Hydrological Sciences -Journal- des Sciences Hydrologiques,39,3, June 1994 2 4 5

Land subsidence, sinkhole collapse and earth fissure occurrence and control in China*

DENG AISONG & JU JIANHUA Department of Geological Environment, Ministry of Geology & Mineral Resources, Beijing 100812, China

Abstract There are many varieties of geological hazard in China which are widely dispersed. Land subsidence is one such hazard and occurs mainly in the coastal cities of east China. Sinkhole collapses are mainly found near those cities of east and middle China that have karst water and karst water-impregnated ore deposits. Earth fissures also occur in east and middle China. All these geological hazards have a great influence upon the construction of engineering works. Though the causes of the hazards may be different, a major contributor is human economic activity. Therefore, a major countermeasure is to control human economic development.

Subsidence du sol, effondrements karstiques et fissuration des sols et leur maîtrise en Chine Résumé La Chine est soumise à de nombreux risques géologiques. Parmi ceux ci la majorité des cas de subsidence se situe dans les villes cotières de l'est de la Chine. Les effondrements d'origine karstique se répartissent sur une plus vaste étendue dans les villes de l'est et du centre de la Chine, s'alimentant en eau d'origine karstique ou possédant des gisements minéraux localisés dans des zones karstiques. C'est également en Chine centrale et orientale que l'on peut observer des fissurations du sol. Les travaux de construction sont gravement affectés par ces risques géologiques. Bien que les causes de ces risques soient variées, les activités économiques comptent parmi les plus importantes. La maîtrise de ces risques géologiques passe donc par celle des activités économiques humaines.

INTRODUCTION

China is a country with a large population over a vast territory. Many new cities have been established since the founding of the People's Republic of China. In these cities, it is common that economic construction has developed rapidly. With the increasing industrialization of the cities, human economic activity has had harmful impacts on the environment, such as depletion of water sources, environmental pollution, geological hazards, etc.

'Paper presented at the Fourth International Symposium on Land Subsidence, held at Houston, Texas, 12 - 17 May 1991.

Open for discussion until I December J 994

246 Deng Aisong & Ju Jianhua

A brief summary of the study of several geological hazards in China is presented in this paper.

LAND SUBSIDENCE

According to data records, the phenomenon of land subsidence in different amounts has been observed in nearly 50 cities in China. Most land subsidence has happened in the large and middle-sized cities where the Quaternary aquifers have been highly developed for groundwater. The subsidence is mainly distributed over the coastal plains in the eastern part of China and the alluvial plains of large rivers (Fig. 1).

Fig. 1 Distribution of land subsidence in major cities and areas of China.

The cities located over the eastern coastal plain, such as Shanghai, Tianjin, Suzhou, Wuxi, Changzhou, Jiaxing and Ningbo are the most densely populated regions of China. Following a large exploitation of groundwater, the decline in water levels and the consolidation and compression of the unconsolidated sediments resulted in land subsidence. From 1921 to 1965, the maximum subsidence was 2.63 m in Shanghai city. In Tianjin city, the total subsidence was 2.46 m from 1959 to 1985. In most parts of the urban area, the subsidence rate was above 50 mm per year, but it was 231 mm in Tanggu district. Because some of the subsidence areas were connected together, the area of influence was large.

Land subsidence, sinkhole collapse and earth fissure in China 247

Hebei plain is situated in an area where the activities of the present tectonic movements are very strong. Subsidence in the area of Handan, the Zhaoqiang-Hengshui-Julu area and the Cangzhou and Baoding areas appeared to result from the present crustal subsidence as well as from excessive pumping of groundwater. So far, areas where the subsidence rate is more than 10 mm per year occupy about 25 000 km2, or about 33% of the total area of the Hebei plain.

Because large amounts of groundwater have been pumped, in cities such as Beijing, Fuyang, Huainan, Xi'an, Kaifeng, Jinan, Changsha, Nanchang, Kunming and Guangzhou (Fig. 2) land subsidence has occurred and has become a geoenvironmental problem even though subsidence rates were less than those of the cities along the eastern coast of China.

Fig. 2 Land subsidence in Cuihu Park, Kunming city, Yunnan Province.

For example, in Fuyang city of Anhui Province, which is a new industrial city, land subsidence started in the 1970s and the subsidence rate has been increasing year by year.

