. Land subsidence developed slowly from 1923 to 1958 when groundwater exploitation was 1.2×109 km3/a and the rate of subsidence per year had risen from several millimeters to 20 mm gradually. Land subsidence developed rapidly from 1959 to 1985  when the maximal exploitation of groundwater reached 1.2×108 km3/a and the maximal rate of subsidence exceeded 100 mm/a.

Many subsidence centers were formed. As a result of the efforts to reduce exploitation of groundwater, the quantity of pumped groundwater has been limited in (0.15–0.2)×108 km3/a in urban district and the rate of subsidence has declined to only 10–15 mm/a from 1986 to 2002 (Cui et al. 2002). At present, land subsidence in Tianjin has joined with the subsidence area of Hebei Province. In this region, there are several subsidence centers. Though the subsidence has been controlled in the urban area of Tianjing and Tanggu district, it is still serious in the suburb and new centers of subsidence appear and develop rapidly. The maximal accumulative subsidence from 1959 to 1999 was 2.83 m in the area of 540 km2 of Tianjin and its suburb (Cui et al. 2002).

Excessive groundwater withdrawal in Hebei plain has resulted in nine huge drawdown cones and nine main subsidence areas, including Cangzhou and Hengshui cities. The subsidence in Cangzhou and Dacheng covers 9,363 km2 and the accumulative subsidence at the center exceeded 2 m. Land subsidence has also been found in Dezhou and Heze of Shandong Province, Xuchang,Puyang, and Xinxiang of Henan Province, as well as
Fuyang of Anhui Province. It occurs almost all over the Huang-huai-hai Plain. The rate of subsidence varies in different cities. Besides land subsidence, there are earth fissures in the Hebei plain. Until 2004, more than 200 fissures have been reported and their lengths range from several hundred meters to 1 km and widths range from 1cm to 2m.

River valleys and intermontane basins

Xi’an is located in the Weihe River valley and its land subsidence was first found at the end of the 1950s. After that, the range of land subsidence increased along with the exploitation of groundwater. Until the beginning of the 1990s, the area where the accumulative subsidence exceeded 200 mm reached to150 km2 and the area where the accumulative subsidence exceeded 1,000 mm reached to 42 km2. The maximal accumulative subsidence came to 2.6 m (only 1.33 m in 1988) until 2002 (Yan 1998; Suo and Li 2002). The land subsidence in Xi’an is characterized not only by its correspondence to the drawdown cone but also, more importantly, to its assignment with 11 NEE earth fissures. The land subsidence was confined within the blocks which are cut by the earth fissures, and it distributed along with NNE. This is not observed in other areas. Land subsidence accelerated the motion of earth fissures and increased their vertical movements.

Land subsidence has also been found in Taiyuan, Linfen, and Yuci of the Fenhe River valley, accompanying many earth fissures. Datong, a mountain basin, developed 10 earth fissures. Their total length is 34.5 km and their principal direction is approximately NE50°. There is not only subsidence, but also horizontal displacement for the fissures in Datong city.

 

Subsidence causesback to top

The common characteristics of land subsidence in China are slow, accumulative and irreversible. Compared with the settlement caused by engineering construction, the land subsidence is slow because the groundwater level changes slowly. For instance, the maximal rate of groundwater level changing in Shanghai is 10 m/a. In spite of being slow, the accumulative land subsidence can achieve a great value because it lasts for a very long time. Soils are of plasticity and creep. It can only develop a little or no rebound when the effective stress exerted in it decreases. For example, though a large amount of water was recharged into the aquifers (approximately 6.4 ×106 km3/a) from 1962 to 1971 in Shanghai, only a little rebounding (3 mm/a) was reported in the period from 1966 to 1971.

