Analysis of soil cushion buffering characteristic for rockfall impact force through discrete element numerical simulation

Rock sheds is one of the main engineering solutions for mitigating rockfall disaster in the mountainous regions of western China. Typically, the roof of a rock shed is covered with a soil cushion composed of sand or gravel. The function of soil cushion is to avoid the direct impact of rockfall on the shed and absorb the impact force of the falling rocks. For a long time, there has been limited studies on the influence of soil cushion thickness on its buffering effect, leading to a lack of a unified theory guiding the design of soil cushion thickness. In this study, the discrete element method was employed to establish a numerical model of rockfall impacting onto soil cushion, and the influence of cushion thickness and rockfall falling height on the buffering characteristics of soil cushion for the rockfall impact force was investigated. The results show that there is a power function relationship between the peak of rockfall impact force and the rockfall falling height, along with a linear positive correlation between the peak of roof center force and the rockfall falling height. The peak of rockfall impact force decreases with increasing cushion thickness. Once the cushion thickness reaches 1.0 times of the diameter of rockfall, the peak of rockfall impact force becomes independent of cushion thickness as cushion thickness increases, the ratio of the peak bottom center force to the peak rockfall impact force decreases, indicating an enhancement in the soil cushion's buffering effect. However, when the cushion thickness reaches 1.5 times of the rockfall diameter, the enhancement in buffering effect becomes less significant. Therefore, the recommended cushion thickness is 1.5 times the rockfall diameter.