Ground Subsidence Evolution from 1000 m Deep Mining: A Case Study in Fengfeng Mining Area

The mining of coal resources in eastern China has entered the stage of deep mining, and many mines have reached the depth of 1000 meters. Diﬀerent from shallow and moderate depth mining, the temporal and spatial evolution regulation of surface movement and deformation under deep mining has its particularity. Combining with the geological and mining conditions of Fengfeng mining area, this paper systematically studies the characteristics of surface movement under the condition of shallow, moderate, and near kilometer mining depth. By means of ﬁeld measurement, InSAR monitoring, we get the subsidence data under diﬀerent mining depth and get the relevant subsidence parameters by inversion. Through comparative analysis, the special law of subsidence under the mining depth of 1000 meters is obtained. The results show that under the condition of nearly 1000 meters mining depth, the surface movement and deformation have thecharacteristics of large displacement angle, small displacement deformation value, and large main inﬂuence radius. The regulation of small proportion of active period of maximum subsidence point, gentle shape of surface movement basin, and low mining adequacy are obtained. The research results provide technical references for deep mining under buildings, railways, and water bodies and provide basis and reference for scientiﬁc mining and safe recovery of coal pillars in kilometer deep mine.


Introduction
Among the total coal reserves in China, 53.17% are buried deeper than 1000 m. e average mining depth of stateowned coal mines increases at the rate of 10-25 m/a. Some mining areas in the east of China, such as Fengfeng, Kailuan, Beipiao, Huainan, Xinwen, and Xuzhou, have entered the deep mining one after another, and the mining depth of some coal mines has reached 1,000 m [1].
According to mining subsidence theory, it is generally believed that the mining depth of coal seam less than 400 m is shallow mining, the mining depth of coal seam 400-700 m is moderate depth mining, and the mining depth of coal seam greater than 700 m is deep mining [2]. In the 1000 m mining depth, the mechanical environment of the mines changes greatly relative to shallow and moderate depth mining [3,4], and the spatio-temporal evolution law of strata and surface movement is different from that of shallow and moderate depth mining [5][6][7]. It is of great significance for the sustainable development of national resources to study the surface movement regulation under the condition of nearly 1000 m mining depth.
International research on deep mine began in 1983. e former Soviet Union proposed a special study on mining of more than 1600 m deep (coal) mines [8]. e University of Idawa, the University of Michigan Technology, and the Southwest Research Institute of the United States carried out a research on deep well mining [9]. At present, most of the researches on deep mining focus on the change of surrounding rock. Zhang [10] studied the water inrush mechanism of the floor induced by deep mining based on the coupled mining pressure and confined pressure. De Santis et al. [11] evaluated the seismic and aseismic rock deformation through the in situ monitoring and 3D geomechanical numerical modelling under the condition of deep mining. Kouame et al. [12] pointed out that a major problem of deep mining is high ground stress, which is the major factor of rock burst. Due to the complexity of the rock burst mechanisms, complexity of induced factors as well as suddenness and randomness of rock burst occurrences, studies of rock burst prediction, and control for safe mine exploitation is far from satisfying. Wang et al. [13] studied the automatic roadway formation method by roof cutting with high-strength bolt grouting with the deep mining. Zhou et al. [14] used the distributed fiber optic sensing technology to monitor the deformation and failure of the extrathick coal seam in deep mining. Grygierek and Zieba [15] analyzed the damage to the highway in the surface linear discontinuous deformations area induced by deep mining.
In recent years, there are more and more articles on the mechanism and law of surface movement in deep mining. Li et al. [7] pointed out that the surface subsidence velocity during active movement is less than 50 mm/month under the condition of large mining depth, and the damage of surface buildings is relatively slow. Based on the analysis of the characteristics of surface subsidence caused by deep mining and shallow mining, Xu et al. [16] proved that the overburden failure of deep mining has the characteristics of uniform, integral deformation, and the surface movement and deformation are continuous, slow, and long. Yu et al. [17] pointed out that when the mining width was fixed, the ground movement and deformation values in deep mining were less than those in shallow mining and proved that the overall compression and deformation had a great influence on the angular parameters of strata movement. Guo and Li [18] calculated the safe depth of deep mining and the limit depth of strip mining, and the relationship between subsidence coefficient and mining thickness of deep strip mining was studied. Mikroutsikos et al. [19] carried out the stability analysis of the landslide at the weak surface under the condition of deep mining. Yang et al. [20] established the relationship between surface subsidence coefficient and width-depth ratio under deep mining conditions by field measurement and predicted mining subsidence by modifying subsidence coefficient. Zhao et al. [21] used the microseismic data to analyze the mechanical response of rock masses.
However, up to now, there are limited literatures on the surface subsidence law under near 1000 meters mining depth. erefore, taking Fengfeng mining area as the research object, this paper studies the surface movement regulation at the mining depth of near 1000 meters by means of field measurement, InSAR monitoring, and comparative analysis.

