Dynamic Changes in Surface Damage Induced by High-Intensity Mining of Shallow, Thick Coal Seams in Gully Areas

1is study proposes a novel approach to study the mechanism of mining and dynamic changes in surface subsidence and geological hazard-prone regions caused by shallow, thick coal seam mining in gully areas. 1is approach combines field observation, threedimensional modeling, numerical simulation, and theoretical analysis based on the conditions of the Chuancao Gedan coal mine. 1e in situ stress field of coalbeds is influenced by the gully terrain. Shear stress becomes concentrated on the surface, causing geological disasters such as landslides and collapse of gully slopes. High-intensity mining activities increase the concentration and are more likely to cause such geological disasters. 1e influence area and severity vary dynamically with the expansion of the excavation area. With the continuous expansion of coal seam mining, the amplification ratio η (the ratio of the maximum impact range of surface subsidence and the mined-out area) first increased to 3.35, then decreased, and finally reached a constant value of 2.1. 1e principle of road line selection is proposed based on an analysis of surface subsidence and gully slope stability on the goaf edge. 1e principle of subsection reinforcement of the gully slope under the dynamic influence of coal seam mining is also determined.


Introduction
As Western China has become the focus of coal mining activity, the mining scale of shallow, thick coal seams has gradually increased [1]. Intensive mining activities in these areas have increased the overburden motion range and exaggerated surface damage, such as surface collapse, landslide, and soil erosion, resulting in serious damage to the fragile ecological environment [2,3]. A large number of studies have been conducted to analyze the safe and sustainable mining of shallow coal seams. Qian and Huang established a mechanical structure model of the main roof in shallow coal seam mining, providing a basic method to support the design of shallow coal seam working faces [4][5][6]. Xu et al. classified key strata structures and analyzed surface damage caused by shallow coal seam mining [7][8][9]. Zhang et al. proposed a preliminary system for water conservation mining in shallow seams. is system was defined in terms of the structure, motion, and fissure distribution of the key strata and aquifuge [10][11][12]. Yu et al. studied the formation mechanism of mining-induced fissures in shallow coal seam mines and proposed preliminary control techniques [13,14]. In addition, other scholars in this field have carried out research on strata movement, mining subsidence, ecological restoration, etc. [8,[15][16][17][18][19][20]. e characteristics of mines in Western China include large coal seam thicknesses, shallow occurrences, and complicated topography. Dynamic strata pressure disasters and geological disasters are frequently observed in gully areas (see Table 1) [21][22][23]. To investigate this problem, Lai et al. analyzed the surrounding rock stress distribution and the strata behaviors of shallow, thick coal seam mining in gully areas [24,25]. Wang et al. studied the overlying strata stability and the support parameter determination of shallow coal seam mining in gully areas [26][27][28]. Liu et al. studied mining-induced fissures in shallow, thick coal seam mining [29,30]. Research on the surface subsidence caused by mining activity under similar geological and geographical conditions has also attracted the attention of scholars [31][32][33]. Related research mainly focuses on the optimization of mining subsidence models and the field measurement of surface subsidence data [34,35].
Based on the analysis of relevant literature, research on the overburden movement and on the mining-induced damage is based on the two-dimensional space of the stope advancing direction. Existing studies generally use two-dimensional plane similarity simulation and numerical calculation modeling to simplify the surface gully terrain.
ere are also few studies on the evaluation of surface geological hazards. e mining of shallow, thick coal seams under gully terrain in Western China is constantly expanding. e mining subsidence and surface damage in this gully area are dynamic and changing. Understanding the mechanisms involved in gully slope landslides, subsidence, and other geological disasters under the conditions in this large mining area is critical for predicting surface damage and developing methods for controlling these disasters [36][37][38]. Compared with other coal seam mining areas, the bedrock in the gully area is thinner and the cementation is poor. e distribution of mining stress on the overburden is affected by both the coal seam mining and the surface gully terrain. However, there are not effective methods for the quantitative expression and evaluation of the distribution and evolution of surface mining stress field and displacement field. In addition, shallow coal seam mining in gully areas is an activity in three-dimensional space involving the surface and subsurface. erefore, combining the research methods of three-dimensional modeling and theoretical analysis is an appropriate approach. Related research has important theoretical and practical value.
In this study, field measurements, numerical simulations, and theoretical analyses were implemented to investigate the in situ stress field and displacement field caused by mining processes under the geographical conditions of gully terrains.
is study serves as a reference for the evaluation of surface geological hazard-prone areas and can help guide reasonable road line selection under the dynamic influences of similar mines.

