IAH ZlsS Congress KAHST HYDBOGEOLOGY AND KARST ENVIRONMENT PROTECTION 10-15 October 1988 GUiUN.CHIHA

ON THE SURFACE COLLAPSE IN KARST REGIONS

Miao Zhongling

Guilin College of Geology, Guilin, Guangxi, China

ABSTRACT The collapse, a common feature of the surface unsta- bility in karst region, can be classified into two types—natural and artificial collapses. The natural collapse has been recorded since ancient time. The 19 events' recorded in the history of Guangxi show that collapses -became more frequent and intensive during the past century. There are many valuable ways to predict collapse, among which the sensitive direction and multi­ variate statistical analysis are the most effective.

INTRODUCTION More and more people are paying a' good deal of attention to collapse hazards which are highly destruc­ tive to the surface completeness and engineering constru­ ctions and imperilling to environment quality and ecolo­ gical balance.The area of karst distribution in China is 3.40 million km2, of which the exposed karst typeloccu- pies 0.91 million km , being distributed mainly in south and southwest China. This paper deals chiefly with the formation, development,controlling factors and prediction methods of the collapse in karst regions.

NATURAL COLLAPSE Natural collapse, one of the geological phenomena resposible for the change of the earth's surface, results from the enlargement and consequent cave-in of underground caves. China is one of the countries having first recorded

1178 and studied the natural collapse. Taking Guangxi as an example, the earliest collapse was recorded in 222 A.D.. The collapse records since the 15th century have been rather complete. Now we quote those collapses happening in Guangxi in order to show the feature of its develop­ ment. (Note: The ancient measuring unit has been reduced into modern system.)

1. June 30, 1498. Guilin. 9 collapse pits, 3-18 m in diamètre. 2. January 21, 1526. Guilin. 2 collapse pits, 6.7 m in depth. 3. March 9, 1571. Guilin. Several collapse pits. 4. February 17, 1578. Lengshui Village, east to Guilin. One collapse pit, 10 m in diamètre. 5. March 17, 1578. Lingui Village, east to Guilin. One collapse pit, 3 m in depth, smoke appearing first, followed by collapse with trees and stones. 6. August 11, 1605. Lingchuan County. One collapse pit, more than 10 m in diamètre and 4 m in depth. 7. February 26-March 26, 1751. Bulu Village in Tianyang County, collapse and landslide. 8. 1744. Desheng Village in Yishan County. Several tens of collapse pits. 9. August 22, 1796. Yishan County. Internal quake of mountain causing sensible earthquake. 10. June 18, 1806. Rongshui County. Several collapse pits. 11. 1853. Luoming Village in Pingnan County. Three collapse pits. 12. 1873. Shuidong Village in Guilin. One collapse pit. 13. 1879. Xiaodong Village in Quanzhou County. One collapse pit bringing several houses down. 14. May 7, 1883. Mengyangxin Village in Binyang County. More than 3 0 collapse pits. 15. August 29, 1890. Sanjinbao in Bobei County. One collpase pit, forming a deep pond. , 16. 1902. Guilin. One collapse pit. 17. July 13, 1911. Xunluo Village in Huanjiang County. Collapse bringing the whole village submerged. 1179 18. 1936, Anhexiang Village in Boshou County. Collapse, forming a deep pond about 0.04 km2 with a loud crash spreading several tens of km away. 19. January 4, 1981. Fenjie Village in Yulin County. Large scaled collapse in an area of 1.7 km long and 0.7 km wide, giving rise to about 400 pits.

The collapses mentioned above are mainly the natural collapse, although artificial factors cannot be ruled out. These records reveal that the collapse has increased obviously since the 18th century in Guangxi. In the 20th century, the collapse has become more often as a result of increase of engineering construction,land utilization and groundwater exploitation. Large-scaled collapses took place in such areas as Huanj iang and Boshou where whole villages were submerged. This may be induced by the two factors as follows: I. The increase of acid materials in environment

Taking C02 for example, the concentration of C02 in atmosphere was less than 300 ppm before .1850, while it got to 330 ppm in 19,78, and even reached 800-1000 ppm in some big cities as well as in the surrounding areas of some large industrial regions. The increase of acid mate­ rials will stimulate limestone denudation thus bringing about more often larger-scaled collapses. II. The speed-up of water circulation The population growth, urban expantion, and industry and agriculture development, have brought about a sharp increase in the demand for water supply. The overdraft of groundwater, the main water source, will hasten the groundwater recharge and shorten the period of water circulation and renewal, thus having stimulated up the karstification around the water source.' Therefore, this is also one of the factors accounting for the intensification of natural collapse.