Geological Mining Conditions in Research Area
Fengfeng mining area is a large coal production base in China, which has 11 pairs of production mines currently, located in the southern part of Hebei province. e total area of the mining area is 1260 km 2 , and the ground elevation is +105 m∼+280 m. ere are 6 stable coal seams in Fengfeng mining area, including coal 2, coal 4, coal 6, coal 7, coal 8, and coal 9. However, due to the extremely complex hydrological conditions, only coal seams 2, 4, and 6 are mined at present, coal seams 7, 8, and 9 have not been mined. According to the mining knowledge, when the coal seam thickness is greater than the minimum minable thickness and the change of coal seam thickness has a certain law, the coal seam is a stable layer. And when the coal seam thickness changes greatly and most areas are minable, the coal seam is a relatively stable coal seam. e characteristics of each coal seams are shown in Table 1. Coal 2 is the main mineable coal seam in Fengfeng mining area, located in the lower part of Shanxi Formation of Permian System. e average thickness is 4.04 m, the occurrence depth is 90∼1200 m, the dip angle is 4∼26°, the bulk density is 1.40 t/m 3 , the roof is mainly sandstone and sandy mudstone, and the floor is generally mudstone and sandy mudstone. Overburden is medium-hard rock with loose layer thickness of 0-30 m. Mining methods mostly adopt strike long-wall light top-caving mining, and all caving methods were used to manage the roof.

Measured Data and Results
More than 10 working faces in coal seam 2 were monitored by traditional surveying technology (level + traverse) and InSAR surveying technology.
Based on the long time series image data of the radar satellite Sentinel-1A in the study area, SBAS-InSAR technology was used to obtain large-scale land subsidence information.
On the contrary, ground mobile observatories were set up to monitor surface subsidence regularly. Generally, an observation line was laid along the direction and dip orientation of the working face, and the control point was selected outside the mining influence area. e subsidence of the observation point was monitored by leveling, and the horizontal movement was monitored by traverse measurement. e observation frequency during active period of surface movement is every 15 days. Figure 1 shows the surface subsidence range and cloud map of a mine in Fengfeng mining area based on 71 Sentinel-1A images and SBAS-InSAR technology. ere is no subsidence data in the central area of the subsidence basin in the subsidence nephogram, which is due to the large subsidence velocity and the decorrelation phenomenon in SAR image interferometry. Figure 2 is the layout of the ground observation station in a mining area of mine D, Fengfeng mining area. Figure 3 shows the surface movement curve along dip orientation of the mining area, drawn from the leveling and traverse measurement data.

Characteristic Analysis of Angular Parameters of Surface Movement.
e angular parameter is an indicator of the law of surface movement. e commonly used angular parameters are boundary angle and moving angle. When the gob has reached the critical size, or nearly so, on the major cross section of a movement basin, the angle between the horizontal line at the panel edge and the line connecting the panel edge and the edge of the movement basin is the boundary angle, the angle between the horizontal line at the panel edge and the line connecting the panel edge and the point of critical deformation on the surface is the moving angle. Table 2 shows the angular parameters obtained from measured data under different mining depths in Fengfeng mining area. Due to the limitation of field observation conditions and the destruction of some observation marker piles, some angular parameters cannot be obtained, so the corresponding data in the table are vacant.
From Table 2, it can be seen that the angular parameters of surface movement and deformation in deep mining of Fengfeng mining area have the following characteristics compared with shallow and moderate depth mining: (1) With the increase of mining depth, the boundary angle and moving angle values increase. is is the remarkable feature of surface movement in deep mining, which is mainly caused by the increase of mining depth and the decrease of the severity of surface deformation. (2) Due to the increase of mining depth, insufficient mining has little effect on the surface. In some cases, the surface movement deformation value is less than the critical value, and there is no moving angle value.