Natural conditions and man-made activities are two main factors that cause land subsidence. The former includes the neotectonic movement and natural settlement of under-consolidated soils. The latter includes excessive groundwater, oil, gas and warm groundwater withdrawal, engineering construction, and other activities.Of the natural factors, the tectonic subsidence caused by the neotectonic movement is primary. In some regions, the neotectonic movement is so small (1–2 mm/a) that tectonic subsidence can be ignored in the research period of tens or one hundred years. Land subsidence in China can be divided into two types: land subsidence caused by excessive groundwater withdrawal, for example, Yangtse Delta; and land subsidence caused by groundwater, oil, warm groundwater withdrawal, which is superposed on the land subsidence caused by the neotectonic movement and natural settlement of underconsolidated soils, such as Tianjin city. The rate of the tectonic subsidence was about 1–2 mm/year in recent 10,000 years in Tianjin, but the subsidence reached 85 mm/a in the urban area and 100 mm/a in Tanggu district in 1985 (Cui et al. 2002). After some measures were taken to reduce the groundwater pumpage, the subsidence declined to 11 mm/a in the urban area and 13 mm/a in Tanggu. From this, it can be concluded that excessive groundwater withdrawal is the main reason for land subsidence in Tianjin. In addition, the land subsidence from the natural settlement of under-consolidated soils is approximately 10–20 mm/a in the recent more than 10 years. The land subsidence due to the extraction of warm groundwater and deep gas and oil is greater than 6 mm/a (Cui et al. 2002). So is the land subsidence in the Huang-huai-hai Plain. Land subsidence is also caused by oil, gas and warm groundwater withdrawal in some place such as Renqiu in Hebei Province. The minor factors in rapid development stage of land subsidence, such as engineering settlement caused by high buildings, natural compaction of stratum, neotectonic movement, sea level rising etc., are important when the land subsidence comes into slow development stage after various artificial measures are in place.

As ground-water pumping increases, land subsidence also will increase. In many aquifers, ground water is pumped from pore spaces between grains of sand and gravel. If an aquifer has beds of clay or silt within or next to it , the lowered water pressure in the sand and gravel causes slow drainage of water from the clay and silt beds. The reduced water pressure is a loss of support for the clay and silt beds. Because these beds are compressible, they compact (become thinner), and the effects are seen as a lowering of the land surface. The lowering of land surface elevation from this process is permanent. For example, if lowered ground-water levels caused land subsidence, recharging the aquifer until ground water returned to the original levels would not result in an appreciable recovery of the land-surface elevation.

With economy developing at a high speed in the recent years, the settlement resulting from engineering construction, especially super high buildings, is coming to light gradually in Shanghai, Tianjin and other cities. For instance, after three high buildings in Lujiazui, Shanghai were finished, the average settlements near these three buildings from April 2000 to April 2001 were 23.1, 26.4 and 63.3 mm, respectively.

Monitoring land subsidence back to top

China has been experiencing significant land subsidence and ground fissure activities since 1920s, which have brought various severe geohazards including damages to buildings, bridges and other facilities. Monitoring of land subsidence and ground fissure activities can provide useful information for assessing the extent of, and mitigating such geohazards. A systematic monitoring network includes underground components, ground components and air components (as shown in figure 23).

The underground components: benchmarks on bedrock, sets of extensometers in different stratahundreds of groundwater observation wells

The grounding components: leveling marks

The air components: GPS stations, InSAR application (corner reflectors)

Figure 23.Different monitoring measures at land subdisence.

Damage of land subsidence back to top

The damage of land subsidence mainly includes: (1) loss of the altitude of ground and loss of the ability of food-control and drainage project to resist foods; (2) crack of building foundation and serious effect on the usage and the life span of buildings, and adverse impact on the safety of railway, highway, subway, tunnel, bridge and underground pipeline, especially when the land subsidence is differential; (3) loss of the under-clearance of bridges and passing capacity of inland river;(4) failure of water supply wells; (5) failure of bench marks. Severe land subsidence occurred in the well developed eastern region in China. In Shanghai, the total loss due to land subsidence has exceeded 35 billion dollars for the recent 40 years.

Reference
Cui XD, Niu XJ, Bai JW (2002) Prevention and control of land subsidence in Tianjin (in Chinese). In: Wei ZX, Li QF (eds) Proceedings of the national symposium on land subsidence, Shanghai Institute of Geology Survey, Shanghai, pp 338–345.

Sun WS (2002) Investigation report on prevention and control of land subsidence in Yangtse Delta (in Chinese). In: Wei ZX, Li QF (eds) Proceedings of the national symposium on land subsi-
dence, Shanghai Institute of Geology Survey, Shanghai, pp 1–12.Zhang AG (2002) Continuable development of Shanghai and the prevention and control of land subsidence (in Chinese). In: Wei ZX, Li QF (eds) Proceedings of the national symposium on land subsidence, Shanghai Institute of Geology Survey, Shanghai, pp 17–26

Xue YQ, Zhang Y, Ye SJ, Li QF.(2005). Land subsidence in China. Environ Geol (2005) 48: 713–720.

Xue YQ, Zhang Y, Ye SJ, Li QF (2003) Land subsidence in China and its problems (in Chinese). Quaternary Sci23 (6):585–593.