ARTIFICIAL COLLAPSE The artificial collapses are closely related to the following factors: (1) Geomorphologically, they mostly take place in negative relief such as depressions, basins, poljes, U-shaped valleys, broad peak cluster

1180 plains, and river terraces and graded surfaces overlying limestone basement and rarely occur in mountainous terrains and plateau slopes; (2) Geologically, the karst areas with thin cover composed of loose Quaternary sediments or weathered sandstone and shale are mostly liable to collapse; and (3) Hydrogeologically, the covers of the areas with confined karstic aquifers, which are supported by confined water, are easily to collapse when losing balance as the pressure of groundwater drops down. The disturbance of man's activity may be either short- or long-term, or repeatedly imposed, and moreover, its intensity is also changeable. So' the occurrence of collapse can also be either abrupt,or.over a long period or multiperiodic.

I. Factors controlling collapse intensity 1. The collapse intensity becomes stronger with the increase of disturbing intensity. The disturbing intensi­ ty refers to drawdown and discharge (pumping rate or water invasion). When it increases, the suffosional process and vacuum negative pressure acting upon the soil will great­ ly enhance, thus making the collapse of soil take place more frequently (Tab.l).

Table 1 Relation between interference and collapse intensity ... —! : : Economic Grade Disturbance intensity Collapse intensity loss EJrawdow n Discharge Area Collapse Diameter (million 3 2 of pit (m) (m / h) (km ) points yuan) 3 nxlO nxio2 1 >100 >1800 >_10 1~10 -nxlO2 -nxlO1 1 2 nxlO

0.1-1 3 <20 <360 0 <1

2. The collapse intensity is in direct proportion to the karstification intensity. The cave density is actually the index scaling the intensity of karstification. The zonation of karst'distribution in eastern China is obvious. As shown in Fig.l, karst development gradually becomes weaker from south to north, namely, from the tropical zone through subtropical and

1181 temperate zones further to cold-temperate zone. So the cave density also shows a tendency of reducing from south to north. In southern Guangdong Province,the average density of collapse pit is several hundred per km2, with the peak value reaching 2000/km2. It is 200-300/km2 in central China and only several to several tens per km2 in horlth China. I • | j ,

N , IgN

.- 100001-4 2 4 § "s • * c •o -" 1000 f- 3 0 ®b 6

3 • 10 Sa ®» 1it1 s "" 12 "3 10 H 13 U 1 West-south I South I Central | North / Local Fig.l Geographic distribuuon collapse intensity in. Chian 1. Yunnan; 2. Guizhou; 3. 4. Guangdong; 5. 6. 7. Guangxi; 8. Hubei; 9. 10. Jiangxi; 12. 13. Çebei

3. The collapse intensity decreases with the in­ crease of soil thickness. Generally the thicker, harder and more compact the soil is, the more difficult it is for the collapse to take place. But a few intensive collapses can penetrate the soil of 40-50 m thick, or even penetrate the weathering sandstone and shale of 93 m thick. According to the soil thickness, collapse poten­ tial can be divided into three types. (1) An area with intensive collapse potential, where the soil thickness <10 m; (2) An area with medium collapse potential, where thei soil thickness between 10-30 m; (3) An area with weak collapse potential, where the soil thickness >30 m. 4. The relation between collapse and time. Not all the collapse pits can occur irr one disturbance. They increase gradually as the time goes on. In some mines and water source areas, the period of collapse may last 10- 20, or even tens of years. The period of intensive collapse occurs at the beginning of disturbing. ' New

1:182 disturbing source will result in new collapse. During the rainy, season, the collapse pit will increase only when the rainfall intensity is increasing. 5. Tile relation between collapse and soil cave. There |are two models of collapse. The covered karst area is dominated by| the karst cave-sdil cave-collapse model while the exposed karst region is by the karst cave- collapse model. ! The soil cave is a transient stage o,f the collapse formation. So the formation and development of soil cave is a factor controlling the process of collapse. l|n some regions of South China (such as Guilin), the clay changes vertically from hard to hard plastic, plastic, soft plastic, and fluid plastic. And its "moisture content becomes larger with the increase of depth. All of this provides a favourable condition for the soil caves to be formed in deep clay. No doubt such areas are the places where intensive collapses occur.