Characteristic Analysis of Predicting Parameters of Surface
Movement. In China, the most commonly used prediction function is the probability integral method. e prediction parameters of the probability integral method are as follows:      Q66  Q64  Q62  Q60  Q58  Q56  Q54  Q52  Q50  Q48  Q46  Q44  Q42  Q40  Q38  Q36  Q34  Q32  Q30  Q28  Q26  Q24  Q22  Q20  Q18  Q16  Q14  Q12  Q10  Q08  Q06  Q04  Q02   0  300  600  900  1200  1500  1800  2100   Shock and Vibration (1) Subsidence rate η: subsidence rate can be calculated by the following formula: where WO is the maximum surface subsidence, mm; M is the mining thickness, mm; and α is the coal seam dip angle,°. (2) Horizontal displacement coefficient b: horizontal displacement coefficient can be calculated by the following formula: where UO is the maximum surface horizontal displacement, mm. (3) Tangent of main influence angle tanβ: tangent of the main influence angle can be calculated by the following formula: where HO is the average mining depth, m, and r is the main influence radius, m. Based on the measured data of deep and shallow mining in Fengfeng mining area, the predicting parameters under different mining depths are shown in Table 3. Table 3 shows that the predicted parameters of deep mining in Fengfeng mining area have the following characteristics compared with shallow mining: (1) With the increase of mining depth, surface subsidence rate η decreases significantly. When H < 400 m, the subsidence coefficient is 0.78; when 400 m < H < 700 m, the subsidence rate varies between 0.4 and 0.7 (when fully exploited); when 700 m < H < 1000 m, mining is mostly under the Q66  Q64  Q62  Q58  Q56  Q54  Q52  Q50  Q48  Q46  Q44  Q42  Q40  Q38  Q36  Q34  Q32  Q30  Q28  Q26  Q24  Q22  Q20  Q18  Q16  Q14  Q12  Q10  Q08  Q06  Q04  Q02   0  300  600  900  1200  1500  1800  2100  2400   e subsidence rate is an important indicator for evaluating mining damage. With the increase of mining depth, the adequacy degree of mining decreases, mining is mostly inadequately exploited or extremely inadequately exploited, and the subsidence rate decreases accordingly. rough correlation analysis and calculation, the relationship between subsidence rate η and width-depth ratio D/H satisfies the Boltzmann function, as shown in the following equation, and the fitting curve of the function is shown in Figure 4: From Figure 4, it can be seen that the surface subsidence rate of Fengfeng mining area has the following characteristics: (1) With the increase of width-depth ratio, the subsidence rate increases rapidly, then increases slowly, and finally reaches the maximum value. is is a remarkable feature of surface subsidence in layered mineral mining.
(2) Under the condition of nearly one kilometer mining depth, the width-depth ratio is small and the subsidence rate of the surface is small; under the shallow condition, the width-depth ratio is large and the subsidence rate is large. e main reason is that with the increase of mining depth and overburden thickness, overburden structure and separation play a controlling role in surface subsidence during the process of subsidence transmission, which makes the subsidence rate decrease.

Analysis of Active Period of Maximum Subsidence Point.
e subsidence velocity is greater than 1.67 mm/d as the criterion of active period. According to the measured data of several observatories, the proportion of active stage to the total moving period of the maximum subsidence point is calculated. Figure 5 shows the changing trend of the proportion of the active stage of the maximum subsidence point in different mining depths. e analysis shows that with the increase of mining depth, the proportion of active subsidence stage to the total movement period gradually decreases, and at about 900 m (working face 157 of mine G), there was no active stage on the surface. is is due to the large overburden thickness, the small amount of subsidence propagated to the surface, which makes the subsidence rate of the surface decrease correspondingly, and the active period shorten or disappear.

