Evolution Mechanism of Water-Conducting Fissures in Overlying Rock Strata with Karst Caves under the Influence of Coal Mining

There are a large number of karst caves in the limestone rock strata in the karst mining area, and the karst water in rock strata seriously restricts the safe and efficient recovery of coal resources. In order to reveal the influence law of karst caves of rock strata on the evolution of water-conducting fissures in coal seam mining, the numerical simulation method in UDEC software is employed to analyze the development characteristics of water-conducting fissures in coal seam mining under the conditions of karst caves of different sizes. By comparing and analyzing the development characteristics of overlying fissures in rock strata and the displacement fields in coal seam mining under the conditions of different karst caves, the evolution law of water-conducting fissures in coal seam mining is obtained. The research results show that the size of karst cave directly affects the maximum height of the water-conducting fissures in the overlying rock strata in coal seam mining; when the thickness of the coal seam is 3.0m, the larger the radius of karst cave is, the higher the development height of waterconducting fissures in the overlying rock strata is, and the greater the overall sinking displacement in the Yulongshan limestone is. When the radius of karst cave is between 10m and 15m, a through-type water-conducting channel is formed between the working face and the karst cave after the coal seam is mined, and the numerical simulation results are consistent with the field monitoring results. The research results provide an important reference for the prevention and treatment for water inrush disaster in karst mining areas.


Introduction
At present, roof water disaster in coal mine is one of the major disasters in China, and roof water disasters cause casualties every year, which poses a serious threat for the safe and efficient production in coal mine [1,2]. Besides, the karst landform area is developed in coal mining zone, and there are a large number of karst caves in rock strata, especially in Southwest China [3]. Meanwhile, a large amount of atmospheric rainfall enters the karst caves through the karst fissures. When the mining fissures and the waterbearing karst caves are connected, the roof water inrush disaster occurs in working face of coal mine [4,5]. Therefore, it is of great practical and engineering significance to study the evolution mechanism of water-conducting fissures in overlying rock strata with karst caves under the influence of coal mining.
In recent years, experts and scholars at home and abroad have done a lot of research on the development law of water-conducting fractured zones in overburden. Wang et al. [6] studied the influence of the structural stability of the main key layer on the evolution of roof water diversion fissures and aquifer water level and obtained the conclusion that the development characteristics of water diversion fissures are different with different fissure forms of the main key layer. Hu et al. [7] studied the relationship between the height of the waterconducting crack and many factors, such as coal seam mining height, hard rock lithology coefficient, working face slope length, mining depth, and mining advance speed. Liu [8] analyzed the relationship between the height of water conducting crack and the mining thickness of coal seam and drawn the conclusion that there is a positive proportional relationship between the height of the square root of mining thickness. Yang et al. [9] analyzed the height of water-conducting fractured zone of overlying strata under different mining sequences of coal seams. Rong et al. [10] studied the influence of valley slope angle on the movement of roof overburden and the development law of water-conducting fissures in mining the shallow and extrathick coal seam. Wu et al. [11] studied the influence of mining width, mining depth, and advancing speed on the development height of the fractured zone under the condition of fixed mining thickness. Hu et al. [12] studied the relationship between the development height of waterconducting fault zones and coal thickness and working face length during coal seam group mining. Zhu et al. [13] studied the development of mining fissures and the failure characteristics of karst caves in karst mining areas by employing the method of physical similarity simulation. Zhang et al. [14] obtained the failure height of overburden in fully mechanized mining with a large mining height in a deep and thick coal seam in Western Mongolia by employing the methods of field measurement, numerical simulation, and physical similarity simulation. Guo et al. [15] obtained the height of the fractured zone in top coal caving mining under the condition of soft and hard interactive overburden by employing the methods of onsite ground drilling flushing fluid leakage and theoretical analysis. Yang et al. [16] obtained the development height of the fractured zone in fully mechanized top coal caving mining under thick loose layer and soft overburden by comprehensively employing the underground borehole water injection leakage observation, borehole television, and numerical simulation technology. Shi et al. [17] established a PCA-GA-Elman optimization model for predicting the development height of fractured zones, by employing the methods of principal component analysis (PCA), genetic algorithm (GA), and optimized Elman neural network. Li et al. [18] analyzed the ecological, mining, structural, and hydrological conditions in Southwest China and explained the process of rocky desertification driven by coal mining activity in karst mountainous areas. Shi et al. [19] studied the karst development law and groundwater occurrence characteristics of the roof limestone aquifer in Xintian coal mine, by the methods of the controlled-source audiofrequency magnetotelluric method, the water release test, the connectivity test, and water chemical analysis.
At present, most of research results focus on the influence of coal mining on the development height of fissures, and there are some studies on the development law of water-conducting fissures in the karst mining area [20,21], but there is few research on the influence of the size of karst caves in rock strata on the development height of waterconducting fissures in the karst mining area. Therefore, based on the geological conditions of the Xintian coal mine, this paper adopts the research methods of theoretical calculation and numerical simulation to study the movement of overlaying rock strata and the evolution law of waterconducting fissures under the conditions of karst caves of different sizes, which provides a theoretical basis for the safe and efficient production in karst mining areas.