Because land subsidence is a geoenvironmental problem which develops slowly, the danger is not obvious over a short time period. It is difficult for people to see the early stages of land subsidence and yet the economic loss after a long time is surprising. The subsidence may result in tidal inundation along the coast, streets being covered with water, bridge piers being lowered with the clearance under bridges being reduced, and in buildings being deformed or damaged.

At present, land subsidence is considered an important geoenvironmental problem and even a kind of geological hazard in some cities. Some effective

248 Deng Aisong & Ju Jianhua

steps and measures of prevention have been taken. For example, in order to control subsidence in Shanghai and Tianjin cities, where the study of subsidence first started, the following measures have been taken: (a) through joint planning, supervision and administration, to compute the

limits of exploited groundwater, and limit groundwater use; (b) to combine reasonable pumping of groundwater with artificial recharge,

increasing the storage in aquifers, and to adjust the exploitation of the sequence of aquifer layers; and

(c) to monitor land subsidence through the establishment of bedrock survey control points in order to forecast when subsidence will reach a point of serious damage. In Shanghai city, land subsidence has been controlled basically by

restricting the amount of groundwater exploited, adjusting the exploited aquifer layer sequence and by having artificial recharge since 1965.

In Tianjin city, subsidence has been controlled by reducing excessive pumping of groundwater. Primary control measures consisted of shutting down some industrial wells, increasing artificial recharge, and lengthening the water pipeline to more distant wells. As a result, the subsidence rate decreased from 86 mm per year in 1985 to 14 mm per year in 1990.

SINKHOLE COLLAPSE

Sinkhole collapse pits are mainly distributed over the eastern and middle part of China, mostly in Shandong, Hunan, Guangxi and Guizhou provinces. They also occur occasionally in Hebei, Shanxi, Anhui, Jiangxi, Hubei, Guangdong and Sichuan provinces (Fig. 3).

The causes of collapses can be divided into artificial ones and natural ones. In accordance with (incomplete) records, sinkhole collapses happened at over 780 sites, and the number of collapse pits is more than 30 000. The collapses are usually distributed over areas where the carbonate rocks are shallow, the thickness of the Quaternary layer is not more than 30 m and human engineering activities are relatively strong.

Sinkhole collapse pits caused by engineering activities

In the well fields of cities such as Qinhuangdao, Zaozhuang, Taian, Wafangdian, Xuzhou, Huaibei, Xicheng, Yulin, Liupanshui and Zunyi, sinkhole collapses have been caused by pumping groundwater.

In Liujiang, the well field for Qinhuangdao city, the upper stratum is loose Quaternary alluvium 8 to 23 m thick, and the lower strata are Cambrian and Ordovician limestone where karst is rather well developed. Since the well field came into production in June 1987, the pumping of large amounts of groundwater combined with heavy rain has resulted in a collapse area of

Land subsidence, sinkhole collapse and earth fissure in China 249

sinkhole collapse caused by f J j natural factors ^-^

Fig. 3 Distribution of sinkhole collapse in major cities and areas in China

0.34 km2 with 286 collapse pits. Most of the sinkholes have a diameter of 2-5 m, the largest being 12 m in diameter, and a depth of 2-4 m.

In Taian city, because of the excessive exploitation of groundwater, 25 collapse sites appeared between 1977 and 1983, with the diameter of the largest collapse pit being 10 m. Those collapses seriously affected the local railway transportation.

In Wafangdian, 25 collapse pits appeared within an area of 1.2 km2. Railway beds were broken and some houses were cracked.

In many mines of south China, water pumping for draining mines or water gushing into the mines has caused collapse pits. The collapses at Siding in Guangxi Province, Daguangshan in Hubei Province, Doulishan (Figs 4, 5 and 6) and Shuikoushan in Hunan Province and Chongqing in Sichuan Province are typical. For example, draining of the Siding mine resulted in over 600 collapse pits and the area of collapse occupied 2.9% of the mine area. The surface has been turned into a honeycomb-shape and a large area of farmland was destroyed. Residents living in endangered houses were evacuated.

