(Jiangsu Geological Survey and Research Institute, Nanjing, Jiangsu, 2 100 18)

Since 1980s, the national economy in Suzhou, Wuxi and Changzhou has developed at a high speed, but the over-exploitation of groundwater resources has destroyed the balanced geological environment and induced a series of geological disasters, such as land subsidence, ground fissures and water accumulation, especially the uneven land subsidence, which has developed rapidly and destructively, causing unstable factors and negative impacts on society. This paper focuses on the formation mechanism of ground fissures in order to better predict ground fissures and achieve the purpose of disaster reduction and prevention.

Keywords: research on ground fissures in Suchang area

Since the reform and opening up, the economy of Suzhou-Changzhou region has developed by leaps and bounds, and now it has become one of the most developed areas in the eastern coastal areas of China. However, compared with the rapid development of economic construction in Suzhou, Wuxi and Changzhou, environmental protection is obviously lagging behind, especially the awareness of geological environmental protection is indifferent. Long-term over-exploitation of groundwater resources has caused regional land subsidence geological disasters.

The land subsidence in Suzhou-Wuxi-Changzhou area mainly occurred in the last 30 years, with the central city earlier and the peripheral counties and cities later, which is basically consistent with the history of groundwater exploitation. Before the mid-1980s, it mainly occurred in three central cities and Xixi area. After the mid-1980s, with the expansion of groundwater exploitation area and the sudden increase of exploitation intensity year by year, the problem of land subsidence also rapidly expanded to the region (Figure 1), and the occurrence degree became more and more serious (Table 1).

Figure 1 Development and Change Map of Land Subsidence in Suzhou, Wuxi and Changzhou Area

At present, the area of accumulated subsidence greater than 200mm is nearly 6000km2, accounting for12 of the original total area of Su Changping. The 500mm contour continuously surrounds three central cities, with an area exceeding 1500km2.

At the same time, in the process of regional land subsidence, due to the existence of specific geological background conditions (bedrock buried hill, ancient buried terrace, uneven distribution of water-bearing sand layer, etc. ) and unreasonable ways and means of artificial exploitation of groundwater, in some areas of Suchang area, geological disasters of uneven ground subsidence have developed and manifested in the form of ground fissures on the surface. It destroyed the ground buildings and underground pipelines and seriously affected the sustainable development of the local economy.

Table 1 Statistical Table of Land Subsidence Development and Change in Suzhou, Wuxi and Changzhou Area

The ground fissures in Suchang area first appeared in the late 1980s and reached its peak in 1990s. At the beginning of this century, they also happened one after another.

1 Distribution and development characteristics of ground fissures

The distribution and development of ground fissures in Suzhou, Wuxi and Changzhou are regular in space and time.

Spatial distribution characteristics of 1. 1

(1) plane feature

The plane forms of ground fissure geological disasters in Suchang area are linear, linear, curved and geese. Fault zones with a certain width are mostly distributed on both sides of the main fault, generally less than 100m, and ground fissures extend from tens of meters to thousands of meters.

(2) Profile characteristics

The geological hazard profile for ground fissures in Suzhou-Wuxi-Changzhou area is generally unclear; and most of them are staggered up and down on both sides for the cracks; forming scarp-like or stepped cracks on the surface. Some are V-shaped cracks, the width of surface cracks is generally about 2 ~ 80 mm, and the depth of cracks is generally about 20 ~ 40 cm. After profile excavation and geophysical surface wave (SWS) measurement, the fracture depth in the excavated profile is 3m (Figure 2), and the surface wave exploration results show that the Quaternary strata with the same fracture on both sides are affected by cutting up to 36m. According to the analysis of three-dimensional seismic exploration results, the influence depth of ground fissures can reach the bedrock surface, and the influence depth can reach 60 ~ 80m.

(3) Directional characteristics

The distribution and development of geological disasters of ground fissures in Suchang area have obvious directionality, and most of them are distributed in northeast or northwest direction. There are also some ground fissure disasters that are distributed in a ring shape. After statistical analysis, the development direction of cracks is not obvious (Figure 3).

Fig. 2 Schematic diagram of the causes of ground fissures on both sides of Shitangwan in Wuxi.

