Study on Mechanical Evolution Characteristics of Overburden Rock in “Knife Handle”-Type Fully Mechanized Top-Coal Caving Face

College of Safety Science and Engineering, Liaoning Technical University, Fuxin, Liaoning 123000, China Key Laboratory of Mineermodynamic Disasters and Control of Ministry of Education, Liaoning Technical University, Fuxin, Liaoning 123000, China Information Institute of the Ministry of Emergency Management of PRC, Beijing 100000, China Shenhua Shendong Coal Group Corporation Limited, Ordos, Inner Mongolia 017000, China Hanjiawa Coal Industry Co., Ltd, Datong, Shanxi 037000, China


Introduction
Since the conception of the wall-type coal mining system, a regular arrangement has been followed for the working face, which has a constant tendency length, to ensure the safe and e cient mining of the working face. Since the beginning of the twenty-rst century, the increasing demand for the coal has led to an increased intensity and depth of coal mining, and "knife handle"-type mining sites have emerged, such as the Shigejie coal mine in Lu'an, Xutuan coal mine in Huaibei, and Shendong mine in Inner Mongolia [1].
Scholars in China and abroad have conducted several studies from di erent perspectives by adopting di erent methods and theories on the movement law of the overburden rock for mining regular working faces with constant tendency lengths. e masonry beam theory developed by Academician Qian et al. [2] states that with the advancement of the working face, the masonry beam structure formed after the breakage of the overburden rock is a determinate structure. Furthermore, the magnitude of the horizontal thrust required to form the masonry beam structure is directly proportional to the breakage length and load of the overburden, inversely proportional to the layer thickness and sinking amount, and independent of the stress distribution in the mining area. Zhang [3] analyzed the overburden damage law under the couple effects of stress field and seepage field at the mining site of a shallow coal seam covered with a thick loose layer and rich groundwater using RFPA2D-Flow numerical simulation software. e results indicated that when the mine roof approaches collapse and destabilization under mining stress and self-weight, the water pressure of the seepage field notably influences the overburden damage. When the overburden breaks, it interacts with the seepage field, leading to abrupt changes of seepage coefficient, groundwater flow, and groundwater head of the seepage field. Zhang et al. [4] used similar simulation experiments in conjunction with the borelaneway resistivity method to accurately obtain the height of the overburden collapse zone and the hydraulic conductivity fracture zone during the mining process. Wang [5] used FLAC 3D numerical simulation software to analyze the relationship between stress evolution and advancement distance of the regular working face and "positive-handle" and "negative-handle" fully mechanized mining faces. e results indicated that mining stress increases sharply when the working face is advanced to an integer multiple of the inclination length of the working face, and the accuracy of the conclusions is verified through engineering examples. Liu et al. [6] took the close-range coal seam as the research object, combined with the geological conditions of the Xieqiao coal mine, calculated the failure depth of the floor when the upper coal seam was mined, and provided the corresponding surrounding rock deformation control measures for the safe mining of the lower coal seam. Li et al. [7] analyzed the ground stress distribution under the regenerated roof geological conditions by means of numerical simulation and put forward the technical measures to strengthen the monitoring of surrounding rock activities and timely pressure relief during the mining process, which ensure the safe mining of the working face.
Compared with studies on regular mining sites with constant inclination length, fewer studies have investigated the overburden damage and stress evolution in irregular mining sites with abrupt changes in inclination length, especially in the fully mechanized top-coal caving face. is study investigated the evolution characteristics of the overburden mechanics of the knife handled fully mechanized top-coal caving field by combining the geological conditions of the top and bottom of coal seam 22# in Hanjiawa mine. FLAC3D software was used to numerically simulate the abutment pressure, horizontal stress, and vertical displacement of the "knife handled" fully mechanized top-coal caving field.

General Situation of the Mine
e Hanjiawa 22401 fully mechanized top-coal caving face is the first mining face in the Western Region; the total coal-sea thickness is 9.2-14.8 m, with an average thickness of 11.6 m. e coal seam inclination angle is 4°-11°, the average inclination angle is 6°, which is near horizontal coal seam. e ground elevation of the working face is 1550-1580 m, the underground elevation is 1263-1290 m, and the coal seam burial depth is about 200 m. e absolute gas gush from recovery is 0.82 m /min and the gas grade is low. e strike length of the working face is 833 m. When mined toward the middle of the working face, the inclination length of the working face is abruptly reduced from 170 m to 120 m; at the working face inclination of 170 m, the strike length is 528 m, and at the working face inclination of 120 m, the strike length is 305 m. e single long-arm backward low top-coal caving mining method is adopted in the working face. e coal mining height is 3 m, the coal caving height is 8.6 m, and the mining and caving ratio is 1 : 2.87. e direction of mining advance is from west to east along the floor of the coal seam, and the fully mechanized caving method is used to manage the goaf. e layout of the working face is illustrated in Figure 1