In the Fankhou mine in Guangdong Province, more than 1600 collapse pits have been formed. The diameter of the largest collapse was 40 m and its depth was about 30 m. For this mine, collapses in the range of 5 km2 caused buildings of 0.07 km2 area and more than 0.066 km2 of farmland to be affected. In the Daguangshan mine in Hubei Province, water gushing into the mine resulted in over 370 collapse pits. Near the mine, the river was cut off

250 Deng Aisong & Ju Jianhua

five times by collapse pits. More than 4000 m2 of buildings were wrecked. In the Enkou mine in Hunan Province there were over 5800 collapse

pits; much farmland was destroyed and houses had to be removed. Sinkhole collapses from mines in the northern part of China are less than

in the southern part, but some of them are rather serious. For example, draining of the Laiwu mine in Shangdong Province caused 44 collapses, covering an area of 0.057 km2. Some farmland was destroyed and houses collapsed. In addition, river water entered the mine through the collapse pits,

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Fig. 4 Collapse pit in Dolishan mine, Hunan Province.

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Fig. 5 Collapse pit in Qijia mine, Hunan Province.

Land subsidence, sinkhole collapse and earth fissure in China 251

Fig. 6 Collapse pit in Enkou mine, Hunan Province.

which in turn impaired the safety of the mine. Similar collapses have also appeared in the Jinzhou mine in Liaoning Province and the Huainan mine in Anhui Province. The collapse often resulted from the development of a worked-out section of a mine and underground chamber, water filling the mine, dynamite explosions , etc. (Fig. 7).

For example, at the Liangwu village in Gui Country, Guangxi Province, the pumping of water, with mine-dynamiting in the sinkhole fractures, resulted in collapses at 157 sites with the average size about 0.4 km2. Liangwu Village had to be moved because of the sinkholes.

Sinkholes and collapse pits caused by natural factors (gravity, earthquake, flood, rainstorm, etc.)

Among these, the sinkhole collapse braccia pipe is typical. Braccia pipes represent paleo-sinkhole collapses which formed before the Quaternary period. They are features where rock layers overlying the carbonate sank into a paleo-sinkhole cave. They often appear in the northern coal districts of China. Sinkhole collapse features are mainly distributed over the Xishan coal field in Shanxi Province, the coal fields along the banks of the Feng River and the coal field in the middle Taining Mountains, Hebei Province.

The distribution of sinkholes is concentrated in some regions. For example, in the Xishan mine there are about 1300 sinkhole collapse features which average 70 sinkhole collapse features per square kilometre. About 2800 sinkhole features in 45 sites have been discovered in more than 20 coal fields in Shandong, Shaanxi, Henan, Jiangsu and Anhui Provinces. They endangered safe production in some mines. For example, in the Fagezhuang mine in

252 Deng Aisong & Ju Jianhua

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F/g. 7 Sinkhole collapse in Benxi city, Liaoning Province.

Kailuan, Hebei Province, gushing water from a sinkhole collapse feature, with a maximum amount up to some 34 m6 s4 resulted in submergence of the shaft and serious loss. Besides, the Haicheng earthquake in 1975 and the Tangshan earthquake in 1976 both caused sinkhole collapses.

The incidence of sinkhole collapse has been generally increasing in recent years. The major causes are intensifying human economic activities, resulting in increasing demand for water and excessive pumping of water. Therefore, the main method for prevention of collapse is to limit the amount of groundwater exploited, as well as to fill up the collapse pits.

More attention should therefore be paid to prevention, as well as comprehensive administration and control, as follows: (a) for the karst groundwater fields wells should be placed carefully and the

amount of groundwater pumped restricted to prevent the rapid and large-scale decline of groundwater levels, which could result in karst collapse. For instance, in the Liujiang well field, the most important method is to limit the total amount of groundwater pumped and work out a plan for groundwater pumping according to the low-water season, the high-water season and the normal-water season. Secondly, wells have been too concentrated, so the wells should be more distributed and located away from residential areas and in the valley zone where good conditions of hydrogeology and supply are available;

(b) for karst water-filled mines, the karst water should be drained slowly. In the collapsed stream segment, some measures such as filling, obstructing, damming and diverting should be taken. In some areas with favourable conditions, a grouting screen could be used to limit the expansion of the cone of depression and thus decrease or prevent large scale karst collapse. For example, in the Matun iron mine in Shandong

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Land subsidence, sinkhole collapse and earth fissure in China 253

Province, good results have been obtained by preparing a grouting screen to protect the groundwater field;

(c) in urban areas, karst collapse should be assessed and forecast in accordance with city construction and development plans. The distribution of city development should then be adjusted as a result of this assessment; and

(d) for the possible collapse areas traversed by railways and highways, karst collapse should be investigated, the collapse and trend of collapse forecast, and preventive measures carried out. For example, good results have resulted from using drill-hole grouting for consolidating road foundations in the Taian region.

