# Granular Flow Impact on Shed Tunnels and the Buffering Effect of Cushion Layers
## 3 Results
### Influence of the Cushion Layer
#### 3.2.1 Thickness of the Cushion Layer  
The thickness of the cushion layer directly affects its buffering effect. Therefore, in this study, the thickness of the cushion layer is roughly divided into two types. The first type is only laid between the flume and the shed tunnel, with a thickness not exceeding the height of the shed tunnel, as shown in Figure 8a-c. We selected three thicknesses: 50 mm, 100 mm, and 150 mm. The second type is laid between the flume and the shed tunnel and on the top surface of the shed tunnel. The thickness of this type is 200 mm, and the particle size of the cushioning layer is selected to be 10 mm to 20 mm, as shown in Figure 8d.  
Figure 8: \(Cont\).  
Figure 7: Impact force time history of the granular flow on the shed tunnel at different slope angles. (**a**) Normal impact force on the frontal surface; (**b**) normal impact force on the top surface.  
Figure 9a shows the time history curves of the frontal normal impact force on the shed tunnel for different cushion layer thicknesses. Except for the cases without a cushion layer and with a 50 mm cushion layer, where the differences in impact force peak values are not significant, it is evident that the peak impact force as well as the residual value decrease with an increase in cushioning layer thickness. When the cushion layer thickness reaches 200 mm, the peak value and residual value of the frontal normal impact force on the shed tunnel are approximately 102 N and 89 N, respectively. Compared to the case without a cushion layer, the peak value and residual value decrease by approximately 50.7% and 50.0%, respectively. Therefore, it can be concluded that increasing the thickness of the cushion layer can effectively reduce the impact of the granular flow on the frontal section of the shed tunnel.  
Figure 9b depicts the time history curves of the normal impact force on the shed tunnel top for different cushion layer thicknesses. When the cushion layer thickness is 0 mm, 50 mm, 100 mm, and 150 mm, there are no significant differences in the peak impact force, residual value, or their variations. However, when the cushion layer thickness reaches 200 mm, there is a noticeable decrease in the impact force. The peak value and residual value of the impact force are approximately 128 N and 64 N, respectively. Compared to the case without a cushion layer, the peak value and residual value of the impact force decrease by approximately 19.5% and 41.8%, respectively. This indicates that laying a cushion layer only between the shed tunnel and the flume has little effect on reducing the impact force on the top surface. It is only when the cushion layer covers the top surface of the shed tunnel that it can effectively provide buffering. Therefore, in practical engineering, it is necessary to increase the cushion layer thickness appropriately and lay cushion layers on both the frontal and top surfaces of the shed tunnel to better protect the shed tunnel from granular flow impact.  
Figure 8: Schematic of cushion layers with different thicknesses. (a) 50 mm; (b) 100 mm; (c) 150 mm; (d) 200 mm.  
Figure 9: Impact force time history of the granular flow on the shed tunnel at different thicknesses of cushion layers. (a) Normal impact force on the frontal surface; (b) normal impact force on the top surface.