Field Study on the Lawof Surface Subsidence in theHigh-Intensity Fully Mechanized Caving Mining Working Face with Shallow Thick Bedrock and Thin Epipedon in Hilly Areas

School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China State Collaborative Innovation Center of Coal Work Safety and Clean-Efficiency Utilization, Henan Province, Jiaozuo 454000, China Xi’an University of Science and Technology, State Key Laboratory of Coal Resources in Western China, Xi’an University of Science and Technology, Xi’an 710054, China State Key Laboratory of Water Resource Protection and Utilization in Coal Mining, Beijing 100011, China Shanxi Yamei Daning Energy Co.,Ltd, Shanxi Province, Jincheng 048000, China College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030000, China School of Civil Engineering, Shaoxing University, Shaoxing 312000, China


Introduction
Before an underground coal seam is mined, the rock remains in a relatively balanced state under the action of in situ stress field. When the coal seam is partially mined, a gob area is formed inside the rock, and the stress equilibrium state of the surrounding rock gets destroyed, causing stress redistribution [1][2][3]. ereafter, the rock undergoes movement, deformation and failure until a new balance is reached. Finally, surface subsidence occurs, which causes damage to surface buildings and structures [4,5]. Many researchers have carried out a large number of laboratory experiments on the mechanical properties and deformation characteristics of coal and rock. For example, Song et al. [6,7] investigate the evolution of deformation and damage related parameters as well as the hysteresis behavior on Tibet marble. e test results show that an increase of FT cycles and fatigue load level both accelerate the damage rate of marble. Based on two different cyclic loading strategies, it is concluded that the maximum load level has a more pronounced effect on energy dissipation than the minimum load level [8]. Scholars [9][10][11][12][13][14] found that the laws of surface subsidence caused by different mining geological conditions differ significantly. Useful conclusions were drawn through a series of studies on the mining-induced surface subsidence from the aspects of bedrock thickness, buried depth, mining sufficiency, and so on. Meanwhile, it is also found in relevant researches [15][16][17] that due to the influence of surface topography, the surface topography damage, subsidence law, and construction protection of hilly areas are quite different from those of plain areas. Gao [18] studied the surface subsidence law under conditions of thick bedrock and large mining depth and found that the boundary angle value is small and the surface subsidence influence range is large due to the influence of multiple groups of thick and hard sandstone layers. Peng and Song et al. [19,20] analyzed and summarized the surface movement and deformation characteristics of loess gully landform and the key technologies of ecological restoration. erefore, the study on the law of surface subsidence in HIFMCMWFSTBTE in hilly areas is of great significance for improving the basic theory of mining subsidence and guiding on-site production [21][22][23][24][25][26][27].
Shallow and thick coal seams occur extensively in hilly areas in Shanxi Province and Shaanxi Province, China. In view of this fact, with the fully mechanized caving mining P2 working face with shallow thick bedrock and thin epipedon in a mine taken as the engineering research object, the temporal and spatial evolution laws of surface movement in HIFMCMWFSTBTE in hilly areas under the same mining and geological conditions were summarized by combining the methods of field measurement and theoretical analysis. Important research results were obtained.

Geological Mining Conditions.
e designed production capacity of the mine is 4.0 Mt/a, and its terrain is mainly low mountains and hills, of which the surface topography of P2 working face belongs to low mountains. e working face is covered by thin epipedon and partially exposed bedrock ( Figure 1). In P2 working face, no. 3 coal seam, whose average thickness, dip angle, and buried depth are 4.8 m, 5.0°, and about 200 m, respectively, is mined. e working face is 220 m wide, has a mining length of 1138 m, and advances at the speed of 4 m/d. e method of fully mechanized caving towards long wall on strike is adopted for coal mining, and the method of total caving is used for roof management. ere are 36 measuring points, among which points C7-C17 are near bedrock-exposing , while the rest of the points are near loess-covering surface. Line C is entirely located in a basin. e layout of observation points in P2 working face is exhibited in Figure 2