II. The feature of collapse distribution The collapse pits are distributed around the disturbing source in zones and in certam directions. From centre to edge, three zones can be identified according to its intensity, i.e. the intensive, medium and weak collapse zones. The width of each zone is con­ trolled by the disturbing intensity. The total width of the three zones is called as the radius of collapse pit distribution. The value of it usually ranges from n-102 to n-103 m, and few can reach 20 km, which is less than or cloSie to the influence radius formed by drainage or pumping. As mentioned above, the collapse pits are concen­ trated in certain directions. And such directions are called as sensitive direction. Each cave that can cause collapse is called as sensitive point. The sensitive direction includes:

* The direction of the main shear plane(joint) in the region; * Fracture zone and active fault belt; * The flow direction of groundwater recharge and runoff; 1183 * The contact zones between soluble and non-soluble rock; * The contact zones between soluble rocks and igneous rocks ; * The axial direction of depression or poljes; * The projection of underground river trace on the surface; * Intensive .karst runoff zone or developed zones.

The sensitive points are controlled by the regional stress field. They often spread at equal distance along the major geological structure line or on the nodes of structure network. Besides, some other structucs can be also regarded as sensitive points, including the intersection point of fault, turning point of a structure, the lowest point of plain or basin, the places near springs, and old depressions.

PREDICTION OF COLLAPSE It is necessary to make collapse prediction in some cities and places adjacent to large factories, mines and water source- .areas within the karst region. Much work has been done in this regard. The prediction techniques available are as follows?

I. The prediction with sensitive direction (or point) When the location of a project is decided, the sites and intensity of disturbing source, such as pumping well and mine, could be located and estimated. On the basis of this, an analysis can be made into its hydrogeological map or karst distribution map, and then the sensitive directions and points are marked on the maps. Finally a comprehensive study is made according to the thickness and physical-mechanical properties of soil, thus identifying the p'cTs sible collapse zone. The identification can be also done with the help of geophysical exploration, air photo interpretation, micro- géomorphologie investigation as.well as radiactivity.

II. Multivariate statistical analysis If the factors indicating the possibility of collapse are regarded as a function, and those inducing 1184 the formation of collapse, such as the thickness and mechanical property of soil, the fluctuation, decline and buried depth of water level, the precipitation and rainfall intensity are taken as independent variables, the variables can reach about 20. Each variable has different effect on the formation of collapse. So we can use the method of multivariate statistics analysis to find out the variable with the maximum influence. The following two ways are commonly used: 1) stepwise regression analysis 2) stepwise discriminant analysis.

CONCLUSION The natural collapse takes place either in the exposed type or the covered type of karst regions. It is most serious in the provinces and regions within South and Southwest China including Guangxi, Guizhou, the boundary between Sichuan, Hunan and Hubei, and the boundary between Sichuan, Guizhou and Yunnan. It threatens the environmental quality, the people's life and the economic construction, and affects the change of landscape. Since the Industrial Revolution the collapse has been occurring more frequently, and it is more diffi­ cult to study it because more artificial factors have participated in. The key measure to prevent artificial collapse is to control the drawdown of groundwater level and pumping rate, and in other words, to avoid the rapid descending or the repeated fluctuation of water level which will batter soil. More attention should be paid to the identi­ fied sensitive points and directions, and collapse pits should be treated properly, so that soil could be kept from erosion.

REFERENCES Yuan Daoxian. 1983. Problems of environmental protection of karst area, Paper Presented to the Annual Meeting of AAAS, p.1-14 (separate). Zhang Yingjun, Miao Zhongling etc.. 1985. Applied karstology and speleology(in Chinese). Guizhou People's Publishing House, p.287. 1185-