Engineering Background
e Chuancao Gedan mine is located on the Loess Plateau and is noted for its complicated topography, which includes large surface variations. e mining field is covered by unconsolidated sediments, resulting in almost complete coverage of old layers formed before the Cenozoic Erathem and the presence of bedrock only in large gullies. e Chuancao Gedan mine exploits Seam no. 6. Seam no. 6 has an average thickness of 12.8 m, an average inclination of 5°, and an average burial depth of 116.6 m. e mining sequence of Seam no. 6 is as follows: working face 6206 ⟶ working face 6205 ⟶ working face 6204 ⟶ working face 6203 ⟶ working face 6202 ⟶ working face 6201 (see Figure 1).
High-intensity mining activities lead to overburden movements and severe surface damage. During the mining period of working faces 6206 and 6205, geological disasters such as landslides and subsidence occurred in the gully area (Figures 2(a) and 2(b)), resulting in the destruction of surface buildings and roads (Figure 2(c)). Due to serious damage to road A in the mining area, the line of surface roads in the mining area needs to be reconfigured and the roads need to be reconstructed. Road B has been rebuilt on a hillside approximately 200 m from the boundary of the mined-out area (Figure 2(d)). Consideration of the influence range of surface subsidence is important for the reinforcement of gully slopes near road B.
For the core at point O (Figure 2(d)), the physical and mechanical parameters of each stratum are shown in Figure 3.

Three-Dimensional Modeling
To establish a three-dimensional analysis model, Fast Lagrangian Analysis of Continua (FLAC) was used as the simulation software. In this paper, the three-dimensional version was used to simulate all processes [39,40].
e Mohr-Coulomb model is commonly used to describe shear damage to soil and rocks. Considering that coal seam mining is a typical nonlinear deformation problem, the Mohr-Coulomb model was used in the calculation. is model uses the following yield criteria to determine the destruction of the rock mass: In this formula, σ 1 and σ 3 are the maximum principal stress and the minimum principal stress, respectively, and c and φ are the cohesive force and internal friction angle, respectively. When f > 0, the material undergoes shear destruction. When f t ≥ 0, the material undergoes tensile destruction.

