# Granular Flow Impact on Shed Tunnels and the Buffering Effect of Cushion Layers
## 4 Discussion  
### Effectiveness of Cushioning  
In this section, the buffering effects are analyzed based on the morphology of the granular flow and the cushion layer. Figure 11 presents the impact states, which are observed under different cushion layer thicknesses and particle sizes at six time points: 0.75 s, 0.80 s, 0.85 s, 0.90 s, 1.00 s, and 1.30 s.  
When the cushion layer is composed of coarse particles with a thickness of 50 mm (Figure 11a), the interaction between the granular flow and the cushion layer is not significant due to the small thickness of the cushion layer. As a result, the granular flow quickly impacts the shed tunnel. This corresponds to the results of the frontal normal impact force peak values without a cushion layer and with a 50 mm cushion layer, as discussed in Section 3.2.1.  
When the cushion layer consists of coarse particles with a thickness of 100 mm (Figure 11b), significant changes can be observed in the surface morphology of the cushion layer as the granular flow impacts it. Depressions appear near the flume, resulting in an overall convex shape. During this deformation process of the cushion layer, some of the energy of the granular flow is dissipated due to the friction between particles. However, since the thickness of the cushion layer is smaller than the height of the shed tunnel, the granular flow can still directly impact the shed tunnel after hitting the cushion layer. This can explain why, in Section 3.2.1, there is still a peak in the frontal normal impact force when using a 100 mm cushion layer.  
As the thickness of the cushion layer reaches 150 mm, aligning its top surface with the shed tunnel top, as shown in Figure 11c, the deformation of the cushion layer after being impacted by the granular flow is similar to that of a 100 mm cushion layer. It eventually takes on a convex fan-shaped form. Since the thickness of the cushion layer is equal to the height of the shed tunnel, the granular flow cannot directly impact the frontal portion of the tunnel after hitting the cushion layer. Instead, it directly impacts the top of the shed tunnel along the cushion layer. In this case, the time history curve of the frontal normal impact force is relatively smooth, without any significant peaks.  
When the thickness of the cushion layer is 200 mm, as shown in Figure 11d,e, the cushion layer undergoes greater deformation and more particles are splashed upon impact. The granular flow is unable to directly impact the shed tunnel. In both cases, the time history curves of the normal impact force are smaller compared to the previous three cases. In the case of a fine particle cushion layer, the smaller particle size results in a larger surface area, allowing for more friction between the particles, which leads to a better buffering effect. The final surface morphology of the cushion layer is more irregular compared to the case with coarse particles.  
Figure 11: _Cont._Figure 11: Variations in the morphology of cushion layers with different thicknesses and particle sizes during the impact process. (**a**) 50 mm coarse cushion layer; (**b**) 100 mm coarse cushion layer; (**c**) 150 mm coarse cushion layer; (**d**) 200 mm coarse cushion layer; (**e**) 200 mm fine cushion layer.