Numerical Simulation
3.1. Modeling. FLAC 3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) is a numerical calculation program developed by ITASCA, USA, and is based on the 3D explicit finite difference method. e program has an internal nested null unit model and 18 intrinsic models, including three elastic models and 15 plastic models, and can be used to satisfactorily simulate the 3D mechanical properties of materials. FLAC 3D is extensively applied in the mining field, especially for the simulation of coal recovery and collapse processes in fully mechanized top-coal caving [8].
FLAC 3D numerical simulation software was used to establish a model by combining the top and bottom rock layer column diagrams of the 22401 fully mechanized topcoal caving face with the corresponding physical and mechanical parameters of the coal rock body; and the model rock layer distribution visualization is illustrated in Figure 2. e model size is designed to be 800 m × 400 m × 100 m, with 297882 zones and 303211 grid points. e grids and nodes of the model are divided densely to reflect the stress distribution in the working face accurately. To eliminate the influence of the boundary conditions, the model is designed to be large enough, and 110 m of protective coal pillars are constructed at each end in the direction of the working face. In the direction of the inclination of the working face, 110 and 135 m of protective coal pillars are constructed on each side of the working face when the working face is large and small, respectively, with an inclination length of 170 and 120 m, respectively. e four sides of the model, front, back, left, and right, are selected as displacement constraint boundaries; that is, no displacement occurs in either the horizontal or vertical direction. e top and bottom of the model are selected as free boundaries, and their horizontal and vertical displacements are not constrained. A uniform load of 5 MPa is applied to the top of the model to simulate the weight generated by the overlying rock layer at burial depth of 200 m [9].

Parameter Design and Failure Criterion.
is simulation uses Fish Language to control the model for simulating the mining activities of the roadway and working face. e Mohr-Coulomb intrinsic model in FLAC 3D is selected for simulating the deformation and damage of the coal rock body, and its mechanical expression as follows: where σ 3 is the minimum principal stress, σ 1 is the maximum principal stress, c is the cohesive force of the material (MPa), and φ is the internal friction angle of the material (°). e physical and mechanical parameters of the coal rock body at the 22401 working face are displayed in Table 1.

Analysis of Simulation Results
As the working face advances, the elastic-plastic energy accumulated in the overlying rock layer begins to release, and the stress induces redistribution and thus a new equilibrium state. Under the influence of mining action, the overburden stress approaches the mining goaf, thus forming a new abutment pressure zone and horizontal stress zone around the mining goaf and a mining stress field with overburden stress formed in front of the working face. e roof and floor are influenced by the double action of selfweight and mining stress field in each rock layer between the vertical and horizontal direction of movement, and a displacement field is formed according to the law of stress evolution [10].

Evolution Law of Abutment Pressure.
In the mining process, the evolution law of abutment pressure in the fully mechanized top-coal caving face with variable face length can be obtained according to the distribution of vertical stress in the roof at different propelling distances. As illustrated in Figure 3

Mathematical Problems in Engineering
Within the propelling range of 0-280 m, the abutment pressure increases with the increase in the propelling distance and is distributed symmetrically along the axis of the working face. e abutment pressure in the rock mass of the two grooves is notably higher than that in the coal seam roof. e maximum abutment pressure is distributed in the rock mass within the range of 0.75-1.25 m from the coal walls of the two grooves, and the minimum abutment pressure is distributed near the interface of the two roadways and the roof and floor.
While the working face is propelled to the vicinity of the knife handle, the abutment pressure increases abruptly, the static pressure transforms into dynamic pressure [11,12], the roof moves intensely, and the pressure is abnormal. erefore, roadway maintenance becomes difficult. Furthermore, large separation cracks appear between some layers of the overlying strata, thereby causing interlayer dislocation. Compared with the roof stress on the air roadway side, the roof stress on the transport roadway side is more concentrated and exhibits an irregular distribution.
When the working face gradually transitions into the small face, the abutment pressure considerably decreases, the numerical fluctuation is small, and the rock strata activity is relatively mild. e stress is symmetrically distributed along the central axis of the working face, and the stress distribution of the roof is approximately arched. e stress in the rock mass on the air roadway side is slightly larger than that on the transport roadway side.

