Study on the Law of Earth’s Surface Movement on Ultra-Long Working Face

High-intensity mining effect induced by long working face mining causes dynamic disasters to safe and efficient coal mining. For the purpose of in-depth study of the movement law of earth’s surface on ultra-long working face, this paperused to build a numerical model, analyzes the law of roof-to-floor strata behaviors in coal bed during mining, and studies the weakening of roof-to-floor parameters in coal bed, bearing pressure and the law of roof caving and rock strata movement. Analyzes the face length effect resulted from the difference in face lengths. It has been recognized that the sphere of influence of bearing pressure changes from small to large and then to small, and changes in distribution of arch bottom and arch height at the plastic failure zone are further intensified over advancing of the working face, moreover, the movement of rock strata is subject to the cumulative influence of the advancing direction of the working face and the face length effect. The above-mentioned research results can serve as a theoretical basis for practical engineering.


Introduction
Double-length working face mainly refers to the working face with ultra-long advance and super face length. The ultra-long advance distance of working face is generally more than 3000 m, and the length of working face is generally more than 300 m. A working face can only be called double-length working face when its above two indexes meet the above standards.
In the context where coal mining machinery and equipment have become more reliable, double-length working face has gradually become the mainstream trend of coal mining in the world. Yang et al. (2020) found that the surrounding rock stress of the withdrawal channel in the working face before mining would transfer from the stoping fall to the stander fall. Fu et al. (2019) analyzed the stoping technology of ultra-long working face in terms of rib fall of coal wall, large-scale roof instability, and uneven gas emission. Guo et al. (2018) explored the causes of deformation by analyzing the behavior law of mine pressure during gob-side entry driving. Wang et al. (2017aWang et al. ( , 2017b) studied how to realize sequential mining along the goaf with a double-lane arrangement of ultra-long driving distance. Kang et al. (2020) applied the support-modification-pressure relief coordinated control technology to reducing roadway deformation and the rupture rate of bolts and cables. Wang et al. (2020) focused on analyzing the characteristics of mining stress rotation in the ultra-long working face of kilometer-deep well, and proposed the principle of applying mining-induced stress rotation in surrounding rock control. Based on the law of initial mining pressure in working faces of different length,  studied the behavior law of mine pressure and the law of overlying strata movement. In the final mining pass-through stage of fully mechanized coal mining face, Yao et al. (2017) found that the application of lateral large-aperture water injection fracturing roof pressure relief technology can significantly reduce the probability of jammed support accidents caused by dynamic load ground pressure. Fan et al. (2017) analyzed the structural characteristics of overlying rock caving in deep-seated ultra-long island working face. Based on the study of roof strata rupture regulation of ultra-long well working face, Wang et al. (2019) built a theoretical analysis model and determined the roof break criterion. Ding et al. (2021) by means of various methods, analyzed the rule of changes in the working resistance of hydraulic supports, the periodic weighting on working face and lead abutment pressure, checked the working strength and reasonable working resistance range of hydraulic support at the existing working face. According to the theory of clamped beam and cantilever beam, Jin et al. (2019) analyzed the behavior law of underground pressure after ultra-long fully mechanized caving face in ultra-thick coal seams is mined. Based on the geological occurrence conditions of ultra-long fully mechanized caving face, Liu (2019) analyzed the support strength, rock pressure and coal wall stability on ultra-long fully mechanized caving face. Yu et al. (2019aYu et al. ( , 2019b conducted indoor analog simulation experiments to analyze the impact of high stress on roadway stability. Shi (2020) studied the characteristics of surrounding rock deformation and support by means of numerical simulation.
The problem of stoping on ultra-long working face is one of challenges for making coal mining safe and efficient. Strong mining effect induced by long working face mining causes a number of coal or rock dynamic disasters, such as gas outburst, rock burst It is necessary to analyze the distribution law of roof and floor support pressure induced by face length effect, the height of caving zone and the law of rock movement. For the purpose of in-depth study of the movement law of earth's surface on ultra-long working face, this paper analyzes the law of roof-to-floor strata behaviors in coal bed during mining, and studies the weakening of roof-to-floor parameters in coal bed, bearing pressure and the law of roof caving and rock strata movement. This paper also analyzes the face length effect resulted from the difference in face lengths.The above-mentioned research results can serve as a theoretical basis for practical engineering.