Dynamic Distribution Characteristics of Subsidence Curve of Line A on Strike (Loess-Covering Surface).
e observation line A on strike is arranged on the side of the stopping line of the working face. e analysis of the strike dynamic subsidence curve in Figure 3 demonstrates that, first, the ground observation starts from A1 towards A41 gradually (in Figure 3, the horizontal axis distance is the distance from each observation point to point A41) and ends after 8-13 observations are completed. After subsidence starts at each monitoring point on line A, the subsidence value gradually increases with the advancement of the working face. Afterwards, as the working face moves away from each point, the subsidence value gradually reaches the maximum under the geological mining conditions. en, this value no longer grows. e point that reaches the maximum subsidence value moves forward from A1 to A41 with the advancement of the working face. According to the results of the last four observations, the minimum and maximum subsidence values of points A18-A21 are 3335 mm and 3362 mm, respectively. Besides, the subsidence value is 27 mm within six months. e results indicate that the surface subsidence tends to stabilize, and the flat bottom of the moving basin appears from A1 to A21. As the working face advances, the flat bottom of the moving basin stretches forward, and the subsidence value no longer rises. Point A32 is located in the ravine whose surface is uplifted.

Dynamic Distribution Characteristics of Subsidence Curve of Line B on Inclination (Bedrock-Exposing Surface)
. Line B, a curved observation line, is arranged along the ravine (Figure 4). Point B17 is the first to be affected by mining and to experience surface subsidence (during the 7th observation) because it is the closest to the open-off cut. e maximum subsidence (the maximum subsidence value 2115 mm and the average subsidence speed 45 mm/d) is observed at point B15 on line B during the 8th and 9th observations, which last 47 d. e results of the last four observations which last 153 d suggest that the maximum subsidence difference is 12 mm, and the surface subsidence basically tends to stabilize, with the final maximum subsidence value being 3167 mm.

Dynamic Distribution Characteristics of Subsidence Curve of Line C on Inclination.
e maximum subsidence (the maximum subsidence value 2363 mm and the average subsidence speed 118.2 mm/d) is observed at point C16 on line C during the fifth and sixth observations, which last 20 d ( Figure 5). During the eighth and ninth observations which last 47 d, the maximum subsidence value is consistent, and the surface subsidence basically tends to stabilize, with the final maximum subsidence value being 3419 mm. e dynamic distribution characteristics of tilt curve are shown in Figures 6-8. It can be seen from the curve that the surface inclination value changes from small to large with the advancement of the working face. During the 21st observation, the maximum value of inclination is −41.7 mm/m (point A29). From the 19th observation to the 21st observation, it can be seen that the moving deformation curve shows the antisymmetric characteristics of the inclined curve from the overall trend. e final distribution is different from that of flat land, which is mainly related to topographic factors. e maximum dynamic tilt deformation value in the strike direction is −41.7 mm/m.
From the 13th observation to the 19th observation, the inclination change of inclination B observation line was small, and the regularity of observation results was not strong. Several observations after the 21st observation showed that the inclination reached the maximum value, and the maximum dynamic inclination deformation value of inclination direction was 55.6 mm/m obtained from the 17th observation. e tilt of observation line C changed little from the 13th observation to the 17th observation, and the regularity of observation results was not strong. Several observation results after the 21st observation showed that the surface movement deformation tended to be stable and reached the maximum tilt value. e maximum dynamic tilt deformation value in the trend direction was 48.6 mm/m obtained from the 25th observation. e dynamic distribution characteristics of curvature curve are shown in Figure 9. With the advancement of the working face, the surface curvature value gradually increases, and the negative curvature has double peak and multipeak phenomena. e distribution law of each observation is basically the same and only increases in value, but the positive and negative changes of the curvature value in the center of the basin have a large jump. It can be seen from the later observation curves that the curve shape is consistent and the value is basically stable. e maximum dynamic positive curvature deformation value in the strike direction is +1.23 mm/m 2 and the maximum dynamic negative curvature deformation is −2.70 mm/m 2 .