Fig. 3 Rose diagram of the development direction of ground fissures in Zhou Nan Village, Changzhou University Town

Temporal development characteristics of 1.2

According to the statistics of the development time of ground fissures in Suzhou, Wuxi and Changzhou, there is obvious correlation with the peak development period of land subsidence disasters in this area. The ground fissures in this area originated from 1989. In the next 20 years, ground fissures will occur almost every year. The peak period was in the 1990s, especially 1995. There are 6 ground fissures in this area, which have slowed down at the beginning of this century.

1.3 Different types of ground fissures occur under different geological background conditions.

Different geological environment background is the internal factor of ground fissures. Therefore, in the areas where buried hills, ancient buried terraces and buried bedrock cliffs are distributed and developed, linear ground fissures are usually developed and have certain extensibility. For example, the ground fissures in Changjing-Tanghe-Zhangjingyangli in Jiangyin City belong to this type of ground fissures. In the Quaternary sediments above the main mining layer of groundwater, due to the influence of groundwater discharge factors, semi-circular ground fissures related to soil structure differences are often formed, such as Caoqiao ground fissure disaster in Changzhou City. In the areas where the main water-bearing sand layers in Quaternary sediments are undeveloped or poorly developed, people usually use the method of comprehensive exploitation of upper and lower aquifers to extract groundwater resources, and then the groundwater level in some areas forms a local drop funnel, which makes the hydraulic gradient in some areas drop sharply and produces an annular ground fissure disaster on the surface, such as the ground fissure disaster in Zhou Nan Village, Changzhou University Town.

1.4 Ground fissures have the characteristics of sustainable development.

The ground fissures in Suzhou, Wuxi and Changzhou have the characteristics of sustainable development in a certain period of time. They generally appear at the beginning of flood season or rainy season, and once formed, they continue to fall and intensify along the fault surface, which is an unstable development state. According to the field investigation, there are still five ground fissures in Suchang area with the characteristics of further development, 15 ground fissures are in a relatively stable development stage, and five are in a stable stage.

Study on formation mechanism of ground fissures

The main factors that affect the formation of ground fissures in Suzhou, Wuxi and Changzhou are: objective geological background conditions (undulating characteristics of bedrock surface, lithology of bedrock, ancient buried terraces, structural differences of Quaternary strata, structural characteristics of aquifers, etc. ) and the destructive effect of human beings on disorderly and over-exploitation of groundwater resources for the purpose of developing economy. The formation mechanism of different types of ground fissures is the result of the interaction of different influencing factors in different regions and under different geological background conditions.

2. 1 buried hill type

Land subsidence is mainly caused by the comprehensive action of external factors such as Quaternary stratigraphic differences, paleobasement fluctuation and groundwater overexploitation in this area. Long-term over-exploitation of groundwater causes the head of aquifer sand and underground reservoir to drop, which leads to water release and compression of aquifer sand itself and overlying soil layer, resulting in land subsidence; When the environmental geological conditions such as the structural difference of soil layer itself or the fluctuation of sedimentary basement are uneven, there will be obvious differential settlement in the process of land settlement caused by soil layer compression, which will form lateral tensile stress in soil; When the lateral tensile stress reaches or exceeds the ultimate tensile strength of soil, it will be manifested in the form of ground fissure disaster on the surface (Figure 4).

Fig. 4 Geological model of ground fissure formation in bedrock buried hill

2.2 Comprehensive exploitation of groundwater

There are deep wells with concentrated mining near the ground fissure zone, and the mining volume is large. The first, second and third confined aquifers are used as water resources for mining. Due to the undeveloped confined aquifers II and III, the water levels of confined aquifers II and III in the area have dropped to -60 ~-70m, and the main mining layer of deep wells is confined aquifer I near the surface, which leads to the underground water aquifers in the area to connect up and down, especially the shallow water (phreatic water and confined water I) water level drops sharply, forming a local water level drop funnel centered on deep wells (Figure 5). This is caused by the compression deformation of soft soil layer and confined water-bearing sand layer near the surface. I confined water layer has the characteristics of shallow burial, fine particles and poor permeability. Under the strong mining action, sediment particles are lost with groundwater, and the sand grains are rearranged, forming a steep water level drop funnel shape with large hydraulic gradient, which often leads to geological disasters of uneven ground settlement.