Establishment of an Analytical
Model. Taking Region A as an example, the three-dimensional modeling of shallow, thick coal seams in gully areas included steps such as the calculation and processing of surface contour data, the modeling of FLAC3D surface topography, the fitting of layered coal strata, and the division of mining areas. e flowchart of the model generation is shown in Figure 4.
First, three-dimensional coordinate data of the control point on the contour of the surface topography in Region A were derived. Second, the kriging interpolation method was used to organize the exported data into grid node coordinates corresponding to only one elevation form. e processed threedimensional topographic map is shown in Figure 5. e map size is 1360 m × 1050 m (X × Y), and the map is divided into 137 rows and 106 columns with widths of 10 m, with a total of 14,522 surface elevation data points. ird, the elevation data of each point were saved as DAT files, which contain the threedimensional coordinates of all grid nodes. e data storage form is {x i , y j , z ij }, where i is the number of rows, j is the number of columns, and z ij is the corresponding point elevation. Finally, the FLAC3D numerical simulation software was used to generate the three-dimensional model of Region A by sequentially reading out each data point in the DAT file and creating tiny hexahedral elements.
e three-dimensional model generated by the hexahedron element above an elevation of 883 m is shown in Figure 6.
Based on the data acquisition and processing of surface contour data, surface geomorphology modeling via FLAC3D, and formation and modeling of the underlying strata, a three-dimensional calculation model for the study area was established, as shown in Figure 7 [41]. e model size is 1360 m × 1050 m×292 m (X × Y × Z); the horizontal grid size X × Y of the model is 10 m × 10 m. e vertical dimension of the model is divided into 14 layers, and the vertical grid size Z is divided according to the thickness of each layer. e model has 614,040 zones and 551,836 grid points. e dimension parameters of the mining face are shown in Table 2.
According to the actual conditions, the boundary constraints adopted in this model are as follows:  (1) e side of the model is a rolling branch, which is used to limit the horizontal movement.
(2) e lower boundary of the model is defined as a fullconstraint boundary, which is used to limit horizontal and vertical displacements.   Advances in Civil Engineering distribution of mining stress.
e mining coal seam is shallow (depth less than 200 m), the mining thickness of the coal seam is large at one time (mining thickness is 12.8 m), and the overlying strata are mostly weakly cemented strata.
us, the empty unit is used to simulate the goaf state in the numerical simulation. is approach can better reflect the actual conditions of coal seam mining.
In the simulation experiment using the Mohr-Coulomb model, the initial stress field needed to be calculated. Because the numerical simulation study area is shallow and there is no geological structure, the original rock stress is mainly caused by the self-weight of the rock mass. Considering the overall in situ stress distribution in the study area and the effect of the surface gullies, the horizontal stress is 1.1 times the vertical stress [25].
According to the measured results, the basic expression of the stress of the original rock in the three directions of the mining area is as follows: In this equation, ρ is the average rock mass of 2,550 kg/ m 3 , g � 10 N/kg, and h is the depth of burial (the elevation difference from the surface level to a certain point).

Material Properties.
e numerical calculation was performed using the Mohr-Coulomb failure criterion. e material parameters of the Mohr-Coulomb model included cohesion, internal friction angle, bulk modulus, Poisson's ratio, bulk modulus compressive strength, and tensile strength.
e physical and mechanical parameters of the rock formations are shown in Figure 3 and Table 3.
An initial equilibrium state was achieved after model establishment, and the vertical stress distribution of the seam and layers under the influence of the gully terrain is shown in Figure 7(c). e effect of the gully terrain on the in situ stress field is significant. e degree of the effect decreases with increasing depth of the gully. e vertical stress distribution in Seam no. 6 is shown in Figure 7(d). e vertical stress contour in the coal seam is similar to that in the surface contour. us, the gully terrain has an impact on the mining of Seam #6.

Geological Hazard-Prone Area in the
Mining Area e mining area is covered by thick, sticky loess with poor mechanical properties. e coal seam overburden mainly consists of sandstone and sandy mudstone with low mechanical strength. e geological conditions in the mining area are simple and have no obvious geological structure. Because of the large size of the excavation area and the lower depth of the coal seam, the main influencing factor of mining pressure is vertical stress under gully influence. As a result of the mining process, collapse of the overlying strata was observed and led to the subsidence phenomena, as shown in Figure 2. Surface deformation and destruction were mainly caused by the increase in the shear strength during the mining process.
According to the Mohr-Coulomb criterion, the primary cause of slope landslide is the shear stress on the surface reaching a threshold shear strength and disrupting the equilibrium state. e shear stress τ at a certain point can be calculated by the maximum principal stress and the minimum principal stress at this point: In the formula, σ 1 is the maximum principal stress at a given point, MPa; σ 3 is the minimum principal stress at a given point, MPa; and β is the angle of the shear stress τ acting face and maximum principle stress σ 1 acting face,°.

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When β � 45°, according to formula (1), the maximum shear stress is e shear stress distribution of the surface gully slope is shown in Figure 8. e maximum shear stress was concentrated from the slope top to the gully bottom. e stress concentration increased with increasing slope angle, indicating a threat for landslides. e probability increases with increasing slope angle. Landslide, collapse, and other geological hazards occur on the acting face of maximum shear stress because of the disruption of the stress equilibrium state during mining activities. To avoid the occurrence of surface geological disasters, the variation in the stress field and displacement field should be given greater attention.

