# An FDM-Based Dynamic Zoning Method for Disturbed Rock Masses above a Longwall Mining Panel
## 4 Discussion  
The four zones of overlying disturbed rock strata are dynamically developed during the mining process, and the mechanical properties in different zones are correspondingly  
Figure 18: Contour maps of surface displacement: (**a**) X direction; (**b**) Y direction; (**c**) Z direction. changed. The dynamic zoning characteristics of disturbed rock masses should not be neglected. Otherwise, the numerical simulation will lead to deviations from the actual results. To predict the dynamic development characteristics and the height of the four zones more accurately, an FDM-based dynamic zoning method for the disturbed rock masses above the longwall mining panel is proposed. The proposed method is applied to the Taixi coal mine, and the dynamic development characteristics of the four zones of overlying disturbed rock masses are analyzed and predicted according to the dynamic zoning results. The coal-seam roof above the mined-out area does not fall immediately with the advancement of the excavation face but caves periodically during the mining process. When the excavation face advances a short distance, the coal-seam roof can be considered a cantilever beam (cantilever plate), and bending deformation occurs due to gravity. When the excavation face advances to 30 m, the coal-seam roof caves for the first time. With the advance of \(L_{p}\) (\(L_{p}\) = 10-30 m), the coal-seam roof caves periodically until the end of mining. The periodic caving characteristics of the coal-seam roof conform to the caving phenomenon in actual mining [2] and other research work [3]. More specifically, [PERSON] et al. [3] used UDEC to reveal the progressive caving of strata above a longwall panel. Their numerical results indicated that the immediate roof acts like beams and collapses periodically. The features of progressive caving fit reasonably well with the field observations in the Ruhr coalfield. [PERSON] et al. [11] conducted a physical modeling experiment to simulate the overlying strata destruction induced by mining and revealed the different roof failure zones. Their results showed that the overburden failure evolution was accompanied by periodic collapse in the layer group and that the developing height of fractures discontinuously jumps. The formation of the four zones is a dynamic process and strongly relevant to coal-seam roof caving. With the advancement of the excavation face, the coal-seam roof periodically caves, and the four zones will also periodically expand upwards and forwards. When the mined-out area reaches a certain extent, the heights of the four zones are nearly constant at maximum, and then the four zones will only develop along the mining direction. The development characteristics of the four zones are in accordance with results in the literature [6,14,27]. After finishing the entire mining process, the heights of the caved and fractured zones are 12 m and 102.8 m, respectively. By using the empirical formula [1] to calculate the maximum heights of the caved and fractured zones, the calculated maximum heights are 7-28 m and 105-175 m, which are consistent with the numerical modeling results. After the completion of mining, the final subsidence basin can be roughly divided into three subzones, i.e., the central zone, the compressive zone, and the tensile zone. The maximum surface subsidence is approximately 2.86 m, and the subsidence coefficient is 0.82 (Equation (4)). As the Taixi coal mine has not yet been mined, the measured surface subsidence cannot be obtained. There are several coal mines close to the Taixi coal mine with similar geological conditions, including the Yanzhou coal mine (80 km away from the Taixi coal mine in the southeast), Zaozhuang coal mine (140 km away from the Taixi coal mine in the southeast), and Fengfeng coal mine (180 km away from the Taixi coal mine in the west). The subsidence coefficients of these neighboring coal mines are in the range of 0.78-0.88 (Table 2).