Geological and Mining
Conditions. The Xintian coal mine is located in Gantang Township, Qianxi County, Guizhou Province. This area is a typical karst landform area. The karst fissures in ground surface in the mining area are developed. Some surfaces have karst forms such as sinkholes and karst funnels. The comprehensive histogram is shown in Figure 1.
The mining area is characterized by karst development, densely covered sinkholes, karst caves, and rocky desertification. The Yelang formation (T 1 y) is divided into the Yulongshan section (T 1 y 2 ) and the Shababaowan section. The Yulongshan section is the whole section of limestone. The upper and middle water diversion channels are developed, and a large amount of precipitation is gathered. It is a strong aquifer, and the lower water diversion fissures are not developed, which creates a weak aquifer; the Shabaowan section is a mudstone layer with good water barrier performance. The limestone of the Changxing Group (P 2 c) is located in the lower part of the marl, the shabaowan section, and the development degree of fissures is weaker than that of the Yulongshan limestone, which is a weak aquifer.
The Xintian coal mine is about 8 km long from east to west and 4.6 km wide from north to south. The mining area is 33.4523 km 2 . The mine is developed by the inclined shaft, and the main mining 4# coal seam is now 29.7 m away from the limestone weak aquifer of the Changxing Group and 84.6 m away from the limestone aquifer of the Yulongshan section. The working face adopts the inclined longwall mining method and backward inclined mining, with an annual production scale of 1200000 tons/year.

Mine
Inflow. By comprehensively analyzing the water inflow of the Xintian coal mine from April 2015 to October 2019 and the corresponding atmospheric rainfall in ground surface, it is obtained that the water inflow of the mine is positively correlated with the atmospheric rainfall.
The changing trend of mine water inflow is consistent with the atmospheric rainfall, and the curve change of mine water inflow follows the change of atmospheric rainfall. It is judged that the water inflow of the mine is closely related to the atmospheric rainfall. Atmospheric rainfall is the makeup water source of the mine water, and the mine is connected to the ground. The relationship between mine water inflow and atmospheric rainfall is shown in Figure 2.
There are showers and rainstorms on June 1 and 2, 2019, and the water inflow of the working face is 42 m 3 /h and 50 m 3 /h, and the water inflow of the mine surged to 102 m 3 /h on June 3, 2019. There is less rainfall from June 6 to 15, and the water inflow at the working face shows a steady decline on the whole. However, after the rainfall on June 16, the water inflow at the working face begins to increase. The above analysis further shows that the conduction between the working face and the ground exists. The relationship curve between water inflow and precipitation at the 1402 working face is shown in Figure 3.
The inclined mining method is adopted in the working face of the mine, and the karst water from the roof overburden flows into the working face through the goaf, which not 2 Geofluids only reduces the coal quality and causes serious economic losses but also poses a serious threat to safety production in coal mine. Therefore, it is necessary to study the influence of karst caves on the evolution of water-conducting fissures in overburden under the influence of mining.

Geofluids
where H li is the height of the water-conducting fractured zone and ΣM is the mining thickness of coal seam.
The cumulative mining thickness of the 4# coal seams is 3.0 m, and the empirical equations (1) and (2) are adopted to calculate the height of the water-conducting fractured zone, which is 30.1-41.3 m and 44.6 m, respectively. Therefore, the maximum height of the water-conducting fractured zone is obtained. And it is 44.6 m. According to the comprehensive column chart 1 of the mine, it is obvious that the waterconducting fractured zone develops to the middle of the Changxing Group limestone, which is not connected to the Yulongshan limestone aquifer and is positively correlated with the mine water inflow and atmospheric rainfall. The distance from the 4 # coal seam to the Yulongshan limestone aquifer is 84.6 m, which is much larger than the calculated height of the maximum water-conducting fractured zone of 44.6 m. It is connected to the limestone aquifer of Changxing Group on the roof and the limestone aquifer of Yulongshan Member. Therefore, the calculation equation is no longer suitable under the karst roof conditions in Xintian coal mine.    Table 2.

Numerical Calculative Models
To ensure the uniqueness of the study variables, each model contains 12 karst caves, and the center of each karst cave is a fixed point. The horizontal distance between the center points of circular caves is 100 m, and the vertical distance is 67 m. The schematic diagram of the numerical model test scheme is shown in Figure 5.