Fig. 5 Geological model diagram of ground fissure formation in groundwater comprehensive exploitation type

2.3 Types of soil structure differences

The Quaternary sedimentary structure near the surface in the ground fissure development area is obvious, especially under the surface hard soil layer, there is a highly compressible soft soil layer (muddy clay layer) with uneven thickness, and the liquefaction index, natural porosity ratio and permeability coefficient of the muddy clay layer are relatively large. When the year of less precipitation comes, the groundwater recharge conditions are poor. Due to long-term over-exploitation, the groundwater head drops sharply, which urges the high-compressibility soft soil to release water tightly, forming plastic deformation, causing continuous ground subsidence and eventually inducing ground fissure disasters (Figure 6).

Fig. 6 Geological model of ground fissures with different soil structures.

2.4 Types of buried terraces

The formation mechanism of ground fissures is similar to that of bedrock buried hill type, which is mainly caused by the comprehensive action of external factors such as ancient buried terraces or bedrock cliffs (with linear distribution characteristics), Quaternary stratigraphic differences and groundwater overexploitation in the area. Long-term over-exploitation of groundwater causes the head of aquifer sand and groundwater reservoir to drop, which leads to water release and compression of aquifer sand itself and overlying soil layer, resulting in land subsidence; On the edge of buried terrace or bedrock cliff, soil compression causes obvious differential land subsidence with linear distribution characteristics, which is manifested as linear ground fissure disaster on the surface (Figure 7).

Fig. 7 Geological model map of fracture formation in buried terrace

2.5 Karst types

At present, the formation mechanism of this type of ground fissures in Suzhou-Wuxi-Changzhou area is similar to that in buried hills, which is mainly caused by the comprehensive action of external factors such as bedrock uplift (bedrock lithology must be limestone strata with salt-dissolving characteristics), karst development, Quaternary strata difference and groundwater overexploitation in the area. Long-term over-exploitation of groundwater causes the water-bearing sand layer and groundwater level to drop, which causes the water release and compression of the underground water-bearing sand layer itself and the overlying soil layer, resulting in land subsidence; In the karst development area, the soil layer loses water and compacts, which leads to the collapse of overlying loose deposits and induces uneven ground settlement, especially at the edge of karst collapse, which is prone to annular ground fissures (Figure 8).

Fig. 8 Geological model of karst ground fissure formation

3 GIS-based division of ground fissure prone areas

At present, GIS spatial analysis method has been increasingly applied to quantitative or semi-quantitative analysis and evaluation of geological environment. Study on Ground Fissures in Suzhou, Wuxi and Changzhou Using Arc/Info to simulate the spatial characteristics of ground fissures in this area.

3. Determination of1factor

The division of ground fissure disaster-prone areas is mainly based on five factors, such as the undulating shape of bedrock surface, the thickness difference of Quaternary sediment distribution, groundwater level, the spatial characteristics of groundwater aquifer and land subsidence, which play an obvious role in the development of ground fissures.

3.2 Evaluation model

The data expression of GIS-based evaluation model is:

Essays on Geological Disaster Investigation and Monitoring Techniques and Methods

In which: I-risk index, which represents the superposition result of comprehensive effects of various influencing factors;

Wi-the weight of the i-th factor;

Ci-the quantitative assignment of the i-th factor;

N- the number of influencing factors.

3.3 Evaluation procedure

The process of spatial distribution zoning of ground fissures assisted by GIS is shown in Figure 9.

Spatial overlay in Arc/Info is actually the overlay of layers, and all spatial analysis is based on digital maps. After many adjustments, the zoning model of ground fissure evaluation in Suchang area is: I=0.33×C bedrock +0. 12× c Quaternary +0. 19× c groundwater level +0. 16× c aquifer +0.2× c ground subsidence.

According to the total comprehensive impact score of impact factors, the evaluation results are qualitatively classified, and the classification reflects the severity of current disasters in principle, and the corresponding area statistics are carried out. See Table 2 for statistics.

Fig. 9 Risk assessment process of ground fissures based on GIS.

Table 2 Evaluation results and statistical results of each division

3.4 geological background analysis of ground fissure disaster-prone areas at all levels

From the evaluation results, it can be seen that the geological disaster area and the potential danger area of ground fissures in Suchang area are mainly distributed in the eastern part of Qishuyan in Changzhou and the western part of Huangdai in Wuxian. The Quaternary basement in this block is uplifted on the opposite sides, and the buried depth of bedrock surface is small, and its fluctuation range is just within the development depth of II and III confined water-bearing sand layers, which objectively has the specific geological environment conditions of ground fissure disaster, but the geological background of different grade areas varies from place to place.