Effect of Mining Activity on the Surface Stress and Displacement Field Changes
Seam no. 6 working face in the three-dimensional model (Figure 7(a)) was successively mined. e mining sequence of Seam no. 6 was as follows: working face 6206 ⟶ working face 6205 ⟶ working face 6204 ⟶ working face 6203 ⟶ working face 6202 ⟶ working face 6201. Working face mining is a dynamic process. e mining speed not only affects the instability movement characteristics of overlying rock, the appearance of working face mineral pressure, and the bearing characteristics of the support, but also dynamically influences the surface subsidence and destruction [42]. Considering the actual propulsion speed of the working face (Table 2), each working face was designed for multiple circular excavations in the numerical calculation, where each excavation cycle was 1.5 m and the cycle time step was 2000 steps. e distributions of the stress field,  After the mining of working face 6203, the shear stress concentration increased, and the maximum value reached 1.4 MPa (Figure 10(b)). When the overlying strata became steady, the plastic failure zone was distributed in an inverted "C" shape. e plastic failure zone was distributed on the side of the unexploited seam (Figure 10(c)). Plastic damage in the gully slope at the edge of the mined-out area affected the stability of surface buildings and roads. When the mined-out area became approximately square, the ground displacement contour was expanded from an "O" shape to an approximate square (Figure 10(d)).
After the mining of working face 6201, the surface shear stress concentration area (Figure 11(b)), plastic failure area (Figure 11(c)), and surface displacement contour (Figure 11(d)) reached maximum values. e maximum shear stress increased to 1.5 MPa, the maximum surface subsidence was 10.0 m, and the maximum values appeared near one side of working face 6201. e surface displacement values of working faces 6206 and 6205 did not increase with increasing mining area, indicating that the surface subsidence in this area tends to be stable.
A comprehensive analysis of Figures 9-11 shows that under the influence of mining stress, the overlying rock layer in the goaf area produces plastic damage, causing overlying rock instability movement and dynamic changes in the surface stress field and displacement field. In particular, the tension stress distribution area appears around the surface of the mining area, and surface deformation and destruction occur under tensional stress. Shear stress concentration occurs on the surface above the goaf area, and step subsidence occurs on the surface.

Influence of Expanded Mining Range on the Dynamic Changes in Surface Damage
With the sequential mining of each working face, the minedout area of Seam no. 6 constantly expanded. To analyze the dynamic influence of the expanding area on surface subsidence, the displacement curves of the slice (Y � 495 m) along the mining sequence direction and the slices (X � 310 m, X � 490 m, X � 660 m, and X � 850 m) along the advancing direction were examined. e dynamic changes in surface displacement are shown in Figures 12 and 13. Figure 12 shows that the surface subsidence area and the maximum subsidence in the X-direction increased with the expansion of the mined-out area. e maximum subsidence value appeared in the mining of the working face. When the overlying strata movement became steady, the surface subsidence tended to be constant. e surface subsidence of 0-130 m in the X-direction tended to be stable during the exploitation of working face 6205. e surface subsidence of     0-240 m in the X-direction tended to be stable during the exploitation of working face 6204. e surface subsidence of 0-310 m in the X-direction tended to be stable during the exploitation of working face 6203. e surface subsidence of 0-360 m in the X-direction tended to be stable during the exploitation of working face 6202. e surface subsidence of 0-380 m in the X-direction tended to be stable during the exploitation of working face 6201. Figure 13 shows that the surface subsidence area in the Y-direction was basically unchanged after the mining of each working surface. e surface subsidence value gradually increased from the edge of the mined-out area to the middle. e maximum subsidence value generally appeared in the middle of the mined-out area. Due to the compaction of caving gangue, the surface subsidence of the previous working face was unchanged when the subsequent working face was excavated. e surface subsidence area was larger than the goaf range [43][44][45]. With the continuous expansion of the coal mining area, the dynamic changes of surface subsidence are of great importance to the prediction of mining damage and the evaluation of surface geological hazards. To analyze the dynamic changes in the surface subsidence influence area with constant expansion of the mined-out area, assuming that the ratio of the maximum impact range of surface subsidence (S max ) and the mined-out area (s max ) is the amplification ratio η of the surface influence range, then In this formula, S imax is surface subsidence influence area after working face i is mined out, m 2 ; s imax is goaf area after working face i is mined out, m 2 .
e amplification ratio η can show the trend of surface movement and deformation of the adjacent working face under the influence of the goaf and the dynamic change of surface subsidence with the continuous expansion of the coal mining area. e dynamic change curve of the amplification ratio η after each working face is mined out is shown in Figure 14  surface subsidence area increased in an "O" shape after working face 6205 was mined out. e surface influence area was larger. e surface subsidence area expanded in the shape of an inverted "C" to the side of the unexploited coal seam on the basis of an "O" shape upon expansion of the mined-out area. e increasing amplitude of the surface influence area decreased as the denominator S imax in formula (3) constantly increases with the increasing mined-out area. e amplification ratio η of surface damage first increased to a maximum, then decreased, and finally remained stable with the continuous expansion of coal seam mining. e analysis results have certain reference value for surface movement and deformation prediction, land reclamation, and ground building protection in mining areas under the conditions of repeated mining in adjacent working faces.
After the mining of working face 6206, the angular parameters of surface subsidence were measured and sorted, and the angular parameters of surface stabilization are shown in Table 4. e mining area of working face 6206 is 57,750 m 2 , and the average buried depth is 200 m e average amplification ratio η of working face 6206 is 3.2. e field measurement results are in good agreement with the numerical simulation results.