The Influence of Karst Caves on Evolution of
Water-Conducting Fissures           (2) The radius of the karst cave is 10 m When the working face is advanced by 45 m, the waterconducting fissured zone develops to a height of 30 m, and the fissures develop to the interior of the Yulongshan limestone; when the working face is advanced by 120 m, the water-conducting fractured zone develops to a height of 95 m, and the fissures develop to the interior of the Yulongshan limestone; when the working face is advanced by 280 m, the water-conducting fissure is connected to the karst cave in the lower part of Yulongshan, and a perturbation through the fissure is formed between the upper and lower karst caves, and the water-conducting channel from the karst cave to the working face is formed. Water-conducting fissures are generated around karst caves affected by mining disturbances, and the karst caves are connected through hor-izontal and vertical water-conducting fissures to achieve internal hydraulic exchange. The development characteristics of water-conducting fissures when the karst cave is 10 m are shown in Figure 8. Through the comparative analysis, it is obvious that the maximum height of overlying fissures developed in coal seam mining is positively correlated with the radius of the karst cave. The development height of the waterconducting fractured zone formed by the mining disturbance gradually increases as the radius of the karst cave increases. When the cavern radius is 10 m and 15 m, the height of water-conducting fissures is 185 m and 195 m, and the split mining ratio is 61.7 and 65, respectively. Under the influence of mining, the water-conducting fissures between the water-bearing cavern and the mined-out area are connected, and the water-gushing disaster occurs immediately at the working face. The development heights of water-conducting fissures under karst caves with different RADII are shown in Figure 10.

Influence of Karst Cave on Overburden Displacement.
Displacement measuring lines are arranged on the roof of the coal seam and the limestone roof of Yulongshan. The subsidence curve of the top interface of the limestone in Yulongshan is shown Figure 11. With the increase of the radius of the karst cave, the maximum subsidence displacement value continues to increase. Therefore, the existence of karst caves affects the development of water-conducting fissures in the Yulongshan limestone aquifer, and the subsidence of the ground corresponding to the working face. Besides, the roof subsidence curve after 280 m of excavation of the 4# coal seam is obtained. With the change in the radius of the karst cave, there is not much difference in the amount of subsidence. The main reason is that the karst cave is far away from the 4# coal seam, and the stress change generated by the karst cave does not affect the change in the roof displacement of the coal seam.

Analysis of Water Inrush Disaster.
The aquifer under the roof of coal seam in the mining area is made up of Yulongshan limestone and Changxing limestone, and the water supply of the aquifer is mainly atmospheric rainfall. Based on the analysis of mine water consumption and numerical simulation results, it is obvious that the atmospheric rainfall in the karst area mostly enters the underground karst aquifer through water drop holes and karst fissures, which becomes the potential threat of coal mining in the karst area. With the continuous increase of coal mining space, the waterconducting fissure gradually develops upward. The mining disturbance influence on the karst cave in the roof gradually strengthens, and the karst fissure around the cave gradually expands outward. When the mining-induced fissure and karst fissure around the cave conduct, forming a water channel between the cave and the goaf. Due to the influence of mining disturbance, the karst fissure between the karst caves leads to the formation of hydraulic exchange. Therefore, a water channel between the ground surface, the karst cave, and the goaf is formed, shown in Figure 12

Conclusions
(1) Through the curve analysis of mine water inflow, working face water inflow, and atmospheric rainfall, it is obtained that both mine water inflow and working face water inflow are positively correlated with atmospheric rainfall, which proves that the working face and the ground form a through-type water channel. At the same time, it is verified that the empirical calculation formula for water-conducting fractured zones is not suitable for karst mining areas, and the actual development height of water-  Figure 12: Schematic diagram of water inrush process in karst area. 9 Geofluids conducting fractured zones in karst areas is larger than the calculation value of the empirical formula (2) The numerical simulation results show that with the increase of the radius of the karst cave, the development height of the water-conducting fractured zone of the overlying rock strata increases. The more the karst cave is disturbed by coal mining, the more karst fissures are generated around it. The change in the radius of the karst cave does not affect the subsidence displacement of the coal seam roof and has little effect on the overall structure of Yulongshan. The ground subsidence increases slightly with the increase of the radius of the karst cave (3) When the radius of the karst cave is 10 m and 15 m, the split-mining ratio is 61.7 and 65, respectively, and the mining-induced fissure intersects with the karst fissure around the cave. The mining-induced fissure is connected between the cave and the mined-out area of the working face and between the adjacent caves. The water diversion channel is formed between the ground surface, the karst cave, and the mined-out area. Therefore, the atmospheric precipitation flows into the coal mine through the water diversion channel, which verifies the correctness of the positive correlation between the mine water inflow and atmospheric rainfall

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.