# Effect of Thick-Hard Main Roof Fracturing on the Spatiotemporal Evolution of Overburden Fractures
## 3 Advanced Abtment Pressure Response Characteristics of the Main Roof Fracturing Weakening  
### Numerical Simulation Model and Fracturing Scheme  
According to the actual conditions of the engineering, a discrete element numerical simulation model with a length of 400 m and a height of 310 m was established by using UDEC 7.0 (Universal Distinct Element Code) numerical simulation software. Table 1 shows the selected parameters for each rock formation, and Figure 2 displays the model schematic. The thickness design of each rock layer was simplified according to the actual distribution of the strata. To avoid the influence of boundary conditions on the simulation effect, a 100 m coal pillar was reserved on both sides of the model. The Mohr-Coulomb constitutive model was used for the contact elements of the model, and the Coulomb slip area contact model was used for the contact model between the blocks.  
To study the influence of different fracturing modes on the advanced abtment stress of the working face and the spatiotemporal evolution of overburden fractures, four different numerical simulation schemes were designed based on the actual situation of the 6503\(\#\) working face. Scheme 1, without involving any treatment on the main roof, simulated the on-site working condition of the main roof reaching the first break length when the working face advanced by 100 m. In fact, Scheme 1 was used as the control group to study the weakening effect of different fracturing schemes on the advanced abtment pressure, as shown in Figure 3a. In Scheme 2, the main roof was vertically fractured with a spacing of 10 m, and the treatment range was confined to the overlying main roof when the working  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Stratum** & **Thickness** & **Density** & **Bulk Modulus** & **Shear Modulus** & **Friction** & **Cohesion** & **Tension** \\  & **(m)** & **(kg/m\({}^{3}\))** & **(GPa)** & **(GPa)** & **(\({}^{\circ}\))** & **(MPa)** & **(MPa)** \\ \hline Soft rock 3 & 90 & 2000 & 3.58 & 2.75 & 15 & 2.15 & 2.25 \\ KS 3 & 15 & 2500 & 9.56 & 7.63 & 35 & 7.25 & 4.20 \\ Soft rock 2 & 70 & 2200 & 5.60 & 4.20 & 28 & 2.24 & 3.57 \\ KS 2 & 15 & 2600 & 15.80 & 12.60 & 42 & 8.54 & 6.00 \\ Soft rock 1 & 85 & 2300 & 5.50 & 3.80 & 26 & 3.06 & 3.76 \\ KS 1 & 10 & 2500 & 12.50 & 9.60 & 35 & 6.63 & 5.50 \\ Immediate roof & 10 & 2200 & 6.20 & 4.50 & 22 & 3.32 & 1.85 \\ Coal seam & 5 & 900 & 4.65 & 3.20 & 21 & 3.13 & 2.10 \\ Coal floor & 10 & 2350 & 4.20 & 2.60 & 30 & 3.50 & 2.45 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Numerical simulation of rock formation parameters.  
Figure 2: Schematic diagram of the model.  
face advanced by 60 m, as shown in Figure 3b. In Scheme 3, the main roof was processed with bisect horizontal fracturing treatment when the working face advanced by 60 m, as shown in Figure 3c. In Scheme 4, the overlying main roof was processed with Voronoi fracturing when the working face advanced by 60 m to simulate the irregular fracturing of rock formations into rock blocks during hydraulic fracturing, as shown in Figure 3d.