Project Profile
The No.9105 working face above the ground is located about 200 m north of Beili Village and Dongshi Village. In terms of its underground location, there is a no-mining reservoir area in the east, unmined areas in the south and north, and No.540 belt roadway in the west. The ground elevation is 903-932 m, and the working face elevation is 377-520 m. The 3# coal seam in which this faced is mined occurs in the middle and lower parts of Shanxi Formation of the Permian System. The seam is a continental lacustrine deposit. Within the scope of this working face, the thickness of the coal seam (6.6 m) is stable, and the thickest layer of dirt band is 0.1 m. The designed length of No. 9105 haulageway is 3650 m, the air way is 3580 m, the strike length is 340 m, and the stope length is 3432 m. The coal bulk density is 1.45 m 3 /t, and the recovery rate is 95%.
Regarding design parameters, the No.9105 working face is the most special one encountered since the mining of Wangzhuang Mine. Its major distinctive features include: the longest working face, the longest working face advancement, and simultaneous use of top coal caving, called double-length working face. Due to the superposition of the above factors, the mining of the No.9105 working face suffers from numerous uncertain challenges, such as evaluation of the production system of the working face, the law of earth's surface movement on the working face, etc. Therefore, the project studies the actual situation of the No.9105 working face and meets the technical requirements of coal mine safety production.
According to the on-site geological data, the No. 9105 working face currently adopts the top-coal caving.The working face's design length is 340 m, and its advance length is 2819 m. The designed production capacity is 3 million t/a. The histogram of the strata in the No.9105 working face of Wangzhuang Coal Mine is shown in Fig. 1.
Wangzhuang Coal Mine is a supergiant modern production mine of Lu'an Group. During the construction and production of the mine, scientific research institutes measured the uniaxial compressive strength, elastic modulus and Poisson's ratio of coal and conducted related experiments. The measurement results are shown in Table 1.

Numerical Calculation Modeling
Numerical calculation modeling is conducted with relevant simulation parameters set. As the average inclination of the working face is 5°, a near-horizontal model is built as shown in Fig. 3. In its coordinate system, the roadway axis serves as the Z axis, the X axis is horizontal, and the Y axis is vertical. Length*width*height = 420 m * 380 m * 91.5 m. The working face is 340 m long, and the reserved boundary coal pillar is 20 m. There are 6 groups of similar lithology in a top-down order, including sandstone (its material strength is slightly lower than packsand), packsand, mudstone, coal, mudstone, medium sandstone, and argiloid (its material strength is slightly higher than mudstone). In the x direction, the width of every grid near the roadway wall is every 1.35 m, and that of every grid far away from the roadway wall is 5 m. In the model, except for the irregular 8-node hexahedral element near the roadway wall, the rest elements are all regularly hexahedral. The total number of elements in the final model is 241,350 and the total number of nodes is 252776. The three-dimensional space model of the ultra-long working face is shown in Fig. 2a below, and its three-dimensional schematic diagram is shown in Fig. 2b below. The side surface of the model is of normal constraint, the top surface is of free boundary of stress and displacement, and the bottom surface is

Determination of Numeric Simulation Scheme
In the simulation, parameters such as length of the working face and mining advancement distance are changed to explore the distribution characteristics of the stope fissure zone and behavior law of mine pressure on the working face under different conditions. The specific simulation scheme covers the following three stages: The first stage: Analysis of the law of initial and periodic weighting on the working face under the influence of face length effect.
The second stage: Analysis of the characteristics and distribution law of roof caving of working faces with different face lengths.
The third stage: Research on the relationship between the length of the working face and strata pressure behavior. To be specific, the following approach is adopted to simulate the face length effect caused by mining the ultra-long working face: Staged excavation is carried out for different facelength models. The excavation length in the first step is 20 m, and the advancing distance of the working face is displayed in cross-sectional view, as shown in Figs. 3a, 4a, 5a. The excavation length in the second step is 40 m, and the advancing distance of the working face is displayed in cross-sectional view, as shown in Figs. 3b, 4b, 5b. In reference to the distribution law of lead abutment pressure on working face under the influence of effect of face length during normal advancing period, the change law of stress field of the overlying strata of the main roof can be preliminarily determined according to past experience. It is found that there are two peaks of lead abutment pressure on the working face in the main roof stratum. There is a peak area in the middle of back abutment pressure on working face in the direction of face length. The area between these two is of low abutment pressure. Compared with the mechanical mining horizon, the abutment pressure peak of the top-coal seam horizon is greater, but the impact range of abutment pressure is smaller, and the limit equilibrium zone of top-coal seam is wider. In addition, the limit equilibrium zone of abutment pressure on working face of great face length is wider during normal advancing period.

