## 5 Case Studies
### Case Two
#### 5.2.2 Test Process and Analysis of Results
After the roadbed is placed and the secondary settlement of the coal mine goat is stable, the dynamic load is applied by the actuator to observe the fracture development and surface settlement of the coal mine goat site. The dynamic load is divided into a group according to 100 train load cycles. As depicted in Figure 17, during the application of the dynamic load, it can be observed that the separation position is continuously developed upward and some cracks are closed. Figure 16: Completed settlement of goat before loading. Figure 17: Model after application of dynamic load. When the load is applied to the 900-1000 th cycle, the coal mine goaf site mutates, as depicted in Figure 18, the boundary cracks of the coal mine goaf penetrate the surface, and the middle of the coal mine goaf site is separated and the cracks are closed. The subgrade is inclined as a whole, the inclination angle is about 12\({}^{\circ}\), and the maximum settlement of the subgrade is 1230.74 mm. Figures 19 and 20 correspond to the overall settlement deformation cloud map under the action of \"M\" wave 250,000 times and 500,000 times, respectively. During the test, the subgrade has uneven settlement, with the deformation value being more prominent on the right side compared to the left side. As the number of cycles for the \"M\" wave increases, the uneven settlement experiences a substantial rise. In such instances, the influence depth of settlement deformation is also significantly greater than that of the left side. This is because the ground on the right side of the subgrade was broken during the loading process, which makes the dynamic stress more borne by the lower geotechnical body, thus making the accumulated settlement deformation of the right side of the ground significantly larger than the left side. As shown in Figures 21 and 22, the development of the separation layer and the trend of surface subsidence show three stages: slow development-rapid development-mutation. The slow development stage may demonstrate that under the impact of dynamic load, the upper space of the initial separation layer constitutes a simply supported beam structure serves the purpose of bearing the load and resisting deformation, so that the separation layer develops slowly. Due to an escalating number of cycles and the influence of vibration, the rock beam is layered, unable to bear the upper load, and its thickness decreases, which makes the development speed of the separation layer faster. When the thickness of the rock beam is reduced to a certain value, the upper load is much larger than the load it can bear,  
Figure 19: Nephogram of settlement deformation under 250,000 cycles of dynamic load. Figure 18: Sudden instability of coal mine goaf. resulting in the bending or even breaking of the rock beam; thus, the surface of the coal mine goaf is unstable and the subgrade falls into the ground. Figure 13:0 Development height of overburden separation under dynamic load. Figure 13:1 Surface subsidence curve under dynamic load. From the analysis of the above test results, it becomes evident that the main reason for the activation deformation of the coal mine goat site of the high-speed railway is that the upper rock mass structure of the separation layer cannot bear the subgrade load and the dynamic load generated by the train. If the separation layer is grouted before the construction, the ground of the coal mine goat site will be strengthened, the height of the separation layer will be reduced, and the top of the separation layer rock (soil) beam structure thickened, so that it is sufficient to bear the subgrade and the train load can effectively reduce the possibility of the activation deformation of the coal mine goat site. The same can also be carried out before roadbed construction using methods such as the strong ramming on a pre-built site to compress the dense coal mine goat site and closing the separation layer closer to the surface, thus reducing the activatable space of the coal mine goat.