# Granular Flow Impact on Shed Tunnels and the Buffering Effect of Cushion Layers
## 1 Introduction  
Granular flow disasters, such as mudslides, landslides, and rock avalanches, frequently occur all over the world with characteristics of large scale, high mobility, long runout distances, and significant destructive power [1; 2; 3]. They can be triggered by earthquakes, extreme climate, and various human factors and result in significant casualties and economic losses [4; 5]. For example, in April 1903, a massive landslide with a volume of approximately 30 \(\times\) 10\({}^{6}\) m\({}^{3}\) occurred on Turtle Mountain in southwestern Alberta, Canada. The landslide traveled more than 3000 m, blocked a river, and devastated the town of Frank, resulting in the deaths of over 10,000 people [6; 7; 8]. In October 1963, a rockslide with a volume of 200-300 million m\({}^{3}\) occurred near the Vajont dam in Italy with a speed of 20-30 m/s, leading to 2500 deaths [9; 10]. In April 1974, an enormous catastrophic rocklide with an estimated volume of 10\({}^{9}\) m\({}^{3}\) occurred in the Peruvian Andes of Peru. The rockslide traveled a vertical distance of 1.9 km at an average velocity of about 130 km/h, causing about 451 deaths and severe destruction [11]. In April 2000, a catastrophic landslide damming event occurred on the Yigong River in Tibet, China. The landslide traveled 10 km within 10 min, with a vertical drop of over 3300 m. The volume of the landslide was estimated to be 28 \(\times\) 10\({}^{7}\)-30 \(\times\) 10\({}^{7}\) m\({}^{3}\), resulting in the formation of a massive landslide dam. This led to a rapid rise in water levels upstream, causing widespread flooding in surrounding villages and posing a significant threat to the lives and properties of downstream residents [12; 13; 14]. In 2008, the Wenchuan earthquake in China triggered thousands of landslide disasters. These earthquake-induced landslides were highly destructive andresulted in approximately 20,000 deaths and significant economic losses [15; 16; 17]. Granular flow and landslide disasters are sometimes caused by their inherent factors, including topography, slope gradient, rock mechanics characteristics, and internal joints [18]. To protect mountainous roads from granular flow disasters, shed tunnels are often installed in sections prone to geological hazards. Shed tunnels serve as protective measures and effectively safeguard the road, vehicles, and pedestrians in the event of a granular flow disaster. Traditional shed tunnels can be classified into different types based on their shape and function, such as portal sheds, rectangular sheds, arched sheds, and semi-arched sheds. Usually, a cushion layer is laid on the mountain side of the shed tunnel as a suitable and effective measure of disaster mitigation. The cushion layer often consists of gravel, sand, or other buffering materials. It can absorb a portion of the impact energy from the granular flow, thereby protecting the shed structure [19; 20]. However, when subjected to intense granular flow impact, the shed tunnel may still sustain damage due to excessive forces. Therefore, investigation into mitigation measures to reduce the impact force of granular flows on shed tunnels is of practical significance. Extensive research has been conducted on the dynamic impact of rolling and falling rocks on shed tunnels, as well as the dynamic response of shed tunnels [21; 22; 23; 24; 25]. Compared to the impact of rolling and falling rocks, granular flows have more complex mechanical properties, and they can cause damage to shed tunnels even without reaching the ultimate bearing capacity. The dynamic effects of granular flow impact on shed tunnels and the cushioning effects of the cushion layer have been investigated through methods such as model experiments, numerical simulations, and theoretical analysis [26; 27; 28; 29; 30; 31]. Indeed, although there have been significant advancements in research on the dynamic impact of granular flow, there are still many unresolved challenges regarding the underlying mechanical mechanisms.