Particularity of Law of Surface Movement in Hilly
Areas. e mining-induced surface movement in hilly areas can be classified into two forms. One is continuous movement deformation, that is, mining-induced slides in hilly areas, and the other is discontinuous movement deformation, that is, mining-induced fractures, landslides, and collapses in hilly areas. Compared with mining in the plain, mining in hilly areas exhibits different regularities which are displayed in the following aspects.

Characteristics of Surface Movement and Deformation in Hilly
Areas. Because of the slopes in hilly areas, the rock slide or landslide caused by underground mining will result in an increase in horizontal movement towards the downhill direction. Meanwhile, due to the occurrence of mininginduced slide, the upper part of the slope slides downhill under the action of tensile stress or push stress, thus promoting horizontal movement and subsidence towards the downhill direction. Affected by mountain sliding, deformation and extrusion occur, leading to the surface uplift at the ravine. Damaged fractures caused by tensile stress in the upper part of the slope are depicted in Figure 12, from which  Advances in Materials Science and Engineering it can be seen that the whole slope moves downhill. Extrusion-induced surface uplift at the ravine is shown in Figure 13.

Range of Surface Movement in Hilly
Areas. Due to the influence of mountain topography, the horizontal movement, the horizontal deformation, and the surface uplift, occurring when the movement boundary is close to the plain or ravine, expand the range of movements in the uphill and downhill directions, thus reducing the boundary angle and movement angle of movement deformation. As can be known from the comprehensive histogram of the coal seam in the mine, the overlying stratum belongs to medium-hard rock. According to Table 1 in the literature [1] and the actual condition (dip angle 5.0°) of P2 working face, the boundary angle on strike, dip boundary angle, rise boundary angle, movement angle on strike, dip movement angle, and rise movement angle of P2 working face can be acquired (Tables 1 and 2). Compared with those of the plain, the measured boundary angle and movement angle of P2 working face are reduced to varying degrees. Specifically, compared with the theoretical values, the boundary angle on strike is reduced by 5°-10°, and the rise boundary angle is reduced by 2°-7°; the movement angle on strike is reduced by 8°-13°, the rise movement angle is reduced by 0°-10°, and the dip movement angle is reduced by 2°-16°, as listed in Tables 1  and 2.   In light of the literature [28], the surface is fully mined when the length or width of the gob area reaches or exceeds H 0 , namely, the average mining depth (ratio 1.

Advances in Materials Science and Engineering
P2 working face with thick bedrock can achieve full mining more easily than those of general geological conditions. It can achieve critical full mining at the ratio of mining width to mining depth of 1.07.

Development and Morphology of Fractures.
According to the field fracture investigation in the whole mining process of P2 working face, many large crisscross fractures appear on the surface, and these fractures exhibit obvious regularity in the strike direction. On the whole, they can be divided into two types. e first type is network fractures generated in bedrock-exposing areas (mainly occurring in the vicinity of A1-A6, A17-A19, and A28-A32 of line A, the whole line B, and C7-C17 of line C), and the other type is large fractures in loess-covering areas (mainly occurring near A7-A11 of line A and C1-C6 and C8-C36 of line C). As shown in Figure 14, the widths of bedrock surface fractures mostly lie in the range of 20-200 mm, with the maximum width being up to 300 mm. Depths of the fractures differ, with the maximum measured depth being about 10-20 m. Generally, the directions of the fractures are related to the mountain topography; most of the fractures are perpendicular to the downhill direction of the slope and develop approximately along the contour, forming a crisscross fracture network.
As shown in Figure 15, the slope, there also occurs an overall slump towards the downhill direction. For example, an overall slump occurs at point A11 of line A, where the root system of vegetation is exposed ( Figure 12). Loess-covering surface fractures at nonslope positions often collapse to fill in the fractures ( Figure 15).  Hence, maximum surface stabilization time is deduced to be 133 d. As shown in Figure 5, point C16 in line C does not begin to subside at the second observation and basically stabilizes at the sixth observation. e stage lasts 108 d. erefore, the maximum surface stabilization time is inferred to be 108 d.