Excavation at 40m in the 3D Model of Face Length 260m
Working face length 260m Excavation 20m Working face length 260m Excavation 40m The following research is carried out to analyze the distribution of roof abutment pressure on working faces of different face lengths during advancing: First, the law of distribution of overlying rock abutment pressure on working faces during normal advancing is analyzed, and the specific points are excavation at 20 m and 40 m. Finally, the influence of face length effect on routines of activity of pressure on the roof of working face is analyzed. To be specific, for the same mining advance distance, the difference in the roof pressure distribution corresponding to working faces of face length of 260 m, 300 m, and 340 m is analyzed, as shown in Figs. 6, 7 and 8.
Routines of activity of pressure on the roof of working face during normal advance.During normal advance, the old roof strata is carried in the form of floor. With the increase of advance, the surrounding bearing reaction of the old roof strata structure gradually increases. As far as the elastic foundation effect of top coal and immediate roof are concerned, the resultant force point of bearing reaction of the simply supported edge moves forward. The peak value of the abutment pressure is increasing, namely, the peak coefficient is increasing. The distance from the two peaks to belt transporter tunnel and air way decreases, implying that the peak area moves closer to the two roadways. When the old roof structure is broken for the first time, the supporting reaction force acting on the periphery of the original plate structure decreases. In sum, the width of limit equilibrium area of working face strike abutment pressure changes from small to large, and then to small. The range of influence of abutment pressure also changes from small ? big ? small.
Routines of activity of pressure on the roof of working faces of different face length. In the influence range of abutment pressure along the 260 m working face under the superimposed influence of the lateral abutment pressure of working face, the distribution of lead abutment pressure in the middle of the forked working face in the direction of face length is shown in Fig. 6. At the position of 20 m, the abutment pressure on the overlying strata and the two sides is 15.624 MPa, and the distribution height of abutment pressure is 8.0 m. At the position of 40 m, the abutment pressure on the overlying strata and the two sides is 19.160 MPa, and the distribution height of abutment pressure is 8.8. The changes in the essential characteristics of the distribution of lead abutment pressure in the middle part of the lengthened working face are shown in Fig. 7. The limit equilibrium zone of abutment pressure on the top-coal seam is wider than that of the mining layer, and the top-coal seam's peak value is higher. The abutment pressure of the top-coal seam near the coal wall is lower than that of the mechanical mining layer. The abutment pressure of the mechanical coal mining body at the coal wall of the 300 m working face and the top-coal seam is greater than the 260 m working face. It is related to the superimposed influence of the lateral abutment pressure of the working face and the broken structure of the overlying strata main roof of the coal seam. In sum, ultra-length working face wall is subject to wall caving, but is conducive to op-coal crushing and improvement of top-coal caving properties. At the position of 20 m, the abutment pressure on the overlying strata and the two sides is 16.324 MPa, and the distribution height of abutment pressure is Leading concentrated stress at slope Trajectory of concentrated distribution of pressure on roof

Distribution of Pressure on the Roof of Working Face with Face Length 300m, Excavation at 40m
Leading concentrated stress at slope Trajectory of concentrated distribution of pressure on roof

Analysis of Roof Caving Features of Working Faces of Different Face Lengths and Regularities of Distribution
The working face length effect of the macroscopic caving features of surrounding rock is identified by means of systematical analysis and study on the mechanical characteristics of surrounding rock with different working face lengths, with numerical simulation conducted. According to previous empirical studies, changes in working face length have a significant impact on caving characteristics of overlying strata. With the increase of working face length, overlying strata caving of working face is intensified in the vicinity of the coal wall margin in front of the working face and areas near upper and lower roadways. In addition, the caving height gradually rises and the flatness rate keeps increasing. Working face is the focus of roof fracture development. The vertical displacement of the surrounding rock decreases, while the horizontal displacement increases. It indicates that reasonable adjustment of working face length can improve the dynamic balance of surrounding rock caving and plays a positive role in protecting the stope and reducing pressure on the mine. The following research is carried out to analyze the distribution of roof fracture in working faces of (a)

Distribution of Pressure on the Roof of Working Face with Face Length 300m, Excavation at 20m
Leading concentrated stress at slope Trajectory of concentrated distribution of pressure on roof Leading concentrated stress at slope Trajectory of concentrated distribution of pressure on roof

Conclusion
Based on the analysis of a numerical model of the roof and floor during coal mining, the weakening of roof and floor parameters, the size of abutment pressure, and the laws of roof caving and rock strata movement are studied. In addition, the face length effect resulted from the difference in working face lengths is analyzed to study the law of rock strata movement on ultra-long working faces. 1. The width of limit equilibrium area of working face strike abutment pressure changes from small to large, and then to small. The range of influence of abutment pressure also changes from small ? big ? small. The essential characteristics of the distribution of lead abutment pressure in the middle part of the lengthened working face changes. The limit equilibrium zone of abutment pressure on the top-coal seam is wider, and its peak value is higher. The abutment pressure of the top-coal seam near the coal wall is higher. 2. With advancing of working face, the arch bottom distribution in the plastic failure zone and changes in arch height are obvious, as the height increases by about 3m. The face length effect has a significant impact on roof caving, and the maximum sphere of influence is about 5m. 3. The study on rock strata movement is subject to the superimposed influence of working face advance and face length effect. To be specific, face length effect has a greater influence, as the maximum movement of rock strata increases by 30cm in the context of mining advancement of 20m. Rock strata movement triggered by the increase of face length effect increases by 47cm.
In particular, rock strata movement reaches the peak when the face length is 340m.
4. The initial roof weighting step is the advancing distance of the working face with the first roof fracture, and the periodic roof weighting step is the advancing distance of the working face with periodic roof fractures. According to simulation, the initial roof weighting step of working face is about 40m, and periodic roof weighting step is about 20m. 5. The height of roof caving after ultra-long working face mining is larger than that of ordinary working faces. The height of caving zone usually reaches 15.5m when No.9105 working face is weighted for Fig. 13 Influence of 300 m working face length on roof fracture development the first time, and the height of the caving zone is about 13.3m during periodic roof weighting step.