Characteristics of Horizontal Stress Distribution.
e evolution law of the horizontal stress of the knife handle comprehensive mining site at the advancing distance can be obtained according to the horizontal stress distribution of the surrounding rock during the mining process. As illustrated in Figure 4,  Within the advancing range from 0 to 200 m, the horizontal stress is symmetrically distributed along the central axis of the coal mining face, and the maximum horizontal stress is distributed in the rock body from 1.13 to 1.51 m away from the coal wall of the two roadways in the shape of a tadpole. e maximum horizontal stress is distributed on the side near the coal wall of the roadway. e stress concentration in the roof plate is relatively low, and the stress concentration gradually increases along the roof plate from the bottom to the top in an arch shape.
Advancing is continued until the working face passes through the knife handle. e horizontal stress is still symmetrically distributed along the central axis of the working face, but the location of the maximum horizontal stress concentration shifts from the interior of the rock bodies of the two roadways to the roof. e stress concentration gradually decreases along the roof from bottom to top, and the shape of the horizontal stress contour gradually changes from an arch to a flat circle. Furthermore, the stress concentration gradually decreases from the interior to the exterior of the circle.
When the working face is penetrated at the knife handle until the end of the recovery process, the degree of stress concentration does not decrease significantly, and the stress distribution state is notably asymmetric. e horizontal stress concentration in the roof on the air roadway side is considerably larger than that on the transport side. However, as the working face continues to advance, the asymmetry of the stress distribution decreases, the stress concentration also decreases, and the workface gradually enters a stable stage.

Variation Law of Overburden Displacement Field.
Under the double action of mining stress and self-weight of the overburden rock, the layers of the roof and floor undergo relative misalignment and vertical movement, leading to the sinking of the roof and bulging of the floor [13]. Figure 5 illustrates the sinking intensity of the roof and the bulging intensity of the bottom slab when the working face advances to the vicinity of the knife handle. When the working face     Compared with the regular working face with constant tendency length, the working face in the vicinity of the knife handle causes the sinking intensity of the roof to increase significantly and causes the roof to move violently; then, the roof is destabilized and deformed, requires joint support, or weak pressure for reduced stress concentration and safe mining. Furthermore, the floor drum displacement does not fluctuate and is rather stable. e contour map of the top and bottom plate displacement is basin-shaped, with the displacement distributed symmetrically along the central axis of the coal mining face. e displacement of the top and bottom plate gradually decreases from the middle to either side.

Conclusion
is study investigated the distribution of abutment pressure, horizontal stress, and the displacement of the roof and floor in the mining process in the 22401 "knife-handle" fully mechanized top-coal caving face of the Hanjiawa mine. FLAC 3D numerical simulation software was used for simulation and analysis. e conclusions drawn based on the study results are as follows: (1) During the mining process, when the working face is far away from the knife handle, the abutment pressure increases with the increase in mining advance distance; within the range 20-30 m before and after the knife handle, the abutment pressure and horizontal stress both increase abnormally, and the stress distribution is notably asymmetric. e maximum abutment pressure and horizontal stress are 18.41 and 16.45 MPa, respectively, and the stress concentration is sufficiently high. When the working face leaves the knife handle and smoothly transitions to a small face, the abutment pressure and the fluctuation in the values decrease, and the mining abutment pressure at the small face is slightly larger than at the large. A certain correlation is noted between the evolution characteristics of abutment pressure and horizontal stress with the mining advance in the working face; both abutment pressure and horizontal stress influence the transport activities in the overlying rock layer.
(2) e evolution trend of the horizontal stress and supporting stress is different. e advancing distance when the horizontal stress increases abnormally is considerably smaller than that when the abutment pressure increases abnormally. us, during the construction process, the problem of excessive concentration of horizontal stress should be addressed through prior intervention.
(3) e roof sinks and the floor bulges because of the mining stress and self-weight. When the working face advances to the vicinity of the knife handle, the activity in the top plate is intense and the sinking intensity increases notably. e floor is more stable, with small fluctuations in the values. e contour map of the roof and bottom displacement is basin-  shaped, with the displacement distributed symmetrically along the central axis of the coal mining face. e displacement of the top and bottom plate gradually decreases from the middle to either side. e maximum sinkage of the roof is 268.9 mm and the maximum bottom drum displacement is 10.01 mm.
Data Availability e data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest
e authors declared no potential conflicts of interest with respect to the research, author-ship, and/or publication of this article.