Surface Movement Duration and
With reference to literature [66], the duration of surface movement can be determined based on the measured data in this mining area. When there is no measured data, the duration (T) of surface movement can be calculated according to the following formula: It is worked out that 208 * 2.5 � 520 d, 200 * 2.5 � 500 d, and 206 * 2.5 � 515 d are required for lines A, B, and C. As revealed by the previous analysis, the surface subsidence stabilization times of lines A and C are closer to the actual values than that of line B. eir average subsidence stabilization time and theoretically calculated subsidence stabilization time are (108 + 103) ÷ 2 � 105.5 d and (520 + 515) ÷ 2 � 517.5 d, respectively. Obviously, the actual subsidence stabilization time is much shorter than and only 105.5 ÷ 517.5 � 20% of the theoretically calculated value. is result manifests that the fully mechanized caving mining working face with thick bedrock can achieve surface stabilization more easily. In addition, the time required for subsidence stabilization of bedrock-exposing surface is short, with the required time ratio being 108/103 � 1.05. e subsidence stabilization times of thick bedrock-exposing surface and thick bedrock surface with thin epipedon are basically the same.

Maximum Surface Subsidence Speed.
Surface subsidence speed refers to the subsidence amount per unit time caused by underground mining. Each point (A1-A21) on the observation line on strike of the surface movement observation station has reached the maximum subsidence, so the calculation of the surface subsidence speed can be figured out by using multiperiod observation results at any point. e observation values and calculation results of each period are disclosed in Table 3.
As given in Table 3, the subsidence difference from the fifth observation to the sixth observation is ∆W � 2363 mm, and the observation interval is t � 20 d. e maximum surface subsidence speed can be calculated according to the following formula: In fact, except for the maximum subsidence speed 118.20 mm/d, the subsidence speeds at other times are  is indicates that considerable surface subsidence occurs in a short period of time, and overburden subsidence is sudden and violent. Given the characteristics in hilly areas, when serious discontinuous damage appears on the surface, the possibility of sudden discontinuous movement and deformation such as large fractures, landslides, and collapses will increase ( Figure 12).

Conclusions
(1) e upper part of the slope in hilly areas slides towards the downhill direction under the action of tensile stress or push stress. As a result, the ranges of horizontal movement and subsidence towards the downhill direction both increase. Affected by the sliding in the ravine of the mountain, the ground may be extruded and deformed, leading to the surface uplift at the ravine, an increase in the range of ground surface movement, and decreases in the movement angle and the boundary angle. In addition, the surface movement duration of HIFMCMWFSTBTE in hilly areas is relatively short. Considerable subsidence will occur in the active stage, and the surface subsidence is sudden and violent. e measured surface stabilization time of P2 working face is only 20% of the calculated value in the Specification. (2) HIFMCMWFSTBTE can achieve full mining more easily than those of other geological conditions. Its critical value of full mining is smaller than the ratio (1.2-1.4) of mining width to mining depth in traditional experience. According to the field measurement, critical full mining can be achieved in P2 working face when the ratio of mining width to mining depth is 1.07. (3) HIFMCMWFSTBTE is prone to serious sudden discontinuous and strongly regular damage. Fractures on the gully region surface develop along the contour, forming a crisscross fracture network. Fractures on the horizontal surface are approximately parallel to the working face. Fractures on the

Data Availability
Some or all data, models, or codes that support the findings of this study are available from the corresponding author upon reasonable request.

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