# Permeability Enhancement Induced by Fracture Shear Dilation During Close-Range Coal Seam Mining
## 4 Evolution of Fractures of Close-Distance Coal Seam Group
### Fracture Evolution of Overlying Strata  
In this section, the 3 DEC V4.1 software was used to simulate the overlying strata collapse and fracture field development process after mining in coal seam 8#, to obtain the overburden collapse state and the damage degree of the surrounding rock at different times, which were used to calculate the opening value of the fracture. The simulation process was divided into three steps. The first simulation pushed forward to 65 m and then waited for 10 days to obtain the evolution process of the overburden collapse and fracture zone. Thereafter, the simulation pushed forward to 120 m to observe the overburden state changes, and finally pushed forward to 180 m. The overlying strata exhibited a periodic collapse, forming a collapse zone, a fracture zone, and a bending subsidence zone. Figure 5 shows the distribution of the overburden displacement and fracture opening after 65 m advancement of the working face. In the initial stage, the direct roof bending subsidence was the largest at the center of the goat, and the subsidence at the mining boundary gradually decreased. As the fracturing of the old roof continued, the overall subsidence in the caved area kept increasing. As shown in Figure 6, the bottom plate also bulges with a displacement of 12 cm. In the initial stage, the distribution of the fractures between rock strata was relatively uniform. In the vertical direction, the fracture aperture is mainly determined by the vertical displacement of the rock stratum. The bending and subsidence of the overburden led to the separation of rock strata and a channel for gas to flow through. \begin{table}
\begin{tabular}{l l} \hline
**Parameters** & **Values** \\ \hline Constants, \(k\) & \(7\times 10^{-6}\) m/N \\ Hurst coefficient for a fracture surface, \(H\) & 0.85 \\ Joint normal stiffness of coal & 2000 MPa/m \\ Joint normal stiffness of sandstone & 2900 MPa/m \\ Joint normal stiffness of mudstone & 2200 MPa/m \\ Joint shear stiffness of coal & 1900 MPa/m \\ Joint shear stiffness of sandstone & 2500 MPa/m \\ Joint shear stiffness of mudstone & 2000 MPa/m \\ Tension capacity of coal & 0.07 MPa \\ Tension capacity of sandstone & 0.1 MPa \\ Tension capacity of mudstone & 0.09 MPa \\ \hline \end{tabular}
\end{table} 
Table 1: Cont. Figure 4: Overview diagram. When the excavation reached 120 m, a wide range of bending subsidence occurred, leading to periodic pressure variation. From the displacement curves of the rock strata at different positions, as shown in Figures 7 and 8, we find that the roof strata above goal present overall subsidence before the position of the working face. Due to the cantilever beam support, the amount of subsidence within 10 m behind the working face was small, but the amount of subsidence gradually increased in the area where the overburden was fractured. Meanwhile, the floor continued to heave, and the displacement was 15 cm. As shown in Figure 6(b), the fracture opening at the goat boundary position was large, and the overburden separation at the top increased after 120 m, resulting in an increase in the fracture opening. At the original working face position of 65 m, due to roof collapse, the cracks were compressed in the center but remained open at the 120 m position. The roof  
Figure 5: Distribution of displacement and fracture aperture after working face advanced to 65 m. (Line a, b, c, d, and e are monitor lines). Figure 6: The curves of vertical displacement at different heights of rock strata after working face advanced to 65 m.  
fracture caused the increases in the crack density and aperture in the goat boundary. At this stage, the gas flow channel was mainly located within 30 m behind the working face. As the working face continued to advance, the overlying strata collapsed periodically. Behind the new caved strata, the old fractures closed, the collapsed rock mass in the middle of the goat was compacted, and the collapse swelling rate of the caved strata was reduced.