Surface Influence in Geological Disaster-Prone Areas Induced by High-Intensity Mining of the Shallow, Thick Coal Seam in the Gully Area
To analyze the dynamic changes in the surface subsidence area induced by high-intensity mining, the dynamic change in surface influence area after each working face was mined out was examined, as shown in Figure 15. e final surface subsidence influence area is 0-1280 m in the X-direction and 100-900 m in the Y-direction. Road A is in the subsidence influence area and was destroyed. Damage area D was located at the edge of the mining subsidence of the working face. Tensional stress concentration occurs in the surface area, and surface deformation and destruction occur under tensile stress. e line selection of road B is located outside the influence area and was not affected by the collapse caused by seam mining.

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Because the line selection of road B is located in the gully slope (Figure 2(d)), the analysis of the stress distribution of the gully slope caused by seam mining is conducive to assessing gully slope stability and taking targeted measures to ensure that landslides, collapses, and other geological disasters do not occur. e dynamic changes in the maximum shear stress near road B after each working face is mined out are shown in Figure 16.
With the sequential mining of each working face, the maximum shear stress of the slope near surface road B increased to different degrees. e increasing range of shear stress changed dynamically with working face mining. Key monitoring and appropriate reinforcement of the slope should be adopted to ensure the stability of the slope according to the dynamic changes in the maximum shear stress near road B.
In order to control the surface subsidence and surface damage of shallow coal seam mining in gully areas, mining technologies to reduce damaging ground fissures should be employed, including mining with narrow coal pillars, limited thickness extraction, backfill mining, and rapid advancing mining [46].

Conclusions
(1) e maximum shear stress is concentrated from the slope top to the gully bottom, indicating a threat of landslides. e probability of a landslide increases with increasing slope angle. Landslide, collapse, and other geological hazards occur on the acting face of maximum shear stress because of the disruption of the stress equilibrium state during mining activities. (2) High-intensity mining activities exacerbate the concentration of maximum shear stress, which is more likely to cause landslides, collapses, and other geological disasters. (3) e amplification ratio η of surface damage first increases to a maximum, then decreases, and finally remains stable with the continuous expansion of coal seam mining. is ratio provides a scientific basis for the determination of the surface repair distance under the condition of high-intensity exploitation. (4) e line selection of road B is located outside the influence area and will not be affected by the collapse caused by seam mining. However, the gully slope near road B is affected by seam mining. Key monitoring and appropriate reinforcement of the slope should be adopted to ensure the stability of the slope near road B during mining.

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

Conflicts of Interest
e authors declare no conflicts of interest regarding the publication of this paper.