## 5 Physical Model Experiment on Roof Blasting Pressure Relief
### Experiment Results and Analysis  
During the mining process, the collapse of the immediate roof occurred 110 cm from the initial working face. In addition, as the advancing distance increased, the breaking movement of the immediate roof gradually extended to the upper position, as shown in Figure 9.  
The lower immediate roof collapsed in a mess on the floor, as illustrated in Figure 9a. Following the breakage of the upper portions of hard roof 1, as the working face continued to advance, the micro fissures developed on the immediate roof and its overlying rock stratum. The development of micro-fractures after the effect of mining influence was coherent, and the internal structure was more fragmented. The main roof began to move periodically with the height of the overburden rock fractured zone increasing upward as the mining continued. At 130 cm, the separation fracture developed to hard roof 2. The barrier of hard roof 2 temporarily inhibited the vertical development of the fracture. Subsequently, following the fracturing of the rock layers below hard roof 2, a fracture line, with a certain angle exhibiting a bias towards the mining zone, was formed. The overburden strata formed a stable \"ladder\" type structure, as illustrated in Figure 9b. When excavating to 150 cm, the hanging roof area of hard roof 2 reached the limit collapse distance, the cantilever beam structure was unstable and collapsed. With the advancement of mining, the height of collapse rose, and the rotary space of hard roof 2 increased. As demonstrated in Figure 9c, the rotary space expanded as excavation progressed, causing the voussoir beam structure to destabilize. Figure 8: Layout of the model and the location of stress monitoring capsules. Figure 9d illustrates that as mining continued, the collapsed rock formed an effective filling support in the goat, which mitigated the impact power and energy of the collapse. Furthermore, the articulation of overburden strata reduced stress concentration between the roof and its overlying rock layer on the unbroken side. Figure 10 delineates the distribution of overburden strata fissures following the excavation. It is evident that the fissures exhibited a discernible spatial distribution pattern within the goat. The separation area and compacted area are affected by coal pillar abutment pressure. The height of the caving zone at the broken side was 100 cm, and the height of the fractured zone was 130 cm; the fissures between hard roof 1 and hard roof 2 were essentially undeveloped, with only a few minor fissures presented above hard roof 2. The caving zone at the unbroken side measured 30 cm in height, while the width of the fractured zone was continuously expanding with the excavation process, reaching a maximum width of 200 cm. The caving zone elevation on the broken side was 3.3 times that observed on the unbroken side, while the elevation of the fractured zone was 0.52 times. It meant that the implement of blasting technology can diminish the stress concentration around the roadway by increasing the height of the collapse zone. To compare the roof displacement on the side of the cut roof and the uncut roof, survey lines were arranged every 10 cm at close distance and every 20 cm at long distance above the coal seam of the physical model. There were 14 survey lines in total. Measuring points were arranged every 10 cm on the survey line. The high-resolution SLR camera (Canon EOS70D, Tokyo, Japan) equipped in the three-dimensional static deformation measurement system was used to record the experimental model from multiple angles. After the recording was completed, the pictures were imported into the computer for processing. Due to the movement of the overlying strata, the subsidence of each measuring point was discontinuous. The data of 4 measuring lines arranged at 70 m, 90 cm, 110 cm and 120 cm away from the coal seam were selected to draw the final subsidence curve in Figures 11 and 12. Figure 9: Schematic diagram of the change in the state of motion of the roof as the excavation advances.