EARTH FISSURES

Earth fissures are mainly distributed over the eastern and middle plain and the basin and hill plateau of China (Fig. 8).

Earth fissures are caused jointly by the activities of fault structures and human activities etc. Presently, about 750 sites with nearly 1000 earth fissures have been found in over 200 cities and countries in more than 10 provinces, including Shaanxi, Shanxi, Hebei, Henan, Shandong, Anhui and Jiangsu Provinces. For instance, the Weinan-Fenhe earth fissure zone begins in Baoji and Xi'an cities and ends in the Datong in Weihe valley, Yellow river,

Fig. 8 Distribution of earth fissure in China.

254 Deng Aisong & Ju Jianhua

Yuncheng city and Taiyuan plain. The earth fissure zone is about 1000 km long and from tens to a hundred km wide. The most serious earth fissures are in Xi'an and Datong. In Xi'an there were 11 earth fissures aligned in the NNE direction within an area of 155 km2, with a maximum displacement of 45 mm, from July 1989 to July 1990. This rapid extension of earth fissures has harmed the city construction. For instance, in the area crossed by earth fissures, the roads became deformed, underground pipelines were broken, and buildings were damaged. Along the Datong earth fissure zone, an active period of earth fissure formation took place from 1979 to 1986. It was stable in 1987 but again the earth fissures developed along their trend in 1989. Now the earth fissure zone is 3 km long and 1.6 km wide. The relative vertical displacement of earth fissure was 20-50 mm and buildings were destroyed (Figs 9, 10 and 11).

After the earthquakes of 1966 and 1976, the length of earth fissures in Hebei plain which had happened in 1963 was extended from 0.5-1 km to 3-8 km. In this area, 77 earth fissures were found from 1978 to 1983 in 16 cities and countries, including Shijiazhuang, Baoding, Langfang Cangzhou, Hengshui and Tianjin, and about 228 earth fissures were found in 39 cities and countries by 1989. In Handan city, earth fissures caused the roads to be deformed, the pipeline broken and buildings damaged. Earth fissures occurring in rural areas not only damaged irrigation ditches, farmland, roads and house foundations, but also broke tree roots. In certain areas, unusual phenomena such as spouting oil and variations in levels in water or oil wells appeared near earth fissures.

Because the occurrence of earth fissures seriously harmed construction, it has been important to find out the causes and any measures of prevention. For example, investigation of the major cause of the earth fissure occurring in the Xi'an area showed that the city lies in the Weihe Basin, and continued

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Fig. 9 Crack in a house wall resulting from an earth fissure.

Land subsidence, sinkhole collapse and earth fissure in China 255

Fig. 10 Earth fissures at Chenzhou mine, Hunan Province.

Fig. 11 Earth fissures resulted in a crack in this building at Chenzhou mine, Hunan Province.

movement activity of faulted structures resulted in earth fissures. Extensive pumping of groundwater caused rapid land subsidence, which aggravated the development of earth fissures. Therefore, preventive measures should include: (a) monitoring the movement of earth fissures to forecast where earth

fissures could possibly occur; (b) exploiting groundwater carefully to limit the amount of confined

groundwater pumped and exploring for new groundwater fields; and

256 Deng Aisong & Ju Jianhua

(c) planning new projects to avoid the zone where earth fissures are likely to occur, and determining the proper avoiding distance according to the height of buildings.

SUMMARY

The data mentioned above show that there is a variety of types of geological hazards related to some types of land subsidence in China. These hazards are mainly distributed over the eastern and middle part of China, and those regions also form the major area developed for economic construction. The study and prevention of geological hazards related to land subsidence are promoted at present in China.

Acknowledgement Dr Haitao Hu is thanked for useful discussion in preparing this paper. Wholehearted thanks are especially due to Dr A. Ivan Johnson for his kind suggestions and generous help.

Received 23 December 1993; accepted 15 February 1994