Numerical simulation of realistic top coal caving intervals under different top coal thicknesses in longwall top coal caving working face

In the process of longwall top coal caving, the selection of the top coal caving interval along the advancing direction of the working face has an important effect on the top coal recovery. To explore a realistic top coal caving interval of the longwall top coal caving working face, longwall top coal caving panel 8202 in the Tongxin Coal Mine is used as an example, and 30 numerical simulation models are established by using Continuum-based Distinct Element Method simulation software to study the top coal recovery with 4.0 m, 8.0 m, 12.0 m, 16.0 m, 20.0 m and 24.0 m top coal thicknesses and 0.8 m, 1.0 m, 1.2 m, 1.6 m and 2.4 m top coal caving intervals. The results show that with an increase in the top coal caving interval, the single top coal caving amount increases. The top coal recovery is the highest with a 0.8 m top coal caving interval when the thickness of the top coal is 4.0 m, and it is the highest with a 1.2 m top coal caving interval when the coal seam thickness is greater than 4.0 m. These results provide a reference for the selection of a realistic top coal caving interval in thick coal seam caving mining.

Top coal caving mining technology started in the 1980s and continues to this day 1,2 . Longwall top coal caving (LTCC) mining consists of two processes: shearer cutting in front of a hydraulic support and top coal caving behind a hydraulic support. The parameters of the top coal caving interval (TCCI) in the process of top coal caving behind a hydraulic support have an important impact on the top coal recovery 3,4 . The top coal recovery is an important index to evaluate the success of top coal caving mining. Therefore, improving the top coal recovery is an important technical problem faced by LTCC mining [5][6][7] .
Wang et al. derived an equation of a possible caving ellipsoid and an actual caving ellipsoid on the basis of caving ellipsoid theory and analyzed the optimization calculation method of caving parameters along the dip direction of the working face [8][9][10] . Sun et al., using a similar simulation, constructed 15 coal drawing models according to different TCCIs, coal drawing sequences, coal drawing methods and coal seam dip angles; they compared and analyzed the influence of different coal drawing conditions on the top coal recovery and the movement of top coal and gangue, and they concluded that the top coal recovery of the working face was the highest when a single round sequential coal drawing method from bottom to top was adopted in the working face dip direction 11,12 . Li et al., with the method of orthogonal experimental design, used the top coal crushing coefficient as the index and the hydraulic support height, hydraulic support length and TCCI of the hydraulic support as the influencing factors to carry out range and variance analyses on the simulation results of PFC 2D and obtain the sensitivity of each influencing factor on the top coal recovery 13 .
Yang et al., by using the method of numerical simulation, analyze the top coal recovery rate of the dynamic group caving method (DGCM) and single-opening sequence caving method (SSCM) under different mining and caving ratios. When the mining caving ratio is less than 1:1, the recovery ratio of DGCM is obviously higher than that of SSCM, indicating that DGCM can greatly improve the recovery ratio. While when the mining-caving ratio is larger than 1:1, the recovery ratio of two caving methods is basically equal; however, the advancing time is significantly shortened under DGCM 14 .

Numerical simulation model
Establishment of the numerical simulation model. In this study, CDEM software is used to simulate the top coal caving during the mining process of panel 8202. The CDEM software was developed by the Institute of Mechanics, Chinese Academy of Sciences, and its algorithm couples the finite element method (FEM) and the discrete element method (DEM) 18 .
To research the influence of the TCCI on the top coal recovery under different top coal thickness conditions, according to the specific size parameters of the ZF15000/27.5/42 low-level caving hydraulic support used in the 8202 LTCC working face of the Tongxin Coal Mine, simplified processing is first carried out to establish a twodimensional numerical simulation model of the caving hydraulic support, as shown in Fig

Model parameters.
According to different forces on the top coal in different rock layers, the degree of particle fragmentation increases from top to bottom. The top coal particles of the established numerical simulation model decrease sequentially from top to bottom. The particle size setting of each rock layer in the model is shown in Table 1, and the mechanical parameters of the particles in the model are shown in Table 2.

Numerical simulation of the top coal caving interval
The selection of the TCCI in the LTCC working face has a great influence on the top coal recovery. Both too large and too small of a TCCI leads to a decrease in the top coal recovery. Based on the current LTCC working face, the shearer cutting depth is mainly 0.6 m and 0.8 m, while very few adopt 1.0 m, and the integer multiple of the shearer cutting depth is selected as the TCCI. A large TCCI is poorly applied in production practice  Figure 3 shows that there is no obvious difference in the first three steps of the top coal recovery of 4.0 m thick top coal under different TCCIs. When there is no gangue boundary on the left side of the caving window, the first step of the top coal caving amount under different TCCI conditions is relatively large, and the caving body is cut into an oval-like shape.
In the subsequent second and third top coal caving steps, due to the large top coal caving amount in the first step, the gangue above the top coal moves to the vicinity of the top coal caving window. After the first top coal caving is completed, the gangue is close to the upper left of the top coal caving window. When the top coal caving window is opened and the top coal is cleaved again, it is easy to cause the gangue near the top coal caving window to mix in advance and end the current top coal caving process. Therefore, the recoveries in the second and third top coal caving steps are greatly reduced compared with the recovery in the first top coal caving step. Only a few top coal particles are recovered before the gangue reaches the recovery area of the rear scraper conveyor, and there is no obvious shape of the top coal drawing body.
The       Fig. 8. Due to the large thickness of the top coal and the long interface of the coal-gangue interbeds, during the caving process, the gangue above the caving window easily reaches the caving window before it reaches the top coal above the caving window, leading to the end of top coal caving. Therefore, the absolute amount of top coal loss is relatively large in the process of thick top coal caving. The horizontal distance between two adjacent coal gangue interlayers is 16-20 m.      ity of hydraulic support, the width of scraper conveyor and the mining height of LTCC working face increase, the top coal caving ratio may break through the limit of 1:3, and the optimization of top coal caving interval will more consider the cooperation with mechanical equipment.