Analysis of Shape of Surface Moving Basin and Sufficiency of Exploitation.
By analyzing the measured data, we can get the contrast map of surface subsidence curves at different mining depths ( Figure 6). From the graph, the following characteristics and laws of surface movement under different mining depths can be seen.
(1) With the increase of mining depth, the shape of surface moving basin changes from "steep basin margin, fast deformation velocity, large subsidence value, and small influence area" to "flat basin, slow deformation velocity, small subsidence value, and large influence area." Under the condition of shallow mining, the margin of subsidence basin is steep, the boundary converges quickly, and the influence scope is small. After full mining, the flat bottom appears in the center of the basin. Under the condition of nearly one kilometer mining, the subsidence basin is gentle, the boundary converges slowly, and the mining influence scope increases. (2) Under certain conditions of mining width, the greater the mining depth, the more difficult it is to achieve full mining, and the lower the mining degree. Under the condition of inadequate mining, the movement of strata and surface is small, and the subsidence rate of surface decreases with the decrease of mining degree, which shows that mining degree has obvious control effect on surface subsidence.

Discussion
With the mining of coal seams, the overlying strata are broken gradually, and the influence of mining is transmitted to the surface layer by layer, resulting in the surface subsidence. e surface subsidence value is often less than the underground coal production. ere are the following relationships among underground coal production V 1 , surface subsidence volume V 2 , rock pressure relief expansion volume V 3 , overburden inner separation layer, and void volume V 4 [22]: With the gradual increase of mining depth, the vertical pressure of the overlying strata increases, and the lithology of the strata becomes hard in the long-term sedimentary environment. Driven by the underground coal mining, the strata movement will have the following three characteristics ( Figure 7): (1) With the coal mining, the initial and periodic fracture distance of the strata increase, and the suspension distance of the strata increases. When the mining affects the rock, the amount of separation between the rock and the adjacent rock increases. (2) When the advancing distance reaches a certain amount, the strata will break. e harder the lithology is, the longer the broken block is, and the larger the gap between the broken blocks is.
(3) With the increase of mining depth, the number of strata is more, the number of weak interfaces is more, and the number of separated layers in overburden is more.
at is, with the increase of mining depth, V 4 increases gradually. However, V 3 remains unchanged, V 2 decreases gradually. Under the condition of horizontal coal seam mining, the subsidence rate η (Formula 1) can also be simply calculated with the following formula.
erefore, with the increase of mining depth, the separation layer between strata and the gap between blocks in overburden increases, which leads to the reduction of mining impact transmitted to the surface, that is, the reduction of subsidence rate.
In addition, the strata have the digestion and diffusion function on the mining influence. In the amount of mining influence, the strata have digestion and absorption function. In the influence range of mining, the strata have diffusion effect on it. erefore, with the increase of mining depth and the number of strata, the digestibility and absorption rate of the strata affected by mining increase, and the range of the strata affected by mining Shock and Vibration 7 increases rapidly. erefore, with the increase of mining depth, the range of mining influence increases, and the amount of mining influence decreases. Compared with shallow and deep mining, when the mining depth reaches 1000 m, the surface movement range is larger and the surface subsidence value is smaller. Reflecting on the main cross section of the surface movement basin, the basin shape will become slower and the basin boundary angle will become larger.

Conclusions
rough analysis of field measured data, the characteristics of surface subsidence induced by near 1000 m deep mining were summarized. e main conclusions are as follows: (1) e regulation of gentle surface movement basin, large displacement angle, small displacement deformation value, and large main influence radius under the condition of nearly kilometer depth are obtained. (2) e Boltzmann function relationship between subsidence rate and width-depth ratio is determined. (3) e strata have the digestion and diffusion function on mining influence. With the increase of mining depth, the proportion of active stage of maximum subsidence point to total movement period decreases, the mining adequacy decreases, the range of mining influence increases, and the amount of mining influence decreases.

Data Availability
e data used to support the findings of this study are available from the first author upon request.

Conflicts of Interest
e authors declare no conflicts of interest.   Shock and Vibration