# Granular Flow Impact on Shed Tunnels and the Buffering Effect of Cushion Layers
## 5 Conclusions  
This study investigated the impact force of granular flow on a shed tunnel through model experiments and PFC simulations. The effects of slope angle, cushion layer thickness, and particle size on impact forces and the buffering effect of the cushion layer were discussed. The main findings of the study are as follows:  
1. As the slope angle increases, both the peak velocity and the impact force of the granular flow on the shed tunnel increase significantly. Increasing the slope angle from 40\({}^{\circ}\) to 60\({}^{\circ}\), the peak velocity rises from 3.92 m/s to 4.94 m/s, indicating an increment of about 25%. Concurrently, the impact force also amplifies by 2-3 times. Additionally, an increase in slope angle leads to a rapid escalation in the kinetic energy of the granular flow. The peak kinetic energy at 60\({}^{\circ}\) is 2.74 times greater than that at 40\({}^{\circ}\). A larger slope angle results in higher dashpot energy, indicating more intense collisions between particles and the shed tunnel.
2. The thickness of the cushion layer directly affects the buffering effect. The augmentation of the cushion layer thickness enhances resistance and damping, resulting in a significant reduction in the kinetic energy of the granular flow and reducing the interaction between the granular flow and the shed tunnel. This helps to reduce the impact of granular flow on the front face of the shed tunnel, thus protecting its structural integrity. When the cushion layer thickness reaches 200 mm, the peak and residual values of the impact force are reduced by 50.7% and 50.0%, respectively, compared to the case without a cushion layer. When the cushion layer covers the top of the shed tunnel, the buffering effect of the cushion layer will be better.
3. A smaller particle size of the cushion layer effectively reduces the impact force of the granular flow on the shed tunnel structure. For a cushion layer with particle sizes ranging from 3 mm to 5 mm, the peak and residual values of the impact force on the front wall of the shed tunnel are reduced by 22.5% and 18.0%, respectively, compared to the case with particle sizes ranging from 10 mm to 20 mm. The smaller particle size of the cushion layer generates more friction with the granular flow, resulting in a significant change in its surface morphology, which can effectively reduce kinetic energy and provide stronger buffering effects in the later stage of movement.  
Conceptualization, Z.D.; methodology, L.W. and J.W.; software, J.W.; validation, L.W.; investigation, L.W. and J.W.; data curation, L.W. and J.W.; writing--original draft preparation, L.W. and J.W.; writing--review and editing, L.W. and Z.D.; supervision, Z.D.; funding acquisition, Z.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant number 42102318), the Shanghai Science and Technology Innovation Action Plan (21 DZ1204202) and theProgram for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (grant number TP2019037).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.