3.5 Division of ground fissure disaster-prone areas

According to the operation results of the above model and the current understanding, based on the undulating shape of bedrock surface (buried hill and buried steep rock), the distribution and thickness difference of Quaternary sediments, and the linear boundary (mostly ancient river boundary) where the thickness of pore water-bearing sand layer changes obviously, the division principle of ground fissure disaster-prone areas is preliminarily determined. Six areas prone to geological disasters of ground fissures and areas where there may be hidden dangers of geological disasters of ground fissures are preliminarily delineated (Figure 10).

(1) Geological disaster-prone areas of ground fissures in Henglin

With Henglin Town as the center, along the NE-trending structural line, to Hutangqiao, Malaysia Airlines and Qishuyan in the southwest, it is a special zone controlled by Jiangnan fault, with an area of about 30km2.

(2) The ground fissures of Hengshan Bridge are prone to geological disasters.

Hengshan Bridge is located in front of Fang Mao Mountain, where the bedrock surface changes from bare to steep. It is inferred that the buried cliff controls the Middle Pleistocene ancient river channel, and the land subsidence on both sides is obviously uneven, which is an area prone to ground fissures and disasters.

(3) The geological disaster-prone area of ground fissures along the bedrock shoreline on the south side of Wuxi-Suzhou ancient river channel.

This zone extends from northwest to southeast for a long time, starting from Jiaxiang in Xishan in the west, roughly along the Beijing-Hangzhou Canal in the southeast, and extending to Huguan in Suzhou through downtown Wuxi. This zone is prone to ground fissures and geological disasters in Suzhou, Wuxi and Changzhou. At present, there are two ground fissures found in Jiaxiang and Maocunyuan.

(4) The south boundary of Jiangyin ancient river is prone to ground fissures and geological disasters.

There is a bedrock fault block uplift between Yanqiao, Chang 'an, Houqiao and Anzhen in Xishan Mountain. Under its influence, it is speculated that buried hills and cliffs are developed, which is a zone with great changes in environmental geological background conditions, and in fact, it is a zone prone to cracks in the region. The ground fissures in Qin Xiang, Yanqiao, Tanghe and Shitangwan Changjing are all in this area.

Figure 10 Zoning Map of Geological Disaster-prone Areas of Ground Fissures in Suchang Area

(5) Geological disaster-prone areas of ground fissures in Dongting.

In the western suburb of Xishan New Town (Dongting), the exploration results of the area where ground fissures have been formed show that it may be related to island residual hills or shallow strata, but the induced reason is still related to over-exploitation of Class II confined groundwater.

(6) The geological disaster-prone area of ground fissures in Tangqiao, Zhangjiagang.

In towns and villages such as Tangqiao, Tang Shi and Xizhang in the southeast of Zhangjiagang, from the signs, the land subsidence is serious, but uneven, and disastrous ground fissures have appeared in many places.

(7) Check the geological disaster-prone areas of Qiaoqiao ground fissures.

The zonal interval, which is located in the northeast of the piedmont plain on the west side of Houshan Mountain, has occurred in many places and has a further serious development trend.

4 ground fissure disaster prevention and control countermeasures

Geological disasters such as land subsidence and ground fissures have seriously restricted the sustainable development of economy and society in Suchang area, and effective measures must be taken to prevent them. In the prevention and control of geological disasters, we must implement the principle of combining prevention with avoidance, further strengthen leadership, raise awareness, plan scientifically and strengthen management in accordance with national and provincial regulations on the prevention and control of geological disasters, and reduce the losses caused by geological disasters to a minimum. According to the investigation, the ground fissure disaster in this area is mainly a catastrophic form after serious land subsidence, so the most fundamental thing is to control land subsidence. In areas with serious land subsidence, we should study all kinds of influencing factors in depth and make a scientific and reasonable evaluation of the uneven development trend of land subsidence.

Especially in the area that is initially designated as prone to geological disasters of ground fissures, urban construction and engineering project construction must also be listed as one of the possible geological disasters, and serious risk assessment should be carried out. In the disaster area where ground fissures have occurred, it is necessary to find out the reasons, accurately define the dangerous areas, set up monitoring points and implement specific avoidance measures.

5 conclusion

The research results of this paper are the crystallization of the collective work of the Delta Project Team of Jiangsu Geological Survey and Research Institute. Here, I would like to express my heartfelt thanks to all the teachers and colleagues who participated in the project!