## 3 Results
### Overlying Stratum Collapse Law and Porosity Simulation  
The coal seam was divided into 10 blocks in the caving direction and the blocks were sequentially extracted in the caving direction to simulate the overlying stratum collapse and porosity evolution during caving. As shown in Figure 5(a), the first block caving already makes the immediate roof loose, resulting in fractures, yet it still can provide support to the overlying stratum. The primary roof and the height of each floor remain unchanged. As shown in Figure 5(b), the caving of the second block enhances the stress on the primary roof. The roof cannot provide the supporting function and collapses. The number of cracks in the overlying stratum continuously increases. As shown in Figure 5(c), after the caving of the third block, the immediate roof of the first block has collapsed and that on the second block is loosened yet not collapsed. As shown in Figure 5(d)-i, starting from the third block, the roofs and overlying strata in the previous gob blocks collapse after each block caving, but the immediate roof of the previous block of the current block is loosened but does not collapse. As shown in Figure 5(j), the simulation is finished at the caving of the 10 th block. The growth rate of fracture in each stratum slows down but the number of fractures continues to increase. Once the collapse in the gob is finished, the fallen rock piles up and the number of fractures decrease to a stable value. Figure 5: Oxygen concentration distribution at each measuring point (own elaboration). Figure 6: _Cont._Figure 6: _Cont._  
Based on the simulation results of overlying stratum collapse situation, the gob of the 31,116 working face can be regarded as porous medium space. The relevant parameters of this porous medium are then determined and the corresponding equations are established for simulation, calculation, and analysis. The simulation of caving face dynamically tracks the changes in the model porosity during the caving process using measurement circles and the data at each time node are processed for dynamic porosity change of the entire model to summarize the gob porosity evolution law. The overlying stratum collapse and gob porosity in each caving step is analyzed as shown in Figure 6. The porosities of the unmined coal seam and overlying stratum are small, ranging from 0.15 to 0.3, and their distribution is relatively uniform. As the  
Figure 6: Overlying stratum collapse and porosity change pattern during coal seam caving (own elaboration). (**a**) First caving step. (**b**) Second caving step. (**c**) Third caving step. (**d**) Fourth caving step. (**e**) Fifth caving step. (**f**) Sixth caving step. (**g**) Seventh caving step. (**h**) Eighth caving step. (**i**) Ninth caving step. (**j**) Tenth caving step. working face is caved, the overlying stratum keeps collapsing and accumulates with the advancement of the working face, which changes the gob porosity. The porosity changes during the 10 caving steps in Figure 6 suggest that the coal seam is in an unexploited state at the first caving step. The porosity shows the blue original state with the value of 0.1, and there are no obvious changes observed in the overlying stratum and key floors. By the fourth caving step, e.g., almost halfway through, the porosity is greatly changed. In the coal seam, the working face is in the stress-reduced zone and the immediate roof dose not collapse, resulting in the red-yellow area with porosity of about 0.75-0.95. The uncollapsed ranges at two ends of the working face are longer than other positions due to the supporting effect of the coal pillars on the roof and, thus, the red and yellow areas at the two ends are more obvious. The roof collapses at other positions in the gob are basically stable, resulting in porosities ranging from 0.25 to 0.55. The upper immediate roof near the working face shows no collapses but obvious cracks are formed, resulting in the porosity increasing from the original 0.12 to 0.21.